型内ランナー切替装置による段差部流動樹脂挙動の可視化解析

抄録

Flow-marks are known to occur due to inconsistent transcription with step-change portions for injection molding, in the step-change area and area opposite to it. It has been clarified that external defects such as these flow-marks are closely related to flow front behavior. The authors developed a rotary runner exchange system which is able to instantaneously switch between flow resins of two colors before the gate, and proposed a new visualization method which enables observation of the internal flow behavior. In this study, we focused on the asymmetric fountain flow behavior in the step-change portion in stepped cavity where cavity thickness changes from the thin part to thick part, and visualized the internal melt behavior near the stepped portion for GPPS. As a result, distribution of colored materials was seen corresponding to the generation of flow-marks at the side of the step-change portion. These colored materials were confirmed to be resins exposed to the outer surface of flow front by asymmetric diverging flow of high viscosity resins flowing near the skin layer of the step portion. And, this flow front was seen to contact the mold surface in the step-change portion, move as if sliding over it, and further elongate. Such flow behavior suggested that the rough surface of molded sample, which was formed by the above diverging flow, elongation, and sliding behavior, corresponded to the flow-mark generation area. This flow mark area spread down to the downstream side with increase in thickness of the thick cavity. This is because the position at which the flow front contacted the mold surface at the step-change portion was distant from the stepped corner. On the other hand, at the flat area opposite the step-change position, a large stagnant area was formed inside the flow front. The resin of shear flow area near skin layer in the thin part was found to ride onto this stagnant area, forming a pattern which turned back to the mold wall side.