Rethinking the injection molding process with thermosets and rubber

Abstract

AbstractTo optimize the processing, a process model has to be developed. To be able to use a model of this kind, we must also have material data (thermodynamic material data, viscosity, processing time) and must install sensors on the machine and in the mould.The flow properties of thermosets/elastomers under processing conditions is determined not only by pressure, temperature and throughput, but also to a considerable extent by the boundary conditions.At a given throughput, it is possible to calculate the pressure loss for passing through a runner system by dividing up the flow into a plug flow and a boundary shear flow. This takes into account the pronounced temperature profile, which brings about a low‐viscosity outer layer and a high‐viscosity core.With thermosets/elastomers, the compounding process influences the flow properties to a considerable extent. This means that the continuous monitoring of the material state must be included in the process control. This monitoring can be carried out during the mould filling period. Since the mould is filled over a wide area at constant velocity, the gate can be regarded as a “rheometer” and the pressure loss between two pressure sensors installed in the gate used as a measure of the viscosity.Starting from this measurement of the flow properties, moulding properties such as weight and surface texture are influenced in the compression phase. The form of the pressure build‐up in the compression phase affects the flash formation; this pressure buildup should, in fact, be carried out in such a way that the pressure maximum is not reached until the outer layers have passed through their viscosity minimum.After the compression, the moulding is crosslinked in the crosslinking phase far enough for it to be demoulded or for the moulding properties, which are connected with a given crosslinking profile over the cross‐section, are reached.