A film spilling out of the parting surface, ejectors and vents. The clamping force, pressure peaks and sealing surfaces are checked.
Flash is thin excess plastic formed by the melt leaking out through the parting surface of the mold. Its basic cause is simple: at the moment of filling, the force trying to open the mold has exceeded the clamping force. But behind this there may be mold maintenance, holding pressure and melt flowability as well as machine tonnage; where the flash forms tells you which possibility applies.
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 |
|---|---|
| Dimensional and Weight Deviations | Dimensional deviation is seen across the whole part and points to process instability. Flash forms only on the parting surface, in a specific area. |
| Overpacking | Excessive holding produces both flash and a stressed part; a sink mark, in the opposite direction, is a sign of insufficient holding. If both are seen at the same time, there is a problem in the holding profile. |
It may solve it temporarily, but it is often the wrong move. First check the holding pressure and injection speed, then the melt temperature. If there are deposits or wear on the mold parting surface, no tonnage will close it; moreover, excessive clamping blocks the venting and produces new defects.
Calculate the required tonnage from the projected area of the part and the cavity pressure of the material; add the runner for a multi-cavity mold. If the calculated value is close to the machine's capacity, the problem really is tonnage.