On the use of interfacial tension parameter to predict reduction of friction by mold coatings in injection molding of polyamide 6

Abstract

The first contact of liquid hot polymer melt to mold surface while injection is a pressure-supported wetting process. After filling is completed, the melt is cooled down to a solid part, which is finally ejected. This separation of solid polymer from solid mold surface is considered to be a dewetting process. The wetting parameter interfacial tension γ12 estimates the resistance of a liquid-solid interface to be separated under external load. If γ12 also predicts the resistance of solid-solid interface to be separated under external load, γ12 could be used to improve mold coatings for reducing friction in injection molding. Based on the analysis of γ12 equation, the following hypothesis is stated: Low γ12 of a polymer - mold coating interface indicates elevated friction and vice versa. This has been examined in a series of experimental tests. First, from 40 to 200°C, contact angles of test liquids bromonaphtalene and ethylencarbonate were captured on solid Polyamide 6 (PA6, Durethan B30S) and on mold coatings a-C:H, CrC/a-C:H, CrN, NbC, NbN, and MoN. Every specimen’s surface energy was determined using the OWRK method. All materials show a distinct temperature dependence of surface energy and its polar and disperse fractions. Then, γ12 of the mold coatings to PA6 was calculated for a temperature of 90°C. Finally, same PA 6 and the coated specimen were tested in a friction test injection mold. The static coefficient of friction μ0 was determined at a friction contact temperature of 90°C. As expected, most coatings rank in friction to PA6 as γ12 suggests, i.e. friction decreases from MoN, a-C:H, NbC, NbN, CrN, to CrC/a-C:H, while γ12 increases from MoN, a-C:H, NbN, NbC, CrN, to CrC/a-C:H.