Gas-Assist Injection Molding — How Does Hollowing a Part with Nitrogen Save Weight, Remove Sinks and Cut Clamping Force?

Last updated: 2026-09-04 · Reading time: about 6 min

Gas-assist injection molding injects nitrogen into the melt after partial fill. The gas follows the path of least resistance — the thickest section, the hottest core — and hollows it out, pushing material into under-filled areas. Weight reduction: 10–25 %; sink elimination on thick ribs; 20–40 % lower clamping force because the hollow section has less projected area. Added mold complexity: gas channels, gas pins and a nitrogen delivery system.
See also related article.

Key facts

Gas-assist works because nitrogen follows the hottest, thickest section of the flow path. The gas displaces melt from the core outward, creating a hollow channel with a solid skin — the same geometry that resists bending but at lower weight.

Weight reduction of 10–25 % is typical on ribbed housings and handles. The hollow section also removes the thermal mass that causes sink marks on thick ribs, because there is no solid core to shrink and pull the surface inward.

Lower clamping force follows from lower projected area: a hollow section displaces less melt, so the cavity pressure during packing drops. On a large part, this can free up press capacity or allow the part to run on a smaller machine.

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.

How does nitrogen hollow out the part?

Four stages, each one controlled by a timing and pressure parameter.

Stage	What Happens	Key Parameter
Short shot (melt injection)	Melt fills 70–95 % of the cavity — deliberately not 100 %	Injection speed and volume; the remaining space is for gas to push into
Gas injection	Nitrogen enters through a gas pin or nozzle and follows the hottest, thickest flow path	Gas pressure (typically 5–30 MPa) and delay time between melt and gas
Gas hold / packing	Gas maintains pressure while the skin freezes; the hollow channel forms	Hold pressure and hold time — must keep gas active until the skin is rigid
Gas exhaust / vent	Gas is released before the mold opens; the part is ejected with a hollow core	Exhaust timing — too early and the channel collapses; too late and cycle lengthens

Gas-assist injection molding (GIT). A process in which nitrogen gas is injected into the melt after a partial fill, creating a hollow channel inside thick sections. The gas follows the path of least resistance — the hottest, thickest area — and pushes excess melt into under-filled regions. The result is a part with a solid skin and hollow core: lighter, sink-free on thick sections, and requiring less clamping force than a solid equivalent.

Weight, sink and clamping force all stem from the same mechanism: removing solid material from the core while keeping a rigid skin. A thick rib that would sink becomes a hollow rib with a frozen skin that cannot shrink inward. A heavy handle needing 300 tonnes of clamping force becomes one needing 200 tonnes. Less solid material means less mass, less shrinkage and less pressure on the mold walls.

How do you set the process window?

Four parameters, each one linked to a specific defect if set wrong.

Set the short-shot size first. The melt must fill enough to establish the flow path but leave room for the gas to penetrate. A typical short shot is 70–95 % of the full cavity volume. Too much melt (over 95 %) and the gas cannot penetrate — the part fills solid and you lose the weight benefit. Too little melt (under 70 %) and the gas breaks through the skin or fingers into thin sections, causing blow-through or gas marks on the surface.
Set the gas delay to control where penetration starts. The delay is the time between the end of melt injection and the start of gas injection. A short delay means the melt is still hot and fluid — the gas penetrates quickly and deeply. A long delay lets the skin thicken — the gas penetrates less and may not the end of the channel. The correct delay gives full penetration without breakthrough: the gas reaches the end of the thick section but does not escape through the skin.
Set gas pressure to match the section thickness. Thicker sections need higher pressure to push the melt forward and form a clean channel. Too low a pressure and the gas does not fully penetrate — the channel is short and the weight savings are incomplete. Too high a pressure and the gas cuts through the skin or creates an irregular, oversized channel. Start from the resin supplier’s recommendation and adjust at T1 based on the actual channel cross-section.
Set gas hold time to keep pressure until the skin is rigid. The gas must maintain pressure while the outer skin freezes solid. If the gas is vented too early, the hollow channel is still soft inside and the skin collapses inward — a sink-like defect on the surface. If the gas is held too long, the cycle lengthens without benefit. The correct hold time is the minimum that keeps the channel pressurized until the skin temperature drops below the resin’s heat-deflection point.

“Gas-assist is not a trick to remove weight — it is a process that changes the internal geometry. The hollow channel is the product; the weight saving, the sink removal and the clamping-force reduction are what the hollow geometry gives you for free.”

— Tooling Engineer, aCatMould

Frequently asked questions

Key parameter identified — critical variable confirmed against spec.

Tolerance within achievable range — commercial ±0.15mm or tighter as needed.

Material matched to application — resin grade selected for end-use environment.

Process window validated at T1 — parameters locked after first trial.

Inspection method defined — CMM, visual, or functional test specified.

Does gas-assist work on every part geometry?

No — it works best on parts with a clear thick-to-thin transition, where the gas can follow a defined channel.
Ribbed housings, handles, flat panels with reinforcing ribs and thick boss features are the standard applications. Parts with uniform thin walls have no thick path for the gas to follow, so there is nothing to hollow. Parts with complex, intersecting thick sections can be run but require careful gas-pin placement and often multiple gas injection points to ensure full, controlled penetration.

The design rule is: if the part has a thick section that would otherwise sink or require excessive packing, gas-assist is worth considering. If the walls are uniform and thin, the process adds cost without benefit.

How much weight can gas-assist actually save?

On a typical ribbed housing or handle, 10–25 % weight reduction is achievable. On a very thick, heavily ribbed part with large bosses, savings can 30 %. The actual number depends on how much of the part volume is in the thick sections that the gas can penetrate — a part with thin walls and a few thick ribs saves less than a part where most of the cross-section is thick.

The weight saving also depends on the short-shot percentage. A 90 % short shot leaves less room for gas penetration than a 75 % shot, so the hollow channel is smaller and the weight saving is less. The optimum is the largest hollow channel that does not cause surface defects or structural weakness.

What defects are unique to gas-assist?

Three defects are specific to the process. Gas blow-through: nitrogen breaks through the surface skin, usually from too small a short shot or too high gas pressure. Internal fingering: gas penetrates thin sections where it should not go, from incorrect gas-pin placement or too long a delay. Skin sink-after: the hollow channel collapses after venting because the skin was not rigid enough — from too short a hold time or too high melt temperature.

All three are fixable by adjusting the four parameters — short shot, delay, pressure and hold time — in combination. A mold-flow analysis predicts the gas path before the tool is cut, which is why gas-assist DFM should always include a simulation step.

Key takeaways

Gas-assist hollows the thick core with nitrogen — the same geometry that resists bending, at lower weight, with no sink on the surface and less clamping force required.

Set four parameters: short-shot size (70–95 %), gas delay (controls penetration start), gas pressure (matches section thickness), and gas hold time (keeps pressure until skin is rigid).

Gas-assist works on parts with thick-to-thin transitions — ribbed housings, handles, thick bosses.
Uniform thin walls have no path for the gas and gain no benefit.

Gas-assist is not a shortcut — it is a geometry change. The hollow channel is the product; the weight saving, the sink removal and the clamping-force reduction are what the hollow geometry delivers. Set the four parameters from the resin data sheet and the mold-flow prediction, then fine-tune at T1 — and the part comes out lighter, cleaner and on a smaller press.