Last updated: 2026-09-05 · 5 min read

Jetting in Injection Molding — What Causes the Snake-Like Surface Defect and How Do You Eliminate It?

Jetting occurs when melt enters an open cavity at high velocity without contacting a wall — the stream folds into a snake-like pattern, creating visible lines and surface roughness that packing pressure or cooling time cannot fix. Root cause: a gate shooting melt into open space. Fix: redirect melt to contact steel immediately. One of the few defects process tuning alone cannot correct.

Key Facts

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  • Jetting produces a snake-like or serpentine pattern starting at the gate, extending 10–50 mm into the cavity. Folded melt creates overlapping layers — glossy ridges on matte surface, rough texture on glossy surface. Localized to gate area with distinctive folded, chaotic pattern.
  • Four conditions create jetting: edge gate facing open cavity (most common), gate too small for cavity thickness, high injection speed through restricted gate, low-viscosity resins (high-MFI PP, PA6/PA66, POM).
  • Five fixes ranked by reliability: overflow well (+$200–500, most reliable), gate relocation to impinge on wall (+$500–1,500), fan/tab gate conversion (+$300–800), reduce injection speed (no cost, may increase cycle), increase gate size (+$100–300).

    • Cycle time — The total time for one injection molding cycle: injection + packing + cooling + mold open/close + ejection. Typical range: 15–60 seconds. Cycle time determines production rate and cost per part — reducing cycle by 5 seconds saves 10–15% annual production cost. The related guide covers adjacent topics.

    How Do You Distinguish Jetting from Weld Lines, Flow Marks and Silver Streaks?

    Defect Appearance Location Fix
    Jetting Snake-like folds, chaotic ridges At the gate, extending into open cavity Gate redesign, overflow well
    Weld line Linear mark where two flow fronts meet Behind cores, at flow convergence Gate relocation, venting, melt temp increase
    Flow mark Concentric ripples following flow path Along entire flow path, at thickness transitions Injection speed, melt temp, runner size
    Silver streaks Fine linear marks in flow direction Throughout the part, often near gate Drying, back pressure, venting

    If the defect starts at the gate and looks like a folded ribbon, it is jetting. If it appears where two flow fronts meet, it is a weld line. If it follows the entire flow path in concentric ripples, it is a flow mark. The distinction matters because the fixes are completely different.

    Why Does Jetting Happen and What Gate Conditions Create It?

    1. An edge gate facing an open cavity. If the gate is at the edge of a large, flat part and the melt exits directly into open space without hitting a wall, core or rib, the melt will jet.
      This is the most common jetting configuration.
    2. A gate too small for the cavity thickness. A pin gate or sub-gate on a thick part creates a high-velocity jet — melt accelerates through the small orifice and then expands into the thick cavity.
      The velocity differential causes the melt to fold.
    3. High injection speed through a restricted gate. Even a properly sized gate can produce jetting if injection speed is too high — melt exits at a velocity exceeding the critical threshold for stable flow, and the stream breaks up into a jet.
    4. Low-viscosity resins. Low-viscosity resins (high-MFI PP, PE, PA) are more prone to jetting because the melt flows more easily and is less resistant to folding.
      High-viscosity resins tend to spread and contact the wall immediately.

    Warpage — Part deformation after ejection caused by uneven shrinkage. Amorphous resins warp less (0.4–0.7% shrinkage). Semi-crystalline resins warp more (1.0–2.5% shrinkage). Fix: uniform wall thickness, balanced cooling, optimized gate location, adequate packing pressure. The related guide covers adjacent topics.

    Jetting: melt exits gate into open space, folds over itself, creates snake-like surface defect. Cannot be fixed by process tuning alone — gate must be redesigned. Most reliable fix: overflow well ($200–500). Alternatives: gate relocation, fan/tab gate conversion, gate enlargement. Jetting traps air, reduces impact strength 20–40%.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Can jetting be fixed by adjusting process parameters without modifying the mold?

    In some cases, reducing injection speed or increasing melt temperature can reduce jetting severity — but these adjustments rarely eliminate it completely. The root cause is a gate geometry that directs melt into open space, and no process parameter change can redirect the melt to contact a wall.

    If the jetting is visible and the part is cosmetic, the mold must be modified.

    Does jetting affect the mechanical strength of the part?

    Yes — the folded layers created by jetting trap air between them, creating voids and weak interfaces in the part wall. A jetting defect at a structural area (near a boss, along a load path) can reduce impact strength by 20–40% compared to a part with stable flow.

    For safety-related parts, jetting at the gate is a reject condition regardless of cosmetic appearance.

    Which resins are most prone to jetting?

    Low-viscosity resins are most prone: high-MFI PP (MFI above 30), PA6 and PA66 (especially unfilled grades), and POM. These resins flow so easily that the melt stream does not spread on contact with the mold wall — it bounces off and continues as a free jet.

    High-viscosity resins like PC, ABS and low-MFI PP tend to contact the wall immediately and fill in a stable pattern. If molding a low-viscosity resin, design the gate to impinge on a wall from the start.

    Key Takeaways

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  • Jetting: melt exits gate into open space, folds over itself, snake-like defect.
  • Causes: edge gate facing open cavity, undersized gate, high speed, low-viscosity resin.
  • Cannot be fixed by process tuning — gate must be redesigned to contact steel.
  • Best fix: overflow well ($200–500).
    Jetting reduces impact strength 20–40%.

    • Jetting is preventable at the DFM stage by checking whether the gate faces an open cavity or impinges on a surface. If the gate faces open space, add an overflow well or relocate the gate before the mold is cut — $200–500 in the mold base vs. $500–1,500 to re-cut after T1.

    Sources: Society of Plastics Engineers (SPE) — Gate Design and Flow Defect Papers · Beaumont, Runner and Gating Design for Injection Molding · ASTM International — Injection Molding Defect Standards · ENGEL — Injection Molding Troubleshooting Guide · Osswald, Injection Molding Handbook (3rd Ed.)

    — Arthur, Tooling Engineer at aCatMould