CNC Machined Delrin & Acetal Parts

Karl Gillihan

⚪ DELRIN / ACETAL

CNC Machining Delrin and Acetal: Delrin 150, Copolymer and Homopolymer

Ask a machinist for their favorite plastic and the answer is usually acetal. It cuts like a dream, holds dimensions better than almost any other polymer, produces clean chips with no gumming, and leaves a glossy finish straight off the tool. Delrin is DuPont's homopolymer brand of acetal, and the small but real distinction between homopolymer and copolymer acetal is the one thing buyers actually need to get right.

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The plastic machinists actually like

Acetal (polyoxymethylene, POM) machines better than any other common engineering plastic, and the reasons are practical. It is rigid rather than soft, so it does not deflect and smear the way nylon or soft plastics do; it forms clean, well-broken chips instead of stringy swarf; it does not gum up or melt easily because it cuts cool and clears heat well for a polymer; and it leaves a naturally glossy, low-friction surface. You can run it fast with standard carbide tooling, often the same tools used for aluminum, and get crisp threads, fine features and excellent finishes with minimal fuss. This machinability translates directly into low cost and fast turnaround for precision plastic parts. Where PEEK demands annealing and careful heat control and nylon demands tricks to combat gumming and moisture, acetal mostly just cuts. Sharp tools, good chip clearing and reasonable speeds are all it asks. For buyers, acetal is the default when you need a precise, low-friction, dimensionally stable plastic part at moderate cost: gears, bearings, bushings, rollers, manifolds, insulators, fittings and wear parts. It is the plastic equivalent of 6061 aluminum, the sensible first choice that covers a large share of applications.

Homopolymer (Delrin) versus copolymer acetal

Both are acetal and both machine superbly; the difference is structural and matters for specific applications. Homopolymer acetal, of which Delrin is the well-known DuPont brand, has slightly higher strength, stiffness and hardness and a marginally better surface finish, making it the choice for high-load mechanical parts, gears and wear components. Its one quirk is a tendency toward centerline porosity in extruded rod, a small void or looser structure at the core of large-diameter bar, which can show up if you machine into the very center of thick stock. For most parts this is irrelevant, but it matters for sealing surfaces or parts using the bar centerline. Copolymer acetal (such as Acetal-C and brand names like Hostaform/Celcon) trades a touch of strength for better chemical resistance to hot water and alkalis, more uniform structure with no centerline porosity, and often better performance in continuous hot-water or steam environments. It is preferred for plumbing, food-contact and parts exposed to hot water or harsh chemicals, and FDA-compliant copolymer grades are common for food and medical use. The buyer rule: choose homopolymer (Delrin) for maximum strength, stiffness and the best mechanical wear performance, accepting the centerline-porosity caveat on thick bar; choose copolymer for chemical and hot-water resistance, void-free structure, and food or potable-water contact. Delrin 150 is a standard general-purpose homopolymer grade widely used for machined parts.

Tolerances, limits and where acetal fits

Acetal holds excellent tolerances for a plastic, +/-0.005 in (0.13 mm) is comfortable and tighter is achievable, helped by its rigidity, low moisture absorption and good dimensional stability, far better than nylon, which swells with humidity. It is not metal: its thermal expansion is several times that of steel, so large dimensions shift with temperature and parts should be measured at controlled temperature, and like all plastics it can carry some residual stress, though far less troublesome than PEEK, so very tight-tolerance thin parts may benefit from stress-relieved stock. Surface finish is naturally glossy and low-friction, which is part of why acetal makes such good bearings and gears. Applications cluster around precision moving parts and low-friction needs: gears, cams, bearings, bushings, rollers, slides, valve components, manifolds and fluid-handling parts, electrical insulators, and consumer and automotive mechanisms. Acetal's low friction, wear resistance and dimensional stability make it ideal where plastic must move against other parts reliably. The honest limits: acetal has lower temperature capability than PEEK (continuous use to roughly 80-100 C), poor resistance to strong acids and to UV without stabilization, and it is difficult to bond or glue (the same inertness that gives chemical resistance resists adhesives), so parts are usually mechanically fastened or designed as one piece. It is also flammable. When a part needs higher temperature, chemical resistance or biocompatibility beyond acetal's range, PEEK or another high-performance polymer is the step up; for the large middle ground of precise, low-friction plastic parts at sensible cost, acetal is the right call.

Frequently Asked Questions

Delrin is a brand name, specifically DuPont's brand of acetal homopolymer, while acetal is the generic name for the polymer polyoxymethylene (POM), which comes in two types: homopolymer and copolymer. So all Delrin is acetal, but not all acetal is Delrin. The meaningful distinction is between homopolymer acetal (Delrin and equivalents) and copolymer acetal (brands like Hostaform and Celcon). Homopolymer has slightly higher strength, stiffness, hardness and a marginally better surface finish, making it preferred for high-load gears, bearings and wear parts, but extruded homopolymer rod can have centerline porosity, a small void or looser structure at the core of large-diameter bar, which matters only if you machine into the very center of thick stock for a sealing surface. Copolymer acetal gives up a little strength in exchange for better resistance to hot water and alkaline chemicals, a more uniform void-free structure, and common FDA-compliant food-contact grades. For buyers, when a drawing says Delrin it means homopolymer acetal; if your part needs maximum mechanical strength, specify homopolymer/Delrin, and if it needs hot-water and chemical resistance or void-free structure for sealing or food contact, specify copolymer acetal.

Acetal earns that reputation because nearly every machining property works in its favor. It is rigid rather than soft, so it does not deflect, smear or push away from the tool the way nylon and softer plastics do, which lets it hold crisp features and tight tolerances. It forms clean, well-broken chips instead of stringy swarf that wraps around tools, and it cuts cool for a polymer without gumming or melting, so heat management is far less fussy than with PEEK or nylon. It leaves a naturally glossy, low-friction surface straight off the tool, often needing no secondary finishing. You can machine it fast with standard carbide tooling, frequently the same geometry used for aluminum, getting clean threads, fine features and excellent finishes with minimal effort. Compared with other engineering plastics that demand annealing, special tooling or anti-gumming tricks, acetal mostly just cuts well. For buyers, this translates to low machining cost and fast turnaround for precision plastic parts, which is why acetal is the default first choice for gears, bearings, bushings, rollers and wear components, essentially the plastic equivalent of 6061 aluminum as the sensible, cooperative go-to material.

Choose homopolymer acetal (Delrin) when mechanical performance dominates, because it offers slightly higher strength, stiffness, hardness and surface finish, making it the better choice for load-bearing gears, cams, bearings and wear parts. The one caveat is centerline porosity: extruded homopolymer rod can have a small void or looser structure at the very core of large-diameter bar, so if your part requires a sealing surface or feature machined right at the bar centerline of thick stock, that porosity can be a problem and you may prefer copolymer or specially cast stock. Choose copolymer acetal when the environment is demanding rather than the loads: copolymer resists hot water, steam and alkaline chemicals better, has a uniform void-free structure throughout the cross-section, and is widely available in FDA-compliant grades for food and potable-water contact. It is the right pick for plumbing components, hot-water valve parts, food-handling equipment and parts exposed to harsh chemistry. For the large middle ground of general precision parts at moderate temperature with no special chemical exposure, either works and homopolymer/Delrin is the common default for its strength and finish. Match the choice to whether your priority is mechanical strength (homopolymer) or chemical and hot-water resistance and void-free structure (copolymer).

Acetal holds excellent tolerances for a plastic, with +/-0.005 in (0.13 mm) comfortable and tighter achievable on stable stock, which is one of its main advantages over other polymers. Its rigidity means it does not deflect and smear under the cutter, its low moisture absorption keeps it dimensionally stable in humidity (unlike nylon, which swells noticeably with moisture), and it carries far less troublesome residual stress than PEEK, so warping after machining is minimal. That said, acetal is still a plastic, not a metal: its coefficient of thermal expansion is several times that of steel, so dimensions shift meaningfully with temperature, and precise parts should be machined and measured at a controlled temperature, with metal-tight tolerances avoided across large spans. For the very tightest tolerances on thin or critical parts, using stress-relieved stock and a rough-then-finish approach helps, though acetal needs this far less than PEEK does. Its naturally glossy, low-friction finish supports precise bearing and gear fits without extra polishing. For buyers, the practical guidance is that acetal is the plastic to choose when you need the best achievable dimensional precision and stability in a machined polymer, while still specifying only the tolerances the function requires and recognizing it cannot match metal's absolute precision.

Acetal is excellent for precise, low-friction, dimensionally stable parts, but it has clear limits that should steer some applications elsewhere. Its temperature capability is moderate, continuous use to roughly 80-100 C, so for parts running hotter you need PEEK or another high-temperature polymer. Its chemical resistance is good against many solvents and fuels but poor against strong acids, and homopolymer in particular is attacked by hot water and alkalis where copolymer does better. It has poor UV resistance unless stabilized, so unprotected outdoor parts degrade. It is flammable and not suitable where flame-retardancy is required without special grades. A practical gotcha is bonding: acetal's chemical inertness, the same property that gives chemical resistance, makes it very difficult to glue or solvent-weld reliably, so acetal parts are normally mechanically fastened, snap-fit, or designed as single machined pieces rather than bonded assemblies; surface treatments exist but adhesion is never as easy as with many other plastics. It is also not biocompatible at the level of implant-grade PEEK, though FDA food-contact grades exist. For buyers, the guidance is to choose acetal for the broad middle ground of precision mechanical plastic parts, but to step up to PEEK or a specialty polymer when high temperature, aggressive chemicals, biocompatibility, UV exposure or reliable bonding are required.

Last updated: July 2026

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