Today, with the development of new polymeric materials, new forming methods have been developed to produce plastic products. Thermoforming as a plastic forming technology in which a thermoplastic sheet is heated in order to deform again, has found a wide place in packaging industry and disposable container production. The main problem with this process is to predict the thickness distribution of final products. Generally, the thickness distribution is found by try and error, which are almost costly and time consuming. Implementation of finite element method prevents to pay any extra cost before production. Hence, in this study, the effect of various parameters including; mold type, the raw material of the sheet, initial thickness of the sheet and evacuation power were investigated on the thickness distribution of a thermoformed cup. To achieve this goal, a homemade thermoforming machine and a pair of male and female mold, which are used to vacuum and drape forming respectively, were built. The raw materials used in this study were two types of polymers, thermoforming grade high impact polystyrene (TF grade HIPS) and traarent polyvinyl chloride (TSP-PVC), with 0.25 and 0.5 mm in initial thickness. To investigate the deformation behavior, whether biaxial or uniaxial stretching are dominant, a square grid with circles on the intersections of the grid line were drawn on the surface of the sheets before begin thermoforming process. Also, by computer simulation of vacuum thermoforming process sheets using ABAQUS software, a simple and accurate model of the whole process is presented. A hyper-viscoelastic constitutive model employed to simulate the material behavior during forming process. Ogden model and Prony series used to define the different parts of the employed constitutive model, respectively. Stress relaxation tests were performed to extract the Prony series parameters of high impact polystyrene used in this study. This work was done by curve fitting procedure in MATLAB software package. The experimental results show that although the evacuation power has no significant effect on the thickness distribution, but higher evacuation power resulting more complete form of parts. In other words, the cups produced with higher evacuation power have complete corners than the cups produced with lower evacuation power. Experimental observations show that the cups manufactured by male mold have thinner wall area than the bottom area of the cup. Contrast the cups produced by female mold have thicker wall area than the bottom area of the cup. An interesting phenomena observed during experimental tests was the return of thermoplastic cups to its initial flat geometry by reheating of it. It means that configure of the cups change into its original shape as it was a thin sheet. This point expresses the dominant hyper -viscoelastic behavior of polymeric sheet during thermoforming process. By comparing the simulation results with experimental results, it was found that the thickness distribution predicted by the simulated model matched well with the measured thickness distribution of empirical tests. This implies a good selection of material model to simulate the behavior of polymeric sheet during thermoforming process and the accurate modeling of the process. Keywords: Thermoforming, Computer Simulation, Hyper -Viscoelastic, HIPS, PVC, Thickness Distribution.
