Jetting in Injection Molding: The Snake-Like Flow Mark

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

In a stable fill, molten plastic enters the cavity and immediately contacts the walls, forming a smooth, expanding flow front. Jetting occurs when the melt shoots through the gate into an open volume—like water from a hose—before the cavity backfills around it. That initial jet cools in flight. When the rest of the melt finally packs in, the cooled strand does not fully re-melt and bond.

The result is a visible snake on the surface, but more importantly, it is a poorly bonded region inside the part. Jetting is a structural flaw akin to a weld line wrapped into a squiggle. If a load passes through a jetted area, the part’s integrity is compromised.

The Root Causes: Gate Sizing and Placement

The reflex fix for jetting on the shop floor is to slow down the initial injection speed. While this often suppresses the symptom, it quietly commits the buyer to a longer cycle time for the life of the tool. Persistent jetting is usually a geometry problem.

Gate Placement: The classic cause of jetting is a gate that fires directly into an open cavity. The canonical fix is to redirect the gate so the melt immediately impinges on a cavity wall or core, breaking the jet and forcing a stable flow front to form.

Gate Sizing and Velocity: Jetting is fundamentally driven by exit velocity. Eastman’s mold design guidelines state that a gate should be sized at approximately 50 to 80 percent of the wall thickness at that location, establishing a practical floor of 1.65 mm (0.065 in) for their polyester materials. A gate significantly below this ratio forces the melt to exit at a much higher velocity for the same fill rate, creating a jet.

Shear Management: The same guidance stresses the importance of smooth transitions. Where a thick sprue or runner transitions into a thin wall, it must be radiused smoothly. Sharp corners in the delivery system generate shear, and excessive shear at the gate provides the energy that becomes a jet.

Material Substitution Risks

Jetting frequently appears when a tool designed for one material is repurposed for another. As Eastman notes, different polymer viscosities require different gate sizes. A gate that filled perfectly with a low-viscosity resin may act as a high-velocity restriction for a high-viscosity replacement, inducing jetting on a tool that hasn’t changed.

Buyer Action Plan at T1

If jetting appears at the T1 trial, treat it as a tooling discussion, not just a process tuning exercise.

Ask the supplier to demonstrate a stable correction, not just a slowed-down first stage. Require them to verify the gate size against the material supplier’s recommendations. If the gate fires into open space, ask what steel changes would be required to impinge the flow against a wall. Finally, do not treat jetting as merely a cosmetic finding; if the affected area is load-bearing, require mechanical testing to confirm the 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.