Views: 0 Author: Site Editor Publish Time: 2026-05-28 Origin: Site
If you've ever seen small burn marks on the edge of a molded part, or noticed incomplete filling at the end of a rib, you've likely encountered trapped gas. It's one of the most common defects in injection molding, yet many shops struggle to solve it consistently.
The good news? Most trapped gas issues can be fixed without expensive mold modifications. The bad news? Many operators reach for the wrong solution first.
Let me walk you through what trapped gas actually is, why it happens, and most importantly — how to fix it, starting with the simplest adjustments.
During injection, the mold cavity is full of air. As molten plastic rushes in, that air needs somewhere to go. If it can't escape, it gets compressed into the last corner of the cavity.
Here's the problem: when air is compressed rapidly, it heats up — dramatically. Temperatures can spike to 300°C (572°F) or higher. At those temperatures, plastic doesn't just flow poorly; it actually burns or degrades.
The result is a small dark or yellowed spot on the part, typically at the end of flow or where two flow fronts meet. In severe cases, the part won't even fill completely.
Not all trapped gas is the same. Identifying the source saves hours of trial and error.
Source | Typical Location | Visual Sign |
|---|---|---|
Air trapped by flow | Last fill point, deep ribs | Single burn mark at flow endpoint |
Flow convergence gas | Weld line areas | Burn marks along a V-shaped line |
Material decomposition gas | Anywhere (gas-rich materials like POM, PA) | Yellow-brown streaks, not isolated spots |
Most operators immediately want to open mold vents. That's sometimes necessary, but it's not the first step. Here's the logical order:
Before touching the mold, try these process changes. They often solve 50-70% of trapped gas problems.
1. Reduce final-stage injection speed
This is the single most effective process adjustment. Trapped gas gets compressed when the melt front moves too fast at the end of fill. Slow it down dramatically — sometimes to 5-10% of maximum speed — in the last 5-10mm of screw travel.
Most modern injection machines allow multi-stage injection profiles. Use it.
2. Use mold breathing / decompression (underrated!)
Many operators don't know their machine has this feature. On some presses (notably Sumitomo, Nissei, Engel), there's a function that briefly reduces clamp force during the last stage of injection. The mold "breathes" open by a few microns — just enough to let gas escape — then clamps fully for packing.
If your machine has this, learn how to use it. It can eliminate burn marks without any mold modification.
3. Adjust melt and mold temperature
Increasing mold temperature slightly slows the frozen layer formation, giving trapped air more time to escape through existing gaps. Decreasing melt temperature reduces gas from material decomposition.
4. Dry the material
This sounds obvious, but moisture becomes steam at injection temperatures. Steam takes up far more volume than air. If you're molding nylon, PC, PET, or any hygroscopic material, verify drying first.
If process adjustments don't fully solve the problem — or if they cost too much cycle time — modify the mold.
1. Add venting slots at the parting line
This is the standard solution for a reason: it works. The key is getting the depth right.
Material | Vent Depth (mm) | Note |
|---|---|---|
PP, PA, POM (high flow) | 0.01 - 0.03 | Too deep = flash |
ABS, PC, PMMA (medium flow) | 0.04 - 0.06 | |
PC+GF, PPS (low flow) | 0.06 - 0.10 |
After the initial 5-10mm of vent depth, back-cut the slot to 0.5-1.0mm so gas can escape freely to atmosphere.
2. Use vented pins or inserts
For trapped gas at deep ribs or around small cores — places where parting line vents don't reach — use dedicated venting components:
Vented ejector pins : Grind a small flat on the pin's shank to create an air path along the pin-mold clearance
Porous vented steel : Material like PM-35 has interconnected pores that let air pass but block plastic
Diamond-cut core pins : Cut small axial grooves on the non-engaging portion of the pin
3. Relocate or redesign the gate
Sometimes the gate itself causes the problem. A poorly placed gate can create flow wrapping — the melt flows around a core and traps air in the middle.
Re-gating to a different location can completely eliminate certain types of trapped gas without any other changes.
If the problem keeps returning — or if mold modifications are impossible — the part design itself may be at fault.
What causes design-related trapped gas:
Abrupt wall thickness changes : Melt races through thick sections, bypasses thin sections, and traps air
Deep, narrow ribs : The rib acts like a dead-end pipe — air has nowhere to go
Unbalanced flow paths : One side fills first, wraps around, and seals off the remaining air
Solutions:
Add wall transitions (tapered, not stepped)
Reduce rib depth or increase base radius
Run a Moldflow analysis before cutting steel — it will show you exactly where trapped gas will occur
90% of trapped gas problems stop somewhere in this flow. The remaining 10% require part redesign or advanced venting like porous steel.
Common mistakes that waste time or make things worse:
Don't increase injection pressure — This only compresses the gas harder, making burns worse
Don't vent too deep — 0.05mm works for ABS; 0.05mm on PP creates beautiful, razor-thin flash
Don't ignore the ejector pins — They're often the best natural vents in the mold
Don't assume it's always air — Decomposing material (especially POM and nylons) produces its own gas
Symptom | Most Likely Cause | First Action |
|---|---|---|
Single dark spot at flow endpoint | Air trapped at last fill | Reduce final injection speed |
Burn marks at weld line | Air trapped between two flow fronts | Add mold breathing or slow both fronts |
Yellow-brown streaks (not spots) | Material decomposition | Lower melt temp, verify drying |
Burn at deep rib tip | No vent path | Add vented ejector pin at rib base |
Burn moves when speed changes | Air compression | Find the speed threshold, stay below it |
Trapped gas is frustrating because it appears suddenly, burns parts, and seems impossible to fully eliminate. But in most cases, it follows predictable patterns with logical fixes.
Start with speed reduction and mold breathing — they're free and often sufficient. Only then move to venting modifications. And if you're designing a new mold, run a simulation first. Finding trapped gas in software costs nothing. Finding it in production costs time, money, and reputation.
The best shops don't eliminate trapped gas by guessing. They follow a method. Now you have one.
