Jetting is a wavy, snake-like line on the part that starts at the gate. The melt shoots through the gate as a thin stream into open space, where it should spread out as a smooth front. The stream cools and folds, and the melt that follows flows around it without fusing fully. Look at fill speed and the gate first. This is rarely a resin problem.
Also described as: snake marks, worming, squiggly line from the gate.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| A wavy line that starts at the gate and wanders into the part | The gate fires into an open cavity. The stream has no wall to hit, so it does not form a flow front. | Where the gate points: at a wall or core, or into open space |
| The line appears at high injection speed and fades when you slow down | Melt velocity through the gate is too high at the start of fill. | Injection speed profile; a slow first stage through the gate |
| Jetting on a thick-walled part fed by a small gate | The gate is small compared with the wall it feeds, so the stream is much thinner than the cavity. | Gate depth against wall thickness at the gate |
| The line is dull and clearly outlined, worse with a cold mold or melt | The stream skins over before the rest of the melt reaches it, so the two do not fuse. | Measured melt temperature; mold temperature near the gate; nozzle temperature |
| Jetting came and went with a change of lot or grade | Melt flow moved, and with it the way the melt behaves as it leaves the gate. | Melt flow on the COAs of the good and bad lots |
| The part cracks along the line | The jetted stream is surrounded by weak fusion lines. It is a structural defect as well as a cosmetic one. | The same checks as above; do not accept it on a loaded part |
Checks in order, cheapest first
- Make a series of short shots. Fill the cavity a little more each shot and look at how the melt leaves the gate. A stream or squiggle in the first short shots confirms jetting and shows where it starts.
- Slow the first stage of fill. Profile the injection speed: slow until a flow front has formed past the gate, then faster for the rest of the fill. This is the most common fix and it costs nothing.
- Check melt temperature against the supplier’s range. A melt at the low end of the range skins over faster. Raise it in small steps, staying inside the range for the grade.
- Raise the mold temperature near the gate. A warmer cavity surface gives the stream more time to fuse with the melt behind it.
- Check the nozzle and the cold slug well. A cold nozzle tip sends a cold slug into the cavity ahead of the melt. Confirm nozzle temperature and that the cold slug well is catching it.
- Compare lots. If jetting started with a new lot, compare melt flow on the COAs before you change the tool.
- Change the gate. Move the gate so the melt hits a wall or a core pin straight away, enlarge it, or change to a tab, fan or overlap gate. This is the lasting fix, and the expensive one, so confirm the free checks first.
- Change the resin. Treat this as a last resort. A resin change seldom cures jetting that the gate is causing.
The numbers
Jetting shows most on clear, glossy and thick-walled parts. These are the melt and mold temperature ranges to check your process against.
| Resin | Melt temperature | Mold temperature |
|---|---|---|
| Polycarbonate | 536–653°F / 280–345°C | 158–248°F / 70–120°C |
| Acrylic (PMMA) | 340–520°F / 171–271°C | 100–205°F / 38–96°C |
| ABS | 425–525°F / 218–274°C (flame-retardant grades never above 475°F / 246°C) | 80–180°F / 27–82°C |
| SAN | 428–500°F / 220–260°C | 104–176°F / 40–80°C |
| GPPS | 356–500°F / 180–260°C | 50–150°F / 10–66°C |
| Nylon 66 | 527–580°F / 275–305°C | 140–212°F / 60–100°C |
| PBT, unfilled | 460–536°F / 238–280°C (above 536°F it decomposes) | 100–212°F / 38–100°C |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 50–203°F / 10–95°C |
| Rigid PVC | 380–410°F / 193–210°C | 40–130°F / 4–54°C |
Typical values from published processing guides. Confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart. Rigid PVC has a narrow range and gives off corrosive hydrogen chloride gas when overheated, so do not raise its melt temperature to chase jetting.
Why jetting happens
- Normal fill is a front, not a stream. In a good fill the melt touches the cavity walls as soon as it leaves the gate and advances as a rounded front.
- Jetting is a stream with nothing to stop it. When a small gate opens into a much thicker section with no wall in front of it, the melt crosses the cavity as a strand, then piles up and folds.
- The strand cools on its own. By the time the cavity fills behind it, the strand has a skin. The boundary between the two is a weld line that runs the length of the mark.
- Speed, temperature and gate size set the threshold. A faster stream, a colder melt and a smaller gate all make jetting more likely.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A grade with a different melt flow | It changes how the melt swells and spreads as it leaves the gate. Which direction helps depends on the gate and the part, so it has to be trialed. | Toughness and other properties move with melt flow. A new grade for your customer to approve, with no certainty that it works. |
| A tighter melt flow range from lot to lot | Useful when jetting comes and goes with lots on a process that is close to the edge. | Nothing in the part. It may narrow your choice of sources. |
| An opaque color or a textured surface in place of clear or gloss | Makes the mark harder to see. | It hides the line and does not remove it. The weak fusion is still there, so this is not acceptable on a loaded part. Your customer has to approve the appearance change. |
None of these is as reliable as changing the gate. If the short shots show a stream leaving the gate, the tool is where the lasting fix is.
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Weld lines. A weld line is a single, fairly straight line where two flow fronts meet, usually behind a hole or core and away from the gate. Jetting starts at the gate and wanders. See weld lines.
- Gate blush. A dull or cloudy halo right around the gate, with no snake-like line, is gate blush. It also responds to a slower first stage.
- Splay. Many fine silver streaks fanning out along the flow come from moisture or gas in the melt. Jetting is one thick, wandering line. See splay and silver streaks.
- Flow marks. Evenly spaced ripples like the grooves of a record, running across the flow, come from a melt front that hesitates because it is too cold or too slow.
Quick answers
What causes jetting in injection molding?
Jetting happens when the melt shoots through the gate as a thin stream into open space when it should spread out as a front. The stream cools and folds, and the melt that follows does not fully fuse with it. A faster stream, a colder melt and a smaller gate all make it more likely. Resin is rarely the cause.
How do I fix a snake-like line at the gate?
Start by slowing the first stage of fill until a flow front has formed past the gate, then go faster. This is the most common fix. Then check melt, mold and nozzle temperature. The lasting fix is the gate: move it so the melt hits a wall or core pin, enlarge it, or use a tab, fan or overlap gate.
Why does the melt squirt into the cavity?
The gate fires into an open cavity, so the stream has no wall to hit and does not form a flow front. This is most likely when a small gate opens into a much thicker section, or when melt velocity through the gate is too high at the start of fill. A series of short shots shows where it starts.
Is jetting only a cosmetic defect?
No. The jetted stream is surrounded by weak fusion lines, so it is a structural defect as well as a cosmetic one, and the part can crack along the line. An opaque color or a textured surface makes the mark harder to see, but the weak fusion is still there. Do not accept jetting on a loaded part.
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