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The main factors affecting the plasticization quality of the screw

The main factors affecting plasticization 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. Larger length to diameter ratio makes it easier to eat uniformly;
b. Plastics with better thermal stability can use longer screws to improve mixing properties without scorching. Plastics with poor thermal stability can use shorter screws or no screws at the end of the screw. Considering the characteristics of plastics, the general flow length is as follows: Thermosetting is 14 \ ’16, rigid PVC, high viscosity PU and other thermosensitivities are 17 \ ’18, general plastics are 18 \ ’22, and PC, POM and other high temperature stable plastics are 22 \ ’24.

2. Compression ratio: It is the ratio of the depth of the last spiral groove in the feed section to the depth of the first spiral groove in the metering section.
a. Considering the compressibility of the material, the degree of filling, and the effect of reflow, the product must be compact, heat transfer and exhaust;
b. Appropriate compression ratio can increase the density of plastics, make the molecules closer to each other, help reduce air intake, reduce temperature rise due to pressure, and affect the difference in output. Improper 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, the better the mixing uniformity of the plastic during plasticization, and the relative output is greatly reduced.
d. High compression ratio is suitable for non-fusible plastics, especially with low melting viscosity and thermally stable plastics; low compression ratio is suitable for fusible plastics, especially with 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 unmelted plastic particles, and increase the density and uniformity of the raw materials in the tube;
b. The back pressure is used to increase the barrel temperature, and the effect is most significant;
c. If the back pressure is too large, it will be easy to decompose to plastics with high heat sensitivity. It may cause drooling to low-viscosity plastics. If the back pressure is too small, the injection-molded 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 relatively shallow 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, which is suitable for high-speed rotation and increased plasticizing capacity;
c. Large screws should not be rotated quickly to avoid uneven plasticization and excessive frictional heat.
d. For plastics with high heat sensitivity, if the screw speed is too large, the plastic will be easily decomposed;
e. Generally, the screw of each size has a certain speed range, the general speed is 100 \ ‘150rpm; too low will not melt the plastic, and too high will burn the plastic.

5.Electric heating temperature setting
a. Melt the cold hard plastic staying in the barrel and screw to facilitate the screw rotation, and provide a part of the heat required for the plastic to get molten;
b. Set the temperature 5 \ ’10 ℃ ​​lower than the melt temperature (partly provided by frictional heat energy);
c. The adjustment of the nozzle temperature can also be used to control problems such as drool, condensate (plug), and wire drawing;
d. General temperature control of crystalline plastics.

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