How to specify screws and barrels for corrosive materials

Corrosive, abrasive, and high-temperature resins can degrade Screw barrel 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 inefficiencies and potential part-quality problems. A preventive maintenance (PM) program that includes frequent inspection for corrosive and abrasive wear is required to maintain processing 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 standard, 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.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 hydrochloric acid; acetals, which produce formic acid; and fluoropolymers, which produce hydrofluoric acid. Standard bimetallic, 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 equipment 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 contact with the melt when processing its Teflon fluoropolymers. The materials of choice are nickel-based alloys such as Hastelloy, Inconel, Monel, and Haynes, which are not affected by the acids.DuPont also notes that hardened electroless-nickel plate can be used, but even small holes, chips, or cracks in the plating can degrade performance.

Chrome-plated barrels and screws are not recommended for processing fluropolymers.NatureWorks, manufacturer of Ingeo bioplastics, recommends use of stainless steel for all processing equipment to minimize corrosion, noting that PLA should not be left in the extruder, polymer filter, transfer lines, or other part of the extrusion system at melt temperatures for extended periods.Corrosion-resistant materials have a lower coefficient of thermal expansion (CTE) than standard steel, which can cause trouble during processing high-temperature resins such as fluoropolymers. When the CTE of the barrel is different from that of the screw, the screw/barrel clearance changes, which can cause the screw to bind and damage the barrel. Matching the screw and barrel is important due to the lower thermal conductivity of corrosion-resistant materials. Screw and barrel manufacturers can provide expertise in pairing these components.