Chapter 6 - Effects of melt processing

Abstract

This chapter explains the effects of melt processing on plastic products, microstructural effects such as crystallinity changes, and macroscopic effects such as product shrinkage. These effects are permanent since plastics are never annealed to cause re-crystallization; it is also possible for cold-worked metal sheet. The cooling rate of the polymer in the later stages of processing and the stress in the melt, as solidification occurs, are the main process parameters. Non-uniform shrinkage can cause residual stresses or the product to warp. The effects of processing on the particle microstructure of PVC and ultra high molecular weight polyethylene (UHMWPE) are also considered in the chapter. The cooling rate in all plastic processes is relatively fast. It varies with position and can be orders of magnitude larger in the outer layers than in the centre of a product. The chapter shows the variation in cooling rate through the glass transition temperature, in a sheet cooled from both sides by a medium with an infinite heat transfer coefficient. The chapter explains how pipes, cooled on the outside, contain residual tensile hoop stresses at the bore. Consequently, cracks tend to start near the bore of the pipe from a point of weakness. PVC pipes used for water distribution are subjected to an internal pressure that can vary according to the water demand. The resulting fluctuation stresses the pipe wall causing fatigue loading. Fatigue stresses, if high enough, may eventually initiate small cracks at weak particle boundaries in the PVC. Subsequent fatigue cycles cause the crack to grow slowly until failure occurs.