Process-induced long-term deformation behavior of semicrystalline PBT

Abstract

Processing conditions during injection molding always play a major role in influencing the properties of a finished part as it produces a unique structural gradient because of the complex thermorheological history. Therefore it is highly desirable to anticipate the effect of process parameters on the resulting microstructure and mechanical properties of the finished part in the long run. In this work, therefore, the influence of mold temperature, injection flow rate, and holding pressure on the deformation behavior of semicrystalline Polybutylene terepthalate (PBT) has been monitored through creep. The resulting internal structures and stresses due to processing have been determined and the deformation behavior has been analyzed. It has been observed that only the rate of cooling shows a remarkable effect on the long-term viscoelastic behavior of an injection molded semicrystalline PBT part, as it influences not only the crystalline phase but also the fractional free volume, whereas the different states of frozen-in orientations and residual stresses have only a negligible effect. A slower cooling rate as well as longer aging time lead to a reduction in free volume and hence reduced tendency to creep. POLYM. ENG. SCI. 46:882–888, 2006. © 2006 Society of Plastics Engineers