Gas-Assist Injection Moulding: Design, Trials and Acceptance

Updated: 2026-10-22

Gas-assist injection moulding forms hollow regions inside a moulded plastic part using controlled gas pressure. It is worth assessing when a handle, rail or frame needs a substantial section that is difficult to make as a uniform shell. It does not guarantee a sink-free surface, a fixed weight reduction or a lower price. Compare a defined gas-assisted design with realistic alternatives and an agreed acceptance plan.

How is the hollow region formed?

Nitrogen enters the melt through a controlled injection point and forms an internal cavity; WITTMANN's AIRMOULD overview describes this process. The gas maintains pressure during part of cooling and must be released safely before the mould opens. The equipment supplier and moulder must define the pressure sequence, interlocks and venting arrangements.

The proposed process may start with a partial or full plastic shot and use a different route for displaced melt. Ask the supplier to identify the variant, gas entry points and any overflow or melt-return arrangements. ENGEL's process overview distinguishes gas injection, water injection and foam moulding: these are different processes, with different equipment and part outcomes.

Which geometry should be reviewed before tooling?

Mark the intended hollow path on sectioned CAD, together with load paths, fasteners, sealing surfaces and visible faces. Ask how the gas will reach each branch, where it should stop and how the remaining wall will be checked. A universal channel-to-wall ratio cannot establish that the channel will form correctly in a particular grade and geometry.

Discuss abrupt section changes, junctions and thin regions with the tool designer. Record permissible gate and gas-pin marks, overflow removal, access for inspection and the areas that must remain solid. A hollow feature may interfere with a screw, insert or later drilling operation. If a sealed internal space is a requirement, specify it explicitly rather than assuming that a gas-assisted cavity is sealed.

Use flow analysis where it helps compare concepts, then verify the predicted channel in moulded samples. An acceptable external surface does not establish the internal wall distribution. Agree where to section sacrificial samples or whether another inspection method is appropriate. The inspection method needs enough resolution and access for the feature being accepted.

What should the trial prove?

Set limits for external dimensions, visible sink or witness marks, part mass, minimum remaining wall and functional load before the trial. These limits come from the application and engineering assessment. They are not standard values supplied by the process name. Record material batch, cavity and process revision with each result, including the relevant plastic-shot and gas-control settings.

Compare repeated parts and agreed start-up or restart conditions. Look for gas breaking through the surface, unintended branches, incomplete channels, local thin walls and changing channel ends. Do not judge repeatability from one attractive sample or accept part mass as a substitute for internal geometry inspection. Similar mass can conceal a different distribution of material.

Fictional acceptance example: two trial handles have the same measured mass and both look acceptable. Section inspection shows that one channel extends too close to a screw seat. Keep the trial on hold, investigate the flow and gas sequence, and repeat the dimensional and load checks after correction. The example does not set a permissible wall thickness or demonstrate a tested Nordmould product.

How do you compare cost and alternatives?

Quote the gas equipment, tool modifications, nitrogen, maintenance, trials and inspection alongside the resin and cycle assumptions. Include removal of overflows, rejects and downstream operations. Compare total expenditure per accepted finished part at the same quantity and duty. Weight, clamp-force or cycle benefits must be established for the proposed design; do not apply a generic savings percentage.

Compare a cored shell, two joined shells and other appropriate concepts against the same load, appearance, assembly and access requirements. A lighter part is not automatically strong enough, and a heavier conventional part may avoid specialised equipment. Confirm process availability and responsibility for the production cell in the written proposal.

Send sectioned CAD, the exact resin grade if selected, annual and lifetime quantities, visible faces, loads and inspection requirements for a project review. See the wall-thickness guide for conventional coring and the sample approval pack for recording dimensional checks and unresolved deviations.