Injection molding defect│How to solve the fogging on the surface of high-gloss products?

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

 I. Phenomenon

Figure 1 before improvement

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 PP material production, the surrounding surface shows large and small white spots, like cloud-like symptoms, here categorized as white fog.

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 be physically changed internally, from the glass state to the high elastic state, from the high elastic state and then to the viscous flow state.

This change process may only be a few seconds - tens of seconds, (depending on the size of the machine or product) in such a short period to change a complex process, injection molding defects are then generated, the gap between the plastic is compressed to the melt inside the 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 exhaust of the mold is good, it produces fogging trapped gas or burning phenomenon.

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.

III. Solution

How to make the best gas discharge and not produce the 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 is very effective in practice.

(1) such as reducing the screw rotation speed, can make the melt temperature uniform, the chance of entrained air to be small, such as the screw rotation speed is too fast, the chance of entrained air to more; melt by the screw shear force, friction heat leads to uneven temperature of the melt, melt decomposition of gas.

2）Improve the mold temperature, 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 glue.

(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.