Abstract
Thermoformed natural fiber filled polyolefin composites are becoming increasingly important because of their combination of low cost, good formability and high strength-to-weight ratio. During thermoforming, ex truded sheets are stretched into a mold by either mechanical force or pressure/ vacuum. As the sheet stretches into the mold it thins. Hence, it is important to optimize the process in order to form an acceptable final part. Often, optimiza tion is an expensive and time-consuming trial-and-error process. However, with microcomputers, numerical simulation of the thermoforming process is becom ing increasingly accurate and powerful. In this study, a one-dimensional finite element simulation was developed to predict the thinning of axisymmetric and rectangular thermoformed parts. This simulation uses an empirical model de veloped from experimental data to represent the temperature-dependent stress-strain behavior of wood flour filled polypropylene. To predict sheet thin ning, the simulation solves the virtual work equation including normal, bend ing and shearing stresses in the sheet during stretching. To solve the governing equations, one-dimensional finite element theory is used. Based on the ex perimental results, an empirical model that represents the material behavior of wood flour filled polypropylene was developed. The simulation was compared to an analytical solution and experimentally measured thickness profiles. Also, the effects of non-isothermal heating and elastic versus elastic-plastic material behavior were studied and a part was optimized using the simulation.
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