Screw wear problem

Screw wear is usually caused by metal-to-metal contact.

The wear of the screw leads to a larger gap between the screw and the barrel, thereby producing a thicker melt layer in the inner layer against the barrel.

If you are not using abrasive packing, screw wear is usually the result of metal-to-metal contact between the barrel liner and the screw. Wear is faster or slower, depending on the contact forces between the screw and barrel lining, and the lubricating effect of the polymer.

With proper screw/barrel alignment and a filled screw, wear typically occurs in the melting or compression section of the screw. In the compression section, the polymer is compressed into a smaller and smaller area; as the polymer flows through this area, the melt is often compressed by a factor of 2-5:1. If all the melt is extruded, the channel will fill with solids immediately. Because a screw with a tapered root is actually a powerful blended helical wedge that can build up enormous pressure when filled with incompressible solids. This is true whether it is a conventional or barrier screw.

When the screw groove is completely filled with solids to form a solid bed, a radial or unbalanced side pressure will act on the screw, giving the screw a thrust or bending force, sometimes there is a huge reaction to the barrel wall, which squeezes the lubricating film of the polymer, and as a result, metal-to-metal friction occurs on the surface. At this point little to no lubricating polymer film exists between the screw and the barrel liner under high pressure.

It's not until the screw/barrel clearance becomes quite large that you realize your throughput is significantly affected. More likely, the first thing you notice is a loss in melting velocity, evidenced by a gradual decline in melt quality. Using a model applicable to most polymers, the melt film is formed primarily from the shearing of the solid material by the barrel, thereby converting mechanical energy into thermal energy (melting).

As the gap increases, the screw will not scrape the inner wall of the barrel tightly when it rotates. This leaves a thick layer of melt against the inner wall of the barrel. This layer of melt gets hotter, but cannot transfer heat efficiently to the underlying bed of molten polymer solids because polymers do not conduct heat well. In this way, the thickness of the molten film greatly reduces the velocity of heat accumulation in the film, which can represent the polymer melting velocity.

This wear in the melting section of the screw is often not easy to detect because it may be corrected over time by small increases in screw speed. Ultimately, the most effective effect is that the melt quality deteriorates to such an extent that it causes quality problems on the extruded product. Since wear usually occurs over an extended period of time, melt quality problems are often attributed to other causes, thereby losing valuable time to take appropriate action. Additionally, if a screw design inherently has excess melting capacity, such as screw length, depth, and other design features, or if it contains a shear dispersing-mixing section, it should be judged as wearable by a number of criteria, but it may not be detected in performance for a long time.

So, how to solve the wear problem of the compression section or the melting section without pulling out the screw frequently?

“It is recommended to test each screw with each polymer as a plant management tool. As the screw wears and the melt film thickness increases, the film in that area tends to get hotter. This can reflect two things, either the temperature of the barrel in the melting zone is too high, or the degree of cooling in that zone is too high. Perhaps more importantly, melt quality deteriorated compared to the baseline record. In addition, there will be a small reduction in yield relative to the original conditions. "

Nanjing Haisi Extrusion has our own finishing factory, which ensures high quality and high precise of screw element.

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