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1、 Several important geometric parameters of screw: 1. Screw diameter (d) A. It is related to the required injection volume: injection volume = 1 / 4 * π * D2 * s (injection stroke) * 0.85; B. generally speaking, screw diameter D is inversely proportional to the maximum injection pressure and directly proportional to the plasticizing ability. 2. Conveying section a is responsible for conveying, pushing and preheating plastics, which should be preheated to the melting point; B. crystalline plastics should be long (such as POM and PA), followed by amorphous materials (such as PS, PU and ABS), with the shortest thermal sensitivity (such as PVC). 3. Compression section a is responsible for the mixing, compression and pressurized exhaust of plastics, through which almost all the raw materials have been melted, but not necessarily mixed evenly; B. in this area, the plastic gradually melts, and the screw groove volume must be reduced accordingly to correspond to the decrease of plastic geometric volume, otherwise, the material pressure is not solid, the heat transfer is slow, and the exhaust is poor; C. generally, it accounts for more than 25% of the screw working length, However, the compression section of nylon (crystalline material) screw accounts for about 15% of the screw working length, plastic screw with high viscosity, fire resistance, low conductivity and high additives accounts for 40% - 50% of the screw working length, and PVC screw accounts for 100% of the screw working length, so as to avoid intense shear heat. 4. Metering section a, generally 20-25% of the screw working length, to ensure that all plastic melting and temperature uniformity, mixing uniformity; B, metering section long, mixing effect is good, too long is easy to make the melt stay too long and produce thermal decomposition, too short is easy to make the temperature uneven; C, PVC and other heat sensitive plastic materials should not stay too long, to avoid thermal decomposition, can use a shorter metering section or not metering section . 5. A. the deeper the depth of the feeding screw groove, the larger the conveying capacity, but the screw strength should be considered. The shallower the depth of the metering screw groove, the higher the plasticizing heat and mixing performance index. However, if the depth of the metering screw groove is too shallow, the shear heat and self generated heat will increase, and the temperature rise will be too high, which will cause discoloration or scorch of the plastic, especially not conducive to the thermosensitive plastic The depth of screw groove = Kd = (0.03-0.07) * D. if D increases, K will be selected as a small value. 2、 The main factors affecting the plasticizing quality are: length diameter ratio, compression ratio, back pressure, screw speed, barrel heating temperature, etc. 1. Length diameter ratio: the ratio of effective working length and diameter of screw.

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a. If the ratio of length to diameter is large, the feed is easy to be uniform; B. for plastics with better thermal stability, a longer screw can be used to improve the mixing performance without scorching; for plastics with poor thermal stability, a shorter screw or screw end without thread can be used. Considering the characteristics of plastics, the general flow length ratio is as follows: thermosetting is 14-16, heat sensitivity of rigid PVC, high viscosity Pu is 17-18, general plastics is 18-22, high temperature stability plastics such as PC, POM is 22-24. 2. Compression ratio: the ratio of the depth of the last screw groove in the feeding section to the depth of the first screw groove in the metering section. a. Considering the compressibility, filling degree, reflux and other effects of the material, the product should be dense, heat transfer and exhaust; B. appropriate compression ratio can increase the density of the plastic, make the combination between molecules more closely, help to reduce the absorption of air, The higher the compression ratio is, the higher the temperature rise in the plasticizing process of the plastic in the pipe will be, the better the mixing uniformity of the plastic in the plasticizing process will be, and the relative discharge quantity will be greatly reduced; D. high compression ratio is suitable for non fusible plastics, especially for low melting Low compression ratio is suitable for fusible plastics, especially for high melting viscosity and heat sensitive plastics. 3. Back pressure A. increasing the back pressure can increase the work done by the screw on the molten resin, eliminate the unmelted plastic particles, and increase the density and uniformity of raw materials in the tube; B. back pressure is used to increase the barrel temperature, and the effect is most significant; C. too large back pressure is easy to decompose the plastic with high heat sensitivity, and salivation may occur on the plastic with low viscosity There may be bubbles in the finished product. 4. Screw speed a, screw rotation speed directly affect the plastic shear in the spiral groove; B, small screw groove shallow, absorb heat quickly, enough to promote the plastic softening in the compression period, the friction heat between screw and barrel wall is low, suitable for high-speed rotation, increase the plasticization energy; C, large screw is not suitable for rapid rotation, so as to avoid uneven plasticization and excessive friction heat; d. For plastics with high heat sensitivity, if the screw speed is too high, the plastics will be easy to decompose; e. generally, the screw of various sizes has a certain speed range, generally 100-150rpm; if the screw speed is too low, the plastics cannot be melted; if the screw speed is too high, the plastics will be scorched. 5. Electric heating temperature setting A. make the cold and hard plastic remaining in the barrel and screw melt to facilitate screw rotation and provide part of the heat required for plastic melting; B. set 5-10 ℃ lower than the melt temperature (part of which is provided by friction heat energy); C. adjust the nozzle temperature to control problems such as salivation, condensate (plug) and wire drawing; D. general temperature of crystalline plastic Control.