Melt that has frozen in the nozzle/gate being carried into the part with the next shot; it leaves a comet-tail mark. Solutions via the nozzle temperature and a cold slug well.
A cold slug forms when a piece of melt left over from the previous cycle and cooled at the nozzle tip is swept into the mold with the next shot. It sits on the part surface as an unfused spot that looks different from its surroundings. Its source is not the mold but temperature management in the nozzle area; that is why corrections made on the mold side do not work.
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 |
|---|---|
| Tiger Stripes | Tiger stripes take the form of bands repeating from the gate. A cold slug is a single, irregularly positioned spot. |
| Weld Line | A weld line forms a sharp line and is always in the same place. A cold slug can land in a different place with each shot. |
The cooling at the nozzle tip depends on cycle pauses and residence time. It is typical for the problem to be invisible while the cycle runs smoothly and to appear in the first shots after a pause.
It is often the first and most effective step, but overdoing it leads to stringing. The permanent solution is to review the nozzle type; using a shut-off nozzle largely eliminates cold slugs.
It can be used in most applications, but it creates pressure loss and is prone to wear with filled materials. Material can also build up and degrade in the mechanism; it requires periodic cleaning.