Damage and repair of screws and barrels

The combined working quality of the two parts, the screw and the barrel, has an important impact on the plasticization of materials, the quality of products and the production efficiency. The quality of their work is related to the manufacturing accuracy of the two parts, the assembly clearance. When the two parts are seriously worn and the output of the extruder decreases, the repair of the screw and barrel should be arranged.
1. Causes of damage to the screw and barrel
1. The screw rotates in the barrel, and the friction between the material and the two makes the working surface of the screw and the barrel gradually wear out: the diameter of the screw gradually decreases, and the inner hole diameter of the barrel gradually increases. In this way, the gap between the fitting diameter of the screw and the barrel increases little by little with the gradual wear of the two. However, since the resistance of the head and manifold in front of the barrel does not change, this increases the leakage flow of the extruded material as it advances, i.e., the flow of the material from the diameter gap to the feed direction. As a result, extruder production has decreased. This phenomenon in turn 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 produced by the decomposition strengthens the corrosion of the screw and barrel.
2. If there are fillers such as calcium carbonate and glass fiber in the material, it can accelerate the wear of the screw and barrel.
3. Because the material is not plasticized evenly, or there are metal foreign substances mixed into the material, the screw rotation torque force increases suddenly, and this torque exceeds the strength limit of the screw, causing the screw to break off. This is an unconventional accident damage.
Second, the repair of the 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 hot-sprayed, and then it is ground to size. This method is generally processed and repaired by a professional spraying plant, and the cost is relatively low.
3. Weld wear-resistant alloy in the threaded part of the worn screw. According to the degree of screw wear, the thickness of 1~2mm is surfacing welded, and then the screw is ground to size. This wear-resistant alloy is composed of materials such as C, Cr, Vi, Co, W and B, which increases the screw's ability to resist wear and corrosion. Professional surfacing welding factories have a high cost for this kind of processing, and they are rarely used except for special requirements of screws.
4. The repair screw can also be repaired by hard chrome plating on the surface, chromium is also a wear-resistant and corrosion-resistant metal, but the hard chromium layer is relatively easy 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 comes later than that of the screw. The obsolescence of the barrel means that the inner diameter increases due to wear and tear over time. Here's how it can be fixed:
1. If there is still a certain nitriding layer in the diameter of the barrel due to wear, the inner hole of the barrel can be directly bored, ground to a new diameter size, and then a new screw can be prepared according to this diameter.
2. The inner diameter of the barrel is machined and trimmed and re-cast with a thickness of 1~2mm, and then finished to size.
3. Under normal circumstances, the homogenization section of the barrel wears quickly, and this section (taken 5~7D long) can be bored and trimmed, and then equipped with a nitriding alloy steel bushing, and the inner hole diameter refers to the diameter of the screw, leaving it in the normal fitting gap for processing and preparation.
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 diameter and a long hole, their machining and heat treatment processes are relatively complex, and the guarantee of accuracy is more difficult. Therefore, whether to repair or replace 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 screw, it is decided to repair, which is not necessarily the right choice, and the comparison of the repair cost with the renewal cost is only one aspect. In addition, it is necessary to look at the ratio of the repair cost to the time of using the screw after repair to the renewal cost and the time of using the renewal screw. It is the right choice to use a scheme with a small ratio to be economical.
4. Materials for the manufacture of screws and barrels
Manufacture of screws and barrels, at present, the commonly used materials in China are 45, 40Cr and 38CrMoAIA,
The manufacturing materials of screw and barrel in imported extruders are commonly used alloy steels such as 34CrAlNi7 and CrMoV9. The yield strength of this material is about 900 MPa. After nitriding treatment, the hardness is above 1000HV, both