Last updated: 2026-09-01 · 5 min read

What Is Decompression (Suck-Back) in Injection Molding — and When Should You Use It?

Decompression (also called suck-back) is a screw-retraction movement after plasticizing and before injection — the screw pulls back a small distance (typically 3–10 mm) to relieve pressure in the melt pool ahead of the check ring. The purpose is to prevent drool (melt leaking from the nozzle tip between shots) and to reduce the pressure in the runner system before the mold opens.
Decompression is useful for low-viscosity resins and open-tip nozzles but counterproductive for high-viscosity resins, valve-gated nozzles, and hot-runner systems — where it introduces air into the melt and causes splay, voids, or short shots.

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

  • Decompression is a screw-retraction movement — the screw rotates to plasticize the resin, then pulls back linearly (without rotation) by 3–10 mm. This creates a negative pressure in the melt pool, pulling the melt back from the nozzle tip and relieving the pressure that causes drool.
  • Decompression is not the same as suck-back during injection (which is a different parameter on some machines). Decompression happens after plasticizing and before injection; suck-back during injection is a holding-pressure release at the end of fill. The two parameters are independent.
  • Too much decompression introduces air into the melt pool. When the screw pulls back, the volume ahead of the check ring increases — if the nozzle is open, air can be drawn in through the nozzle orifice. This air becomes a bubble in the next shot, producing splay marks, voids, or short shots.

    • 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.

    When Decompression Is Needed

    Condition Decompression Needed? Reason
    Low-viscosity resin (PA, POM, TPU) yes (3–6 mm) low viscosity causes drool from open nozzle between shots
    High-viscosity resin (PC, PPS, PEEK) usually not needed high viscosity resists drool; decompression introduces air
    Open-tip nozzle yes (if drool occurs) melt can flow freely through open nozzle when pressurized
    Shut-off nozzle (valve-gate) no mechanical shut-off prevents drool without decompression
    Hot-runner system no hot runner maintains its own pressure balance; decompression disrupts it
    Cold-runner with sprue bushing yes (if stringing) decompression breaks the melt string when the mold opens

    How to Set Decompression Correctly

    1

    Start with zero decompression

    Run the first shots without decompression and observe the nozzle tip between shots. If there is no drool (melt leaking from the nozzle), decompression is not needed. If there is drool, proceed to step 2.

    2

    Add decompression in 2 mm increments

    Start at 2 mm screw retraction. Run 5 shots and observe the nozzle tip. If drool persists, increase to 4 mm. Continue in 2 mm increments until the drool stops. The minimum effective distance is the correct setting.

    3

    Verify no air entrapment

    After setting the decompression distance, inspect the molded parts for splay marks (silver streaks on the surface), internal voids (visible in transparent parts or by cross-sectioning opaque parts), or short shots. If any of these appear, the decompression is drawing air into the melt — reduce the distance by 2 mm and re-check.

    4

    Set decompression speed

    Decompression speed (screw retraction speed) should be slow — typically 5–15 mm/s. Fast decompression (50+ mm/s) creates a sudden pressure drop that can vaporize dissolved gasses in the melt (cavitation) or draw air through the nozzle orifice. Slow retraction relieves pressure without creating a vacuum.

    What Happens When Decompression Is Wrong

  • Too little decompression — drool continues. The melt that leaks from the nozzle between shots solidifies on the nozzle tip and is pushed into the next shot as a cold slug.
    The cold slug creates a visible defect (a circular mark or a streak) on the part surface, typically near the gate.
  • Too much decompression — air enters the melt pool through the nozzle orifice (on open-tip nozzles) or through the gap between the check ring and the barrel (on worn check rings).
    The air becomes a bubble in the melt, producing splay marks on the part surface, internal voids in thick sections, or short shots if the air volume is large enough to displace the melt.
  • Decompression speed too fast — cavitation occurs. The sudden pressure drop causes dissolved gasses in the melt (moisture, monomers, degradation byproducts) to form bubbles inside the barrel.
    These bubbles are injected into the cavity and produce voids or splay marks that look identical to moisture-related splay — but the root cause is decompression, not drying.
  • Decompression on a hot-runner system — the hot runner maintains a specific pressure balance in the manifold and nozzles. Decompression from the machine barrel disrupts this balance, pulling melt back from the hot-runner nozzles and creating a void in the manifold.
    The next shot fills the void with degraded material (melt that sat in the manifold at processing temperature for an extended time) and produces a burn mark or a discoloration at the gate.

    • Frequently Asked Questions

    Is decompression the same as back pressure?

    No.
    Back pressure is the resistance applied to the screw during plasticizing (rotation) — it controls melt density, mixing quality, and color dispersion. Decompression is the screw retraction after plasticizing — it relieves pressure in the melt pool before injection. Back pressure acts during rotation; decompression acts after rotation. They are independent parameters that affect different aspects of melt quality.

    Does decompression affect cycle time?

    Marginally. The decompression movement itself takes 0.2–0.5 seconds, which adds to the overall cycle time. However, the main cycle-time impact is indirect: if decompression eliminates drool, the operator does not need to purge the nozzle between shots (which takes 5–15 seconds). If decompression is set incorrectly and causes splay or voids, the reject rate increases, which reduces effective output (good parts per hour) even though the cycle time per shot does not change.

    Can decompression replace proper nozzle selection?

    No. Decompression is a process adjustment; nozzle selection is a hardware decision. If a low-viscosity resin (PA, POM) consistently drools from an open-tip nozzle, the correct fix is to install a shut-off nozzle (spring-loaded or pneumatically actuated) that mechanically prevents melt flow between shots. Decompression is a workaround that masks the drool symptom; a shut-off nozzle eliminates the root cause. For production molds running 24/7, the shut-off nozzle is the more reliable solution.

    Key Takeaways

    1. Decompression (suck-back) is a screw-retraction movement after plasticizing that relieves melt-pool pressure to prevent drool. It is needed for low-viscosity resins with open-tip nozzles and is counterproductive for high-viscosity resins, shut-off nozzles, and hot-runner systems.
    2. The correct decompression distance is the minimum that stops drool — start at zero, add 2 mm increments until the drool stops, then verify that no air-related defects (splay, voids, short shots) appear. More decompression is not better — it introduces air.
    3. Decompression speed must be slow (5–15 mm/s). Fast retraction causes cavitation (dissolved gasses forming bubbles) that produces defects indistinguishable from moisture-related splay. Before increasing drying time, check the decompression settings.

    A process engineer troubleshooting splay marks or voids should check the decompression setting before assuming that the resin needs more drying. Decompression-induced air entrapment produces the same visual defect as insufficient drying — and the fix (reduce decompression distance or speed) takes 2 minutes, while adding drying time adds 2–4 hours to the preparation cycle.

    Sources
    Plastics Technology — injection process parameters
    Beaumont Technologies — process optimization
    RJG Inc — scientific molding training

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