Details
Material hierarchy: Polymer - Engineering Thermoplastic
Acetal is a highly crystalline material and is therefore is an opaque white color in its natural state. Acetal is available as a homopolymer or copolymer, although both classes provide excellent mechanical, chemical, and electrical properties. They are able to do so over a wide temperature range and over long periods of time. Unlike nylon, acetal is not hygroscopic and will not change dimensionally in a humid environment. They also provide superior creep and fatigue resistance. Acetal is well known for its friction and wear properties and is one of the strongest and stiffest plastics out there. Additives can be used to increase these and other properties even more so. Acetal requires protection from UV by using additives, such as carbon black, in order to keep it from degrading with prolonged exposure. You will find acetal in many plumbing applications, such as impellers, pumps, showerheads, nozzles etc., typically where water exposure is high. They are also common in industrial and electrical applications for things such as gears, cams, switches, buttons, and handles. Spiratex has experience extruding profiles, filled and unfilled, for various industries.
Acetal is the popular name given to polyoxymethylene, an engineering polymer prepared from formaldehyde (CH2O) with high stiffness, strength, toughness, and wear resistance. In addition, it has a high melting point, low moisture absorption, and is insoluble in common solvents at ambient temperatures. Because of this combination of properties, acetal resins are competitive with certain metals (e.g., brass and zinc) in automotive components such as door handles, pump housings, and similar parts; appliance hardware; and machinery components.
Polymers of formaldehyde, correctly called polyoxymethylenes (POM), acetals are among the strongest (tensile strength: 10,000 psi) and stiffest (modulus in flexure: 410,000 pxi) thermoplastics; acetals also are characterized by excellent fatigue life and dimensional stability. Other outstanding properties include low friction coefficients, exceptional solvent resistance, and high heat resistance for extended use up to 220°F.
At present there are two basic types of acetal: homopolymer and copolymer. The homopolymers are somewhat tougher and harder than the copolymers but suffer from instability in processing. Stabilization improvements have reduced odor and mold deposit problems and widened the processing temperature range.
The fact that acetals are highly crystalline thermoplastics accounts for their excellent properties and predictable long-range performance under load. In creep resistance, acetal is one of the best thermoplastics; however, the apparent modulus falls off consistently with long-term loading.
Acetals have an excellent fatigue endurance limit; at 100% RH it is 5000 psi at 77°F, and it is still 3000 psi at 150°F. Furthermore, lubricants and water have little effect on the fatigue life.
The impact strength of acetals does not fall off abruptly at subzero temperature like that of many other thermoplastics, and their hardness is only slightly reduced by moisture absorption or temperature below 215°F. Although less good than that of nylons, the abrasion resistance of acetals is better than that of many other thermoplastics. Like nylons, acetals have a slippery feel.
Acetals are especially notable among thermoplastics because of their resistance to organic solvents. However, when in contact with strong acids, acetals will craze.
In addition to good mechanical properties, acetals have good electrical properties. The dielectric constant and the dissipation factor are uniform over a wide frequency range and up to temperatures of 250°F. Aging has little effect on an acetal's electrical properties.
Acetls are available as compounds for injection molding, blow molding, and extrusion. Filled, toughened, lubricated, and UV-stabilized versions are offered. Grades reinforced with glass fibers (higher stiffness, lower creep, improved dimensional stability), fluoropolymer, and aramid fibers (improved frictional and wear properties) also are on the market.
Many applications involve replacement of metals where the higher strength of metals is not required and costly finishing and assembly operations can be eliminated. Typical parts include gears, rollers, and bearings, conveyor chains, auto window lift mechanisms and cranks, door handles, plumbing components, and pump parts.
Acetal is a semi-crystalline thermoplastic, with a continuous use temperature range of up to 180°F (82°C). It has a low difficulty to machine. It has good dimensional stability, good strength, good stiffness, and good chemical resistance. It has low moisture absorption. It has good wear resistance. It has a low coefficient of friction.