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.