Meaning
Molding process performance metrics track the cycle-to-cycle consistency of the mass of plastic injected into a mold cavity. This measurement is known as shot weight variance, which governs the stability of the polymer melt delivery. The metric does not measure the dimensions of the part or the rate of cooling.
Process Stability
Consistent cycle times and pressures are necessary to keep the weight of each molded part stable. A high shot weight variance indicates that the injection molding machine is experiencing fluctuations in melt temperature or screw performance. This instability can cause cosmetic defects like sink marks or flashing.
Tracking this metric allows process engineers to identify mechanical wear in the non-return valve.
Defect Detection
Weighing the molded parts at regular intervals provides an direct check of the molding process. When the shot weight variance exceeds the control limits, it usually means that some of the parts are either underfilled or overpacked. These parts must be quarantined for detailed inspection.
The cost of running the machine with high variance is the loss of material and the time spent sorting through suspected batches.
Control Strategy
Advanced closed-loop controllers adjust the injection parameters in real time to minimize weight fluctuations. By reducing shot weight variance, the factory can achieve a higher yield of compliant parts and reduce scrap. This control is particularly important in multi-cavity molds where even tiny variations in flow distribution can cause some cavities to produce bad parts while others produce good ones.
