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EXTRUSION Several variations of flight wedges, such as chamfers, radii, and the like, are offered as devices to reduce screw wear. Screw flights wear when they make contact the barrel wall because the lubricating film of polymer in the flight clearance is missing. Without the lubricating film, the flights are in metal-to-metal contact with the barrel. The screw is centered in the barrel at the drive end by the reducer—assuming proper alignment of the feed throat and barrel—but is free to move at the other end. The polymer-filled channels tend to hydraulically support the screw when rotating. The screw isn't just bouncing around, but still continuously contacts the barrel wall due to moving radial pressures developed in the screw. The screw flights provide the main centering and will not wear as long as a lubricating polymer film is maintained between flight and barrel wall. When that film is squeezed out, the flights will gall, causing wear, even with the most advanced materials of construction. A concept widely used to maintain shaft centering and reduce wear is a hydrodynamic bearing, as shown in Fig. 1. Any eccentricity of the shaft "drags" the lubricant into the gap between the shaft and bearing and creates an increase in film pressure. The magnitude of the pressure development increases with the relative velocity, film viscosity and decreasing film How 'Hydro Wedges' Reduce Screw Wear Get more insights on Extrusion from our expert authors. short.ptonline.com/extrudeKH Learn more at PTonline.com KNOW HOW EXTRUSION Generally, it is difficult to use a hydrodynamic bearing in single screws without restricting the output. Using them only in the melting section provides for greater design freedom and negates the impact these devices can have on output and melt temperature. By Jim Frankland Pressure Development Rotating Screw Flight with Hydro Wedge in Stationary Barrel A "hydro wedge" has to be on the pushing side of the flight to have any effect, but testing confirmed there is pressure development due to the moving surfaces. In general, the effect is useful from a flight helix angle of 10-20°, and a wedge angle of 3-5° off the flight or mixer centerline seems to develop the most pressure. Rotation Force Screw Flight Pressure Development Pressure 3-5° Chamfer Hydrodynamic bearings are used to maintain shaft centering and reduce screw-flight. Typical Hydrodynamic Bearing The hydro wedge can be relegated to only the sections of the screw showing rapid wear, typically the melting section and Maddock mixer, where there is more discretion in the design and the dimensions can be more easily manipulated to compensate for the slight reductions in output and elevation in melt temperature. Here, on the Maddock mixer, a series of wedges is applied to both the bands at both ends and to the sealing flights. Maddock Mixer with Hydro Wedges Rotation Flow 3-5° Section B-B FIG 1 FIG 2 FIG 3 Rotation Force 3x Scale 3-5° Section A-A B A B A 32 APRIL 2017 Plastics Technology PTonline.com K now How