What is the cause of screw and barrel damage and how to fix it?


The combined working quality of the two parts, the scre […]

The combined working quality of the two parts, the screw and the barrel, has an important influence on the plasticization of the material, the quality of the product and the 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 of the screw and barrel should be arranged.

1. Reasons for damage to the screw and barrel
1. The screw rotates in the barrel, and the friction between the material and the two gradually wears the working surface of the screw and the barrel: 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. However, since the resistance of the nose 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.

2. 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.

3. Because the material is not plasticized evenly, or there is metal foreign matter mixed into the material, the rotational torque of the screw suddenly increases, and this torque exceeds the strength limit of the screw, causing the screw to break. This is an unconventional accident damage.

2. Repair of screw
1. The twisted screw should be considered according to the actual inner diameter of the barrel, and the outer diameter deviation of the new screw should be given according to the normal clearance with the barrel.

2. After the thread surface with the reduced diameter of the worn screw is treated, the wear-resistant alloy is thermally sprayed, and then ground to size. This method is generally processed and repaired by a professional spraying factory, and the cost is relatively low.

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 surface of the screw can also be plated with hard chrome. Chromium is also a wear-resistant and corrosion-resistant metal, but the hard chrome layer is easier to fall off.

3. Repair of the barrel
The inner surface hardness of the barrel is higher than that of the screw, and its damage is later than that of 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:

1. 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.

2. The inner diameter of the barrel is machined and trimmed to re-cast the alloy, the thickness is between 1~2mm, and then finish to size.

3. Under normal circumstances, the homogenization section of the barrel wears quickly. This section (5~7D long) can be trimmed by boring, and then equipped with a nitrided alloy steel bushing. The normal fit clearance is processed and prepared.

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. . Therefore, whether to repair or replace the new parts after the wear of these two parts must be comprehensively analyzed from an economic point of view. If the repair cost is lower than the cost of replacing the 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.