Meaning
Consistency in the weight of finished plastic parts is used to measure process repeatability. In polymer injection moulding, mass stability indicates that the cavity has been filled and packed with the same amount of resin in each successive cycle. High stability of mass correlates with dimensional accuracy and consistent mechanical properties in the molded components.
Process Influence
Precision in the switchover point between the injection and holding phases is the primary driver of weight consistency. Variations in the cushion size or screw recovery can alter the amount of polymer delivered, reducing the mass stability of the parts. When the cushion fluctuates, the pressure transmitted to the cavity changes, leading to shot-to-shot variation.
A constant non-return valve shut-off action is required to maintain this stability, as any leakage past the ring during injection will decrease the injected volume and cause the mass of the parts to drop.
Material Influence
Regrind usage alters the viscosity of the resin and degrades the weight consistency of the run. Because recycled polymer has experienced multiple thermal histories, its flow behavior is less predictable than that of virgin resin. This variability reduces the mass stability and leads to part rejection because of density changes.
Control Feedback
Monitoring the weight of molded parts in real time allows the system to detect process drift. If the mass stability drops below the statistical limit, the controller can adjust the pack pressure to compensate for the change. This feedback loop ensures that the parts remain within the dimensional specifications throughout the entire production run.
