Abrasive wear

Extruded, especially components of the formula, such as fillers (calcium carbonate) and pigments (titanium dioxide), can damage the outer diameter of the screw, the teeth, the root of the screw, and the inner wall of the barrel. The first stage of wear indicates the so-called After the “blasting effect”, depending on the time of use and the amount of filler used, the flowing extrudate grinds grooves of a depth of a few millimeters into the screw teeth, the outer edge of the screw root, and the inner wall of the barrel.

Damages of the above nature occur mainly in the plasticizing zone immediately adjacent to the exhaust zone, especially when the lubricant in the formulation is excessive. Due to too much lubricating components, the degree of plasticization of the extrudate is poor (solid state), so that the filler cannot be wrapped in the molten material and the filler grinds the screw and the barrel like an abrasive.

2. Adhesive wear (contact wear)

Due to the separation forces introduced by the material (roll flow) in the groove, the screw is forced into contact with the upper and lower inner walls of the barrel. The magnitude of this force is directly proportional to the pressure produced by the stream. In any case, there will always be metal contact between the screw and the barrel, resulting in wear.

If excess lubricant is used in the formulation or if the operating temperature is set incorrectly (the cylinder temperature in zones 1 and 2 is too low) and the material begins to plasticize at the end of the discharge section, severe wear will occur in this zone. This damage will eventually lead to screw fracture. Usually:

1) Good uniform material: low viscosity, low pressure, low separation force, low wear;

2) Inhomogeneous materials with poor quality; high viscosity (hard particles in the material), high pressure, high separation force, and high wear;

3) Granular materials (hard particles), excessive recycling of pellets, more severe wear.

3. Corrosion

In the processing of PVC, PVC is to decompose HC1. Different environmental influences (such as humidity) can cause different corrosion. With the appearance of "paste" (decomposition of PVC), large areas of erosion will occur. Especially when using formulations containing tin stabilizers (transparent plates), localized pits are often found during processing.

In summary, wear on the screw and barrel is usually the result of interaction and mutual promotion of the above three forms of wear. In most cases, the contact wear that results in a reduction in the diameter of the screw (or an increase in the inner diameter of the barrel) is the main reason that affects the normal function of the plasticizing unit of the twin-screw extruder. In extreme cases, continuous bending stresses (screws are pushed to the upper and lower sides of the barrel during each rotation, resulting in a corresponding bending of the screw) can cause one or two screws to break.