Delrin (POM Homopolymer) vs Acetal Copolymer (POM-C) Injection Molding Guide

In mechanical engineering drawings, “Delrin” and “Acetal” are routinely written as interchangeable synonyms for Polyoxymethylene (POM). In polymer chemistry and on the injection molding floor, they represent two distinct molecular architectures with very different failure modes:

• Acetal Homopolymer (POM-H)—invented by DuPont and sold under the trademark Delrin (and Asahi Kasei Tenac homopolymer)—consists of a pure, uninterrupted repeating chain of oxymethylene (-CH2-O-) units. That uniform chain packs into 75%–85% crystallinity, delivering 10%–15% higher room-temperature tensile strength, stiffness, hardness, and gear tooth fatigue resistance.
• Acetal Copolymer (POM-C)—sold under trade names such as Celcon / Hostaform (Celanese), Ultraform (BASF), and Duracon (Polyplastics)—inserts random ethylene-oxide (-CH2-CH2-O-) comonomer “stoppers” along the polymer chain. Those comonomer links lower crystallinity slightly (65%–75%), but stop thermal “unzipping” (formaldehyde outgassing) in the molding barrel, prevent hydrolysis in continuous > 80 °C hot water and strong alkalis (pH > 9), and eliminate centerline microporosity in thick sections.

Use this engineering matrix alongside our POM Acetal overview and Nylon guide to specify explicitly on your 2D print whether a part requires POM-H, POM-C, or may accept either.

1. Head-to-Head Property & Processing Matrix: Delrin (POM-H) vs Acetal Copolymer (POM-C)

Property / Engineering Metric	Acetal Homopolymer (POM-H, e.g., Delrin 500P)	Acetal Copolymer (POM-C, e.g., Celcon M90 / Ultraform N2320)	Winner & Practical Buyer Impact
Crystallinity & Specific Gravity	75% – 85% | 1.42 g/cm³	65% – 75% | 1.41 g/cm³	POM-H: Higher crystallinity drives peak mechanical strength and wear hardness.
Crystalline Melting Point ($T_m$)	175 – 178 °C (347–352 °F)	165 – 168 °C (329–334 °F)	POM-H melts 10 °C higher, though POM-C resists long-term oxidative thermal aging better.
Tensile Yield Strength (23 °C, ISO 527)	68 – 72 MPa (~10,000 psi)	60 – 64 MPa (~8,800 psi)	POM-H (+12%–15% stronger): Allows thinner gear teeth and higher snap-fit spring forces.
Flexural Modulus (23 °C, ISO 178)	2.9 – 3.1 GPa	2.5 – 2.7 GPa	POM-H (+15% stiffer): Lower beam deflection under short-term mechanical load.
Notched Izod Impact (23 °C / -40 °C)	80 – 120 J/m (65–90 J/m at -40 °C)	60 – 85 J/m (50–65 J/m at -40 °C)	POM-H: Superior impact toughness and repeated flex-fatigue endurance (10^7 cycles).
Continuous Hot-Water & Steam Limit	≤ 60 °C continuous water (degrades over months above 70–80 °C)	80 – 90 °C continuous hot water (survives intermittent 100 °C boiling water)	POM-C by a wide margin: Mandatory for domestic hot-water plumbing valves, shower mixers, and coffee-machine manifolds.
Chemical / pH Resistance (Acids vs Bases)	pH 4.0 to 9.0 (attacked by strong alkalis, bleach, and strong acids)	pH 4.0 to 13.0 (excellent resistance to strong bases, caustics, detergents, and fuels)	POM-C: Comonomer Carbon-Carbon bonds stop alkaline chain-end depolymerization (caustic cleaners, automotive fuel pumps).
Centerline Porosity in Thick Walls (> 4 mm)	Prone to Centerline Microporosity (voids/chalky core due to 12% volumetric contraction during crystallization)	Minimal / Absent (lower crystallinity contraction yields dense, non-porous cores)	POM-C: Preferred for food/medical fluid blocks where porous cores trap bacteria or leak under pressure.
Thermal Processing Window & Outgassing	Narrow (205–218 °C melt); decomposes into irritating formaldehyde gas ($CH_2O$) if held > 225 °C	Wider (190–225 °C melt); comonomers arrest chain unzipping, reducing mold deposit & gas	POM-C: Molders strongly prefer POM-C because it runs with fewer gas-burn interruptions and longer barrel residence tolerance.
Mold Shrinkage (2.0–3.0 mm wall)	1.9% – 2.3% (isotropic)	1.8% – 2.2% (isotropic)	Identical (~2.0%): Both grades run in the exact same mold cavity without changing steel dimensions.
Relative Resin Cost (per kg)	1.15x – 1.35x	1.00x (Baseline)	POM-C saves 15%–25% on raw resin cost and is supplied by multiple global producers.

2. When You Must Specify Delrin (POM-H) vs When You Must Specify Copolymer (POM-C)

Specify Delrin (POM-H) Exclusively When:

1. Peak Gear Tooth Root Strength & Fatigue Life Drive Part Size: In small, heavily loaded spur or planetary gearboxes, conveyor chains, and seatbelt mechanisms, Delrin’s 12%–15% higher tensile yield (70 MPa vs 62 MPa) and flexural fatigue endurance prevent tooth root shear where copolymer would require stepping up the gear module.
2. High-Cycle Spring Clips & Cantilever Latches: When a molded spring arm must hold constant return force without creep relaxation and survive severe cold-drop impact at -40 °C, high-viscosity homopolymer (e.g., Delrin 100P) outperforms standard copolymer.

Specify Acetal Copolymer (POM-C) Exclusively When:

1. Hot Water (> 60 °C), Coffee/Beverage Systems, or Alkaline Detergents: In plumbing mixing cartridges, dishwasher spray arms, irrigation valves, and fuel-tank sender hulls, alkaline water (pH 9–13) or hot water (80 °C) attacks the end-caps of homopolymer chains and unzips them. Copolymer (POM-C) resists alkaline hydrolysis up to pH 13 and 85–90 °C continuous water.
2. Thick Cross-Sections (> 4–6 mm) or Post-Mold CNC Machining: Because homopolymer shrinks more violently upon crystallizing (~80% crystallinity), thick molded gear hubs or valve bodies often develop a spongy, micro-porous centerline core (centerline porosity). If a secondary machining operation drills or threads into that core—or if pressurized fluid contacts it—the porous center weeps and fails hygiene tests. Copolymer molds dense and void-free.
3. Low-VOC Automotive Interiors: Automotive cabin specs (VDA 275) strictly cap formaldehyde emissions (< 2–5 ppm). Low-emission POM-C grades easily pass automotive cabin VOC limits where standard POM-H struggles.

3. Injection Molding & Tooling Rules for Both POM Grades

Whether molding POM-H or POM-C, acetal’s high crystallinity (~2.0% shrinkage) and formaldehyde decomposition chemistry require three non-negotiable press controls:

• Hot Molds (80–105 °C) Are Mandatory for Dimensional Stability: Molding POM in a cold water-chilled tool (30–40 °C) quenches the skin in an under-crystallized state. When that part later enters a 70 °C environment in the field, it undergoes secondary post-crystallization, shrinking an extra 0.3%–0.6% and seizing tight shafts! Running the mold at 85–105 °C completes crystallization inside the cavity so field dimensions stay rock-stable.
• Shallow Perimeter Venting (0.010–0.020 mm) + 420SS Corrosion Resistance: Formaldehyde outgassing leaves white paraformaldehyde plate-out in unvented pockets and corrodes non-stainless steel if combined with moisture. Specify 0.012–0.020 mm vents with wide relief channels.
• Never Mix POM with PVC or Flame-Retardant Resins in the Same Barrel!: This is a critical shop-floor safety rule. Even trace amounts of PVC (hydrochloric acid) or acidic brominated flame-retardants in a barrel heated to 200 °C with POM will catalyze violent, exothermic formaldehyde gas decomposition that can blow the nozzle or hopper off the press. Always purge barrels with neutral HDPE or PP before and after running POM.

Buyer FAQs

Is Delrin the same as Acetal?

All Delrin is acetal (polyoxymethylene, POM), but not all acetal is Delrin. Delrin is the trade name for acetal homopolymer (POM-H), which has 75%–85% crystallinity and 12%–15% higher room-temperature tensile strength, stiffness, and impact resistance. Most generic “acetal” quoted by molders is acetal copolymer (POM-C, such as Celcon, Hostaform, or Ultraform), which costs 15%–25% less, processes over a wider thermal window without formaldehyde outgassing, and offers far superior resistance to hot water (> 60 °C), strong alkalis (pH 9–13), and centerline porosity.

Can Delrin (POM-H) and Acetal Copolymer (POM-C) be molded in the same injection mold?

Yes—POM-H and POM-C share virtually identical isotropic mold shrinkage (1.8% to 2.2%, with 2.0% or 0.020 mm/mm as the standard tooling nominal). A precision gear or housing mold cut for Delrin 500P can run Celcon M90 or Ultraform N2320 copolymer with dimensional shifts typically under ±0.001–0.002 mm/mm, tunable via holding pressure and mold temperature (80–105 °C).

Which is better for hot water and plumbing valves: Delrin or Acetal Copolymer?

Acetal Copolymer (POM-C) is mandatory for continuous hot-water (> 60–85 °C), steam, and alkaline detergent (pH > 9) applications. In continuous hot water or chlorinated/alkaline plumbing systems, acetal homopolymer (Delrin / POM-H) suffers hydrolytic chain unzipping and loses mechanical strength over time, whereas the carbon-carbon comonomer links in POM-C block chain-end degradation.

What causes centerline porosity in thick acetal parts?

Centerline porosity (a chalky or spongy micro-void zone in the center of walls thicker than 4–6 mm) occurs when the outer skin of the acetal part freezes against the mold steel before the gate can pack enough molten resin to compensate for the 10%–12% volumetric contraction as the core crystallizes. It is much more severe in high-crystallinity homopolymer (POM-H) and is resolved by switching to acetal copolymer (POM-C), thickening the gate (≥ 65%–75% of wall thickness), running a hotter mold (90–105 °C), and coring out walls to uniform 2.0–3.5 mm thickness.

Disclaimer

PlasticsTechnologyAlliance.com is an independent buyer resource. It does not manufacture parts, sell materials, or certify suppliers. Delrin is a registered trademark of Delrin USA, LLC; Celcon and Hostaform are trademarks of Celanese; Ultraform is a trademark of BASF. Verify potable-water (NSF-61), food (FDA), and mechanical specifications against the producer’s grade datasheet.