Determination of the structural capabilities of thermoformed and blow-molded components.

摘要

In the thermoforming processes, a polymer membrane is heated well above the glass transition temperature and forced into a mold. This is accomplished by applying a pressure differential where the pressure on the non-mold side of the membrane is higher than that of the mold side. Upon contact with the mold surface, the polymer is rapidly cooled and subsequently removed from the mold. During the process, stresses are induced into the polymer that must be included in the determination of the load bearing capabilities of the component. Moreover, the stresses are the cause of significant undesirable warpage. In this investigation, the residual stress and warpage of a thermoformed component made from ABS (Acrylonitrile Butadiene Styrene) plastic were investigated. The research was both experimental and theoretical in nature in that the viscoelastic material behavior first had to be measured experimentally, followed by correlation to a finite strain constitutive model. A finite element analysis of the thermoforming process was also performed. To facilitate measuring the viscoelastic behavior of the polymer, a heated tensile testing machine was constructed. This machine was equipped with a high speed digital data acquisition and control system to obtain the stress and strain data during rapid stretching. The data obtained from the testing was correlated to a non-linear finite strain viscoelastic material model. The constitutive model used was time-strain separable and was a viscoelastic generalization of rubber elasticity. Results for both the Moony-Rivlin and the five-term polynomial strain energy functions are presented. Stress relaxation experiments were also carried out and the results are given at a variety of temperatures. Once the constitutive behavior of the polymer was determined, a finite element analysis was performed to model the inflation of the polymer into a simple mold. The final thickness distribution of the formed component and the stresses upon contact with the mold surface were determined. Finally, the residual stresses and the warpage were determined by performing a second finite element simulation of the polymer after contact had been made with the mold surface. The finite element simulation was performed for various forming conditions.