Gas assist
- Keeps a smooth, closed outer shape, like a round handle
- Needs gas equipment, pins, and process development
- Wall thickness inside the channel varies with the process
Process guide
Gas Assist Injection Molding
A thick handle molded solid will sink, warp, or take forever to cool. Gas assist leaves the outside where the drawing wants it and takes the middle away by blowing a channel through the melt before it sets.
How it works
Fill most of the way, then push with gas
The gas does the packing that a screw cannot do through a thick section.
The press injects less resin than the cavity holds. Right after, nitrogen goes in through the nozzle or through gas pins in the mold. The gas takes the path of least resistance, which is the hot, still liquid core of the thickest sections. It pushes that core resin ahead to finish filling the cavity and leaves a hollow channel behind it.
The gas then holds pressure from the inside while the part cools. Because the pressure acts right where the resin is shrinking, the outer wall stays pressed against the steel. That is why gas assist parts show so little sink over thick areas. Before the mold opens, the gas is vented and the part comes out with a sealed or open channel, depending on the design.
Variants
Three ways gas gets used
They share the name and little else. Know which one a supplier is proposing.
| Variant | What happens | Typical use |
|---|---|---|
| Short shot | Partial fill, then gas pushes the core resin to the end of the cavity | Handles, rods, and tube like parts that should end up hollow |
| Full shot with overflow | Cavity filled completely; gas displaces core resin into a side overflow well | Parts where a short shot would leave a visible hesitation mark |
| External gas | Gas is let in between the steel and one face of the part, not into the melt | Large panels with ribs, to press the show face flat and hide sink |
Design
Designing the path the gas will take
Gas does not go where you want. It goes where the resin is thickest and hottest, so the thick section has to be drawn as a channel on purpose.
- Give the channel a rounded section. Gas cores come out roughly round, so a square thick rib ends up with uneven walls
- Run the channel from the gas entry toward the last place to fill, with no dead ends it has to back out of
- Keep the channel clearly thicker than the walls around it, or the gas will wander into them
- Avoid sharp turns. Gas cuts the inside corner, which leaves the outside of the bend thin
Troubleshooting
Defects that only gas parts have
Standard molding defects still apply. These come on top of them.
| What you see | What usually happened |
|---|---|
| Fingering: thin gas tracks spreading into flat walls | The walls next to the channel were too close to its thickness, so the gas had more than one easy path |
| Blow through: gas bursts out at the end of flow | Too little resin ahead of the gas, or gas pressure came in too early |
| A dull band across the surface | The flow front paused between the end of injection and the start of gas |
| Thin wall on the outside of a bend | Gas took the short way around the corner |
Alternative
Gas assist or coring the section out
The first question on any thick part is whether it needs to be thick at all.
Coring out with ribs
- Works in a standard mold on a standard press
- Wall thickness is set by steel, not by gas behavior
- Leaves an open, ribbed back face, which is fine where nobody sees or holds it
Keep reading
Related guides
Other answers to thick sections, and the defects they are trying to avoid.
Structural foam molding
A foamed core instead of a hollow one.
Structural foam moldingMuCell foam molding
Microcellular foam for flatter parts.
MuCell foam moldingSink marks
Why thick sections pull the surface in.
Sink marksVoids
Hollow pockets nobody asked for.
VoidsRibs
Stiffness without a thick wall.
RibsHandles
Grip shapes and where the mass goes.
HandlesProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about gas assist molding
Why nitrogen and not air?
Nitrogen is dry and inert, so it does not react with or burn the hot resin, and it is easy to supply at steady pressure.
Can water be used instead of gas?
Yes. Water assist works the same way and cools the channel from inside, which shortens the cycle on long hollow parts. The water has to be fully removed afterward, and the equipment is less common.
Does the hollow channel make the part weaker?
Less than you might expect. A hollow round section keeps most of the bending stiffness of a solid one, the same reason tubes are used for frames. The real risks are thin spots where the gas cut a corner.
Can a part be switched to gas assist after the mold is built?
Sometimes, if the thick section already works as a channel and there is room for a gas pin. Usually both the part and the mold need changes, so it is better decided before tooling.
Next step
Show where the part is thick
Upload CAD and point out the heavy sections. The review looks at coring, ribs, and wall changes so a standard injection mold can do the job. NetProto manufacturing is ISO 9001 certified.