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Screens and Screening

SpecialChem / Jun 26, 2007

Typically, the screens are supported on a breaker plate, a perforated steel disk that also serves as the seal between the head section (die and adapter) and the extruder barrel. This keeps the screens from being pushed into the head and die. The screens are usually square mesh - that is, same number of wires per inch or cm in both directions. That makes a stronger screen with rectangular openings, and only one of these is usually enough. In most extrusions, though, a screen pack is used, which is made up of several screens - a 20-mesh (20 wires per inch) is put up against the breaker plate, then perhaps a 40-mesh, then an 80 if that much filtration is wanted. Some very critical applications such as fibers, filaments and thin film use even finer screens, such as 200-350 wires per inch. Beyond this, there are three-dimensional media such as sand beds, sintered metal, and other specialized systems that are common in synthetic fiber spinning but seldom used in conventional extrusion and are beyond the scope of this article.

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