Flow and Solidification of Polymer Melt in the Injection Molding Processes : Optical Observation of the Melt Flow and Growth of Solidified Layers

Abstract

This paper describes an experimental study carried out to investigate the relation between the development of stress in a molded polymer and the injection molding conditions. The mold used in the present study had a rectangular cavity constructed with a transparent window on each side : the birefringence patterns in the polymer during the filling and cooling processes were observed through an application of the photoelastic method. The results clearly show that the development process of birefringence in the polymer can be separated into three successive stages : the flowing stage, the relaxation stage, and the cooling stage. The amount of residual birefringence of the molded polymer is determined by both the unrelaxed shear stress in the near-wall region (skin region) and thermal stress due to cooling is the core region. It is concluded that the thickness of the skin region, where the shear stress due to flow of polymer melt is frozen, can be estimated on the basis of the relation between the relaxation time of shear stress and solidification time of injected polymer melt. This consequence enables us to draw a figure which shows the relation between the pattern of residual stress in the molded part and the injection molding conditions.