Acetal Copolymer Rod

Acetal copolymer - POM-C - is the machinist's plastic. It cuts and turns to a fine finish and holds the tolerance afterwards, it is stiff and strong without being brittle, it runs quietly against metal without lubrication, and it barely notices humidity. For gears, bearings, bushings, rollers, manifolds and precision machined parts it is the default, and it has been for decades.

Published figures for unfilled copolymer: tensile strength around 8,800 psi, flexural modulus about 370,000 psi, Rockwell M80, notched Izod around 1.0 ft-lb/in, and a melting point of 329F with continuous service to roughly 180F in air. Water absorption is about 0.2 percent at 24 hours - low enough that a part machined dry stays the size you cut it, which is exactly what nylon will not do. Specific gravity is 1.41, so a foot of 1 inch rod weighs about half a pound.

Copolymer or homopolymer? The two are sold side by side and the difference is real. Homopolymer is a little stronger and stiffer on paper. Copolymer wins everywhere else that matters in service: it runs pH 4 to 13 against homopolymer's pH 4 to 9, it resists hydrolysis to 85C against 60C, and it is less prone to centerline porosity in larger sections. That chemical window is why copolymer is what goes into wash-down, caustic cleaning and hot-water duty, and why a homopolymer part in a caustic wash is a warranty claim waiting to happen.

On porosity, since it decides whether a part can hold pressure. Centerline porosity in heavy sections is a specified characteristic rather than a defect: ASTM D6100 sets porosity levels and gives a dye-penetration test, so you can specify what you need instead of hoping. Copolymer is the less susceptible type and the mills supply it porosity-free. We pass that on as the mills state it rather than promising zero voids - one distributor reports interconnected pores of 100 to 150 microns even in stock advertised as void-free. If the part seals, tell us and we will specify to D6100.

The warning worth more than any number on this page: CHLORINE ATTACKS ACETAL AT 3 TO 5 PPM. That is ordinary municipal tap water and ordinary sanitizing concentration. Acetal in a chlorinated circuit, a pool system or a sanitized wash-down line will craze and fail, and almost nobody prints this. Strong acids and strong bases attack it too. If chlorine is anywhere in the application, ask us about something else before you machine it.

Machining notes. It is one of the easiest engineering plastics to machine well - sharp tools, positive rake, generous chip clearance, and it takes a fine finish without much coaxing. Coolant is optional on light cuts and helps on heavy ones. It is not notch-sensitive the way polycarbonate is, but its thermal expansion is high for a plastic, so a part machined warm measures differently cold. Let heavy sections normalize before the finishing cut.

One note on the black. Carbon black genuinely does screen ultraviolet in acetal at the loadings used, and one mill states plainly that its black is weather resistant where its natural is not. But standard black pigmented stock is still not a weathering grade, and dedicated UV grades exist. For sustained outdoor service ask us rather than assuming the color is doing the work.

Safety, and this one is specific rather than boilerplate. Acetal evolves FORMALDEHYDE above 446F and also on contact with strong acids or strong bases. OSHA's permissible exposure limit is 0.75 ppm over eight hours with a 2 ppm short-term cap, and it is a sensitizer - repeat exposure matters. Keep tools from overheating the cut, use local exhaust, and never let acetal swarf mix with PVC, because halogenated polymers can trigger sudden formaldehyde release from it. Ask us for the manufacturer's Safety Data Sheet and work to it rather than to this paragraph.

How this is sold: by the foot, cut to the length you need. The mills supply this material in 5 and 10 foot lengths and we cut from them, so tell us the length and the quantity and we will quote the cut rather than making you buy a whole stick.