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The main factors that affect the quality of plasticizing screw

Update:23-06-2020
Summary:

The main factors affecting the quality of plasticizatio […]

The main factors affecting the quality of plasticization are: aspect ratio, compression ratio, back pressure, screw speed, barrel heating temperature, etc.

   1. Aspect ratio: the ratio of the effective working length of the screw to the diameter of the screw.

  A. If the length-to-diameter ratio is large, the food is easy to be uniform;

  B. Plastics with better thermal stability can use longer screws to improve mixing without scorching, and plastics with poor thermal stability can use shorter screws or no screw thread at the end of the screw. Considering the characteristics of plastics, the general flow rate is as follows: thermoset is 14\'16, rigid PVC, high viscosity PU and other heat sensitivity is 17\'18, general plastic is 18\'22, high temperature stable plastics such as PC and POM are 22\'24.

   2. Compression ratio: the ratio of the depth of the last screw groove of the feeding section to the depth of the first screw groove of the measuring section.

  A. Consider the impact of the material's compressibility, filling degree, reflux, etc., the product should be compact, heat transfer and exhaust;

b. Appropriate compression ratio can increase the density of the plastic, make the molecules more tightly bonded, help reduce the absorption of air, reduce the temperature rise due to pressure, and affect the difference in output. Inappropriate compression Than will destroy the physical properties of plastic;

  C. The higher the compression ratio, the higher the temperature rise of the plastic during the plasticizing process in the material tube, and the better the uniformity of the mixing of the plastic during plasticization. The relative output is greatly reduced.

  D. High compression ratio is suitable for non-fusible plastics, especially with low melt viscosity and thermal stability; low compression ratio is suitable for fusible plastics, especially with high melt 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 the raw materials in the feed tube;

  B. Back pressure is used to increase the temperature of the barrel, and its effect is more significant;

  C. Excessive back pressure, easy to decompose to plastics with high thermal sensitivity, and salivation may occur to low viscosity plastics. If the back pressure is too small, the finished product may have bubbles.

   4. Screw speed

  A. The rotation speed of the screw directly affects the shear of the plastic in the spiral groove;

  B. The small screw groove is shallower and absorbs heat quickly, which is enough to promote the softening of the plastic during the compression period. The frictional heat energy between the screw and the barrel wall is low, suitable for high-speed rotation and increasing the plasticizing ability;

  C. Large screw should not be rotated quickly to avoid uneven plasticization and excessive friction heat;

  D. Plastics with high thermal sensitivity, if the screw speed is too large, the plastic will be easily decomposed;

  E. Generally, the screw of all sizes has a certain speed range, the general speed is 100\'150rpm; if it is too low, it will not melt the plastic, and if it is too high, it will burn the plastic.

   5. Electric heating temperature setting

  A. Melt the cold and hard plastic staying in the barrel and screw to facilitate the rotation of the screw, and provide a part of the heat required for the plastic to melt;

  B. Set the temperature 5\'10℃ lower than the melt temperature (partly provided by frictional heat energy);

  C. The temperature adjustment of the nozzle can also be used to control problems such as drooling, condensate (plug nozzle), and wire drawing;

  D. General temperature control of crystalline plastics.

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