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

Injection moulding studies are reported, during which in process measuremen ts 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 injectio n 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 condi tions were kept constant throughout and several process variables were moni tored during injection, including melt pressure, melt temperature, and visc osity index - a specific pressure integral calculated from primary injectio n. Part weights were measured to provide an indication of part quality. Res ults showed that in each case, variations between batches produced a measur able effect on part quality. These variations were detected by in process m easurements, particularly by viscosity index, which tracked significant cha nges in part weight. Several 'problem' batches not identified by the compou nder's internal quality checks were detected, and the influence of regrind and a development compound were also clearly identified. No simple relation ship between viscosity index and part quality was observed for the limited processing range covered. Overall, the studies show the potential of in pro cess measurements to provide a real time, sensitive indication of process v ariation. PRC/1609 (C) 2000 IoM Communications Ltd.