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. Their quality of work is related to the manufacturing precision and assembly clearance of the two parts. When the wear of the two parts is severe and the output of the extruder is reduced, the maintenance of the screw and the barrel should be arranged.
First, the cause of damage to the Screw And Barrel
1. 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 wear gradually: the diameter of the screw is gradually reduced, and the diameter of the inner hole of the barrel is gradually increased. Thus, the matching diameter gap between the screw and the barrel increases little with the gradual wear of the two. However, since the resistance of the front head and the splitter plate of the barrel is not changed, this increases the leakage flow when the extruded material advances, that is, the amount of material flowing from the diameter gap to the feed direction increases. As a result, the extruder production is reduced. This phenomenon causes the residence time of the material in the barrel to increase, causing the material to decompose. In the case of polyethylene, the hydrogen chloride gas produced by the decomposition enhances the corrosion of the screw and the 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. Since the material is not plasticized evenly, or metal foreign matter is mixed into the material, the torque of the screw is suddenly increased. This torque exceeds the strength limit of the screw, and the screw is broken. This is an unconventional accident damage.
Second, the repair of the screw
1. The broken screw should be considered according to the actual inner diameter of the barrel, and the outer diameter deviation of the new screw is given according to the normal gap of the barrel.
2. After the surface of the thread with the reduced diameter of the wear 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 spray factory, and the cost is still relatively low.
3. Surfacing the wear-resistant alloy on the threaded portion of the wear screw. Build 1 to 2 mm thick according to the degree of screw wear, and then grind the screw to size. This wear-resistant alloy consists of materials such as C, Cr, Vi, Co, W and B, which increases the anti-wear and corrosion resistance of the screw. Professional surfacing plants have a high cost for this type of processing, and are rarely used except for special requirements.
4, repair screw can also be surface hard chrome plating method, chromium is also wear-resistant and corrosion-resistant metal, but the hard chrome layer is easier to fall off.
Third, the repair of the barrel
The inner surface of the barrel is harder than the screw, and its damage is later than the screw. The scrapping of the barrel is such that the diameter of the inner diameter increases due to time wear. Its repair method is as follows:
1. The barrel with increased diameter due to wear. If there is a certain layer of nitrided, the inner hole of the barrel can be directly bored, ground to a new diameter, and then the new screw is prepared according to the diameter.
2. The inner diameter of the barrel is machined and trimmed to recast the alloy, the thickness is between 1~2mm, and then finished to the size.
3. Under normal circumstances, the homogenization section of the barrel wears faster. This section (take 5~7D long) can be trimmed by the boring, and then a nitriding alloy steel bushing is used. The diameter of the inner hole is referenced to the diameter of the screw. Normally fit the gap and process and prepare.
It is emphasized here that the two important parts of the screw and the barrel are an elongated threaded rod and a small diameter and long hole. The machining and heat treatment processes are complicated and the precision is difficult to guarantee. . Therefore, after the wear of these two parts is repaired or replaced with new ones, it must be comprehensively analyzed from an economic point of view. If the repair cost is lower than the cost of the new screw, it is decided that repair is not necessarily the correct choice. The comparison between the repair cost and the update cost is only one aspect. Also depends on the ratio of repair costs to screw time and renewal costs after repair and updated screw usage time. It is economical to adopt a scheme with a small ratio and is the right choice.
4, screw and barrel manufacturing materials
Manufacture of screw and barrel. At present, the commonly used materials in China are 45, 40Cr and 38CrMoAlA.
The manufacturing materials of the screw and barrel in the imported extruder, commonly used alloy steel are 34CrAINi7 and CrMoV9. The yield strength of this material is about 900 MPa. After nitriding treatment, the hardness is above 1000 HV, which is both wear-resistant and has good corrosion resistance.
