THE DEVELOPMENT OF A SOLVER FOR THE PREDICTION OF RESIDUAL STRESSES IN THE POLYMER EXTRUSION

During the forced fast cooling process that occurs in the industrial production of plastic products, huge thermal gradients are induced in the material. In these conditions, stresses of thermal origin develop, promoted by the inherent variation of the material density, since its shrinkage is partially inhibited by the already solidified material layers. As a consequence, the non-relaxed thermal stresses, the so-called “thermal residual stresses” are “frozen” in the material/product after the solidification process. The prediction of residual stresses is a topic of major importance. In fact, they affect the mechanical performance of the products (as they are added to the applied ones) and if the service temperature is high enough, they might induce material deformation, with consequent geometry distortion. This subject has been the focus of several works related with injection moulding, but in what concerns to the extrusion process, the available scientific literature is scarce. Therefore, the aim of this work is to develop a new numerical code, in the OpenFOAM® computational library, able to predict the development of the residual stresses in extruded profiles cooled down in calibrators. The strategy employed consists of the detection of the solidification front and the computation of the temperature increment through paths orthogonal to it (defined by orthogonal lines), according to a previous work [Tropsa(2001)]. The frozen-in temperature (and, consequently, the strains) are obtained by the temperature gradient integrations along the detected paths. The complete residual stress analysis includes the resolution of the energy equation providing the temperature field and the momentum equation that considers the residual stresses generated during cooling.