A description and model of the thermally induced residual stress that forms in a ''platelike'' injected molded polymer part is outlined. A linear heat balance integral (LHBI) temperature profile approximation and the concept of a virtual adjunct mold (VAM) is used to arrive at an approximate algebraic model that, upon providing material properties and processing conditions, gives an immediate evaluation of the residual stress through the plate thickness. Results obtained with this model are in very close agreement with a full numerical solution and agree closely with experimental measurements made on a range of starch-based biodegradable polymer samples.