Frederic Neil Cogswell

Rheology and Mechanical Properties: a Study of the Influence of Processing History On the Impact Resistance of Thermoplastics

HE IMPACT RESISTANCE of a plastic article is a combination of the intrinsic Tductility of the material, the particular molecular orientation and texture arising from the specific moulding conditions, and the geometry of the component, rather than a genuine physical property. This is at least partly the reason for the proliferation of methods for impact testing, none of which gives either an unambiguous measure of the inherent strength of the material (even if such a concept is anything more than an abstraction) or, on the other hand, an engineering design datum of general applicability. This paper presents some results acquired recently during the first stages of a search for an improvement in current procedures for the evaluation of impact resistance. Our starting point was an acceptance of the fact that a measured impact datum always relates to a particular specimen and never has the status of a genuine material property. It follows that results obtained on idealized specimens cannot be translated into predictions about the impact resistance of even simple mouldings and therefore a crucial first stage of the experimental program is the development of a series of moulds to provide test pieces that may be regarded as sub-components or simulated end-products. Those test pieces could then be used in a systematic study of the effects of mould geometry, flow discontinuities, regions of molecular alignment, defects and external stress concentrators on impact resistance. Ideally such moulds should embrace the various features that occur in moulded articles and the flow geometries should be predictable and, where possible, controllable. The stress fields during impact should also be analyzable so that any appropriate theories can be utilized. The paper describes two new moulds, one a four-cavity mould for the production of Charpy bars with controlled defects and the other a rectangular