Influence of initial sheet temperature an ABS thermoforming

Understanding the effects of material and processing parameters on the ther moforming process is critical to the optimization of processing conditions and the development of better materials for high quality products. In this study we investigated the influence of initial temperature distribution ove r the sheet on the part thickness distribution of a vacuum snap-back formin g process. The linear viscoelastic properties along with the Wagner two par ameter nonlinear viscoelastic: constitutive model were utilized for numeric al simulation of the thermoforming operation. Simulations of pre-stretched vacuum thermoforming with a relatively complex mold for a commercial refrig erator liner were conducted. The effects of temperature distribution over t he sheet on the part thickness distribution were determined to examine proc ess sensitivity and optimization. Effects of the temperature distribution o n the material rheology and polymer/mold friction coefficient are primarily responsible for the changes in the thickness distribution. We found that e ven small temperature differences over the sheet greatly influenced bubble shape and pole position during the bubble growth stage and played a critica l role in determining the part thickness distribution. These results are di scussed in terms of rheological properties of polymers such as elongational viscosity and strain hardening.