The use of the simplex method and its derivatives to the on-line optimization of the parameters of an injection moulding process

The sequential simplex method and its derivatives, the modified and the super modified simplex methods, are used to optimize on-line the levels of the parameters of an injection moulding process (producing bottle caps) in order to minimize the process cycle time and to reduce the rejects. Nine process parameters are selected to conduct the optimization: screw rotation speed, back pressure, injection rate, injection pressure, cooling time, holding pressure, holding time, nozzle temperature and opening stroke. Optimum conditions are achieved using 17 experiments only. The obtained results clearly show that the process cycle time is very sensitive to the effect of two parameters: cooling time and holding pressure. Low levels of these two parameters are needed in order to minimize cycle time. In addition, the evaluation of rejects in the course of the simplex evolution shows that this additional response is also sensitive to the same parameters. In order to minimize rejects, these two parameters have to be set to low levels. Finally, this study allows us to conclude that the simplex method may be easily used and reused on-line to drive the process continuously towards its optimal conditions without any disruption of production or additional operating costs.