How to Prevent Jetting: Gate Design and Mold-Side Fixes

A 250 mm appliance housing left the press with worm-like streaks on 60% of its gloss surface. Each streak started at the gate and snaked toward the far wall in folds. The mold was new, the resin was PP, and the gate was a pinpoint aimed across 180 mm of open cavity. Reject rate ran 34% across the first shift. The lasting fix was a drawing change plus a 20 °C hotter cavity.

Jetting happens when melt exits the gate as a free jet and snakes across the cavity instead of spreading as a front. The jet folds over itself, leaves worm tracks, and welds each fold skin to skin. Those folds are weak internal weld lines, not cosmetic noise. This guide goes deep on jetting and the flow marks that live beside it. The 9-defect troubleshooting overview covers jetting in one paragraph; here the gate, runner, and cavity detail decide the part.

Read this from the mold side. You are a mold buyer or tooling engineer choosing gate type, gate location, runner layout, and mold temperature before steel is cut. Process tweaks appear last, because they paper over geometry. Values marked typical come from industry practice and published processing guides, so confirm each one against your resin datasheet.

The Snapshot

• Jetting occurs when melt leaves the gate faster than it can grip the wall and fold into a front. The melt snakes across open space as a free jet.
• Gate placement rule: direct the melt at an opposite wall or rib within about 2 to 3 times the gate depth. The jet breaks, folds, and restarts as a normal front.
• Fan, edge, and film gates spread the flow front and cut jetting risk; pinpoint and tunnel gates concentrate velocity and jet more readily.
• A small gate raises exit velocity and shear heating, and both make jetting worse. Enlarge the gate, keep the land short, and polish the exit.
• Raising mold temperature by 10 to 30 °C, shortening free-fall distance, and extra care with low-viscosity PP and PE grades each reduce jetting at trial.

What Jetting Looks Like and Why It Kills Parts

Jetting leaves three fingerprints. First, worm-like tracks: rope folds that start at the gate and wave across the cavity in an unbroken line. Second, surface streaks: gloss changes, silver lines, or dull bands that follow the jet path. Third, hidden damage: every fold that laps over itself creates an internal weld line where two skins met with trapped air between them.

The defect zone is compact and predictable. On long cavities the visible damage usually sits within 20 to 50 mm of the gate, because that is where free flight happens. Beyond that zone the jet finally hits a wall, folds, and fills normally. That is why molders call jetting a near-gate defect, and why inspection starts at the gate every time.

Short shots expose the geometry. A molder runs a clear or dyed resin, stops fill at 70%, and opens the tool. A jetted short shot shows a rope lying across open space with bare cavity around it. A healthy short shot shows a rounded front pressed against both walls with full-width contact. The difference is visible in seconds and costs almost nothing to run.

Jetting kills parts twice. Cosmetics reject a gloss housing on sight, which is the fast kill. Strength rejects later. Each fold is a weld line, and weld lines carry less strength than solid melt. Molecular chains meet skin to skin instead of entangling. A structural part can look clean at the gate and still crack through a fold under load or impact.

Air entrapment rides along as a third failure. A free jet does not push air ahead of a rounded front; it seals air between its folds and the cavity wall. Trapped air shows as voids, silver streaks, or burn marks along the fold path. Venting at the end of fill helps only where the front arrives last, so it cannot vent a pocket that sealed early behind the jet.

The Physics of Jetting: Velocity vs Front Spread

Normal filling is fountain flow. Melt exits the gate, touches the cavity wall, and folds forward continuously, with wall friction dragging the surface into a skin while the core advances. The front spreads across the full channel width as it moves. Jetting starts when gate velocity outruns what that front can absorb, and the melt loses wall contact.

The free-flight rule sets the threshold. Industry practice says jetting becomes likely when free-fall distance from gate exit to first wall exceeds about 2 to 3 times the gate depth. A gate 1.0 mm deep aimed at a wall 8 mm away already breaks the rule. A pinpoint gate at the center of a 180 mm housing breaks it by a factor of 60.

Velocity scales with geometry, not with press settings alone. Volumetric flow divides by gate area, so cutting the gate area in half doubles exit velocity at the same fill rate. Doubling the cross-sectional area of the gate cuts exit velocity in half. Every millimeter of gate cross-section is a velocity decision made on the drawing.

Shear heating makes small gates self-defeating. Melt forced through a small gate sees high shear rate. Shear heats the melt by tens of degrees at pinpoint gates on typical fill speeds. Hotter melt means lower viscosity, and lower viscosity means a thinner, faster jet that flies farther before it folds.

Each fold becomes a knit. The jet lays rope over rope, and two cooled skins meet without true mixing. The fold line behaves like a weld line you never drew on the part. Gate placement decides where those knits form, which is the same logic as any other knit. Our weld line guide covers knit strength and placement in detail; jetting is simply one more way to create one.

Jetting and flow marks share a neighborhood but not a mechanism. Flow marks are surface ripples from uneven front velocity, hesitation, or shear at the gate, and the melt keeps wall contact the whole time. Jetting means the melt lost contact entirely. The fixes overlap, which is why this guide treats both: gate geometry and mold temperature cure a large share of each.

Gate-Side Root Causes and Fixes

Seven root causes cover the jetting cases that reach a mold trial. The first four live in the gate itself, so each fix is a drawing change. The last three sit in the mold body and the resin, and each fix is still made before full production starts.

Root cause 1: the gate aims across a long open cavity. This is the most common cause and the easiest to see. Melt exits toward empty space, flies across the part, and folds only when it finally hits the far wall. Fix it by moving the gate so the melt strikes an opposite wall or rib within about 2 to 3 times the gate depth. In a deep box, aim the gate at the core side wall instead of dropping it through the center. On a flat panel, use an overlap gate that fires at the cavity wall behind the gate.

Root cause 2: the gate is too small. A small gate forces high exit velocity and high shear heating at the same time, and both drive jetting. Fix it by sizing the gate to the wall. Edge gate depth at 50 to 80% of wall thickness is the typical working range, with width sized for the flow length. Doubling the gate cross-section cuts exit velocity in half at the same fill rate, which is the fastest single change a toolroom can make.

Root cause 3: the gate type concentrates the stream. Pinpoint and tunnel gates deliver a narrow, fast rope, so they jet more readily on long open cavities. Fan, edge, and film gates spread the melt across a wider front before it enters the cavity. Fix it by choosing the gate type for the part geometry, not for tooling convenience. A fan gate costs more to cut than a pinpoint and pays for itself in first-shot yield. The risk table later in this guide ranks each type.

Root cause 4: the gate land is too long or the exit is rough. A long land keeps the melt channeled and delays the spread that should happen right after the gate. Fix it by keeping edge gate lands short, roughly 0.5 to 1.5 mm in typical practice. Polish the land and the cavity face just downstream of the gate. Rough steel tears the melt skin and leaves silver streaks that read as jetting on a gloss part. Radius the gate exit where the drawing allows.

Gate decisions belong on the drawing. Trade-offs between gate count, weld lines, and cosmetic zones are covered in our 7 gate design best practices. A jetting fix that survives production almost always starts there.

Mold and Material Root Causes and Fixes

Root cause 5: mold temperature is too low. A cold cavity wall freezes the jet skin before it can wet the steel and fold. The rope keeps its shape, and the defect locks in. Fix it by raising mold temperature by 10 to 30 °C, the typical trial range. Keep the melt near the middle of its window. A hotter cavity keeps the jet skin mobile long enough for the front to re-attach and spread.

Root cause 6: flow length is long enough to let the front stall or hesitate. On a long part, one gate at one end means a long free-fall distance and a slow fill at the far end. Hesitation shows up as flow marks where the front crawls over thin ribs. Fix it by cutting the distance the melt must travel. Add gates along the part, or run cascade or sequential valve gates that open in order. Each front then travels a short, controlled distance at full speed.

Root cause 7: the resin flows too easily for the gate geometry. Low-viscosity semicrystalline resins such as PP and PE jet most readily. So do very low-viscosity grades of any family. The thin melt loses wall contact faster. High-viscosity resins such as PC and ABS jet less often but show their own hesitation marks when the geometry is hostile. Respect the material in the gate choice. PP and PE need the wall-strike rule, fan gates, and hot cavities more than PC does.

Material and mold causes rarely travel alone. A PP housing on a cold mold with a pinpoint gate fails on all three at once, and changing any one of the three helps. Changing the gate geometry first is the industry habit because it fixes the other two at the same time.

Design Fixes, Ranked by Trial Payback

When jetting appears at trial, the toolroom works this list in order. Each fix is ranked by how often it ends the defect on its own. Every one is a drawing or mold change, not a process gamble.

1. Put a wall in the jet's path. Move the gate so the melt strikes an opposite wall or rib within 2 to 3 gate depths. The jet breaks on impact, folds, and restarts as a normal front. On deep boxes aim at the core wall; on panels fire at the cavity wall through an overlap gate. This single change eliminates most jetting.

2. Spread the front with a fan or edge gate. A fan gate widens the stream before it enters the cavity, dropping velocity across the whole front. Edge gates do the same across a narrower width. Both cut the rope effect that pinpoint gates create. Expect a small cost in gate trimming effort and a large gain in surface quality.

3. Enlarge the gate and shorten the land. Size edge gate depth at 50 to 80% of wall thickness, keep the land near 0.5 to 1.5 mm, and polish the exit. Doubling gate cross-section halves exit velocity at the same fill rate, which removes the speed condition that jetting needs.

4. Raise mold temperature by 10 to 30 °C. A hotter cavity keeps the jet skin mobile so it wets the steel and folds instead of freezing in rope shape. Move the mold up in steps of 5 to 10 °C at trial and watch the streak zone shrink before your eyes.

5. Use cascade or sequential valve gates on long parts. Stage the gates so each front fills a short span at controlled speed. Sequential fill also moves weld lines off loaded surfaces, which is a second win on the same drawing.

6. Balance the runner in multi-cavity tools. An unbalanced runner makes one cavity overspeed while another lags, so jetting shows on one station and not its neighbors. Balance runner diameters and gate sizes so every cavity sees the same velocity, then verify with short shots.

7. Polish the gate land and the cavity face downstream. Rough steel tears the melt skin and leaves silver streaks that look like jetting on gloss parts. Diamond-buff the gate zone to a mirror finish on cosmetic tools, and radius the gate exit where wall thickness allows.

Process settings come last for one reason. Slowing the first stage and heating the mold mask jetting, but they also stretch cycle time and leave the part sensitive to every material lot. A gate change costs one tooling revision and removes the defect for the life of the mold.

Gloss Parts: Jetting in the Visible Zone

Gloss surfaces make jetting unmissable. High-polish steel reflects every fold like a mirror, so a wave that is invisible on a textured part rejects instantly on a piano-black housing. The defect zone near the gate is the problem. On long cavities the visible damage typically sits within 20 to 50 mm of the gate. That zone is exactly where a gloss part gets inspected first.

The first answer is to hide the gate. Put the gate on a non-cosmetic face, an inside surface, a rib, or a parting line that the customer never sees. Edge and film gates can run along a hidden edge so the spread happens off the show face. Moving the gate is free at design stage and expensive after steel, so the gate location review belongs in the DFM pass before tooling starts.

When the gate must sit on the show face, geometry carries the load. Use a fan or film gate to spread the front. Aim the melt at a wall within the 2 to 3 gate-depth rule. Run the mold 10 to 30 °C hotter. On very long cosmetic panels, a valve gate with sequential control beats any fixed gate because each front fills a short span.

Mild flow marks on gloss parts need the same treatment. Shear-induced silver streaks near a small gate are the cousin of jetting. They respond to the same fixes: bigger gate, shorter land, polished steel, hotter mold. If the process must slow down to hold a surface, the gate is underdesigned and the tool, not the press, should change.

Gate Type vs Jetting Risk

Gate type sets the starting risk before a single trial shot. The table below ranks common gate types by how readily they jet, with typical best use for each. Risk ratings are typical industry practice for unfilled engineering resins on normal cavity geometry. A bad placement can make any gate jet, and a wall strike can tame the worst.

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Table: Typical gate type behavior on jetting risk for unfilled engineering resins. Risk ratings are industry practice and shift with placement, gate size, and resin viscosity.

Read the table as a placement problem first. Fan, film, and edge gates start with an advantage because they spread the melt before it enters the cavity. Pinpoint and tunnel gates concentrate the stream, so they demand the wall-strike rule, a larger gate, or a hotter mold to stay clean.

No gate type is immune. A fan gate aimed across a long open span can still jet. A pinpoint gate firing at a wall 1.5 mm away can fill clean. That is why the placement rule outranks the type choice, and why both belong in the DFM review before the tool is quoted.

FAQ: Jetting and Flow Marks

Q1. What is the difference between jetting and flow marks?

Jetting means the melt lost wall contact and flew across the cavity as a free jet, leaving worm tracks and folds. Flow marks mean the melt kept wall contact but moved unevenly, leaving ripples, hesitation bands, or shear streaks. They look similar near the gate and share fixes: gate geometry, gate size, and mold temperature. A short-shot check tells them apart in one trial.

Q2. How close must the opposite wall be to stop jetting?

The typical working rule is 2 to 3 times the gate depth. A gate 1.0 mm deep should fire at a wall or rib within about 2 to 3 mm. Beyond that distance the melt is free to snake. When no wall exists nearby, spread the melt with a fan gate, slow the first stage, or run the mold hotter.

Q3. Which resins jet the most?

Low-viscosity semicrystalline resins such as PP and PE jet most readily, and very low-viscosity grades of any family follow them. The thin, fast melt loses wall contact quickly at a small gate. High-viscosity resins such as PC and ABS jet less often but develop hesitation marks when geometry is hostile. The gate rules still apply to every material.

Q4. Can process settings alone fix jetting?

Partially, and only as a stopgap. Slowing the first stage by roughly 30 to 50% and raising mold temperature by 10 to 30 °C can hide jetting at trial. The defect returns with material lot changes, cycle pressure, and thin-wall fills. A gate geometry change removes the defect for the life of the mold, which is why the tooling fix is the production fix.

Q5. Why does one cavity jet in a multi-cavity tool while its neighbors are clean?

Runner imbalance is the usual cause. The cavity nearest the sprue fills at higher velocity, so it jets while the far cavities lag. Balance runner diameters and gate sizes across the tool, then verify with short shots at 50 to 70% fill. If one gate is dimensionally larger or smaller than its drawing, fix the gate first and check the runner second.

The Bottom Line

Jetting is a mold-side defect with a gate-side cure. Every lasting fix is a drawing change made before steel is cut or during the first tooling revision, not a process setting found by trial.

The checklist is short. Aim the gate at an opposite wall or rib within 2 to 3 times the gate depth. Prefer fan, edge, and film gates over pinpoint and tunnel gates on long spans. Size edge gate depth at 50 to 80% of wall thickness, keep the land near 0.5 to 1.5 mm, and polish the exit. Run the mold 10 to 30 °C hotter when the jet freezes in shape, and give PP and PE the most conservative geometry of all. Balance runners in multi-cavity tools so velocity is equal from station to station.

Do the geometry once and the defect stays gone. DieStrike builds injection molds under IATF 16949 across 120+ machines. Molds ship in 2 to 5 weeks with geometry at ±0.005 mm standard and ±0.002 mm precision. Hardened cavity steel runs at HRC 62, and standard parts ship in 3 to 7 days. Connector OEMs including TE Connectivity, Amphenol, and Luxshare run production on the same discipline.