Characterization and Simulation of Material Distribution and Fiber Orientation in Sandwich Injection Molded Parts

Transcription

1 Characterization and Simuation of Materia Distribution and Fiber Orientation in Sandwich Injection Moded Parts Von der Fakutät für Maschinenbau der Technischen Universität Chemnitz genehmigte Dissertation zur Erangung des akademischen Grades Doktor-Ingenieur (Dr.-Ing.) vorgeegt von M. Eng. Somjate Patcharaphun geboren am in Bangkok, Thaiand Gutachter: Prof. Dr.-Ing. G. Mennig Prof. Dr.-Ing. J. Wortberg Prof. Dr.-Ing. Habi. B. Wieage Tag der Einreichung: Tag der Verteidigung: URL: ISBN: ( )

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3 Bibiographic Description Author: Patcharaphun, Somjate Topic: Characterization and Simuation of Materia Distribution and Fiber Orientation in Sandwich Injection Moded Parts A Dissertation submitted to the Facuty of Mechanica Engineering, Institute of Mechanica and Pastics Engineering, Chemnitz University of Technoogy, Pages, 69 Figures, 14 Tabes, 144 References Abstract In this work, the materia distribution, structure of fiber orientation and fiber attrition in sandwich and push-pu injection moded short fiber composites are investigated, regarding the effect of fiber content and processing parameters, given its direct reevance to mechanica properties. The prediction of the tensie strength of conventiona, sandwich and push-pu injection moded short fiber composites are derived by an anaytica method of modified rue of mixtures as a function of the area fraction between skin and core ayers. The effects of fiber ength and fiber orientation on the tensie strength are studied in detai. Modeing of the speciaized injection moding processes have been deveoped and performed with the simuation program in order to predict the materia distribution and the fiber orientation state. The secondorder orientation tensor ( a 11 ) approach is used to describe and cacuate the oca fiber orientation state. The accuracy of the mode prediction is verified by comparing with corresponding experimenta measurements to gain a further basic understanding of the met fow induced fiber orientation during sandwich and push-pu injection moding processes. Key words: Sandwich injection moding, Push-Pu injection moding, Fiber orientation distribution, Fiber ength distribution, Materia distribution, Mechanica properties, Numerica simuation.

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5 Acknowedgements This work is based on research conducted between March 2003 and Apri 2006 at the Institute of Mechanica and Pastics Engineering at the University of Chemnitz. A number of peope have contributed to the competion of this thesis, and deserve to be thanked. First and foremost, I woud ike to express my sincere gratitude to my supervisor, Prof. Dr.-Ing. Günter Mennig for his invauabe guidance and encouragement throughout my studies. His support during my pursuit of the doctor degree wi aways be appreciated. Specia thanks to Dr.-Ing. Hannes Michae for his kindness and encouragement. I enjoyed the many hours of ivey exchanges (technica or not) that we had and ook forward to future coaborations. I woud ike to express my thanks to Dip.-Ing. Hemut Püschner, and other coeagues at the aboratory, who provided invauabe support concerning the experimenta part. Thanks aso go to M.Tech. Kaushik Banik, M.Sc. Bin Zhang, Loic Bouteruche and Anne Hewitson, for interesting discussions, many vauabe experimenta data, and great friendships. I wish to extend my thanks to TARGOR GmbH, BUNA GmbH, BASF AG, and BAYER GmbH, Germany for the cost free suppy of materias. Financia support from the Facuty of Engineering, Kasetsart University, Thaiand is gratefuy acknowedged. Most importanty, my deepest thanks go to my parents, my sisters, and my wife for their dedication and inspiration that enabed me to reach this miestone in my ife. Thank you for keeping me afoat when I was down and thank you, by aways being there when I needed you, for reminding me of the goodness of ife. Chemnitz, 2006 Somjate Patcharaphun

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7 Contents Acknowedgements Bibiographic Description Nomencature V 1. Introduction Conventiona injection moding process Two-component injection moding process Co-injection (Sandwich) moding Gas- and Water-assisted injection moding Overmoding Speciaized injection moding techniques for enhancing properties of thermopastics and composites Mutipe ive-feed injection moding Push-pu processing Sequentia injection moding Simuation of the injection moding and speciaized processes Simuation of the conventiona injection moding process Simuation of some speciaized injection moding processes Research objectives Outine of the thesis 16

8 II Contents 2. Moding of Short Fiber Reinforced Composites Rheoogy of short fiber composites Microstructure of injection moded short fiber composites Fiber orientation Fiber attrition during moding Mechanica properties Predictive methods of tensie strength for short fiber composites Modified rue of mixtures (MROM) Area fraction method Modeing of the Injection Moding Process Governing equations Predicting fiber orientation Characterizing orientation Fow-induced fiber orientation Numerica simuation of fiber orientation for injection moding Experimenta and Simuation Procedures Materias and processing conditions Sandwich injection moding Push-pu injection moding Microstructure anayses Skin/core materia distribution Fiber orientation anaysis Fiber ength anaysis (Fiber attrition) Mechanica testing Process simuation Pre-processing 53

9 Contents III Simuation approach Simuation of skin/core materia distribution in sandwich injection moding Simuation of 3-D fiber orientation distribution in sandwich and push-pu injection modings Experimenta Resuts and Discussion Comparison between conventiona and sandwich injection modings Fiber orientation distribution Fiber ength distribution (Fiber attrition) Mechanica properties Comparison between conventiona and push-pu injection modings Geometry of wedines Fiber orientation in wedine areas Effects of hoding pressure difference and fiber concentration on penetration ength of wedine Fiber ength distribution in wedine areas Wedine strength Prediction of tensie strength for short fiber reinforced composites Comparison between Simuation and Experiment Sandwich injection moding Effect of skin/core voume fraction on the skin/core materia distribution Effect of processing parameters on the skin/core materia distribution Effect of skin and core met temperatures Effect of skin and core injection fow rates Effect of mod temperature 98

10 IV Contents Effect of gass fiber content on the skin/core materia distribution Case study Simuation of fiber orientation in sandwich injection moding Simuation of fiber orientation in push-pu injection moding Concusions References Curricuum Vitae 137

11 Nomencature Symbo Meaning Unit σ CU Utimate strength of the composite MPa σ f Utimate strength of the fiber MPa V f Voume fraction of the fiber - V m Voume fraction of the matrix - σ m Stress deveoped in the matrix MPa f 0 Fiber orientation efficiency factor - f Fiber ength efficiency factor - a n Proportion of fibers making an ange ϕ n with respect to the appied oad or fow direction - Fiber ength μ m c Critica fiber ength μ m d Diameter of fiber μ m τ Interfacia shear strength between fiber and matrix MPa τ m Shear strength of the matrix MPa F C Tota oad sustained by the composite N F L Load carried by ongitudina fibers N F T Load carried by transverse (or random) fibers N A A L C Area fraction between the skin region and the cross-sectiona area of specimen -

12 VI Nomencature Symbo Meaning Unit A A T C Area fraction between the core region and the cross-sectiona area of specimen - σ UL Utimate tensie strength of the skin materia MPa σ UT Utimate tensie strength of the core materia MPa f 0 skin Fiber orientation efficiency factors for the skin ayer - f 0 core Fiber orientation efficiency factors for the core ayer - A skin Cross-sectiona area of skin materia A core Cross-sectiona area of core materia 2 mm 2 mm ρ Density 3 kg / m P Pressure Pa C p Specific heat at constant voume J. kg 1 1. K T Temperature C 3 v Specific voume m / kg S Rate of heat generation due to chemica reaction u Veocity vector - g Body force vector - q Heat fux vector - Gradient operator - D Dt Substantia derivative - τ Extra stress tensor - 3 W / m η Non-Newtonian viscosity Pa. s γ& Strain rate tensor - k Heat conduction coefficient - I Identity matrix - α Compressibiity coefficient -

13 Nomencature VII Symbo Meaning Unit β Therma expansion coefficient - h g Met-mod heat transfer coefficient - ( θ φ) ψ, Orientation distribution function - a ij Second order orientation tensor - a ijk Fourth order orientation tensor - λ Shape factor of partice - r e Aspect ratio of the eipsoid - C I Fiber interaction coefficient - δ b Thickness fraction of the core materia - x i L 0 Measured distance ratio between ength of ϕ i measurement and tota ength of specimen - Ange between the individua fiber and the oca N ϕ i fow direction Number of fibers with a certain ange to the oca fow direction - % Δ Percent difference between the number average fiber ength inside the granues and the overa gass fiber ength inside the moded part % G Average fiber ength inside the granues μ m j Loca fiber ength inside the individua ayers of sectioned part μ m

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15 1. Introduction 1.1 Conventiona injection moding process Injection moding process is one of the most widey used operations in the poymer processing industry. It is characterized by high production rate, high automation, and accurate dimensiona precision. Products ranging from as sma as pastic gears to as arge as automobie bumpers can be injection moded. Injection moding process is accompished in an injection moding machine (Figure 1.1) which basicay consists of two essentia components; the injection unit and the camping unit. The function of the former is to met the poymer and inject it into the mod cavity, whereas the camping unit hods the mod, opens and coses it automaticay, and ejects the finished products. Figure 1.1 Schematic drawing of a typica injection moding machine. [1]

16 2 Introduction The most common type of injection moding machine is the in-ine reciprocating screw type. The screw both rotates and undergoes axia reciprocating motion. When the screw rotates, it acts ike a screw extruder, meting and pumping the poymer. When it moves axiay, it acts ike an injection punger, pushing the poymer met into the mod cavity. The screw is generay driven by a hydrauic motor and its axia motion is activated and controed by hydrauic system. The raw materia is suppied to the injection moding machine through the feed hopper, which is ocated on top of the injection unit. The screw takes in the materia and conveys it to the screw tip. On its way, the pastic passes through heated barre zones, whie the rotation of the screw resuts in a continuous rearrangement of the pastic materia in the fights of the screw. Shear and heating from the barre wa cause a argey homogeneous heating of the materia. The conveying action of the screw buids up the pressure in front of the tip. This pressure pushes back the screw. As soon as there is enough suppy of met in front of the screw, the screw moves forward to inject the moten materia into the mod cavity. The injection moding process can be subdivided into four stages: (a) injection, (b) packing, (c) cooing, and (d) ejection. The cyce begins when the mod coses, foowed by the injection of the poymer into the mod cavity. Once the cavity is competey fied, a hoding pressure is maintained to compensate for materia shrinkage. As soon as the gate is competey frozen, no more materia can be injected and, the packing pressure is reeased and the screw turns, feeding the next shot to the front of the screw. When the part is sufficienty coo, the mod opens and the part can be taken out for further cooing to the ambient temperature.

17 Introduction Two-component injection moding processes During the past two decades, numerous attempts have been made to deveop injection moding process to produce products with specia design features and properties. Two component injection moding being an aternative process derived from conventiona injection moding has created a new era for additiona appications, more design freedom, and specia structura features. These efforts have resuted in a number of processes, incuding: Co-injection (Sandwich) moding Gas- and Water-assisted injection moding (GAIM and WAIM) Overmoding Further two-component injection methods e.g. Insert moding and Rotating mod techniques are beyond the scope of this section Co-injection (Sandwich) moding Sandwich injection moding is an extension of the standard injection moding technoogy which aows for two components to be sequentiay injected into the mod in order to fabricate products with a ayered structure. This processing technoogy was first invented by Garner and Oxey of ICI [2]. Figure 1.2 shows a schematic principe of the sandwich injection moding process. The formation of the skin and the core structure can be expained by the moding process. A given percentage of the skin materia is first injected into the cavity to form the skin ayer. As the fastest materia in the center of the fow reaches the fow front, it spits to the outer wa of the mod and freezes forming a frozen ayer or skin ayer. This is caed Fountain fow as schematicay iustrated in Figure 1.3. Prior to the skin materia s reaching the end of the cavity, the second materia is injected to form the core. This core materia deveops a second fow front pushing the skin materia ahead of it unti the cavity is neary fied and finay a much smaer amount of the skin materia is injected to sea the gate. The ast injection of skin materia is important to cean a core materia out of the gate area and ensure that no core materia wi be injected into the next part during the initia skin materia injection.

18 4 Introduction B B A A (1) (2) B B A A (3) (4) Figure 1.2 The sandwich injection moding process works by first injecting the skin materia (1, 2) then switching to the core materia (3). A sma amount of skin materia can sea the gate to purge the core materia away from the sprue (4). Mod Wa Soidified Skin Layer Fountain Fow Fow Direction Core Materia Skin Materia Wa Thickness Soidified Skin Layer Met Front Figure 1.3 Schematic of poymer met fow profie across the thickness during sandwich injection moding process.

19 Introduction 5 The resuting skin/core geometry of sandwich moded parts provides a number of advantages because the different materia properties can be incorporated into the same part, as demonstrated in Figure 1.4. It is often desirabe for the skin materia to have a superior appearance, whie the strength and rigidity of the part is strongy dependent upon the core materia [3-5]. Typicay, the core materia wi be ess costy than the skin materia, which can yied potentia cost savings. This is often achieved by using recyced materia as the core. Sandwich injection aso expoits to use a foam core. In this case, arge parts with hard and gossy surfaces can be moded without the need for high camping forces since shrinkage is compensated by the expansion of the core materia. In appications, where thin-waed products are more suitabe, ow weight, high stiffness products with reinforcing ribs can be moded economicay [6-7]. Skin materia Core materia Core materia Skin materia Figure 1.4 Sandwich injection modings. [5] Gas and Water-assisted injection moding Gas-assisted injection moding (GAIM) is an important variant of the traditiona technoogy for injection moding of thermopastics. In the simpest terms, gas-assisted moding process begins ike any conventiona injection moding process with the injection of poymer met into a cavity. Ony a partia voume of met is injected and a short shot is purposey produced (see Figure 1.5). At the end of the poymer injection stage, compressed gas, usuay nitrogen (due to its reative inertness and avaiabiity) is injected through the centra core of the met simiar to sandwich injection moding. The gas drives the moten poymer further into the mod, unti it is fied competey. The penetrating gas, acting now as the core materia, eaves

20 6 Introduction a poymer ayer at the mod wa, yieding a product with a poymer skin and a hoow core. The gas can either be injected through a neede in the nozze, or directy into the mod through separate gas injection needes. After the mod has been entirey fied, gas is used to transmit the packing pressure to the poymer that is being cooed. Any shrinkage of the poymer materia near the gas channe is compensated for by an enargement of the gas core. Once a poymer materia has soidified, the gas pressure is reeased. The product is then further cooed unti it has retained sufficient rigidity to be ejected from the mod. The most important characteristic of GAIM is the fact that the pressure drop in the gas core is negigiby sma compared to the pressure drop in an equivaent moten poymer. Consequenty, the pressure can be considered constant throughout the gas core, which accounts for most of the advantages of GAIM, such as reduction of raw materia, weight of product, cyce time, camping force, sink marks and residua stresses, and enhancement of design possibiities [1, 8-10]. Vented cavity Vented cavity (1) (2) P = 0 (3) (4) Figure 1.5 Schematic showing the various stages of the gas assisted injection moding process: (1) Met injection; (2) Gas injection; (3) Packing phase; (4) Part ejection. [8] Water-assisted injection moding (WAIM) appears at the beginning of the 70s but its rea deveopment started at the Institute für Kunststoffverarbeitung (IKV), a pastic processing deveopment center in Germany, in This process is simiar to GAIM except that it uses water instead of nitrogen. The aim of deveoping WAIM is to reduce cooing cyce times in the production of hoow or party hoow parts [11].

21 Introduction Overmoding The overmoding process is a versatie and increasingy popuar injection moding process that provides increased design fexibiity for making muti-coor or muti-functiona products at reduced cost. This technique permits one-step joining of two or more poymers (e.g. rigid/fexibe or rigid/rigid) into parts, which do not require any further finishing operations [12-13]. Besides the economica advantages, the process offers the possibiity of obtaining a broad range of mechanica properties of the end products [14-15]. The typica appications of the overmoding process are the combination of muti-coored areas within one part and the soft-touch appications (e.g. handes). For instance, two-component pastic parts can be produced by a two-component injection moding machine, which introduces sequentiay different poymers into a specia mod through separate runner systems. After moding the preform of the first component, the cavity part for the second component is activated by removing a meta core (core back mod) or opening the mod and transporting the preform into a second cavity (e.g. rotating mod base). The second poymer is then deivered by the second injection unit into the newy formed cavity through its independent runner system and the fina part is ejected after packing and cooing phases. This method is sometimes referred to as in-mod assemby, since the resuting part effectivey acts as an assemby of two materias rather than as a ayered structure. 1.3 Speciaized injection moding techniques for enhancing properties of thermopastics and composites Defects such as wedines, sink marks, and warpage are caused by met fronts coision, unbaanced fow, uneven cooing and non-uniform interna stress. Varying the processing parameters can resut in the modification of the moded part outook, physica and mechanica properties [1, 8, 16]. The modifications, however, are often sight and not quantified, and they aso rey upon the expertise of the operator who uses his experience and art to determine the processing parameters. During the ast decade, severa techniques have been deveoped using different approaches in order to improve the moding properties, e.g. wedine strength, by controing the met fow pattern of the poymer as it is being shaped [17-19]. This concept has been appied to a wide range of thermopastic matrix composites especiay with gass-

22 8 Introduction fiber reinforced thermopastics [20-25]. There have been many such improvements, but three in particuar stand out. The first, mutipe ive-feed injection moding, auxiiary equipment is incorporated into the standard moding machine. For two others, push-pu processing and sequentia injection moding, require specia moding machine and modified tooing for optima success Mutipe ive-feed injection moding The mutipe ive-feed injection moding process, is aso known as Shear Controed Orientation Injection Moding (SCORIM), deveoped at Brune University, and icensed by British Technoogy Group [26]. This process achieves significant improvement and contro over part properties by using a specia injection head that spits the met fow in the mod into two streams (see Figure 1.6). Once the mod is fied or during the packing stage, the mutiive feed system s hydrauic pistons begin moving forward and backward in an aternating fashion. As one ive feed piston pushes downward, it forces met through the runner and cavity up into the second ive feed cyinder. The process then reverses, and the met fows in the opposite direction. The principe advantages of the process are; enhanced and controed orientation of fiber or fake fiers, significant reduction of wedine effects and controed modification of the microstructure of injection moded unfied pastics, especiay in iquid crysta poymers (LCPs) [20-21, 27]. Mutipe ive-feed processing head Conventiona injection unit Wedine Hydrauic cyinders and pistons Runner system Figure 1.6 Schematic of the mutipe ive-feed injection moding process. [8]

23 Introduction Push-pu processing The push-pu injection moding process is a met osciation technique, which is very simiar to SCROIM. It was originay unveied by Köckner Ferromatik Desma at the K 89 show. As shown in Figure 1.7, the push-pu injection moding system incudes two injection units and a two gate mod. The cavity is firsty fied simutaneousy by the met from both the units via the two separate gates. After the two met fronts meet, the wedine is formed and the fiing phase is subsequenty switched to the hoding phase. The materia soidifies starting at the cavity wa but there is sti moten core and then the first push-pu stroke begins. The contro software program aows the definition of severa hoding pressures for one stroke from either the first or the second injection unit. From one of the injection units, poymer met is pressed into the cavity resuting in the moten core being pushed through the gate back into the other injection unit and thus the geometry of wedine is deformed to a tongue shape. As the materia fows back and forth through the mod, moecuar orientation is continuousy created and subsequenty frozen in as the materia soidifies from the outer ayers toward the hot core. By keeping the moten poymer in aminar motion during soidification, the moded parts acquire an oriented structure throughout the voume. If the mod is compex and the met has to fow around obstaces, the motion wi create better mixing in the area behind the obstaces and reduce the weakening effect of the wedine by dispersing them throughout the part and eiminates void, cracks, and micro-porosities in arge cross-section moding. The number of strokes can be seected by taking into account the part s thickness. When a the strokes are competed, cooing phase foows. As the thickness of frozen ayer increases with the number of strokes within the hoding time, the tota cyce time is not notaby increased as compared to conventiona injection moding [23]. Primary runner Overfow runner Mutigated mod Primary injection unit Secondary injection unit Figure 1.7 Schematic principe of push-pu injection moding process. [8]

24 10 Introduction Sequentia injection moding Sequentia injection moding is an increasingy used manufacturing processing technique presenting the advantages over traditiona injection moding. The process is generay used in arge parts, which are difficut to pack from one centra area. Sequentia vave gating is used to contro the fiing of parts and each vave gate is independenty opened and cosed at a predetermined event (time, screw position, cavity pressure, etc.) providing compete contro of cavity fi. This technique can minimize the pressure oss in the system and aso can be used to contro the ocation of wedine, as iustrated in Figure 1.8, in order to ensure that the wed is positioned away from the critica area, and thus improving the product s performance [24, 25]. Resutant Part a) Cassica Wedine b) Midde Disturbance c) Side Disturbance Figure 1.8 Schematic iustration of sequentia injection moding (experimenta mod, deveoped at TU Chemnitz): Fiing study by sequentiay opening and cosing the vave gates. [24, 25]

25 Introduction Simuation of the injection moding and speciaized processes Simuation of the conventiona injection moding process There are severa miestones in the history of Computer-Aided Engineering (CAE). The anaysis of mod fiing in injection moding started with the work of Spencer and Gimore [28] in the eary 1950 s. They empoyed an empirica equation for capiary fow and couped it with a quasi steady-state approximation to cacuate the fiing time. Since then, different methods have been proposed to describe the moding cyce with varying degrees of compexity. One-dimensiona rectanguar fow was proposed by Baman et a. [29] and Staub [30]. Harry and Parrot [31] considered a one-dimensiona quasi-steady state fow anaysis couped with an energy baance equation. Wiiams and Lord [32] made a significant contribution by considering a the components of a one-dimensiona non-isotherma fow. A simiar mode was presented by Thiene and Menges [33] using a different soution technique. In order to study a more representative one-dimensiona fow, a number of anayses were carried out on the radia fiing of a center gated disc mod. Kama and Kenig [34-35] proposed an integrated mathematica treatment of the fiing, packing, and cooing stages of the injection moding cyce. Simiar simuations were carried out by Berger and Gogos [36], and Wu et a. [37]. However, it was not unti the 1970 s when the deveopment and appication of computer simuations to injection moding intensified. In particuar, Stevenson and co-workers [38] anayzed one-dimensiona fow in a center-gated disc. Lord and Wiiams [39] studied the one-dimensiona fiing behavior in rectanguar cavity geometry. Nunn and Fenner [40] modeed one-dimensiona tubuar fow of poymer mets, which was ater extended by Hieber et a. [41] to simuate the poymer fow in a non-circuar tube under non-isotherma condition. The genera characteristics of injection moding are that the part thickness is much smaer than the overa part dimension and the poymer mets are highy viscous due to their ong moecuar chain structure. As a resut, the ratio of inertia force to the viscous forces (as characterized by the dimensioness Reynods number) is in the order of This makes the Hee-Shaw fow formuation [42], which is based on the creeping-fow ubrication mode, an appropriate candidate for anayzing the fow in typica injection moded parts. In addition to negecting the fuid inertia, the Hee-Shaw fow formuation aso omits cacuation of the

26 12 Introduction veocity component and therma convection in the gapwise direction. Compared with heat conduction in the gapwise direction, heat conduction in the panar directions is aso negected. Other commony adopted simpifications incude negecting the transverse fow at the met front region (the fountain fow behavior), viscous convection (drag force) and heat conduction on the atera wa surfaces, and mapping of gapwise soutions at the fow junctions and where the wa thickness changes. Accordingy, the usage of computationa resources incuding computationa storage and CPU time can be reduced consideraby compared with the case of a fu three-dimensiona simuation. In this approach, threedimensiona geometry is represented with one-dimensiona tubuar eements and twodimensiona trianguar thin-she eements for which the wa thickness is impicity specified as an attribute; i.e. a mid-pane mesh has to be created either from coapsed or from an existing three-dimensiona CAD design mode. Those one- and two-dimensiona eements are numericay divided into severa ayers (typicay 8-20) in the gapwise direction for detais of the variabes under consideration. Whie the governing equations of mod fiing and packing are being soved by the finite-eement method (FEM), finite-difference method (FDM) is appied in the gapwise direction and the tempora domain. By doing so, the transient behavior and variation of the variabes in the gapwise direction can be captured. Since the gapwise veocity component is not cacuated and the mesh mode ony represents the shape of the part geometry, the Hee-Shaw fow formuation is sometimes caed 2.5- dimensiona (2.5-D) simuation. Athough the governing equations and the geometry are simpified, the Hee-Shaw fow mode became the standard numerica framework for various commercia software packages and research codes [43-45] and has been extended or incorporated by other researchers [46-49] e.g. simuation of poymer met fow during the fiing and packing phase, fiber orientation, shrinkage and warpage. However, the Hee-Shaw fow formuation has its imitations owing to the inherent creeping-fow and thin-wa assumptions. For exampe, the she eement empoyed in the Hee-Shaw mode needs the construction of the mid-pane, which is time-consuming [50]. Furthermore, it cannot accuratey mode the three-dimensiona fow behaviors, particuary important when moding with fiber reinforced systems [51], within thick and compex geometries or at the met fronts (fountain fows), regions where the part thickness changes abrupty or separate met fronts meet (wedines), and regions around specia part features such as bosses, corners, and/or ribs as compared to those obtained by the three-dimensiona (3-D) simuation mode [50, 52-54].

27 Introduction 13 The interest in 3-D simuation of injection moding has increased tremendousy in the past few years. Severa commercia and research-oriented 3-D CAE simuation programs for injection moding have been deveoped [52-53]. In particuar, Hetu et a. [52] deveoped a 3- D finite-eement program for predicting the veocity and pressure fieds governed by generaized Stokes equations. In addition to the temperature fied, they aso soved the position of fow fronts using the pseudo-concentration method. Zachert and Michaei [53] anayzed poymer fow at the region of a sudden thickness change during injection moding using both a Hee-Shaw fow formuation and a 3-D approach. Chang and Yang [54] deveoped the numerica simuation for 3-D mod fiing based on an impicit finite-voume method (FVM). Their work was ater commerciaized and extended to cover various stages in injection moding and specia moding processes. Pichein and Coupez [55] anayzed the 3-D mod fiing of an incompressibe fuid and the shape of the fountain fow front using an impicit discontinuous Tayor-Gaerkin scheme. Han et a. [56] predicted the fuid fow advancements and pressure variation in the microchip encapsuation process using a 3-D FEM based on a generaized Hee-Shaw formuation. By treating the poymer density as a function of pressure and temperature, Haagh et a. [57] incorporated the compressibiity of the poymer met in a 3-D mod fiing process. Rajupaem et a. [58] and Tawar et a. [59] used an equa-order veocity-pressure formuation to sove the Navier-Stokes equations in their 3-D simuation of mod fiing/packing phases Simuation of some speciaized injection moding processes Over recent years, as for the conventiona injection moding, the numerica simuations of coinjection, gas-assisted injection, SCORIM, and push-pu processing are mosty based on the thin wa, Hee-Shaw approximation. Simuation of the sequentia sandwich injection moding process was first carried out by Turng and Wang [60] in order to predict the skin and core met front progression and the distribution of the two ayers by cacuating the residence time of the partices that enter the mod cavity. Schatter et a. [61-62] used a specia transport equation to characterize the dispacement of the interface between skin and core for the sequentia injection of poymers. Visuaization and simuation of the sandwich mod fiing process have been presented by Lee et a. [63-64]. They deveoped a simuation approach based on the Hee-Shaw approximation and kinematics of interface to cacuate the two-phase fow and the interface evoution during fiing in simutaneous sandwich moding. Jaroschek [65] studied the distribution of the core materia during the fiing process of sandwich

28 14 Introduction injection moding using mutipe gates, a variation of cascade contro, with specific vaves and standard hot runner manifods. The experimenta resuts were aso compared with those obtained by the Modfow simuation package using a ayered 2.5-D fow approach and FEM grid representing the housing geometry (oudspeaker box). His findings suggest that it is possibe to use a doube hot runner system that aows even arge components to be produced by sandwich moding using mutipe gating in case where the simuation program provides sufficient accuracy. Chen et a. [66-67] utiized an agorithm based on the contro voume finite-eement method combined with a partice-tracing scheme using a dua-fiing-parameter technique to predict the advancements of both met front and gas front during the gas-assisted injection process. Gao et a. [68] aso used this voume tracking technique with the Gaerkin Finite Eement mode to simuate the fiing stage of the gas-assisted injection moding process, particuary the gas penetration phenomenon invoving the gas-poymer interaction. Wang et a. [69] compared the experimenta and simuation resuts of gas penetration in terms of different shot sizes, deay time and gas pressure. They suggested that an improper modeing can cause artificia fast cooing in the gas channes, which wi hinder the gas penetration in the numerica simuation. Pittman et a. [70] simuated the cooing and soidification of poymer met during SCORIM by using the one-dimensiona transient mode. A non-newtonian, temperature-dependent viscosity is used, together with temperaturedependent therma properties and atent heat of soidification. Recent work [71] investigates the wedine strength and the fiber orientation in the wedine region of push-pu processed parts, with respect to the number of push-pu strokes and the hoding pressure differences between both the injection units. The experimenta resuts are aso compared with those obtained by the simuation package using a 2.5-D mode (based on Hee-Shaw approximation). A good agreement has been obtained between the predictions and the measurements, thus showing the usefuness of the commercia software in heping the design engineer to identify the ocation of wedine and fiber orientation state within the wedine areas. Initia work on the 3-D simuation of the gas-assisted injection moding process was done by Khayat et a. [72] which used a boundary-eement method (BEM). Their contribution reduces to the anaysis of isotherma, incompressibe, Newtonian fuid fow in simpe 3-D geometries. Haagh et a. [73] presents a 3-D mode based on the finite-eement method and a pseudoconcentration technique for tracking the fow interfaces. Iinca et a. [74] used a pressure stabiized Petrov-Gaerkin method to sove the Navier-Strokes equations in their 3-D

29 Introduction 15 numerica mode for gas-assisted injection moding. An additiona pressure stabiization term was incuded compared with the standard Gaerkin method. The position of the poymer/air interfaces was aso tracked using the pseudo-concentration method. Iinca et a. [75] aso used this numerica mode to simuate 3-D numerica mode to simuate 3-D co-injection moding. The poymer/air and skin/core poymer interfaces were tracked by soving two additiona transport equations. 1.5 Research objectives The main objective of this work is to investigate the capabiity of the sandwich injection moding technique for enhancing the orientation of fibers within the moded parts. The infuences of gass fiber concentration and processing parameters on the materia distribution, fiber orientation and fiber attrition are examined. Additionay, one of the speciaized injection moding techniques push-pu processing is empoyed in order to improve the fiber orientation within the wedine area. The effect of processing parameters incuding the number of push-pu strokes and the hoding pressure differences between both the injection units have been studied. The degradation of the fiber ength caused by the aternating shear fied is aso investigated. The prediction of the tensie strength of conventiona, sandwich and push-pu injection moded short fiber reinforced composites are derived by an anaytica method of modified rue of mixtures (MROM) as a function of the area fraction between skin and core ayers. The effects of fiber ength and fiber orientation on the tensie strength of short fiber reinforced composites are aso studied in detai. This mode provides the necessary information to determine what fiber ength distribution and what fiber orientation distribution are required to achieve a desired composite strength. Materia distribution and fiber orientation structures of sandwich and push-pu processed parts are predicted by the 2.5 and 3-D numerica anayses. The predictions sove the fu baance equations of mass, momentum, and energy for a generaized Newtonian fuid. The second-order orientation tensor ( a 11 ) approach is used to describe and cacuate the oca fiber orientation state. The accuracy of mode predictions is extensivey evauated by

30 16 Introduction comparing with corresponding experimenta measurements to gain a further basic understanding of the reationship between the processing conditions, the fiber orientation distribution and the properties of the fina injection moded part. 1.6 Outine of the thesis In Chapter 2 the fundamentas of rheoogy, genera behavior and predictive methods for short fiber reinforced composites are introduced. The infuence of parameters on mechanica and physica properties is described. The mathematica formuations used for the fow mode and the basics of fiber orientation prediction are summarized in Chapter 3. Chapter 4 detais the experimenta procedures incuding materias, processing conditions, morphoogy observation and measurement of mechanica properties. The process simuations of sandwich and pushpu injection moding processes are aso presented. In Chapter 5, the morphoogy deveopments with different processing types are discussed. It provides the evidences of mechanica properties, fiber orientation, and fiber ength distribution within sandwich and push-pu injection moded parts compared with those obtained using conventiona injection moding. In Chapter 6, predictions of the skin/core materia distribution and fiber orientation are compared with the experimenta resuts. A number of simuations of sandwich and pushpu injection moding in a dumbbe part are carried out in order to investigate the infuence of various parameters. Finay, the concusions are presented in Chapter 7.

31 2. Moding of Short Fiber Reinforced Composites A short fiber composite consists of a poymer matrix reinforced by fibers of much smaer ength as compared with the overa dimensions of the fabricated structure. These reinforced poymers have been deveoped to fi the mechanica property gap between the continuous fiber aminates used as primary structures by the aircraft and aerospace industry and the neat poymers used in ow-oad-bearing appications. Athough short fiber composites do not achieve the characteristic mechanica vaues which can be obtained with continuous fiber aminates [76]. However, they can be processed with the same techniques used for unfied thermopastics, e.g. injection moding of short fiber composites for the high voume production purposes and the abiity to be moded into compex shapes. Furthermore, their intrinsic recycabiity is rapidy being recognized as a strong driving force for their further appication [77]. The moding processes of short fiber composites requires the compounded materia to be heated and then forced to fow under the appication of a high pressure in order to conform to the shape of the mod cavity. If injection moding is the chosen fabrication route, then the fow processes invoved in mod fiing can be very compex and resut in a marked orientation of the fibers in the fina part. The microstructure and morphoogy that are produced depend on how we the fibers are dispersed, the range of fiber engths and diameters, how the fibers interact with the mod was and each other during fow, the heat transfer in the mod, and the geometry of the mod. This in turn wi have a major effect on the anisotropy of the mechanica and physica properties of the injection moded component [76].

32 18 Moding of Short Fiber Reinforced Composites This chapter wi introduce a short review of rheoogica properties and microstructure of short fiber reinforced composites incuding the fiber attrition during processing operations. Topics aso incude the mechanica properties and the predictive methods for short fiber reinforced composites. 2.1 Rheoogy of short fiber composites The rheoogica properties of short fiber composites may differ in detai from those of norma unfied poymers, but they are not consideraby different. In practice the methodoogy for measuring the basic fow properties of fiber fied mets foows cosey that used for characterizing the fow properties of unfied materias. Indeed, the shear viscosity can be measured using most of the norma techniques, i.e. cone and pate, capiary, dynamic mechanica, etc. Detaied experimenta techniques used for characterizing rheoogica properties can be found esewhere [78]. Much of the eary work on the rheoogy of fied poymers was concerned with diute suspensions. The fow properties of such mets wi differ significanty from those of fied thermopastics of commercia grades, where individua partices are in cose proximity to each other. Figure 2.1 shows a pot of og shear viscosity versus og shear rate for three grades of poycarbonate, containing 0, 20 and 35 by weight of short gass fibers (%wt), respectivey. At ow shear rates the presence of the fibers causes an appreciabe increase in viscosity, but it is noteworthy that the viscosity vaues for the three materias converge to a very simiar vaue at the higher shear rates. Essentiay the same pattern of behavior is shown by many other fiber reinforced grades of poymers [79]. The simiarity in viscosity vaues at high shear rates, for fied and unfied poymers, is an important factor in expaining the successfu expoitation of these materias, since very itte additiona power wi be required to process the fied materias [80]. The basic rheoogica data for fiber fied poymers are derived from tests performed using very simpe geometries. In practice, however, the fow geometries occurring in technoogica processing equipment wi be very compex. The fiber orientation distribution (FOD) in a moded part can be quaitativey interpreted through an appreciation of the fiber orientation resuting from certain basic categories of fow:

33 Moding of Short Fiber Reinforced Composites 19 Shear fow as wi occur in a straight tube or duct Convergent fow in the simpest case this wi occur when a fuid passes from a wide to a narrow cross-section. Divergent fow (aso referred to as extensiona fow) in the simpest case this wi occur when a fuid passes from a narrow to a wide cross-section. Convergent fow eads to an aignment of fibers parae to the fow direction. Divergent fow, on the other hand, tends to orient the fibers orthogonay to the fow direction. Frequenty, this is observed as fiber fied mets pass from a narrow gate into the mod cavity PC (Makroon 2800) PC fied with 20 wt% short-gass-fiber (Makroon 8020) PC fied with 35 wt% short-gass-fiber (Makroon 8030) Viscosity (Pa.s) Temp. = o C Shear Rate (1/s) Figure 2.1 Apparent viscosity versus shear rate for three poycarbonates containing different amounts of short gass fibers. (Modfow database)

34 20 Moding of Short Fiber Reinforced Composites 2.2 Microstructure of injection moded short fiber composites Fiber orientation As the fiber suspension fows into the mod, one must know the fiber orientation structure not ony to determine its rheoogica behavior but aso to estimate its mechanica performance. Fow and deformation of the suspension change the orientation of the fibers fowing in it. These orientations are subsequenty frozen in as the materia soidifies and become a key feature of the microstructure of the finished composite. If the fibers are randomy oriented, the mechanica and physica properties wi be isotropic. If the fibers are aigned in one direction, the composite wi be stiffer and stronger in that direction compared with any other direction. The distribution of orientations in a moded part coud be quite diverse. One region may have random fiber orientation, whie others may have preferred aignment in certain directions. In the injection moded short fiber composites, a characteristic ayer structure is observed, with the fibers oriented in quite different manners according to their ocation through the thickness, as schematicay demonstrated in Figure 2.2. These genera features are apparent in studies of fiber orientation distribution found in the iterature [80-89]. In the skin region, the fiber orientation is predominatey parae to the fow direction. This is due to, as the met fis the mod, there is fountain fow which initiay orients the fibers perpendicuar to the main fow direction. Fountain fow causes the met to be deposited on the mod wa with the aignment direction parae to the mod fi direction. Here it soidifies rapidy and this aignment is retained in the soid artice. Further behind the met front, shear fow dominates and produces fairy uniform eves of fiber aignment. The fiber orientation and the thickness of this region are infuenced by non-isotherma effects and by injection speed [80-81]. In contrast, the core of the moding contains fibers mainy aigned perpendicuar to the fow direction due to a sower cooing rate and ower shearing.

35 Moding of Short Fiber Reinforced Composites 21 Z X Y (Fow direction) Skin ayer Core ayer Skin ayer Figure 2.2 Schematic diagram of moding indicating the fiber orientation in the skin and core ayers. During the injection moding process of short fiber composites, the distribution of fiber orientation is governed by a variety of factors. These incude the type and/or shape [82, 90], concentration of fibers [83-84], the gate designs and/or fow geometry [85-86]. The processing conditions such as injection fow rate, injection pressure, and met temperature can aso significanty ater the proportions of the oriented regions [87-88]. It has aso been shown that the mechanica and physica properties of injection moded short fiber composites depend criticay on the fiber orientation distribution in the fina product [82-89] Fiber attrition during moding One of the major concerns in producing fiber reinforced parts by injection moding is fiber breakage during processing. Many experiments have estabished the fact that the fibers get damaged during processing and fiber ength may be reduced by an order of magnitude [91-93]. This reduction in ength can potentiay reduce the reinforcing efficiency of the fibers, thus substantiay reducing the mechanica properties of the composite [94-95]. The reduction

36 22 Moding of Short Fiber Reinforced Composites in fiber ength during compounding and moding can be expained by the three common mechanisms incuding; fiber-fow, fiber-fiber and fiber-wa interactions. Fiber-fow interactions: most fows in injection moding process are a combination of extensiona and shear deformations. In a purey eongationa fow, fibers tend to aign aong the stretching direction and hence are under tension, fibers are rather unikey to break under this mode. However, in shear fows, fibers rotate across the streamines and may bend to their critica radius of curvature and bucke under viscous forces transmitted by the poymer met. Fiber-fiber interactions in concentrated suspensions can cause fiber overaps, which wi induce bending stresses in the fibers, resuting in breakage. The effect of fiber voume fraction on breakage was studied by von Turkovic and Erwin [91], who found that gass fibers in poystyrene had the identica ength distribution at the exit of an extruder for voume fractions ranging from 1 to 20%. Their resuts indicated that the fiber ength was reduced by an order of magnitude after processing and that the average fina ength was insensitive to initia fiber ength distribution. Fiber-wa interactions: in injection moding, the effect of mod and screw geometries pays a crucia roe in fiber ength reduction. Wider mod channes have been shown to reduce breakage [84-86]. Aso, the gate region of the mod may be a key factor in fiber degradation. Baiey and Rzepka [96] studied ong fiber materias under various materia and processing conditions in a punger moding machine, a conventiona injection moding machine, and an extruder. Mod configurations with a sma gate and a generous gate were studied. Fiber oadings from 30-60% were used. Their study showed that a arger gate produced fina fiber engths with a mean of 0.99 mm, whereas a smaer gate had a mean fiber ength of ony 0.49 mm. They aso found that there was substantiay more damage in the skin region than in the core. This coud be attributed to a high shear rate near the mod surface couped with fiber interactions with the mod wa.

37 Moding of Short Fiber Reinforced Composites Mechanica properties The genera stress-strain curves observed in gass fiber reinforced thermopastics are iustrated in Figure 2.3. It is we known that the addition of gass fibers resuts in an enhancement of the stiffness and strength of the composite [82-85, 89, 94-95]. It aso can be seen that both the stiffness and strength increase with fiber concentration. However, one very important consequence of utiizing arge voume fractions of fibers is that athough stiffness is increased significanty, there is not necessariy a pro rata change in strength [85, 89, 95]. Furthermore, the work to fracture decreases rapidy as the concentration of fibers is increased [82-83, 85], i.e. the composite wi ony toerate sma impact energies. As with a materias that are uniaxiay oriented, the mechanica properties of a highy aigned composite depend criticay on the ange between the appied stress and fiber orientation direction. Figure 2.3 aso shows some experimenta tensie strength data obtained on moderatey we-aigned composites of gass fiber in poypropyene. It can be seen that a arge composite strength is ony obtained when the stress direction is cose to the fiber orientation axis. This sensitivity to ange becomes much more acute as the ratio of fiber strength to matrix strength increases. The anisotropy in the mechanica properties has important impications for the behavior of partiay aigned short fiber composites. A crosssection cut from a simpe tensie test specimen, moded in short gass fiber reinforced poycarbonate, reveas a compex fiber orientation distribution (see Figure 2.4). It is cear that when a stress is appied to this bar many of the fibers wi be at quite arge anges with respect to the stress direction. Overa, then the stiffness of the bar wi be significanty ess than for fuy aigned fibers.

38 24 Moding of Short Fiber Reinforced Composites 70 Tensie stress, (MPa) σ SFRPP30 parae to fow direction SFRPP30 transverse to fow direction SFRPP40 parae to fow direction SFRPP40 transverse to fow direction LFRPP30 parae to fow direction LFRPP30 transverse to fow direction LFRPP40 parae to fow direction LFRPP40 transverse to fow direction Fow direction ε Strain, (%) Figure 2.3 Typica stress-strain curves for gass fiber reinforced poypropyene at various fiber voume fractions, fiber engths, and testing ocations. [89] (The description of abbreviations is shown in section 4.1) Another parameter of key importance in infuencing the mechanica properties of composites is the fiber ength. As can be seen from Figure 2.3, when the stress is appied parae to the fiber axis in a uniaxiay aigned fiber reinforced composite, the greatest stiffness and strength occur when the fibers are very ong compared with their diameter. With short fiber composites, however, the fibers are usuay of the order of haf a miimeters in ength so as to enabe the composites to be processed easiy using, e.g. injection moding. In this case the stiffness and strength of the composite wi be ower [86, 89, 95]. Hence the strength of any given injection moded composite part wi depend criticay on both the fiber ength distribution and fiber orientation distribution.

39 Moding of Short Fiber Reinforced Composites 25 Fiber orientation parae to the fow direction Skin ayer Random-in-pane aignment of fibers Core ayer Fiber orientation parae to the fow direction Fow direction Skin ayer Figure 2.4 Optica photomicrograph showing the fiber orientation pattern across the thickness of tensie test bar. 2.4 Predictive methods of tensie strength for short fiber composites Tensie strength is one important property of engineering materias. One of the basic motivations for the use of composite materias as engineering materias is the high tensie strength that can be achieved by incorporating high strength fibers into a matrix since the fibers carry most of the oad. Over the ast decade, severa theoretica modes have been proposed in order to predict the moduus and strength of short fiber composites. One is the aminate anaogy, which combines the micro-mechanics of joining different phases with the macro-mechanics of amination theory. The success of the aminate approximation is strongy dependent upon the assumption of physica voume averaging combined with an abiity to estimate the properties of the individua pies, each of which contains uniaxiay oriented fibers. This approach has been used successfuy to predict strength, moduus, stress-strain behavior [97-98], and fexura stiffness [99]. The other major approach is the modified rue of

40 26 Moding of Short Fiber Reinforced Composites mixtures (MROM), which has been mosty used to predict the moduus and strength of short fiber composites by taking into consideration the effects of fiber ength and orientation distribution [ ]. In genera, a of the proposed methods have shown good agreement with experimenta resuts. Athough the aminate and MROM methods are usuay used to estimate the strength for short fiber composites, the procedures to estimate the strength of sandwich and push-pu injection moded parts have not been estabished. In this section, therefore, the mode used for predicting the utimate tensie strength (UTS) of sandwich and push-pu injection moded part wi be introduced. This predictive method is aso based on a MROM as a function of the area fraction between skin and core ayers (so caed area fraction method). The advantage of this method over the traditiona method is that the wedine strength of push-pu processed part and the UTS of sandwich injection moded part, containing different fiber concentration between skin and core materia, being abe to estimate. In order to take into account the infuence of fiber ength as we as fiber orientation, the fiber orientation efficiency factor ( f ) and fiber ength efficiency factor ( f ) aso can be accommodated Modified rue of mixtures (MROM) The modified rue of mixtures is often used to predict the tensie strength of short fiber composites. The formua of MROM is given by σ = f 0 f V σ + V σ (2.1) CU f f m m where σ CU and σ f are the utimate strength of the composite and fiber, respectivey; V f and Vm denote the voume fraction of the fiber and matrix; σ m is the stress deveoped in the matrix; f and f are the fiber orientation and fiber ength efficiency factors, which depend 0 on various parameters such as fiber voume fraction and processing conditions, and are ony fitted empiricay [95]. By using the Voigt average [79] and dividing the reinforcement into groups of uniaxiay aigned fibers, f 0 is determined by f = a 4 0 n cos ϕn (2.2) n

41 Moding of Short Fiber Reinforced Composites 27 where an is the proportion of fibers making an ange ϕ n with respect to the appied oad or fow direction. The efficiency of fiber reinforcement for severa situations is presented in Tabe 2.1, this efficiency is taken to be unity for an oriented fiber composite in the aignment direction, and zero perpendicuar to it. Tabe 2.1 Reinforcement efficiency of fiber reinforced composites for severa fiber orientations and at various directions of stress appication. [79] Fiber Orientation Stress Direction Reinforcement Efficiency, (f 0 ) A Fibers parae Parae to fibers 1 Perpendicuar to fibers 0 Fiber randomy and uniformy distributed within a specific pane Any direction in the pane of the fibers 3/8 Fiber randomy and uniformy distributed within three- in dimensions space Any direction 1/5 If the fiber ength ( ) is uniform, the fiber ength efficiency factor can be obtained from f f = for < c (2.3) 2 c c = 1 for c (2.4) 2 where c the critica minimum fiber ength. This critica ength is given by σ d = f c 2τ (2.5) where d is the fiber diameter and τ the interfacia shear strength between fiber and matrix. In the case of a strong interfacia bond, τ is imited by the shear strength of the matrix ( τ m ). Assuming isotropy of the matrix this resuts in σ τ = m (2.6) 3

42 28 Moding of Short Fiber Reinforced Composites If the fiber ength is not uniform, the mode can be given by V σ σ CU = f + f f i c 0 + f0 V f σ f Vm m 1 σ c (2.7) i 2 i 2 c i The first and second terms in this expression represent the contributions of the fiber ength being shorter and onger than c, respectivey. As aways one shoud be fuy aware of a assumptions that ie behind any mode, which in this case are: Stress transfer across the interface increases ineary from the tips of the fiber inwards to some maximum vaue No fiber-matrix deboding occurs The fiber orientation factor is independent of strain and is the same for a fiber engths The composite matrix properties are the same as the resin properties The fiber strength is known (which may aso be different from a textbook vaue or even a measurement on the fibers used to produce the test sampes) τ is independent of oading ange Fiber diameter is monodisperse Area fraction method The deviation of this mode to predict the tensie strength of short fiber composite can begin by considering the tota oad sustained by the composite ( F C ) is equa to the oads carried by ongitudina fibers and transverse (or random) fibers, which was proposed by Akay and Barkey [85], defined as F = F + F (2.8) C L T

43 Moding of Short Fiber Reinforced Composites 29 From the definition of stress ( F = σ A ) and the expression for F, F and F in terms of C L T their respective stresses, the utimate tensie strength of the composite ( σ as: CU ) can be rewritten σ A = σ A + σ A (2.9) CU C UL L UT T or A L AT σ = + CU σ UL σ UT (2.10) AC AC where A A L C is the area fraction between the skin region and the cross-sectiona area of specimen; and A A T C is the area fraction between the core region and the cross-sectiona area of specimen. The UTS of the skin region, σ generay aigned in the fow direction or tensie axis, is given by: UL, where the fibers near the part surface are V σ σ UL = f + f f i c 0 f V f f V skin + 0 σ skin m m 1 σ c (2.11) i 2 i 2 c i The utimate tensie strength of the core region, σ UT, where the fiber orientation is predominatey transverse or random to the fow direction, this equation can be written as: V σ σ UT = f + f f i c 0 f V f f V core + 0 σ core m m 1 σ c (2.12) i 2 i 2 c i Therefore, the UTS of short fiber reinforced composites ( σ fiber ength and fiber orientation can be evauated with foowing equation: UC ) with respect to the effects of

44 Moding of Short Fiber Reinforced Composites = C T m m i c f f c i f f C L m m i c f f c i f f CU A A V V f V f A A V V f V f c i core c i core c i skin i skin σ σ σ σ σ σ σ (2.13) where and are the fiber orientation efficiency factors for the skin and core ayers, respectivey. The schematic diagram of cross-sectiona area for conventiona injection moded composites is depicted in Figure 2.5a. skin f 0 core f 0 Equation (2.13) can be expressed in terms of the UTS for the sandwich injection modings as beow: = C core m m i c f f c i f f C skin m m i c f f c i f f CU A A V V f V f A A V V f V f c i core c i core c i skin i skin σ σ σ σ σ σ σ (2.14) where and skin A core A are the cross-sectiona area of skin and core materias (see Figure 2.5b). Equation (2.13) aso can be empoyed for sandwich injection moded composites, containing different fiber concentration between skin and core materia (see Figure 2.5c). For exampe, when the skin and core materias fied with 40 and 20 wt% of fiber, the expression can be written as foows: = C T m m i c f f c i f f C core m m i c f f c i f f C skin m m i c f f c i f f CU A A V V f V f A A V V f V f A A V V f V f c i core sub c i core sub c i core c i core c i skin i skin σ σ σ σ σ σ σ σ σ σ (2.15)

45 Moding of Short Fiber Reinforced Composites 31 A L A T AC = A L + A T Skin materia Core materia A Skin A Core AC = A Skin + A Core Skin materia Core materia A Skin A Core AC = A Skin + A Core + A T A T (a) (b) (c) Figure 2.5 Schematic iustration of cross-sectiona area indicating the skin and core regions: (a) Conventiona injection moded short fiber composite, (b) Sandwich injection moded part, and (c) Sandwich injection moded short fiber composite. In addition, the prediction of wedine strength, WL σ, of short fiber reinforced composites for conventiona and push-pu injection modings aso can be cacuated by rewriting the Equation (2.13) as : = C Core m m i c f f c i f f C Skin m m i c f f c i f f WL A A V V f V f A A V V f V f c i core c i core c i skin i skin σ σ σ σ σ σ σ (2.16)

46 32 Moding of Short Fiber Reinforced Composites where A and A are the cross-sectiona areas where the fibers are aigned randomy and skin core perpendicuar to the fow direction, respectivey (see Figure 2.6). In the case of push-pu injection moded composites, fow direction. A core is the region with the predominant fiber orientation in the A Skin A Core A C A Skin = + A Core Conventiona injection moded part A Skin A Core A C A Skin = + A Core Push-pu injection moded part Figure 2.6 Schematic iustration of cross-sectiona area at the wedine position for conventiona and push-pu injection moded short fiber composites.

47 3. Modeing of the Injection Moding Process 3.1 Governing equations The poymer met fow is governed by the three fundamenta aws of physics, i.e. the principes of conservation of mass, momentum, and energy, which are expressed as ρ t Continuity: + ( ρ u) = 0 (3.1) Momentum: Du ρ = P + τ + ρ g (3.2) Dt DT P ρ p = + τ : u + S (3.3) Dt T Energy: C q T ( u) v where ρ is the density, P is the pressure, C p is the specific heat at constant voume, T is the temperature, v is the specific voume, S is the rate of heat generation due to chemica reaction, u is the veocity vector, g is the body force vector, q is the heat fux vector, is the gradient operator, D Dt is the substantia derivative, and τ is the extra stress tensor. To equate the numbers of unknowns and equations, it requires additiona reationships among the variabes, such as rheoogica constitutive equation, an equation of state for poymer, a therma constitutive equation and/or equations for cure and kinetics [45]. For exampe, the

48 34 Modeing of the Injection Moding Process rheoogica constitutive equation of poymer mets is typicay modeed by the generaized Newtonian viscosity mode: τ = 2η & γ (3.4) T [ ] with u + ( u) 1 γ& = (3.5) 2 where η is the non-newtonian viscosity, γ& is the strain rate tensor, and u is the veocity vector. Due to the typicay arge number of eements and nodes associated with 3-D simuation, it is necessary to negect ess significant terms in the governing equations. Otherwise, the computationa time and memory requirement woud become too excessive to justify for the 3-D simuation. For exampe, if the poymer mets are assumed to be incompressibe during fiing, the conservation equations of mass and momentum in equations (3.1) and (3.2) become the Navier-Stokes equations [57]: u = 0 (3.6) ( η γ ) ρ g Du ρ = P + 2 & + (3.7) Dt To take into account the effect of inertia on the poymer met fows (e.g. branching fows or jetting), the inertia term in the eft-hand side of equation (3.3) may not be ignored [59]. Whether the body force (e.g. gravity) term, ρ g, in the momentum equation can be negected depends on the geometry and materia of the system [57]. If the inertia is negigibe, the Navier-Stokes equations can be further reduced to the Stokes equations: ( 2η γ ) + ρ g 0 = P + & (3.8) Through the dimensiona anaysis (see [73]), the energy equation can be reduced to a simper form as:

49 Modeing of the Injection Moding Process 35 T ρ C p + u T = ( k T ) + 2η &: γ & γ (3.9) t where k is the heat conduction coefficient. After the entire cavity is competey fied, the pressure throughout the cavity increases rapidy as additiona poymer met is packed into the cavity to compensate for the voumetric shrinkage. During the packing stage, the met compressibiity can no onger be negected. Haagh et a. [73] used equation (3.1) as the governing equation to account for poymer shrinkage in both fiing and post-fiing stages. Hetu et a. [52] empoyed the foowing governing equations for the compressibe fow during the packing stage. The compressibiity effects are incuded by incorporating the equation of state in the continuity equation. P T u = α + u P β + u T (3.10) t t 2 0 = P + 2η & γ η I u (3.11) 3 T P ρ C p + u T = β T + u P + ( k T ) + 2ηγ& : & γ (3.12) t t where I is the identity matrix, α is the compressibiity coefficient, and β is the therma expansion coefficient. The density and the coefficients α and β can be simpy cacuated from the equation of state [45]. After the gate freezes off, no more poymer met enters the cavity and the cooing stage begins. In this stage, the convection and dissipation terms in the energy equation can be negected since the veocity of a poymer met in the cooing stage is amost zero [52].

50 36 Modeing of the Injection Moding Process 3.2 Predicting fiber orientation As pointed out earier, knowing an average fiber orientation direction for the compete part is not sufficient because the oca spatia orientation aso pays an important roe in determining properties such as strength and toughness. Hence, considerabe research has been done with the intent of earning how to predict fiber orientation in injection moded parts. Mathematica modes have been deveoped and incorporated into simuations of injection moding with the emphasis on predicting the infuence of mod geometry, processing conditions, and materia properties on the fina orientation pattern. Most recent works on the fiber orientation anaysis have their origin from Jeffery [104], who derived the equation of orientation change of an eipsoida partice immersed in the homogeneous fow fied based on hydrodynamics. This equation is avaiabe ony in a diute suspension regime where the interaction between fibers is negigibe. In a non-diute suspension regime, statistica consideration of the fiber orientation is required to mode the ensembe of interacting fiber partices. In this case, the orientation distribution function is the ony mode that can describe the distribution of fiber orientation competey. For an efficient numerica simuation of the orientation state of fibers, Advani and Tucker [105] reviewed the orientation tensor, which was originay introduced by Hand [106]. The meaning of the orientation tensor is simiar to the Fourier series of the orientation distribution function. In this approach, ony a few components are required to represent the state of orientation at each spatia point. This advantage has made the orientation tensor, especiay the second order tensor, to be widey used in cacuation of the fiber orientation [47, ] and materia property prediction [ ] in short fiber composites. The orientation tensor is independent of the coordinate system, making it advantageous for numerica simuation and evauations of orientation. The ony weakness of the orientation tensor approach is that a cosure approximation is required to cose the governing equation which is the main reason for causing errors between cacuated and measured vaues. Hybrid cosure approximation proposed by Advani and Tucker [105, 112] has been considered in many appications. Recenty, severa approaches have been introduced to propose more accurate cosure approximations [ ].

51 Modeing of the Injection Moding Process 37 The foowing section wi review the present state-of-the-art modeing of fiber orientation in moding short fiber composites. Topics covered incude characterizing orientation, fiber orientation mechanics for a coection of fibers, cosure approximation and incorporation of these ideas in manufacturing process mode for injection moding Characterizing orientation A compete description of the distribution of fiber orientation begins by considering a singe fiber whose orientation can be represented by a unit vector p as shown in Figure 3.1. Components of the vector p are described by anges θ and φ in spherica coordinates as foows: p 1 = sinθ cosφ p φ 2 = sinθ sin p θ 3 = cos (3.13) Describing the orientation of individua fibers is ineffective since the composites contain numerous short fibers. Thus, the concept of probabiistic distribution was introduced to fuy describe the distribution of fiber orientation in three dimensions. The probabiity distribution function for orientation, aso known as the orientation distribution function, ( θ φ) ψ,, is defined as the probabiity of fiber ying between anges θ and θ + dθ, φ and d φ. The probabiity distribution function must satisfy two physica conditions. First, one end of the fiber is indistinguishabe from the other end, so ψ must be periodic: ( θ φ) = ψ ( π θ, φ π ) ψ, + (3.14) Second, every fiber must have some direction, so the integra over a possibe directions or the orientation space must be equa to unity: 2ππ 0 0 ψ ( θ, φ ) sinθ θ dφ = 1 d (3.15) This is known as the normaization requirement. If the orientation statistics change with position, ψ is a function of x, y, z in addition to θ and φ.

52 38 Modeing of the Injection Moding Process 3 p θ φ 2 1 Figure 3.1 Characterization of the fiber orientation in a coordinate system The distribution function can be approximated by measuring the orientations of a arge number of fibers seected from a region where the distribution function is a compete and unambiguous description of the fiber orientation state. However, this distribution function depends not ony upon the anges θ and φ but aso upon spatia position. The resuting approach woud be highy computationay intensive with arge storage requirement, making it somewhat inappropriate for numerica simuation. For a more efficient method of numericay simuating the orientation state of fiber, Advani and Tucker [105] used orientation tensors. Such tensors are defined as the dyadic products of the unit vector p averaged over a possibe directions, with ψ as the weighting function. The definitions of second ( a ) and fourth ( a ) order orientation tensors can be defined as: ij ijk a ij = p p ψ ( p)dp (3.16) i j and a ijk = p p p p ψ ( p)dp (3.17) i j k There are a number of physica interpretations of these tensors. They can be thought of as a generaization of orientation parameters, as moments of the distribution function, or as a series expansion of the distribution function. Advani and Tucker [105] provide a compete review. They have shown that these tensors are free from a priori assumptions about the

53 Modeing of the Injection Moding Process 39 shape of the distribution function and can be readiy transformed from one coordinate frame of reference to another compying with the rues of tensor transformation, that is a ij = a ji (3.18) The normaization condition such as Equation (3.15) impies that the trace of a ij is unity: a = 1 (3.19) ii The tensor description substitutes ess number of scaar quantities for the distribution function to describe orientation. For exampe, for panar orientation, ony two of the four components are independent. For the 3-D case, ony five of the nine components are independent. Viscosity and eastic stiffness are normay fourth-order tensors, and one woud normay require ony a fourth-order orientation tensor to predict the effect of orientation on these fourth-order properties [105]. By approximating the fourth-order tensor in term of secondorder tensors, it is possibe to predict the effect of orientation on fourth-order properties using just the second-order orientation tensor [112] Fow-induced fiber orientation Jeffery modeed accuratey the motion of a singe fiber immersed in a arge body of incompressibe Newtonian fuid [104]. However, fibers in concentrated suspensions behave somewhat differenty. A number of researchers have observed fiber orientation in concentrated suspensions [80-89, 96]. A of them reported fiber orientation behavior that is quaitativey simiar to singe-fiber motion. In eongationa fows, fibers aign aong the direction of stretching, aigning with the streamines in converging fows and norma to the streamines in diverging fows. Shear fows tend to orient fibers in the fow direction. However, these researchers do not observe perfect aignment of fibers, which the theory predicts. Fibers fowing in concentrated suspension are so cose to each other that they not ony interact hydrodynamicay but may physicay coide with each other, causing more erratic motion that vioates Jeffery s assumptions. The dynamic behavior of a concentrated suspension is quite compex, as the rheoogy is cosey couped with the fiber orientation structure. So far, a phenomenoogica mode

54 40 Modeing of the Injection Moding Process proposed by Fogar and Tucker has proven usefu [116]. They mode the interaction between the fibers by introducing an additiona term in the equation of motion for singe-fiber motion. This term is simiar to a diffusive term, and the effective diffusivity is made proportiona to the strain rate, as interactions take pace ony when the suspension is deforming. A dimensioness interaction coefficient C I term, typicay of the order of 10-2, served to match their experimenta resuts [ ]. The equation of motion can then be combined with the equation that conserves fibers in the orientation space to produce the equation of change for fiber orientation in terms of distribution function and orientation tensors [105, ]. For exampe, the equation of change for the second-order tensor is given by: D a Dt ij 1 ( ω ik akj aik ωkj ) + λ (& γ ik akj + a & ik γ kj 2 & γ k aijk ) + 2C & I γ ( ij 3aij ) (3.20) 1 = δ 2 2 where δ ij is the unit tensor, v v j i ω ij = and xi x j v v j i γ& ij = + are the vorticity and the xi x j 2 ( re 1) rate of deformation tensors, respectivey. A shape factor of partice is defined as λ = 2 ( r + where r is the aspect ratio of the eipsoid, C is the experimenta interaction coefficient e & γ ij & γ ij depending on the partice geometry and concentration, and & γ = is the effective shear 2 rate. I e Numerica simuation of fiber orientation for injection moding Over the ast decade, severa numerica techniques have been deveoped to determine the fiber orientation in injection moded composites by using the generaized Hee-Shaw (GHS) formuation [47, 108, 113, ]. However, because of the simpifying assumptions used in GHS fow mode [42], the so-caed 2.5-D mode reaches its imits in the simuation of the thick-waed modings, compex geometrica configurations (such as bosses, corners, and ribs), and at the met front (fountain fow) regions. Therefore, the fuy 3-D simuation mode shoud be abe to generate compementary and more detaied information reated to the fow characteristics in injection moded parts than the one obtained when using a 2.5-D mode. This wi be particuary important whie moding with fiber reinforced systems.

55 4. Experimenta and Simuation Procedures 4.1 Materias and processing conditions Sandwich injection moding To investigate the infuence of processing parameters and gass fiber content on the materia distribution in sandwich injection moding, the poystyrene (PS 165H), unfied poycarbonate (Makroon 2800), and poycarbonate fied with 20 and 35 wt% short gass fiber (Makroon 8020 and Makroon 8345) were used. The materias were suppied in granuar form by BASF Chemica Co., Ltd. and BAYER GmbH, Germany, respectivey. The sandwich injection moded part (dumbbe shape) was carried out on an ARBURG ALLROUNDER twocomponent injection moding machine (Mode: 320S ). The core materias were coored prior to injection to faciitate the identification of the interface between skin and core materias. A the specimens were moded after the machine had attained a steady state with respect to the preset met and mod temperatures. The seection of the variation range for each injection parameter was based on the fiing technique recommended by the materia suppier and ARBURG s operating instructions [123]. The effects of the skin/core ratio, moding parameters, and gass fiber content on the core thickness were varied on three eves, whie other variabes were maintained at a constant eve throughout this study. Tabe 4.1 and 4.2 summarize the materias and the various parameter settings. These four materias are respectivey designated as PS, PC, SFRPC20, and SFRPC35. The sandwich combinations considered are PC/PC, PC/SFRPC20, PC/SFRPC35, SFRPC20/PC, and SFRPC35/PC. In this nomencature, the first constitutes the skin whie the second designates the core.

56 42 Experimenta and Simuation Procedures Tabe 4.1 Materia and variation of parameter settings used for investigating the effect of processing conditions on the skin/core materia distribution in sandwich injection moding. Materia Grade Suppier Poystyrene PS 165 H BASF Processing Conditions Sandwich Moding 1 st -Pasticator (Skin) 2 nd -Pasticator (Core) Nozze temperature ( C) 210 / 230 / / 230 / 260 Injection fow rate (ccm / s) 9.25 / 18.5 / / 13.5 / 27.0 / 54.0 * Injection voume (%by voume) 35 / 40 / / 60 / 55 Mod temperature was set at 40 C Tota cooing time = 40 sec. * Tota voume of part = 37.0 ccm Tabe 4.2 Materias and processing parameters used for investigation of the effect of mod temperature and gass fiber content on the skin/core materia distribution in sandwich injection moding. Sandwich Moding Sampe Code Skin Materia Core Materia (Skin/Core) PC PC PC/PC PC PC+SGF 20 wt% PC/SFRPC20 PC PC+SGF 35 wt% PC/SFRPC35 PC+SGF 20 wt% PC SFRPC20/PC PC+SGF 35 wt% PC SFRPC35/PC Processing Conditions Sandwich Moding 1 st -Pasticator (Skin) 2 nd -Pasticator (Core) Nozze temperature ( o C) Injection fow rate (ccm/s) * Injection voume (% by voume) Mod temperatures were set at 40 C / 80 C /120 C Tota cooing time = 40 sec. * Tota voume of part = 37.0 ccm

57 Experimenta and Simuation Procedures 43 Poypropyene compounded with short gass fiber is one of the most commerciay important materias and exhibits the most significant deveopment and growth. Therefore, to investigate the effect of processing technique on the fiber orientation, fiber attrition and mechanica properties, the unfied poypropyene (PP-H 1100 L, suppied by TARGOR) and poypropyene fied with 20 and 40 wt% short gass fiber (PP32G10-0 and PP34G10-9, suppied by BUNA) were used. The test specimens were aso moded on the same machine which can be empoyed both for conventiona injection moding and sandwich moding. Injection speed and skin/core voume ratio were chosen as foows: the speed of the first injection unit (skin materia) was kept higher in order to achieve a good surface finish and to prevent premature soidification of the met, whereas ower speed was used for the second injection unit (core materia). The atter was done in order to assess the uniform core extension aong the fow direction without the breakthrough of the core materia at the far end of the bar [ ]. Severa settings were tried and those eading to an overa satisfying quaity with regard to visua properties were finay chosen. The mod temperature was 55 C and the five heating zones (from nozze to feed zone) were set to 250 C, 240 C, 230 C, 220 C, and 210 C, respectivey. The materias and processing parameters used for singe and sandwich moding specimens, containing different short gass fiber contents between skin and core materias, are given in Tabe 4.3 and 4.4. Tabe 4.3 Materias used for investigation of the effect of processing technique on the fiber orientation, fiber attrition, phase separation, and mechanica properties. No. Singe Moding Sampe Code 1 PP PP 2 PP+SGF 20 wt% SFRPP20 3 PP+SGF 40 wt% SFRPP40 Sandwich Moding Sampe Code Skin Materia Core Materia (Skin/Core) 4 PP+SGF 20 wt% PP SFRPP20/PP 5 PP PP+SGF 20 wt% PP/SFRPP20 6 PP+SGF 20 wt% PP+SGF 20 wt% SFRPP20/SFRPP20 7 PP+SGF 40 wt% PP SFRPP40/PP 8 PP PP+SGF 40 wt% PP/SFRPP40 9 PP+SGF 40 wt% PP+SGF 20 wt% SFRPP40/SFRPP20 10 PP+SGF 40 wt% PP+SGF 40 wt% SFRPP40/SFRPP40

58 44 Experimenta and Simuation Procedures Tabe 4.4 Processing conditions used for investigation of the effect of processing technique on the fiber orientation, fiber attrition, phase separation, and mechanica properties of singe and sandwich injection modings. Processing Conditions Singe Moding Sandwich Moding 1 st -Pasticator 2 nd -Pasticator Injection pressure (bar) Hoding pressure (bar) Hoding time (sec) Back pressure (bar) Cooing time (sec) Injection fow rate (ccm/s) Screw speed (m/min) Injection voume (ccm), (%) 37 (100%) 14.8 (40%) 22.2 (60%) Push-pu injection moding Compared to the conventiona injection moding process, the push-pu technique is different in the way that the mod is fitted with at east two gates. The cavity is firsty fied simutaneousy by the met from both the units via the two separate gates (see Figure 4.1). After the two met fronts meet, the wedine is formed and the fiing phase is subsequenty switched to the hoding phase. The materia soidifies starting at the cavity wa but there is sti moten core and then the first push-pu stroke begins. The contro software program aows the definition of severa hoding pressures for one stroke from either the first or the second injection unit. From one of the injection units, poymer met is pressed into the cavity resuting in the moten core being pushed through the gate back into the other injection unit and thus the geometry of wedine is deformed to a tongue shape. One of these movements is termed as push-pu 1 stroke. The number of strokes can be seected by taking into account the part s thickness. When a the strokes are competed, cooing phase foows. As the thickness of frozen ayer increases with the number of strokes within the hoding time, therefore the tota cyce time is not notaby increased as compared to conventiona injection moding [23].

59 Experimenta and Simuation Procedures 45 Second pasticator Wedine First pasticator Mod unit (a) (b) (c) Figure 4.1 Schematic principe of push-pu injection moding process (a) First step, (b) Second step, and (c) Third step. The materias used in this study were unfied poycarbonate (Makroon 2800) and poycarbonate fied with 20 and 35 wt % short-gass-fiber (Makroon 8020 and Makroon 8345). The dumbbe-shaped specimens were injection moded on ARBURG two-component injection moding machine. Besides the push-pu processing, conventiona injection moding was aso carried out for reference purpose. The materia injected from both the units was same with an addition of a sma amount of pigment in one of them to be used as a tracer materia. A the specimens were moded ony after the machine had attained a steady state with respect to the preset met and mod temperatures. The processing parameters used for push-pu moding are given in Tabes 4.5 and 4.6, respectivey.

60 46 Experimenta and Simuation Procedures Tabe 4.5 Processing conditions used in this study. Processing Conditions 1 st -Pasticator 2 nd -Pasticator Nozze temperature ( o C) Injection speed (ccm/s) Injection pressure (bar) * Injection voume (% by voume) Tota cooing time = 45 sec. * Tota voume of part = 24.0 ccm Mod temperature = 100 o C Tabe 4.6 Parameter settings for push-pu 1, 2, and 3 strokes Sampe Code (# 1 st / # 2 nd ) Push-Pu 1 stroke Push-Pu 2 strokes Push-Pu 3 strokes Difference of Hoding Difference of Hoding Difference of Hoding Hoding Pressure Time Hoding Pressure Time Hoding Pressure Time ΔP (bar) (sec) ΔP (bar) (sec) ΔP (bar) (sec) PC / PC SFRPC20 / SFRPC SFRPC35 / SFRPC Microstructure anayses Skin/core materia distribution For investigating the skin/core materia distribution, tensie specimens were cut aong the fow direction through the midde at five different ocations, as shown in Figure 4.2. The sections were then mounted on a stage, after poishing with the hep of a metaurgica

61 6 mm Experimenta and Simuation Procedures 47 technique. The thickness fraction of the core materia ( δ b ) was assessed by optica microscopy (OLYMPUS mode PMG3) and computer aided image anaysis (a4i Anaysis version 5.1 and Image-Pro Pus). The measurements were taken at every 6 mm from the gate, which corresponded to the measured distance ratio ( x i L0 and tota ength of specimen (170 mm). ) between ength of measurement Fow direction Skin materia δ Core materia b Longitudina area 170 mm 4 mm Figure 4.2 Location of sections for materia distribution anaysis [126] Fiber orientation anaysis Poarized ight microscopy and computer aided image anaysis were aso utiized in order to investigate the fiber orientation distribution. For observing fiber orientation, tensie specimens were cut parae to the fow direction into various ayers parae at the midde of specimen (or wedine position for push-pu moded part) as shown schematicay in Figure 4.3. The sections were then poished using a metaurgica technique and mounted on a stage. In the present case, 500 fibers per sampe were measured, estabishing histograms and cacuating the fiber orientation variation across ony haf the thickness of the sections assuming the symmetry of fow. In order to determine panar fiber orientation in the skin and

62 48 Experimenta and Simuation Procedures core ayers, the second order orientation tensor, introduced by Advani and Tucker [105], was cacuated using the foowing equation: a 11 ϕi 1 2 a11 = cos ϕi N N ϕ n= 1 i (4.1) where ϕ i is the ange between the individua fibers and the oca fow direction and N ϕi is the number of fibers with a certain ange ϕ i to the oca fow direction. For perfect aignment aong the fow axis, the orientation average ( randomy oriented to the fow direction it woud be 0.5. a 11 ) woud be equa to 1, whereas when fibers are X Z Y (Fow direction) Wedine Position 20 mm Figure 4.3 Location of areas for fiber orientation anaysis [23, 127].

63 Experimenta and Simuation Procedures Fiber ength anaysis (Fiber attrition) For the investigation of fiber engths within the skin and core ayer, the tensie specimens were cut into seven sections, as shown in Figure 4.4. For the separation of skin and core materias microtome technique was empoyed. Short-gass-fibers were isoated from the composite materias by using an incineration method, according to DIN EN 60. Magnified fiber images were then digitized semi-automaticay with the hep of Image-Pro Pus software running on a persona computer. The fiber ength distribution (FLD) was determined by the average fiber ength which was cacuated from a minimum of 500 ength measurements on fibers recovered from the incineration of the specimen sections. The percent difference between the average fiber ength inside the granues and the overa gass fiber ength inside the moded part ( %Δ ) was used to describe the resuts. For this purpose the foowing equation was empoyed: j G % Δ = 100 (4.2) G with being the average fiber ength inside the granues and the oca fiber ength inside G the individua ayers (skin and core ayers) of the sections of the parts. j 25 mm 25 mm 25 mm 25 mm 25 mm 25 mm 1 20 mm mm Figure 4.4 Location of sections for fiber ength distribution anaysis.

64 50 Experimenta and Simuation Procedures 4.3 Mechanica testing Geometry of tensie specimens is shown in Figure 4.5a, according to the recommendation of DIN EN ISO 527-1/1A/5. Impact bars were obtained from the tensie bars by removing the camping parts; these rectanguar specimens are of thickness 4 mm, width 10 mm and ength 80 mm. A V-notch of 45 o ± 1 o and a root radius of 0.25 ± 0.05 mm were made by sawing with a razor bade. The dimension of the Charpy V-notch impact specimen is iustrated in Figure 4.5b, referring to the standard of DIN EN ISO 179/1 e A. Gate Runner Sprue Test specimen (a) (b) Figure 4.5 (a) Dimensions of the injection moded specimen and (b) Geometry of Charpy V- notch impact. The moded tensie specimens were tested on Zwick 1464 mechanica tester at a crosshead speed of 5 mm/min for a sampe gage ength of 50 mm. For each moding condition, five dumbbe-shaped specimens were tested and the average vaues of the maximum tensie stress were used for anaysis. Charpy impact tests were conducted on a CEAST impact tester mode 6545 using the specimens with a V-notch. The tests were carried out with impact energy of 1 Joue and a sampe span ength of 80 mm. The average vaues of notched Charpy impact strength (kj/m 2 ) were obtained again from a group of five specimens.

65 Experimenta and Simuation Procedures Process simuation Pre-processing A simpe simuation project can be subdivided into three sections. In the first stage, a mode is created, the boundary conditions are assigned and the cacuation is set up. In the subsequent processing step, the cacuation agorithm is appied to the meshed mode and resuting fies are automaticay saved. This means that the resuts are not visibe immediatey. When dispaying resuts (post-processing), first one has to decide about which information contained in the extensive resut data shoud be presented. Then, the corresponding data can be read from the resut fies and can be dispayed graphicay. Resuts from the first cacuation are interpreted. Then the decisions to modification of the part geometry or the process parameters are made and a new cacuation is performed. Additionay, the resuts from an anaysis can be the basis for the foowing simuation. For exampe the resuts from a fiing anaysis are used as a basis for the packing anaysis. A cooing anaysis can be attached to a packing anaysis. Figure 4.6 shows the typica sequence performed when simuating the injection moding process. Open a new project Create report Create mod mode Open / Create a part mode Perform further Anaysis if required Mesh mod mode Create and edit the mesh Change part geometry If yes If required create runner system If no Are the resuts reasonabe? Assign boundary conditions Change boundary conditions Pre-processing Set up anaysis Run anaysis Create and dispay resuts Figure 4.6 Typica sequence performed in injection moding simuation.

66 52 Experimenta and Simuation Procedures Simuation approach Simuation of skin/core materia distribution in sandwich injection moding To investigate the effects of processing parameters and gass fiber content on the skin/core materia distribution, the commercia software package, Modfow, has been utiized in order to predict the fow behavior and thickness fraction of the core materia in the sandwich injection moding process. The midpane mode was first created by a CAD program (Soidwork) and was meshed by creating trianguar eements on the surfaces (3517 nodes, 6296 eements) as demonstrated in Figure 4.7. In the present impementation for the sandwich injection moding process, each poymer obeys the governing equations for generaized Hee-Shaw fow of ineastic, non-newtonian fuids under non-isotherma conditions [44]. Injection Point Mesh thickness (mm) Figure D mesh mode used for the simuation of skin/core materia distribution in sandwich injection moding.

67 Experimenta and Simuation Procedures Simuation of 3-D fiber orientation distribution in sandwich and push-pu injection modings At present, a commercia software package for 3-D simuation of two-component injection moding incuding the fiber orientation anaysis is not yet avaiabe. However, from the process setting of this program (where the user can input the processing parameters i.e. an injection ocation, seection of materia, definition of mod and met temperature) it is possibe to contro the met fow profie during the fiing phase simiar to the met fow deveopment during sequentia sandwich and push-pu injection moding processes. By using ony one materia and controing the ram speed profie during injection phase for sandwich injection moding process [128] or utiizing the controed vave gate with the hot runner system for push-pu processing [129]. In this study, the 3-D modes of sandwich and push-pu processing have been deveoped and performed with the aid of Modfow simuation program in order to predict the 3-D fiber orientation distribution in the sandwich and push-pu injection moded parts. First, the 3-D mode was created by Soidwork program and was then meshed by creating tetrahedra eements within the CAD mode (see Figures 4.8 and 4.9). This mesh mode was then simuated by using the 3-D finite eement anaysis, which is based on fuid mechanics and heat transfer cacuations. The materia properties used for computer simuation of skin/core materia distribution in sandwich injection moding and for predicting of 3-D fiber orientation distribution in sandwich and push-pu injection modings are summarized in Tabes 4.7 Tabe 4.7 Materia properties (Modfow database). Materias Trade Name Therma Conductivity Specific Heat Gass Transition (W / m C) (J / kg C) Temperature ( C) PP + 40% Short Gass Fiber * Cestran PP-GF at 275 C 2243 at 275 C 138 PS PS 165H at 230 C 1975 at 230 C 92 PC Makroon at 300 C 1700 at 300 C 139 PC + 20% Short Gass Fiber Makroon at 300 C 1530 at 300 C 150 PC + 35% Short Gass Fiber Makroon at 300 C 1400 at 300 C 150 * The materia properties of PP fied with 40% short gass fiber from TARGOR (PP34G10-9) are not avaiabe in Modfow database

68 54 Experimenta and Simuation Procedures Injection Point Figure D mesh mode used for simuation of fiber orientation during sandwich injection moding (59,798 nodes, 337,358 tetrahedra eements). Overfow Cavity # 1 Vave Gate # 1 VaveGate# 2 Injection Point Vave Gate # 3 Main Cavity Hot Runner System VaveGate# 4 Overfow Cavity # 2 Figure D mesh mode used for simuation of fiber orientation during push-pu processing (22,352 nodes, 119,355 tetrahedra eements).

69 Experimenta and Simuation Procedures 55 Sandwich injection moding The sandwich injection moding simuation was performed by injecting 40 % vo. of materia (Cestran 40 wt% GF) into the cavity with the injection fow rate of 18.5 ccm/s. After approximatey 1 second, the rest of materia (60 %vo.) was then injected into the cavity with a sower injection fow rate of 8.88 ccm/s unti the end of the injection shot. The comparison of simuation resuts between singe and sandwich injection moding processes is shown in Figure %vo. of materia Singe injection moding Sandwich injection moding Figure 4.10 Three-dimensiona simuation resuts of the met fow front during the fiing stage for singe and sandwich injection modings.

70 56 Experimenta and Simuation Procedures Push-pu processing simuation To simuate the conventiona injection moded part with wedine, vave gates # 2 and # 3 were initiay opened whereas vave gates # 1 and # 4 were cosed. The simuation started when the poymer met was injected from the injection point and passed vave gates # 2 and # 3 into the main cavity and a wedine was formed by the merging of two met fronts at the midde of the part, as demonstrated in Figure 4.11a. After the main cavity had been competey fied, the simuation was switched to the packing phase and the push-pu processing coud be simuated by aternativey opening and cosing the vaves as described previousy [71,129]. Figure 4.11b to 4.11d demonstrate the simuation resuts of the push-pu process, for the number of push-pu 1, 2, and 3 strokes, respectivey. In the case of the pushpu 1 stroke, vave gate # 3 was cosed after the main cavity had been fied and a wedine had been produced and at the same time vave gate # 4 was opened. During this period, depending on the setting time of the vave gate, the moten poymer fowed continuousy from the main cavity through vave gate # 4 into overfow cavity # 2. For the push-pu 2 strokes, after the first stroke was competed, vave gates # 2 and # 4 were cosed and vave gates # 1 and # 3 were activated. Then the poymer met fowed in the reverse direction from the main cavity through vave gate # 1 into overfow cavity # 1. In the case of the push-pu 3 strokes, the third stroke of the push-pu processing simuation started after the second stroke was competed by cosing vave gates # 1 and # 3 and opening vave gates # 2 and # 4. The poymer met was again pushed back through the corresponding cavity system.

71 Experimenta and Simuation Procedures 57 VaveGate# 3 VaveGate# 2 VaveGate# 3 VaveGate# 2 VaveGate# 4 VaveGate# 1 VaveGate# 4 VaveGate# 1 Overfow Cavity # 2 Overfow Cavity # 1 Overfow Cavity # 2 Overfow Cavity # 1 Wedine Position Push-Pu 1 stroke (a) (b) VaveGate# 3 VaveGate# 2 Vave Gate # 3 Vave Gate # 2 VaveGate# 4 VaveGate# 1 VaveGate# 4 VaveGate# 1 Overfow Cavity # 2 Overfow Cavity # 1 Overfow Cavity # 2 Overfow Cavity # 1 3 rd stroke 1 st stroke 2 nd stroke (c) 1 st stroke (d) 2 nd stroke Fi time (sec) Figure 4.11 Evoution of the met fow front during the simuation: (a) Conventiona injection moding with wedine, (b) Push-pu injection moding 1 stroke, (c) Push-pu injection moding 2 strokes, and (d) Push-pu injection moding 3 strokes.

72 58 Experimenta and Simuation Procedures

73 5. Experimenta Resuts and Discussion 5.1 Comparison between conventiona and sandwich injection modings Fiber orientation distribution As Figure 5.1a shows, it was found that there are a number of distinct regions within the modings with different fiber aignments. This has aso been identified by severa other studies [17, 20, 47, 80-89,120]. The ayer at the mod wa, referred to as the surface ayer, tends to have fibers randomy oriented or sighty fow aigned. This randomy oriented region is caused by fountain fow near the met front [113]. In particuar, fibers from the core region near the met front move outward to the wa passing through the fountain fow region. In the skin region, the fiber orientation is predominatey parae to the fow direction. This is due to eongationa forces arising during fountain fow at the front and to shear fow after the front has passed. In contrast, a random in pane aignment of fibers is observed in the core ayer due to a sower cooing rate and ower shearing. Moreover, the micrographs ceary revea that voids are mosty ocated within the core ayer. This observation has aso been reported in previous works [85, 96,130]. The presence of voids is mainy attributed to shrinkage during the cooing stage of injection moding where the moten core undergoes shrinkage away from the soidified skin ayer [130]. Figure 5.2 shows the measured vaues for the fiber orientation tensor ( a 11 ) vs. the reative thickness (z i / h). From the measured resuts obtained in this study, it is interesting to note that the core of the moding appears to contain a random aignment of fibers ( a ) rather than highy aigned transverse to the fow direction ( a 11 0). The expanation for this woud be reated with the effect of geometry such as a camping part of tensie specimen (at the entrance region) where the converging fow fied is estabished. This can ead to a higher veocity gradient of the fowing met aong the fow path, and thus resuting in an increase of the fiber orientation within the core ayer. It

74 60 Experimenta Resuts and Discussion aso can be observed that the a 11 in the core region of SFRPP20 is higher than in the core of SFRPP40. These resuts are in agreement with the work carried out by previous researchers [83-85] in that an increase of the thickness of the core ayer of injection moded short fiber reinforced thermopastics appears to be more pronounced as the gass fiber content increases. Surface Layer Skin Layer Core Layer Voids Z Y Fow Direction (a) SFRPP20 SFRPP40 Y Fow Direction (b) Z SFRPP20/SFRPP20 SFRPP40/SFRPP40 Figure 5.1 Optica micrographs in the Z-Y pane of (a) singe and (b) sandwich injection modings.

75 Experimenta Resuts and Discussion Orientation Tensor Component (a 11 ) Surface Experimenta Resuts SFRPP20 SFRPP40 SFRPP20/SFRPP20 SFRPP40/SFRPP40 Reative Thickness (z i / h) Midpane Orientation Tensor Component (a 11 ) Figure 5.2 Variation of the a 11 component of orientation tensor through the haf thickness of singe and sandwich injection moded specimens. Figure 5.1b shows the fiber orientation inside the skin and core ayers of sandwich moded specimens with different gass-fiber contents. For SFRPP20/SFRPP20 and SFRPP40/SFRPP40, it can aso be seen in Figure 5.2 that the vaues for a 11 within the core ayer are higher than those obtained for singe injection modings. This can be due to two possibe reasons. Firsty, it can be the resut of the met fow front of the first injected materia (which wi become the skin materia) deveops a paraboic veocity profie, near the mod wa the fibers are generay aigned in the fow direction due to the high veocity gradient. Prior to the skin materia s reaching the end of the cavity, the second materia is injected to form the core. This materia deveops a second fow front, pushing the skin materia ahead of it. As shown in Figure 5.3, the veocity at the center of the core materia is higher than that at the skin fow front, because the materia injected first soidifies as it comes into contact with the cod wa of the mod. The soidified skin materia can act as a second mod wa inside the mod cavity, narrowing of the fow channe. Thus the higher the veocity gradient of core materia, the higher is the fiber orientation in the core ayer. Secondy, the sower the injection speed of the second materia during the fiing stage, the higher the thickness of the soidified skin ayer restricting the cross-sectiona area avaiabe for the

76 62 Experimenta Resuts and Discussion fowing met. This eads to a higher veocity gradient that tends to increase the fiber orientation of the adjacent met ayer. These resuts are in agreement with the work carried out by previous researchers [47, 81, 119] in that the fibers at the mid-pane become more fow aigned and the thickness of the core region decreases with the thickness of the cod boundary ayer increasing. Primary Skin Layer Fow Direction Core Layer Veocity Profie of Skin Materia Soidified Skin Layer Mod Wa Fow Direction Core Materia Skin Materia Veocity Profie of Core Materia Primary Skin Layer Secondary Skin Layer Tota Skin Layer Fow Direction Core Layer Figure 5.3 Schematic of poymer met fow profie and resuting fiber ayer structure in sandwich injection moded short fiber composites. [127]

77 Experimenta Resuts and Discussion 63 The photomicrographs of ongitudina area of sandwich specimens are iustrated in Figures 5.4a through 5.4e. These pictures ceary indicate that the fibers are highy oriented parae to the oca fow direction within the skin region, as presented in Figures 5.4a to 5.4c. Furthermore, in the core region, the higher degree of fiber orientation and esser voids (Figures 5.4c-e) of sandwich moded parts are thought to be caused by the shape of the veocity profie and the thickness of the frozen ayer near the wa, as stated earier. SFRPP20 SFRPP40 SFRPP40 PP PP SFRPP20 (a) (b) (c) PP PP SFRPP20 SFRPP40 Y (Fow direction) Z (d) (e) Figure 5.4 Optica micrographs of ongitudina area (Z-Y pane) of sandwich injection moded parts: (a) SFRPP20/PP, (b) SFRPP40/PP, (c) SFRPP40/SFRPP20, (d) PP/SFRPP20, and (e) PP/SFRPP40.

78 64 Experimenta Resuts and Discussion Fiber ength distribution (Fiber attrition) Figure 5.5 shows the percent differences between the mean fiber ength of the granues and the overa gass fiber ength inside the moded part ( % Δ ). In a the cases, the fiber ength is much ower in the injection modings than in the granues, which is due to the fact that fiber ength is aways reduced to a imiting vaue depending on met viscosity, the intensity of the shear fied and the residence time [91-92]. Furthermore, it is obvious that the mean fiber ength for each subdivision of tensie specimens decreases with the increase of the gass fiber concentration. This has been observed by many authors [82, 87, 92, 94-95, 99, 102] who mainy attribute a higher fiber concentration to a higher degree of fiber-fiber interaction and increased fiber-wa contacts. Moreover, it can be seen that the fiber attrition inside the skin ayer of the injection modings is higher than that in the core ayers. Our experiments with simpe moded geometry showed that the reduction of fiber ength for SFRPP20 is approximatey 10-15% in the core and about 20-25% in the skin ayer. As pointed out by previous researchers [83, 131], the fiber ength is obviousy higher within the core ayer than in the skin ayer. This is due to the foowing mod fiing characteristics. The core component is fied with reativey ow shear force compared to the skin component, where the met begins to soidify as soon as it comes into contact with the cod mod wa. Therefore, ess deformation is appied to the fibers at the center which resuts in a higher average fiber ength in the core region. For the higher fiber concentration, a higher degree of fiber degradation inside the skin and core ayers, which accounts for approximatey 30% in the core ayer and 40-45% in the skin ayer, can be observed. The occurrence of more pronounced fiber ength degradation for the higher fiber concentration is beieved to arise from an increased fiber-fiber interaction in the more viscous met. With respect to fiber attrition in the ongitudina direction of the bar, it can be noted that the effects of different processing types and gass-fiber concentrations do not ead to significant changes of fiber ength. In a the cases, ony insignificant differences between the subdivisions were observabe. Probaby the effect of the simpe mod geometry, used for this investigation, on fiber ength destruction is smaer than that of compicated mod geometry [132]. Comparing the effect of sandwich and singe injection moding processes on the fiber ength inside the skin region, ony minor differences were observed. As mentioned earier, this is due to a higher shear rate near the mod surface and fiber interactions with the mod wa. The effect of different processing types on fiber ength, however, is more pronounced in

79 Experimenta Resuts and Discussion 65 the core region. The fiber ength distribution within the core ayer of the sandwich moding (SFRPP20/SFRPP20) is sighty ower than the vaues obtained for the singe injection moding (SFRPP20). For a higher fiber oading, the fiber ength inside the core region of SFRPP40 and SFRPP40/SFRPP40 becomes higher. This can not ony be expained by the narrower fow channe and the higher shear rate occurring during the sandwich moding process, but aso by the higher fiber oading itsef, which resuts in more frequent fiber-fiber interactions and, thus, higher fiber destruction in the core region of sandwich modings %Δ SFRPP20 (Core ayer) SFRPP20 (Skin ayer) SFRPP20/SFRPP20 (Core ayer) SFRPP20/SFRPP20 (Skinayer) (a) 0 %Δ SFRPP40 (Core ayer) SFRPP40 (Skin ayer) SFRPP40/SFRPP40 (Core ayer) SFRPP40/SFRPP40 (Skin ayer) (b) Position Figure 5.5 Fiber attrition in the skin and core ayers at various positions of tensie specimens: (a) Singe and sandwich moded parts containing 20 wt% short gass fibers and (b) Singe and sandwich moded parts containing 40 wt% short gass fibers Mechanica properties Figure 5.6 iustrates the tensie and impact properties of sandwich moding specimens containing different gass fiber concentrations within the skin and core materias in comparison to those of singe injection moding specimens. It is generay known that the addition of gass fibers resuts in an enhancement of the tensie and impact properties [76, 82-

80 66 Experimenta Resuts and Discussion 89, 95-96, ]. The mechanica properties of PP co-injected with gass-fiber reinforced PP are generay at an intermediate eve between those of PP and gass-fiber reinforced PP aone [133]. It is interesting to note that, for the sandwich injection modings (SFRPP20/SFRPP20 and SFRPP40/SFRPP40), the maximum tensie stress and impact strength are higher than for the singe injection modings (SFRPP20 and SFRPP40). This improvement of the mechanica properties is considered to be due to a higher degree of fiber orientation within the core ayer (the infuence of voids is negected since the area fraction between the tota area of voids and the cross-sectiona area of the specimen is approximatey ess than 0.25%). However, comparing the mechanica properties of sandwich moding and singe injection moding, it can be observed that the maximum tensie stress and the impact strength of sandwich specimens are not as high as one might have expected. This is probaby due to the higher fiber attrition that occurs during sandwich moding process and this fiber shortening can reduce the fiber reinforcing efficiency [ ] PP SFRPP20/PP Maximum Tensie Stress (MPa) SFRPP40/PP PP/SFRPP20 PP/SFRPP40 SFRPP20 SFRPP20/SFRPP20 SFRPP40/SFRPP20 SFRPP40 SFRPP40/SFRPP40 Maximum Tensie Stress Impact Strength Impact Strength (kj/m 2 ) 0 Figure 5.6 Maximum tensie stress and impact strength of conventiona and sandwich injection moded parts containing different gass fiber contents.

81 Experimenta Resuts and Discussion Comparison between conventiona and push-pu injection modings Geometry of wedines Figure 5.7 shows the wedine geometries of conventiona and push-pu processed specimens for SFRPC35 / SFRPC35, which are moded at severa hoding pressure differences. In the case of conventiona injection moding process, a reativey straight wedine is produced when the hoding pressures on both sides are kept same. The geometry of wedine is deformed to the tongue shape with difference in hoding pressure. It has been found that the position of the wedine at the mod surface does not change even when the pressure difference is increased. The tongue-shaped wedines can aso be observed for SFRPC20 / SFRPC20 and PC / PC in the same way. The geometry of wedine for push-pu 2 and 3 strokes processed specimens can be easiy observed in the ongitudina area (Z-Y pane), (as presented ater in Figures 5.10 and 5.11). Hoding Pressure Difference (ΔP) Wedine Position ΔP P = 0 bar Conventiona ΔP P = 70 bar Push-Pu Pu 1 stroke ΔP P = 120 bar Push-Pu Pu 1 stroke ΔP P = 220 bar Push-Pu Pu 1 stroke ΔP P = 120 bar Push-Pu Pu 2 strokes ΔP P = 120 bar Push-Pu Pu 3 strokes Figure 5.7 Wedine geometry of SFRPPC35/SFRPC35.

82 68 Experimenta Resuts and Discussion Fiber Orientation in Wedine Areas A typica resut of fiber orientation in the wedine area is shown in Figure 5.8. As expected, it can be seen that fiber orientation in the wedine region consists mainy of fibers which are parae to the wedine surface, i.e., perpendicuar to the fow direction. This is associated with the fountain fow phenomena at the met fow front, as investigated by previous works [ ]. As one moves away from the wedine region, the orientation pattern is simiar to that observed in the non-weded specimens. Fountain fow at the met fow front Wedine region Figure 5.8 Optica micrographs showing the fiber orientation distribution in the wedine region of the PC fied with 35 wt% of short-gass-fiber at the midpane ocation (X-Y pane).

83 Experimenta Resuts and Discussion 69 The micrographs of the wedine geometries and the fiber orientation pattern across the thickness of the push-pu processed specimens are iustrated in Figures and the measured vaues of orientation tensor (a 11 ) within wedine area are shown in Figure It has been found that in the surface ayer of wedine position, the fibers sti show perpendicuar aignment to the fow direction. This is due to the fact that the met begins to soidify as soon as it comes into contact with the cod mod wa. For the push-pu 1 stroke and 2 strokes processed specimens, it can be ceary seen that the fibers within the wedine region are highy aigned parae to the oca fow direction and the degree of fiber orientation in the core ayer of push-pu 1 stroke processed specimen increases with increasing hoding pressure difference (see Figure 5.12). However, in the case of the pushpu 3-strokes processed sampe, it shoud be noted that the third stroke of push-pu does not produce any major changes in the fiber orientation within the wedine region, particuary in the core ayer, where the fibers are preferentiay oriented perpendicuar to the fow direction, as presented in Figures 5.11a. This can be attributed to the heat transfer characteristic of moten poymer, as reported by Modfow database, where the therma conductivity of the unfied PC (Makroon 2800) was W/m.K and W/m.K for PC fied with 35 wt% short-gass-fibers (see Tabe 4.7). In the case of PC fied with 35 wt% short-gass-fibers the viscosity increases more rapidy and reaches its no-fow temperature sooner than for the unfied PC, and thus it coud not be manipuated as much as the unfied PC with the ower therma conductivity (see Figure 5.11b). Therefore, the moten core is expected to become thinner during the push-pu process as stroke by stroke soidified ayers deposit on the mod wa, so that the resistance against the screw motion increases.

84 70 Experimenta Resuts and Discussion Figure 5.9 Optica micrographs showing the fiber orientation pattern across the thickness (Z- Y pane) of the push-pu 1 stroke processed specimen.

85 Experimenta Resuts and Discussion 71 1 st 2 nd Figure 5.10 Optica micrographs showing the fiber orientation pattern across the thickness (Z-Y pane) of the push-pu 2 strokes processed specimen.

86 72 Experimenta Resuts and Discussion Fiber aignment perpendicuar to the fow direction 1 st 3 rd (a) 1 st 3 rd (b) Figure 5.11 (a) Optica micrographs showing the fiber orientation pattern across the thickness (Z-Y pane) of the push-pu 3 strokes processed specimen and (b) Wedine geometry of push-pu 3 strokes processed specimen for unfied PC.

87 Experimenta Resuts and Discussion Orientation Tensor Component (a 11 ) Conventiona Push-pu 1 stroke (ΔP = 70 bar) 0.2 Push-pu 1 stroke (ΔP = 120 bar) Push-pu 1 stroke (ΔP = 220 bar) 0.1 Push-pu 2 strokes (ΔP = 120 bar) Push-pu 3 strokes (ΔP = 120 bar) Surface Experimenta resuts Reative Thickness (z i / h) Midpane Orientation Tensor Component (a 11 ) Figure 5.12 Variation of the a 11 component of orientation tensor through the haf thickness of conventiona and push-pu processed specimens in the wedine area Effects of hoding pressure difference and fiber concentration on penetration ength of wedine The optica micrographs of penetration ength of wedine for the unfied PC and PC fied with different gass-fiber contents (20 and 35 wt %) using various hoding pressure differences are shown in Figures 5.13 to It can be seen that the higher the difference in hoding pressure, the onger is the penetration ength of wedines, as expected. However, it shoud be noted that the reationship between the pressure difference and the penetration ength of wedine is not inear, (see Figure 5.16) suggesting that this increase in the penetration ength of wedine is not soey due to an increase in hoding pressure differences, but may invove some other parameters incuding the compressibiity of moten poymer during the hoding stage and aso the effect of pressure on the viscosity of met, in that the greater the pressure the higher the met viscosity [143]. In comparing the penetration ength

88 74 Experimenta Resuts and Discussion of wedine for fied and unfied PC, it can be observed that the higher the gass fiber content added into the poymer met, the shorter the penetration ength of the wedine. For the maximum hoding pressure difference, it has been found that the maximum penetration ength of the unfied PC met is around mm, whie in the case of fied PC met (20 and 35 wt%), it is around mm away from the initia wedine position. The decrease in penetration ength of wedine in fied PC met can be attributed to an increase in viscosity in the fowing poymer met [144]. This appears to contradict resuts of a previous work on the fied and unfied PC in which the shortest penetration ength of wedine for unfied PC was reported [145]. ΔP = 0 bar Penetration ength of wedine ΔP = 70 bar ΔP = 120 bar ΔP = 220 bar Figure 5.13 Penetration ength of wedine across the sampe thickness of the moded part for the unfied PC at various hoding pressure differences.

89 Experimenta Resuts and Discussion 75 ΔP = 0 bar Penetration ength of wedine ΔP = 70 bar ΔP = 120 bar ΔP = 220 bar Figure 5.14 Penetration ength of wedine across the sampe thickness of the moded part for the PC fied with 20 wt% short gass fibers at various hoding pressure differences. ΔP = 0 bar Penetration ength of wedine ΔP = 70 bar ΔP = 120 bar ΔP = 220 bar Figure 5.15 Penetration ength of wedine across the sampe thickness of the moded part for the PC fied with 35 wt% short gass fibers at various hoding pressure differences.

90 76 Experimenta Resuts and Discussion Penetration Length of Wedine (mm) PC / PC SFRPC20 / SFRPC20 SFRPC35 / SFRPC Difference of Hoding Pressure, ΔP (bar) Figure 5.16 Reationship between the hoding pressure difference and penetration ength of wedine for PC with various gass-fiber contents Fiber ength distribution in wedine areas The histogram representing the fiber ength distribution for the granues of PC fied with 20 and 35 wt% short-gass-fibers (20G and 35G) is shown in Figure 5.17a. In genera, it can be seen that the higher the fiber oading, the shorter the fiber ength, as has been reported by many authors [82, 87, 92-95, 99]. A further fragmentation of fibers aso occurred during injection moding as presented in Figure 5.17b. Our experiments with simpe geometry showed that the reduction of fiber ength is approximatey 8-14 %. This is due to the high shear rates during injection moding when the met containing fibers has to pass through gates which are primariy narrow channes of fow. Tabe 5.1 shows the percent differences between the mean fiber ength of the granues and the mean fiber ength ( % Δ ) within the wedine area of conventiona and push-pu processed parts. It has been found that the effects of hoding pressure difference and the number of push-pu strokes do not ead to significant changes of fiber ength. This may be due to the fact that fiber ength is aways reduced to a

91 Experimenta Resuts and Discussion 77 imiting vaue depending on met viscosity, the intensity of the shear fied and the residence time [91-92] and thus the fibers can not undergo further fragmentation during push-pu processing G20 μ = μm Reative Fiber Length Distribution Average aspect ratio (L/D) = G35 μ = μm Average aspect ratio (L/D) = (a) 0.00 Fiber ength (μm) Fiber Length (μm) SFRPC20 μ = μm Reative Fiber Length Distribution Average aspect ratio (L/D) = SFRPC35 μ = μm Average aspect ratio (L/D) = (b) 0.00 Fiber ength (μm) Fiber ength (μm) Figure 5.17 Histograms representing fiber ength distribution (a) within PC granues containing 20 and 35 wt% short gass fibers and (b) within injection moded artice without wedine.

92 78 Experimenta Resuts and Discussion Tabe 5.1 Fiber ength in the wedine area of conventiona and push-pu processed parts. Sampe Code Push-Pu 1 stroke Difference of Hoding Average Fiber Average Aspect (# 1 st / # 2 nd ) Pressure, ΔP (bar) Length (μm) Ratio (L/D) % Δ SFRPC20 / SFRPC SFRPC35 / SFRPC Sampe Code Push-Pu 2 strokes Difference of Hoding Average Fiber Average Aspect (# 1 st / # 2 nd ) Pressure, ΔP (bar) Length (μm) Ratio (L/D) SFRPC20 / SFRPC % Δ SFRPC35 / SFRPC Push-Pu 3 strokes Sampe Code Difference of Hoding Average Fiber Average Aspect % Δ (# 1 st / # 2 nd ) Pressure, ΔP (bar) Length (μm) Ratio (L/D) SFRPC20 / SFRPC SFRPC35 / SFRPC

93 Experimenta Resuts and Discussion Wedine strength The photographs of the sampes broken during the tensie testing are iustrated in Figure In the case of SFRPC20/SFRPC20 and SFRPC35/SFRPC35, the tensie faiure was britte without any neck formation and it occurred at the wedine position. The expanation for this woud be reated to the existence of the perpendicuar fiber aignment around the wedine surface, which can be a source of stress concentration within the part surface and thus it woud be easy to break at this position when compared to another position away from the wedine area. On the other hand, for the unfied PC specimens, the faiure was ductie with necking initiated across the gauge ength. This behavior is aso in accordance with previous observation [145]. Hoding Pressure Difference (ΔP) Wedine Position ΔP P = 0 bar Conventiona ΔP P = 70 bar Push-Pu Pu 1 stroke ΔP P = 120 bar Push-Pu Pu 1 stroke ΔP P = 220 bar Push-Pu Pu 1 stroke ΔP P = 120 bar Push-Pu Pu 2 strokes ΔP P = 120 bar Push-Pu Pu 3 strokes Figure 5.18 Appearance of fractured specimens of SFRPC35 / SFRPC35. The maximum tensie stresses of conventiona and push-pu processed parts, for PC containing different short-gass-fiber contents (0, 20, and 35 %wt) are shown in Figure For the unfied PC, it is found that the presence of the wedine and the effect of push-pu processing do not have any significant infuence on the maximum tensie stress. Whie, in the case of short gass fiber reinforced PC, the maximum tensie stress of the sampes with

94 80 Experimenta Resuts and Discussion wedine is significanty ower than the vaues obtained for the sampes without wedine. Furthermore, the wedine strength of injection moded PC composites is found to decrease with the content of reinforcing fibers in the composites. These behaviors are aso in accordance with previous investigations using different composite materias [71, , ] Wedine PPP 1 stroke (ΔP=70 bar) PPP 1 stroke (ΔP=120 bar) PPP 1 stroke (ΔP=220 bar) PPP 2 strokes (ΔP=120 bar) PPP 3 strokes (ΔP=120 bar) Without Wedine Maximum Tensie Stress (MPa) PC / PC SFRPC20 / SFRPC20 SFRPC35 / SFRPC Maximum Tensie Stress (MPa) 0 Figure 5.19 Maximum tensie stress of conventiona and push-pu processed specimens containing different gass fiber contents. In comparing the wedine strength of injection moded parts produced by conventiona and push-pu techniques, it can be seen that the wedine strength of the push-pu 1 stroke processed parts increases with increasing the hoding pressure differences. In case of the highest hoding pressure difference (P = 220 bar), the wedine strength is higher than that of the conventiona moding. In fact the differences are in the range of 40-45%. For the pushpu 2 strokes processed part, it is aso observed that the wedine strength is superior to the one produced by the conventiona injection moding (approximatey %). This increase in the wedine strength is supposed to be caused by the higher degree of fiber orientation

95 Experimenta Resuts and Discussion 81 aong the fow direction within the wedine areas, as mentioned earier. However, there is ony minor difference in wedine strength observed between push-pu 3 strokes and conventiona injection modings (around 7-10%). This is due to the perpendicuar fiber aignment in wedine area particuary in the core ayer of push-pu 3 strokes processed part, as stated earier. 5.3 Prediction of the tensie strength for short fiber reinforced composites The purpose of deveoping a theoretica mode is to expain and predict the experimenta resuts. Additionay, the theoretica mode shoud aso be abe to be verified by the existing experimenta resuts. As described in the section 2.4.1, the critica fiber ength ( c ) and the interfacia shear strength between fiber and matrix (τ ) can be cacuated by Equations 2.5 and 2.6 for given composites system if the fiber ength distribution, the fiber diameter ( d ) and the matrix strength ( σ ) are given. The required parameters for predicting the strength of m short fiber reinforced composites are given in Tabes 5.2 to 5.4. Since the fiber voume fraction, the matrix strength and the area fraction between skin and core ayer have been given experimentay (the measured data of fiber orientation and ength, gass fiber content of granues and area fraction of skin and core ayers are summarized in Appendices A to F), then the predicted vaues of UTS for the conventiona, sandwich and push-pu injection moded composites can be estimated foowing Equations 2.13 to The theoreticay cacuated resuts, together with the experimentay determined UTS and wedine strength are shown in Figures 5.20 to It can be seen from Figure 5.20 that the mode predictions show a reasonabe agreement with the experimenta vaues, athough there are sti some differences in both the cases. The cacuated resuts indicate that the UTS increases with the increase of fiber voume fraction and the UTS of PP sandwich injected with gass fiber reinforced PP (PP/SFRPP and SFRPP/PP) are at an intermediate eve between those of PP and gass fiber reinforced PP aone. In addition the predicted resuts aso show the UTS of sandwich injection modings are higher than that of singe injection modings. This is due to a higher degree of fiber orientation within the core ayer (or a higher area fraction of skin ayer, A L ) of sandwich injection moded composites, as mentioned in section 5.1.4, though the A C

96 82 Experimenta Resuts and Discussion fiber ength within the core ayer of the sandwich modings are sighty ower than the vaues obtained for the singe injection modings. 120 Experimenta Resuts Theoreticay Cacuated Resuts SFRPP20/PP PP/SFRPP20 SFRPP20 SFRPP20/SFRPP20 SFRPP40/PP PP/SFRPP40 SFRPP40/SFRPP20 SFRPP40 SFRPP40/SFRPP40 Maximum Tensie Stress (MPa) Maximum Tensie Stress (MPa) 0 Figure 5.20 Comparison of experimenta and theoreticay cacuated UTS resuts for conventiona and sandwich injection moded short gass fiber reinforced poypropyene.

97 Experimenta Resuts and Discussion 83 Tabe 5.2 Parameters used in the theoretica cacuation of UTS for singe and sandwich injection moded short gass fiber reinforced PP composites. Sampes Parameters Source SFRPP20/PP PP/SFRPP20 SFRPP20 SFRPP20/SFRPP20 SFRPP40/PP PP/SFRPP40 SFRPP40/SFRPP20 SFRPP40/SFRPP40 SFRPP40 σ m (MPa) Measurement τ (MPa) Cacuated [79] σ f (MPa) [76] d (μm) Measurement c (μm) Cacuated [79] V f / Measurement V m / Measurement A L /A C _ Measurement A T /A C _ _ Measurement A Skin /A C A Core /A C Measurement Measurement

98 84 Experimenta Resuts and Discussion Tabe 5.3 Parameters used in the theoretica cacuation of wedine strength for conventiona and push-pu injection moded short gass fiber reinforced PC composites. SFRPC20/SFRPC20 Parameters Source PPP 1 stroke PPP 2 strokes PPP 3 strokes Without Wedine (ΔP = 70 bar) (ΔP = 120 bar) (ΔP = 220 bar) (ΔP = 120 bar) (ΔP = 120 bar) Wedine σ m (MPa) Measurement τ (MPa) Cacuated [79] σ f (MPa) [76] d (μm) Measurement c (μm) Cacuated [79] V f Measurement V m Measurement A Skin /A C Measurement A Core /A C Measurement SFRPC35/SFRPC35 Parameters Source PPP 1 stroke PPP 2 strokes PPP 3 strokes Without Wedine (ΔP = 70 bar) (ΔP = 120 bar) (ΔP = 220 bar) (ΔP = 120 bar) (ΔP = 120 bar) Wedine σ m (MPa) Measurement τ (MPa) Cacuated [79] σ f (MPa) [76] d (μm) Measurement c (μm) Cacuated [79] V f Measurement V m Measurement A Skin /A C Measurement A Core /A C Measurement

99 Experimenta Resuts and Discussion 85 Tabe 5.4 Mean gass fiber ength of injection moded composites. Singe modings μ (μm) Push-pu processed parts SFRPP20 (Skin ayer) SFRPC20/SFRPC20 SFRPP20 (Core ayer) SFRPP40 (Skin ayer) Conventiona (Wedine) SFRPP40 (Core ayer) Push-pu 1 stroke (ΔP = 70 bar) Push-pu 1 stroke (ΔP = 120 bar) μ (μm) Sandwich modings μ (μm) Push-pu 1 stroke (ΔP = 220 bar) Push-pu 2 strokes (ΔP = 120 bar) SFRPP20/SFRPP20 (Skin ayer) Push-pu 3 strokes (ΔP = 120 bar) SFRPP20/SFRPP20 (Core ayer) Conventiona (Without Wedine) SFRPP40/SFRPP40 (Skin ayer) SFRPP40/SFRPP40 (Core ayer) SFRPC35/SFRPC35 μ (μm) SFRPP20/PP (Skin ayer) Conventiona (Wedine) PP/SFRPP20 (Core ayer) Push-pu 1 stroke (ΔP = 70 bar) SFRPP40/PP (Skin ayer) Push-pu 1 stroke (ΔP = 120 bar) PP/SFRPP40 (Core ayer) Push-pu 1 stroke (ΔP = 220 bar) SFRPP40/SFRPP20 (Skin ayer) Push-pu 2 strokes (ΔP = 120 bar) SFRPP40/SFRPP20 (Core ayer) Push-pu 3 strokes (ΔP = 120 bar) Conventiona (Without Wedine) A comparison between the experimenta and predicted resuts of wedine strength for conventiona and push-pu injection moded short gass fiber reinforced PC composites are iustrated in Figures 5.21 and The predicted resuts are in satisfactory agreement with the experiments in that the wedine strength of injection moded composites decreases with the increase of the fiber voume fraction and the UTS of wedine-containing parts is very much ower than the vaues obtained for the parts without wedine. Furthermore, it is evident that the wedine strength increases with the increase of hoding pressure difference (P) and the increasing of number of strokes does not have any significant infuence on the UTS. However, it shoud be noted that the predicted UTS resuts are sti higher than the experimenta ones. The reasons for this are twofod; firsty, the parameters used in the cacuation ( σ and τ ) are given by various independent methods from iterature [76, 79]. f The conditions of the tests may be different, which woud ead to an error in the cacuation. It is observed in Equations 2.2 to 2.5 that a the orientation measures are independent of the fiber strength. Ony the fiber ength efficiency factor ( f ) depends on the fiber strength,

100 86 Experimenta Resuts and Discussion which is the vaue known with the east degree of accuracy. Therefore, if σ f is not known with sufficient accuracy, the absoute vaue of f may be inaccurate. Secondy, the errors may arise due to the consideration of the uniform fiber aignment, faw-free moding, and both the ongitudina and transverse ayers experience the same strain, whereas these assumptions are difficut to obtain in thermopastic composites. 140 Experimenta Resuts for SFRPC20/SFRPC/ Maximum Tensie Stress (MPa) Theoreticay Cacuated Resuts for SFRPC20/SFRPC Maximum Tensie Stress (MPa) 0 0 Wedine PPP 1 stroke (ΔP=70 bar) PPP 1 stroke (ΔP=120 bar) PPP 1 stroke (ΔP=220 bar) PPP 2 strokes (ΔP=120 bar) PPP 3 strokes (ΔP=120 bar) Without Wedine Figure 5.21 Comparison of experimenta and theoreticay cacuated wedine strength for conventiona and push-pu injection modings with 20 %wt short gass fiber reinforced poycarbonate.

101 Experimenta Resuts and Discussion Experimenta Resuts for SFRPC35/SFRPC Theoreticay Cacuated Resuts for SFRPC35/SFRPC Maximum Tensie Stress (MPa) Maximum Tensie Stress (MPa) 0 0 Wedine PPP 1 stroke (ΔP=70 bar) PPP 1 stroke (ΔP=120 bar) PPP 1 stroke (ΔP=220 bar) PPP 2 strokes (ΔP=120 bar) PPP 3 strokes (ΔP=120 bar) Without Wedine Figure 5.22 Comparison of experimenta and theoreticay cacuated wedine strength for conventiona and push-pu injection modings with 35 %wt short gass fiber reinforced poycarbonate.

102 88 Experimenta Resuts and Discussion

103 6. Comparison between Simuation and Experiment 6.1 Sandwich injection moding Effect of skin/core voume fraction on the skin/core materia distribution In sandwich injection moding, one of the major tasks is to find out the proper ratio between the skin and the core materias which is needed to obtain an optimum skin/core sandwich structure in the moded part. In this study, the virgin poystyrene (PS) was empoyed in order to investigate the effect of processing parameters on the skin/core materia distribution. In the case of a simpe mod geometry (dumbbe shaped parts), three voume fractions of the core materia (in terms of cavity voume percentage, vo.%), ranging from 55 to 65 vo.% were injected with the met temperature of 230 C. The mod temperature was set at 40 C and the skin and core injection fow rates were kept at 18.5 and 27.0 ccm/s, respectivey. Figure 6.1 shows the experimenta and numerica resuts of the effect of varying core voume fractions on the skin/core materia distribution. It is found that the core voume fraction of 60 vo.% produces sandwich injection moded parts without any defect, whereas the owest core voume fraction (55 vo.%) shows a arge amount of skin materia at the far end of the cavity due to ess penetration of the core met aong the fow direction. On the contrary, increasing the core voume fraction from 60 to 65 vo.%, resuts in a breakthrough phenomenon because the amount of skin materia is too ow and the core met can easiy catch up with the fow front of the skin materia, which generates a defective part and eads to the moding being discarded.

104 90 Comparison between Simuation and Experiment 55 vo.% of Core Materia 60 vo.% of Core Materia 65 vo.% of Core Materia (a) Experimenta Resuts 55 vo.% of Core Materia Breakthrough of core materia at the end of the bar Distribution of poymer A and B Poymer A 60 vo.% of Core Materia 65 vo.% of Core Materia Thickness fraction, poymer B Poymer B (b) Simuation Resuts Figure 6.1 Infuence of core voume fraction on the materia distribution at the end of fiing process: (a) Experiment and (b) Simuation. For purpose of comparison, a heat transfer coefficient () of 1,200 W/m 2 -K [147] is used in this cacuation and the wa thickness of the mode is divided into 20 ayers. It can be seen from Figure 6.1b that the resembance between numerica simuation and experiment is strikingy good. Moreover, the prediction by simuation program is aso in accordance with the measured data obtained from the Image-Pro Pus Anaysis software, as shown in Figure 6.2, which represents the thickness fraction of the core materia at various positions aong the bar. However, it shoud be noted that the vaues for the thickness fraction of the core materia are sighty different in the simuation and the experimenta resuts. The measured vaues are higher than the predicted vaues. Foowing are some possibe reasons for these discrepancies. Firsty, the therma conductivity of poymer (k) used in the cacuations is assumed to be constant [44]. However, it was found that this property varied consideraby with temperature

105 Comparison between Simuation and Experiment 91 [148]. Secondy, the heat transfer coefficient between the poymer and the mod meta is more significant for thin wa modings. In this study, the cacuated vaues of thickness fraction were sti ower than the measured ones, athough the suggested vaue of 1,200 W/m 2 -K was empoyed in the simuation [147]. According to a recent measurement of the heat transfer coefficient [149], it was found that this vaue is approximatey 550 W/m 2 -K. Thirdy, the errors may arise due to the use of dimensiona anaysis to simpify the governing equations, which assumes the cavity to be thin and fat in that the ratio of cavity thickness to cavity ength is much ower than unity ( δ = H L 1) [44]. Therefore, errors can occur in the region containing out-of-pane fow, such as thick sections of the part, which cannot be accuratey modeed by 2.5-D anaysis. Finay, the errors may arise due to the assumption of a steady state and incompressibiity of the fuid in the cacuations, whereas these are difficut to achieve during the processing of the poymeric materias. Thickness Fraction of Core Materia (δ / b) Simuation Resuts 55 vo.% of Core Materia 60 vo.% of Core Materia 65 vo.% of Core Materia Breakthrough of Core Materia Experimenta Resuts 55 vo.% of Core Materia 60 vo.% of Core Materia 65 vo.% of Core Materia (a) (b) Reative Cavity Length (x i / L 0 ) Figure 6.2 Effect of core voume fraction on the thickness fraction of core materia at various positions of specimen: (a) Experiment and (b) Simuation.

106 92 Comparison between Simuation and Experiment Effect of processing parameters on the skin/core materia distribution Effect of skin and core met temperatures The effect of skin and core met temperature on the thickness fraction of the core materia is shown in Figures 6.3 and 6.4. The injection fow rates of skin and core met were set at 18.5 and 27.0 ccm/s, respectivey, whie the injection voumes of skin and core poymers were kept at 40 and 60 vo.%, respectivey. Experimenta resuts are in good agreement with the numerica simuation resuts. In Figures 6.3a and 6.4a, ony the skin met temperature is varied and the core met temperature is kept unchanged. It can be seen that an increase in the skin met temperature induces a decrease in skin thickness, which resuts in a thicker core ayer near the gate region and a arger amount of skin poymer in areas that are remote from the gate. In turn, for the ower skin met temperature, the thicker skin ayer is formed near the gate area. This can aso ead to an increase in the penetration ength of the core met at the far end of the cavity. Experimenta resuts Simuation resuts Skin / Core = 210 o C / 230 o C Skin / Core = 210 o C / 230 o C Skin / Core = 230 o C / 230 o C Skin / Core = 230 o C / 230 o C Skin / Core = 260 o C / 230 o C Skin / Core = 260 o C / 230 o C (a) Thickness fraction, poymer B Skin / Core = 230 o C / 210 o C Skin / Core = 230 o C / 210 o C Skin / Core = 230 o C / 230 o C Skin / Core = 230 o C / 230 o C Skin / Core = 230 o C / 260 o C Skin / Core = 230 o C / 260 o C (b) Figure 6.3 Infuence of skin/core met temperature on materia distribution at the end of fiing process: (a) Effect of skin met temperature and (b) Effect of core met temperature.

107 Comparison between Simuation and Experiment 93 Thickness Fraction of Core Materia (δ / b) Experimenta Resuts Skin Temp./Core Temp. = 210 / 230 o C Skin Temp./Core Temp. = 230 / 230 o C Skin Temp./Core Temp. = 260 / 230 o C Simuation Resuts Skin Temp./Core Temp. = 210 / 230 o C Skin Temp./Core Temp. = 230 / 230 o C Skin Temp./Core Temp. = 260 / 230 o C Reative Cavity Length (x i / L 0 ) (a) Thickness Fraction of Core Materia (δ / b) Experimenta Resuts Skin Temp./Core Temp. = 230 / 210 o C Skin Temp./Core Temp. = 230 / 230 o C Skin Temp./Core Temp. = 230 / 260 o C Simuation Resuts Skin Temp./Core Temp. =230 / 210 o C Skin Temp./Core Temp. =230 / 230 o C Skin Temp./Core Temp. =230 / 260 o C Reative Cavity Length (x i / L 0 ) (b) Figure 6.4 Comparison of predicted and experimenta resuts: (a) Effect of skin met temperature and (b) Effect of core met temperature on core thickness fraction of moded part at various positions of tensie specimens.

108 94 Comparison between Simuation and Experiment The effect of the core met temperature on materia distribution is shown in Figures 6.3b and 6.4b. The resuts show that the effect of the core met temperature on materia distribution is more pronounced than that of the skin met temperature. The core ayer near the gate region is thicker when the core met temperature is ower. In addition, when the core materia is ess viscous, the core met front advancement increases substantiay. This can not ony be expained by the higher core met temperature and ower core met viscosity, but aso by the oca rate of cooing at the mod center which is reativey sow when compared to that near the mod wa, this can be expained by the ow therma conductivity of the poymer met. Both of the effects just mentioned can ead to the fow front of the core materia stretching easiy which resuts in a reduction of the thickness of the core near the gate, whie the core thickness away from the gate is increased Effect of skin and core injection fow rates Figures 6.5 and 6.6 show the effect of skin and core injection fow rate on the thickness fraction of the core poymer at the end of the fiing process. Both the skin and core met temperatures were set at 230 C and the injection voume of skin and core poymers were 40 and 60 vo. %, respectivey. It was found that the effect of the skin injection fow rate does not ead to significant changes in the skin/core configuration of moded parts, as presented in Figures 6.5a and 6.6a. In a the cases, ony insignificant differences with regard to the thickness fraction vaues for the core materia can be noted between the measured positions. Probaby this can be attributed to the higher cooing rate at the mod wa, which has a greater effect on the skin met viscosity than the rise in temperature caused by shear heating. From the simuation resuts, it can aso be seen that the higher the skin injection fow rate, the ower the thickness fraction of the skin ayer near the gate. This is associated with the shear thinning behavior of the poymer met.

109 Comparison between Simuation and Experiment 95 Experimenta resuts Simuation resuts Skin / Core = 9.25 / 27.0 ccm/s Skin / Core = 9.25 / 27.0 ccm/s Skin / Core = 18.5 / 27.0 ccm/s Skin / Core = 18.5 / 27.0 ccm/s Skin / Core = 37.0 / 27.0 ccm/s Skin / Core = 37.0 / 27.0 ccm/s (a) Thickness fraction, poymer B Skin / Core = 18.5 / 54.0 ccm/s Skin / Core = 18.5 / 54.0 ccm/s Skin / Core = 18.5 / 27.0 ccm/s Skin / Core = 18.5 / 27.0 ccm/s Skin / Core = 18.5 / 13.5 ccm/s Skin / Core = 18.5 / 13.5 ccm/s Skin / Core = 18.5 / 6.5 ccm/s Skin / Core = 18.5 / 6.5 ccm/s Core injection fow rate 54.0 ccm/s 27.0 ccm/s 13.5 ccm/s 6.5 ccm/s Distribution of poymer A and B Poymer A Breakthrough of core materia at the end of the bar (b) Poymer B Figure 6.5 Infuence of skin/core injection fow rate on materia distribution at the end of fiing process: (a) Effect of skin injection fow rate and (b) Effect of core injection fow rate.

110 96 Comparison between Simuation and Experiment Thickness Fraction of Core Materia (δ / b) Thickness Fraction of Core Materia (δ / b) Experimenta Resuts Skin / Core Injection Fow Rate = 37.0 / 27.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 27.0 ccm/s Skin / Core Injection Fow Rate = 9.25 / 27.0 ccm/s Simuation Resuts Skin / Core Injection Fow Rate = 37.0 / 27.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 27.0 ccm/s Skin / Core Injection Fow Rate = 9.25 / 27.0 ccm/s Reative Cavity Length (x i / L 0 ) Experimenta Resuts Skin / Core Injection Fow Rate = 18.5 / 54.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 27.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 13.5 ccm/s Skin / Core Injection Fow Rate = 18.5 / 6.5 ccm/s Simuation Resuts Skin / Core Injection Fow Rate = 18.5 / 54.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 27.0 ccm/s Skin / Core Injection Fow Rate = 18.5 / 13.5 ccm/s Skin / Core Injection Fow Rate = 18.5 / 6.5 ccm/s Reative Cavity Length (x i / L 0 ) (a) (b) Figure 6.6 Comparison of predicted and experimenta resuts: (a) Effect of skin injection fow rate and (b) Effect of core injection fow rate on core thickness fraction of moded part at various positions of tensie specimens.

111 Comparison between Simuation and Experiment 97 A variation of the core injection fow rate has a more significant effect on the materia distribution than a change in the skin injection fow rate, which is evident from Figures 6.5b and 6.6b. A decrease in the core injection fow rate eads to a significant reduction of the core thickness fraction near the gate region, whie the penetration ength of the core poymer increases substantiay and breakthrough occurs at the far end of the cavity. The reason for this phenomenon can be reated to the sower core injection fow rate, eading to the skin poymer gaining more time to soidify against the mod wa, which resuts in a thicker soidified skin ayer cose to the gate (see Figure 6.7). This can ead to an increase in the core met front advancement. In the case of breakthrough, the skin materia does not reach the far end of mod. Consequenty, the core met front catches up with the skin met front and ends up at the far end of the cavity. In contrast, the higher core injection fow rate resuts in a higher thickness fraction of the core materia near the gate eaving a arge amount of skin poymer at the end of the cavity. This can be expained by the fact that the faster the core injection fow rate, the higher the shear rate near the mod wa; hence the shear thinning behavior is more pronounced for the skin poymer. Gate Thicker soidified skin materia Skin / Core injection fow rate = 18.5 / 6.5 ccm/s Gate Thinner soidified skin materia Skin / Core injection fow rate = 18.5 / 54.0 ccm/s Figure 6.7 Effect of core injection fow rate on thickness distribution of soidified skin materia.

112 98 Comparison between Simuation and Experiment Effect of mod temperature The effect of the mod temperature variation on the skin/core materia distribution is shown in Figure 6.8. In this study the unfied PC was used for both the skin and core materias, which were injected with the same met temperature of 300 C. The injection voume of core materia was kept at 50 vo.% and the injection fow rate of skin and core met were maintained at constant eves of and ccm/s, respectivey. Both of experimenta and predicted resuts indicate that increasing mod temperature induces an increase in the core thickness. This is due to the fact that higher mod temperature aows sower cooing of the poymer met, resuting in a thinner frozen ayer of the skin materia, i.e. higher thickness fraction of core materia. On the other hand, at the ower mod temperature, the thicker skin ayer is formed near the gate region. This can aso ead to an increase in the penetration ength of the core met front at the end of the cavity. Thickness Fraction of Core Materia (δ / b) Experienta Resuts Mod Temperature = 40 o C Mod Temperature = 80 o C Mod Temperature = 120 o C Simuation Resuts Mod Temperature = 40 o C Mod Temperature = 80 o C Mod Temperature = 120 o C Reative Cavity Length (x i / L 0 ) (a) (b) Figure 6.8 Effect of mod temperature on the thickness fraction of core materia at various positions of specimen: (a) Experiment and (b) Simuation.

113 Comparison between Simuation and Experiment Effect of gass fiber content on the skin/core materia distribution Figure 6.9 shows the effect of the gass fiber content within the skin materia on the thickness fraction of core materia. In this investigation, the unfied PC, PC fied with 20 and 35 wt% were injected with the skin fow rate of ccm/s and ccm/s for the core materia. The mod and met temperature were set at 300 C and 80 C, respectivey. It can be seen that the higher the gass fiber content in the skin materia, the thicker the frozen ayer of skin materia is formed. This can aso be attributed to the heat transfer characteristic of moten poymer, as mentioned in section 5.2.2, where the therma conductivity of the PC fied with 35 wt% short-gass-fibers was higher than that of unfied PC. Thus, the viscosity of PC fied with 35 wt% short gass fibers increases more rapidy and soidifies sooner than for the unfied PC, resuting in the thicker soidified skin ayer. Thickness Fraction of Core Materia (δ / b) Experimenta Resuts PC/PC SFRPC20/PC SFRPC35/PC Simuation resuts PC/PC SFRPC20/PC SFRPC35/PC Reative Cavity Length (x i / L 0 ) (a) (b) Figure 6.9 Effect of gass fiber content within the skin materia on core thickness fraction of moded part at various positions of tensie specimens: (a) Experiment and (b) Simuation.

114 100 Comparison between Simuation and Experiment The effect of the gass fiber content within the core materia on the thickness fraction of core materia can be observed further in Figure The resuts indicate that the thickness fraction of core materia increases with increasing the gass fiber content. This is associated with the higher the gass fiber content added into the poymer met, the higher the viscosity of the fowing poymer met [78, 144]. The viscosity versus shear rate curve for three materias (PC, SFRPC20, and SFRPC35) is shown in Figure 2.1. Thickness Fraction of Core Materia (δ / b) Experimenta Resuts PC/PC PC/SFRPC20 PC/SFRPC35 Simuation Resuts PC/PC PC/SFRPC20 PC/SFRPC Reative Cavity Length (x i / L 0 ) (a) (b) Figure 6.10 Effect of gass fiber content within the core materia on core thickness fraction of moded part at various positions of tensie specimens: (a) Experiment and (b) Simuation.

115 Comparison between Simuation and Experiment Case study This experimenta study deas with the sandwich injection of a housing part as iustrated in Figure In this case, the sandwich moding technoogy aows to combine a conductive fied skin materia with a cheaper unfied or recyced core materia based on the same materia for good skin-core adhesion. The industria goa is to fi a cheaper or recyced poymer in the core as much as possibe and without breakthrough phenomenon. In this study, the sequentia injection was performed in the foowing steps, as described in section 1.2.1, a given percentage of the skin materia (PS, transparent) is first injected into the cavity, foowed by the injection of the core materia (PS, bue coor) and finay a much smaer amount of the skin materia is injected to sea the gate. The materia and processing parameters used for the computer prediction are given in Tabe 6.1 and the amounts of injected materia (in term of cavity voume percentage) are varied according to Tabe 6.2. Mesh thickness (mm) Figure 6.11 Housing and 2.5-D meshed mode used in this study.

116 102 Comparison between Simuation and Experiment Tabe 6.1 Materia and processing conditions. Materia Grade Suppier Poystyrene PS 165 H BASF Processing Conditions 1 st -Pasticator 2 nd -Pasticator (Skin Materia) (Core Materia) Met temperature ( o C ) Injection fow rate (ccm/s) Injection pressure (bar) Hoding pressure (bar) _ 800 Hoding time (sec.) _ 25 Mod temperature = 40 o C Tota cooing time = 40 sec. Sandwich Moding Tabe 6.2 Amount of injected materia. Injection Voume (%by voume) Run # 1 st -Pasticator 2 nd -Pasticator 1 st -Pasticator (Skin Materia) (Core Materia) (Skin Materia) 1 50% 45% 5% 2 55% 40.5% 4.5% * Tota voume of part = 30 ccm A comparison between the experimenta and simuation resuts for the housing part is iustrated in Figures 6.12 to In Figure 6.12b the simuation resut of the core materia distribution is given in terms of core thickness fraction. The bue area designates the area which is ony occupied by the skin materia, whie the yeow-green areas indicate the areas occupied by both the skin and core materias. The simuation resuts of skin and core materia distribution for different injection voume percentage are given in Figure In the case of Run # 1 (A-B-A = % by voume), the bue areas are the breakthrough areas as predicted by the numerica simuation where the core component has broken through the skin component and can be seen at the part surface, which woud ead to discarding of moded part (see Figure 6.13a). On the other hand, by sighty changing the content of skin and core materias (Run # 2, A-B-A = % by voume), it can be seen that the entire surface is occupied by skin materia (see Figure 6.13b). Finay, at the end of fiing, one observes a very good agreement between prediction and experiment.

117 Comparison between Simuation and Experiment 103 Skin materia (a) (b) Skin materia Thickness fraction, poymer B Figure 6.12 Sandwich injection of a housing part: (a) Experimenta and (b) Simuation resuts. Distribution of poymer A and B The breakthrough of core materia Poymer A Poymer B Figure 6.13 (a) Materia distribution of skin and core poymers at the end of fiing: Injection voume of A-B-A = % (Run # 1).

118 104 Comparison between Simuation and Experiment Distribution of poymer A and B Poymer A No breakthrough Poymer B Figure 6.13 (b) Materia distribution of skin and core poymers at the end of fiing: Injection voume of A-B-A = % (Run # 2). 6.2 Simuation of fiber orientation in sandwich injection moding Comparison between 2.5-D and 3-D simuation of fiber orientation and measurement In comparing the 2.5-D simuation resuts for the fiber orientation tensor ( a 11 ) across the haf thickness for singe and sandwich injection modings, as depicted in Figure 6.14, there is no significant discrepancy in both cases. Furthermore, as can be seen from Figure 6.15 the predicted vaues of a 11 obtained in 2.5-D mode are not in accordance with the measurements. The simuation resuts show the higher vaue of a 11 within the core region compared to the skin region, whist the measured resuts show the ower vaue of a 11 within the core ayer. This may be probaby caused by some factors. Firsty, it can be associated with the effect of geometry such a camping part of tensie specimen where the converging fow fied is estabished. This can ead to a higher veocity gradient of the fowing met aong the fow path, and thus resuting in an increase of the fiber orientation within the core ayer, as mentioned in section Secondy, the errors may arise due to the use of dimensiona anaysis to simpify the governing equations, which omits the cacuation of veocity

119 Comparison between Simuation and Experiment 105 component and therma convection in the gap wise direction [44-45]. Finay, the Hee-Shaw fow formuation aso negects the transverse fow at the met front region (the fountain fow behavior) which has a significant effect on the evoution of fiber orientation during injection mod fiing [150]. Fow direction Converging fow Fiber orientation tensor Reative thickness = 0 (Surface) Reative thickness = 0 (Surface) Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = Reative thickness = (Midpane) Reative thickness = (Midpane) Singe Moding Sandwich Moding Figure D simuation resuts for the orientation tensor ( a 11 ) component at different ayers over the haf thickness of singe and sandwich injection moded parts.

120 106 Comparison between Simuation and Experiment Orientation Tensor Component (a 11 ) Surface 2.5-D Simuation Resuts Singe Moding Sandwich Moding Reative Thickness (z i / h) Experimenta Resuts SFRPP20 SFRPP20/SFRPP Midpane Orientation Tensor Component (a 11 ) Figure 6.15 Comparison of the 2.5-D mode prediction and measurement of fiber orientation tensor ( a 11 ) across the haf thickness at the midde position of singe and sandwich injection modings (tensie specimen) for PP fied with 20 wt% of short gass fiber. The 2.5-D mode of the rectanguar bar (without camping part) was used in order to negect the effect of converging channe. It can be seen from Figure 6.16 that the fiber orientation predictions of the rectanguar bar give a better quaitative agreement with the experimenta measurements for the orientation profies as compared to that of the tensie geometry, in which the vaues of a 11 become ower in the core region. However, there is sti no significant difference in the predicted vaue of a 11 between singe and sandwich modings and the predicted vaues of a 11 within the skin ayer obtained from 2.5-D mode are sti underestimated.

121 Comparison between Simuation and Experiment Orientation Tensor Component (a 11 ) Surface 2.5-D Simuation Resuts Singe Moding Sandwich Moding Reative Thickness (z i / h) Experimenta Resuts SFRPP20 SFRPP20/SFRPP Midpane Orientation Tensor Component (a 11 ) Figure 6.16 Comparison of the 2.5-D mode prediction and measurement of fiber orientation tensor ( a 11 ) across the haf thickness at the midde position of singe and sandwich injection modings (rectanguar specimen) for PP fied with 20 wt% of short gass fiber. Figure 6.17 represents the 3-D simuation resut of fiber orientation tensor ( a 11 ) at the end of fiing for singe injection moding process (injection fow rate of 18.5 ccm/s). It can be seen that the predicted vaues of a 11 agree reasonaby we with the measurements. The simuation resuts show the higher degree of fiber orientation in the skin ayer as compared to the core ayer, where the fibers are oriented randomy to the fow direction ( a ). This woud be associated with the effect of geometry i.e. a camping part of tensie specimen, as stated earier. It is aso indicated in Figure 6.18a that the cacuated vaues of a 11 within the core region of SFRPP20 are higher than that of SFRPP40. This is due to an increase in the viscosity of the poymer met, which has more fiber content [78, 144]. The higher the viscosity of poymer mets the narrower the shear zone during the fiing phase, and thus resuting in a ower degree of fiber orientation in the core region. Furthermore, from the 3-D simuation resuts as iustrated in Figures 6.18b to 6.20, it is interesting to note that the

122 108 Comparison between Simuation and Experiment sandwich injection modings show higher vaues of a 11 within the core region as compared to the singe injection modings. The change in the a 11 vaues is thought to be caused by the thicker soidified skin ayer which deveops during the fow of sandwich injection moded short fiber composites, as mentioned in section Therefore, the veocity profie of the foowing core met can be sharper and can cause the fibers in the core ayer to be more aigned in the fow direction. Fiber orientation tensor (a 11 ) Figure D simuation resut of fiber orientation tensor ( a 11 ) for singe injection moded part.

123 Comparison between Simuation and Experiment Orientation Tensor Component (a 11 ) Singe Injection Modings Experimenta Resuts 3-D Simuation Resuts 0.5 SFRPP20 SFRPP20 SFRPP40 SFRPP Orientation Tensor Component (a 11 ) (a) 1.0 Surface Reative Thickness (z i / h) Midpane 1.0 Orientation Tensor Component (a 11 ) Experimenta Resuts 3-D Simuation Resuts 0.5 SFRPP40 Singe moding SFRPP40/SFRPP40 Sandwich moding Surface Reative Thickness (z i / h) Midpane Orientation Tensor Component (a 11 ) (b) Figure 6.18 Comparison of the 3-D mode prediction and measurement of a 11 across the haf thickness at the midde position: (a) singe injection moded specimens for PP fied with 20 wt% and 40 wt% of short gass fiber (b) singe and sandwich injection moded specimens for PP fied with 40 wt% of short gass fiber.

124 110 Comparison between Simuation and Experiment Singe Injection Moding Sandwich Injection Moding Figure 6.19 Comparison of the orientation tensor ( a 11 ) component across the thickness for singe and sandwich injection modings. Fow direction Singe Injection Moding Fow direction Sandwich Injection Moding Figure 6.20 Comparison of the orientation tensor ( a 11 ) component at the mid-pane ayer (z = 2.0 mm) for singe and sandwich injection modings.

125 Comparison between Simuation and Experiment Simuation of fiber orientation in push-pu injection moding Effect of hoding pressure difference and the number of push-pu strokes on 3-D fiber orientation in wedine areas Figures 6.21 and 6.22 represent the 3-D simuation resuts of fiber orientation tensor ( a 11 ) for conventiona and push-pu injection moding processes. In the case of conventiona injection moding with wedine, the predicted resuts indicate that the fibers near the part surface are randomy oriented. Near the midpane of the part (core ayer), the fibers are mainy aigned perpendicuar to the fow direction, which is caused by the fountain fow effect at the met front [ ]. For the push-pu injection modings, the simuation resuts show that not ony the degree of fiber orientation increases with increasing hoding pressure differences, but aso an increase in the number of strokes does not produce any major changes in fiber orientation within the wedine area compared to push-pu 1 stroke. These findings are aso in good agreement with previous experimenta work concerning the fiber orientation distribution within the wedine areas of push-pu processed parts. In addition, it can be seen from Figure 6.22 that the predicted vaues of orientation tensor components ( a 11 ) agree reasonaby we with corresponding experimenta measurements. The predicted vaues of a 11 across the part thickness show the same tendency as the measured ones, athough there is sti a sight discrepancy in both cases. One possibe factor that may cause the differences between cacuation and measurement is the fiber-fiber interaction coefficient (C I ). According to previous findings [ , 122] concerning the comparison between the numerica and experimenta fiber orientation in injection moded part, it has been found that the fiber orientation predictions are quite sensitive to this coefficient especiay near the surfaces of the part. They aso suggested that C I = 0.01 gives a good agreement for the skin ayer, whereas C I = is a better choice for the core ayer.

126 Comparison between Simuation and Experiment 112 Wedine Position Fow Direction Fow Direction 1st PushPush-Pu 1 stroke 1st 2nd PushPush-Pu 2 strokes 1st 2nd 3rd PushPush-Pu 3 strokes (a) Z Wedine Z Y X X 1st sh Pu PushPush-Pu 1 stroke 1 Pu Pu 2nd 1st ke ro st es 3 k ro st u -P sh 2 Pu u -P sh PushPush-Pu 2 strokes 1st k ro st 3rd 2nd PushPush-Pu 3 strokes es (b) Figure D Simuation resuts of fiber orientation tensor ( a11 ): (a) at midpane ayer and (b) across the part thickness of conventiona and push-pu injection modings, for PC with 35 wt% short gass fibers.

127 Comparison between Simuation and Experiment Orientation Tensor Component (a 11 ) Simuation Resuts Wedine Push-Pu 1 stroke (ΔP = 70 bar) Push-Pu 1 stroke (ΔP = 120 bar) Push-Pu 1 stroke (ΔP = 220 bar) Push-Pu 2 strokes (ΔP = 120 bar) Push-Pu 3 strokes (ΔP = 120 bar) Surface Reative Thickness (z i / h) Midpane Orientation Tensor Component (a 11 ) Orientation Tensor Component (a 11 ) Experimenta Resuts 0.3 Wedine 0.2 Push-Pu 1 stroke (ΔP = 70 bar) Push-Pu 1 stroke (ΔP = 120 bar) 0.1 Push-Pu 1 stroke (ΔP = 220 bar) Push-Pu 2 strokes (ΔP = 120 bar) Push-Pu 3 strokes (ΔP = 120 bar) Orientation Tensor Component (a 11 ) Surface Reative Thickness (z i / h) Midpane Figure 6.22 Predicted and measured vaues of a 11 orientation tensor component across the haf thickness at the wedine position of conventiona and push-pu injection modings, for PC with 35 wt% short gass fibers.

128 114 Comparison between Simuation and Experiment

129 7. Concusions The present study was concentrated on investigating the capabiities of the sandwich and push-pu injection moding processes to enhance the orientation of fibers and its effect on the attrition of fiber ength since these are critica to the mechanica performance of short fiber composites. The accuracy of the mode prediction was verified by comparing with the corresponding experimenta measurements. Resuts and concusions obtained during this study can be summarized as: A sandwich injection moding technique was empoyed to improve the mechanica properties of short gass fiber reinforced thermopastics (SFRTs) with respect to the fiber orientation and fiber attrition within the skin and core ayers. The resuts confirmed the expected rise in the maximum tensie stress and impact strength as the concentration of the short gass fibers was increased. The mechanica properties of sandwich injection modings were observed to be higher than those of singe injection modings, which can be attributed to the higher fiber orientation within the core ayer. The resuts obtained by anayzing the fiber attrition inside the skin and core regions in the ongitudina direction of tensie specimens showed that the degree of fiber degradation inside the skin ayers was higher than in the core ayers. There were ony minor differences in the skin region fiber ength observed between sandwich and singe injection moding processes, this effect was more pronounced in the core region and for the higher fiber concentration. A theoretica mode derived by an anaytica method of modified rue of mixtures (MROM) as a function of the area fraction between skin and core ayers has been introduced to predict the utimate tensie strength (UTS) of conventiona, sandwich and push-pu injection moded composites. The effects of fiber ength, fiber orientation and fiber content on the tensie strength of SFRTs were studied in detai. The present mode was verified to

130 116 Concusions existing experimenta resuts, and for a cases, the predictions showed satisfying agreement. The differences between cacuated and experimenta data may resut from some parameters and assumptions made in the derivation of the equations, which woud ead to errors in the cacuations. A push-pu injection moding technique was empoyed to enhance the wedine strength of short gass fiber reinforced poycarbonate with respect to the fiber orientation and the fiber ength distribution in the wedine areas. The effects of processing parameters incuding the number of push-pu strokes, hoding pressure differences between both of the injection units and the effect of gass fiber concentration have been studied. It was found that the wedine strength of the push-pu 1 stroke processed parts increases with increasing penetration ength of wedine. An increase of the number of strokes did not produce any major changes in the wedine strength compared to that of push-pu 1 stroke processed part. The fiber attrition within the wedine area was not significanty affected by the hoding pressure difference and the number of push-pu strokes. An expected noninear reationship between the hoding pressure difference and the penetration ength of wedine was aso observed, which can be associated with the rheoogica behavior and heat transfer characteristics of poymer met in the cavity. The effects of processing parameters and gass fiber concentration on the skin/core materia distribution in sandwich injection moded parts were investigated and extensivey verified against the predicted resuts performed by the commercia simuation package (Modfow). Both the simuated and the experimenta resuts indicated that in order to obtain an optimum encapsuated skin/core structure in the sandwich injection moded part, it is necessary to seect proper core voume fraction and processing parameters. The resuts suggest that the most important processing parameter for controing the breakthrough phenomenon was the core injection fow rate, whie the skin injection fow rate did not have any significant infuence on the thickness fraction of the core materia. The thickness fraction of core materia increases with either an increase in mod temperature and skin met temperature or a decrease in core met temperature. An increase in soidified thickness of skin materia or an increase in thickness fraction of core materia, which has higher gass fiber content can be associated with the heat transfer characteristic and the viscosity of moten poymer. If a higher proportion of core materia is needed for industria purposes, either the skin met temperature or the core injection fow rate has to be increased, or the core met

131 Concusions 117 temperature has to be decreased. A good agreement between simuation and experimenta resuts indicate that the simuation program can be used as a vauabe too for the prediction of met fow behavior during sandwich injection process. Structure of fiber orientation in sandwich and push-pu injection moded short fiber composites have been predicted by the 2.5-D and 3-D numerica anayses. The predictions sove the fu baance equations of mass, momentum, and energy for a generaized Newtonian fuid. The second-order orientation tensor approach was used to describe and cacuate the oca fiber orientation state. Changing the numerica vaues of orientation tensor ( a 11 ) ceary shows the difference in the capabiities of simuation mode. The predicted resuts obtained from the 2.5-D was found to be ess accurate than that of 3-D mode. This is due to the use of dimensiona anaysis to simpify the governing equations, which omits the cacuation of veocity component and therma convection in the gap wise direction. In addition, the Hee- Shaw fow formuation aso negects the transverse fow at the met front region (the fountain fow behavior) which has a significant effect on the evoution of fiber orientation during injection mod fiing. For the 3-D mode, the predicted and measured resuts of a 11 were found to be in a good agreement. However, sight discrepancies were observed at the center and cose to the mod wa which may resut from the fiber-fiber interaction coefficient used in the cacuation.

132 118 Concusions

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150

151 Curricuum Vitae Persona data Name: Somjate Patcharaphun Date of birth: 25 December 1972 Birthpace: Bangkok, Thaiand Nationaity: Thai Marita status: Married Education Primary schoo, Santa Cruz Suksa Schoo, Bangkok, Thaiand Secondary schoo, Taweetapisek Schoo, Bangkok, Thaiand Bacheor of Engineering (B.Eng.), Facuty of Engineering, King Mongkut s Institute of Technoogy Thonburi (KMITT), Bangkok, Thaiand Master of Engineering (M.Eng.), Schoo of Energy and Materias, King Mongkut's University of Technoogy Thonburi (KMUTT), Bangkok, Thaiand Professiona experience Project engineer, Isuzu Motor (Thaiand) Co., Ltd Technica consutant and instructor, Itathai Deveopment Co., Ltd Project engineer, Teecom Asia Co., Ltd. since 2000 Lecturer, Department of Materias Engineering Facuty of Engineering, Kasetsart University, Thaiand.