Abstract
Keywords Moulding simulation, rubber, experimental validation, processing crosslinking
Abstract Despite potential cost and time savings, previous bad experiences of poor predictions discourage the application of mould filling simulation in the manufacture of elastomeric automotive components. A specially designed instrumented mould tool was used to investigate the source of the poor predictions and hence, identify potential solutions.
Injection moulding experiments and mould filling simulations were carried out under the same conditions. Measured and predicted values of injection pressure, cavity pressure and degree of cure were compared to assess accuracy of the simulation. The input parameters required to attain good simulations were identified and the sensitivity analysis carried out.
A rather unexpected finding was that machine input parameters generally had a much greater effect on the quality of the simulation than the material properties. To obtain accurate predictions in injection pressure, besides introducing the real speed profile, it is necessary to correct the factor used to convert hydraulic system pressure to injection pressure. In addition, the accuracy of cure prediction depends on the use of actual tool temperature, rather than set platen temperature.
Apart from the machine input parameters the viscoelastic nature of elastomers had a significant effect on the prediction of pressures. The Cross viscosity model used in the flow simulation software cannot predict the time dependent behaviour of elastomers. Real stress growth and decay occurs more gradually than in the simulation, resulting in lower and broader peaks of pressure than predicted.
It is concluded that mould-filling simulation is potentially powerful for mould tool and process design of elastomeric components. However, it is necessary to carry out a certain amount of preliminary work in order to benefit from accurate predictions. The method of modelling and handling input data described above was validated when an accurate simulation of mould filling for a brake booster diaphragm was obtained. This diaphragm is an example of automotive component where the most important design restrictions are imposed by the moulding feasibility and in consequence where this simulation methodology gives great benefits in terms of time and cost savings.