When there is pitting on the surface of PVC plastic , we will think of whether the mold is rusted, but after careful inspection, it is found that the mold is still quite bright, and there is no small flaw. After the melter temperature and back pressure are increased to improve the quality of the melt, the situation will improve, but sometimes it is still difficult to completely remove the pitting. In fact, this is the reason why the toner is not completely melted. Because the mold surface is too smooth and bright, the injection molded parts are also very bright, and the small undissolved toner particles will be faithfully reflected on the surface of the injection molded part.
Therefore, the color is used to match the color, the pitting point disappears automatically, and the surface of the molded part is bright like a mirror. In actual production, we found that this is the case with black molded parts. Therefore, when there is a problem with the surface of the bright spot, consider the problem of toner.
Black spot defect analysis and troubleshooting:
1, the melt temperature is too high
If the material temperature is too high, the molten material will be decomposed by heat to form carbides. In order to avoid thermal decomposition of the molten material, the temperature of the tail of the cylinder must not be too high for the thermosensitive thermoplastic materials such as polyvinyl chloride. When black spots and streaks appear on the surface of the plastic part, immediately check whether the temperature controller of the barrel is out of control, and properly lower the barrel and mold temperature. However, it is worth noting that if the material temperature and mold temperature are too low, it will also produce 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 confluent material stays in the barrel, causing the retained melt to locally decompose and generate black spots and streaks. In this regard, you can first reduce the temperature of the barrel slightly to see if the fault can be eliminated. Secondly, the barrel, the nozzle and the mold should be inspected for dead spots and smooth.
After taking the above measures, if the fault has not been eliminated, the equipment should be repaired in time to adjust the gap between the screw and the barrel.
3, the melt and the mold wall friction overheat
If the injection speed is too fast, the injection pressure is too high, and the relative movement speed of the melt and the cavity wall is too high when filling the mold, and it is easy to generate friction and overheat, which causes the melt to decompose to generate black spots and texture. In this regard, the injection speed and injection pressure should be appropriately reduced.
4, the barrel and mold exhaust failure
If the barrel or mold is poorly vented, the gas remaining in the melt will burn due to adiabatic compression, causing the melt to decompose and generate black spots and streaks. In this regard, the injection speed can be appropriately reduced, and the structure of the discharge port of the barrel can be improved under conditions suitable for the particle size and uniformity of the raw material.
For the poor exhaust of the mold part, check whether the gate position and the vent position are correct, whether the gate type is suitable, remove the volatile substances such as rust inhibitor adhered in the mold, and reduce the amount of the release agent. . Under the premise of no flashing of the flash, the clamping force can be appropriately reduced and the exhaust gap can be increased. In addition, check for leaks in the barrel and thimble.
5, accumulation of coking
When the nozzle is in poor fit with the main flow path of the mold, coking will be generated near the gate and injected into the cavity with the flow material, forming black spots and streaks on the surface of the plastic part. In this regard, the relative position of the nozzle and the main channel of the mold should be adjusted in time to make it fit well.
In addition, if the hot runner of the mold is poorly designed or produced, the melt will not flow smoothly in the flow passage, and black spots and streaks will be generated on the surface of the plastic part. In this regard, the surface luminosity of the hot runner should be increased to lower the heating temperature of the runner.
6, the raw materials do not meet the molding requirements
If the content of volatiles in the raw materials is too high, the water-sensitive resin is poorly dried, the amount of recycled materials is too much, the fine powder is too much, the raw materials are unevenly colored, and the lubricants are not selected correctly or used excessively, which will lead to different degrees. Black spots and streaks are formed on the surface of the plastic part. In this regard, different measures should be taken to take appropriate measures and exclude them separately.
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