Reaction kinetics and injection molding of liquid silicone rubber

Conventional silicone rubber is processed by several steps in series: milling, performing, molding, postcuring, and finishing. These steps are combined in the liquid silicone rubber (LSR) injection molding process, where the final shape is molded and set in a closed cavity. This paper will examine the curing kinetics and heat transfer of LSR molding. First, a kinetic model for the curing will be developed from the chemical mechanism of hydrosilation. The model will be tested with curing data at different temperatures and concentrations of inhibitor and catalyst. Then this new kinetic model is applied to a curing analysis of rubber-injection molding. The curing analysis will predict gel time in a cavity of known thickness and mold temperature. Comparisons will be made to experimental gel time data in a rectangular plaque mold. It was found that inhibitor consumption follows a zero-order kinetic model, and the subsequent polymerization kinetics are first order in vinyl concentration and concentration of available catalyst. The gel conversion was predicted using the recursive method, and thermal diffusivity was determined by transient temperature measurements in a cured slab. The predicted gel times were in good agreement with experimental demolding data. Because of its simplicity and accuracy, this analysis can be a useful tool for formulation optimization and mold-temperature selection.