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.
Abrasive wear, which is caused when gritty reinforcements and other hard particles scour the barrel and screw surfaces, can be combated by using hardened, wear-resistant alloys and coatings. Tungsten alloys for barrel linings and screw hardfacings offer excellent protection against abrasive wear.
Screws often are surfaced with hard cobalt- or nickel-based weldments or manufactured with tool steels that can be case-hardened or through-hardened. Colmonoy 56, a nickel-chrome-boron alloy, is often used to protect screws against mildly corrosive and mildly abrasive resins. Colmonoy 83 hardfacing offers more protection at a higher price.
Bimetallic barrel liners of abrasion-resistant alloys offer protection against various wear conditions and provide a range of cost/performance options. Among the best in terms of corrosion resistance are HIP (hot isostatic pressing) barrels for injection molding and extrusion. Even fluoropolymers are no threat to these barrels, which have a protective layer of nickel-rich boron alloy containing molybdenum and a matrix of borides and carbides.
When used with corrosion-resistant screws, HIP barrels are said to reduce substantially the absorption of iron particles from the barrel bore into the melted polymer. Of course, this degree of protection comes with a much higher price tag. Nickel-chrome and tungsten barrels can be used as long as the barrel discharge end is cast in Inconel and the surfaces of the transducer holes and flanges are made with Inconel as well. Those barrels can be bought for considerably less than HIP barrels.