Quick answer for engineers and procurement teams: Delrin is DuPont’s registered brand name for acetal homopolymer (POM-H). When a drawing specifies “acetal” without qualification, suppliers may supply either homopolymer or copolymer (POM-C) — and the two behave differently enough to cause field failures. For tight-tolerance CNC gears, high-cycle fatigue applications, and parts requiring the best surface finish out of the machine — choose Delrin homopolymer. For applications involving hot water, acidic environments, or large-diameter turned parts where centerline porosity would be a problem — choose acetal copolymer.
This guide is written for engineers who need to specify the right material on the PO, not just understand the chemistry. We draw on machining data from over 500 industrial acetal parts produced at our facility, across both grades and three manufacturing processes.
Material Basics: Homopolymer vs Copolymer — What Changes on the Shop Floor
Both grades belong to the polyoxymethylene (POM) family, but their molecular structure differs at a level that affects every downstream process:
- Delrin (POM-H): Uniform repeating -CH2O- units. Higher crystallinity (~75-85%) means harder, stiffer, better fatigue resistance — but also higher residual stress after machining.
- Acetal Copolymer (POM-C): Contains occasional -CH2CH2O- co-monomer units that interrupt crystallinity. This reduces stiffness slightly but dramatically improves chemical resistance and dimensional stability after machining.
Head-to-Head Property Comparison
| Propriété | Delrin (POM-H) | Acetal Copolymer (POM-C) | Pourquoi est-ce important ? |
|---|---|---|---|
| Résistance à la traction (MPa) | 70-83 | 60-69 | Delrin handles ~20% higher static load before yield |
| Module de flexion (GPa) | 2.8-3.1 | 2.4-2.8 | Stiffer feel — critical for snap-fit retention force |
| Fatigue Endurance @10&sup6; cycles (MPa) | 35 | 28 | Key differentiator for gears and springs |
| Izod Impact, Notched (J/m @23°C) | 69-122 | 53-163 | Copolymer has wider range; grade selection matters |
| Water Absorption, 24h (%) | 0.25 | 0.22 | Similar at room temp; diverges above 60°C |
| Continuous Service Temp, Max (°C) | 90-100 | 100-110 | Copolymer wins for hot environments |
| Creep Strain after 1000h @23°C, 10MPa (%) | ~0.5 | ~0.8 | Delrin holds dimension better under constant load |
| Coefficient of Friction (dry vs steel) | 0.2-0.35 | 0.25-0.40 | Delrin is slightly more slippery — better for bearings |
Design Rules for Delrin and Acetal Parts
These rules come directly from our CNC machining cell and apply equally to prototype quantities and production runs. Adapt them for molding by adding draft angles (1-2° minimum for both materials).
Design Rules for Acetal Parts (from the Shop Floor)
Based on machining data from 500+ industrial acetal parts produced at our facility (Q1 2026). Apply these directly in your CAD drawings.
| Paramètre | Delrin (POM-H) | Acetal Copolymer (POM-C) |
|---|---|---|
| Minimum wall thickness | 1,0 mm | 0.8 mm |
| Recommended corner radius | R ≥ 0.5 mm | R ≥ 0.3 mm |
| Hole diameter tolerance (CNC) | ±0.025 mm | ±0.05 mm |
| Maximum aspect ratio (depth/dia) | 6:1 | 5:1 |
| Surface finish as-machined (Ra) | 0.8 μm | 1.2 μm |
| Post-machining warpage risk | Moderate (stress relief recommended) | Low (better dimensional stability) |
These values assume standard machining conditions at 20°C, sharp carbide tooling, and dry cutting. For injection molding design rules, refer to our engineering team consultation.
Why Centerline Porosity Changes Part Design
One of the least-discussed differences between the two materials is centerline porosity in Delrin extruded rod. As the rod diameter increases beyond 40 mm, POM-H extrusion typically develops a porous center core (0.5-2 mm diameter) due to shrinkage during cooling. If your part design places a critical sealing surface or tight-tolerance bore on the rod centerline, this porosity causes leaks or out-of-spec bores. Copolymer does not have this problem — the co-monomer disrupts crystallization enough to eliminate centerline voids.
Règle pratique : If your part has a through-hole or sealing surface passing through the rod center and the rod diameter exceeds 40 mm, specify acetal copolymer. One customer saved 18% on a returned batch for a water valve component by making this single material switch.
Industry Applications: Where Each Grade Wins
| L'industrie | Pièces courantes | Preferred Grade | Selection Rationale |
|---|---|---|---|
| Automobile | Fuel system valves, seat belt components, window regulator gears | Delrin (POM-H) | Higher fatigue strength for 100k+ cycle durability; low fuel permeability |
| Dispositifs médicaux | Inhaler mechanisms, insulin pen bodies, surgical instrument handles | Acetal Copolymer (POM-C) | Better resistance to sterilization chemicals; lower extractables |
| Machines industrielles | Conveyor rollers, bearing cages, pump impellers, wear strips | Delrin (POM-H) | Superior wear resistance and low friction (self-lubricating); higher hardness |
| Électronique grand public | Keyboard mechanisms, printer gears, camera lens barrels | Acetal Copolymer (POM-C) | Better hot water resistance for dishwasher-safe applications; lower creep under constant spring load |
| Plumbing & Fluid Handling | Valve seats, shower mixer components, water meter internals | Acetal Copolymer (POM-C) | Lower hot water swelling; avoids centerline porosity issues that can cause leaks in POM-H |
| Food Processing | Filling nozzles, conveyor guides, cutting board liners | Both (FDA-compliant grades) | Use homopolymer for dry-contact apps; copolymer for wet/hot-contact per NSF 51 |
The pattern is clear: Delrin dominates in dynamic mechanical applications (gears, bearings, springs) where fatigue life and low friction are primary requirements. Acetal copolymer dominates in applications involving fluids, chemicals, or sterilization — and anywhere the part geometry makes centerline porosity risky.
Tolerance Comparison by Manufacturing Process
The achievable tolerance depends as much on the manufacturing process as the material choice. Below is a practical reference for engineers writing drawing callouts.
| Manufacturing Process | Delrin (POM-H) Tolerance | Acetal Copolymer (POM-C) Tolerance | Meilleur pour |
|---|---|---|---|
| Usinage CNC | ±0.025 mm | ±0.05 mm | Prototypes, low-mid volume (<1,000 pcs), tight-tolerance parts |
| Moulage par injection | ±0.10 mm | ±0.08 mm | High volume (>5,000 pcs); copolymer molds more consistently |
| 3D Printing (FDM) | ±0.20 mm | ±0.15 mm | Rapid prototypes only; not recommended for functional testing |
Tolerances are achievable under standard shop conditions. Tighter tolerances available on request with additional process controls.
Note that copolymer’s lower post-machining stress relaxation gives it a slight tolerance advantage in injection molding — the part comes out of the mold closer to its final dimensions and drifts less over the first 72 hours.
Cost Decision Framework: When to Pay for Delrin
Cost Decision Framework: Delrin vs Acetal Copolymer
Material cost is only ~15-20% of total part cost for machined components. The real difference is in process efficiency and rejection rate:
- Raw material price: Delrin rod is typically 10-15% more expensive than generic acetal copolymer rod at the same diameter.
- Machining time: Delrin cuts cleaner and faster — expect 8-12% shorter cycle time per part vs. copolymer at equivalent tolerance.
- Rejection rate (internal data): Copolymer parts have higher visual defects (center porosity streaks) — 3-5% rejection vs. ~1% for Delrin in CNC turning operations.
- Break-even point: If your annual volume exceeds 2,000 parts, the reduced rejection rate of Delrin offsets the raw material premium. Below 2,000, generic copolymer is more cost-effective.
- Hidden cost: Copolymer proposed for dimensional-critical assemblies may accumulate 1-2% scrap from dimensional drift post-machining due to residual stress release over the first 72 hours.
The cost decision often comes down to a simple question: “What happens if this part fails?” For non-critical cosmetic or spacing components, copolymer saves money. For load-bearing, safety-critical, or high-cycle components, the Delrin premium pays for itself in reduced field returns.
Common Defects and How to Fix Them
When an acetal part fails or is rejected at incoming QC, the root cause is often a material mismatch. Here are the four most frequent issues we see when customers bring parts to us for rework:
| Problem | Likely Material Cause | Corriger |
|---|---|---|
| Gear tooth wear after < 50k cycles | Copolymer used where homopolymer needed (28 MPa vs 35 MPa fatigue limit) | Switch to Delrin 100 or 150 grade for gears; add PTFE-filled grade (Delrin 500AF) if unlubricated |
| Surface cracking after 6+ months in service | Environmental stress cracking from acidic environment (common in POM-H) | Switch to POM-C; verify chemical compatibility with your process fluid |
| Dimensional growth in hot water application | POM-H absorbs more water at elevated temperature (~0.8% at 80°C) | Use POM-C (0.6% absorption); pre-condition parts by soaking in 60°C water for 24h before final QC |
| Porosity visible on turned surfaces | Centerline porosity — inherent to POM-H extrusion process | Specify POM-C for parts with large turned diameters (>50 mm); request “low-porosity” grade from supplier |
Specifying Acetal Correctly on Your Purchase Order
Avoid the most common procurement mistake: writing “Acetal” on the PO without specifying the grade. A correctly specified PO should include:
- Material family and type: “Acetal Homopolymer (Delrin 150)” or “Acetal Copolymer (equivalent to Celcon M90)”
- Form: “Extruded rod, diameter 50 mm” or “Injection molding pellets”
- Tolerance requirement: “Per ISO 2768-m” or specific callout
- Any special grade requirements: “FDA-compliant,” “UV-stabilized (Delrin 527),” “PTFE-filled (Delrin 500AF)”
This level of specificity prevents the supplier from substituting copolymer for homopolymer when the drawing only says “acetal” — a practice that is more common than most engineers realize.
Conclusion et recommandations
The Delrin vs acetal decision is not about which material is “better” — it is about matching the right grade to the right application. Use Delrin homopolymer for dynamic mechanical parts where fatigue life, surface finish, and low friction are critical. Use acetal copolymer for fluid-contact applications, hot or chemically aggressive environments, and large-diameter turned parts where centerline porosity is a risk.
Still unsure which grade fits your design? Our engineering team reviews material selection as part of every quote. Send your drawing or 3D file and we will recommend the optimal grade for your application at no charge.
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Questions fréquemment posées
Puis-je remplacer le Delrin par un copolymère d'acétal dans une conception d'engrenage existante sans modifier les dimensions ?
Yes, with caveats. Copolymer has approximately 20% lower fatigue endurance (28 MPa vs 35 MPa at 10&sup6; cycles) and slightly higher creep under constant load. For static-load gears operating under moderate torque and thin walls, direct substitution works. For high-cycle dynamic-load gears or non-lubricated bearing sleeves, expect up to 30 microns of additional radial play after the first month of operation. Dimensionally, identical tooling can be used — but if switching from homopolymer to copolymer, consider increasing wall thickness by 8-10% to compensate for lower stiffness.
Pourquoi ma pièce en Delrin usinée par CNC se déforme-t-elle après être restée une semaine sur une étagère ?
Il s'agit d'une libération de contraintes résiduelles — une caractéristique bien connue de l'homopolymère d'acétal. Lors de l'extrusion, la surface extérieure de la tige refroidit plus rapidement que son cœur, ce qui crée un gradient de contraintes. Lorsque vous enlevez de la matière de manière asymétrique lors de l'usinage, ce déséquilibre des contraintes provoque un gauchissement dans les 24 à 72 heures. Trois solutions : (1) procéder d'abord à un usinage grossier, laisser la pièce reposer pendant 24 heures, puis effectuer l'usinage de finition ; (2) recuire la tige à 160 °C pendant 1 heure par tranche de 25 mm d'épaisseur avant l'usinage ; (3) opter pour un copolymère d’acétal pour les pièces nécessitant un enlèvement de matière fortement asymétrique. Nous recommandons généralement le recuit pour toute pièce en Delrin dont la planéité ou la cylindricité doit rester dans une tolérance de 0,05 mm.
Quelle nuance offre les meilleures performances dans des environnements exposés à l'eau chaude ou à la vapeur ?
Le copolymère d'acétal (POM-C) présente de meilleures performances que le Delrin (POM-H) dans l'eau chaude à plus de 60 °C. Le copolymère absorbe moins d’eau à température élevée (~0,61 TP3T contre ~0,81 TP3T à 80 °C), gonfle moins et résiste nettement mieux à l’hydrolyse, c’est-à-dire à la dégradation chimique de la chaîne polymère par l’eau. Pour des applications telles que les composants de mitigeurs de douche, les pièces de lave-vaisselle ou les éléments internes de machines à café, le copolymère est le choix standard. Si l’application nécessite également la conformité aux normes de la FDA, spécifiez un copolymère de qualité alimentaire tel que le Celcon M25 ou un équivalent.
Quels sont les traitements de surface disponibles pour les pièces en Delrin et en copolymère d'acétal ?
Le Delrin à l'état brut d'usinage offre la meilleure finition naturelle (Ra 0,8 μm avec des outils en carbure tranchants à une vitesse de rotation comprise entre 5 000 et 8 000 tr/min). Le copolymère d’acétal à l’état brut présente généralement un Ra compris entre 1,0 et 1,2 μm. Aucun de ces deux matériaux n’admet bien la peinture ou le placage en raison de leur faible énergie de surface. Le grenaillage produit une finition mate uniforme sur les deux matériaux (Ra 2,0-3,0 μm) et constitue la finition secondaire la plus courante. Pour obtenir un aspect brillant, le polissage à la flamme convient aux deux types de matériaux, mais nécessite un contrôle minutieux de la température afin d’éviter la fusion de la surface. Aucun de ces matériaux ne doit être soumis à un polissage à la vapeur (pratique courante pour l’acrylique) : les solvants utilisés attaquent l’acétal.
