Main factors of screw influencing plasticizing quality


The main factors affecting the plasticizing quality are […]

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 the effective working length of the screw to the diameter of the screw.
a. If the ratio of length to diameter is large, the feed is easy to be even;
b. The plastics with better thermal stability can use longer screw to improve mixing without burning, while the plastics with poor thermal stability can use shorter screw or screw end without thread. Considering the characteristics of plastics, the general flow length ratio is as follows: thermosetting property is 14 \ '16, rigid PVC, high viscosity Pu and other thermosensitive properties are 17 \' 18, general plastics are 18 \ '22, PC, POM and other high temperature stable plastics are 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 influence of material compressibility, filling degree and reflux, the products 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 the air absorption, reduce the temperature rise caused by pressure, and affect the difference of output. Improper compression ratio will destroy the physical properties of plastic;
c. The higher the compression ratio is, the higher the temperature rise of plastic in the plasticizing process is, the better the mixing uniformity of plastic in the plasticizing process is, and the relative discharge amount is greatly reduced.
d. High compression ratio is suitable for non fusible plastics, especially low melting viscosity and heat stable plastics; low compression ratio is suitable for fusible plastics, especially high melting viscosity and heat sensitive plastics.
3. Back pressure
a. Increasing the back pressure can increase the work done by the screw to the molten resin, eliminate the unmelted plastic particles, and increase the density and uniformity of raw materials in the material pipe;
b. The back pressure is used to raise 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 the plastic with high heat sensitivity. If the back pressure is too small, the injection product may have bubbles.

4. Screw speed
a. The rotation speed of screw directly affects the shear of plastic in the spiral groove;
b. The small screw groove is shallow and absorbs heat quickly, which can make the plastic soften in the compression period. The friction heat energy between the screw and the barrel wall is low, which is suitable for high-speed rotation and increases the plasticizing ability;
c. Large screw is not suitable for fast rotation to avoid uneven plasticization and excessive friction heat;
d. For the plastics with high heat sensitivity, if the screw speed is too large, the plastics will be easy to decompose;
e. Generally, the screw of all sizes has a certain speed range, generally the speed is 100 \ '150rpm; if it is too low, it cannot melt the plastic, and if it is too high, it will scorch the plastic.
5. Electric heating temperature setting
a. The cold and hard plastics retained in the barrel and the screw are melted to facilitate the rotation of the screw and provide part of the heat needed for the melting of the plastics;
b. It is set to be 5 \ '10 ℃ lower than the melt temperature (part of it is provided by friction heat energy);
c. The adjustment of nozzle temperature can also be used to control salivation, condensing material (plug), draw wire and other problems;
d. General temperature control of crystalline plastics