A concentric matte corona around the gate. These are micro-notches caused by excessive shear; eliminated with the speed profile and gate geometry.
A dull ring forming near the gate is the reflection on the surface of the sudden change in speed and pressure the melt experiences as it enters the mold. It appears as a circular area around the gate with a different gloss from its surroundings. The defect is cosmetic, but on glossy, visible parts it is a direct reason for rejection.
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 |
|---|---|
| Gloss Differences | A gloss difference spreads over wide areas of the part. A dull ring is only immediately around the gate and has a circular boundary. |
| Jetting | Jetting leaves a worm-like mark extending inward from the gate. A dull ring, on the other hand, is an area surrounding the gate. |
It usually improves it, because the shear rate in the gate area decreases. But slowing down only in the first stage and speeding up afterward is more effective — a staged speed profile is the standard solution for this defect.
Yes. A hot mold surface lets the melt stay fluid for longer and surface replication improves. If there is a local cold area around the gate, the layout of the cooling channels should be checked.
Textured (EDM) surfaces largely hide the difference; on glossy surfaces, even the smallest replication defect is visible. If the part has a visible surface, moving the gate to a non-visible area at the design stage is the most lasting solution.