Abnormal black spots and pits of rubber and plastic products

1、 Black spot
1. What is "black dot" (appearance)?
Black spots refer to the phenomenon of black spots or stripes in molded products. The black spots on the surface of molded products are BS (black spots). For some reasons, its composition is estimated to be coking resin or carbonized resin.
2. Causes of black spots
Resin decomposition
Because plastic is a chemical substance, it will gradually decompose when it is heated above the melting point. The higher the temperature and the longer the time, the faster the decomposition will proceed. In addition, there are check rings, thread grooves and other parts easy to retain resin inside the barrel. After coking or carbonization, the resins that remain in these parts will fall off at a certain rhythm and mix into the molding products, thus forming black spots.
Insufficient cleaning
The situation that the resin used before is left in the molding machine due to insufficient cleaning is also the cause of black spots. As mentioned in the previous item, due to the existence of check rings, thread grooves and other parts that are easy to retain resin, the corresponding cleaning amount and times should be used when replacing this part. In addition, the cleaning method suitable for each material must also be used. If it is the cleaning of PBT and other similar resins, it is relatively easy to carry out, but if it is the cleaning of different kinds of materials, due to the different melting points or decomposition temperatures and the compatibility (affinity) between resins, even if it is cleaned, it can not be completely discharged in many cases.
3. Countermeasures for black spots
First, clean thoroughly until the black spots no longer appear. Black spots tend to stay in the check ring and thread groove in the barrel. If there have been black spots, it is estimated that the cause of black spots is likely to remain in the barrel. Therefore, after the black spots appear, they must be fully cleaned before taking countermeasures (otherwise the black spots will never disappear).
Shorten the retention time. Even if the setting temperature of the molding machine is within the recommended temperature range of various resins, long-term retention will still lead to resin deterioration and easy to appear black spots. If the molding machine has the delay setting function, please make full use of this function. In addition, the molding machine suitable for the mold size should also be used.
2、 Pitting
1. What is "pitting" (appearance)?
"Pits" mainly refer to "pits" on thick walls. "Pitting" is caused by the resin not adhering to the cavity surface. There are many reasons for the occurrence of pitting, but the main reason is insufficient pressure.
2. The cause of "pockmarks"
Insufficient pressure holding
After the resin is injected into the mold, its surface has solidified, but the middle part has not fully solidified. If the pressure retaining at this stage is insufficient, the semi-cured surface will not cling to the mold cavity, which is easy to produce a pitted appearance.
Resin temperature and mold temperature
When the resin temperature or mold temperature is low, the curing of the molded product surface will be accelerated, and sometimes it is impossible to apply pressure retaining. Results It is easy to produce bad appearance such as pitting.
Liquidity and injection rate
When the fluidity of the material is poor, the filling time will be extended, and the surface layer of the molded product has solidified before the pressure-holding process, so it is sometimes impossible to apply pressure-holding to the resin. Results It is easy to produce bad appearance such as pitting. When the injection speed is slow, it is easy to produce pitting for the same reason.
3. Countermeasures for "pockmarks"
Try to increase the pressure holding
First, try to gradually increase the set value of pressure maintaining. This will basically solve the problem. If there are other faults (such as flash, etc.) after increasing the pressure maintaining, the next "condition easy to apply pressure maintaining" needs to be set:
1. Increase the resin temperature 2. Increase the mold temperature 3. Increase the injection speed 4. Expand the gate (* a) 5. Adjust the thickness (1t~2t) 6. Use materials with good fluidity (* b)