The combined performance of the screw and barrel significantly affects material plasticization, product quality, and production efficiency. This performance is influenced by the manufacturing accuracy of both components and the clearance between them. When wear on these parts becomes severe and the extruder’s output declines, maintenance of the screw and barrel should be scheduled.

I. Causes of Damage to the Screw and Barrel

1. As the screw rotates within the barrel, friction between the material and the two surfaces gradually wears down their working surfaces: the screw diameter diminishes, while the inner diameter of the barrel expands. Consequently, the clearance between the screw and the barrel increases incrementally with this wear. However, since the resistance offered by the die and the flow‑diverting plate at the barrel’s exit remains unchanged, the leakage flow of the extruded material—i.e., the amount of material that bypasses the screw–barrel interface and flows toward the feed end—also rises. This, in turn, leads to a reduction in the extruder’s throughput. Moreover, this phenomenon prolongs the residence time of the material inside the barrel, promoting thermal degradation. In the case of polyethylene, the gaseous decomposition products further exacerbate corrosion of both the screw and the barrel.

2. If the material contains fillers such as glass fibers, it can accelerate wear on the screw and barrel.

3. When the material is not uniformly plasticized or when metallic foreign objects are mixed into the feed, the torque required to rotate the screw can increase suddenly. If this torque exceeds the screw’s strength limit, the screw may fracture. This constitutes a highly unusual type of failure.

II. Repair of the Screw Rod

1. The broken screw should be designed based on the actual inner diameter of the barrel, with the new screw’s outer diameter tolerance determined to ensure a proper clearance relative to the barrel.

2. For worn screw rods with reduced diameter, the threaded surface is first treated by thermal spraying with a wear‑resistant alloy, followed by grinding to the specified dimensions. This method is typically performed by specialized coating shops and remains relatively cost‑effective.

3. Apply a wear‑resistant alloy overlay to the threaded portion of the worn screw. Depending on the extent of wear, build up 1–2 mm of material, then machine the screw to its final dimensions. This wear‑resistant alloy comprises elements such as carbon, chromium, vanadium, cobalt, tungsten, and boron, enhancing the screw’s resistance to wear and corrosion. However, the cost of this overlay process is quite high, so it is seldom employed except when specifically required for certain screws.

4. The screw can also be repaired by surface hard-chrome plating; chromium is a wear‑ and corrosion‑resistant metal, but the hard chrome layer tends to flake off relatively easily.

III. Repair of the Barrel

The inner surface of the barrel has higher hardness than the screw, so it tends to wear out later. The barrel is considered scrapped when its inner diameter increases due to wear over time. Its repair methods are as follows:

1. For a barrel whose diameter has increased due to wear, if it still retains a certain nitrided layer, the inner bore can be directly re-bored and lapped to a new diameter, after which a new screw can be machined to match that diameter.

2. The inner diameter of the barrel is machined and re‑cast with an alloy, with a wall thickness ranging from 1 to 2 mm, followed by precision finishing to the specified dimensions.

3. Under normal operating conditions, the homogenization section of the barrel tends to wear relatively quickly. This section—typically 5 to 7 times the screw diameter in length—can be reconditioned by boring and then fitted with a nitrided alloy‑steel liner. The inner diameter of the liner should be matched to the screw diameter, leaving an appropriate clearance for proper fit, and then machined accordingly.

It is important to emphasize here that the screw and the barrel—two critical components—are, respectively, a slender threaded shaft and a relatively narrow, long bore. Both undergo complex machining and heat‑treatment processes, making it difficult to ensure high precision. Therefore, when these parts wear out, whether to repair them or replace them should be evaluated from an economic standpoint. If the cost of repair is lower than that of a new screw, one might opt for repair—but this is not necessarily the right decision. Comparing repair costs with new‑part costs is only one factor. One must also consider the ratio of the repair cost to the service life of the repaired screw, versus the ratio of the new‑part cost to the service life of a new screw. Only the option with the lower ratio is economically viable and represents the correct choice.

4. Materials Used for Manufacturing Screws and Barrel Assemblies

Screw and barrel manufacturing: Currently, the commonly used materials in China include 45, 40Cr, and 38CrMoAlA.

The materials commonly used for manufacturing the screw and barrel of an extruder include alloy steels such as 34CrAINi7 and CrMoV9. These materials exhibit a yield strength of approximately 900 MPa. Following nitriding treatment, their hardness exceeds 1000 HV, providing both excellent wear resistance and superior corrosion resistance.