Injection Molding of Rubber

Abstract

Abstract The outline of a strategy for the injection molding of a safe mix is described. Safe accelerators, safe curing systems, and mixes are suggested with machine conditions which should yield short curing times for a wide range of products. Comparisons have been made of the vulcanizate properties of injection and compression moldings prepared from the same mix in the same mold and press at the same temperature. Results have shown that, although anisotropy is more pronounced in injection moldings, the mean of values for modulus along and at 90° to radial mold flow lines of an optimum modulus cure are the same (within experimental error) as the mean of values for modulus along and at 90° to mill flow lines in a compression molding also cured to its optimum modulus. Injection moldings are similar to compression moldings in volume swelling and hardness but they have marginally higher rebound resilience and marginally lower compression set. The effect of an increase in mold temperature is to give a valuable reduction in cure time, but it also causes a reduction in modulus and modulus dependent vulcanizate properties. Optimum values for maximum-minimum Monsanto Rheometer torque, relaxed modulus, and 300% modulus are reduced to 77– 79% of their optimum values at 160°C when cured at 200°C, but this reduction can be compensated for by adding higher levels of accelerator or accelerator and sulphur. Although a certain degree of anisotropy may have to be tolerated, the analysis of vulcanizate properties has provided a means of understanding and correcting an injection molding mix which fails to pass a specification. It gives confidence that NR can be cured at relatively high temperatures such as 180–190°C, which make injection molding profitable.