Extruders Extrusion system
Single screw extruder is mainly used for extrusion soft and hard polyvinyl chloride, polyethylene, thermoplastics, machinable a variety of plastic products, such as blown film, squeeze tubes, pressure plate, wire drawing, etc., can also be used to melt granulation. Since single screw extruder is widely used, what should the single screw extruder pay attention to when processing the hard PVC powder? The following four points should be noted on the main machine when the single screw extruder is processed with hard PVC powder.
(1) with PVC is a kind of "heat-sensitive plastic" of heat sensitive, screw shear is too big or long screw is likely to cause material overheating decomposition, so the screw length to diameter ratio with 20 ~ 25:1 advisable, don't be too long.
(2) taper screw with gradual taper of spiral groove, no pin type, separate screw. The compression ratio is 2 ~ 5, which should be selected according to the material model and the melting viscosity. The general principle is that the more viscous the material is, the smaller value will be taken, and the high value of the liquid is good. Such as: for Ф 65 extruder, desirable conveying screw groove depth 9 mm, and the homogenization section of spiral groove take 3 mm deep.
(3) because the powder is not easy to transport, the groove can be opened vertically in the feeding port of the cylinder (hopper seat), so as to make the material flow smoothly and improve the delivery efficiency by increasing the friction force.
(4) in the barrel to the molding process temperature Settings on the nose, can be set to: at the beginning of 150 ~ 160 ℃ and 160 ~ 160 ℃, 165 ~ 165, 175 ~ 180 ℃ and 180 ~ 190 ℃. In general, the temperature of PVC material will be overheating decomposition after more than 180 ℃, so after the set temperature, but also according to different, formulation of PVC products, in the processing to adjust. Until you adjust to the best fit. The high temperature is set at the head of the machine tool. The main purpose is to make the material smooth and smooth, while the surface is good, but the temperature should be controlled. Single screw extruder is composed of transmission system, extrusion system, heating system, cooling system, control system, etc. (there are also some auxiliary equipment). The extrusion system is the key part of extrusion molding, which plays an important role in extrusion molding quality and yield. The extrusion system mainly includes feeding device, drum, screw, head and die, etc. Let's talk about the extrusion system.
(1) feeding section
The plastic hopper enters into the cylinder of the extruder, which moves forward due to the friction of the inner wall of the cylinder and the surface of the screw. In this segment, the function of the screw is mainly to provide the plastic compaction to provide the driving force; The material still exist in solid state, due to the effect of friction heat of the strong, in close connection with the inner wall of the cylinder at the end of the material to plastic flow temperature, but the molten still not start, this area is called hysteresis area.
(2) homogenizing section
The material from the melt passage into the homogenization segment is the fused viscous fluid. The forward viscous fluid is further mixed and plasticized under the resistance of the head mould, and some of it is squeezed out from the die of the machine head.
From the above individual working principle of single screw extruder, it is not hard to see in single screw extruder, plastic, forward extrusion flow, the main source of power and charging period of solid conveying, plasticizing degree of homogenization. There have been many new structures in improving the structure of screw mixing, but it is suitable for polymer with good thermal stability, but it is not suitable for PVC resin production.
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