Delrin and Acetal Sheet: The Machinist's Plastic in a Fabrication Process

Acetal comes in two families that look identical and behave differently in ways that matter. Delrin is DuPont's brand of acetal homopolymer, which offers slightly higher mechanical strength, stiffness, and hardness, plus better fatigue and creep resistance, making it the premium choice for high-load mechanical parts, gears, and wear components. Delrin 150 is a common general-purpose homopolymer grade. Acetal copolymer (POM-C, sold under names like Acetron and Ultraform) trades a small amount of peak strength for two real advantages: better chemical resistance, especially to hot water and strong bases, and, critically, the absence of centerline porosity. Homopolymer rod and thick sheet can have a small zone of low-density porosity along the centerline from how it is extruded, which can become a problem in sealed, FDA, or thin-wall parts machined from the center of the stock; copolymer does not have this. So the rule of thumb is homopolymer (Delrin) for maximum mechanical performance and copolymer for chemical exposure, FDA and food contact, sealed parts, or anything machined from thick stock where centerline porosity would be a defect.

Acetal is prized because it machines like a dream. It is stiff and rigid enough to hold a sharp edge and tight tolerance, it cuts cleanly without melting or gumming if speeds and feeds are reasonable, and it produces good surface finish, so machinists can hold tolerances comparable to soft metals, often a few thousandths of an inch on well-controlled parts. Combined with its naturally low coefficient of friction and good wear resistance, this makes it the standard for machined bushings, bearings, gears, rollers, manifolds, and precision mechanical components. In a sheet context, acetal sheet stock is cut to blank and then routed, milled, drilled, and turned into flat plates, slides, wear strips, insulators, and machined details. Like all thermoplastics it is not bent on a press brake; the geometry is machined from solid. The main process caution is thermal expansion: acetal expands and contracts with temperature far more than metal (roughly ten times steel), so tight-tolerance parts must account for the temperature at which they are measured and used, and clamping during machining must avoid locking in stress that releases as warp.

Acetal's standout trait among plastics is dimensional stability. Unlike nylon, it absorbs very little moisture, so it holds its size in humid and wet environments where nylon parts swell and bind, which is a major reason acetal beats nylon for precision parts that see water or humidity. That said, it still moves with temperature, and machined parts can relieve internal extrusion stress after cutting, so for the tightest tolerances an annealing step (a slow heat cycle near 150 C and slow cool) before final machining stabilizes the part. There is one important limitation buyers must respect: acetal has poor resistance to strong acids and to oxidizing agents, and it is sensitive to chlorine and to prolonged UV without stabilization. It also has a relatively low continuous service temperature, around 80 to 90 C, well below PEEK or PEI. So acetal is the right machined-plastic choice for precision mechanical parts at moderate temperature with neutral chemistry, and the wrong choice for hot, strongly acidic, or chlorinated environments, where copolymer helps with bases but not strong acids, and a step up to PEEK or PVDF is warranted.