Injection defect │ How to solve the fog on the surface of high-gloss products

- Apr 16, 2019-

        High-gloss and high-brightness products are indeed a difficult product for injection molding enterprises, because it can not hide external defects, even if the dust in the workshop will cause different degrees of defects in the product, such as trachoma, pitting, dip Pollution, etc., so the environment is also one of the key factors affecting such products.

First, the phenomenon
The surface of the product is white mist. From the visual point of view, it is white, so it is not obvious in white products. The black highlight is the most obvious. The product is made of PP material, and the surrounding surface presents large and small white markings. Like cloud-like symptoms, it is classified as white fog.

Second, the theory
According to the above theoretical basis, there is a certain gap between the plastic and the plastic in the blanking stage, which is taken into the cylinder after being sheared by the screw. During the screw shearing process, physical changes occur inside the material, from the glass state. Transition to a high-elastic state, from a high-elastic state to a viscous flow state, the process of this change may be only a few seconds - tens of seconds, depending on the machine or product size, to transform a complex in such a short period of time In the process, injection molding defects are also generated, and the gap between the plastics is compressed to generate gas inside the melt. If the gas is not discharged to the outside of the cylinder during plasticization, the existing gas is wrapped in the melt and injected into the mold. Whether the cavity can be smoothly discharged depends on whether the exhaust of the mold is good or not, and the phenomenon of fogging or scorching occurs.
  Obviously, white fog is also a phenomenon of trapped gas. Many people who are engaged in injection molding technology mistakenly believe that only charring is trapped, or it is uncomfortable to open the exhaust slot when it is trapped. It is because of burning. The trapped gas is too serious, the gas is compressed under high pressure and high speed, and the local combustion is not released. The combustion temperature can reach 600-800 °C instantaneously, which is the highest level of trapped air.

Third, the solution
How can we let the gas discharge best without white fog?
Exhaust gas is not completely solved by the mold. For the characteristics of PP material, the temperature range is very wide, and its flow performance is superior. If the exhaust groove of the mold is too large or the position is wrong, it cannot be properly vented, and it is easy to fly. Bad side, so proper exhaust is useful.

2. Process
Mastering the right tuning skills is very effective in practical work.
1) If the screw rotation speed is reduced, the melt temperature can be made uniform, and the chance of entraining air is small. For example, the screw rotation speed is too fast, and there is a greater chance of entraining air; the melt is subjected to the shearing force of the screw, and the friction is generated. The heat causes the melt temperature to be uneven, and the melt decomposes to generate gas.
2) Increasing the mold temperature can be beneficial to high pressure and slow filling to win time. If the mold temperature is too low, the surface of the product is easy to freeze prematurely, and slow filling cannot be achieved, which may result in lack of glue.
3) Reduce the injection speed. The faster the speed, the faster the gas is compressed and the higher the temperature. The slower the speed, the slower the gas is compressed and the more favorable the gas is discharged.
4) The clamping force, the larger the clamping force, the worse the exhaust effect on the surface of the cavity, and the smaller the clamping force, the better the effect of exhausting the surface of the cavity.

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