Jetting in Injection Molding: The Snake-Like Flow Mark

Most flow defects look like what they are—lines, streaks, halos. Jetting is the strange one: a wormy, snake-like squiggle wandering across the part from the gate, sometimes visible as a distinct strand lying on the surface. It looks like someone piped a thin bead of plastic onto the part and molded over it—because mechanically, that’s close to what happened. This guide covers jetting from a buyer’s standpoint, as part of the injection molding defects section.

What Jetting Is

In a well-behaved fill, melt enters the cavity and immediately contacts the walls, forming a smooth, expanding flow front that fills outward under pressure. Jetting happens when the melt instead shoots through the gate into open space—an unrestrained jet squirting across the cavity like water from a hose—before the cavity backfills around it. That early jet cools as it flies and lands, and when the rest of the melt packs in around it, the cooled strand doesn’t fully re-melt and weld. The result is the visible snake on the surface, and a poorly bonded region inside the part.

That second half matters: jetting isn’t only cosmetic. The jetted strand is a weld-quality problem—like a weld line wrapped into a squiggle—so a jetted area can also be a weak one.

Why It Happens

Cause	How it produces jetting
Gate placement into open volume	The gate fires melt across a cavity with nothing to land on—the classic cause
Gate too small	High velocity through a small gate turns the melt into a jet
Injection speed too high (early)	Fast first-stage filling launches melt instead of building a front
Melt/mold too cold	A cold jet freezes before the pack can re-melt and blend it
No flow obstruction near the gate	Nothing to break the jet and establish a front

Gate design and placement are important patterns to verify, while the initial fill profile, material condition, temperature, and cavity geometry can also influence the result. The gate design guide covers the tooling-side questions.

Technical Patterns and Corrections to Verify

• Gate into a wall, not open space. Redirecting the gate so melt immediately impinges on a cavity wall or core breaks the jet and forms a front—the canonical fix.
• Enlarge or re-style the gate. A bigger gate or a type that spreads flow (fan, tab) lowers exit velocity.
• Slow the first stage. A slower initial injection speed lets the front establish before speeding up—a standard profiled-injection move.
• Warm things up. Higher melt and mold temperature help a jet blend back in, though this treats the symptom more than the cause.

For a buyer, the important question is whether the supplier has demonstrated a stable correction. Ask for the proposed mechanism, before/after samples, the process conditions used, and whether a gate change was evaluated before approving a tuned-around production process.

What a Buyer Should Do

• Flag likely geometries early. Parts where the gate would fire into an open volume deserve a gate-placement conversation before tooling.
• Treat jetting at trial as a tooling discussion, not just a process one—ask what gate change would eliminate it rather than suppress it.
• Check the jetted area’s strength, not just its look, if any load passes through it.
• Distinguish it from its cousins. Wavy concentric patterns are flow lines; a straight seam where fronts meet is a weld line. The snake from the gate is jetting.

This is an independent buyer resource and not a substitute for engineering analysis. Whether a specific jetting problem is best solved by gate, process, or design depends on the part and tool—work the fix with your supplier.

Buyer FAQs

What does jetting look like on a molded part?

A wormy, snake-like squiggle or strand pattern wandering across the surface, starting at the gate—quite different from the smooth concentric patterns of flow lines or the straight seam of a weld line. It often looks like a thin bead of plastic lying on the surface that was molded over, which is essentially what happened.

What causes jetting in injection molding?

Melt shooting through the gate into open cavity space instead of immediately contacting a wall and forming a flow front. The jet cools in flight, and when the cavity packs around it, the cooled strand doesn’t fully re-weld. Root causes are usually gate placement firing into an open volume, an undersized gate creating high exit velocity, too-fast initial injection speed, and cold melt or mold compounding the poor re-weld.

Is jetting just cosmetic?

No. The visible squiggle is also a poorly bonded region—the jetted strand didn’t fully re-melt and weld to the surrounding material, similar to a weld line wrapped through the part. If structural load passes through a jetted area, strength is a legitimate question, not just appearance. That’s worth raising explicitly at sampling.

Can jetting be fixed by process adjustment alone?

Sometimes. A controlled first-stage fill profile, material and mold-temperature review, or other process change may correct the observed pattern. Gate size, type, and location are also important possibilities to verify. If jetting appears at trial, ask the supplier to compare the proposed process and tooling explanations and demonstrate a stable correction rather than assuming either one is the durable answer.

Evidence Box

The guidance on this page—covering jetting—summarizes molding failure mechanisms, causes, and corrective directions that are consistently documented across industry troubleshooting references and supplier technical literature, combined with buyer-side review logic. No defect can be diagnosed from a description alone: cause and fix on a real part depend on the specific tool, resin, and process, and should be confirmed with your supplier against physical evidence.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, diagnose production problems remotely, or certify suppliers. Confirm defect causes and corrective actions with your supplier against your specific part, tool, and process.