A computational study on the influence of the rheological behavior of polystyrene and its blends on their thermoforming ability
F. M. Duarte1,
V. C. Barroso2, J. M. Maia1 and J. A. Covas1,*
1 IPC - Institute for Polymers and Composites, Department
of Polymer Engineering, University of Minho, Campus Azurém, 4800-058 Guimarães,
Portugal
2 Institute of Polymer
Materials, University Erlangen-Nürnberg, Martensstr. 7, 91058 Erlangen, Germany
Abstract
The
present work aims at understanding the relationship between heating conditions,
rheological behavior and thickness distribution that lead to the optimization
of the latter in thermoforming. The materials used in this study were
polystyrene, PS, high-impact polystyrene, HIPS, and a 50/50 w/w % blend of the
two. The study was done by investigating computationally the influence of the
material thermo-rheological properties on sheet temperature and final thickness
distribution of a vacuum-produced part and relating the sheet heating
conditions with the forming stage.
When
sheet temperature is uniform, the degree of strain hardening and the failure
behavior in extension are the most important parameters in controlling the
kinetics of the process and the thickness profile. In the case of non-uniform
sheet temperature, the results show that an increased degree of
strain-hardening is more relevant to the dynamics of the process than
relatively small differences in sheet temperature.
However,
the solution of the inverse thermoforming problem (determining the heater
temperature that induces a certain thickness distribution) showed that under
practical processing conditions the effect of differences in thermal properties
are predominant over the rheological ones.
Keywords:
Thermoforming, polystyrene, high-impact polystyrene, inverse problem, thickness
distribution