(1) The temperature of the melt is too high.
Too high material temperature will cause the melt to decompose overheated, forming carbides, so in order to avoid overheating decomposition of the melt, especially for heat-sensitive thermoplastic materials such as polyvinyl chloride, the temperature at the tail of the barrel must be strictly controlled not too high.
Especially when black spots and stripes appear on the surface of the plastic parts, it is necessary to immediately check whether the temperature controller of the barrel is out of control, and it is also necessary to appropriately reduce the temperature of the barrel and mold.
However, it is worth noting that if the material temperature and mold temperature are too low, it will also cause bright streaks on the surface of the plastic part.

(2) The barrel gap is too large.
If the wear gap between the screw and the barrel is too large, the convergence melt will be trapped in the barrel, resulting in the local overheating and decomposition of the retained melt to produce black spots and streaks.
In this regard, the barrel temperature can be slightly lowered to observe whether the fault can be eliminated.
Secondly, check whether there are dead corners in the barrel, nozzle and mold and polish it smoothly.

(3) The melt and the mold wall are frictioned and overheated.
If the injection speed is too fast, the injection pressure is too high, and the relative movement speed between the melt and the cavity wall is too high during mold filling, it is easy to cause friction and overheating, causing the melt to decompose and produce black spots and patterns.
In this regard, the injection speed and injection pressure should be appropriately reduced.