Injection molding defects: how to solve the surface fogging of high-gloss products?

High-gloss, high-gloss products for injection molding enterprises is indeed more difficult to cope with product because it can not hide external defects, even the dust in the workshop can cause different degrees of defective products bad, such as trachoma, pitting, staining, etc., so the environment is also one of the key factors affecting these products.

I. Phenomenon

Injection molding defects: high-gloss product surface fogging how to solve it?

Product surface white fog, from a visual point of view, presents a white shape, so it is not obvious when the white products, black highlights are the most obvious.

As shown in Figure 1, the product is produced by PP material, and the surrounding surface shows large and small white spots, like a cloud-like symptom, which is categorized as white mist here.

II. Theory

According to the theory derived from the above is based on the following: there is a certain gap between the plastic and plastic in the downfeed stage after the screw shear is brought into the barrel, in the screw shear process, the material will undergo physical changes inside. From the glass state into a high elastic state, from the high elastic state, and then to the viscous flow state, the process of this change may only be between a few seconds - tens of seconds, (depending on the size of the machine or product) in such a short period of time to transform a complex process, injection molding defects are then generated.

The gap between the plastic is compressed to the inside of the melt to produce gas if the gas is not discharged to the outside of the barrel during plasticization, the presence of gas is wrapped into the melt is injected into the mold cavity, whether the mold can be smoothly discharged depends on whether the mold exhaust is good, then the phenomenon of fogging trapped gas or burning.

Obviously, a white mist is also a phenomenon of trapped gas, many people engaged in injection molding technology mistakenly believe that only scorched is trapped gas, or trapped gas to open the exhaust tank, which is not scientific, scorched because trapped gas is too serious, the gas is compressed at high pressure, the high-speed state can not be released to produce local combustion, combustion temperature can instantly reach 600-800 ℃, which belongs to the highest realm of trapped gas.

Third, the solution

How can we make the best gas discharge without producing a white mist phenomenon?

1. Moulds

Exhaust gas does not completely rely on the mold to solve, for PP material characteristics, the temperature range is very wide, its flow performance is superior, if the mold exhaust slot opens too large or incorrect location, can not be correctly exhausted, but also easy to produce bad flying edge, so the correct exhaust is useful.

2. Process

Mastering the correct tuning skills in the actual work is very effective.

1) such as reducing the screw rotation speed, can make the melt temperature uniform, the opportunity to entrain air to be small, such as the screw rotation speed is too fast, the opportunity to entrain air to more; melt by the screw shear force, frictional heat generation leads to uneven melt temperature, melt decomposition to produce gas.

2) Increase the temperature of the mold, can facilitate the high pressure, slow filling to win time, if the mold temperature is too low, the product surface is easy to freeze prematurely, the slow filling can not be achieved, may lead to a lack of rubber.

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

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