Guide
Delrin vs acetal: can a drawing that calls out Delrin accept acetal copolymer (POM-C)?
Delrin vs acetal is the comparison between Delrin, the best-known trade name for acetal homopolymer (POM-H), and the wider acetal family, which on most stock shelves means acetal copolymer (POM-C). Both are polyoxymethylene: the homopolymer is stronger, stiffer and more creep resistant, while the copolymer lasts longer in hot water and strong alkalis and has far less centerline porosity in thick rod and plate. A drawing that says “Delrin” can accept POM-C only when the part does not depend on the homopolymer’s extra strength, stiffness or short-term heat resistance, and the drawing owner agrees in writing.
By Saola Manufacturing engineering team · Published · Updated
Most pages on this topic stop at “Delrin is a brand of acetal.” That is true, but it does not answer the question a design engineer or buyer actually faces: the print says Delrin, the quote offers acetal copolymer, so is that a conforming part or a deviation? This guide answers it from the regulations, the material standards and published property data, then gives a decision rule you can write on the drawing and the RFQ, plus the machining notes that matter for tolerance.
On this page
Is Delrin the same as acetal?
Delrin is an acetal, but acetal is not always Delrin. Delrin is a trade name for acetal homopolymer (POM-H), while “acetal” on a quote or a stock label most often means acetal copolymer (POM-C), a related but chemically different polymer.
Acetal, polyacetal, polyoxymethylene and POM are generic names for the same polymer family, built from repeating oxymethylene units. US food-contact law describes the two types precisely. The homopolymer is polymerized formaldehyde (21 CFR 177.2480). The copolymer is the reaction product of trioxane with ethylene oxide or with a small amount of butanediol formal (21 CFR 177.2470). Those comonomer units interrupt the chain, which is what gives the copolymer its better thermal and chemical stability and costs it some crystallinity, strength and stiffness.
The standards treat them as one material family. ASTM D6100, the US specification for extruded, compression molded and injection molded acetal stock shapes such as rod, plate and tubular bar, is written for polyoxymethylene shapes in general rather than for one brand, and its requirements include tensile properties, flexural modulus, Izod impact and porosity level. The ISO designation system for POM resins, ISO 29988-1, likewise covers homopolymers, copolymers and blends. So on paper, “acetal” alone is ambiguous: it is satisfied by either type unless the drawing narrows it.
How do Delrin (POM-H) and acetal copolymer (POM-C) properties compare?
Homopolymer is the stronger and stiffer of the two, with the higher melting point and heat deflection temperature. Copolymer gives up some strength in exchange for longer life in hot water and alkalis, a slightly higher continuous-use temperature in air in at least one maker’s data, and much lower centerline porosity in thick stock shapes.
One stock-shape maker’s comparison gives the homopolymer roughly a 15% edge in hardness and stiffness and in both tensile and compressive strength, plus about 10% more load capacity before creep sets in (published POM-C vs POM-H data). The same property data rate the homopolymer’s resistance to hydrolysis only up to about 60 °C, against about 85 °C for the copolymer, and recommend the homopolymer only from pH 4 to pH 9. The same stock-shape data list a continuous operating temperature in air of about 90 °C for the homopolymer and about 100 °C for the copolymer, so the homopolymer’s heat advantage is a short-term one. A 2002 materials overview in Machine Design points the same way: it rates the homopolymer’s fatigue endurance as the best among unfilled commercial thermoplastics, advises against long contact with strong acids or bases beyond the pH 4 to 9 window, and reports that the copolymer stands up to strong bases. Strong acids attack both types.
| Property | Delrin type, POM-H | Acetal copolymer, POM-C |
|---|---|---|
| Tensile strength (ISO 527), MPa | 78 | 66 |
| Flexural strength (ISO 178), MPa | 106 | 91 |
| Flexural modulus (ISO 178), MPa | 3,450 | 2,660 |
| Melting point (DSC, ISO 11357), °C | 180 (CFR range 172–184) | 165 |
| Heat deflection temperature (ISO 75), °C | 110 | 100 |
| Hydrolysis resistance, hot water up to, °C | 60 | 85 |
| Recommended pH range | 4–9 | 4–13 |
| Thermal expansion, 23–60 °C, µm/(m·K) | 95 | 110 |
| Centerline porosity in thick rod and plate | Higher, most visible in thick plate and large rod | Significantly lower |
| Continuous operating temperature in air, °C | 90 | 100 |
Two cautions on reading the table. First, these are typical values for unfilled extruded shapes from one source; filled grades (PTFE-lubricated, glass-reinforced) and other makers’ data differ, so design against the datasheet of the grade you will actually buy. Second, the gaps are real but moderate. For most bushings, spacers, fixtures, insulators and wear pads at room temperature, either type is far stronger than the application needs, and the deciding factors are the environment and the stock size, not the tensile column.
What is centerline porosity, and why does it matter for thick Delrin rod?
Centerline porosity is a core of small voids that forms along the center of extruded acetal rod and thick plate, and it is much more pronounced in homopolymer than in copolymer. It matters whenever a machined feature cuts into that core, such as a center bore, a sealing face or a thin wall made from the middle of the bar.
The voids come from how thick sections solidify. The outside of an extruded rod freezes first and becomes rigid, while the core is the last to crystallize and shrink, and when the shell can no longer move inward the shrinkage opens up as voids along the axis. The effect grows with diameter and thickness. The same maker describes copolymer shapes as having far less of this core porosity and ties that to lower outgassing and a wider chemical-resistance window (POM-C vs POM-H data).
On a finished part the porosity shows up in predictable ways. A bore through the center of a large rod can expose voids that leak gas or fluid, trap cleaning agents or food soil, or simply look like a defect at incoming inspection. A thin-walled bushing turned from the core of a large bar carries the voids in its wall. Parts cut from the outer zone of the same bar are unaffected. That is why porosity level is among the requirements for POM shapes in ASTM D6100, so a callout can ask for it. If porosity matters, say so: specify POM-C or a stated low porosity level on the stock certificate, and state which features must be free of visible voids.
When should “Delrin” on a drawing be held, and when can it be opened to “acetal or equal”?
Hold the homopolymer when the part is designed close to its strength, stiffness, fatigue or creep limits, sees short heat peaks near its deflection temperature, or is already qualified. Open the callout to acetal copolymer when the part sees hot water, steam, alkaline cleaners or porosity-sensitive features, or when it is a lightly loaded room-temperature part where either type is far beyond the requirement.
The table turns that into a rule a reviewer can apply line by line. Work down it and stop at the first row that matches the part; a qualified or regulated part is the first check because it overrides all the engineering ones.
| If the part… | Callout decision | Why |
|---|---|---|
| Is already qualified (medical, aerospace, a validated assembly, a customer-approved PPAP) | Hold the exact grade; any change goes through the drawing owner | A material change voids the qualification evidence, whatever the datasheets say |
| Touches food or drinking water | Either type, but the grade must comply with the matching 21 CFR section and the certificate must say so | Homopolymer and copolymer are listed in separate CFR sections |
| Is a gear, cam, snap-fit or spring arm sized near its stress limit | Hold POM-H | Higher strength, stiffness and fatigue endurance |
| Carries sustained load (clamp, preloaded seat, press fit) | Hold POM-H, or recheck creep with POM-C data | Homopolymer carries more load before creep |
| Sees short heat peaks under load, near the heat deflection temperature | Hold POM-H | Higher melting point and heat deflection temperature |
| Runs hot in air for long periods | Check the continuous-use rating of both types | One maker rates POM-C slightly higher for continuous service |
| Sees hot water, steam, dishwashers or alkaline cleaners | Specify POM-C | Homopolymer is rated for hot water only to about 60 °C and for pH 4 to 9 |
| Has a center bore, sealing face or thin wall cut from thick stock | Specify POM-C, or require a low porosity level on the stock certificate | Centerline porosity is concentrated in thick homopolymer |
| Is a lightly loaded spacer, bushing, insulator, fixture or wear pad at room temperature | Open to “acetal, POM-H or POM-C” | Either type exceeds the requirement; wider choice of stock |
Avoid a bare “Delrin or equal”. It leaves the supplier to decide what “equal” means, and a reasonable supplier and a reasonable inspector can disagree. Write the type and the reason instead. Three examples that leave no room for argument:
MATERIAL: ACETAL HOMOPOLYMER (POM-H), DELRIN OR EQUAL HOMOPOLYMER PER ASTM D6100. NO COPOLYMER SUBSTITUTION.MATERIAL: ACETAL COPOLYMER (POM-C) PER ASTM D6100, NATURAL. LOW CENTERLINE POROSITY REQUIRED AT BORE.MATERIAL: ACETAL (POM-H OR POM-C) PER ASTM D6100, BLACK. SUPPLIER TO STATE TYPE ON CERTIFICATE.
If the drawing already says “Delrin” and you cannot change it, the copolymer is a deviation, not an equivalent. Approve it the same way you would approve any material substitution: in writing, against a part number and revision, with the reason recorded.
Is acetal copolymer cheaper than Delrin?
Copolymer stock is often priced below branded homopolymer and is widely stocked in large sizes, but the difference varies with size, color and market. On most machined parts the stock is only one part of the price, so the material choice should follow function first.
The price of a machined acetal part is driven by machine time, setups, inspection and scrap as well as by stock. Machine time is close for the two types, since both cut cleanly and form chips well. Where the choice does move cost is on thick parts: if a center bore in a large homopolymer bar opens up voids at inspection, the scrap and re-sourcing cost far more than any saving on raw material. Ask for a quote on both types when the drawing allows either, rather than assuming which will be lower.
How do you hold tolerance when machining Delrin or acetal copolymer?
Acetal machines well, but extruded stock carries internal stress that is released as material is removed, and it expands with temperature several times faster than metals. Close tolerances therefore need stress relief between roughing and finishing and measurement at a controlled temperature.
Stress first. Extruded rod and plate are cooled from the outside in, so the skin and the core end up with different residual stresses. Removing a large share of the cross section on one side lets the part bow or ovalize over the following hours. The standard remedy is to rough machine, leave a finishing allowance, let the part relax or anneal it per the stock maker’s annealing guidance, and then finish machine. Buying stock that the mill has already stress relieved helps, and so does machining both sides of a plate symmetrically.
Temperature second. At the typical acetal expansion rate of about 100 µm per meter per kelvin (published POM data), a 100 mm feature changes size by about 0.1 mm over a 10 °C swing. A part measured warm off the machine and again in an air-conditioned inspection room can disagree by more than its tolerance band. Dimensional measurements are referenced to 20 °C per ISO 1, so state on the drawing that plastic dimensions apply at that reference temperature after the part has stabilized.
Moisture is a smaller issue than with nylon, because acetal absorbs little water, which is one reason acetal is chosen for precise plastic parts. Two more habits help: specify tolerances that reflect the material rather than copying metal-part tolerances, and keep thin walls and long unsupported features away from the core of thick homopolymer stock. Why tight tolerances cost more on any material is covered in the tolerance cost guide.
What should an RFQ for an acetal part state?
State the type (POM-H, POM-C or either), any named grade, the stock form, the color, and whether the part is food-contact, medical or already qualified. Also say which features must be free of porosity and what certificate you need.
Those few facts settle most of the questions a supplier would otherwise have to ask. When we review a drawing that says only “Delrin” or only “acetal”, we ask which type is required and why, and we flag center bores and sealing faces cut from thick stock before quoting, because those are where the two types behave differently. Acetal parts are produced by the same CNC turning and CNC milling processes described on those pages; if your part is plastic, name the material type on the RFQ so the quote addresses stock form and stress relief directly. Ask for a certificate of conformance that names the type and grade, and a stock certificate where traceability matters; the certificate of conformance guide lists what that document should contain. Drawings can be sent through the quote form.
Frequently asked
What is another name for Delrin?
Acetal homopolymer, POM-H, polyoxymethylene homopolymer or polyacetal. The generic names “acetal” and “POM” also cover the copolymer, so they are broader than Delrin; only “homopolymer” or “POM-H” narrows the callout to the same polymer type.
Is POM material the same as acetal plastic?
Yes. POM (polyoxymethylene), acetal and polyacetal are the same family. Each name includes both the homopolymer and the copolymer, which is why a drawing or a quote that says only “POM” or “acetal” does not tell you which type you will get.
What are the disadvantages of Delrin?
Centerline porosity in thick rod and plate, poor resistance to hot water and to strong bases and acids, and difficult bonding, since acetal’s low surface energy makes adhesives hold poorly without surface treatment. Like all unmodified acetals it is flammable, and long outdoor exposure calls for a UV-stabilized grade.
Which plastic is stronger than Delrin?
Glass-reinforced grades of acetal and nylon are stiffer and stronger than unfilled Delrin, and high-performance plastics such as PEEK and polyamide-imide are stronger still and keep their strength at much higher temperatures. Each trades away something acetal does well, such as low moisture uptake, low friction or machining cost.
Can you tell Delrin from acetal copolymer after machining?
Not reliably by eye or by feel. A lab can separate them by differential scanning calorimetry per ISO 11357-3, because the homopolymer melts at a higher temperature than the copolymer, typically by more than 10 °C. If the type matters, require it on the certificate rather than relying on testing finished parts.
Sources
- 21 CFR 177.2480, Polyoxymethylene homopolymer, US Food and Drug Administration via eCFR, accessed 2026
- 21 CFR 177.2470, Polyoxymethylene copolymer, US Food and Drug Administration via eCFR, accessed 2026
- ASTM D6100-25, Standard Specification for Extruded, Compression Molded and Injection Molded Polyoxymethylene Shapes (POM), ASTM International, 2025
- ISO 29988-1:2019, Plastics: Polyoxymethylene (POM) molding and extrusion materials, Part 1: Designation system and basis for specifications, ISO, 2019
- POM plastics: copolymer and homopolymer acetals, property data, stock-shape manufacturer product page, accessed 2026
- Acetal, Machine Design, 2002