Injection molding jetting is a snake-like flow pattern created when a fast melt stream enters an open cavity without forming a stable advancing front. The defect may look like a serpentine line, wave, or flow mark and is most visible on cosmetic surfaces.
Jetting is usually a flow-front problem, not simply a machine-speed problem. The gate, runner, cavity geometry, resin condition, melt temperature, mold temperature, and injection-speed profile must be considered together.
How jetting forms
When the melt exits a small or poorly aimed gate into a larger open region, it can travel forward as a narrow stream before spreading against the mold wall. The stream cools and may not remix smoothly with the following melt. The resulting surface pattern can remain visible after packing and cooling.
Jetting is more likely when the gate points into open space, the gate is restrictive, the initial injection speed is too high, the melt or mold is too cold, or the material has an unsuitable flow behavior. Moisture, contamination, or degraded resin can create similar-looking streaks, so the defect must be identified before changing the tool.
Jetting versus other surface defects
| Observed pattern | Possible defect | First investigation |
|---|---|---|
| Snake-like line beginning at the gate | Jetting | Gate direction, initial speed, gate size, and short shot |
| Concentric rings or halo at the gate | Flow mark or gate blush | Gate freeze, melt/mold temperature, and packing |
| Silver streaks following flow | Moisture, gas, or material degradation | Drying record, residence time, and material condition |
| Dark mark at the end of fill | Burn or diesel effect | Vent location, trapped gas, and fill speed |
Root-cause troubleshooting sequence
- Map the defect relative to the gate, runner, weld lines, inserts, ribs, and end of fill.
- Run short shots to observe whether the stream travels freely before the flow front spreads.
- Confirm resin grade, drying, regrind ratio, colorant, moisture, and purge condition.
- Review gate size, gate land, gate angle, runner restriction, cold-slug well, and venting.
- Use a staged injection-speed profile and change only the section associated with the defect.
- Verify the result with appearance, dimensions, weld-line strength, and repeatability.
Process adjustments
- Reduce the initial fill speed where the melt first enters the cavity, then increase speed after a stable front is established if required.
- Adjust melt and mold temperatures only within the resin supplier’s processing window.
- Check screw recovery, residence time, back pressure, and nozzle condition for material degradation or inconsistent melt.
- Do not compensate for a poor gate design by making the whole shot slower; this may create short shots or cold flow marks elsewhere.
Gate and part design solutions
A gate that feeds against a wall, fan, tab, or gradual transition can help establish a broader flow front. Relocating the gate may move the cosmetic witness or weld line, so the part’s appearance and structural requirements must be reviewed together. A cold-slug well and adequate venting can also prevent cold or trapped material from disturbing the front.
Use a fill simulation or DFM review for large cosmetic parts, thin walls, abrupt area changes, and parts with multiple gates. The engineering team can evaluate whether a gate change is safer than repeated process adjustments.
RFQ and production validation
For a new tool, provide resin grade, color, surface specification, gate preference, wall map, expected volume, and acceptable cosmetic limits. During T1, retain short-shot parts and record the fill profile, pressure, temperature, and speed changes. Approve the process only after a repeatable window is established.
The injection molding team and mold-making team should review jetting as a combined material, process, and tool issue. Contact Cavity Mold with photos and short-shot data for a focused troubleshooting review.
Technical reference
The Covestro defect-prevention guide identifies jetting causes related to gate/throat design, injection speed, and molding conditions.