Predicting residual stresses due to solidification in cast plastic plates

Abstract

Thermal processing is an economical and efficient way of manufacturing polymeric products. However, it suffers from an important side effect that may considerably affect product performance: during non-uniform cooling to room temperature, strains become ‘frozen in’ the material. These frozen in strains lead to undesirable distortions and residual stresses within the final product. This work is concerned with cast filled PMMA plates for domestic applications. In a typical casting process, the material is first polymerised at a high temperature and then cooled to the ambient temperature. After cooling, the plate has distorted to an extent dependent on the cooling conditions.

A method has been developed for quantitative prediction of the distortions and consequent residual stresses caused by non-uniform solidification. A ‘residual’ temperature field is introduced, which provides the relationship between the thermal history and frozen in strains. When applied as an actual temperature distribution, it results in distortions and residual stresses. Laboratory tests indicative of the actual casting process validated the frozen in strain model. Preheated plates were cooled in a controlled environment while temperatures were recorded at a number of locations on the plate surfaces. The measured temperature histories were then used in the numerical prediction of the residual temperature field. Experimental and numerically predicted distorted shapes agree very well.