Annealing as a Method of Improving the Quality of Moldings from Amorphous and Partially Crystalline Thermoplastics.

Studies on tubs molded from polystyrene and polyethylene show that the density differentials within the molding and, thus, the cooling stresses can be reduced by annealing. In the case of the PS tubs investigated, which show fairly high anisotropy of the structure near the gating point, the relaxation effect was always higher than that of the anisotropy of shrinkage. This can also be explained by the fairly low shrinkage data of amorphous thermoplastics, which generally are about 0. 5%. On the tubs made of high-density polyethylene, a slight decrease of the inherent stresses could only be proved after mild annealing at about 50 degree C; at temperatures around 80 degree C, the inherent stresses increased considerably as a result of the anisotropy of shrinkage, and annealing eventually led to an increase rather than a decrease of the inherent stresses.