Last updated: 2026-09-04 · 5 min read
Gas-Assisted Injection Molding — How Does It Work, What Parts Benefit from It, and When Is the Additional Tooling Cost Justified?
Gas-assisted injection molding (GAIM) injects nitrogen into the melt after initial fill — the gas pushes melt to the cavity walls and creates a hollow channel inside the part. The result: 20–40% lighter, no sink marks on thick sections, and gas pressure packs from the inside where melt cannot . The process does not change the geometry — it changes how pressure is delivered. Investment: $30,000–70,000; justified above 100,000 shots per year.
Key Facts
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Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.
How Does the Gas-Assisted Process Work?
Two variants, each solving a different surface-quality problem:
| Variant | How It Works | When to Use |
|---|---|---|
| Short-shot GAIM | Inject 70–85% of cavity volume with melt, then gas pushes melt to walls and fills remaining 15–30% | Thick structural parts — handles, rails, tubes — where the gas channel is internal and unseen |
| Full-shot GAIM | Inject 100% of cavity volume with melt, then gas creates the hollow channel and packs the part | Parts with cosmetic surfaces where gas must not break through the skin |
In both variants, the gas is injected through a gas injection nozzle (GIN) or a gas pin — a small pin in the cavity that releases gas into the melt. The gas follows the path of least resistance: the hottest, thickest section of the part. It creates a hollow channel along that path. The gas pressure is maintained until the part solidifies, then vented and the part ejected. The hollow channel is internal — the outer surface looks identical to a solid part.
The key physics: the gas replaces packing pressure with internal pressure that reaches where melt pressure cannot. In a thick section, the core stays molten longest — conventional packing pressure drops off as the flow path freezes. Gas pressure, injected directly into the core, continues packing from the inside. That is why GAIM eliminates sink marks: the gas holds pressure on the core until it solidifies, not just until the flow path freezes.
What Parts Benefit and When Is the Investment Justified?
Three steps to decide:
- Check the geometry. Parts with thick sections (6mm+): handles, rails, tubes, structural ribs. Parts with varying wall thickness — the gas follows the thick section naturally, packing areas that would otherwise sink. Long flow-length parts — the gas pushes melt to the end of the cavity, reducing injection pressure and allowing molding of long parts (e.g., 600mm handles) that would otherwise require very high pressure or multiple gates.
- Check what to exclude. Thin-walled parts below 3mm — the gas breaks through the skin. Optical parts — the hollow channel disrupts light transmission. Parts with tight dimensional tolerance on internal dimensions — the hollow channel size varies with process conditions. Medical or food-contact parts where the internal surface cannot be inspected.
- Price the investment honestly. Gas injection unit: $25,000–60,000. Mold modifications (gas channels, venting): $2,000–8,000. GIN nozzles: $500–2,000 each, 1–4 per mold. Nitrogen per shot: $0.01–0.05 — negligible. Total: $30,000–70,000. The savings: 20–40% material reduction, 10–30% cycle time reduction on thick parts. At 100,000+ shots per year, the savings pay for the system within 12–18 months. Below 10,000 shots, the investment is rarely justified.
| Component | Cost | Notes |
|---|---|---|
| Gas injection unit | $25,000–60,000 | Retrofittable to existing machines; nitrogen generator or bottled nitrogen |
| GIN nozzle | $500–2,000 each | 1–4 per mold, installed at nozzle location or in runner |
| Gas pin | $300–1,000 each | 1–4 per mold, installed in cavity when GIN cannot be used |
| Mold modifications | $2,000–8,000 | Gas channels to guide the gas path, venting at gas exit points |
| Nitrogen per shot | $0.01–0.05 | Generator pays for itself in 6–12 months at high volume |
Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.
Gas-assist does not change the part geometry — it changes how pressure is delivered. The gas packs from the inside, reaching where melt pressure cannot.
Frequently Asked Questions
Does gas-assist leave visible marks on the part?
The GIN leaves a small mark (2–3mm) where gas enters the melt — typically on a non-cosmetic surface inside a rib or on the back. The gas exit point also leaves a small mark. With careful design, both marks can be hidden on non-visible surfaces. The gas pin leaves a smaller mark (1mm) suitable for semi-cosmetic surfaces.
Full-shot GAIM works on parts with one cosmetic surface — the gas channel is internal, and the cosmetic surface shows no marks. But GAIM is not recommended for A-A class parts with two cosmetic surfaces — the gas channel may create a visible witness line on the opposite side.
What materials work best with gas-assist?
Most thermoplastics work: PP, ABS, PC, PC/ABS, PA, POM and PBT. The key requirement is a frozen skin layer — the outer surface must solidify before the gas breaks through. Semi-crystalline materials (PP, PA, POM) form a strong skin quickly and work well. Amorphous materials (ABS, PC) also work but require higher gas pressure.
Difficult materials: highly filled resins (GF50+ — glass fibers block the gas channel), TPE/TPU (too soft — gas breaks through the skin), and high-temperature resins like PEEK or PSU (gas pressure cannot penetrate the high-viscosity melt). If the part can be molded in an unfilled or lightly filled grade, GAIM is viable.
Does gas-assist really reduce cycle time?
Yes — 10–30% on thick-section parts. The hollow channel reduces the amount of material that must cool. For a 10mm thick handle: cycle time drops from 45 seconds (solid) to 30 seconds (hollow) — a 33% reduction. For a 6mm rib: from 25 seconds to 20 seconds — 20% reduction.
The reduction is most significant where cooling dominates the cycle: thick sections and slow-crystallizing materials. On thin-walled parts below 3mm, the reduction is negligible — the part cools quickly regardless. The cycle time saving, combined with material reduction, is what makes GAIM pay for itself at high volume.
Key Takeaways
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Gas-assist injection molding is a proven process for thick-section parts where sink marks, weight and cycle time are the primary concerns.
The investment is significant but justified at high volumes where material savings and cycle time reduction pay for the system within 12–18 months. If your part has thick sections above 6mm and annual volumes above 100,000 shots, GAIM is worth evaluating during the DFM phase.
Sources
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Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet