Delrin & Acetal Turning Services

Karl Gillihan

⚪ DELRIN / ACETAL

Turning Delrin and Acetal: The Plastic That Machines Like Brass

Among engineering plastics, acetal is the one machinists actually enjoy on the lathe. Often called by the brand name Delrin, it cuts cleanly into crisp chips, holds an edge on threads, and finishes bright without the gumming or melting that plagues softer plastics, which is exactly why it is the default choice for high-volume turned plastic parts like gears, bushings, and fittings. The decisions here are mostly about homopolymer versus copolymer and respecting plastic's dimensional quirks.

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Why acetal is the best-machining engineering plastic

Acetal (polyoxymethylene, POM) is a semi-crystalline thermoplastic with high stiffness, good strength, low friction, excellent dimensional stability for a plastic, and good fatigue resistance, the natural choice for precision mechanical parts. On the lathe it behaves about as well as a plastic possibly can: it cuts cleanly with sharp tooling into well-broken chips rather than stringy ribbons, it does not gum or smear the way softer plastics do, and it produces bright, crisp finishes and clean threads. It machines fast with low cutting forces, high spindle speeds, and standard sharp tooling, often the same positive-rake geometry used for aluminum and brass. It can be cut dry for light work, though air blast or coolant helps clear chips and manage heat on heavier or high-speed cuts. Acetal's relatively higher melting point and stiffness compared to soft plastics mean it tolerates machining heat better, so the melting and gumming that ruin parts in softer materials are much less of a concern. This combination, low cost, easy machining, good mechanical properties, is why acetal dominates turned-plastic production. For a gear, bushing, roller, valve component, or insulating fitting, acetal turns quickly and cheaply to good tolerances, and it is frequently the first material a designer should consider for a non-high-temperature precision plastic part.

Homopolymer (Delrin) vs copolymer acetal

There are two families, and the distinction matters. Acetal homopolymer, of which Delrin is the well-known DuPont brand (Delrin 150 being a common grade), has slightly higher strength, stiffness, and hardness, and a higher crystallinity that gives excellent mechanical properties and surface finish. Its one notable drawback is a tendency toward centerline porosity, a small zone of voids that can form along the central axis of extruded rod, which can be an issue for parts machined from the very center of large-diameter stock or for sealing applications. Acetal copolymer (brands like Celcon and Hostaform) has marginally lower strength and stiffness but better chemical resistance, particularly to hot water and strong bases, better resistance to porosity (no significant centerline porosity issue), and better long-term stability in some environments. For parts exposed to hot water, harsh chemicals, or where the centerline-porosity risk of homopolymer is a concern, copolymer is often the safer choice. Both machine essentially the same way and beautifully, so the selection is driven by the application, not by machinability. Homopolymer Delrin for maximum strength, stiffness, and the best finish on solid sections; copolymer for chemical and hot-water resistance and to avoid centerline porosity in parts bored through the center of large rod. Medical and food-contact applications often have specific grade requirements (FDA-compliant or USP-class grades) that further constrain the choice.

Tolerances, thermal movement, and finishing realities

Turned acetal holds good tolerances for a plastic, ±0.002 in is comfortable and ±0.001 in achievable on smaller features, but plastic tolerances simply cannot match metal, and the reason is thermal expansion and moisture and creep behavior. Acetal's coefficient of thermal expansion is high, roughly 47 to 68 µin/in/°F, around ten times that of steel, so a part that measures perfectly in a cool inspection room can be meaningfully different at the temperature of a warm machine or a hot application. Tolerances must be specified and measured at a controlled temperature, and very tight tolerances on large plastic parts are often simply not realistic. Acetal absorbs very little moisture (a major advantage over nylon, which swells noticeably with humidity), so it is more dimensionally stable than most plastics, but it can still creep under sustained load and relax stress over time. Parts machined from large stock may also move slightly as machining relieves internal stress, though acetal is far less prone to this than PEEK. Surface finish is excellent, acetal turns bright and smooth with sharp tooling, and burrs are soft and easily removed though they do form and may need deburring, especially around threads and cross-holes. The practical guidance: machine with sharp tools, keep chips clear, specify realistic plastic-appropriate tolerances at a defined temperature, and remember that the same low cutting forces that make acetal easy to cut also mean slender parts can deflect under tool pressure and need support.

Cost, applications, and when to pick something else

Acetal is inexpensive among engineering plastics and machines fast, so turned acetal parts are economical, often the lowest-cost route to a precision plastic component. Stock is widely available in rod and the material is forgiving, so prototype acetal parts turn quickly and ship fast, and high-volume parts run efficiently on lathes and screw machines. The cost is dominated by machining time and stock, with few of the special process steps (like PEEK's annealing) that drive up high-performance-plastic costs. The applications play to acetal's strengths: gears, bushings, bearings, rollers, cams, valve and pump components, insulating fittings, manifolds, and precision mechanical parts across automotive, consumer products, medical devices, and industrial equipment. Its low friction and wear resistance make it excellent for moving parts, and its dimensional stability suits precision components. Know when to choose differently. Acetal's continuous service temperature is only around 180°F, so for higher temperatures you need PEEK, Ultem, or another high-temp plastic. It has limited resistance to strong acids and oxidizers, so aggressive chemical service may call for PTFE or PVDF. It is flammable and not suitable where flame resistance is required without a special grade. And it cannot be solvent-bonded easily, so assemblies relying on adhesives may favor other plastics. But within its temperature and chemical envelope, for a precision turned mechanical part, acetal is usually the right and most economical answer, which is exactly why it is the workhorse of turned-plastic production.

Frequently Asked Questions

Delrin is a brand name, not a separate material. Delrin is DuPont's trade name for acetal homopolymer (polyoxymethylene, POM), so all Delrin is acetal, but not all acetal is Delrin. The meaningful technical distinction is homopolymer versus copolymer acetal. Homopolymer acetal (Delrin, with Delrin 150 a common grade) has slightly higher strength, stiffness, hardness, and the best surface finish, but it can have centerline porosity, a small zone of voids along the central axis of extruded rod, which matters for parts machined from the very center of large stock or for sealing surfaces. Copolymer acetal (brands like Celcon and Hostaform) has marginally lower strength but better chemical resistance, especially to hot water and strong bases, no significant centerline porosity, and good long-term stability. Both machine essentially identically and very well, so you choose by application: homopolymer Delrin for maximum strength, stiffness, and finish on solid sections; copolymer for hot-water and chemical resistance or to avoid centerline porosity in center-bored parts. When a drawing simply says Delrin, it means acetal homopolymer; when it says acetal, confirm whether homopolymer or copolymer is required.

Because it combines easy, clean machining with genuinely useful mechanical properties at low cost. On the lathe, acetal cuts cleanly with sharp tooling into well-broken chips rather than the stringy ribbons or gummy mess that softer plastics produce, it does not melt or smear the way low-melting plastics do, and it gives bright finishes and crisp, clean threads. It machines fast with low cutting forces and high spindle speeds using standard sharp tooling, often the same geometry used for aluminum and brass, and can be cut dry for light work. Its relatively higher stiffness and melting point mean it tolerates machining heat far better than soft plastics, so the gumming and melting that ruin parts in other materials are much less of a concern. On top of that, acetal has high stiffness, good strength, low friction, excellent dimensional stability for a plastic, low moisture absorption, and good wear and fatigue resistance, making it ideal for precision mechanical parts like gears, bushings, and rollers. This rare combination of being both easy to machine and mechanically useful, at a low material cost, is exactly why acetal is the default workhorse for high-volume turned plastic parts.

Comfortably around ±0.002 in, with ±0.001 in achievable on smaller features, but plastic tolerances genuinely cannot match metal, and the reason is thermal expansion. Acetal's coefficient of thermal expansion is roughly 47 to 68 µin/in/°F, about ten times that of steel, so a part that measures perfectly in a cool inspection room can be meaningfully larger at the temperature of a warm machine or a hot application. Tolerances must be specified and measured at a controlled, stated temperature, and very tight tolerances on large acetal parts are often simply unrealistic because thermal movement swamps them. The good news is acetal absorbs very little moisture, unlike nylon which swells noticeably with humidity, so it is more dimensionally stable than most plastics, but it can still creep under sustained load and relax stress over time, and slender parts can deflect under tool pressure during machining because cutting forces, while low, act on a flexible material. So for precision acetal parts, design to plastic-appropriate tolerances, specify the inspection temperature, support slender features during machining, and avoid demanding metal-level precision on large or thin sections where thermal expansion and deflection make it impractical.

Choose a different plastic when your application exceeds acetal's temperature or chemical envelope, or needs properties it lacks. Acetal's continuous service temperature is only about 180°F, so for higher temperatures step up to PEEK, Ultem (PEI), or another high-temperature plastic. Acetal has limited resistance to strong acids and oxidizers, so aggressive chemical service may require PTFE or PVDF instead. It is flammable and not suitable where flame resistance is mandated unless you use a special flame-retardant grade. It cannot be solvent-bonded easily, so assemblies that rely on adhesive bonding may favor other plastics. And where you need higher toughness and impact resistance with some flexibility, nylon or a tougher polymer may suit better, though nylon swells with moisture. Within its limits, though, acetal is usually the right and most economical choice for a precision turned mechanical part, low friction, good stiffness, dimensional stability, easy machining, and low cost, so the main reasons to switch are heat above 180°F, aggressive chemicals, flammability requirements, or a need for adhesive bonding. Match the material to the actual service conditions rather than defaulting either way.

Last updated: July 2026

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