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Wear environments

Commonly identified wear environments encountered in feed screw and cylinder processes.

Abrasive wear is damage caused to the components by hard abrasive fillers such as fiberglass, and mineral fillers. These particles pass between the screws outer flight and the barrel ID. They gradually plow away small amounts of metal from the screw and barrel causing wear. Hard filled plastic and even unfilled plastic under extreme pressure can abrade and erode the root of the screw at the leading flight radius, this generally occurs in the compression zone at the center of the plasticating assembly. The barrel inner diameter will often wear opposite the location of screw wear. In processes that exhibit rapid wear rates it is common to find heavily filled materials, high compression screw design, high back pressure, high screw speed, short fast cycles and heavy through puts.

Adhesive wear or better known as galling is caused by metal-to-metal contact. Metals that experience momentary contact can weld to each other under pressure and friction, upon continued rotation of the screw the welded points tear apart and deposit metal at various other locations on the screw and barrel and or into the melt. Insufficient heat settings can result in solid plastic being transported down the screw faster than melting can occur. This results in side loading in the screw channel forcing the screw flight against the barrel ID under pressure and can cause adhesion type wear. Straightness of the screw and barrel, machine alignment, proper screw and barrel clearance, correct heat settings, hardness and compatibility of metal materials help to prevent adhesion wear.

Corrosive wear is a chemical attack on the screw and barrel resulting when resins are degraded by over heating or excessive residence time. PVC's release hydrochloric acid when degraded. Other resins that can release corrosive elements are Fluoropolymers, Sulfides, ABS, polycarbonate, cellulosics, flame-retardants and other additives. The best preventive measure for this problem is to avoid overheating and excessive residence time. In these applications corrosion resistant screw and barrel construction should be specified.

Machine and process factors affecting wear on the screw and barrel Straightness of screw and barrel.

Screw design.

High screw rpm.

High screw backpressure.

Combination of screw surface and barrel wear.

Screw and barrel liner materials.

Improper support of the barrel.

Screw, barrel, and drive alignment.

Excessive loads on barrel discharge end.

Uniformity of barrel heating.

Improper heater settings.

Excessive residence time.

Material being processed.

Abrasive fillers. Corrosion.


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