Hold Pressure
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Definition of Hold Pressure
Hold pressure (often called pack pressure) applies after the cavity is volumetrically filled. It pushes additional molten material into the mold to offset shrinkage as the plastic cools and solidifies, helping maintain dimensions and reduce sink marks.
Hold time must be long enough for the gate to freeze off so material cannot back-flow out of the cavity. Too little hold leads to sinks and size variation; too much can overpack, increase stress, or worsen flash at weak shut-offs.
Why It Matters for Plastic Injection Molding
Pack and hold settings strongly influence part weight, dimensions, and internal density. AMP validates hold profiles during process qualification so production stays within customer tolerance bands.
Adequate hold reduces voids and sink marks behind ribs and bosses. For glass-filled or semi-crystalline materials, hold windows are tighter and require careful tuning with cooling and gate freeze time.