Polymer wear evaluation

Polymer wear plays an increasing role in manufacturing of machine parts for e.g. medical devices. Some of these have an expected lifetime of five to eight years during which very little wear of the components is acceptable. Too much wear compromises the dosage accuracy of the device and thereby the safety of the patients. Prediction of the wear of polymers is complicated by the low thermal conductivity of this kind of material. It implies that any acceleration of testing conditions by increased contact pressure and/or sliding
velocity will make the polymer fail due to exaggerated heat buildup. This is not the kind of wear observed in medical devices. In the present work a method was developed capable of evaluating the wear progression in polymer-polymer contacts. The configuration of the setup is injection moulded specimens consisting of an upper part having a toroid shape and a lower flat part. The sliding contact is then a line contact deformed by the load. In this way it is possible to have a pressure distribution ranging from a very high value in the middle to zero at the borders according to Hertzian contact mechanics. The wear is evaluated after 10 s of sliding in the beginning increasing to 30 min. at the end. One test has a duration of maximum two hours. The wear is initiated in the highly loaded centerline of the contact. It then propagates to the lower pressure areas. The width of the wear track then expresses the wear extent. The method was calibrated with both wear resistant and easily worn polymers and it showed good ability to predict the wear extent seen in medical devices after five years of use.