| Injection molding of thermoplastics is widely used nowadays for automotive, aerospace, electrical and consumer goods. Tight tolerances in part dimensions and part weight are among the important requirements in these competitive industries. Accurate control of the injection molding process and its parameters is needed to achieve tight tolerance products. The gate freeze off time is one of the most important process control parameter in injection molding. The present study investigated the phenomenon of the gate freeze off during the process with the objective of characterizing the external indicator of reciprocating screw displacement, with the objective of application in a future process control system. The study, using High Impact Polystyrene (an amorphous polymer), developed information for a future adaptive process control system, concerning the instant at which the gate actually freezes.;The gate freeze process was modeled using a simple, first approximation, stepwise procedure, to estimate part weight change as a function of time and temperature during the holding phase as the gate freeze progressed. Measured part weights from the experiments were compared to predicted values. The study showed reasonable agreement in the comparisons. The study showed that a holding time longer than the gate freeze over packs the sprue and the gate which is a waste of material, cycle time and energy. The model, although a simple one, provided evidence of possible utility of the use of screw velocity as an external indicator for future process control applications. |
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