This presentation was made at the NAFEMS Americas Seminar - Confidence in Engineering Simulation: The Next 10 Years of CAE in Mexico.

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Resource Abstract

Warpage is a common and unavoidable flaw of injection-molded parts, mainly associated with non-uniform shrinkage phenomena, which is inherent of the macromolecular structure of polymeric materials. This defect is influenced, in more or less extent, by part/tool design, material properties and injection-molding process conditions. The control of these parameters becomes important towards warpage minimization to improve part quality and dimensional stability, factors of capital importance to fulfil tolerance requirements in assemblies.



Plastic parts with low warpage are a constant goal for plastic transformers, who look for practical and feasible ways to manage warpage issues. Computer-aided Engineering (CAE) software Autodesk Moldflow Insight can be used to simulate different scenarios of injection molding process, in order to minimize warpage in plastic parts.



Warpage control becomes more challenging when dual-shot overmolding is used. This process is a variant of injection molding process that allows a secondary material to be injected partially or fully over another material previously injected (known as substrate or polymeric insert) to manufacture one single part. This is required when different properties are required (mechanical, ergonomical and aesthetical as the most important properties). In the case of overmolded plastic parts, the warpage behavior becomes more complex, due to the interaction of two plastic materials with different properties, each one with its own processing history.



In this study, a 3D mesh model of a dual-shot automotive pillar was analysed. The first shot, considered as a polymeric insert for the purposes of this study, was simulated with Poly(carbonate)/Acrylonitrile-Butadiene-Styrene (PC/ABS), while the second shot was Poly(methylmetacrylate) (PMMA).



The 3D mesh model was made considering the same mesh on shared surfaces between both shots. A 5-level 6-factor 25-experiments (L25 56) Taguchi orthogonal array design of experiment (DOE) was used to analyse the warpage dependence on second shot molding conditions. The molding conditions considered were: melt temperature, mold temperature, packing pressure, packing time and cooling time. In the DOE was also considered the temperature of the plastic insert as a factor. The experiments suggested by DOE were simulated in Moldflow to obtain warpage results as an output. An analysis of variance (ANOVA) was used to statistically find the relative effect of studied process parameters on warpage.