Injection Pressure and Holding Pressure in the Molding Cycle

Injection pressure

The pressure actually applied during injection is higher than the pressure needed to fill the cavity. Cavity pressure rises sharply to a peak, and that peak is what is usually called injection pressure.

Holding pressure

After the cavity is filled and until the gate has frozen, the plastic in the cavity still needs substantial pressure — this is holding pressure. It:

• Packs additional material near the gate and prevents unsolidified material from flowing back toward the gate under residual pressure before the gate freezes.
• Prevents shrinkage and sink marks and reduces voids.
• Reduces sticking, cracking or warping caused by excessive injection pressure.

Holding pressure is therefore usually set at 50–60% of injection pressure. If holding pressure is too high or held too long, cold material from the gate and runner can be pushed into the part, leaving bright cold-slug marks near the gate and needlessly lengthening the cycle.