Jetting in Injection Molding: The Snake-Like Flow Mark

Jetting appears on an injection-molded part as a distinct worm- or snake-like squiggle radiating outward from the gate—looking as though a thin strand of toothpaste was squeezed onto the empty mold cavity first and then flooded over by the rest of the plastic.

Mechanically, that visual impression is exact. In a healthy injection molding cycle, molten polymer exits the gate, touches the adjacent cavity and core walls, and develops a stable, smoothly expanding fountain flow front. Jetting occurs when a high-velocity stream of melt shoots through a restricted gate directly into an open, unobstructed cavity volume before establishing contact with the cavity walls. That free-flying rope of plastic folds back and forth across the cold steel and begins to freeze. Moments later, when the bulk flow front catches up and packs around the cooled strand, the two temperatures fail to fuse completely—locking both a visible snake mark onto the surface and a jagged internal weld line inside the wall.

Why Jetting Is Both a Cosmetic and a Structural Defect

Buyers reviewing T1 sample parts sometimes wave off jetting on a non-cosmetic or painted housing assuming that a coat of primer or a matte texture will cover the squiggle. That assumption creates two serious downstream failures:

1. Internal Delamination and Impact Weakness: Because the cooled polymer jet never fully re-melts when the warm main flow surrounds it, the boundary around every coil of the “snake” behaves like a cold weld line. Under drop impact, flexural bending, or internal fluid pressure, cracks initiate along the outline of the jetted strand at a fraction of the base resin’s rated strength.
2. Differential Gloss and Paint Telegraphing: Even after sanding or painting, the jetted strand has a different molecular orientation, density, and shrinkage profile than the surrounding plastic matrix, causing the worm pattern to telegraph through coatings after thermal cycling.

Root Causes: Gate Geometry, Exit Velocity, and Shear

When jetting shows up during a press trial, the technician’s immediate reflex is to slow down the initial injection velocity so the melt dribbles out of the gate slowly enough to form a puddle before accelerating. While a slow-fast velocity profile is a legitimate diagnostic test, relying on ultra-slow initial fill alone can lengthen cycle time or cause hesitation lines if the underlying gate geometry is flawed.

Persistent jetting traces back to three physical tooling and material factors:

Root-Cause Factor	Physical Mechanism at the Gate	Permanent Tooling / Design Solution
1. Unobstructed Gate Line-of-Sight (“Free-Stream Firing”)	An edge or pin gate points straight down the length of a long, open cavity or steps down suddenly from a thin gate into a thick cavity pocket without hitting a wall.	Impinge the Flow: Re-orient the gate location or add an overlapping tab/core pin so incoming melt immediately strikes a steel wall or core face at ~45°–90°, breaking momentum and forcing radial fountain flow.
2. Undersized Gate Cross-Section (Excessive Exit Velocity)	For a fixed volumetric fill rate ($Q = A \times v$), making the gate area ($A$) too small forces exit velocity ($v$) to spike, turning laminar flow into a high-speed jet.	Scale Gate to 50%–80% of Wall: Per Eastman Chemical’s mold design guidelines, open the gate depth to 50% to 80% of the nominal wall thickness (and avoid depths below 1.65 mm / 0.065 in on shear-sensitive polyesters where wall thickness allows).
3. Abrupt Step Transitions & High Shear Heating	Sharp 90° steps between the gate land and a thick part wall—or high-filler/low-melt-swell resins (such as mineral/glass-filled grades or PVC)—prevent the polymer stream from swelling to touch the walls.	Radius Transitions & Use Fan/Tab Gates: Machine a smooth, sloped transition or fan/tab gate that gradually expands the flow channel from the runner into the cavity wall.

Why Switching Resins Often Triggers Jetting on an Existing Mold

Jetting frequently appears when a buyer asks a molder to run an alternate resin grade in an existing tool that previously ran cleanly.

As Eastman’s processing guidelines emphasize, polymers differ in both melt viscosity and die swell (extrudate swell)—the natural tendency of entangled polymer chains to expand radially as they exit a narrow gate restriction. Unfilled amorphous resins with high elasticity swell outward as they exit the gate, helping the melt stream touch the top and bottom cavity walls and establish fountain flow. Highly filled compounds (such as 30%–40% glass-filled or mineral-filled resins) have much lower die swell; the stream exits the gate like a stiff rod without expanding to touch the cavity walls. Consequently, a gate depth that worked for an unfilled resin often requires opening up or converting to a tab/fan gate when running a filled or higher-viscosity grade.

Buyer Action Plan at Pre-Steel DFM and T1 Review

Use these checkpoints to prevent or resolve jetting without accepting a fragile processing band-aid:

1. At Pre-Steel Mold Design Review: Inspect the 3D gate detail in the mold design review. Verify that the gate depth is specified at 50% to 80% of the wall thickness (possibly cut steel-safe at 50% for T1) and check where the gate vector points. If an edge gate fires straight down a long open channel or drops into a thick pocket without hitting a core or wall, ask the toolmaker to add a radiused transition, a tab gate, or an impinging flow path before cutting steel.
2. At T1 Sample Review: If T1 parts exhibit snake-like marks near the gate, ask the supplier whether the gate is still at its initial steel-safe minimum depth. Enlarging the gate depth on the toolroom bench (for example, moving from 50% to 70% of wall thickness) combined with a two-stage “slow across the gate, fast through the cavity” velocity profile solves most jetting cases permanently without sacrificing cycle time.
3. For Load-Bearing or Pressure-Containing Parts: Never approve a part with visible jetting in a structural zone on cosmetic waivers alone—require destructive mechanical or burst testing to verify that the knit boundary has been eliminated.

Buyer FAQs

What is the difference between jetting and normal flow lines?

Flow lines (or hesitation marks) are concentric, ripple-like rings or Halo bands that follow the contour of the expanding melt front when injection speed fluctuates or the melt cools prematurely against cold mold walls. Jetting is a distinct, erratic worm- or snake-like squiggle that shoots straight out from the gate before folding onto itself, caused by a high-velocity stream of plastic flying into open cavity space without touching the mold walls first.

Can jetting be fixed just by slowing down injection speed?

Slowing down the initial injection velocity profile—so the screw moves slowly just until the melt front clears the gate orifice and establishes wall contact, then accelerates to normal fill speed—often helps suppress jetting. However, if the gate depth is fundamentally undersized (well below 50%–80% of the wall thickness) or points directly into a deep open cavity pocket, relying on slow injection speed alone can starve thin walls downstream, increase cycle time, and leave a narrow process window. Permanent correction usually pairs a profiled first-stage velocity with opening the gate depth or adding a radiused tab transition in the steel.

What gate types are best for preventing jetting?

Tab gates, fan gates, and overlap/flash gates are specifically engineered to prevent jetting because they spread the incoming melt stream or force it to impinge against a perpendicular wall before entering the main cavity. Conversely, small-diameter pin gates, submarine (tunnel) gates, and narrow edge gates firing directly into a thick, unobstructed cavity section carry the highest risk of jetting unless angled against a core face or cavity wall.

Why do glass-filled or mineral-filled plastics jet more easily than unfilled resins?

Unfilled polymers exhibit significant viscoelastic “die swell”—as the compressed polymer chains exit the narrow gate orifice, they relax and expand outward, helping the melt stream touch the cavity walls and establish smooth fountain flow. Adding 30% to 40% glass fibers or mineral fillers suppresses die swell so the exiting melt stream stays narrow and shoots forward like a rigid rope, making proper gate depth (50%–80% of wall) and impinging gate placement even more critical for filled resins.

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.