Processor Tips



Oversizing Screws and Barrels

By:
30. December 2012 20:54

I have written in the past about screw/barrel wear, and also about screw rebuilding, but there are some screw and barrel shops that offer an alternative for excessive barrel wear: honing the barrel oversized and then rebuilding the screw oversized also to fit.
 
Back in the 1980s, honing barrels oversized 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 0.030 in. on small extruders and 0.060 in. on large extruders). The repair shops would then hone the barrel, typically, 0.030-in. oversizd, rebuild the screw 0.030-in. oversized and maintain the original screw flight to barrel ID clearance, which is normally 0.001-in. to 0 .0015-in. /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. That’s because in injection molding the non-return valve would also have to made oversize, which 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, say 4.5-in. and up; but with small extruders, it can be a big problem. For example, if a 6-in. barrel is honed 0.030-in. oversized or 0.015-in. side, this makes the metering section of the screw 0.015-in. deeper. So, if the original metering depth was 0.275-in. and it is rebuilt 0.030-in. oversized, the new metering depth would be 0.290-in. 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-in. screw is built oversized 0.030-in. and the original metering depth was 0.150-in. deep, the oversize screw’s new metering depth would be 0.165 in. or 10% greater. Since many small extruders are used for processing 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 in. screw the mixer barrier land may be 0.025 in., and if the screw is rebuild 0.015-in. per side, the modified screw’s mixer clearance is now 0.040 in. 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 0.060 in. thick. If the barrel is honed 0.030 in. oversized, now the residual thickness of the liner is reduced to 0.045 in.; 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 has 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 0.015 in. per side oversized, the tip of the transducer will protrude into the bore of the barrel 0.015 in. And on a 2.5-in. the clearance between the screw and barrel is only 0.004 in. to 0.005 in. per side, so the transducer will extend 0.010 in. into the screw flight path. The result is destruction of an $850 pressure transducer.
 
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.
 
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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com

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