What Is Jetting in Injection Molding — and How Do You Stop It?

Last updated: 2026-09-01 · Reading time: about 4 min

Jetting is what happens when the melt stops flowing and starts flying: instead of advancing as a front that hugs the walls, it squirts through the gate as a rope, shoots across open space, and folds its frozen coils wherever it lands.
The signature is a worm-like, wrinkled ribbon growing out of the gate — and the cure is to make the melt grip the wall again: aim the gate at steel, soften the first-stage speed, or widen the gate until the jet becomes a front. It is a flow-state problem, which is why material swaps rarely fix it and geometry usually does.

Key facts

<>

  • Molding wants wall contact: a stable front advances by laying melt against the cavity surface. Jetting is the opposite regime — a free stream that cools in the air of the cavity before it ever touches anything.
  • The trigger is a trio: a small gate shearing the melt to high speed, a first-stage velocity left high, and an open space directly beyond the gate with nothing to catch the stream.
  • The worm-shaped mark is frozen history: jet coils solidify in flight, then the slower bulk flow arrives and folds them against the wall — a wrinkle pattern that repeats on every shot in the same place.

    • Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. See also related guide.

    Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

    What does jetting look like — and why does it happen?

    The tell is the shape: a snaking, blistered ribbon that starts at the gate and wanders across the part, unlike the straight bands of ordinary flow marks or the halo of blush — the flow-mark page draws that distinction. The mechanism explains the shape. Push melt through a small gate fast enough and it exits as a jet, a rope of material moving faster than a front can form.
    The rope does not touch the walls; it sails through the open space ahead of the gate, cooling as it goes, until it hits the far steel and folds back on itself like a dropped garden hose. By then the rope’s skin has chilled, so when the bulk of the melt finally catches up and packs the cavity, it presses the frozen coils into the surface — the wrinkle pattern you see.
    The trigger is always the same trio, which is why the diagnosis asks three questions: how small is the gate, how fast is the first stage, and what sits in front of the gate — an open cavity invites the jet, a wall a few millimeters away stops it before it starts.

    How do you stop the jet?

    Give the stream something to meet. The classic cure is geometric: relocate or re-aim the gate so the melt enters against a wall, a pin or an existing surface — an impingement a few millimeters out turns the jet into a spreading front before it can form.
    Soften the launch: a multi-stage profile that starts slow at the gate and accelerates once material is on the walls keeps the exit velocity below the jetting threshold — the cheapest fix in the set, tried before any steel moves. Widen the gate if the sizing is the author: a gate that shears melt to jetting speed for its own sake is a gate-design conversation, balanced against vestige and packing.
    And predict rather than chase: the flow study flags jets before steel is cut, which is the least expensive place to re-aim a gate. The verdict is the same in every case — the worm disappears and a clean front fills behind it, confirmed on parts at the first trial and written into the acceptance record.

    Which jetting checks belong in the review?

  • The worm located and traced — the ribbon’s origin and path recorded; it always points back at its gate.
  • Gate aim inspected — what sits directly beyond the gate: open space invites jetting, nearby steel prevents it.
  • First-stage speed reviewed — launch velocity eased down until the melt exits as a front, not a rope.
  • Gate size cross-checked — small gates shear melt to jetting speed; the balance with vestige is revisited, not assumed.
  • Prediction consulted — the flow study’s jet indicators checked before the tool is cut, when re-aiming is still free.
  • The cure documented — profile and gate changes recorded in the process file so the fix survives operator changes.

    • Frequently asked questions

    Is jetting the same thing as flow marks?

    No — they share a surface and nothing else.
    Flow marks are records written by a front that is behaving normally but unevenly; jetting is a flow regime that has failed outright, with melt folding in mid-air before it packs. The shapes differ — bands and halos versus a three-dimensional worm — and so do the cures: profiles tune flow marks, but a jet needs its geometry answered, because no setting makes a free stream hug a wall it has already passed.

    Why does jetting show up with some materials and not others?

    Rheology decides how easily a stream holds together: melts that resist stretching and stay coherent longer jet at lower speeds, while materials that break up quickly into a dispersed front are more forgiving. That is why the same gate and profile can run clean on one resin and worm on another, and why a material change is worth a profile check — the flow-index page explains the wider sensitivity.
    But the geometry rule holds across all of them: give the stream a wall to meet and the question mostly disappears.

    Doesn’t slowing the fill cause other problems?

    Only if the whole fill slows. The jetting cure is a staged profile — slow at the gate for the first moments, then full speed along the walls — so the melt exits gently and travels briskly. Run the entire shot slowly and you invite short shots, hesitation marks and long cycles; run the first stage alone gently and none of those arrive. The profile is the balance point, and it is found on the machine in minutes.

    Key takeaways

    1. Jetting is a failed flow regime — a free stream instead of a wall-hugging front — and the worm mark is its frozen fold pattern.
    2. The trigger trio is small gate, fast first stage and open space beyond the gate; answer any one and the jet collapses.
    3. Cure by geometry first (aim the gate at steel), then by profile; the flow study predicts the jet before the steel is cut.

    If worms or wrinkled ribbons are wandering across your parts, send a photo and note which way the ribbon runs. You will get the gate aim, the profile shape and the sizing check in one review — by a named tooling engineer, before the next batch runs.

    Sources
    Autodesk Moldflow — injection molding simulation, reference for jetting prediction
    Plastics Industry Association — industry resources on injection molding practice
    HASCO — moldmaking components, reference for gate systems and inserts

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet