Analysis and elimination methods of black spots on PVC particles

When black spots appear on the surface of PVC particles, it is easy to associate them with whether the mold is rusted or not. But upon careful inspection, it was found that the mold was still quite shiny, with no small marks present. After increasing the temperature and back pressure of the adhesive, and improving the quality of the adhesive, the situation may improve, but sometimes it is still difficult to completely remove the pitting. In fact, this is the reason why the color powder has not completely melted.

Due to the smoothness and brightness of the mold surface, the injection molded parts are also very bright, and small undissolved powder particles left on the surface of the injection molded parts will be truthfully reflected. So when switching to color schemes, the spots disappear automatically, and the surface of the injection molded parts becomes as bright as a mirror. In actual production, a relatively high proportion of black injection molded parts are found to have this situation. Therefore, when there are spots on the surface of bright parts, it is advisable to consider the issue of color powder.

If the temperature of the molten material is too high, it will overheat and decompose, forming carbides. In order to avoid overheating and decomposition of the molten material, for thermosensitive thermoplastic materials such as polyvinyl chloride, it is necessary to strictly control the temperature at the end of the barrel not to be too high. When black spots and stripes appear on the surface of plastic parts, the temperature controller of the material barrel should be checked immediately to see if it is out of control, and the temperature of the material barrel and mold should be appropriately reduced. However, it is worth noting that if the material temperature and mold temperature are too low, it will also cause bright stripes to appear on the surface of the plastic part.

If the wear gap between the screw and the barrel is too large, the molten material will be stuck in the barrel, causing local overheating and decomposition of the stuck molten material, resulting in black spots and stripes. You can first slightly lower the temperature of the barrel and observe if the problem can be resolved. Secondly, the material barrel, nozzle, and mold should be checked for any dead corners of stored materials and polished smooth. After taking the above measures, if the fault is still not eliminated, the equipment should be repaired in a timely manner and the gap between the screw and the material barrel should be adjusted.

If the injection speed is too fast, the injection pressure is too high, and the relative movement speed between the molten material and the mold cavity wall during mold filling is too high, it is easy to cause friction and overheating, causing the molten material to decompose and produce black spots and patterns. The injection speed and pressure should be appropriately reduced.

When the nozzle and the main flow channel of the mold do not match well, material coking will occur near the gate and be injected into the mold cavity with the flowing material, forming black spots and stripes on the surface of the plastic part. Timely adjust the relative position between the nozzle and the main channel of the mold to ensure a good fit. In addition, if the hot runner design or production of the mold is poor, the molten material will flow poorly in the runner and remain coking, which will also cause black spots and stripes on the surface of the plastic part. Improve the surface brightness of the hot runner and reduce the heating temperature of the runner.

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