Injection defects: how to solve the surface fog of high gloss products?

Highlight and highlight products are really difficult products for injection molding enterprises, because they can’t hide external defects. Even the dust in the workshop will cause different degrees of defects, such as trachoma, pitting and contamination. Therefore, the environment is also one of the key factors affecting such products.

1、 Phenomenon

The white fog on the surface of the product is white from the visual point of view, so it is not obvious when the product is white, and the black highlight is the most obvious

As shown in Figure 1, the product is made of PP material, and the surrounding surface shows large and small white stripes, like cloud like symptoms, which is classified as white fog here.

2、 Theory

According to the above theoretical basis, there is a certain gap between the plastic and the plastic in the blanking stage, which is brought into the barrel after being sheared by the screw. In the process of screw shearing, physical changes will occur in the material. From glass state to high elastic state, from high elastic state to viscous flow state, this change process may be only a few seconds to tens of seconds. It is a complex process to change in such a short time (according to the machine or product size), and injection molding defects also occur immediately.

The gap between the plastics is compressed to the inside of the molten material to produce gas. If the gas is not discharged to the outside of the barrel during plasticization, the existing gas is wrapped in the molten material and injected into the mold cavity. Whether the mold can be discharged smoothly depends on whether the exhaust of the mold is good, resulting in fogging, gas trapping or charring.

Obviously, white fog is also a phenomenon of trapped gas. Many people engaged in injection molding technology mistakenly believe that only coke is trapped gas, or open the exhaust slot when trapped gas occurs, which is unscientific. Coke burning is because the trapped gas is too serious, the gas is compressed under high pressure and high speed, and the local combustion is generated, and the combustion temperature can instantly reach 600-800 ℃. This belongs to the highest state of trapped gas.

3、 Solution

How can we best discharge the gas without white fog?

1. Mould

The exhaust 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 slot of the mold is too large or in the wrong position, it can not exhaust correctly, and it is easy to produce poor flash, so the correct exhaust is useful.

2. Process

Mastering the correct adjustment skills is very effective in practical work.

1) If the screw rotation speed is reduced, the melt temperature can be uniform, and the opportunity of entrained air is small. If the screw rotation speed is too fast, the opportunity of entrained air is more; The melt is greatly sheared by the screw, and the friction generates heat, resulting in uneven melt temperature and gas generated by melt decomposition.

2) Increasing the mold temperature can help high pressure and slow filling win time. If the mold temperature is too low, the product surface is easy to freeze prematurely, and slow filling cannot be realized, which may lead to glue shortage.

3) Reduce the injection speed. The faster the speed, the faster the gas compression and the higher the temperature. The slower the speed, the slower the gas compression, which is more conducive to the discharge of gas.

4) Clamping force: the greater the clamping force, the worse the exhaust effect on the cavity surface, and the smaller the clamping force, the better the exhaust effect on the cavity surface.