Plastics Technology

SEP 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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ABOUT THE AUTHORS: Mark A. Spalding is currently a Fellow in the Materials & Parts Processing Group at The Dow Chemical Company, Midland. Mich. His expertise is in single-screw extrusion and related polymer processing technologies. Spalding has designed extrusion systems for most of Dow's major customers for virtually every resin that Dow produces. He is a Fellow and an Honored Service Member (HSM) of the Society of Plastics Engineers (SPE). Contact: 989-636-9849;; Eddy Garcia-Meitin is a technologist leader for Dow in Freeport, Tex. He has been leading optical and electron microscopy efforts towards product development and failure analysis for nearly 40 years. Stephen L. Kodjie is a senior analytical technologist at Dow in Lake Jackson, Tex. He is an expert in compositional and failure analysis of polymers and has been at Dow for over 10 years. Xiaofei Sun is a senior engineer in Dow's Polyethylene Products R&D group in Midland. He is active in troubleshooting single-screw extruders and optimizing Maddock-style mixers. Gregory A. Campbell is president and chief technical officer at Castle Associates, Jonesport, Me., where he consults on extrusion and polymer processing issues. He taught chemical engineering for 24 years at Clarkson University in Potsdam, N.Y., where he also served as chair of the Chemical Engineering Dept., Dean of Engineering, and University Chief Information Officer. Timothy Womer is a recognized authority in plastics processing and machinery with a career spanning more than 40 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;; 100 C 125C 126 C 135 C 135 C (pressure smeared) A Axial Mixer B Mixer Cross Section Mixing Flight Mixing Flight Wiper Flight Mixing Flight Clearance (u + λ) In Flute Wiper Flight Flow Out Flute FIG 6 Photographs of an unmixed gel at select temperatures using a hot-stage microscope. The unmixed gel melted at about 135 C. When the gel was smeared by moving the glass cover slip, the stress was enough to disentangle the polymer chains such that the gel would not reappear upon cooling. FIG 7 Schematic of a Maddock-style mixer. the degradation is occurring is to remove the screw from the extruder while hot. For this procedure, pellet flow to the hopper is stopped while screw rotation is continued. The screw is rotated until resin flow out of the die stops. Next, screw rotation is stopped and the transfer line is removed from the discharge end of the extruder. The hot screw should be pushed out about three diameters and then photographed and studied for indications of resin degradation. The metal surfaces should appear clean with only mild discoloration. If a stag- nant region exists, then dark colored, degraded material will occupy the space. Once the segment is studied and photographed, the hot resin should be removed from the screw using brass tools. Another three diameters are then pushed out and the process is repeated. If the process is running a natural resin and a colored masterbatch, the extruder should be first purged with the natural resin until the extrudate is essentially free of colorant. The screw shown in Fig. 3 was removed from the extruder using this technique. The trouble- shooter must be able to identify the character- istics of the gel and recognize the likely source. 66 SEPTEMBER 2018 Plastics Technology Trouble shoot ing

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