APPROXIMATE MODEL OF THERMAL RESIDUAL STRESS IN AN INJECTION MOLDED PART

Abstract

A description and model of the thermally induced residual stress that forms in a ``plate-like’ ’ injected molded polymer part is outlined. A linear heat balance integral (LHBI) temperature pro®le approximation and the concept of a virtual adjunct mold (VAM) is used to arrive at an approximate algebraic model that, upon providing material prop-erties and processing conditions, gives an immediate evaluation of the residual stress through the plate thickness. Results obtained with this model are in very close agree-ment with a full numerical solution and agree closely with experimental measurements made on a range of starch-based biodegradable polymer samples. Injection molding is used in the fabrication of a wide range of plastic products. In the injection-molding process, molten polymer is injected into a mold and then is packed under pressure, solidi®ed, and then cooled to a uniform temperature. Figure 1 schematically illustrates the solidi®cation stage of a ``platelike’ ’ part. Injection molding introduces residual stresses, a stress state that exists in the bulk of a material without application of any external load, in the ®nal product [1, 2]. It is important to know the residual stress state of the part to predict its performance under load. Depending on the situation, residual stresses can be either detrimental or