Screw extruder is the core equipment of plastic forming and blending modification. In the actual blending modification production process, the screws of the extruder are exposed to harsh high pressure and high temperature environment, and are subjected to huge friction and shear forces.
Due to the special working environment, the extruder screw is not a common metal-to-metal friction, but a metal-polymer, so the wear of the screw surface is often serious.
The wear of the screw increases the spacing between it and the barrel, which affects the compression and shearing of the material by the screw, which will lead to a decrease in product quality. On the other hand, frequent replacement of worn and failed screws not only increases costs, but also delays production schedules, resulting in reduced production efficiency.
Screws are generally scrapped due to long-term wear, and the gap between the screw and the barrel is too large to be extruded normally, but there are also examples of failure due to improper design or operation of the working stress beyond the strength limit. The following briefly introduces the reasons for the wear of the extrusion screw:
Each plastic has an ideal plasticizing processing temperature range, and the barrel processing temperature should be controlled to be close to this temperature range. Plastic particles from the hopper into the barrel, will first reach 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 the screw surface wear increase.
Similarly, in the compression and homogenization sections, if the melt state of the plastic is uneven, it will also cause faster wear.
The speed should be properly adjusted. Because some plastics are added with reinforcements, such as fiberglass, minerals or other fillers. These substances tend to have much greater friction against metals than molten plastics.
If a high speed is used, while increasing the shear force on the plastic, it is easy to shred the reinforcing fiber, and the shredded fiber contains a sharp end, which greatly increases the wear force. When inorganic minerals glide at high speed on the metal surface, their scraping effect is not small. Therefore, the speed should not be adjusted too high.
Clearance between screw and barrel
The screw rotates in the barrel, the friction between the material and the two causes the working surface of the screw and the barrel to wear gradually, the diameter of the screw gradually decreases, and the diameter of the inner hole of the barrel gradually increases. In this way, the gap between the mating diameter of the screw and the barrel increases little by little with the gradual wear of the two.
However, since the resistance of the machine head and manifold in front of the barrel does not change, this increases the leakage flow of the extruded material when it advances, that is, the amount of material flowing from the diameter gap to the feed direction increases. As a result, the production of plastic machinery has declined. This phenomenon increases the residence time of the material in the barrel, resulting in the decomposition of the material. In the case of polyvinyl chloride, the hydrogen chloride gas produced by decomposition strengthens the corrosion of the screw and barrel.
Uneven plasticization of materials or mixing of metal foreign body
Because the material is not uniformly plasticized, or there is a metal foreign matter mixed into the material, the torque force of the screw rotation suddenly increases, and this torque exceeds the strength limit of the screw, so that the screw is broken. This is an unconventional accident damage.
The above is the more common causes of screw wear, and we hope that through the above content, everyone can have a further understanding of the extrusion screw.
Reducing the wear of the screw and extending the working life of the screw will greatly reduce the maintenance cost of the equipment, ensure the quality of the product, and bring higher economic benefits to the enterprise.