Feed Screws and Mixer Screws Information
Feed screws (feedscrews) and mixer screws are horizontally-oriented screws or augers placed in the bottom of a bin, silo, extruder, hopper, or molding machine to provide uniform flow. Feed screws with pitches varying along their lengths may be used to provide uniform flow. As the feed screw turns, process material is dispensed.
Most feed screws and mixer screws are made of alloy steel, tool steel, or stainless steel and then heat-treated, annealed, hardfaced with tungsten, triple-plated with chrome, or otherwise strengthened or finished. Plastic feed screws may be suitable for some applications. Both metal feed screws and plastic feed screws consist of a helically-flighted shaft that rotates inside a barrel or cylinder to advance the process material. The barrel’s thick, outer wall provides strength and may contain an integrally-formed or replaceable liner. There are several barrel choices for feed screws. Twin barrels are designed to house two extruder screws side-by-side. Depending on the design, the feed screws may or may not intermesh. Two-piece barrels usually have an annealed, outer shell and a centrifugally-cast second section that provides added strength. Vented barrels are designed for two-stage feed screws used in hot melt processing. These two-stage screws are usually longer than standard feed screws.
Selecting feed screws and mixer screws requires an analysis of product specifications and features. The following are important parameters to consider:
- screw inventory
- channel width
- screw diameter
- screw speed
Screw inventory or enclosed channel volume is measured from the forward edge of the feed opening to the discharge end of the screw channel. Channel width is the distance across the screw channel perpendicular to the flight as measured at its periphery. Clearance (radial clearance, diametrical clearance) is the difference in the diameters of the screw and the bore. Screw diameter is defined by the cross-section of the feed screw bounded by its flight-lands. This measurement is usually expressed as a nominal diameter, but can also be a range. Screw speed is the number of feed screw revolutions per minute (RPM). Abrasive wear, adhesive wear, and corrosive wear are additional factors to consider when selecting feed screws. Adhesive wear is caused by continuous contact between the process materials and the feed screw itself. Adhesive wear occurs during the landing of the screw flight. Corrosive wear, a problem caused by the formation of acids in the processing of plastics, can pit the surfaces of untreated metal feed screws.