Last updated: 2026-09-02 · 5 min read

What Is Cushion in Injection Molding — and Why Does a Stable One Stabilize Weight?

Cushion is the small pad of melt — typically two to six millimeters of screw stroke — left in the barrel at the end of injection so the machine always has packing pressure in reserve. A stable cushion means the screw finishes each shot in the same place, which means every cavity receives the same compressed volume of melt, which means part weight holds. When cushion wanders, shot weight wanders with it, and every downstream check gets harder.

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.

Key Facts

  • What cushion actually does: during packing, hydraulic or electric pressure acts through that remaining melt pad to feed the part as it cools and shrinks. Zero cushion means pressure decays the moment injection ends; too much cushion means a lazy melt column that cools, builds degradation risk, and weakens pressure transmission.
  • The stability chain: consistent shot size (metering stroke) creates a consistent cushion, which creates consistent packing, which delivers consistent part weight and dimensions. Cushion variation is one of the first signals that something upstream — check ring wear, melt temperature drift, resin moisture — has changed.
  • How it is monitored: cushion is read in millimeters (or inches) at the end of each shot, and its variation shot-to-shot is tracked in process monitoring. A cushion that drifts in one direction points to machine wear; one that jumps shot-to-shot points to resin or check-ring trouble.

    • 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. The related guide covers adjacent topics.

    Where Cushion Problems Point

    Cushion Signature Likely Cause What to Check
    Steady drift toward zero over hours Leaking check ring or worn screw flight Non-return valve condition; recovery consistency
    Jumps between consecutive shots Resin feed or drying inconsistency Moisture reading, regrind ratio, feed throat bridging
    Different value on the same tool across presses Setup transfer without process data Melt viscosity state, transfer documentation
    Cushion present but weight still varies Pressure transmission or gate issue Hold pressure profile, gate land condition, venting

    Setting and Holding a Stable Cushion

    1

    Size the shot honestly

    The total shot weight should sit comfortably inside the machine’s usable shot capacity — heavily undersized shots make metering unstable and cushion control coarse. For multi-cavity tools, the balanced fill is verified first; cushion discipline comes after the fill is balanced, not instead of it.

    2

    Transfer to position control

    Once the part fills and packs correctly, lock injection and packing to position or fixed stroke so the screw terminates at a defined point every cycle. The cushion value then becomes a monitored output — the record of how consistently the melt system delivers.

    3

    Watch the trend, not the single shot

    Plot cushion over time. A flat line is the goal; a slope is maintenance information; noise is process information. Setting an alarm band around the cushion catches check-ring wear weeks before it shows up as scrapped parts or short shots.

    4

    Document it in the setup sheet

    Cushion setpoint, observed range, and the trend at sign-off belong in the process sheet alongside melt temperature and hold profile. When the same tool runs on another press, the cushion record is what makes the transfer an engineering exercise instead of a fresh search.

    What a Stable Cushion Buys You

  • Weight control: shot-to-shot weight variation tightens visibly once cushion holds steady — the first quality metric most auditors pick up.
  • Dimensional control: packing consistency feeds directly into dimensions that depend on pressure, especially around bosses and thick sections.
  • Early warning: cushion is often the first signal to move when the check ring or the resin changes, before defects the parts.
  • Faster tool transfer: a cushion record plus its trend lets a qualified process be rebuilt on another machine in hours, not days.
  • Cleaner defect conversations: with cushion stable, flash and short-shot arguments move from “machine vs mold” to evidence, because one major variable is closed.

    • Frequently Asked Questions

    What cushion value should I run?

    Enough to transmit packing pressure for the full hold time — commonly a few millimeters on small shots, larger on big ones — but small enough that the melt column does not sit hot in the barrel longer than necessary.
    The right answer for a specific tool comes from the pressure-and-part-weight response during process setup, recorded with the setup sheet.

    Why does cushion collapse toward zero over a run?

    A leaking non-return valve is the classic cause: it fails to trap the full metered volume, so the screw lands closer and closer to the forward position. Recovery stroke grows to compensate, weight holds for a while through the growing stroke, and then control is lost. The fix is valve service, not more position setpoint.

    Is cushion relevant on electric machines?

    Yes — the concept is identical and the control is typically tighter, which makes the cushion reading even more useful as a diagnostic: when an electric machine’s cushion drifts, the suspect list narrows quickly to the melt system or the resin, because position repeatability is not the weak link.

    Key Takeaways

    1. Cushion is the packing reserve: a small, deliberate melt pad that lets pressure keep feeding the part through the hold phase.
    2. Stable cushion is the chain link between metering consistency and part weight consistency — track its trend, not just its value.
    3. Drifting cushion diagnoses machine wear; jumping cushion diagnoses resin or feed trouble; a documented cushion record is what makes tool transfers predictable.

    Ask for cushion setpoint, observed range and trend in every trial and production process sheet — aCatMould delivers them with each process report, because a cushion that is documented is a variable that never has to be argued about later.

    Sources
    Plastics Technology — process setup and machine control
    Kistler — process monitoring in injection molding
    Husky — injection molding systems guidance

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