Jetting in Injection Molding: The Snake-Like Flow Mark

Jetting appears as a worm-like squiggle from the gate. It often looks like a thin bead of plastic was piped onto the surface and molded over. Mechanically, that is what happened.

In a stable fill, melt enters the cavity, hits the walls, and forms a smooth expanding flow front. Jetting happens when melt shoots through the gate into open volume (like water from a hose) before the cavity backfills around it. That jet cools in flight. When the rest of the melt packs in, the cooled strand does not fully re-melt and bond.

You get a visible snake on the surface and a poorly bonded region inside. Jetting is a structural flaw, like a weld line wrapped into a squiggle. If a load passes through a jetted area, integrity is compromised.

The Root Causes: Gate Sizing and Placement

The shop-floor reflex is to slow the first injection stage. That often suppresses the symptom and quietly locks a longer cycle into the life of the tool. Persistent jetting is usually geometry.

Gate placement: The classic cause is a gate that fires into open cavity. The usual fix is to redirect so melt immediately hits a cavity wall or core, breaks the jet, and forces a stable front.

Gate sizing and velocity: Jetting is exit-velocity driven. Eastman’s mold design guidelines size a gate at about 50 to 80 percent of the wall thickness at that location, with a practical floor of 1.65 mm (0.065 in) for their polyester materials. A gate well below that ratio forces much higher exit velocity for the same fill rate and creates a jet.

Shear management: The same guidance stresses smooth transitions. Where a thick sprue or runner meets a thin wall, radiusing is required. Sharp corners in the delivery system generate shear, and excess shear at the gate supplies the energy that becomes a jet.

Material Substitution Risks

Jetting often appears when a tool built for one material is run with another. As Eastman notes, different viscosities need different gate sizes. A gate that filled well with a low-viscosity resin can act as a high-velocity restriction for a high-viscosity replacement, inducing jetting on an unchanged tool.

Buyer Action Plan at T1

If jetting shows at T1, treat it as a tooling discussion, not only process tuning.

Ask for a stable correction, not only a slowed first stage. Require gate size checked against the material supplier’s recommendations. If the gate fires into open space, ask what steel changes would make the flow hit a wall. Do not treat jetting as only cosmetic. If the area is load-bearing, require mechanical testing to confirm weld quality.

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.