Jetting — Causes and Solution

Jetting occurs when the melt spurts from the gate into the mold cavity as a free jet. This jet, advancing without sticking to the mold surface, cools and does not fuse fully with the melt that comes after it; it leaves a worm-like mark extending into the part from the gate. This mark is both a visible defect and a mechanical weakness in that area.

How to Recognize It

A serpentine/snake-shaped matte or rough line starting from the gate and spreading into the part.
There may be gloss and color differences around the line; it can be confused with the record groove effect.

Physical Causes

The melt spurts into the cavity as a free strand WITHOUT FORMING a frontal flow front; the surface of the strand cools and cannot bond homogeneously with the melt that comes after it.
A sudden increase in cross-section + high injection speed is the typical combination.

Solution Steps at the Machine

Apply the steps in order and one at a time: after each change, wait a few cycles and assess the result. If there is no improvement, undo the change and move on to the next step.

Reduce the injection speed or use a profile (start slow → speed up).
Increase the melt temperature.
In rare cases the opposite works: hiding the mark by raising the speed VERY high.

Mold, Design and Material Solutions

Round off the gate/part transition; ENLARGE the gate.
Position the gate so that the melt strikes a wall or obstacle (impact die).
Avoid filling the cavity from top to bottom.
With amorphous materials, dynamic temperature control can remove the visual mark.

Which defect is it confused with?

A wrong diagnosis leads to turning the right parameter in the wrong direction. This defect can be confused with the ones below; the right-hand column gives the practical way to tell them apart.

Confused with	How to tell them apart
Dull Ring Near the Gate	A dull ring is a circular area surrounding the gate. A jetting mark, on the other hand, is a line extending inward from the gate.
Cold Slug	A cold slug is a separate piece coming from the nozzle and its position varies. A jetting mark always starts from the gate.

Frequently asked questions

Does lowering the injection speed always solve it?

A profile that is slow in the first stage and fast afterward eliminates the problem in most cases. But the real cause is the gate geometry: as soon as the melt enters the cavity, it should strike a surface and spread out.

What should change in the gate design?

Positioning the gate close to the opposite wall or so that it strikes a surface physically prevents jet formation. Enlarging the gate cross-section is also effective, by reducing the speed of the melt.

Prepared by:Uğur Çamlıca· Teknik Part · Source: manufacturer documentation and field practice

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