A simulation test for the selection of coatings and surface treatments for plastics injection moulding machines

Abstract

Glass-filled polymers are known to produce considerable wear on the screws and barrels of injection moulding machines and several coatings and surface treatments have been used to combat this ranging from chromium plating to High Velocity Oxy Fuel (HVOF) WC/Co. However, quantification of the benefits of such coatings and treatments has proceeded on an ad hoc basis and there is little data available to aid selection. In this study we have developed a novel wear tester to simulate the conditions of wear which occur in the barrel of an injection moulding machine and used it to rank the coatings and surface treatments which are often used in this application. The tester concept is similar to that of the ASTM rubber wheel abrasion test except that the rubber wheel is replaced by a steel wheel heated to a fixed temperature (up to 200°C) by hot air and the sand is replaced by pelletised plastic. The plastic sticks to the hot wheel and is softened and dragged past the sample which is also heated to the same temperature. The sample is pressed against the plastic-coated wheel with a small load and a wear scar is produced at the contact point by a combination of abrasive wear from the glass filler and sliding wear from the plastic. In the case of glass filled nylon the best surface treatments were based on nitriding. Coatings tend to not perform very well in this test but are still better than uncoated steel. This paper discusses these results and the mechanisms of failure observed in the test.