High-gloss and high-gloss products are indeed a difficult product for injection molding companies, because they cannot hide external defects. Even the dust in the workshop will cause product defects to varying degrees, such as trachoma, pitting, and staining. Pollution, etc., so the environment is also one of the key factors affecting such products.
1. Phenomenon
White fog on the surface of the product, from a visual point of view, it presents a white shape, so it is not obvious when the product is white, 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 large and small white markings, like symptoms of cloud and fog, which are classified as white fog here.
2. Theory
According to the above-mentioned 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. During the shearing process of the screw, the inside of the material will change physically. From the glass state to the high elastic state, from the high elastic state to the viscous flow state, the process of this change may only take a few seconds to tens of seconds, (depending on the size of the machine or product) it will change in such a short time. In a complicated process, injection molding defects also occur immediately.
The gap between the plastics is compressed to produce gas inside the melt. If the gas cannot be 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 Depending on whether the exhaust of the mold is good, the phenomenon of fogging or scorching occurs.
Obviously, white fog is also a phenomenon of trapped air. Many people engaged in injection molding technology mistakenly believe that only burnt is trapped, or if trapped air occurs, they go to open the exhaust slot. This is unscientific. The burnt is due to The trapped gas is too serious, and the gas is compressed under high pressure and high speed and cannot be released. The combustion temperature can reach 600-800℃ instantly, which is the highest state of trapped gas.
Three, the solution
How can the gas be best discharged without producing white fog?
1. Mould
Exhaust is not completely solved by the mold. In terms of the characteristics of the PP material, the temperature range is very wide, and its flow performance is superior. If the exhaust groove of the mold is opened too large or the position is incorrect, the exhaust cannot be properly exhausted, and it is easy to cause flying. The edges are bad, so correct exhaust is useful.
2. Craft
Mastering the correct tuning skills is very effective in actual work. 1) If the screw rotation speed is reduced, the temperature of the melt can be made uniform, and the chance of entrained air will be small. If the screw rotation speed is too fast, there will be more opportunities for the entrainment of air; the melt will be subjected to a large shear force by the screw, resulting in friction. The heat causes the temperature of the molten material to be uneven, and the molten material decomposes to produce gas.
2) Increasing the mold temperature can benefit the high pressure, and the slow filling will gain time. If the mold temperature is too low, the product surface is prone to premature freezing, and the 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 compression, the more conducive to the discharge of the gas.
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.