Investigation of process parameters for an Injection molding component for warpage and Shrinkage

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1 Investigation of process parameters for an Injection molding component for warpage and Shrinkage Mohammad Aashiq M 1, Arun A.P 1, Parthiban M 2 1 PGD IN TOOL & DIE DESIGN ENGINEERING-PSG IAS 2 ASST.PROFESSOR DEPARTMENT OF MECHANICAL ENGINEERING-PSG CT Coimbatore , India Abstract The purpose of the research is to explore the influence of different mold temperatures on the warpage & shrinkage of the injection molded component s. The simulation software MOLDEX 3D was used for this study, the simulations were done by varying different mold temperatures and their corresponding warpage & shrinkage were collected. It was found that the different mold wall temperature causes the asymmetrical polymer flow in the cross-section due to which the asymmetrical structure in the parts cross-section occurs and this was observed using the flow analysis software. So it is required to assure homogeneous mold wall temperature across the entire cavity during the production of injection molded parts. This research finally concludes that warpage and shrinkage decreases for increased values of mold temperature. Keywords: Injection molding, Warpage, Shrinkage, Flow Analysis. I. INTRODUCTION Injection molding is a process where by a solid thermoplastic material is heated until it reaches a state of fluidity, is then transferred under pressure(injected) into a closed hollow space (mold cavity),and then cooled in mold till it reaches a solid state, conforming the shape of mold cavity. Assuring the proper mold temperature for a specified polymer is a very important issue as well as keeping the temperature constant and equal across the cavity surface. Differences in mold temperature can lead to the problems with manufactured parts like warpage [1-3]. The reason of this are stresses in the parts.since polymer with higher temperature exhibits more intensive shrinkage than in lower temperature, the temperature differences created during part cooling in the Mold lead to part distortion. The deformation of a simple injection molded part due to less and high temperatures are shown in Fig. 1. Part becomes concave from the Mold hot side and convex from the Mold cold side. The main cause of warpage is the difference in internal stresses produced in the material due to differential shrinkage.it needs to be addressed early on during the molding process as the defects are hardly rectifiable later. Fig. 1.Warpage of injection molded part due to different mold temperature. The non-uniform mold temperature in the mold plate can be also problematic in case of multi cavity Molds. If the parts are formed in different temperature they can differ in structure and properties after manufacturing process [2]. The differences in properties are significant for semi crystalline polymers that crystallize during solidification process. The problem of different properties among the parts from one mold is especially important in case of molding small parts in molds with many cavities. Moreover, the different temperature can also affect polymer flow in runners of multi cavitymold and this can lead to non-simultaneous filling of the mold cavities [2]. Computer simulation programs are very good tools for prediction of quality problems with injection molded parts [2]. It is possible to predict polymer flow inside the cavity as well as other physical properties distribution of melt across the entire cavity like pressure, shear stress, shear rate, temperature etc. Weldlines and air traps are also identified. Minimum warpage optimization can be done by CAE analysis by processing parameters optimization [2]. Gate location analysis also can be done to achieve this. The warpage is also dependent on presence of filler in the plastic as well as on the kind of filler. Particularly it can be different across the part depending of the filler orientation [3]. The warpage is dependent also on processing conditions. Computer simulations are helpful with warpage optimization. ISSN: Page 1237

2 In case of thin-shell parts the melt temperature and holding pressure are important parameters. The holding pressure is especially important with minimizing the shrinkage of semicrystalline polymers, like POM.The shrinkage can be also the reason of warpage. To minimize warpage it is important to assure the uniformity of the temperature across the part. It will also result in preventing sink marks and different shrinkage in the parts [2]. The cooling of the parts in injection molding plays an important role. It is done by cooling channels in the mold. These channels can be designed in many ways in the same Mold but uniform mold temperature should be assured. When there is only one and long cooling channel the coolant heats up in the mold and is not efficient. One of good solutions is to use special manifolds and design many coolant inlets and also many outlets in the mold [3]. Sometimes special injection molding techniques are used to avoid quality problems with parts. For example to avoid sink marks in parts with thick walls polymer with foaming agent is used. It makes a possible manufacturing thick-walled part which is very difficult in conventional injection molding because thick layer of polymer is cooled very slowly and exhibits very high shrinkage. This leads to sink marks occurring and to the warpage problems. Minimizing warpage is also possible when using compression injection molding, which is especially recommended for thin-walled parts and parts with application for optics. The reason of minimal warpage after this manufacturing process are very low internal stresses in injection Molded parts [1]. The reason of warpage is usually unequal Mold temperature or differences in part thickness. In addition, it is amplified by intensive shrinkage of fluid core inside the part. If the fluid core is placed not symmetrically in the middle of the part cross-section, one half of the part contracts more because of intensive shrinkage. If melt polymer flows inside a Mold channel (runner or gate) the flow front is formed in a fountain flow [1]. The flow front is symmetrical to axis of symmetry, placed in the half-distance between Mold walls, if only the conditions of flow are the same near both Mold walls equal temperature, surface roughness etc. Fig. 2. The scheme of melt behaviour for less temperature Fig.2. shows how a flow front of plastic material inside the cavity plate will be when the mold temperature is less(for example :30 C).The flow of plastic material will be different for higher mold temperature.the flow (for example :60 C) is shown in Fig.3. The flow is symmetry since the mold temperature is high. Fig. 3. The scheme of melt behaviour for higher temperature The differences in Mold temperature can be caused by different cooling.the location of cooling channels is often determined by shape of molded part cavity in the Mold. The unequal cooling and flow asymmetry causes further problems with molded parts first of all warpage. The Molded part is warped in the way like shown in Fig. 1. Due to Temperature difference part is contracted. Therefore on this part wall the surface is concaved. The theory presented in some publications explains the reasons of the warpage. It is required to do more research in this field to study about warpage.. This could provide some information helpful in more detailed description of warpage phenomenon. II. PROCEDURE A. Materials In this study, Acrylonitrile butadiene styrene (ABS) was used as a polymer material. The grade of the ABS was LG ABS AF-303 and its Melt Flow Index Value is MFI 50 g/10min at 230ᵒC. B. Injection Molding The basic function of a mold is two fold. Firstly it should impart shape to the part and secondly it should cool the molded part. Double cavity injection Mold is designed for performing the tests. The specimens (bars: 55.5x20x2.95 mm) was used for the investigation. The layout of component is shown in Fig. 5.. The sketch of one of molding plates in the Mold showing the layout of cavities and runners is presented in Fig. 4. The mold base was designed in order to accommodate cooling lines that are running around the component.the mold base dimensions are(244x194mm). The important aspect in injection Molding is cooling the part. In the case of experimental Mold used here the cooling system is very simple one circuit in each Molding plate. The details of cooling system in the Mold are sketched in Fig. 6. The entire Mold consists of several plates (mounting plates, ejector plates etc.) but cooling circuit is made only in two plates (Molding plates) that form the parts. One of these plates is flat the plate is with sprue bushing mounted. In the second one the cavities are milled together with the runners and edge gates, as it is shown in Fig4. ISSN: Page 1238

3 Fig. 6.Scheme of mold cooling Fig. 4. Sketch of the injection Mold used for the Analysis Injection temperature: 230 C Mold temperature: C. Injection velocity: 30 cm/s Injection time:.37 s Holding pressure: 60 MPa Holding time: 5 s Cooling time: 17 s For the Mold temperature values that were kept different for each run in Flow Analysis (30 C, 40 C, 50 C, and 60 C) to cause the stress in the parts and, as the result, deformation of the parts(to analyze warpage). The temperature values were changed to compare the warpage to highly deformed parts and to observe the asymmetry of flow front in the Mold. III. RESULTS AND DISCUSSION. Fig. 5. Sketch of the injection mold component C. Processing Parameters The polymer used for investigation was LG ABS AF- 303.This material is used to manufacture parts like caps, blocks and automotive components. Even though ABS plastics are used largely for mechanical purposes, they also have electrical properties that are fairly constant over a wide range of applications. The temperature values were changed to compare the warpage to highly deformed parts and to observe the asymmetry of flow front in the Mold. The Mold temperature values were kept different for each run in Flow Analysis (30 C,40 C,50 C,60 C) to analyse the warpage.all other parameters were kept constant for all the analysis. The difference in value of warpage & shrinkage for different mold temperature values were observed from the simulated results. The parts deformation were showed in the following (Fig.7.a,7.b,7.c,7.d.). A. Observation of parts deformation The parts after processing with different Mold temperatures at 30 C, 40 C, 50 C, and 60 C are deformed. The variation in warpage is noted along X-displacement, Y-displacement and Total displacement. In practice the displacement along X and Y can only be calculated using the Co-ordinate Measuring Machine.So the Total displacement values are noted.the shrinkage variations are also noted down. ISSN: Page 1239

4 Fig. 7b.Warpage Displacement of Component at 40 C Fig. 7a.Warpage Displacement of Component at 30 C Fig. 7c.Warpage Displacement of Component at 50 C ISSN: Page 1240

5 Fig. 7d.Warpage Displacement of Component at 60 C The deformation measured with the results obtained from Flow Analysis run clearly shows that warpage is caused due to Mold temperature differences. The optimum Mold temperature helps to avoid the warpage in the plastic component. Here the optimum mold temperature is considered as 60 C. TABLE I EFFECTS OF PROCESSING PARAMETERS Mold Cooling time Shrinkage (%) Warpage (mm) Fig.9. Measured deformation [mm] in the parts length Temperature( C) (sec) IV. CONCLUSION If the Mold temperature is different, this leads to asymmetry of melt flow. This, in turn, causes the asymmetrical structure development in the part crosssection. This study concludes that the warpage and shrinkage value decreases for higher mold temperature In further research the effect of variation of mold temperature on the warpage and shrinkage obtained using the simulated results will be verified by experimental work. REFERENCES Fig. 8. Measured shrinkage [%] in total volume [1] J.P. Beaumont, R. Nagel, R. Sherman, 2002, Successful Injection Molding, Hanser [2] J.P Beaumont., 2004 Runner and Gating Design Handbook. Tools for Successful Injection Molding, Hanser, Munich, inatti.smorawiski,1989 InjectionMolding technology, WNT, Warsaw,,460 (in Polish). [3] T.A. Osswald, L-S.Turng, P.J. Gramann, 2001 Injection Molding Handbook, Hanser Publishers, Munich. [4] Practical Guide to Injection Molding, 2004 Edited by Vanessa Goodship, Rapra Technology Limited and ARBURG Limited,, [5] ISSN: Page 1241