Detailed introduction

31

2018

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07

Factors Affecting the Quality of Screw Plasticization


 
ImpactScrewThe main factors of plasticizing quality are: aspect ratio, compression ratio, back pressure, screw speed, barrel heating temperature, etc.
1. Length-diameter ratio: the ratio of the effective working length of the screw to the diameter of the screw.
A, large aspect ratio is easy to eat evenly;
B, better thermal stability of the plastic can be longer screw to improve the mixing without burning, poor thermal stability of the plastic can be shorter screw or screw end without thread. Considering the characteristics of plastics, the general flow length is as follows: thermosetting is 14'16, hard PVC, high viscosity PU and other heat sensitivity is 17'18, general plastic is 18'22,PC, POM and other high temperature stability plastic is 22'24.
2. Compression ratio: the ratio of the depth of the last screw groove of the feed section to the depth of the first screw groove of the metering section.
A, considering the influence of material compressibility, filling degree, reflux, etc., the product should be dense, heat transfer and exhaust; B, appropriate compression ratio can increase the density of plastic, make the combination between molecules more closely, help to reduce air suction, reduce the temperature rise due to pressure, and affect the difference of output, inappropriate compression ratio will destroy the physical properties of plastic; C, the higher the compression ratio, the higher the temperature rise of the plastic in the plasticizing process in the material pipe, the better mixing uniformity of the plastic in the plasticizing process, and the relative discharge amount is greatly reduced.
D, high compression ratio is suitable for non-fusible plastics, especially with low melting viscosity, thermal stability plastics; low compression ratio is suitable for fusible plastics, especially with high melting viscosity, heat sensitive plastics.
3. Back pressure
A, increasing back pressure can increase the work done by the screw on molten resin, eliminate unmelted plastic particles, and increase the density and uniformity of raw materials in the material pipe; B, back pressure is used to increase the temperature of the barrel, and its effect is the most significant; C if the back pressure is too large, it is easy to decompose plastics with high heat sensitivity, and salivation may occur for plastics with low viscosity. if the back pressure is too small, the finished products injected may have bubbles.
4. Screw speed
A, the rotation speed of the screw directly affects the shear of plastic in the spiral groove; B, small screw groove shallow absorb heat source quickly, enough to promote plastic softening in the compression period, the friction heat between the screw and the barrel wall is low, suitable for high-speed rotation, increase plasticizing capacity; C, large screw should not rotate quickly, so as not to plasticize uneven and cause excessive friction heat; D, for plastics with higher heat sensitivity, if the screw speed is too large, the plastic will easily decompose. E, usually each size screw has a certain speed range, the general speed is 100 150rpm; Too low to melt the plastic, too high to burn the plastic.
5. Electric heating temperature setting
A, melt the cold hard plastic retained in the barrel and the screw to facilitate the rotation of the screw and provide part of the heat required for the plastic to obtain melting; B, set 10 ℃ lower than the melt temperature (partly provided by friction heat energy); C, nozzle temperature adjustment can also be used to control salivation, condensate (plug nozzle), wire pulling and other problems;