Screw protection | machinery
Screw and barrel wear is a major contributor to reduced extruder efficiency and, ultimately, to compromised product quality. Peter Mapleston looks at the latest metallurgical developments
Screws and barrels can wear it well
Extruder screws and barrels provide their optimum efficiency when they are new and in their “as designed” state. Over time these components will wear, especially when in contact with abrasive or corrosive materials, and efficiency can decrease notably. Machine adjust- ments can be used to compensate for this loss of efficiency but may introduce new problems including high shear rates, thermal degradation of polymers, greater cooling demand, and ultimately product inconsistency. Adjustments can, in some cases, actually accelerate the wear condition. “Many factors determine the best time to repair or
replace a feed screw/barrel but many processors continue operating well past this point,” says one provider of wear resistant solutions. This article takes a look at what some key technology suppliers are doing in terms of developing special materials and surface treatments that can increase the performance and
lifetime of screws, barrels and dies in thermoplastics extrusion equipment. In the area of screw segments, the focus in on improving ductility for the screw segments while maintaining at least the same levels of wear resistance, says Dirk Zimmermann, general manager at Extruder Experts, which special- ises in highly wear-resistant special materials for all wearing parts in extruders. With extruders becoming more and more powerful in terms of throughput and torque rating, he says the area between the splined bore and the ground diameter of the screw segments becomes more and more loaded with forces that need to be transmitted into the processed materials. Zimmermann says his company is researching new
materials that offer good ductility at the same level of wear resistance as the classical PM-HIP (powder metallurgy hot isostatic pressing) steels. Several materials are currently under test, “but only one new steel type showed really good results,” he says. “Extrud- er Experts will do some more testing on these solid (non-bimetal) screw steels [before market introduction]” he says. For highly corrosive processes, the “old-style” tip-welded bimetal solutions are still judged to be the best for twin screw extruders, he says though. Discussing barrels, Zimmermann says that, as the
most expensive wear parts (aside from the screw shaft), these should last at least two to three times as long as the screw segments in the melting zone. “So the search is still on to find very high wear resistant liners or cladding on the surface of the barrels. We at Extruder
www.pipeandprofile.com November/December 2016 | PIPE & PROFILE EXTRUSION 37 Above: Matching
metallurgy to the specific processing
task is critical in managing wear and corrosion,
according to Leistritz
Left: Cross section showing an Xaloy coated screw flight by Nordson
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56