Material Selection for Gas Assisted Injection Molding

Material Guide · Gas Assist Injection · gasassistinjection.com

Gas-assisted injection molding hollows out thick sections from the inside. That simple idea makes the polymer choice more important than in conventional molding: the material has to fill the part, hold a stable skin while gas pushes through, and keep enough stiffness once the core is hollowed. This guide explains what to look for and which materials work best.

Why material matters more in GAIM

In GAIM, the gas pushes the still-molten core aside to form the hollow channel. The material therefore has to do three jobs at once:

1. Fill the cavity well enough that gas only has to move a mobile, predictable melt front.
2. Hold a skin that resists blow-through while the gas displaces the core.
3. Carry the load — because the core is removed, the remaining skin and ribs must be stiff enough.

A material that is great in conventional molding can be frustrating in GAIM, and vice versa. Choosing the grade is not a footnote — it is a design decision.

Key material properties to evaluate

Recommended material families

These are the families we most often mold with GAIM. Within each family, the exact grade still needs to be checked for melt strength, shrinkage and modulus before committing the tooling.

Materials to handle with care

• High-clarity / optical grades — a hollow channel can disturb light path or optics. Evaluate carefully; gas assist is not a default for clear parts.
• Brittle or low-melt-strength resins — the gas can break through the skin, causing blow-through or surface defects.
• Very high-viscosity or heat-sensitive resins — narrower processing window makes gas timing and channel control harder.
• Heavily filled grades — fillers change flow and shrinkage; the hollow channel behaves differently and warp risk needs review.

Fillers, additives & color

• Glass / talc fillers raise stiffness and reduce shrinkage, which often helps GAIM — but they change flow and can increase wear on gas pins and gates.
• Impact modifiers improve toughness but can narrow the processing window slightly.
• Colorants are generally fine; the important thing is to specify the exact filled/unfilled grade so shrinkage and flow are known.

Whatever the recipe, the grade must be defined up front — a GAIM mold is designed around the material’s flow and shrinkage.

How to choose — a quick checklist

Run your candidate material through these questions before locking the tooling.

Bottom line

The best GAIM materials combine good flow, stable melt strength, predictable shrinkage and enough stiffness to survive hollowing. PP, ABS, PC, PC/ABS, glass-filled PA, PBT, PET and HDPE are the families we mold most often. Clarity grades, brittle resins and heavily filled materials deserve a feasibility check before tooling is cut.

At Injection assistée par gaz, we recommend the material as part of the project evaluation — not in isolation — so flow, warp, cost and the gas channel are decided together.