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Abstract This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis for the mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as consideration needed in designing of plastic injection mould are presented. Design of mould was carried out using commercial computer aided design software Unigraphics,Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solved using a commercial finite element analysis software called LUSAS Analyst, Version13.5. The software provides contour plot of temperature distribution for the moldel and also temperature variation through the plastic injection molding cycle by plotting time response curves. The results show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions. This uneven cooling effect at different regions of mould contributed to warpage.20581
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Keywords: Plastic Injection mould; Design; Thermal analysis
1 Introduction
Plastic industry is one of the world’s fastest growing industries, ranked as one of the few billion-dollar industries. Almost every product that is used in daily life involves the usage of plastic and most of these products can be produced by plastic injection molding method. Plastic injection molding process is well known as the manufacturing process to create products with various shapes and complex geometry at low cost.
The plastic injection molding processis a cyclic process. There are four significant stages in the process. These stages are filling, packing, cooling and ejection. The plastic injection molding process begins with feeding the resin and the appropriate additives from the hopper to the heating/injection system of the injection plastic injection molding machine. This is the “filling stage” in which the mould cavity is filled with hot polymer melt at injection temperature. After the cavity is filled, in the “packing stage”, additional polymer melt is packed into the cavity at a higher pressure to compensate the expected shrinkage as the polymer solidifies. This is followed by “cooling stage” where the mould is cooled until the part is sufficiently rigid to be ejected. The last step is the “ejection stage” in which the mould is opened and the part is ejected, after which the mould is closed again to begin the next cycle.
The design and manufacture of injection molded polymeric parts with desired properties is a costly process dominated by empiricism, including the repeated modification of actual tooling. Among the task of mould design, designing the mould specific supplementary geometry, usually on the core side, is quite complicated by the inclusion of projection and depression.
In order to design a mould, many important designing factors must be taken into consideration. These factors are mould size, number of cavity, cavity layouts, runner systems, gating systems, shrinkage and ejection system.
In thermal analysis of the mould, the main objective is to analyze the effect of thermal residual stress or molded-in stresses of product dimension. Thermally induced stresses develop principally during the cooling stage of an injection molded part, mainly as a consequence of its low thermal conductivity and the difference in temperature between the molten resin and the mould. An uneven temperature field exists around product cavity during cooling.
During cooling, location near the cooling channel experiences more cooling than location far away from the cooling channel. This different temperature causes the material to experience differential shrinkage causing thermal stresses. Significant thermal stress can cause warpage problem. Therefore, it is important to simulate the thermal residual stress field of the injection-molded part during the cooling stage. By understanding the characteristics of thermal stess distribution, deformation caused by the thermal residual stress can be predicted.