MEASUREMENT OF RESIDUAL-STRESSES IN INJECTION-MOLDED THERMOPLASTICS

The thermal residual stresses in injection moulded acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) flat plaques were measured by removing thin layers from specimens and measuring the resultant curvature or the bending moment in the remainder of the specimens. A coordinate machine and various mechanical devices or analogues were used to measure the curvature. The accuracy of the measurements depended on the type of mechanical device employed. These devices need to physically touch the specimen surface for measurement and some of them, such as a dial gauge, can exert sufficient pressure on the specimen to change its curvature. Following experimentation with various options, a peg/pegboard arrangement was found to enable accurate reproduction of the specimen curvature. Obviously, a non-contact method such as an optical scanner produced the most reliable curvature measurements. The accuracy of residual stress determinations using curvature data, further depends on the through thickness uniformity of the elastic modulus. Alternatively, the residual stress was determined via bending moment measurements in order to obviate the elastic modulus in calculations. As would be expected, the results showed the existence of the same type of stresses through the thickness of the mouldings as obtained from the curvature measurements but variations were indicated in the stress magnitudes.