The Main Factors Affecting Plasticizing Quality of Extruder Screw

The main factors affecting the quality of plasticization are: length-to-diameter ratio, compression ratio, back pressure, plastic extruder screw speed, barrel heating temperature, etc.


Ⅰ. Length-to-diameter ratio: the ratio of the effective working length of the extruder screw to the screw diameter


1. If the ratio of length to diameter is large, the food will be more uniform


2. For plastics with better thermal stability, longer extruder screw can be used to improve mixing performance without scorching, and plastics with poor thermal stability can be used with shorter extruder screws or without threads at the end of the screw. Considering the characteristics of plastics, the general flow length ratio is as follows: thermosetting is 14\'16, rigid PVC, high-viscosity PU and other heat sensitivity are 17\'18, general plastics are 18\'22, PC, POM and other high-temperature stable plastics are 22\'24.


Ⅱ. Compression ratio: the ratio of the depth of the screw groove of the last extruder in the feeding section to the depth of the first groove of the metering section


1. Consider the impact of plastic compressibility, filling degree, backflow, etc., products should be dense, heat transfer and exhaust;


2. A proper compression ratio can increase the density of the plastic, make the molecule and the molecule more tightly bound, help reduce the absorption of air, reduce the temperature rise due to pressure, and affect the difference in output, improper compression ratio will destroy the physical properties of plastic;


3. The higher the compression ratio of the extruder screw, the higher the temperature rise of the plastic during the plasticization process in the barrel, the better the mixing uniformity of the plastic during the plasticization, and the relative output is greatly reduced. .


4. High compression ratio is suitable for non-meltable plastics, especially low melting viscosity, thermally stable plastics; low compression ratio is suitable for fusible plastics, especially high melting viscosity, heat-sensitive plastics.


Ⅲ. The back pressure of the extruder screw


1. Increasing the back pressure can increase the function of the extruder screw on the molten resin, eliminate unmelted plastic particles, and increase the density and uniformity of the raw materials in the barrel;


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


3. If the back pressure is too large, it is easy to decompose the plastic with high heat sensitivity, and it may cause salivation to the low viscosity plastic. If the back pressure is too small, the injection molded product may have bubbles.


Ⅳ. The speed of the extruder screw


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


2. The screw groove of the small extruder is shallow, and the heat source is absorbed quickly, which is enough to promote the softening of the plastic in the compression section. The frictional heat energy between the screw and the barrel wall is low, which is suitable for high-speed rotation and increases the plasticizing capacity;


3. The screw of a large extruder should not be rotated quickly to avoid uneven plasticization and excessive frictional heat;


4. For plastics with high heat sensitivity, if the screw speed is too high, the plastic will be easily decomposed;


5. Generally, each size screw has a certain speed range, the general speed is 100\'150rpm; if it is too low, the plastic cannot be melted, and if it is too high, the plastic will be scorched.


Ⅴ. The electric temperature setting of the extruder screw


1. Melt the chilled plastics in the barrel and the screw of the extruder to facilitate the rotation of the screw and provide part of the heat required for the melting of the plastic;


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


3. The adjustment of the nozzle temperature can also be used to control salivation, condensate (plug nozzle), wire drawing and other issues;


4. General temperature control of crystalline plastics.

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