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This paper presents the design of a plastic injection mould for producing warpage testing specimen and performing thermal analysis forthe mould to access on the effect of thermal residual stress in the mould. The technique, theory, methods as well as consideration neededin designing of plastic injection mould are presented. Design of mould was carried out using commercial computer aided design softwareUnigraphics, Version 13.0. The model for thermal residual stress analysis due to uneven cooling of the specimen was developed and solvedusing a commercial finite element analysis software called LUSAS Analyst, Version 13.5. The software provides contour plot of temperaturedistribution for the model and also temperature variation through the plastic injection molding cycle by plotting time response curves. Theresults show that shrinkage is likely to occur in the region near the cooling channels as compared to other regions. This uneven cooling effectat different regions of mould contributed to warpage.© 2005 Elsevier B.V. All rights reserved. 34750
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Keywords: Plastic Injection mould; Design; Thermal analysis
1. IntroductionPlastic industry is one of the world’s fastest growingindustries, ranked as one of the few billion-dollar industries.Almost every product that is used in daily life involves theusage of plastic and most of these products can be producedby plastic injection molding method [1]. Plastic injectionmolding process is well known as the manufacturing processto create productswith various shapes and complex geometryat low cost [2].The plastic injection molding process is a cyclic process.There are four significant stages in the process. These stagesare filling, packing, cooling and ejection. The plastic injec-tion molding process begins with feeding the resin and theappropriate additives fromthe hopper to the heating/injectionsystemof the injection plastic injectionmoldingmachine [3].This is the “filling stage” in which the mould cavity is filledwith hot polymermelt at injection temperature.After the cav-ity is filled, in the “packing stage”, additional polymermelt ispacked into the cavity at a higher pressure to compensate theexpected shrinkage as the polymer solidifies. This is followed by “cooling stage” where the mould is cooled until the part issufficiently rigid to be ejected. The last step is the “ejectionstage” in which the mould is opened and the part is ejected,after which the mould is closed again to begin the next cycle[4].The design and manufacture of injection molded poly-meric parts with desired properties is a costly process domi-nated by empiricism, including the repeated modification ofactual tooling. Among the task of mould design, designingthe mould specific supplementary geometry, usually on thecore side, is quite complicated by the inclusion of projectionand depression [5].In order to design a mould, many important designingfactors must be taken into consideration. These factors aremould size, number of cavity, cavity layouts, runner systems,gating systems, shrinkage and ejection system [6].In thermal analysis of the mould, the main objective isto analyze the effect of thermal residual stress or molded-instresses on product dimension. Thermally induced stressesdevelop principally during the cooling stage of an injectionmolded part, mainly as a consequence of its low thermalconductivity and the difference in temperature between themolten resin and the mould. An uneven temperature fieldexists around product cavity during cooling [7].
During cooling, location near the cooling channel experi-ences more cooling than location far away from the coolingchannel. This different temperature causes the material toexperience differential shrinkage causing thermal stresses.Significant thermal stress can causewarpage problem. There-fore, it is important to simulate the thermal residual stress fieldof the injection-molded part during the cooling stage [8].Byunderstanding the characteristics of thermal stress distribu-tion, deformation caused by the thermal residual stress canbe predicted.In this paper the design of a plastic injection mould forproducingwarpage testing specimen and for performing ther-mal analysis for the mould to access on the effect of thermalresidual stress in the mould is presented.2. Methodology2.1. Design of warpage testing specimenThis section illustrates the design of the warpage testingspecimen to be used in plastic injection mould. It is clearthat warpage is the main problem that exists in product withthin shell feature. Therefore, the main purpose of the prod-uct development is to design a plastic part for determiningthe effective factors in the warpage problem of an injection-moulded part with a thin shell.The warpage testing specimen is developed from thinshell plastics. The overall dimensions of the specimen were120mmin length, 50mminwidth and 1mmin thickness.Thematerial used for producing the warpage testing specimenwas acrylonitrile butadiene stylene (ABS) and the injectiontemperature, time and pressure were 210 ◦C, 3 s and 60MPa,respectively. Fig. 1 shows the warpage testing specimen pro-duced.2.2. Design of plastic injection mould for warpagetesting specimenThis section describes the design aspects and other consid-erations involved in designing the mould to produce warpagetesting specimen. The material used for producing the plasticFig. 1. Warpage testing specimen produced.injectionmould for warpage testing specimen was AISI 1050carbon steel.Four design concepts had been considered in designing ofthe mould including:i. Three-plate mould (Concept 1) having two parting linewith single cavity. Not applicable due to high cost.ii. Two-platemould (Concept 2) having one parting linewithsingle cavity without gating system. Not applicable dueto low production quantity per injection.iii. Two-plate mould (Concept 3) having one parting linewith double cavities with gating and ejection system. Notapplicable as ejector pins might damage the product asthe product is too thin.iv. Two-platemould (Concept 4) having one parting linewithdouble cavitieswith gating system, only used sprue pulleract as ejector to avoid product damage during ejection.In designing of the mould for the warpage testing spec-imen, the fourth design concept had been applied. Variousdesign considerations had been applied in the design.Firstly, themouldwas designed based on the platen dimen-sion of the plastic injection machine used (BOY 22D). Thereis a limitation of the machine, which is the maximum area ofmachine platen is given by the distance between two tie bars.