Charpy impact and dart drop impact

In the selection of an appropriate material which satisifes the impact strength requirement for an injection moduled product, the Charpy, Izod or dart drop test methods should be used according to the impact mode displayed by the product—that is, either snap impact on a bar or impact on a shell structure—because generally the ranking of a material determined by the Charpy test differs from that determined by the dart drop test. However, preference for the Charpy test, because of the advantage of simples evaluation work, sometimes leads to erroneous judgement of product characteristics. The breaking behaviour of various materials as determined by Charpy apparatus equipped with instrumentation was studied, and a new design of Charpy test was shown to correlate with the dart drop impact test. The procedure used was to provide a bigger notch tip radius, to break the specimen parallel to the flow direction in the injection moulding process, and to apply the energy absorbed until the beginning of fracture at the notch end.