Delrin vs. Acetal Copolymer: Which Grade Should You Specify?
POM-H vs. POM-C — the difference engineers most often overlook. Centerline porosity, chemical stability, and the correct drawing callout.
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.
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
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 (73°F) 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 |
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.
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
We Review Acetal Grade Specs on Every RFQ
MakerStage reviews material-grade callouts during quote review for acetal RFQs. If your drawing specifies POM-H for a thick-section part where centerline porosity is a risk, we can flag that before production starts.
Get a CNC Acetal QuoteChemical 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 / 176°F) | 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 |
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 D6100, 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
- "…per ASTM D6778" for finished machined parts — D6778 classifies raw molding-grade resin, not extruded or compression-molded shapes; use ASTM D6100 for finished-part callouts
- 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.
Further Reading
- What Is Acetal (POM/Delrin)? Complete Engineer's Guide — hub guide covering all acetal grades, properties, and applications.
- Acetal Filled Grades: PTFE, Glass, Carbon Compared — specialty grades for gears, bearings, and high-wear applications.
- CNC Machining Acetal (POM/Delrin): Speeds, Feeds, and Design Rules — full machining reference with DFM checklist.
Frequently Asked Questions
Ready to Quote Your Acetal Part?
CNC machining in Delrin (POM-H), acetal copolymer (POM-C), and filled grades. Upload your drawing so the quote review can confirm the right acetal grade before production.
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