Jetting is a surface defect in injection molding where the melt shoots into the cavity as a fast, thin stream that freezes before the rest of the fill catches up. It leaves a wavy, worm shaped squiggle near the gate. The main fixes are moving the gate against a wall, opening the gate up, and slowing the fill speed.
Jetting looks small on a first article part, but it is a warning about how the melt enters your cavity. If you chase it on the press without understanding the flow, you burn trial hours and still ship parts with a weak spot near the gate. This guide walks through what jetting is, why it happens, and how we fix it for good.
What is jetting in injection molding?
Jetting is a snakelike surface mark that starts at the gate and trails into the part. It forms when melt enters an open cavity as a fast jet instead of spreading in a smooth wave. The jetted material cools on contact, then the following melt fills around it, so the cold stream never fully bonds.
In normal filling the melt moves by fountain flow. The front rolls forward like a wave, the outer skin freezes against the steel, and the hot core keeps pushing the front along. That gives you a clean, uniform surface. Jetting breaks that pattern. The melt leaves the gate faster than it can spread, so a thin rope of plastic lands in the middle of the cavity, folds on itself, and welds poorly to everything around it. You see it as a squiggle, and under load it acts like a built in crack starter because the bond in that zone is weak.
The tell is direction. A jetting mark points away from the gate and follows the path the jet took. That is different from a cosmetic streak that has no clear origin. Once you learn to read the direction, you can usually trace it back to the gate that caused it.
What causes jetting in injection molding?
Jetting is caused by melt entering the cavity too fast through a gate that points into open space instead of at a wall. Six factors drive it. Most jetting on real programs comes from a mix of two or three of them, not just one.
- Gate aimed at open air. If the gate fires the melt into the middle of the cavity with nothing to impinge on, the stream stays a jet. When the gate points at a nearby wall or a core pin, the melt hits the steel, spreads, and starts fountain flow right away.
- Gate too small. A small gate acts like a nozzle. It speeds the melt up and thins the stream, which is exactly the condition that jets. Opening the gate slows the entry velocity and thickens the front.
- Injection speed too high at the start. A fast first stage pushes melt through the gate before it can spread against a wall. A slow start lets the melt establish contact, then you ramp speed once the flow front is anchored.
- Cold melt or cold mold. Low melt temperature and a cold cavity make the jet freeze faster, so it locks in the squiggle instead of remelting as the fill catches up.
- Sharp transition at the gate. A gate that dumps straight into a thick section with no land or taper encourages a free jet. A tapered land or a gate that overlaps onto the wall guides the melt.
- Wrong gate type for the geometry. A pin or restrictive edge gate into a large open face jets far more than a fan, tab, or overlap gate that fans the melt out as it enters.
On a thin wall housing program we oversaw, the first trial came off the press with a clear jetting trail on the cosmetic face. The processor kept dropping injection speed and the mark faded but never left. The real problem was a pin gate firing straight into an open cavity with no wall to hit. Once our manufacturing partner added a small deflector wall opposite the gate on the next steel revision, the jetting was gone at normal fill speeds. This is why we treat jetting as a design and flow question first, not a press setting.
How do you fix a jetting defect in injection molding?
You fix jetting by making the melt hit a wall as it enters and by slowing the entry velocity. Try process changes first because they are fast and free, then move to gate and tooling changes if the process alone does not clear it. The table below maps each root cause to the fix that addresses it.
| Root cause | Fix | Where it happens |
|---|---|---|
| Gate points into open cavity | Relocate gate against a wall or core, or add a deflector so melt impinges | Tooling |
| Gate too small | Open the gate to slow and thicken the entering stream | Tooling |
| High first stage speed | Profile the fill: slow start, then ramp once the front is anchored | Process |
| Cold melt or cold mold | Raise melt and mold temperature within the datasheet window | Process |
| Sharp gate entry | Add a tapered land or overlap the gate onto the wall | Tooling |
| Wrong gate type | Switch to a fan, tab, or overlap gate that fans the melt | Tooling |
Start with a slow to fast fill profile. Based on MoldMinds experience, cutting the first stage fill speed by 20 to 40 percent until the flow front clears the gate area, then ramping to a normal rate, removes light jetting on many parts without touching the steel. If the mark survives that, the gate itself is the problem and no amount of speed tuning will fully hide it.
Raising melt and mold temperature helps because a hotter jet stays fluid long enough to remelt into the fill behind it. Keep both inside the resin datasheet range. Pushing melt temperature past the datasheet to mask jetting trades one defect for another and can degrade the polymer.
When process alone will not clear it, the fix moves into the tool. The most reliable cure is a gate that points at a wall, a core pin, or a deflector so the melt spreads on contact. A fan gate, tab gate, or overlap gate does the same by fanning the stream wide as it enters. These are geometry decisions, which is why catching jetting before steel is cut saves the most money.
MoldMinds is vendor agnostic. We hold no referral arrangements with any tool shop, so when we call for a gate change or a deflector, it is because the flow needs it, not because a supplier wants the extra work.
How is jetting different from splay and flow lines?
Jetting, splay, and flow lines are all surface defects, but they come from different causes and look different on the part. Confusing them sends you chasing the wrong fix. Jetting is a directional squiggle from the gate. Splay is silver streaking from moisture or gas. Flow lines are faint rings or waves from uneven fill.
- Jetting makes a raised, wavy trail that points away from the gate. Root cause is a fast, unanchored melt stream. Fix it at the gate and the fill profile.
- Splay shows as silver or white streaks spread across the surface, usually from wet resin or trapped gas. Fix it by drying the resin and venting, not by moving the gate. We cover this in our guide to splay marks and silver streaks.
- Flow lines are faint tonal rings or ripples from the melt cooling unevenly as it fills. They point to fill speed and wall thickness, not to a jetting stream.
The fast way to tell jetting apart is to look for a clear origin at the gate and a direction. Splay has no single origin and spreads. Flow lines follow the flow path as smooth bands rather than a folded rope. Good plastic part design reduces all three by giving the melt an even path and a sensible gate location from the start.
Can moldflow analysis predict jetting before you cut steel?
Yes. Moldflow analysis predicts jetting by simulating how the melt enters the cavity and whether the flow front stays anchored to the wall. It shows velocity at the gate, the fill pattern, and where a free jet is likely to form, so you can move the gate or change its type on the screen instead of in a trial.
This is where jetting is cheapest to kill. Once steel is cut, moving a gate means welding, remachining, or a new insert, plus another round of trials. In a simulation you test three gate locations in an afternoon. You watch the fill animation, spot the frame where the melt shoots into open cavity, and pick the gate that puts the melt against a wall from the first millimeter of fill.
We run moldflow analysis on parts with cosmetic faces, thin walls, or open geometry near the gate, exactly the parts that jet. If you are not sure whether your part needs it, our guide on when to use moldflow analysis lays out the cases where the simulation pays for itself. For a part that risks jetting, the answer is almost always before the gate design is locked.
Simulation is not a magic wand. It guides the gate decision and the fill profile, then a real trial confirms it. But catching jetting in the model rather than on the press is the difference between one steel revision and three.
The bottom line on jetting
Jetting tells you the melt outran its own ability to spread. The mark is cosmetic, but the weak bond it leaves is structural, and that is the part that matters under load. Fix it by slowing the first stage fill and, more importantly, by giving the melt a wall to hit as it enters. Most stubborn jetting is a gate pointed at open air, and the cheapest place to catch that is in a moldflow model before the tool is built.
Brandon Henderson is a certified journeyman mold maker and global tooling engineer with 15 years in plastics. As a toolroom manager he managed tooling programs for brands including Hershey’s and Clorox, and now provides US based technical oversight for offshore injection mold sourcing.