[Required Course for Extrusion Production of Corrosive Materials] Screw & Barrel

Corrosive, abrasive, and high-temperature resins can inadvertently degrade screws and barrels. Certain engineered materials, thermoplastic elastomers, and newly developed biopolymers often create corrosive conditions that rapidly degrade equipment. Reinforcement materials such as glass fibers, balls, and other fillers and additives are abrasive and can also cause damage to equipment, especially feed screws and barrels.

Processors must closely monitor the effects of their resins on equipment to avoid production inefficiencies and potential part quality issues. A preventive maintenance (PM) program that includes frequent checks for corrosion and abrasive wear is needed to maintain processing standards. Screws and barrels should be inspected, measured, and pulled out for rebuilding or replacement when necessary, as even the smallest wear can affect production and threaten product quality.

Acid attack on surface

The best way to protect your equipment from abrasive and corrosive resins is to select screws and barrels made of the right materials to prevent wear due to the type of plastic being processed. Wear-resistant barrels and screws made to prevent wear, corrosion, and high-temperature melt processing cost three to four times more than standard specifications because corrosion-resistant alloys are expensive and difficult to machine. But they offer better wear life and longer service intervals.

Corrosive wear such as pitting occurs on barrel and screw surfaces when metals are attacked by acids and acid gases during processing, which dissolve oxide coatings. Highly corrosive polymers include PVC, which produces hydrochloric acid; acetals, which produce formic acid; and fluoropolymers, which produce hydrofluoric acid. Fluoropolymers can severely damage standard bimetallic, nitrided or tool steel barrels in a very short time. Other corrosive melts are those containing flame retardants and foaming agents.

Resin manufacturers are usually happy to provide recommendations for equipment to process their products. DuPont specifies that when processing its PTFE fluoropolymer, all extrusion equipment parts that come into contact with the melt must be made of special corrosion-resistant materials. The materials chosen are nickel-based alloys that are not affected by acids, such as Hastelloy, Inconel, Monel, and Haynes.

DuPont also notes that hardened electroless nickel plate can be used, but even small holes, nicks or cracks in the plating can degrade performance. Chrome-plated barrels and screws are not recommended for processing fluoropolymers.

NatureWorks, the manufacturer of Ingeo bioplastics, recommends using stainless steel in all processing equipment to minimize corrosion, and notes that PLA should not be left in the extruder, polymer filter, transfer line or other parts of the extrusion system at melt temperatures for extended periods of time.

Corrosion-resistant materials have a lower coefficient of thermal expansion (CTE) than standard steel, which can cause problems when processing high-temperature resins such as fluoropolymers. When the CTE of the barrel is different from the CTE of the screw, the screw/barrel clearance changes, which can cause the screw to seize and damage the barrel. Because corrosion-resistant materials have lower thermal conductivity, matching the screw and barrel is important. Screw and barrel manufacturers can provide expertise in pairing these components.

Abrasive wear is caused when grit reinforcements and other hard particles wash against barrel and screw surfaces and can be countered by using hardened wear resistant alloys and coatings. Tungsten alloys for barrel liners and screw case hardening provide excellent protection against wear.

The carbon content affects the hardness of the alloy. For screws, a medium carbon heat treated steel is usually used as the base onto which the hardface is welded to the top of the threads.

Screws are typically surface treated with a tough cobalt or nickel based weldment or are manufactured from tool steels that can be case hardened or through hardened. Colmonoy 56 is a nickel chromium boron alloy that is commonly used to protect screws from mildly corrosive and mildly abrasive resins. Colmonoy 83 weld overlays offer more protection at a higher price.

Bimetallic barrel liners made from wear-resistant alloys offer protection against a wide range of wear conditions and a range of cost/performance options. 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 a nickel-boron-rich alloy containing molybdenum and a matrix of borides and carbides.

When used with a corrosion resistant screw, HIP barrels are said to significantly reduce the absorption of iron particles from the barrel bore into the molten polymer. Of course, this degree of protection comes at a higher price. Both nickel-chrome and tungsten barrels can be used, provided that the barrel discharge end is cast in Inconel and the surfaces of the transducer bore and flange are also made of Inconel. These barrels can be purchased at a much lower price than HIP barrels.

Preventive Maintenance

The consequences of excessive wear can be significant to a processor's bottom line. Excessive clearance between the screw flights and barrel surface can affect machine performance, efficiency and energy consumption. In extrusion, abrasive conditions can lead to reduced throughput, necessitating increased screw speeds and energy consumption. This is often accompanied by higher melt temperatures and resulting quality issues.

Careful monitoring of critical screw/barrel clearances through a good PM program will ensure maximum processing throughput without unplanned downtime. PM programs are equally important when part quality is critical, such as when producing medical tubing and components for medical devices.

The screw should be pulled out while hot and cleaned thoroughly to check the thread OD and barrel ID regularly. Plastic material on the screw and barrel surface can be removed with a wire brush and copper wire mesh. Precision wear measuring instruments are accurate and easy to use and are available in a variety of sizes and functions.

After all plastic has been cleared from the barrel, an electronic bore gauge is inserted into the barrel and readings are taken at every inch mark to generate data on the ID condition. The screw thread diameter and depth are measured using a thread micrometer. Digital versions provide LCD readouts and enable data to be logged to a computer.

Taking these measurements periodically to determine the wear rate will enable the operator to predict when the screw or barrel will need to be replaced or repaired and schedule downtime at a favorable time. The recommended clearance for a new 2-in. screw and barrel is 0.004 to 0.006 in.

The rule of thumb is one thousandth of an inch per side. Screws should be reassembled when they exceed maximum clearance. Repair services can give worn parts new life at a lower cost than complete replacement. Barrels used for injection molding can be resleeved on the worn sections. Extruder barrels usually have to be resleeved along the entire length, which is more expensive and not always cost-effective.

Once the screw wears beyond a certain point, the melt will begin to return to the flights, causing further accelerated wear of the screw and barrel. Severe wear can waste a lot of resin and cause productivity to drop. However, wear can be detected before it reaches this point through regular PM inspections. Keeping the screw/barrel clearance within the recommended values is the best strategy.

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