Injection molding defects
Jetting is a snake-like or worm-track surface defect that often starts at the gate when molten plastic squirts into open cavity space instead of spreading in contact with mold steel. This guide explains the flow physics, how to separate gate design issues from fill-profile issues, and what to change before you accept cosmetic scrap.
Defect troubleshooting
Jetting is a filling defect where the melt front travels as a free stream rather than a wall-contacting fountain flow.
Starting at or near the gate, you may see a rough, matte, or ribbed strand that looks like a snake track on the part surface. Industry troubleshooting references describe this as melt exiting a restrictive gate or nozzle path at high velocity, then entering a larger cross section with nothing immediately in its path to slow and spread the stream. The strand cools on its surface, folds, and later melt may not fuse cleanly, which hurts appearance and can weaken the part locally.
Jetting is not the same problem as a simple flow line, although both relate to how the melt front advances. Jetting is tied to gate aim and initial velocity into open space, while flow lines often follow broader fill-pattern changes. On the floor, confirm the defect origin at the gate, whether tracks disappear when you slow first-stage fill, and whether the gate points into a void rather than impinging on a wall or core pin.
For new parts, gate location and fill strategy should be reviewed in CAD before steel is cut. NetProto returns DFM feedback with instant quotes so teams can discuss gate approach and cosmetic risk on a hardened steel production tool, depending on part size and complexity, while changes are still inexpensive.
Root causes
Use this table to decide whether the next lever is gate geometry, fill profile, melt condition, or part geometry.
| Likely driver | What you see | First lever to try |
|---|---|---|
| Gate aimed into open cavity space with no immediate wall or core contact | Snake track begins at gate and runs straight into a pocket or large volume | Reposition gate to impinge on steel; use overlap, fan, tab, or tunnel-style gate concepts in the next tool revision |
| Excessive initial injection speed through the gate | Jetting worsens when fill rate is high; tracks shorten when first-stage speed is reduced | Profile fill so melt is slow at gate entry, then increase speed after the front contacts steel and spreads |
| Abrupt expansion from runner to part section without a gradual transition | Jetting at every shot from the same gate on a stable process | Round gate transitions, widen gate land gradually, and review runner-to-part geometry in mold design |
| Melt too stiff or freezing early at the gate | Jetting with short fill tendency or cold slug at gate on the same lot | Raise melt temperature within supplier guidance; verify mold temperature and residence time; dry hygroscopic resin |
| Sudden cross-section changes along the first flow path | Tracks follow a path into a thick region after a thin section | Smooth cross-section transitions in part design; move gate closer to uniform sections when possible |
| Hot runner or nozzle restriction amplifying shear at the gate | Jetting after hot-runner maintenance or tip change; blush at gate on the same cavity | Verify tip alignment and flow path; coordinate with hot-runner supplier on gate style suited to the resin |
Fix strategy
Mild jetting can sometimes be masked by slower fill, but recurring snake tracks usually mean the gate strategy fights the cavity geometry.
Troubleshooting
Document gate appearance and fill-only behavior before you commit to gate relocation or steel rework.
Gate origin
Photograph tracks with gate location and flow direction. Compare to blush or gate vestige only at the gate. Jetting extends as a distinct strand into the cavity, not a uniform dull band around the gate.
Geometry check
On the bench, trace where the melt enters relative to walls and cores. If the stream points into open space, note that for mold design. If aim looks correct, suspect speed profile or hot-runner restriction.
Profile test
Reduce initial injection speed through gate entry while keeping adequate end-of-fill. If tracks shrink but short fill appears, coordinate with the short-shot troubleshooting path before raising speed again.
Melt quality
Confirm dryer status and lot identity. Adjust melt and mold temperature within supplier bands if the jet stream looks stiff or folds sharply. Avoid chasing jetting with overheating that invites degradation.
Permanent fix
When conservative process settings still produce snake tracks, schedule gate relocation or style change in the next mold revision. Customer-owned hardened steel tools can be modified, but mold modifications are scoped and quoted separately from production.
Prevention
Most jetting is preventable when gate aim and fill profile are planned with the cavity layout, not tuned only after first shots.
In CAD review, verify the gate sprays into steel within a short flow distance, not into a large unfilled pocket.
Avoid launching flow from a thin section into a sudden thick volume without simulation or trial intent.
Specify a fill profile with slow gate passage on the setup sheet, especially for high-viscosity or filled grades.
For cosmetic surfaces, place gates away from show faces when design allows, or use tab gates that move vestige to trim-friendly locations.
Upload CAD for DFM feedback before tooling release on qualified programs so gate approach and cosmetic risk are flagged early.
Keep fill-only sample photos at startup so operators can tell new jetting from scratches or splay.
Tooling context
Jetting is a flow and cosmetics issue. For general molded dimension bands tied to surface finish class, use the published commercial tolerance table rather than inventing track-specific limits in a troubleshooting guide.
FAQ
Jetting usually appears as a snake-like, worm-track, or ribbed strand on the surface, often starting at the gate. The track may look matte or rough compared with surrounding plastic because the jet stream cooled partially before later melt surrounded it.
Cosmetics are the common complaint, but folded jet streams can act like internal weak planes similar to poor knit fusion. If the track crosses a load-bearing region or seal surface, treat jetting as a structural and function risk, not only appearance.
Slowing initial fill often reduces jetting, but it is not a universal fix. If the gate still aims into open space, tracks may remain unless gate position or style changes. Very slow fill can also cause short shot or sink if pack is not rebalanced.
Gate blush is usually a dull or whitish zone concentrated at the gate from high shear at entry. Jetting extends as a distinct stream mark into the cavity. Both point to gate and fill issues, but jetting implicates aim into open space and velocity profile more than blush alone.
Upload CAD for an instant quote and DFM feedback. Engineers can review gate location, cosmetic surfaces, and fill-sensitive geometry while the hardened steel tool is still in planning, depending on part size and complexity. There is no minimum order quantity once the tool is running.
Ready to quote
Bring CAD and sample photos into an instant quote so DFM review and gate strategy align with your cosmetic targets. One hardened steel tool is built for the life of the part on qualified programs, depending on part size and complexity.
