It is indeed quite challenging to extrude the High-gloss plastic profile products during our plastic extrusion processing. Since it is difficult to hide any surface defects. Even the dust in the workshop can cause a certain degree of defective surface such as trachoma, pitting, staining, etc. Therefore, the environment is also one of the key factors to avoid these problems.
That white fog looks like the special white pots on the extruded white PVC profile, it is not obvious to recognize at first glance. while it looks very vivid on those black rigid PVC extrusion parts with large and small white spots, like cloud-like symptoms, which is categorized as white fog here.
According to the above analysis: in the downstream stage, there is a certain gap between the plastic particles when brought into the screw barrel. In the screw shear processing, the material will undergo internal physical changes, from the glass state to the high elastic state, from the high elastic state, and then to the viscous flow state. There is a certain gap between the plastic particles.
This sudden change may only last for a few seconds or tens of seconds which depends on the size of the plastic extruder or product. During such short seconds, the extrusion forming defects are then generated. The gap between the plastic particles is compressed to the melting pot. If the gas is not discharged to the outside of the barrel during plasticization, then it will be extruded into the mold cavity.it will result in trapped gas or burning phenomenon.
Obviously, the white mist is also a phenomenon of trapped gas. Many technics engaged in plastic extrusion technology mistakenly believe that the scorching is due to the trapped gas. It can be solved by opening the exhaust tank to discharge those gas. However, the issue of scorching is because trapped gas in high pressure, high-speed is too serious. The gas is compressed but not released so as to produce partial combustion. The combustion temperature can instantly reach 600-800 ℃, which belongs to the highest realm of trapped gas.
How to make the best gas discharge without the white mist phenomenon?
Mold: It is difficult to solve the gas discharge by the mold design. For PP material, it has a wide temperature range and superior flow performance. It can not properly exhaust gas if the exhaust slot open too large or incorrect location. It will also easy to produce flying edges.
Process: It is more effective to master plastic extrusion techniques to make sure the best gas discharge.
(1) Reducing the rotation speed of the screw can make the melt temperature stable and the small chance of air entrainment. the screw rotation speed is too fast, the chance of air entrainment should be more; the melting is subjected to the large shearing force of the screw, and the frictional heat causes the uneven temperature of the melt, and the decomposition of the melt produces gas.
(2) Increase the temperature of plastic profile mold, which can facilitate high pressure and slow filling to get more time. If the mold temperature is too low, the product surface is easy to freeze prematurely and slow filling cannot be realized, which 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.
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