Use of non-invasive measurement methods to aid the control of part quality in the injection moulding process

The ability to monitor and detect changes in the morphology of injection moulded parts during the manufacturing cycle is a key issue for improving the process. Knowledge of the extent of solidification of the part during the packing and cooling phase enables better control of the process and good repetition of product quality. A novel way of extracting process information from two existing non-invasive methods is presented here. One method uses piezoelectric transducers generating/recording ultrasound waves that propagate through the polymer melt, while the other utilizes fast-response thermocouples embedded close to the mould cavity. The ultrasound wave speed variation and amplitude attenuation are monitored. These are then utilized with the aid of an off-line data processing algorithm to identify phase changes during the injection cycle (instead of monitoring variations in cavity pressure), as well as to predict accurately the extent of solidification of the produced part. The extent of solidification is also predicted with the thermocouple method. The results of both methods can be used to improve the control of product quality, with the ultrasound method providing significant additional information.