Vacuum extrusion is also called exhaust cold feed extrusion. In the middle of the cold feed extruder, there is an exhaust section, and the exhaust port of the exhaust section is connected with the vacuum pump to form a decompression zone, and the thermal resin material enters the exhaust section in the form of a film, which is easy to exclude the gas inside the plastics.
In the process of plastic extrusion manufacturing, when the material is just out of the die, it is completely softened. After entering the vacuum shaping die, with the effect of negative vacuum pressure, the profile in the softened state and with a certain shape is only adsorbed on the cavity of the vacuum shaping die, and then cooled by the cooling water in the die, and when the profile is led out of the vacuum shaping die, it can obtain the ideal shape and size.
The proper control of vacuum degrees will directly affect the quality of the plastic profile products. Usually, the vacuum level should be more than 67kPa. But too large a vacuum level will increase the load of the traction machine and delay or even prevent the products from entering the vacuum sizing die smoothly, resulting in the blockage of the material between the mouth die and the vacuum sizing die. In addition to reducing the output and shortening the service life of the vacuum pump; while too small a vacuum level will not have enough suction power, resulting in serious deformation or no molding. This cannot guarantee the appearance quality and dimensional accuracy of the products.
The cooling water of the vacuum shaping dies usually flows from the rear of the shaping sleeve and out from the front so that the direction of water flow is opposite to the forward direction of the profile. This can make the profile cool down more slowly and the internal stress is smaller, and at the same time, the temperature at the front of the shaping sleeve is higher and the profile is easy to absorb. The cooling water temperature of hard PVC profiles is required to be below 20℃, with 15-20℃ being more ideal. Since hard PVC profiles are often asymmetrical products, they cannot be cooled by a water bath. It will make the cooling speed of asymmetrical sections different and produce irregular deformation of the profiles, and several vacuum shaping dies should be used to cool and shape the profiles most ideally.
During the plastic profile manufacturing production process, if it is found that the plastic profile is still bent after coming out of the vacuum shaping mold, the cooling water flow and the temperature of the electric heating plate in each part can be adjusted appropriately. Even the local spraying cooling water can be used to correct it.