Jetting in Injection Molding: When the Melt Enters as a Rope

What jetting looks like

Jetting is a snake-like ridge of material on the surface of a part, usually starting at the gate and wandering across the cavity. It looks like a worm that has been pressed into the plastic, sometimes with a glossy track and a matte edge, and it is often accompanied by a small fold or a weld line where the rope has doubled back on itself.

The cause is that the melt does not fill the gate. It shoots through as a free jet, exactly like water from a hose, and lands on the far wall of the cavity before the cavity has any material in it. The jet then coils and folds, and the folds freeze into the surface as it cools.

This is a filling defect, not a packing defect, so it appears early in the shot and it does not respond to hold pressure. That distinguishes it from sink marks and voids, which are the result of what happens after the cavity is full.

The gate-to-depth ratio

Whether the melt jets or spreads depends on the relationship between the gate size and the space it is entering. The useful number is the gate diameter divided by the cavity depth at the gate, and the behaviour changes sharply as that ratio falls.

At a ratio of 0.15 the severity is about 9 on a ten-point scale, at 0.25 it is 6, at 0.35 it is 3, at 0.50 it is 1 and at 0.70 there is no jetting at all. The transition sits between 0.3 and 0.4, which is why a gate that is a third of the cavity depth is the usual rule of thumb.

The physics is straightforward. A jet forms when the momentum of the melt carries it across the gap before viscous forces and the growing melt pool can slow it down. A bigger gate reduces the velocity for the same volumetric flow, and a shallower cavity gives the jet less distance to travel before it hits the opposite wall and becomes a spreading front.

Table 1 — Gate size against cavity depth, and what each band needs.

Speed makes it worse

The second variable is fill speed, and its effect is the opposite of what a production engineer wants. Faster filling means more momentum at the gate, so the jet travels further before it breaks up, and the defect gets worse. Severity rises from 2 at 20 mm/s to 4 at 50, 7 at 100, 9 at 200 and 10 at 300 mm/s.

This puts jetting in direct conflict with cycle time. The obvious way to fill a thin part is fast, and fast is exactly what produces the rope. The resolution is a profile: fill the first 10 to 20 percent of the shot slowly, at 20 to 40 mm/s, so that a pool forms at the gate and the jet is suppressed, then accelerate to the normal speed once the flow front is established.

The slow first stage costs very little cycle time because it covers so little volume. On a 60 gram shot, filling the first 10 grams at a quarter speed adds about 0.15 seconds, which is invisible next to the cooling time and far cheaper than a tool modification.

Fixing it in the tool

The permanent fixes are all geometric. The first is to enlarge the gate, which reduces the velocity for the same flow rate and moves the ratio toward 0.5. The second is to move the gate so that it points at a wall close by, so the jet has nowhere to travel. The third is to add an impingement pin or a small boss directly opposite the gate, which breaks the jet into a spreading flow.

The fourth, and often the cheapest, is to change the gate type. A tunnel or submarine gate can be angled so that it discharges against the cavity wall rather than across it, which converts the jet into a wall-hugging flow without changing the part. A fan or a tab gate does the same thing more thoroughly, at the cost of a gate that has to be trimmed.

Table 2 — Tooling options, what each one changes and what it costs.

Fixing it in the process

When the tool cannot change, three process levers remain. The first is the velocity profile already described: a slow first stage that lets a pool form. The second is melt temperature, raised by 10 to 20 C, which lowers the viscosity and lets the jet spread rather than stay coherent. The third is a slightly larger shot cushion, which changes the flow pattern at the start of injection.

Mould temperature helps less than people expect. A hotter mould keeps the rope soft for longer so it can level out, but it does not stop the jet forming. A 20 C rise in mould temperature typically takes one point off the severity scale, where slowing the fill takes three or four.

What does not work is raising the hold pressure. Jetting happens during filling, before the cavity is full, and hold pressure acts after. A moulder who increases hold pressure to chase a jetting mark will add flash and increase the residual stress without touching the defect.

Telling jetting from other marks

Three defects look similar on a dark, glossy part, and the fix is different for each. Jetting is a raised or glossy track that starts exactly at the gate and wanders. A flow mark is a dull, wavy line that appears where two flow fronts meet or where the front stalled, and it is usually further from the gate.

A silver streak is a bright line caused by moisture or by gas, and it follows the flow direction rather than the jet path. It usually appears in the last area to fill. A drag mark or a scratch is a groove rather than a ridge, and it is on the surface rather than in it.

The quick test is to look at the gate under a low-power lens. Jetting always starts within a millimetre or two of the gate and has a coherent track for the first few millimetres. If the mark starts 10 mm away and has no track, it is a flow or a gas defect, and enlarging the gate will not help.