Delrin vs. Acetal Copolymer: Homopolymer Guide (2026)

The Grade Decision Depends on Stock Thickness, Not Brand Name.

Most engineers pick Delrin by name because that's what they've always used — without considering whether POM-H or POM-C is actually correct for the application. The single most important decision criterion is this: if your part is machined from rod or plate stock thicker than 25 mm (1 in), specify POM-C copolymer to avoid centerline porosity in your machined part. For thin stock, POM-H (Delrin) has a slight mechanical advantage.

Section 1 of 5

Molecular Structure: Why the Chemistry Matters

The performance differences between POM-H and POM-C trace directly to their polymer chain structures. Understanding the chemistry helps you predict behavior — not just look up a data sheet value.

POM-H — Homopolymer (Delrin)

A pure repeating chain of –(CH₂–O)– oxymethylene units. The high regularity of the chain enables a highly crystalline structure (~75–85% crystallinity) which gives POM-H its higher tensile strength and stiffness.

High chain regularity → high crystallinity → higher strength
End groups (hemi-acetals) can degrade thermally → lower thermal stability than POM-C
Solidification from outside inward creates risk of centerline void in thick stock
Higher tensile and flexural modulus than POM-C

POM-C — Copolymer (Celcon, Ultraform)

Incorporates a small amount of comonomer (typically 1,3-dioxolane) randomly along the backbone chain. These comonomer units act as chain stoppers that prevent the end-group depolymerization mechanism that degrades POM-H at elevated temperatures.

Comonomer units interrupt end-group thermal degradation
Slightly lower crystallinity than POM-H → marginally lower strength
Solidifies more uniformly → no centerline porosity in thick sections
Better resistance to thermal oxidative degradation over long service

Section 2 of 5

Full Property Comparison

Use this table to confirm which grade wins on the properties that matter for your part — strength, porosity risk, chemical resistance, or stock availability. All values for unfilled grades at 23°C unless noted.

Property	POM-H (Delrin 150)	POM-C (Celcon M90)	Winner
Tensile Strength (UTS)	69 MPa (10 ksi)	62 MPa (9 ksi)	POM-H +11%
Tensile Modulus	3,100 MPa (449 ksi)	2,690 MPa (390 ksi)	POM-H +15%
Flexural Modulus	2,900 MPa (421 ksi)	2,550 MPa (370 ksi)	POM-H +14%
Elongation at Break	40–75%	60–75%	POM-C (more ductile)
Notched Izod Impact	75–130 J/m (1.4–2.4 ft·lb/in)	65–100 J/m (1.2–1.9 ft·lb/in)	POM-H marginally better
Hardness (Rockwell M)	M80	M76–M80	Essentially equal
Compressive Strength	127 MPa (18.4 ksi)	110 MPa (16.0 ksi)	POM-H
Coefficient of Friction (dry)	0.20–0.35	0.25–0.35	Essentially equal
Moisture Absorption (equil.)	0.9%	0.8%	Essentially equal
Continuous Service Temp	90–100°C (194–212°F)	90–100°C (194–212°F)	Equal (standard grades)
Melting Point	175°C (347°F)	165°C (329°F)	POM-H higher crystalline melt
Thermal Stability	Good	Better	POM-C — comonomer blocks depolymerization
Chemical Resistance	Good	Better	POM-C — comonomer improves alkali resistance
Centerline porosity (thick stock)	Yes — >25 mm (1 in.) diameter	None	POM-C — critical for thick-section parts
FDA food contact (natural grade)	Yes (verify grade)	Yes (verify grade)	Equal — both available in FDA-compliant grades
Stock availability	Excellent	Good	POM-H (Delrin) more common in US distributor stock

Section 3 of 5

Centerline Porosity in POM-H: The Hidden Risk

If your part is machined from POM-H rod stock thicker than 25 mm (1 in.), this hidden porosity risk can put voids right where your bore or sealing surface goes. The single most important structural difference between POM-H and POM-C is one that never shows up in a room-temperature tensile test — but can destroy a precision machined part.

What Centerline Porosity Is

When extruded rod stock solidifies during manufacturing, POM-H crystallizes from the outside surface inward. As the outer material solidifies and contracts, the still-molten core must supply material to fill the volume. In large-diameter rod (typically above 25 mm / 1 in.), the core cools last and can form a continuous void along the centerline — essentially a pipe or seam running the length of the rod.

Void appears along rod centerline — not visible on outside surface

Typically 1–5 mm diameter in 50 mm rod; larger in bigger stock

Surface of part looks sound — void only revealed when bore breaks through it

Can cause part failure: bore wall collapse, O-ring groove leak, bearing pocket failure

Worked Example: When Does the Bore Hit the Void?

You order Ø50 mm (2 in.) Delrin rod and machine a Ø25 mm (1 in.) through-bore on center. The porosity zone in 50 mm POM-H rod typically spans a 2–5 mm (0.08–0.20 in.) diameter region along the centerline.

Your bore wall starts at 12.5 mm from center — well outside the porosity zone. Safe in this case.

Now machine a Ø40 mm (1.575 in.) bore in the same rod. The bore wall is now 5 mm from center — right at the edge of the porosity zone. High risk of intersecting voids. Specify POM-C instead, or inspect the rod stock with ultrasonic testing before machining.

How to Avoid It

Parts machined from stock > 25 mm (1 in.) diameter

Specify POM-C (copolymer) — no centerline porosity by design

Parts requiring central bore or through-hole

Inspect POM-H stock via ultrasonic or machine sacrificial test cut before full production

O-ring grooves, sealing surfaces at part center

Specify POM-C to guarantee material integrity at the critical sealing surface

Thin parts (< 25 mm / 1 in. all dimensions)

POM-H (Delrin) safe to use — porosity issue only occurs in thick stock

We Review Acetal Grade Specs on Every RFQ

MakerStage's free DFM review includes material grade review for every acetal order. If your drawing specifies POM-H for a thick-section part where centerline porosity is a risk, we'll flag it and recommend the appropriate grade before your parts are cut.

Get a CNC Acetal Quote with Free DFM Review

Section 4 of 5

Chemical Resistance: Where POM-C Has an Edge

If your part sees alkaline wash-down, hot water above 80°C (176°F), or long-term elevated-temperature service, POM-C's chemical stability is the deciding factor in your grade selection. For most applications, both grades handle the same chemical environments. The gap opens in alkaline conditions and elevated-temperature service.

Chemical	POM-H (Delrin)	POM-C	Recommendation
Fuels, oils, greases	Excellent	Excellent	Both suitable
Water / deionized water	Excellent	Excellent	Both suitable
Mild acids (pH 4–7)	Good	Good	Both suitable; test specific fluid
Dilute NaOH / KOH (< 5%)	Fair	Good	POM-C preferred in alkaline wash-down environments
Concentrated alkalis (> 5%)	Poor	Fair	Neither ideal; consider PTFE or PVDF for strong alkali contact
Concentrated mineral acids	Poor	Poor	Both degrade rapidly — use PTFE or PEEK
Alcohols (IPA, methanol)	Good	Good	Both suitable at room temperature
Aromatic solvents (toluene)	Fair	Fair	Limited contact acceptable; not for immersion
Hot water / steam (> 80°C)	Fair (depolymerization risk)	Better	POM-C significantly better for hot water service
Thermal oxidative stability (long-term)	Good	Better	POM-C comonomer prevents end-group unzipping degradation

Section 5 of 5

How to Specify Acetal on Engineering Drawings

Correct material callouts give your machine shop clarity and prevent costly grade substitution requests mid-production. Use generic ASTM standard callouts — not trade names only.

Recommended Drawing Callouts

"Acetal Homopolymer (POM-H) per ASTM D6778, natural color"
"Acetal Copolymer (POM-C) per ASTM D6100, natural color"
"Acetal Copolymer (POM-C), FDA-compliant, natural color" for food contact
"Delrin 150 or approved equivalent" — if brand is required
Add color callout explicitly — natural, black, or other

Callouts to Avoid

"Acetal" alone — too generic; does not specify POM-H vs POM-C
"Delrin" alone — implies POM-H, but allows brand ambiguity
"Plastic" — insufficient; provides no grade information
"POM" without H or C designation — grade not specified
Specifying POM-H for thick-section parts without checking for porosity risk

Quick Selection Rule

Under 25 mm (1 in.) all dimensions, benign environment: POM-H (Delrin 150 equivalent) — higher strength and modulus, more available stock.
Over 25 mm (1 in.) any dimension, or elevated temperature, or alkaline environment: POM-C (Celcon M90 equivalent) — no porosity, better thermal stability.

Frequently Asked Questions

What is the difference between Delrin and acetal copolymer?

Delrin is a trade name for acetal homopolymer (POM-H) made by Celanese. Acetal copolymer (POM-C) uses a different backbone structure with small amounts of comonomer (typically dioxolane) that eliminates the end-group instability of pure POM-H. POM-H has higher tensile strength and modulus; POM-C has better thermal stability, better chemical resistance, and avoids centerline porosity in thick sections.

Why does Delrin (POM-H) have centerline porosity?

POM-H solidifies from the outside in during cooling of thick rod or plate stock. As the core cools last, shrinkage voids form along the centerline of the stock — particularly in diameters above 25 mm (1 in). POM-C copolymer has a different solidification behavior that eliminates this porosity, making it the correct choice for parts machined from thick stock where the center material will be exposed.

Which acetal grade machines better, POM-H or POM-C?

Both grades machine equally well. There is no practical machinability difference between Delrin homopolymer and acetal copolymer in standard CNC turning and milling operations. Both produce short, clean chips at equivalent cutting speeds, hold the same tolerances, and respond identically to sharp carbide tooling.

Is acetal copolymer food safe?

Natural (white/ivory) acetal copolymer is available in FDA-compliant grades per 21 CFR 177.2470 for food contact. As with homopolymer, the natural color grade is generally FDA-compliant; black and other colored grades use pigments that may not be. Always specify "natural color, FDA-compliant grade" explicitly and confirm with the material supplier for your specific application.

When should I specify POM-C over Delrin (POM-H)?

Specify POM-C (acetal copolymer) when: (1) your part is machined from rod or plate stock with a diameter or thickness over 25 mm (1 in) — to avoid centerline porosity in the machined part, (2) the service environment involves elevated temperatures above 80°C or aggressive chemical exposure, or (3) the application requires improved thermal oxidative stability over the long term. For small parts from thin stock in benign environments, POM-H (Delrin) has a slight strength and stiffness advantage.

How do I specify the correct acetal grade on a drawing?

Use specific material callouts: "Acetal Copolymer (POM-C) per ASTM D6100, natural color" or "Acetal Homopolymer (POM-H) per ASTM D6778, natural color." Avoid specifying a trade name only (e.g., "Delrin") unless the brand is contractually required — generic callouts using POM-H or POM-C give your machine shop flexibility to source equivalent material from multiple suppliers without requiring a material substitution request.