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Delrin and Acetal Grades for Machined Parts
Homopolymer or copolymer, unfilled or filled, standard or food contact. What each grade does on a machined part and how to name it on the drawing so every quote prices the same material.
Homopolymer vs copolymer: typical properties
Typical values for unfilled grades. Datasheets differ by supplier and test method, so compare like with like and use the supplier sheet for the grade you actually buy.
| Property | Homopolymer (POM-H) | Copolymer (POM-C) |
|---|---|---|
| Density | 1.42 g/cm³ | 1.41 g/cm³ |
| Tensile strength at yield | 60 to 70 MPa | 55 to 65 MPa |
| Tensile modulus | 2,800 to 3,200 MPa | 2,600 to 2,900 MPa |
| Melting point | About 175°C | About 165°C |
| Water absorption, 24 h | About 0.2 percent | About 0.2 percent |
| Thermal expansion | About 110 µm/m·K | About 110 µm/m·K |
| Hot water and alkalis | Limited; avoid long exposure | Better; common choice for washdown |
| Centreline porosity in thick rod | Possible | Rare |
Delrin vs acetal copolymer, in full · Delrin material properties guide
Grades buyers specify most
Delrin is a trademark of its owner and names homopolymer grades only. Copolymer comes from several producers under their own names. Stock shapes are extruded or cast by converters from these resins.
| Grade | Type | What it changes on a machined part |
|---|---|---|
| Delrin 150 | Homopolymer, unfilled | High-viscosity grade that much natural and black homopolymer rod and plate is extruded from. The usual meaning of "Delrin" on a drawing. |
| Delrin 100 series | Homopolymer, unfilled | High-viscosity, tougher homopolymer grades. More common in moulding; check stock availability before specifying for machining. |
| Delrin 570 | Homopolymer, 20 percent glass | Stiffer, lower creep and lower thermal expansion. Harder on tools and more brittle at edges. |
| Delrin AF | Homopolymer, PTFE fibre | Lower friction and less stick-slip for dry-running bearings and slides. Costs more and is usually bought as rod. |
| Acetal copolymer (POM-C) | Copolymer, unfilled | Sold as Hostaform, Ultraform, Duracon, Tecaform AH, Acetron GP and others. More uniform through thick sections. |
| Glass filled copolymer (GF25) | Copolymer, 25 percent glass | The copolymer counterpart to Delrin 570. Stiffness for structural brackets and housings. |
| Food contact POM | Usually copolymer | Supplied with FDA 21 CFR 177.2470 or EU 10/2011 documentation. Blue metal-detectable versions exist for food lines. |
| ESD and conductive POM | Carbon or additive filled | Controlled surface resistivity for electronics handling. Usually black, with lower impact strength. |
Choosing a grade
If the drawing already names a grade, keep it. These notes are for buyers still deciding or checking whether an alternative is worth asking about.
- Load and fatigueGears, snap features and cams under repeated load favour homopolymer for its higher stiffness and fatigue strength.
- Hot water and cleaningParts washed with hot water, steam or alkaline cleaners favour copolymer. Chemical and temperature resistance
- Thick sectionsRod over about 75 mm and thick plate favour copolymer, which is free of the centreline porosity homopolymer rod can show.
- Dry slidingBearings and slides that run without lubricant favour PTFE-filled acetal such as Delrin AF.
- Stiffness and size stabilityBrackets and housings that must not creep or grow favour 20 to 25 percent glass-filled grades.
- Food contactAsk for the compliance statement with the quote. Food grade and detectable acetal
When acetal is the wrong answer
Acetal covers most precision plastic parts at room temperature. A few conditions point elsewhere.
- Above about 100°CContinuous service hotter than that points to PEEK, PPS or PEI. Acetal vs PEEK
- Strong acidsAcetal is attacked by strong acids and oxidisers. PVDF or PTFE suit those media.
- Abrasive wearBulk material chutes and liners wear acetal faster than UHMW polyethylene. Acetal vs UHMW
- Impact at low temperatureNylon and UHMW keep more toughness when cold. Acetal vs nylon
How to write the grade on a drawing
A clear material note stops two shops quoting two different materials for the same part.
- Name the type"Acetal homopolymer" or "acetal copolymer", plus a grade or trade name if one matters.
- Allow or forbid substitution"Delrin 150 or equivalent" invites copolymer quotes. "No substitution" stops them.
- ColourNatural (white), black or blue. Colour changes stock availability and sometimes price.
- DocumentsMaterial certificate, food contact statement or RoHS / REACH declaration, if you need them. What to put on a Delrin drawing
Background: polyoxymethylene on Wikipedia. ASTM D6778 and ISO 9988 classify POM resins.
Questions about acetal grades
What is the difference between Delrin 150 and Delrin 570?
Delrin 150 is unfilled acetal homopolymer, the grade much homopolymer rod and plate is extruded from. Delrin 570 is homopolymer with 20 percent glass fibre, which raises stiffness and lowers thermal expansion but wears tools faster and makes edges more brittle.
Can a shop substitute acetal copolymer for Delrin?
Only if the drawing allows it. Copolymer machines the same way and is often cheaper and easier to source in large sizes, but it is slightly less stiff and strong. If the drawing says "Delrin 150 or equivalent", ask the shop to name the exact equivalent on the quote.
Which acetal grade is best for gears?
Unfilled homopolymer in most cases, for its higher fatigue strength and stiffness. PTFE-filled grades help where the gear runs dry against another plastic. Glass-filled grades are less common for gears because the glass wears the mating gear.
What standard covers POM material?
ASTM D6778 classifies POM moulding and extrusion materials and ISO 9988 does the same internationally. Stock shapes are covered by ASTM D6100. Most drawings name a grade or trade name instead and add "or equivalent".