Black spots

It means that black spots or streaks appear in the molded product. You can see the below marked place that are the black spots on the molded product surface. It is estimated that the composition is charred resin or carbonized resin.

What will cause the black spots?

1.Resin decomposition
When the plastic is heated above the melting point, it will gradually decompose. The higher the temperature and the longer the time, the faster the decomposition will proceed. In addition, there are places such as check ring and thread grooves inside the barrel where resin is easily retained. After these continuously retained resins are coked or carbonized, they will fall off at some point and mix into the molded product, thus forming black spots.

2.Insufficient cleaning
Due to insufficient cleaning, the previously used resin remains in the molding machine, which is also a cause of black spots. Since there are places such as check ring and thread grooves where resin is easily retained, it is necessary to use the corresponding cleaning amount and frequency when replacing these parts. In addition, it must use a cleaning method that is suitable for each material. For example, if it is PBT → PBT and other similar resins, it is relatively easy to clean. However, if it is cleaning different types of materials, since the melting points or decomposition temperatures are different and there is compatibility between the resins, even if it is cleaned, it cannot be completely removed in many cases.

3.Foreign matters mixed in
Contamination is also one of the causes of black spots. If some of the pellets put into the hopper are mixed with other resins with lower decomposition temperatures, black spots are likely to occur due to the decomposition of the mixed resins. In addition, attention should be paid to recycled plastics, because they become easier to decompose after being heated many times. The more times the recycling is repeated, the longer the heating time. Besides, there is a possibility of metal contamination during recycling.

1.Wash thoroughly until the black spots no longer appear.
Black spots tend to stay in the check ring and thread grooves in the barrel. If black spots have ever appeared, the cause of the black spots is likely to be residue in the barrel. Therefore, after the black spots appear, it must be fully cleaned before taking countermeasures, otherwise the black spots will never disappear.

2.Try lowering the resin temperature.
Each resin has a recommended operating temperature, and this information is also included in the product catalog or product packaging. If the set temperature of the molding machine is found to be out of range, please lower the temperature. In addition, the temperature displayed on the molding machine is the temperature at the sensor location, which may be slightly different from the actual resin temperature. If possible, it is recommended to measure the actual temperature with a resin thermometer. In particular, the parts that are prone to generate residual, such as the check ring, are most likely to cause black spots, so pay special attention to the temperature near them.

3.Shorten the residence time
Even if the setting temperature of the molding machine is within the recommended temperature range of various resins, long-term retention will still cause the resin to deteriorate and black spots will easily appear. If the molding machine has a delay setting function, please make full use of this function. In addition, you should use a molding machine that is suitable for the mold size.

4.Check if there is any impurity contamination
Black spots may also be caused by the accidental mixing of other resins or metals. Surprisingly, this is often caused by insufficient cleaning. Please clean thoroughly and completely remove the resin used in the previous molding before working. When using recycled plastics, please check with the naked eye whether there are foreign materials in the pellets.