In-mould process data is essential for high quality injection moulding processes due to its high correlation to multiple quality criteria, such as part weight, dimensions and surface properties. To take advantage of the high correlation of cavity pressure and part quality, we developed a phase-unifying model-based cavity pressure control. This approach allows an approximate prediction and realisation of cavity pressure curves in real-time by calculating the optimal screw velocity adjustment based on a process model. The phase-unifying process control approach avoids discontinuities by eliminating the switch-over point and enables thereby a smooth transition from filling to packing.
Besides the process control concept, the specification of a suitable process setting is the second key factor in achieving high part quality. A process adaption is always required, if process disturbances occur. For example, production interruptions can occur in the injection moulding process when parts are demoulded incorrectly. Furthermore, process disturbances can influence the injection moulding process such that the process parameters leave a predefined monitoring window and cause a machine interruption. During these interruptions, the thermal household of the mould and polymer melt changes. This causes rejection of parts during the subsequent start-up process due to the changed process conditions. The start-up of the process takes a long time until quality criteria can be again fulfilled.
The study aims to shorten the start-up process as use-case for phase-unifying process control. For this purpose, injection moulded parts were produced for various interruption times. The part quality of the first five parts of start-up process was determined by weight measurements. Additionally, we carried out injection moulding trials for different cavity pressure references to quantify the correlation between cavity pressure characteristics and part weight as quality criteria.
Quantitative correlations between cavity pressure characteristics and part quality were determined by calculating an ideal cavity pressure reference for inline process control for the start-up process. This allows high process stability under the occurrence of production interruptions with fast mould filling.
The quality consistency of part weight was increased by 35 % for the first five produced parts. This leads to a highly reproducible process and fewer rejected parts during the production start-up. Part weight fluctuations are significantly lower compared to conventional process control, which shows the high potential of a combined inline and online phase-unifying process control for injection moulding.