Residual stresses in plastics, rapidly cooled from the melt, and their relief by sectioning

Abstract

A theory for the residual stresses in tempered glass plates has been adapted for the cooling of plastics, which have temperature dependent thermal properties. The theory was checked against experimental residual stress distributions found in quenched polycarbonate sheet, and against the analytical solution for temperature independent properties. The heat transfer coefficient for quenching polycarbonate from 170° C into iced water was found to lie between 1000 and 4000 W m⁻² K. It is known that the cutting of thin sections from a sheet relieves the residual stresses, and this is used for transparent plastics to distinguish between orientation and stress bi-refringence. An elastic stress analysis of the sectioning process showed that the section width must be less than 20% of the sheet thickness for the residual stresses to be reduced to 5% of their original values.