The main factors affecting the quality of plasticizing by screw

Update:13 Apr 2019
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. Length to diameter ratio: the ratio of the effective working length of the screw to the diameter of the screw.

a, the long diameter ratio is large, the feeding material is easy to be uniform;

b. The plastic with better thermal stability can be used with a longer screw to improve the kneading property without scorching. The plastic with poor thermal stability can be used without a screw or a screw end. Considering the characteristics of plastics, the general flow length is as follows: thermoset 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 plastics are 22\'24.

2. Compression ratio: the ratio of the last groove depth of the feed section to the first groove depth of the metering section.

a. Considering the compressibility, filling degree, reflow, etc. of the material, the product should be dense, heat transfer and exhaust;

b. Appropriate compression ratio can increase the density of plastics, make the combination of molecules and molecules more tight, help to reduce the absorption of air, reduce the temperature rise caused by pressure, and affect the difference of output, improper compression Than will destroy the physical properties of the plastic;

c. The higher the compression ratio, the higher the temperature rise during the plasticizing process of the plastic in the material tube, and the better the uniformity of mixing in the plasticized plastic, and the relative discharge amount is greatly reduced.

d. High compression ratio is suitable for non-melting plastics, especially low melt viscosity, heat stable plastic; low compression ratio is suitable for fusible plastics, especially high melt viscosity, 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 material tube;

b, back pressure is used to increase the temperature of the barrel, the effect is most significant;

c. The back pressure is too large, and the plastic with high heat sensitivity is easy to be decomposed. For low-viscosity plastics, there may be a flow phenomenon, and the back pressure is too small, and the finished product may have air 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 absorbs the heat source shallowly, which is enough to soften the plastic during the compression period. The frictional heat energy between the screw and the wall of the barrel is low, which is suitable for high-speed rotation and increases the plasticizing ability;

c. Large screws should not be rotated quickly to avoid uneven plasticization and excessive friction 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, and the general speed is 100\'150 rpm; if it is too low, the plastic cannot be melted, and if it is too high, the plastic is burnt.

5, electric temperature setting

a. Melting the chilled plastic retained in the barrel and the screw to facilitate the rotation of the screw, providing a portion of the heat required for the plastic to melt;

b, set lower than the melt temperature 5 \ '10 ° C (partially provided by frictional heat);

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

d, crystalline plastics generally temperature control.

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