The radial clearance between the screw flights and barr […]
The radial clearance between the screw flights and barrel is typically set at of the diameter for screws sized from 2 to 10 in. This gap allows for very good barrel wiping action while providing some tolerance in the screw/barrel manufacturing process. Additionally, this clearance allows for a thin melt film to form between the screw flight and the barrel wall, functioning as a lubricant.The screw is kept centered in the barrel by the support of the polymer surrounding it, as shown in Fig.But even with this “hydraulic” centering, due to various pressure fluctuations.
the screw still tends to wobble and as a result comes into contact with the barrel surface intermittently over much of its length.If the screw is run more or less empty, the melt film can be quickly scraped off, losing the lubricating effect. Consequently, abrasive wear will quickly develop between the two unlubricated, sliding metal surfaces. Ultimately this leads to adhesive wear, where the two sliding metal surfaces actually fuse or gall. During galling the two surfaces actually pull metal from one another, leading to very rapid or catastrophic wear. Numerous metal alloys have been developed to minimize the effect of galling, but none totally eliminate it.
Because of the relatively high viscosity of most polymers, the polymer film captured between the flight and the barrel wall provides a substantial support or normal force against the screw flight, minimizing the contact between it and the barrel, as shown in Fig.Additionally, the polymer film is being constantly renewed as the screw rotates, and it absorbs some of the heat from any sliding friction to further reduce the onset of any adhesive wear. In the absence of the film, there is nothing to prevent metal-to-metal contact and frictional heating, causing rapid wear.Consequently, screws should never be run for an extended time without polymer.
Sometimes the screw is run empty accidentally, such as when the feed is interrupted by bridging, loss of material, or purging. However, in most cases, the extruder is run “dry” because of inattention or lack of understanding of the machine. Many operators would never expect a machine bearing to run very long without lubrication, yet they will routinely run the screw dry.The lubricating film between the screw flight and barrel provides support in that case, preventing full metal-to-metal contact.That’s why it’s very risky to run an extruder dry for more than a few minutes.In the solids-feed area, where there is little melt, the screw is centered by the gear reducer far enough downstream to prevent contact in that area. Yet such centering may be inadequate when crammers or stuffers are used, or when grooved-feed sections, which can develop very high side loads, are deployed. The centering effect of the reducer also does not provide support over the entirety of screw length, putting the flights in direct contact with the barrel.