Plastics Technology

MAY 2012

Plastics Technology - Dedicated to improving Plastics Processing.

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Page 52 of 75

tips and techniques screws and barrels How to Spec' Screws & Barrels For Running Corrosive Materials Highly polished FTXC-4000 bimetallic screw, with overall chromium carbide coating, offers advantages over flight-top hardfacings. By Ron Anderson, Flite Technology Corrosive, abrasive, and high-temperature resins can degrade screws and barrels before you know it. Certain engineering materials, thermoplastic elastomers, plus the wave of newly developed biopolymers, often create a corrosive condition that can quickly degrade equipment. Reinforcements such as glass fibers, spheres, and other fillers and additives are abrasive and also take a toll on equipment, especially feedscrews and barrels. Processors have to closely monitor the effects that their resins are having on the equipment to avoid production inef- ficiencies and potential part-quality problems. A preventive maintenance (PM) program that includes frequent inspection for corrosive and abrasive wear is required to maintain pro- cessing standards. Screws and barrels should be examined, measured, and pulled for rebuilding or replacement when needed because even minimal wear affects production and threatens quality. The best way to protect equipment against abrasive and corrosive resins is to select screws and barrels constructed of the right materials to protect against wear from the types of plastics to be processed. Wear-resistant barrels and screws manufactured to protect against abrasion, corrosion, and high-temperature melt processing cost more than the stan- dard, polyolefin-grade variety—as much as three or four times more—because corrosion-resistant alloys are expensive and difficult to machine. But they provide better wear life and longer intervals between servicing. ACIDS ATTACK SURFACES Corrosive wear such as pitting on the surfaces of barrels and screws occurs when metal is attacked during processing by acids and acidic gases, which can dissolve oxide coatings. Highly corrosive polymers include PVC, which produces hy- drochloric acid; acetals, which produce formic acid; and fluo- ropolymers, which produce hydrofluoric acid. Standard bime- tallic, nitrided, or tool-steel barrels can be severely damaged by fluoropolymers in a very short time. Other corrosive melts are those containing flame retardants and foaming agents. Resin manufacturers are generally happy to provide equip- ment recommendations for processing their products. DuPont makes it very clear that special corrosion-resistant materials must be used for all parts of extrusion equipment that come into PLASTICS TECHNOLOGY MAY 2012 51

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