Analysis of Concentric Flow Mark Occurrence Phenomena in Injection-molded Samples Produced with a Hot Runner System

Abstract

Molded products using a hot runner can have peculiar defects such as flow marks, gate marks, flow imbalance, and the like, caused by melt staying in the manifold channel. These phenomena have been noted, but the systematic analysis of their generation mechanisms have not been adequately shown. In this study, we focused on the generation mechanism of concentric flow marks around the gate that were observed on the surface of molded products using acrylonitrile-butadiene-styrene resin (ABS) with a side-fed type hot runner mold. An analysis was conducted using a dynamic flowing visualization method developed by the authors and a two-color molding method. A correlation between these flow patterns and flow mark generation areas was then derived. A unique stagnant behavior of the flow front was observed in the filling process, and it was thought that this caused a change in the deformation of the rubber particles in the melt during the transcription process. Results indicate a glossy unevenness on the surface. The stagnant behavior of the flow front was presumed to be a phenomenon that occurs due to the reversal of the temperature distribution between the inner and the outer layers of the cavity when the low-temperature, high-viscosity resin stagnating around the valve pin of the hot runner begins flowing into the cavity.