Screw extruder is the core equipment of plastic forming and blending modification. In the actual production process of blending modification, the extruder screw is in the harsh high pressure and high temperature environment, and suffers from huge friction and shear force.
Because of the special working environment,the extruder screwis not the common friction between metal and metal, but metal and polymer, so the wear of the screw surface is often serious.
Screw wear increases the distance between the screw and the barrel, which affects the compression and shear of the screw to the material, thus leading to the decline of product quality. On the other hand, frequent replacement of worn-out screw not only increases the cost, but also delays the production schedule, resulting in reduced production efficiency.
Screw is generally due to long-term wear, screw and barrel clearance is too large to normal extrusion and scrap, but there are also examples of damage caused by improper design or operation of the working stress exceeds the strength limit. The following simple to introduce the reasons for the wear of the extrusion screw.
1. The processing temperature
Each type of plastic has an ideal plasticizing temperature range, and the barrel processing temperature should be controlled close to this temperature range. Plastic particles from the hopper into the barrel, will first arrive at the feeding section, in the feeding section will inevitably appear dry friction, when these plastics are not heated enough, uneven melting, it is easy to cause the inner wall of the barrel and screw surface wear.
Similarly, in the compression section and homogenization section, if the plastic melting state is disordered and uneven, it will also cause wear to increase.
The speed should be adjusted properly. As a result of part of the plastic with strengthening agent, such as glass fiber, minerals or other filling materials. These materials tend to rub against metal materials much more than molten plastic.
If the use of high speed, in improving the plastic shear force at the same time, it is easy to enhance the fiber tear, torn fiber contains sharp end, so that the wear force greatly increased. Inorganic minerals in the metal surface high-speed sliding; its scraping effect is not small. So the speed should not be adjusted too high.
3. Clearance between screw and barrel
The screw rotates in the barrel and the friction between the material and the working surface of the screw and the barrel gradually wears away, the diameter of the screw is gradually reduced, and the diameter of the inner hole of the barrel is gradually increased. In this way, the diameter gap between the screw and the barrel increases with the gradual wear of the two.
However, because the resistance of the nose and the shunt plate in front of the barrel has not changed, it increases the leakage flow of the extruded material when it moves forward, that is, and the material increases the flow momentum from the diameter gap to the direction of the feed. The result is a decline in production of plastic machinery. This phenomenon makes the material stay in the cylinder time increase, resulting in material decomposition. In the case of PVC, the decomposition of hydrogen chloride gas enhances the corrosion of the screw and barrel.
4. Uneven plasticizing of materials or mixed with metal foreign matter
Because the material is not plasticized evenly, or there is a metal foreign matter mixed with the material, so that the screw rotation torque force is suddenly increased, this torque exceeds the strength limit of the screw, so that the screw is broken. This is a kind of unconventional accident damage.
The above is the more common screw wear reasons; I hope that through the above content can let you have a further understanding and understanding of the extrusion screw.
Reducing screw wear and prolonging screw life will greatly reduce the maintenance cost of equipment, ensure the quality of products, and bring higher economic benefits to enterprises.