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How To Improve The Service Life Of Extruder Screw And Barrel?

The screw and barrel are key components of the plastic extruder, playing a crucial role in the extrusion equipment’s service life, the quality, and production efficiency of extruded products. The quality and grade of an extruder depend significantly on these core components. The barrel works in tandem with the screw to crush, soften, melt, plasticize, exhaust, and compact the plastic, ensuring the continuous and even transport of the material to the molding system. Additionally, the barrel acts as a support for the installation of other devices, such as heaters, cooling devices, feeding hoppers, and measuring devices.

How can the effective service life of the extruder screw barrel be improved?

Reducing the residence time of the material in the screw leads to uneven mixing and plasticization. Excessive shearing may cause rapid temperature increases and thermal decomposition of the material. Poor extrusion stability can alter the size of the extrudate, necessitating enhancements in the precision of auxiliary devices and control systems. Addressing how to prolong the lifespan of the reducer and bearings during high-speed operations is crucial. During the extrusion process, the design of the screw and barrel should aim to maximize melting efficiency by maintaining minimal gaps between the barrel’s inner diameter and the screw flights’ outer diameter. These gaps, typically very small, allow for effective heat transfer and shear rates.

Implementing these adjustments can help operators avoid issues such as extended cycles, unmelted materials, higher melt temperatures, or increased scrap rates. These changes often occur gradually and may not be immediately noticeable until they significantly impact product quality or production efficiency.

How Can We Reduce The Wear Of The Screw And Barrel?

First, it’s essential to understand the types of wear that occur.

3 Types of Screw and Barrel Wear:

1、Adhesive Wear

Adhesive wear occurs when the helical surface contacts the barrel’s inner surface and then separates, generating extremely high stresses. To determine if adhesive wear has occurred, check for two signs: burrs or surfacing material at the spiral edge’s end being scratched off or missing, and scratches or wear inside the barrel at the damaged screw points.

2、Abrasive Wear

A common type of wear in plastic machinery, abrasive wear is minimal when processing low viscosity polymers without fillers. However, when rubber particles are transported along the screw and pressed against the barrel wall, the shearing action causes wear on the screw edges and barrel wall. Abrasive wear becomes noticeable when processing resins with fillers, additives, or contaminants, or high-viscosity polymers. Hard materials tend to erode softer surfaces, hence the screw surface is typically made of hard, wear-resistant alloys. Glass and mineral-filled resins can wear down the screw’s root, especially at the back of the feed channel before the additives are fully mixed with the molten plastic.

3、Corrosive Wear:

Corrosion refers to the gradual destruction of materials by chemical attack, where the metal surface atoms are oxidized, damaging the surface. Once eroded, a metal surface becomes more susceptible to mechanical wear. Corrosion is especially common when extruding PVC or fluoropolymers and can aggressively damage parts made primarily of iron.

Wear Of The Screw and Barrel In The 3-Section Area

1、Feed Section

This section is where the raw material enters the screw. The phenomenon of the screw sagging due to gravity and then lifting as the material enters and exerts upward pressure typically occurs in the feed section. Here, raw materials are introduced into the system. The screws, being in a cantilever state with one end supported and the other end free, are prone to sagging under their own weight. As material enters, it fills the gap, pushing the screw upward against the barrel, which leads to wear from the outer diameter of the screw rubbing against the barrel’s inner wall. This wear is particularly pronounced when the material is abrasive or contains fillers, due to the high friction involved in conveying the raw materials.

2、Compression Section

In the compression section, wear can occur due to increased pressure and temperature, which may lead to deformation or erosion of the screw flights and barrel lining. This section processes a changing volume of material, resulting in increased stress and wear on both the screw and barrel. The high-speed operation combined with the forces applied to compress and plasticize the material can cause the screws to bend. This bending is most noticeable in the middle area, where mechanical forces are greatest, causing significant wear on the barrel due to increased contact and friction.

3、Metering Section

The metering section’s design often leads to velocity differences between the material at the screw root and the tip. This velocity gradient can produce high shear rates, causing localized wear on the screw flights and barrel due to the varying forces applied across the material.

Operating at temperatures necessary to maintain the material’s molten state, the metering section is susceptible to thermal wear. This occurs as high temperatures may cause differential expansion between the screw and barrel, increasing the likelihood of contact and subsequent wear.

Solutions

1、Verifying the Dimensional Accuracy of the Screw and Barrel

It is essential to inspect the screw’s straightness and concentricity when preparing to refurbish or install a new one. Upon installing the screw into a new barrel, it should easily slide in and rotate without any interference, confirming its straightness and concentricity. For extruders, pre-calibrating the machine is recommended before processing new parts, even though only a few machines may require calibration during inspections.

2、Screw Design

The screw must be tailored to the specific application. A blockage can occur if the screw’s melting section capacity is insufficient for the unmelted material delivered by the feed, leading to pressure that forces the screw against the barrel, causing wear. This phenomenon, known as solid wedging, often arises when a screw designed for a particular resin is used with different polymers at varying melt rates.

3、Materials of Screw and Barrel

The screw and barrel should be made from wear-resistant materials and alloys that are compatible with each other. Typically, softer materials wear out more quickly when they come into contact with harder substances. Thus, it is essential to carefully select the screw material and the helix cladding material, ensuring they are compatible with the barrel’s lining. The screw should be manufactured from high-strength, corrosion-resistant alloy steel. Meanwhile, the barrel should be constructed from either alloy steel or a composite steel pipe that is lined with heat-resistant alloy steel. Such choices of material guarantee high compressive strength, durability, excellent wear resistance, and corrosion resistance.

4、Enhancing Screw Cleaning and Maintenance

Specialized plastic applications, especially those involving abrasive and corrosive fillers and additives such as fiberglass, minerals, flame retardants, and stabilizers, necessitate a thorough examination of potential abrasive and corrosive wear. These aggressive fillers can markedly diminish the lifespan of screws and barrels. Proper protection of these components can prolong their wear life and lessen the impact of abrasive wear. During operation, some particles may adhere to the surface of the twin-screw extruder’s screw. Excessive buildup can hinder the screw’s normal function and accelerate wear. Consequently, regular cleaning and maintenance of the twin-screw extruder’s screw are essential. It is advisable to use a specialized cleaning solution during this process, with careful attention to temperature control to avoid damaging the screw.

5、Pay Attention to Details When Using

If not in use, the screw should be heated to a certain temperature to avoid corrosion and wear caused by moisture and frost on the screw; before use, carefully check whether all parts are intact and replace them in time if damaged.

By linking production efficiency to the measurements of screws and barrels, processors can establish reference points to express the relationship between wear and various production metrics. Although regular measurement of these components, especially in larger machines, may not always be practical, maintaining good records and monitoring operating parameter adjustments can help predict the need for refurbishments or replacements, optimizing component life, minimizing unplanned downtime, and ensures sustained optimal productivity.

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