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The temperature of each section on the barrel of the plastic sheet extruder increases gradually from the feeding section to the joint between the barrel and the molding die.
The mold temperature is slightly higher than the barrel temperature. 5-10 ℃ above the temperature. The temperature at both ends of the mold is slightly higher than that in the middle of the mold, and the higher temperature is controlled at 5-10 ℃.
The upper surface of the middle roll of the three rollers should be on a horizontal plane with the lower plane of the die lip. The lip end face is parallel to the center line of the intermediate roller, and the distance is 50-100 mm.
The die lip gap should be slightly less than or equal to the thickness of the sheet. The gap between the die lips should be slightly smaller than that between the two sides.
It should be noted that the roughness r of three roll working face should not be greater than 0.2pm. When cleaning the roll surface, it is not allowed to scratch the roll surface with a hard knife, but the copper knife should be used to clean the residual material on the roll surface.
The roller surface should have some middle height, and the gap between the three rollers should be equal to or slightly larger than the thickness of the sheet.
Mold temperature control should be stable. When the temperature is high, the flow speed of the melt in the mold becomes faster, and when the temperature is low, the flow speed of the melt in the mold becomes slow. The instability of the melt flow rate will result in a large error in the longitudinal thickness of the sheet.
Pay attention to control the temperature of the working face of the three rollers. The temperature of the feeding roller is slightly higher than that of the discharging roller. The roller surface temperature is too high, so the slab is not easy to roll off. The surface of the products is easy to produce transverse lines, and the surface of the products is not glossy when the temperature is low. According to this phenomenon, the roller surface temperature control should be adjusted in time.
The running speed of the three rollers is slightly higher than the extrusion speed of the slab from the die mouth, and the speed difference is generally less than 10%. The working speed of the three rollers should be controlled smoothly. Too fast or too slow running speed has great influence on the thickness error of the sheet.
For the extrusion molding of plastic sheet, except that PVC, PE and PP resin are not treated before extrusion, other plastics (such as ABS, polyamide, etc.) should be dried before extrusion. Otherwise, exhaust type extruder should be used for extrusion molding.
