Gas-assisted and foam injection: process options that solve sinking in thick parts

In classic injection, a thick region means long cooling + sink-mark risk. There are two processes used to reduce both problems without changing the geometry: gas-assisted injection (GIM) and foam (microcellular) injection.

Gas-assisted injection

The cavity is partially filled with plastic, then nitrogen gas is injected into the center of the melt. The gas pushes the thick region and the melt's hot core, forming hollow channels.

Advantages

• Sink marks in thick regions are greatly reduced (the gas pressure acts like holding pressure).
• Material and weight savings (hollow handles, arms, frames).
• Reduced warpage, lower cavity pressure and tonnage.

Limitations

• The gas unit and mold (gas pin) require extra investment.
• The process window is narrow; gas breakthrough and part wall thickness need control.
• The design must suit gas channels (e.g. thick ribs/arms).

Typical applications: door handles, grips, frames and long-handle parts, TV and furniture housings.

Foam (microcellular) injection

Supercritical N₂ or CO₂ is mixed into the melt in a controlled way in the barrel; once it enters the cavity, the gas forms small cells. The cells provide pressure without requiring holding.

Advantages

• Sinking and warpage are reduced; cavity pressure and tonnage drop.
• Weight and material savings; cycle time sometimes shortens.

Limitations

• Surface quality (marks similar to silver streaks) can be worse than classic injection; visible parts need extra measures such as mold heating.
• Mechanical properties (impact, fatigue) change; test them.
• Licensing and equipment cost.

Structural foam (chemical foaming agent) is a cheaper alternative to these two methods; used in large, thick parts.

Which one?

Note: These processes require expertise; trials should be run together with the supplier and the mold maker.

If you want to solve sinking first through process and design settings, see the sink mark guide.