What are the most common causes of screw wear in a blown film extruder?

Blown film extrusion often involves the use of fillers (such as talc, calcium carbonate, or glass fibers) and additives (such as colorants, UV stabilizers, or antioxidants) to modify the properties of the polymer. These fillers, especially when added in large quantities, can be abrasive. As the material is processed through the extruder screw, the hard particles within these fillers come into constant contact with the screw's surface, causing abrasive wear. The feed and compression zones of the screw, where the material is under the highest shear and pressure, are especially vulnerable to this type of wear. Over time, this leads to the erosion of screw flights, affecting both the efficiency of the extrusion process and the overall lifespan of the screw.

Blown film extrusion requires precise control of processing temperatures, as excessive heat can cause the resin to soften. A screw subjected to high temperatures for prolonged periods may undergo thermal softening, which reduces its hardness and makes it more susceptible to wear and deformation. Additionally, thermal cycling—the repeated heating and cooling of the screw during startup, shutdown, or frequent processing changes—can create material fatigue, further exacerbating wear. Inaccurate temperature control can lead to uneven material flow, increased friction, and a higher rate of mechanical stress on the screw.

Friction between the screw and the barrel is a natural part of the extrusion process, but excessive friction can accelerate screw wear. When the clearance between the screw and the barrel is too tight, the material is subjected to increased resistance, resulting in more friction. This leads to higher shear forces in the compression and metering zones, causing the screw to wear down faster. In addition, viscous or poorly processed materials that create more resistance in the system can increase friction, further contributing to wear. Lubrication and regular inspection of screw-to-barrel clearance are essential to reduce friction and prolong screw life.

Screw speed has a direct impact on both the performance of the extrusion process and the rate of wear. Operating the screw at a speed that is too high can cause excessive shear stress, generating higher heat and friction. This can result in accelerated wear, especially in the metering and compression zones where the screw is under maximum load. On the other hand, operating at too low a speed can cause inefficient melting and higher residence time, leading to material degradation and additional wear. Maintaining an optimal screw speed is crucial for balancing energy consumption, material flow, and screw wear.

Uneven or inconsistent material flow into the extruder can lead to localized wear on the screw. For instance, improper feeding, inconsistent material pellet size, or contamination in the material can create flow disruptions. These disruptions result in areas of the screw where the material is over-concentrated, creating point loading on specific sections of the screw. The concentrated material can cause more friction and accelerate wear, particularly in areas where the screw is already under mechanical stress. Ensuring consistent material feeding and a steady flow rate helps maintain the uniformity of screw wear and ensures smooth operation.