In process monitoring of polymer batch to batch variation in injection moulding

Injection moulding studies are reported, during which in process measurements have been used to monitor batch to batch variation of several production grade polymers. Three materials were studied: a polyamide 6, a polyacetal, and a flexible poly(vinyl chloride), all of which were commercial injection moulding polymers supplied by industrial collaborators. Instrumented high precision electric and servohydraulic moulding machines were used to mould parts in house from a number of different material batches. Moulding conditions were kept constant throughout and several process variables were monitored during injection, including melt pressure, melt temperature, and viscosity index –a specific pressure integral calculated from primary injection. Part weights were measured to provide an indication of part quality. Results showed that in each case, variations between batches produced a measurable effect on part quality. These variations were detected by in process measurements, particularly by viscosity index, which tracked significant changes in part weight. Several ‘problem'batches not identified by the compounder's internal quality checks were detected, and the influence of regrind and a development compound were also clearly identified. No simple relationship between viscosity index and part quality was observed for the limited processing range covered. Overall, the studies show the potential of in process measurements to provide a real time, sensitive indication of process variation.