Oversize Screw and Barrel
I have written in the past about screw/barrel wear and also screw rebuilding, but there are some screw and barrel shops that offer an alternative for excessive barrel wear and that is honing the barrel oversize and then rebuilding the screw oversize also to fit.
Back in the 1980’s, honing barrels oversize was often offered to a customer when the screw was rebuilt back to the original OEM outside diameter flight size, and the barrel was worn beyond the acceptable clearance (typically .030” on small extruders and .060” on large extruders). The repair shops would then hone the barrel, typically, .030” oversize, rebuild the screw .030” oversize and maintain the original screw flight to barrel ID clearance, which is normally .001” to .0015”/diametrical inch of screw diameter/side.
Note the oversizing of screws and barrels is generally only done for extrusion and blow molding applications and very seldom done for injection molding because in injection molding the non-return valve would also have to made oversize, and this is not very practical.
Since most screw/barrel repair shops do not have screw designers on staff, they do not realize that they may cause the customer processing problems compromising the extruder’s performance by offering this supposedly economic benefit. The problem caused by oversizing screws and barrels is not as problematic on large extruders, 4.5” and up; but with small extruders, it can be a big problem. For example, if a 6.0” barrel is honed .030” oversize or .015”/side, this makes the metering section of the screw .015” deeper. So, if the original metering depth was .275” and it is rebuilt .030” oversize, the new metering depth would be .290” deep or approximately 5% greater. On an extruder of this size this will not have a significant effect, especially if a viscous resin is being processed, i.e., Fractional Melt HDPE.
On the other hand, if a 2.5” screw is built oversize .030” and the original metering depth was .150” deep, the oversize screw’s new metering depth would be .165” or 10% greater. Since many small extruders are used for processing resins of low viscosity resins, the pumping capacity of the screw will be diminished significantly.
The other problem that arises when screws and barrels are oversized is that it affects the mixing capability of the screw. Maddock or UCC mixers are commonly used for many processing applications; many times screw/barrel repair shops only rebuild the main OD lands on the mixer and do not rebuild the barrier land to maintain the original barrier clearance. For example, on a 2.5” screw the mixer barrier land may be .025” and if the screw is rebuild .015” per side, the modified screw’s mixer clearance is now .040”. Therefore, the shear rate is greatly decreased, the shear stress is decreased, and the pressure drop over the barrier land is decreased, all of which will deteriorate the mixer’s performance. Poor mixer performance will be evident visibly as unmelts and possible non-uniform color distribution.
On the barrel side of the equation of oversizing, there are also negative effects. First, it must be noted that the residual thickness of the bi-metallic liner in the barrel is only .060” thick. If the barrel is honed .030” oversize, now the residual thickness of the liner is reduced to .045”; and as thickness of the liner is reduced, the hardness of the liner will diminish.
The other major negative affect when the barrel is honed oversized is if the barrel is a pressure transducer port that is located in the area where the screw flight rotates, the pressure transducer will be destroyed. The reason for the destruction of the transducer is that when the barrel was honed .015” per side oversize, the tip of the transduce will protrude into the bore of the barrel .015” and on a 2.5” the clearance between the screw and barrel is only .004” to .005” per side, so the transducer will extend .010” into the screw flight path. The result is an $850 pressure transducer has been destroyed.
Thus, even though honing your barrel oversize and rebuilding the screw oversize to match sounds like a great economical advantage to the maintenance and purchasing departments of a company, the problems which are manifested for the production and quality control departments are devastating. In the end, the expected savings which were obtained are greatly over-weighed by the overall loss in profits due to poor product quality. Think twice before traveling down the path of oversizing your extruder components.