Influence of clamping force on tie-bar elongation, mold separation, and part dimensions in injection molding

Abstract

In injection molding, the magnitude of clamping force may affect the quality of plastic parts. A small clamping force may produce defects such as flashes and poor geometrical accuracy, whereas a large force could result in insufficient air venting during mold filling/packing, leading to the generation of short shot. Traditionally, clamping force is set at the highest machine specification, which may lead to additional energy consumption. Moreover, heavy loading at the tie bars is detrimental to the durability of processed molds and the machine itself. In this study, we developed a measuring system for recording tie-bar elongation and mold separation during injection molding at each corner of the mold. To determine the consequences of proper and improper clamping force settings on the quality of injection-molded parts, we systematically evaluated the effects of relevant parameters such as cavity layout and gate location on cavity pressure distribution during packing and the reactive force from cavity pressure acting on mold walls. Correlation analysis on experimental results depicted that the maximum increase in tie bar elongation and the maximum mold separation during injection molding are highly correlated with the thickness of plastic parts, and we suggested that the two features are good indicators of part quality. Moreover, cavity layout of generating diverse filling patterns and the distribution of cavity pressure is influential to mold separation as well as tie-bar elongation during mold filling. This phenomenon is further change the degree of mold separation at each corner of the injection mold and should be concerned in practice.