In the injection cycle of small injection molding machines, the actual pressure exerted must exceed the pressure necessary to fill the mold cavity. The pressure regulated by the mold increases rapidly, ultimately reaching a maximum level known as injection pressure. It is important to note that injection pressure is greater than filling pressure.
The function of holding pressure in small injection molding machines includes the following: Once the mold cavity is filled with plastic, and until the nozzle has completely cooled, a significant amount of holding pressure is required to maintain the plastic in the cavity. The key roles include:
Supplementing Material Near the Nozzle: Prior to the solidification and closure of the nozzle, holding pressure helps prevent the unhardened plastic from flowing back towards the material source due to residual pressure, thereby ensuring there is enough plastic in the cavity.
Minimizing Product Shrinkage and Voids: Holding pressure plays a critical role in reducing vacuum bubbles and lessening the potential for mold adhesion or warping caused by excessive injection pressure. As a result, holding pressure is typically set at around 50% to 60% of the injection pressure.
It’s essential to recognize that if the holding pressure or duration is extended too long, it may force cold material from the nozzle and runner into the final product, leading to cold streaks near the nozzle. Furthermore, this can unnecessarily lengthen the injection cycle time of the small injection molding machine.
