In the production process, due to improper operating methods, defective products may inevitably occur. In fact, there is often not a significant difference between defective and qualified products. For example, black spots appearing on the surface of plastic products usually lead to product rejection. So how can this situation be avoided? Let’s analyze it below.
When observing the surface of plastic products, if the black spots are relatively large, they are likely carbonized particles or impurities from the raw materials (such as black masterbatch particles). If the black spots are dark, brittle, blocky, and porous, they are typically carbonized defects. If the black spot density is particularly high, it is necessary to check whether obvious impurities are present in the raw materials.
In most cases, this is related to improper cleaning of raw materials after material changeover; otherwise, the root cause of the raw material itself should be investigated.
Since foreign matter may be mixed into the material, black spots can be generated during molding. Therefore, it is essential to strictly control cleanliness throughout all stages, including production, packaging, storage, transportation, reporting, mixing, and feeding into the barrel.
For carbonized black spots, processing temperature must be strictly controlled. In actual production, even if the nominal processing temperature is the same, differences in back pressure, cycle time, injection volume, and thermocouple position can lead to different carbonization and degradation behaviors.
In general, under the same temperature, lower back pressure, shorter cycle time, larger injection volume, and when the thermocouple is inserted in the upper part of the screw barrel, melt degradation is less likely, reducing the formation of carbonized black spots. To prevent carbonization, material stagnation must be strictly avoided. Dead corners in the screw barrel, nozzle, and runner system should be eliminated, and flow transitions should be smoothed to prevent material retention areas.
Black spots may also be caused by additive degradation or existing carbonized deposits. These deposits on the screw surface and barrel wall may gradually detach under strong shear forces, enter the screw barrel, and be discharged with the melt.
This “discharge process” refers to the occurrence of black spots and impurities in products. For example, during material changeover or color switching, this impurity discharge process is unavoidable. To minimize it, a common method is thorough purging (cleaning).
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