Delrin for Machined Parts: Acetal Homopolymer vs. Copolymer vs. Nylon

Delrin shows up on more machined plastic drawings than almost any other material. It’s stiff, it holds dimensions, it slides against metal without lubrication, and it cuts cleanly. Engineers also use “Delrin” and “acetal” as if they were the same thing, and that shortcut can cost you on the wrong part. Delrin is one specific type of acetal, and the other type behaves differently in heat, chemicals, and thick sections.

This guide covers what Delrin actually is, how it compares to acetal copolymer and nylon, and how to spec it so your CNC machined or injection molded parts come back right.

What Delrin is

Acetal, also called polyoxymethylene or POM, is a semi-crystalline engineering thermoplastic. It comes in two forms:

• Acetal homopolymer (POM-H). Delrin® is the best-known trade name for this form.
• Acetal copolymer (POM-C). Sold under several trade names and often just called “acetal” on stock lists.

Both share the properties that make acetal popular: low friction, good wear resistance, low moisture absorption, and excellent machinability. The differences come from chemistry. Homopolymer has a more uniform molecular chain, which makes it a little stronger and stiffer. Copolymer adds other monomer units along the chain, which trades a little strength for better thermal and chemical stability.

Delrin vs. acetal copolymer

Values are typical published figures for unfilled stock shapes. Check the supplier’s data sheet for design values.

The porosity issue most drawings miss

Thick Delrin rod and plate can have a low-density zone in the center, often called centerline porosity. If your part is machined from the middle of a large bar, a sealing face or a thin wall can land right in that zone. For thick sections, sealing surfaces, or parts that will be inspected for voids, acetal copolymer or a porosity-free homopolymer grade is the safer call. For small-diameter, thin-walled bushings, Delrin’s extra stiffness is usually worth it.

Delrin vs. nylon

Nylon is the other common choice for bushings, gears, and wear parts. The two get swapped often, and they shouldn’t be.

• Moisture. Nylon absorbs significant water, and its dimensions and stiffness change as it does. Delrin absorbs very little. For parts that need to hold a tolerance in humid or wet environments, Delrin wins.
• Toughness. Nylon generally has better impact resistance and is more forgiving under shock loads.
• Friction and wear. Both are good. Delrin is stiffer and has a slicker surface against steel. Nylon handles abrasive conditions well, especially in filled grades.
• Temperature. Some nylon grades hold up at higher temperatures than acetal. Check the heat deflection temperature for your grade.

A simple rule: if dimensional stability matters most, start with Delrin. If impact and toughness matter most, start with nylon. Our plastic injection molding materials guide compares acetal and nylon with ABS, polycarbonate, and polypropylene.

Where Delrin works best

• Gears, cams, and rollers that run against metal without grease.
• Bushings and bearings, especially small, thin-walled ones.
• Fixtures and nests that hold finished parts without scratching them.
• Precision components for robotics and automation equipment.
• Instrument and device parts in medical products. Specific Delrin grades carry FDA food-contact compliance, but medical use depends on the grade and your device’s requirements, so confirm before you specify.

Machining Delrin: what to know before you quote

Acetal is one of the easiest plastics to machine. It cuts cleanly, chips well, and holds tight tolerances compared to most plastics. A few things still affect cost and quality:

• Tolerances. Plastics move more with temperature than metals do. Holding metal-style tolerances on a large Delrin part is possible, but it adds time and inspection cost. Call out tight tolerances only where the function needs them.
• Stress relief. Stock that’s been properly annealed by the extruder stays flat and stable during machining. Heavy material removal on one side of a part can still cause warping, so shops often rough, rest, and then finish.
• Thin walls and long features. Delrin is stiff for a plastic but much less stiff than aluminum. Thin walls can deflect under cutting forces.
• Sharp inside corners. Same rule as metal: add an inside radius that a standard end mill can cut. Our post on drawing callouts that raise your quote covers this and other cost drivers.

Molding vs. machining Delrin

For prototypes and low volumes, machining from stock is usually faster and cheaper because there’s no tool to build. Once volumes climb into the thousands, injection molding usually wins on part price. Acetal molds well, but it shrinks more than many resins, so the mold has to be designed for acetal from the start. Switching resin after the tool is cut is expensive. If you’re planning a path from machined prototypes to molded production, our engineering team can review the design for both processes up front.

How to call out Delrin on a drawing

• Name the form: “acetal homopolymer (Delrin 150 or equivalent)” or “acetal copolymer.” Writing just “acetal” leaves the choice to the shop.
• Say whether “or equivalent” is acceptable. Some programs need a specific brand and grade.
• Specify color. Natural and black are the most common.
• Call out any filled grade by name, such as PTFE-filled or glass-filled.
• Note compliance needs, such as FDA food contact, on the drawing so the shop buys certified stock.

How PE sources Delrin parts

PE’s network machines and molds acetal parts every week for industrial, medical, and aerospace programs. We review the drawing for material callouts, thick-section porosity risk, and tolerances that will drive cost, then match the job to a shop set up for plastics. One project manager owns the order from quote to delivery.