Plastic Mould Defects-Dark Spots

Different factors can cause the formation of dark spots or speckled:

1)  process-related causes:

e.g. melt temperature too high or residence time in the plasticizing unit too long.

Wrong temperature in the hot-runner system

2)      mold-related causes:

e.g. dirty gate system or wear ((dead spot)) in the hot-runner system

3)      machine-related cuases:

e.g. dirty plasticizing unit or wear of screw and cylinder

4)      caused by polymer or dyeing:

e.g. impurities in the granule, high reclaim content or unsuitable dye / master batch

Impurities in the granule? If yes, then

(1) check granule for impurities

Do dark spots appear after a change of material? If yes, then

(1) clean plasticizing unit

Is the melt temperature exceeding the processing range? If yes, then

Reduce melt temperature:

(1) vary cylinder temperature (-)

(2) vary screw speed (-)

(3) reduce back pressure

Is the residence time of the melt within the critical range? If yes, then

(1) reduce cycle time

(2) increase plasticizing time delay

(3) check dimensil on of plasticizing unit

If no, then

(1) check hot-runner temperature

(2) reduce reclaim content

(3) check dye for compatibility

(4) check plasticizing system, gate system and hot-runner for impurities, wear and dead spot