Last updated: 2026-09-02 · 5 min read

How Does Gas-Assisted Injection Molding Work — and When Should You Use It?

Gas-assisted injection molding injects nitrogen gas into the melt after partial cavity fill — the gas pushes the melt to the mold walls and creates a hollow core, producing thick-looking parts with less material, lower clamp force and reduced sink — and it is used for handles, beams, housings and any part where thick sections would otherwise sink or require excessive packing. The process adds equipment cost (gas injection unit, gas nozzles) but reduces per-part material and cycle time for the right geometry.

Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. See also related guide.

Key Facts

  • The gas (nitrogen, generated on-site) enters through nozzles in the mold and pushes the melt to fill the cavity — the gas channel forms in the thickest section, creating a hollow core with solid outer walls.
  • Material savings: 20–40% less resin per part compared to solid molding of the same geometry — the hollow core replaces material that would have been packed solid.
  • Clamp force reduction: the hollow part has lower overall volume, so the injection pressure and required clamp force drop — a part that needs 500 tons solid may need only 300 tons with gas assist.

    • Shrinkage — The dimensional reduction from mold cavity to cooled part. Amorphous resins: 0.4–0.7% (uniform). Semi-crystalline: 1.0–2.5% (directional). Mold makers apply shrinkage factor when machining cavity. Actual shrinkage varies with wall thickness, gate location and packing pressure. The related guide covers adjacent topics.

    Where Gas Assist Works and Where It Doesn’t

    Application Suitability Reason
    Handles, beams, tubes Excellent Long flow paths with uniform thick sections; gas channels follow the part axis
    Housings with ribs Good Gas follows ribs, eliminating sink on the cosmetic surface
    Thin-wall parts (under 2mm) Poor Walls too thin for gas channels; gas breaks through the skin
    Optical or cosmetic surfaces Risky Gas finger marks (flow lines) may appear on the surface

    Design Rules for Gas-Assisted Parts

  • Gas channel must follow a continuous thick path from the gas inlet to the overflow — the gas takes the path of least resistance, which is the thickest section. Discontinuous thick sections cause the gas to break through the skin.
  • Wall thickness ratio: the gas channel diameter should be 2–3× the nominal wall — too small and the gas cannot penetrate; too large and the gas channel is visible on the surface.
  • Gate location: the melt must partially fill the cavity before gas injection — the gate is placed so the short shot fills 70–95% of the cavity, then the gas completes the fill and packs the part.
  • Overflow well: the gas pushes melt ahead of it into an overflow well (not the part) — the well captures the excess material and prevents gas from escaping through the vent.

    • What This Means for Your Quotation

    A gas-assist quotation includes the gas injection unit cost, the nitrogen generator, the gas nozzle locations and the expected material savings per part. The DFM review confirms whether the part geometry supports gas channels and where the nozzles are placed. When the material savings and cycle reduction exceed the equipment cost, gas assist lowers the total program cost; when the geometry is unsuitable, the quotation should recommend standard molding instead.

    Frequently Asked Questions

    Does gas assist weaken the part?

    No — the hollow core has solid outer walls that carry the structural load. The part’s stiffness-to-weight ratio is actually higher than a solid part of the same outer dimensions, because the material is concentrated at the outer surface where bending stress is highest. The hollow core does not reduce structural performance for most applications.

    What resins work with gas assist?

    Most thermoplastics work: PP, ABS, PA, PBT, PC and their glass-filled grades. The key requirement is a melt strength that holds the skin while the gas penetrates the core — most engineering and commodity resins meet this. Highly filled resins (over 40% mineral) can be abrasive on gas nozzles and require hardened nozzle tips.

    How do you control the gas channel inside the part?

    Gas pressure, delay time and melt short-shot percentage control the channel. Higher gas pressure pushes the gas further; longer delay (more melt in the cavity) reduces penetration; higher short-shot (more melt) reduces the hollow volume. The process window is established at trial and documented in the trial report with the gas pressure curve.

    Key Takeaways

    1. Gas-assist molding injects nitrogen into the melt to create a hollow core — saving 20–40% material, reducing clamp force and eliminating sink in thick sections.
    2. The part must have a continuous thick path for the gas channel; thin walls, optical surfaces and discontinuous sections are unsuitable.
    3. The quotation should include gas equipment cost, nozzle locations and expected material savings — so the decision is based on total program cost, not just tooling price.

    Send aCatMould the part drawing, and the DFM review confirms whether gas assist is suitable — with gas channel layout, nozzle locations and material savings projection in the quotation, so the process decision is based on verified geometry, not assumption.

    Sources
    Plastics Technology — gas-assisted injection molding
    SPE — gas assist design guidelines
    MoldMaking Technology — gas assist mold design

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