The combined working quality of the screw and barrel of china extruder machine has an important impact on the plasticization of materials, the quality of products and production efficiency. The quality of their work is related to the manufacturing accuracy and assembly clearance of the two parts. When the two parts are seriously worn and the output of the extruder decreases, the maintenance or replacement of the screw and barrel should be arranged.
Reasons for damage to the screw and barrel
The screw rotates in the barrel, and the friction between the material and the two causes the working surface of the screw and the barrel to gradually wear, the diameter of the screw gradually decreases, and the diameter of the inner hole of the barrel gradually increases. In this way, the matching diameter gap between the screw and the barrel increases little by little with the gradual wear of the two.
Since the resistance of the head and the manifold in front of the barrel has not changed, this increases the leakage flow of the extruded material forward, that is, the flow of the material from the diameter gap to the feeding direction increases. The result is a drop in extruder throughput. This phenomenon increases the residence time of the material in the barrel, resulting in the decomposition of the material. In the case of polyethylene, the hydrogen chloride gas generated by the decomposition enhances the corrosion of the screw and barrel.
If there are fillers such as calcium carbonate and glass fiber in the material, it can speed up the wear of the screw and barrel.
Because the material is not plasticized evenly, or there is metal foreign matter mixed into the material, the torque of the screw rotation suddenly increases. This torque exceeds the strength limit of the screw, causing the screw to twist and break, which is an unconventional accident damage.
When the screw and barrel are installed, the level, straightness and clearance are not adjusted in place, and the fastening screws are not reinforced. There is friction in the screw barrel when starting up, which accelerates the damage of the screw barrel.
The preheating time before starting the china extruder machine is insufficient, the raw materials accumulated in the screw barrel have not been fully softened, the starting torque is too large, or there are large pieces of unsoftened material, causing the screw barrel to be sprained or broken. and damage the gearbox.
China’s high-quality extruders are generally equipped with overload protection for the main motor of the extruder, which can protect the screw barrel to a certain extent and greatly reduce the occurrence of screw sprains and twist breaks. However, if the screw structure at the stress point is weak or the time is sudden, the screw damage may still occur.
Repair of screw
1) If the screw is twisted and broken by the china extruder machine, it is necessary to carefully check whether the barrel is seriously damaged. If the barrel can continue to be used, it should be considered according to the actual inner diameter of the barrel, and the outer diameter of the new screw should be given according to the normal clearance with the barrel. Diameter deviation is manufactured.
2) After the thread surface with the reduced diameter of the worn screw is treated, the wear-resistant alloy is thermally sprayed, and the thickness is generally increased to 2-3mm, and then it is ground to the size. This method is generally sprayed and repaired by professional screw and barrel manufacturers
3) Overlay welding wear-resistant alloy on the thread part of the worn screw. According to the degree of screw wear, surfacing welding is 1~2mm thick, and then the screw is ground and processed to size. This wear-resistant alloy is composed of materials such as C, Cr, Vi, Co, W and B, which increases the wear resistance and corrosion resistance of the screw. Professional surfacing plants have high costs for this kind of processing, and are generally rarely used except for special requirements for screws.
4) The repair screw can also be plated with hard chrome on the surface. Chromium is also a wear-resistant and corrosion-resistant metal, but the hard chrome layer is easier to fall off.
Repair of the barrel
The inner surface hardness of the barrel of the china extruder machine is higher than that of the screw, and its damage will come later than the screw. In the repair of the barrel: the hardness of the inner surface of the barrel is higher than that of the screw, and its damage is later than that of the screw. In addition, the author thinks that it is because the barrel is a cylindrical surface, the force friction area is large, the force area of the screw thread is small, and the raw material in the screw groove is designed to have a thrust surface that moves upwards. The damage to the barrel comes later than the screw. The scrapping of the barrel is the increase in the inner diameter due to wear and tear over time. Here’s how to fix it:
- For the barrel whose diameter is increased due to wear, if there is still a certain nitriding layer, the inner hole of the barrel can be directly bored, ground to a new diameter, and then a new screw can be prepared according to this diameter.
- The inner diameter of the barrel is machined and trimmed to re-cast the alloy, the thickness is between 1~2mm, and then finished to size. This method is expensive.
- Under normal circumstances, the homogenization section of the barrel wears quickly. This section (5~7D length) can be trimmed by boring, and then equipped with a nitrided alloy steel bushing. The diameter of the inner hole refers to the diameter of the screw. With the clearance, processing and preparation. The feeding section of the high-efficiency rod barrel also wears faster. It should be checked frequently during use.
It is emphasized here that the two important parts of the screw and the barrel, one is a slender threaded rod, and the other is a hole with a relatively small and long diameter. Their machining and heat treatment processes are more complicated, and it is difficult to ensure accuracy. . The repaired screw and barrel must be sent to the manufacturer for processing, otherwise there may be a problem that the new screw and the old barrel do not match.
Therefore, whether to repair or replace the new parts after the wear of these two parts must be fully analyzed. If the repair cost is lower than the cost of replacing a new screw, it is decided to repair it. This is not necessarily the right choice. The comparison between the repair cost and the replacement cost is only one aspect. In addition, it also depends on the ratio of the repair cost and the time of using the screw after the repair to the replacement cost and the use time of the updated screw. It is economical to adopt a scheme with a small ratio, which is the correct choice.
Materials for screw and barrel manufacture
For the manufacture of screws and barrels, the commonly used materials in China are 38CrMoAlA, 45 (generally used for alloy barrels) and 40Cr.
To sum up, as the core component of extrusion machinery, screw and barrel require professional and technical personnel to focus on its design, processing, installation, use and maintenance.
As a professional company from screw design and processing manufacturer to complete machine integration, China JWELL Machinery has a long history and rich experience in the processing and manufacturing of screw and barrel.