The Measurements of the Shear Stress Distribution at the Cavity Surface during Melt Filling in an Injection Mold

Measurement of shear stress distribution is important for examining phenomena occurring in the interface between the mold cavity surface and molten resin in injection molding. The authors developed a high precision measurement technique for shear stress distribution using a movable block and 3-component force transducer. In this study, we analyzed the influence of resins, cavity surface properties, and cavity thicknesses on shear stress distribution. The results are summarized below：Ⅰ) Fluctuations were seen in the shear stress profile under some experimental conditions. These fluctuation phenomena were caused by delamination flow in the nozzle. They were removed by a digital filter to correct the shear stress profile.Ⅱ) From typical experimental results, we proposed a model on the shear stress distribution near the flow front (“FF” hereafter). It consisted of (1) area which rises by rapid deformation just after the FF contacts the cavity surface, (2) area in which the shear stress remains constant due to the effects of shear viscosity and no solidified area forms, (3) area in which shear stress increases with the growth of the solidified layer, and (4) area in which shear stress remains a constant value due to the effects of shear heating and shear-thinning around the wall.Ⅱ) The process by which characteristic phenomena occurred in various conditions was estimated as follows based on above model.ⅰ) Area (2) became longer with increasing injection rate (IR) because the distance to be moved by the FF to cool down to form the solidified layer, increases with IR.ⅱ) The larger the roughness of the cavity surface, the higher was the shear stress in areas (3) and (4) because the growth of the solidified layer is accelerated by the increase in the area contacting the resins on grained surfaces.