What is the design life of a screw for a plastic single-screw extruder?

Update:12-10-2018
Summary:

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. 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 the 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 a 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, the 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.