Plastic injection products have white fog defects

High gloss and high-brightness products are indeed a difficult problem for Medical Double-Color Plastic Injection Molding Companies to deal with, because they cannot hide external defects. Even the dust in the workshop can cause defects of varying degrees in the product, such as sand eyes, pitting, stains, etc., so the environment also affects the white fog on the surface of the product;

From a visual point of view, it appears white, so it is not obvious on white products. Black highlights are the most obvious, and the surrounding surface shows white spots, like a cloud-like symptom, which is classified as white fog here.

Cause Analysis
During the injection molding process, there is a certain gap between the plastics at the feeding stage, and they are brought into the barrel after being sheared by the screw. During the screw shearing process, the material will undergo physical changes, from the glass state to the high elastic state, and from the high elastic state to the viscous flow state. This change process may only take a few seconds to tens of seconds. (Depending on the size of the machine or product) In such a short time, a complex process must be transformed, and injection molding defects will also occur;

The gaps between the plastics are compressed to produce gas inside the melt. If the gas is not discharged to the outside of the barrel during plasticization, the existing gas will be wrapped in the melt and injected into the mold cavity. Whether the mold can be discharged smoothly depends on whether the mold exhaust is good, which will cause fogging, trapped air or burning.

Obviously, white fog is also a phenomenon of trapped air. Many people engaged in injection molding technology mistakenly believe that only burning is trapped air, or they open exhaust grooves when trapped air occurs. This is unscientific. Burning is caused by excessive trapped air. The gas is compressed under high pressure and high speed and cannot be released, resulting in local combustion. The combustion temperature can reach 600-800℃ instantly.

Solution

Mold exhaust is not entirely solved by the mold. For the characteristics of PP materials, the temperature range is very wide and its flow performance is excellent. If the exhaust groove of the mold is too large or the position is wrong, it cannot be properly exhausted, and it is easy to produce flash defects, so correct exhaust is useful.

Process
Mastering the correct machine adjustment skills is very effective in actual work.

1) If the screw rotation speed is reduced, the melt temperature can be uniform, and the chance of entrained air should be small. If the screw rotation speed is too fast, the chance of entrained air should be more; the melt is subjected to large shear force from the screw, friction heat causes uneven melt temperature, and the melt decomposes to produce gas.

2) Increasing the mold temperature can facilitate high pressure and slow filling to gain time. If the mold temperature is too low, the product surface is prone to premature freezing, slow filling cannot be achieved, and may lead to glue shortage.

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, which is more conducive to gas discharge.

4) Clamping force. The greater the clamping force, the worse the exhaust effect of the cavity surface. The smaller the clamping force, the better the exhaust effect of the cavity surface.