The working principle of casting film machine
After extrusion, the molten plastic flows out through the gap at the front of the die head to form a film. After leaving the die head, the melt passes through a short gap to reach the low-temperature casting roll surface and is cooled sharply. In order to prevent the plastic melt due to thermal expansion and cold shrinkage caused by excessive "neck" and curling phenomenon, Nanyue Technology on both sides of the casting roll set up an air knife or electronic locking device; That is, compressed air or high pressure ion flow is applied to the plastic film to make it close to the casting roll surface. At the same time, in order to avoid the air brought between the film and the cooling roll when the casting roll is rotating, the air bubble will be generated, the cooling effect will be reduced and the forming quality of the film will be affected, the positive pressure air knife is set on the front of the film, but the air pressure chamber is set. The "pressure" is generated by blowing the compressed air to the extruded molten film through the positive pressure air knife. The double chamber vacuum suction device produces "suction" on the melt film when vacuuming, and this press and suction sucks away the cloud entrain air on the surface of the cooling roll, ensuring the close contact between the molten film and the cooling casting roll, and improving the molding efficiency and quality of the plastic film.
The film forming zone is a key part that affects the quality of the film, and the operation is very important. It is necessary to control the position of the aerobic knife, the air speed and the vacuum degree of the air knife, so that the film curtain is close to the casting cooling roll, and the position is stable, and the location of the cooling roll is as close as possible, otherwise it will produce quality defects such as uneven film thickness and surface gas lines. The air volume of the air knife should be controlled appropriately, the air volume is too small, the pressure is insufficient, the roller effect is poor, and the film is easy to produce shear wave. The Angle of the air knife is also very important, if it is not correct, it will also cause bubbles on the surface of the film.
In the extrusion of casting film, the main cooling process takes place on the casting roll, and the rest of the cooling process takes place on other cooling rolls. The casting roll and cooling roll rely on forced water circulation cooling, in order to improve the cooling effect and reduce the temperature difference of the roll surface, the casting roll and cooling roll are designed as jacket type, the cross flow of the cooling water reduces the temperature difference of the roll surface and ensures the uniform cooling of the plastic film.
In order to further improve the cooling effect of the plastic film, between the casting roll and the cooling roll, some also add a rubber cleaning roll with a "man" spiral groove, its role is to further exclude the air squeezed between the film and the cooling roll, and another role is to remove the precipitates attached to the surface of the film during the plasticizing process to ensure the quality of the film. The installation position can be manually adjusted, and the contact and separation with the cooling roller are achieved by the cylinder action.
The distance between the die lip and the casting roll is called the "air gap". If the distance is too large, the melt is susceptible to fluctuations under the influence of external factors, and the thickness of the film changes accordingly. At the same time, the melt is susceptible to air oxidation, the transparency becomes worse, and the phenomenon of "shrinking neck" and curling is increased. In order to obtain a reasonable distance from the die lip to the casting roll, the casting roll lifting device is set up. The lifting device is composed of a reducer and a lifting screw mechanism. The distance between the die head and the casting rod can be adjusted according to the casting process requirements.
In order to facilitate the operation, cleaning and maintenance of the die head, the forward and backward moving device of the casting part is set up. The forward and backward moving of the casting forming device adopts a variable speed transmission mechanism to drive the axle mounted on two rails. The distance of the forward and backward movement can be determined by the positioning of the travel switch on the rail according to the process requirements. In order to avoid the damage of equipment caused by the collision with the die head due to misoperation when the casting roll moves forward and backward, we set an interlock device in the electrical system, that is, only when the casting part moves forward and backward, can the lifting action of the casting roll be started; Similarly, the forward and backward moving mechanism of the roll can be started only when the lifting mechanism of the roll component is lowered into place.
