Low friction surfaces for plastic injection moulding dies - an experimental case study

Maintaining low friction forces during the ejection stage of polymer injection moulding is becoming of increasing importance in the manufacture of polymer components, because this should reduce energy consumption and wear. The first stage of an experimental programme designed to determine the optimal surface finish/coating for P20 (low alloy) die steel, has shown that magnetron sputtered CrN coatings, when applied to polished P20, causes a significant reduction in the frictional forces that act during the ejection of an acetal polymer test ring. The standard deviation about the mean of the friction forces was also least for this combination of surface treatments. The lowering of the friction forces by the CrN coating, despite having a slightly higher surface roughness, is attributed to a lower chemical wetting of the CrN surface whilst in intimate contact with acetal. In contrast, perished P20 or polished P20 coated with TiN or MoS2 developed higher friction forces with wider standard deviations. Future work will examine the abrasive wear response of coated and uncoated P20. (C) 1999 Elsevier Science S.A. All rights reserved.