1. Requirements for materials
It can be seen from the extrusion process that the extruder screw works under high temperature, certain corrosion, strong wear and high torque. Therefore, the extruder screw must have the following characteristics:
1) High temperature resistance. No deformation at high temperature;
2) Wear resistance and long service life;
3) Corrosion resistance. Because the material is corrosive;
4) High strength: can withstand high torque and high speed;
5) Good cutting performance;
6) Small residual stress and thermal deformation after heat treatment.
2. Common materials and heat treatment of extruder screw
At present, the commonly used extruder screw barrel materials in China include 45 steel, 40Cr, ammoniated steel, 38CrMoAl, etc.
1) 45 steel is cheap and has good processability. However, the wear resistance and corrosion resistance are poor.
Heat treatment: quenching and tempering HB220-270, high frequency quenching HRC45-48.
2) The performance of 40Cr is better than that of 45 steel, but a layer of chromium is often plated to improve its corrosion and wear resistance. However, the requirements for chromium coating are high, and the coating is too thin and easy to wear. If it is too thick, it is easy to peel off. After peeling off, it will accelerate corrosion. At present, it has been rarely used.
Heat treatment: Quenched and tempered HB220-270, hard chromium plating HRC > 55.
3) Nitrided steel and 38CrMoAl have excellent comprehensive properties and are widely used. Generally, the nitriding layer reaches 0.4-0.6 mm. However, this material has low resistance to hydrogen chloride corrosion and high price.
Heat treatment: Quenched and tempered HB220-270, nitriding HRC > 65.
Titanium carbide coating is used to improve the corrosion resistance of extruder screw surface abroad, but it is reported that its wear resistance is not good enough.
In recent years, foreign countries have taken a series of measures to improve the wear and corrosion resistance of extruder screw. One way is to use highly wear-resistant and corrosion-resistant alloy steel. Such as 34CrAlNi, 31CrMo12, etc. In addition, Xaloy alloy is sprayed on the surface of extruder screw. This Xaloy alloy has high wear and corrosion resistance.
When the screw is matched with the main shaft of the reduction gearbox with a long cylindrical surface, the screw can be used as a cantilever beam with one end fixed. The stress state of the screw during extrusion can be simplified as shown in the figure:
1. Stress of screw
1) Self weight g;
2) Torque M required to overcome material resistance;
3) Axial force P generated by material pressure.
The screw is generally scrapped due to long-term wear and the gap between the screw and the barrel is too large to be extruded normally. However, there are also examples of failure when the working stress caused by improper design or operation exceeds the strength limit. Therefore, the screw should also meet certain strength requirements.
4) The dangerous section of the screw is generally at the place with the smallest thread root diameter in the feeding section.
According to the material mechanics, for plastic materials, the composite stress is calculated by the third strength theory. The strength conditions are:
2. Strength calculation
3. Other situations
1) For the case that the screw tail is floating connected with the main shaft of the reduction gearbox, because the screw floats in the barrel, the bending stress caused by the screw self weight is equal to zero, so it is only calculated according to the screw compression stress and shear stress.
2) Because the bending stress caused by the self weight of the screw is very small (the screw is filled with materials), even in the former case, it can be omitted. Therefore, the two methods are actually the same.
3) There are also methods to estimate the screw diameter according to pure torsion.