Three of the most useful plastics for making bearings are Acetal, Delrin AF® and PTFE. Acetal and Delrin AF® are both thermoplastics; meaning that these materials can be heated into a liquid form and then cooled into a solid retaining the shape of either the extrusion nozzle or the mold used to shape it. PTFE has the lowest friction constant of any plastic material. It is not a thermoplastic but it can be extruded. These materials are ideal for use in applications where the sliding action of the parts is needed for things such as plain bearings, gears, and slide plates.
Acetal or polyoxymethylene (POM) is a semi-crystalline, non-amorphous type of thermoplastic. Delrin®, which is the DuPont brand name for Acetal, is a homopolymer. It comes in both black and white. One important difference between Acetal and the homopolymer Delrin resin is that Acetal tends to have centerline porosity. In a slab or sheet it will appear as a white stripe down the middle of a molded sheet of material. Both materials are FDA or 3A (Dairy) compliant.
Delrin has a melting point of 175°C. POM copolymer has a slightly lower melting point of 162-173°C. In general, Acetal has a very low coefficient of friction making it a good candidate for manufacturing light duty bearings. As indicated above, there are two types of Acetal (POM); one is a copolymer and the other is a homopolymer. While the two varieties may look the same and are quite similar, there are some significant differences in their properties.
Acetal copolymer will perform better than the homopolymer in areas such as thermal performance, impact, wear, chemical resistance and it also has a lower water absorption rate. Homopolymer acetal has a high crystallinity, thus improving its mechanical strength and creep resistance. Homopolymer Acetal is generally resistant to a wide range of solvents and common chemicals but does not perform well when exposed to oxidizing agents and strong acids. The copolymer Acetal is used more often than the Acetal homopolymer in faucets, impellers, value stems, values, value steams, and pump parts; in short, where ever moisture is present. Acetal or POM has high strength, hardness and maintains rigidity to -40°C.
Delrin AF is a combination of 10-25% oriented PTFE fluorocarbon fibers uniformly dispersed in Acetal/Delrin resin. The resulting product is one that possesses the strength, toughness, and dimensional stability of Acetal with the added properties of PTFE, which is one of the world’s most slippery solid materials. In fact, PTFE, commonly known as Teflon®, is used as the non-stick material on cookware. When long wear and low friction are needed in an application, bearings made of Delrin AF sustain high operating loads at high speeds and show little wear.
These non-lubricated bearings are also free of slip-stick behavior as the static and dynamic coefficients of friction are equal. Slip-stick is the spontaneous jerking motion that may occur as two surfaces alternately stick to and slide over each other with the corresponding change in the force of friction. Stick-slip can be most simply compared to seismically-active faults where earthquakes are generated during periods of rapid slips. PTFE friction bearings are maintenance-free as are lubricated PTFE bearings. No additional lubrication is needed. Additionally, PTFE has one of the highest continuous service temperatures among plastic.
These materials are used in tiny plastic gears in watches, garden hose nozzles, knife handles, pump parts, zippers, ball bearings, fasteners and ski bindings as well as acoustic guitar picks and many other diverse applications.
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