High-gloss and high-gloss products are indeed difficult products for injection molding companies, because they cannot hide external defects, and even the dust in the workshop will cause defects in different degrees of products, such as trachoma, pitting, stains, etc. Pollution, etc., so the environment is also one of the key factors affecting this type of product.
1. Phenomena
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The white fog on the surface of the product is white from a visual point of view, so it is not obvious when it is a white product, and the black highlights are the most obvious. As shown in Figure 1, the product is made of PP material, and the surrounding surface shows a large Large and small white spots, cloud-like symptoms, here are classified as white fog.
2. Theory
The theoretical basis obtained from the above is that there is a certain gap between the plastic and the plastic during the feeding stage, which is brought into the barrel after being sheared by the screw. During the shearing process of the screw, physical changes will occur inside the material, from the glass state Transform into a highly elastic state, from a highly elastic state to a viscous fluid state.
The process of this change may only be between a few seconds - dozens of seconds, (according to the size of the machine or the product) in such a short period of time to change a complicated process, injection molding defects will also occur immediately, and the gap between the plastics will be compressed to Gas is generated inside the molten material. If the gas cannot be discharged to the outside of the barrel during plasticization, the existing gas will be 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. , the phenomenon of fogging, gas trapping or scorching will occur.
Obviously, white mist is also a phenomenon of trapped air. Many people engaged in injection molding technology mistakenly believe that only burning is the cause of trapped air, or that it is unscientific to open the exhaust groove when trapped air occurs. Burning is caused by Gas trapping is too serious, the gas is compressed under high pressure and high speed and cannot be released, resulting in partial combustion, and the combustion temperature can reach 600-800°C in an instant, which is the highest state of trapped gas.
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3. Solutions
How can the gas be best discharged without producing white fog?
1. Mold
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 excellent. If the exhaust groove of the mold is too large or the position is wrong, it cannot be exhausted correctly, and it is easy to cause flying. Bad sides, so proper venting is all that matters.
2. Craft
Mastering the correct tuning skills is very effective in actual work.
1) If the rotation speed of the screw is reduced, the temperature of the molten material can be made uniform, and the chance of entrained air is small. If the screw rotates too fast, the chance of entrained air is more; The heat causes the temperature of the melt to be uneven, and the melt decomposes to generate gas.
2) Increasing the mold temperature can benefit high pressure and slow filling to gain time. If the mold temperature is too low, the surface of the product is prone to premature freezing, and slow filling cannot be achieved, which may lead to 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 easier it is to discharge the 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.
