Last updated: 2026-09-05 · 5 min read
Jetting in Injection Molding — What Does It Look Like, Why Does the Melt Shoot Through the Gate Without Touching the Walls and How Do You Stop It?
Jetting is a defect where the melt shoots through the gate as a thin, fast stream without contacting the mold wall — it folds into a snake-like pattern. The cause: injection speed is too high, or the gate is too small, so melt exits at high velocity into an open cavity. Fix by reducing speed, enlarging the gate, or adding an overflow tab that forces the melt to impinge on a surface.
Key Facts
- Jetting occurs when melt velocity through the gate exceeds 1.0 m/s into an open cavity — the stream does not spread against the wall, it shoots across and folds back on itself. The folded layers create visible lines, streaks or a worm-like pattern. The defect is most common with pin gates and submarine gates (small cross-section, high velocity) feeding into a large open area. Edge gates rarely jet because the melt spreads against the adjacent wall immediately.
- Jetting is not just cosmetic — the folded layers create a weak zone with poor molecular entanglement. The folded interfaces have 30–50% lower tensile strength than bulk. If the jetting area is load-bearing, the part fails at the fold under impact or fatigue. Even on non-structural parts, fold lines create a rough surface that affects painting and coating adhesion.
- The most reliable fix is a process change: reduce first-stage injection speed by 30–50%. The melt should exit the gate slowly enough to spread against the wall (fountain flow) rather than shooting across. If reducing speed causes a short shot, increase melt temperature by 10–15°C to compensate. The second fix: enlarge the gate — a larger gate reduces velocity at the exit. For a pin gate: increase diameter from 0.8mm to 1.2mm. For a submarine gate: increase width from 1.0mm to 1.5mm.
How Do You Distinguish Jetting from Other Surface Defects?
| Defect | Appearance | Location | Cause | Fix |
|---|---|---|---|---|
| Jetting | Snake-like folds, worm pattern | Opposite the gate | High gate velocity | Reduce speed, enlarge gate |
| Flow marks | Wavy lines following flow path | Along flow, not at gate | Slow flow, cold mold | Raise mold temp, increase speed |
| Splay marks | Silver streaks, gas bubbles | Random, throughout part | Moisture or degradation | Dry resin, lower melt temp |
| Weld line | Linear mark where flows meet | At flow convergence | Two melt fronts meeting | Move gate, raise temp |
Jetting is distinguished from flow marks by location — jetting starts at the gate and extends in a snake pattern. Flow marks appear along the flow path but not at the gate — they are caused by slow flow or a cold mold. If the defect is a snake-like pattern right next to the gate, it is jetting. If it is a wavy pattern further from the gate, it is flow marks.
Fountain flow — the desired flow pattern where melt exits the gate, contacts the mold wall and spreads outward like a fountain. The melt front advances in a smooth, uniform wave — no folding, no jetting. Fountain flow is established when gate velocity is below 0.5 m/s and the melt immediately contacts the wall. The short shots guide explains how flow pattern affects cavity filling.
What Three Fixes Eliminate Jetting?
- Reduce the first-stage injection speed — this is the fastest and most effective fix. Set the injection profile to a slow first stage (10–20mm/s) until melt exits the gate and spreads against the wall, then switch to normal speed. This ensures melt exits below the jetting threshold (below 0.5 m/s). Once fountain flow is established, speed can increase. Most presses support multi-stage profiles — set switchover at 5–10mm from the gate. The weld line strength guide shows how injection speed also affects weld line quality.
- Enlarge the gate — a larger gate cross-section reduces melt velocity at the exit. For a pin gate: increase diameter from 0.8mm to 1.2mm — velocity drops by 44% (inversely proportional to area). For a submarine gate: increase width from 1.0mm to 1.5mm or depth from 0.5mm to 0.8mm. Trade-off: larger gate vestige — but 0.3mm is easier to trim than a jetting defect that makes the part scrap. If the gate cannot be enlarged, add a jetting tab.
- Add a jetting tab (impingement pin or overflow tab) — this forces the melt to hit a surface before entering the cavity. A jetting tab is a small protrusion (3–5mm wide, 2–3mm long) placed opposite the gate. The melt hits the tab and is forced to spread radially — eliminating the free jet. After molding, the tab is broken off (if outside the part) or left in place (if non-functional). An impingement pin is a similar feature on the mold core. Both add small mold cost but eliminate jetting permanently.
Jetting tab (impingement tab) — a small protrusion (3–5mm wide, 2–3mm long) placed opposite the gate to force the melt to hit a surface before entering the cavity. The melt hits the tab and spreads radially — eliminating the free jet. After molding, the tab is broken off. An impingement pin is a similar feature on the mold core. Both add small mold cost but eliminate jetting permanently. The gate vestige guide covers how gate type affects jetting tendency.
Jetting: melt shoots through gate at >1 m/s, folds into snake pattern. Not just cosmetic — folds have 30–50% lower strength. Fix: reduce first-stage speed to 10–20mm/s, enlarge gate (pin 0.8→1.2mm), add jetting tab. Jetting starts at gate; flow marks appear further from gate.
- First-stage speed ≤20mm/s until melt contacts wall — below 0.5 m/s jetting threshold.
- Pin gate ≥1.2mm, submarine gate ≥1.5mm wide — larger gate = lower exit velocity.
- Jetting tab opposite gate if gate cannot be enlarged — forces radial flow, eliminates free jet.
- Defect at gate = jetting; away from gate = flow marks — reduce speed 50% to confirm.
- Mold temp as secondary fix only — +20°C reduces visibility 20–30%, does not fix root cause.
Frequently Asked Questions
Why does jetting only happen with certain gate types and not others?
Jetting requires a small gate feeding into a large open cavity. Pin gates (0.5–1.2mm) and submarine gates (0.5–1.5mm) have small cross-sections — melt exits at high velocity. Edge gates have a large cross-section and are adjacent to the wall — melt exits and immediately contacts the wall, establishing fountain flow. If the design requires a pin or submarine gate, plan for jetting prevention from the start.
Can jetting be fixed by raising the mold temperature?
Raising mold temperature helps marginally — a hotter wall keeps melt fluid longer, allowing folded layers to heal. But it does not eliminate the root cause (high gate velocity). Fold lines are caused by melt folding over itself — the two surfaces have different orientations and do not fully bond. For cosmetic improvement, raising mold temp by 20°C may reduce jetting visibility by 20–30%. For a structural fix, gate velocity must be reduced — use mold temperature as a secondary measure only.
How do I know if the defect is jetting or just a cosmetic flow mark?
Check the location and pattern. Jetting starts at the gate and extends in a snake pattern — lines are irregular, worm-like, often raised. Flow marks appear as regular, wavy lines following the flow path, not connected to the gate. Test: reduce first-stage speed by 50%. If the defect disappears, it was jetting (velocity-dependent). If it remains, it is a flow mark (temperature-dependent).
Key Takeaways
- Jetting: melt exits gate >1 m/s, folds into snake pattern.
- Not just cosmetic — folds have 30–50% lower tensile strength.
- Fix: slow first stage (10–20mm/s), enlarge gate, add jetting tab.
- Jetting starts at gate. Flow marks appear further from gate.
Send the part drawing with the gate location, gate type and photos of the defect — the troubleshooting review identifies whether the marks are jetting or flow marks and specifies the speed profile and gate modification needed to eliminate the defect.
Sources: Plastics Technology — Jetting Defect Diagnosis and Prevention · Plastics Technology Online — Gate Design and Melt Velocity Effects · Society of Plastics Engineers (SPE) — Jetting and Flow Instability Technical Papers · BASF — Processing Guidelines for Engineering Resins · DuPont — Injection Molding Troubleshooting Guide
— Arthur, Tooling Engineer at aCatMould