Thermomechanical contact elements for healing in polymers

Abstract

In injection moulding of polymers flows may split and recombine. The resulting weldlines usually have inferior mechanical properties. A finite element model has been used to study the development of a weldline during the filling and cooling stages of injection moulding. An Arbitrary LagrangeEuler method is applied to describe the moving free surface. For the instationary thermal and mechanical contact behaviour between polymer and mould, use is made of a penalty formulation implemented in a contact element. For the description of contact between two polymer flow fronts (weldlines) the model has been enhanced using reptation theory describing the healing between the two fronts. As a result the strength of the weldline can be predicted. INTRODUCTION Weldlines are present in many injection moulded products. They may be caused by multiple gates or by splitting of the flow, e.g. by inserts in the mould. Two colliding melt fronts will weld to a certain degree. A weldline influences the optical and mechanical quality of the product. It may be visible because of differences in brilliance, colour, distribution of fillers or because of a v-notch. In general this is undesired. The mechanical quality of the product is influenced by imperfect bonding over the weldline and may be worsened by stress concentrations due to a v-notch. The notch may be generated by a number of different mechanisms. Due to trapped air or increasing viscosity near the mould the injection pressure may be insufficient to close the gap between the melt fronts. Or the Transactions on Engineering Sciences vol 1, © 1993 WIT Press, www.witpress.com, ISSN 1743-3533 104 Contact Mechanics gap may have been closed, but torn open due to thermal stresses. Numerical simulation may help to reveal the origin of a v-notch. However experiments show notches of order 1-10 jum, in samples of order 1 mm thickness, a ratio of 1:100-1000. An accurate numerical description of an evolving notch will thus require a very fine grid and is not feasible with our current solution strategies. Disregarding the notch for the time being, we concentrate on the interior of the weldline. Looking on a microscopic scale (due to the fountain flow phenomenon) the macromolecules will be oriented parallelly to the weldline before the melt fronts collide. By a diffusion mechanism the molecules will cross the interface after collision has taken place. Increasing temperature or decreasing pressure will accelerate this process of healing. Experimental and theoretical work can be found in literature. Implementing a model for healing in a contact-element of a finite element package enables simulation of the forming of a weldline. As a result the strength of the weldline may be predicted. HEALING THEORY Using reptation theory (de Gennes [6]) the average interpenetration distance X of the molecular chains over the interface can be modelled as