Guides / Materials

Delrin or Acetal Copolymer: How to Pick the Right POM for a Machined Part

September 18, 2026

Acetal comes in two families. Homopolymer, sold most often as Delrin, and copolymer, sold as Hostaform, Ultraform, Tecaform and others. Both are POM. Both machine cleanly. On a quote they often sit within a few percent of each other. The difference shows up in service, and it shows up in thick sections, so it is worth deciding before you send the drawing out.

What actually differs

Homopolymer has higher crystallinity. That gives it more stiffness, more hardness and better resistance to repeated loading. If a part is a gear tooth that deflects a little on every rotation, or a snap feature that flexes, homopolymer lasts longer.

Copolymer has a slightly more open structure with a more stable backbone. It gives up a little strength and gains resistance to hydrolysis, which is what happens when acetal sits in hot water or alkaline cleaning fluid for a long time. It is also more uniform through a thick section.

Property in practice	Homopolymer (Delrin)	Copolymer (POM-C)
Stiffness and hardness	Higher	Slightly lower
Fatigue under repeated load	Better	Good
Hot water and steam	Degrades faster	Holds up better
Alkaline cleaning	Less tolerant	More tolerant
Thick rod and plate	Centre can be less dense	More even through the section
Common stock forms	Rod, plate, tube	Rod, plate, tube

Centreline porosity, and why shops care

Large diameter acetal rod cools from the outside in. In homopolymer that can leave the core less dense than the skin. Machinists call it centreline porosity. On a part cut from the outer part of the rod you never see it. On a part where the finished bore sits on the rod axis, roughing can open it up and leave a dull, porous band that no finishing pass removes.

This is why a shop machining a thick homopolymer part often roughs it, looks at the core, and only then commits to the finishing cut. It is also why copolymer gets specified for thick plate and large diameter parts even when homopolymer would be stronger.

If your part is under about 40 mm across, it rarely matters. Above that, mention it or let the shop pick.

Environment usually decides

Run through the service conditions before the mechanical numbers:

• Cold water, air, oil, fuel, neutral cleaning: both work. Pick on mechanics.
• Hot water above roughly 60 degrees C, steam, repeated washdown: copolymer.
• Caustic or high pH cleaning: copolymer.
• Strong acid: neither. Acetal is a poor choice in acid at any concentration worth naming.
• Continuous UV outdoors: neither without a stabilised grade, and even then plan on surface chalking.
• Continuous service much above 90 degrees C: look at PEEK, PET or nylon instead.

Then decide on mechanics

If the environment allows both, homopolymer is the default for anything that carries or transmits load. Gears, cams, cam followers, ratchets, drive pins, anything with a tooth or a living hinge. Copolymer is the default for plates, guides, manifolds and wear strips, where the part is thick and the load is spread.

What to write on the drawing

Name the family and the reason, not just the brand. Something like:

Material: acetal homopolymer (Delrin 150 or equivalent). Substitution with copolymer requires approval.

or

Material: acetal copolymer, food contact grade per EU 10/2011. Supply compliance documentation with first shipment.

Writing “Delrin” alone on a drawing gets you whichever acetal the shop has, because most shops read it as a generic word for the material. Writing “acetal” alone does the same. Naming the family and adding a substitution note is what makes the quote comparable across shops.

Filled grades change the calculation

PTFE filled acetal drops friction further and reduces stick slip in dry running bearings. It costs more, it is usually only available as rod and it is slightly weaker than the base grade.

Glass filled acetal is stiffer and moves less with temperature, which helps on parts that have to hold a dimension across a wide temperature range. It is more brittle at thin walls and sharp edges, and it wears tooling faster, so expect a higher price per part on small runs.

Both are worth naming on the RFQ, because not every shop stocks them and lead time on filled rod can add a week.

A short decision path

1. Does the part sit in hot water, steam or alkaline cleaning? If yes, copolymer.
2. Is the part thicker than about 40 mm with a feature on the axis? If yes, lean copolymer, or ask the shop.
3. Does a feature flex or carry repeated load? If yes, homopolymer.
4. Does it need to run dry against steel with the lowest possible friction? Consider PTFE filled.
5. Does it need to hold a dimension across a wide temperature swing? Consider glass filled.
6. Still undecided? Write both on the RFQ and ask the shop what it has in stock. On small runs the grade that is already on the rack often saves more money than the grade difference costs.

Frequently asked questions

Keep reading

• Black vs Natural Delrin: What the Colour Changes and What It Does Not
• Acetal vs PTFE: Seals, Valve Seats and Low-Friction Parts Compared