How the Injection Molding Process Differs: Gas-Assisted vs. Conventional

You already know that gas‑assisted injection molding (GAIM) can eliminate sink marks and reduce weight. But how does the actual process differ from conventional injection molding?

If you put a standard molding machine operator in front of a gas‑assisted job without telling them, they'd likely produce a pile of scrap. The reason? The process steps, parameters, and machine setup are fundamentally different.

Here are six critical differences in how the two processes run on the shop floor.

1. Fill Volume: Full vs. Partial

This is the most basic distinction.

In conventional molding, you shoot until the mold is full. In gas‑assist, you intentionally leave empty space — that space becomes the gas channel.

Tolerance challenge: The shot volume in GAIM must be controlled to within ±0.5–1%, because too much plastic blocks the gas path, and too little leaves unfilled areas.

2. Pressurization: Screw vs. Gas

Conventional molding uses the injection screw to push more material into the cavity during packing. GAIM injects high‑pressure nitrogen after the screw stops. The gas pushes the melt against the cavity walls and creates the hollow core.

Gas pressure range: Typically 10–30 MPa (1500–4500 psi), often with multiple pressure stages.

3. Holding/Packing Phase: Long vs. None

This is where GAIM saves significant cycle time. The gas acts as a perfect pressure‑transmitting medium, pushing uniformly from the inside out.

4. Gas Injection Timing: A Critical New Parameter

Conventional molding has no gas timing. GAIM has a gas delay – the time between screw injection and gas injection.

• Too short (0–0.1 sec): Gas fingers through the melt, causing blow‑through or thin spots.

• Too long (>2 sec): The melt skin freezes, and gas cannot penetrate — no hollow channel forms.

Typical delay: 0.2–1.5 seconds, depending on wall thickness and material.

5. Cooling Time: Thick Section vs. Hollow Section

Example – a 10 mm thick handle:

• Conventional: cools like a 10 mm solid block → long cooling time

• GAIM: internal gas channel leaves only ~3–4 mm of solid plastic → 20–50% shorter cooling time

6. End of Cycle: Vent vs. No Vent

GAIM systems have a gas‑release stage where the internal gas pressure is reduced to near atmospheric. If you skip this, opening the mold creates a dangerous pressure burst and a noisy pop.

Some advanced systems recover the nitrogen for reuse.

Quick Comparison Table: Process Parameters

What Equipment Do You Need to Switch?

Conventional molding only needs a standard injection molding machine.

Gas‑assisted molding requires:

1. A standard injection molding machine (can be the same one)

2. Gas control unit (GCU) – generates high‑pressure nitrogen and controls timing/pressure stages

3. Nitrogen source – membrane generator (preferred) or bottled gas

4. Modified mold with:

   • Gas pin(s) at thick sections

   • Sealed parting lines (gas leaks ruin the process)

   • Gas vent / escape at the end of fill

5. Precise shot volume control – typically a screw with closed‑loop stroke control

A Simple Analogy

• Conventional molding = completely filling a balloon with water, then squeezing it.

• Gas‑assisted molding = putting a little water in the balloon, then blowing air into it — the air pushes the water to the walls, creating a hollow center.

Why Does This Matter to You?

Understanding the process differences helps you:

• Specify the right process for thick, sink‑free parts → choose GAIM

• Troubleshoot defects – gas fingering vs. short shot vs. gas leak

• Quote realistically – GAIM parts cost less material and cycle time, but require mold modifications and a GCU

• Design for manufacturing – GAIM needs gas channels designed into the CAD model