Calculating the temperature and degree of cross-linking for liquid silicone rubber processing in injection molding

Abstract

Processing of liquid silicone rubber (LSR) in the injection molding process has a high economic potential. Since there are some fundamental differences compared to classical thermoplastic injection molding, up to now there is a lack of well-founded knowledge of the process which allows an estimation of the cycle time. Because, in addition to reverse temperature control, LSR processing also involves an irreversible temperature- and time-dependent chemical reaction. In this paper, the complex cross-linking reaction is first modelled phenomenologically using dynamic differential scanning calorimetry (DSC) measurements and the well-fitting Kamal-Sourour model. Afterwards, a temperature and cross-linking simulation is set up, which reliably simulates the time- and travel-dependent temperature profile and degree of cross-linking in the mold. Therefore, the released exothermic cross-linking heat is also taken into account. The simulated temperature values are verified with measurements in the cavity during the injection molding process. The measured values correspond very well with the simulated values at different mold temperatures. It is shown that the influence of the cross-linking heat on the overall temperature profile in the LSR component during the injection molding process is relatively low. Nevertheless, the model is necessary to determine the degree of cross-linking - it can be used to calculate the cycle time at which the component of a certain cross-section can be ejected at a known tool temperature and is fully cross-linked. With this knowledge, existing processes can be optimized in terms of mold temperature and curing time, but also new components can be calculated economically.