Smart Molding International 2-2023

45 www.smart-molding.com measuring technology defect production based on quality prediction models as the ultimate goal. Relevant correlation with part quality What makes cavity pressure such an informative and highly relevant value? As a process parameter, it is captured directly in the cavities with the help of pressure sensors. It precisely describes the processes taking place in the cavities, thus providing a transparent view of the conditions under which the part is being created throughout the injection molding process. Essential advantages: specific quality-related features of the part such as dimensional accuracy, surface characteristics, weight and degree of molding can be attributed to the cavity pressure profile during the injection, compression and holding phases. It follows that the cavity pressure profile can be considered as a fingerprint of the quality of the specific part that is currently being produced – it provides the basis for precise statements about optimal process parameters all through the production process. Good parts (OK) can already be differentiated from bad ones (NOK) while production is still in progress. For this purpose, the upper and lower limits of the key values are determined for validation of the respective process parameters. If the value determined from the curve then goes beyond the defined process window during production, the part concerned is classified as NOK and will be separated automatically. The result is that only good parts continue further into the value chain. But this is not the only benefit: the characteristic values obtained can also be used as input for statistical process control (SPC) (see Figure 1). Validation based on cavity pressure measurement yields even more advantages: for example, significantly less effort is required as compared to methods that exclusively take account of machine process parameters. This is because the processes in the machine cannot adequately describe the formation of the molded part in the cavity, so correlation with part quality is difficult. To achieve consistent part quality, the machine parameters then have to be adapted to new conditions – such as changes in material behavior when batches of different materials are processed. If the fluctuations are so pronounced that the tracked settings are outside the previously validated process window, the process needs to be validated again – which involves a series of time-consuming trials. This can all be made far easier by using cavity pressure measurement, so the correlation between measured values and quality characteristics is known (Figure 2). Smart combination of measurements and statistics A process monitoring solution b a s e d o n c a v i t y p r e s s u r e measurement consists of sensors that are highly precise but also robust, together with a process monitoring system such as ComoNeo from Kistler. In this case, the ComoNeoPREDICT f un c t i on a l i t y c omb i n e d w i t h the STASA QC software enables the system to provide efficient, automated documentation of the test plans (Design of Experiments, or DoEs), and also to perform the corresponding process analyses. This user-friendly functionality can be implemented efficiently on the shop floor, so operators can take a decisive step towards zero-defect produc t i on : ComoNeoPREDI CT makes it possible to determine the expected quality characteristics from the measured values, with no need to expend effort on measuring the parts. Thanks to this solution, a part's quality can be predicted even before it is manufactured – including the necessary documentation. Figure 1. With the help of cavity pressure measurement, good and bad parts can be automatically differentiated and separated during production Figure 2. The cavity pressure values measured during injection molding phases correlate to the parts' quality characteristics