Plastics Technology

SEP 2018

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narrow restriction will disentangle unmixed gels and trap, melt and disperse solid polymer fragments from the upstream melting section. The flow is then passed to the outflow flute and to the downstream sections of the screw. The unmixed gels are removed from the extrudate by increasing the stress level in the Maddock mixer. The stress level is increased by decreasing the clearance on the mixing flight. The stress level required to disperse unmixed gels depends on the resin and the level of chain entanglement. In past experience, the stress level required to disperse PE unmixed gels is about 100 to 200 kPa. The shear stress that the material experiences for flow across the mixing flight of the Maddock mixer can be estimated using Equations 1 and 2. The shear stress is responsible for breaking up the entangled species. This calculation is based on screw rotation physics. Equation 1: Equation 2: τ Μ = ηγ Μ γ Μ is the average shear rate for flow over the mixing flight in 1/sec. N is the screw rotation rate in revolutions/sec. η is the shear viscosity at the temperature and shear rate of the mixing process. D b is the barrel diameter. u is the undercut distance on the mixing flight. λ is the main flight clearance. τ M is the shear stress that the material will experience in flow over the mixing flight. FIG 4 Two-dimensional flows in a screw channel with H/W = 1 (channel depth/channel width). The arrows show the recirculation flows. The shaded area in the lower right corner is expanded to show the Moffat eddy. The main flow is out of the plane towards the reader. -1 -0.6 -0.2 0.2 0.6 1 X X Y Y 2 1 .6 1.2 0.8 0.4 0 0.2 0.1 0 0.8 0.9 1 call 800-328-5088 email info@cometpe.com visit cometpe.com Conveying Systems • Honeycomb Matrix Dryers • Hot Air Dryers • Compressed Air Dryers Color Feeders • Blenders • Granulators • Vision Systems • Mold Dehumidifiers • Material Handling PROTECT IT DRY IT LOAD IT HONEYCOMB MATRIX DRYERS COMPRESSED AIR RESIN DRYERS HOT AIR DRYERS HOPPER LOADERS CONVEYING SYSTEMS CENTRAL LOADERS VENTURI LOADERS EASY, ALL-IN-ONE MOLD PROTECTOR Full Color, Higher Resolution! Detects broken core pins & plugs, insert loading errors, molded parts left in the cavity, both positions of the slide core & top pin centering, metal die casting errors, and residue on the mold. γ Μ = π(D b - 2u - 2λ)N (u+λ) . . . 64 SEPTEMBER 2018 Plastics Technology PTonline.com Trouble shoot ing

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