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34 APRIL 2017 Plastics Technology PTonline.com E X T R U S I O N K now How ABOUT THE AUTHOR: Jim Frankland is a mechanical engineer who has been involved in all types of extrusion processing for more than 40 years. He is now president of Frankland Plastics Consulting, LLC. Contact jim.frankland@comcast.net or (724)651-9196. thickness. The pressure development is maximum near the point of minimum film thick- ness. The whole effect is to re-center the shaft in the bearing. The greater the eccentricity, the greater the pressure development, making it somewhat self-correcting as the force increases. Hydrodynamic bearings have been used in a number of twin-screw applications to support kneading sections, and in melt pumps to support the gears; and there are also a number of fully round mixing sections in which they are used to reduce wear. Generally, it is difficult to use a hydrodynamic bearing in single screws without restricting the output. The concept was thoroughly investigated in the early 1980s by Union Carbide when the first LLDPE polymers were introduced. The prevalent screw designs at that time for polyolefins were conventional screws with Maddock-style mixers. The reduced shear sensitivity of LLDPE compared with LDPE caused very high, unbalanced pres- sures to develop in these screws—particularly in the melting or compression section—and in the Maddock mixers, resulting in high wear. To get a hydrodynamic effect depends on forming a converging wedge between the surfaces moving at different velocities. That is very difficult to achieve with a narrow flight, since any pressure created acts in all directions and tends to squeeze the film out of any wedge before any significant pressure level is reached. The helix angle also negatively affects the pressure development in the flight clearance because the drag force is largely circumferential, pulling the polymer out of the wedge if it is on the trailing side.. The result of the studies done in the 1980s determined that any "hydro wedge" had to be on the pushing side of the flight to have significant effect, and it worked more like a scoop than the classic hydrodynamic support. However, testing confirmed there was pressure devel- opment due to the relative velocity between the surfaces—but it is a daunting task to math- ematically quantify. In general, the effect was tested to be useful from a flight helix angle of 10-20°, and a wedge angle of 3-5° off the flight or mixer centerline seemed to develop the most pressure, as shown in Fig. 2. However, because of the leakage, a slightly wider than normal flight was found to be advan- tageous. That can have a minor negative effect on output and melt temperature, but the hydro wedge can be relegated to only the sections of the screw showing rapid wear, typically the melting section and the Maddock mixer. Fortunately, in these sections there is some 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. For the Maddock mixer, a series of wedges was applied to both the bands at both ends and to the sealing flights, as shown in Fig. 3. The addition of the wedges made no measurable change in performance of the screws, yet the wear resistance was greatly improved in both areas, which supported the theory of developing radial centering pressure. The hydro wedge for screw flights and Maddock mixers was originally patented by Union Carbide but the patents expired many years ago. The designs were supplied to LLDPE proces- sors to promote LLDPE usage until being largely replaced by barrier screws. Hydro wedges still offer benefits with any high-viscosity, crystalline polymer where wear causes a limita- tion in output by restricting the design. Shear-type, dispersive mixers can benefit in almost all cases if the proper wedge geometry can be obtained. Hydro wedges have also been used to reduce wear rates with filled polymers by maintaining a more uniform flight clearance and by "flushing" fillers through the flight clearance to provide improved lubrication. Hydrodynamic bearings have been used in a number of twin-screw applications to support- kneading sections. Make Perfect Parts and Save Money! • Preload cores for perfect parts. • Save $$$$ with a smaller cylinder. • Simplify mold operation and design. • Smaller mold base = low cost parts. • Fully Preloads at only 1,500 psi. • Stays locked without hydraulics. • 400˚ F systems available. • Rush 0-2 day shipping available. pfa-inc.com N118 W18251 Bunsen Drive Germantown, WI 53022 (262) 250-4410 Fax (262) 250-4409 KOR-LOK ® , Hydra-Latch ™ and Hydra-Jaws ™ are trademarks of PFA, Inc. ©2016 PFA, Inc. All Rights Reserved. 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