How Do You Set Holding Time and Packing Pressure?

Last updated: 2026-09-01 · Reading time: about 4 min

Holding is the phase that decides whether a part holds its size: after fill, the press keeps pushing a small amount of melt into the cavity to replace what cooling shrinks away.
Holding time is set by one physical event — the moment the gate freezes — and holding pressure is set by the shrinkage the part is allowed; both are found by measurement, a part-weight curve and the defect list, not by default settings. Get the pair right and dimensions stay centered; get either wrong and the part pays in sinks, flash or stress.

Key facts

<>

  • Packing compensates shrinkage: melt injected during hold replaces the volume cooling takes away — the same shrinkage the shrinkage page describes, but answered at the press instead of in the steel.
  • Holding time has a hard ceiling: once the gate freezes, extra hold time does nothing but waste cycle — the gate-seal time, not the machine timer, defines the useful window.
  • Pressure and time carry different jobs: pressure sets how hard the compensation pushes (sinks versus flash versus stress), time sets how long the compensation lasts (dimensional stability versus wasted seconds).

    • Runner system — The channel network that delivers melt from the machine nozzle to each cavity. Cold runner: solidifies between shots, produces runner waste. Hot runner: stays molten, eliminates runner waste but adds $15k–$40k to mold cost. See also related guide.

    Cavity — The hollow space inside the mold that forms the part shape. Multi-cavity molds produce multiple parts per shot. Cavity count determines production output but also mold cost and complexity. Each cavity must be balanced for uniform fill. See also related guide.

    What does packing actually do — and when does it stop working?

    The sequence explains the limits. Fill brings the cavity to 95–99% full at velocity; the changeover to pressure drops the speed and holds a set pressure instead. During hold, the part cools and shrinks, and the press feeds fresh melt to make up the difference — but only while the path is open. The path is the gate: a small orifice that freezes early in a thin part and late in a thick one.
    While it is open, hold pressure packs material in and dimensions track the cavity; when it freezes, the part is hydraulically sealed and nothing more can enter. Hold time beyond seal is dead time on the clock — which is why gate size and cooling design set the packing budget as much as the process does.
    Pressure does its own work while the gate is open: too low and the last regions to fill starve, showing the classic sink marks over thick sections; too high and the part flashes at the parting line, sticks in the cavity or carries locked-in stress that shows up later as warp.

    How do you find the right hold time and pressure?

    Symptom Points to Adjustment
    Sinks over ribs and bosses Hold pressure too low or hold cut short before seal. Raise pressure in steps; verify hold time covers gate seal.
    Part weight climbs with hold time, then flattens The plateau is the seal point — weight stops responding. Set hold time just past the plateau start, not at maximum.
    Flash at the parting line Hold pressure above what the clamp and steel accept. Step pressure down until flash stops, then verify dimensions.
    Parts short near the gate but heavy elsewhere Overpack near the gate while the far end starves. Profile the hold — higher early, tapering later.
    Dimensions drift run to run Hold window sitting on the edge of the seal curve. Move hold time to the middle of the weight plateau.
    Sticking or ejection marks Overpack presses the part against the core. Reduce second-stage pressure; re-check sinks afterward.

    The weight curve is the instrument behind half of this table: mold a short series stepping hold time up in half-second increments, weigh each part, and plot the points.
    Weight rises while hold still feeds material, then flattens exactly at gate seal — the plateau’s start is the seal time, and the honest setting sits just past it, safely inside the flat region rather than balanced on its edge. Pressure finds its number the same way against defects: high enough that sinks stay inside tolerance, low enough that flash and sticking stay gone, with the working point recorded per the acceptance discipline.
    Parts with widely different wall sections may need a profiled hold — stronger pressure at the start, easing off as the gate narrows — which is a normal refinement, not a sign of trouble.

    Which packing checks belong in the record?

  • Weight curve on file — hold-time steps, part weights and the seal point marked, so the setting has evidence instead of habit.
  • Hold time inside the plateau — comfortably past seal but not lingering seconds that only cost cycle.
  • Pressure bounded by both defects — sink behavior and flash behavior both tested, setting placed between them.
  • Cushion and changeover stable — the switch to hold repeats shot to shot, or every downstream setting floats.
  • Critical dimensions measured at the working point — the tolerance targets verified where the process actually runs.
  • Settings archived with the process sheet — hold profile, seal time and pressure window recorded beside the mold number.

    • Frequently asked questions

    Does a bigger gate mean longer holding time?

    Yes — and that is by design more often than by accident.
    A larger gate stays open longer, so the useful hold window stretches and thick sections get more compensation. The trade shows up elsewhere: bigger gates leave larger vestige and extend degating work. Gate size, hold time and cosmetics are one conversation, not three.

    Can high hold pressure damage the part?

    It can, in two ways. Mechanically, overpack presses the part against the core and stresses it at ejection. Structurally, packed-in stress relaxes later — the part leaves the mold straight and warps over the following hours, a delayed defect that is hard to trace back to a pressure setting made days earlier. Bounding the pressure by flash alone misses this; the stress check is dimensional stability over time.

    Why do settings that worked last month produce sinks now?

    Because the seal point moved. A new resin lot with different flow, a worn gate, a changed melt temperature — any of these shifts when the gate freezes, and a hold window tuned to the old seal now ends early. The weight curve is the diagnostic: re-run it, and the plateau’s new position says where the setting should sit. This is exactly the drift behavior the lot-variation page describes, met at one parameter.

    Key takeaways

    1. Hold time is bounded by gate seal — found with a part-weight curve — and time beyond seal only wastes cycle.
    2. Hold pressure is bounded by two defect walls, sinks on one side and flash or stress on the other; the working point sits between.
    3. Both settings live or die on repeatability: a stable cushion, a documented window and a weight curve on file keep the part centered lot after lot.

    If sinks, flash or drifting dimensions are arguing about your hold settings, send the part and the current process sheet. You will get the weight-curve read and the pressure window named — by a named tooling engineer, before another lot runs on guesses.

    Sources
    ISO 20457:2018 — Plastics moulded parts: Tolerances and acceptance conditions, International Organization for Standardization
    Plastics Industry Association — industry resources on injection molding practice
    Autodesk Moldflow — injection molding simulation, reference for pack and hold phase analysis

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet