Analysis of Flow-mark Occurrence Phenomenon Downstream of Weld-line Area in Injection Molding

Abstract

In injection molding, it is generally recognized that the flow front, which is split into two by the obstacle, re-converges behind the obstacle to form a V-notch-shaped weld-line. However, in certain cases, the appearance of a zonal flow-mark downstream of the weld-line could be noticed even though the V notch disappears. This flow-mark could especially be observed frequently in molded products using high-impact polystyrene (HIPS). The authors developed resin marking methods using a gate magnetization system and a rotary runner exchange system that enable the precise observation of the internal three-dimensional flow behavior with elapsed time. Using these methods, it is statically possible to visualize the freezing flow patterns marked in chronological order by observing the cross-section of the molded samples. In this study, we focused on the melt flow patterns behind the weld-line and visualized the internal melt behavior. Furthermore, we conducted comparisons between these melt flow patterns and the flow-mark occurrence area, and performed measurement of flow-front surface temperature and melt stretching test for HIPS to study the mechanism of the flow-mark occurrence. As a result, the visualization of the melt flow patterns behind the obstacle pin confirmed that resins turning around the pin start to stretch from behind the pin gradually, and reach the flow-front after considerable delay, especially near the cavity walls, generating a temperature drop compared to the surrounding resins. Consequently, it is assumed that when resins with the temperature lower than that of surrounding resins spread near the cavity wall while subjected to fountain-flow extension effects, they form a surface layer, thus causing considerable surface roughness with deformed and protruded rubber particles. This appears as the dull portion of the zonal flow-mark.