A legacy drawing says Delrin: translating it to a current POM spec is a specification-engineering sequence rather than a name substitution. Legacy drawings frequently list only the trade name Delrin, a DuPont polyoxymethylene homopolymer, or a grade-specific notation such as Delrin 500 without quantifying melt flow rate, tensile modulus, or thermal aging behavior. The current translation must therefore map the historical trade name to the ISO 1043-1 material class POM-H, then determine whether the part can accept a POM-C copolymer replacement or whether the specification must retain a homopolymer grade. ASTM D6778 and ISO 9988-1 provide classification frameworks for current polyoxymethylene moulding and extrusion materials. On a manufacturing drawing, a material note such as `MATL: DELRIN` creates a compliance problem when the downstream fabricator purchases from multiple compounders. Without a grade-specific MFR, the injection molding behavior, weld-line strength, and fatigue life are not defined. The translation record should therefore replace the trade name with a four-element designation: polymer type, filler or reinforcement, stabilizer package, and a melt flow rate window. For example, an unfilled UV-stabilized POM-H with MFR measured at 190°C and 2.16 kg per ISO 1133-1 is a complete base callout. The absence of any one element is an incomplete specification.

Why Does a Legacy Delrin Callout Not Equal a Generic POM Designation?

Delrin is a trade name for acetal homopolymer, whereas many current POM compounds sold under generic names are copolymers. The homopolymer chain is essentially linear -CH₂O- repeat units; the copolymer introduces a low fraction of carbon-carbon comonomer units. This molecular difference changes the crystalline melt peak, the resistance to alkaline or hydrothermal depolymerization, and the width of the injection-molding processing window. Differential scanning calorimetry per ISO 11357-3 typically places unfilled POM-H melting near 172–178°C, while POM-C grades melt near 164–170°C. The homopolymer therefore offers a modestly wider crystalline service-temperature margin, but it also unzips more readily at elevated temperature when thermally or chemically attacked. Mechanically, POM-H historically provides higher tensile modulus and higher notched impact strength at room temperature, although modern nucleated POM-C grades have narrowed the difference. Data generated under ISO 527-2 and ISO 179-1/1eA are needed for any substitution. A part dimensioned around POM-H creep resistance will not necessarily survive a POM-C replacement if the copolymer exhibits lower modulus under continuous load. Conversely, a part with long, thin wall sections may benefit from the wider processing window and reduced centerline porosity of a POM-C injection grade. Before releasing a replacement POM specification, the responsible engineer should recover the original grade data. If the drawing does not specify a DuPont grade number, historical procurement records, material certifications, or old product drawings may identify whether the part was molded from a high-viscosity grade or a high-flow grade. The current specification should then record an MFR window that matches the original processing path. A part originally molded in a multicavity hot-runner tool with a long flow length cannot be translated to a very low-MFR extrusion grade without risking short shots or excessive shear heating. Conversely, a structural clamp or gear blank originally extruded in a high-molecular-weight POM-H cannot be replaced with a high-MFR POM-C without reducing tensile impact, fatigue resistance, and weld strength.

Critical Mechanical and Thermal Property Differentials for POM-H and POM-C

Property	Standard method	Unfilled POM-H	Unfilled POM-C
Melt peak, DSC	ISO 11357-3	172–178 °C	164–170 °C
Tensile modulus	ISO 527-2	2700–3200 MPa	2500–2900 MPa
Tensile yield stress	ISO 527-2	65–70 MPa	55–65 MPa
Notched Charpy impact, 23 °C	ISO 179-1/1eA	6–8 kJ/m²	7–12 kJ/m²
Heat deflection temperature, 1.80 MPa	ISO 75-2	95–100 °C	90–100 °C
Water absorption, saturation, 23 °C	ISO 62	0.8–1.2 %	0.8–1.0 %

Published datasheets vary by supplier; these ranges are not a substitute for lot-specific certification data. The comparison should be read as a screen, not as an acceptance test. For bearing, gear, or conveyor-wear applications, a legacy Delrin callout carries implicit tribological expectations. Homopolymer POM offers high crystallinity and often lower wear against steel in dry sliding, but the absence of a defined wear test on the legacy drawing means that equivalence cannot be demonstrated from a tensile property comparison alone. If the replaced part is a gear in a paper-handling mechanism, the current specification should add a wear-factor or mass-loss acceptance criterion using the same test rig geometry as the original part, or at minimum a standardized thrust-washer test. Published data for laboratory-wear equivalence between POM-H and POM-C under identical counterface roughness and contact pressure are limited. Thermal expansion and water absorption also matter for part geometry. POM-H and POM-C have similar coefficients of linear thermal expansion near 100–120 × 10⁻⁶ K⁻¹ depending on filler; a replacement with the same nominal CLTE still requires a mold-shrinkage review because nucleated POM-C can shrink differently. Mold shrinkage should be measured in a comparison tool under ISO 294-4.

If the Part Encounters Moisture, Alkaline Cleaners, or Continuous UV, a Different POM Stabilization Profile Is Required

Legacy Delrin parts used in plumbing, food-processing, or medical devices may see repeated hot-water or cleaning-agent exposure. Polyoxymethylene homopolymer is attacked by strong acids, oxidizing agents, and alkaline solutions. The copolymer is more resistant to hot water and alkali because the comonomer interrupts the chain-unzipping depolymerization mechanism, but greater resistance is not the same as immunity. Long-term exposure must be evaluated by tensile property retention after immersion per ISO 175 in the actual cleaning fluid at the service temperature. For parts in continuous hot water above 60°C, both POM-H and POM-C can lose molecular weight, and published data for long-term performance in this configuration is limited. UV exposure presents another translation trap. A drawing that says Delrin natural does not specify UV stabilization. Natural POM chalks, surface-cracks, and loses tensile impact outdoors. The current specification must add an accelerated weathering requirement, typically ISO 4892-2 or an application-specific automotive OEM weathering procedure, with acceptance criteria for color and mechanical property retention. For food-contact or potable-water components, the legacy Delrin callout may have been selected under historical FDA clearances. The replacement specification must state the applicable regulatory pathway. POM homopolymer and POM copolymer are not interchangeable in every food-contact filing; the current material supplier must provide a letter documenting FDA 21 CFR 177.2470 for POM copolymer or FDA 21 CFR 177.2480 for POM homopolymer, as applicable. For European applications, the compound must comply with the relevant EU 10/2011 migration limits for formaldehyde and any additives used in the stabilization package because POM is sensitive to processing temperature and residual formaldehyde.

Specification element	Required test or designation	Translation action
Base polymer designation	ISO 1043-1, ISO 9988-1	Use POM-H or POM-C as determined by chemistry, not trade name.
Melt flow rate	ISO 1133-1, 190 °C / 2.16 kg	Set tolerance around original grade.
Tensile modulus and yield stress	ISO 527-2	Check snap-fit and gear-tooth stiffness.
Impact resistance	ISO 179-1/1eA or ISO 180/A	Record 23 °C and -30 °C values if low-temperature service applies.
Heat deflection temperature	ISO 75-2, method A, 1.80 MPa	Compare with continuous service temperature.
Weathering	ISO 4892-2	Required only if part has outdoor or UVC exposure.
Regulatory	FDA 21 CFR 177.2470, 177.2480, EU 10/2011	Verify supplier certification for end-use contact.

On the molding floor, the translated POM specification changes machine settings. Homopolymer POM-H melts at approximately 175 °C and is typically processed at 190–210 °C, while POM-C is often processed at 180–210 °C. The actual melt temperature reading should not exceed 230 °C for either material because POM decomposition accelerates sharply, releasing formaldehyde gas and leaving burn marks. Injection molding production experience shows that screw-recovery time, back pressure, and cushion control become critical when a high-flow POM-C replaces a high-viscosity POM-H: the same tool may fill more easily, but the gate freeze-off time often changes, and sink marks can develop if the holding-pressure profile is not re-optimized. Pre-drying is not required for POM in sealed containers with low ambient humidity, but when the resin is exposed to relative humidity above 60% or when regrind is used, a desiccant dryer set to 80 °C for 3–4 h reduces surface moisture effects. POM is not recommended in contact with strong acids, chlorine gas, or certain transition-metal chlorides at elevated temperature; published data for specific chemical incompatibilities with actual process fluids should be required from the material supplier before substituting POM-C for Delrin in an existing chemical-environment part. Medical device applications are outside the scope of a simple material translation because sterilization method, residual formaldehyde, and grade-specific FDA master files introduce constraints that ISO mechanical data alone cannot resolve.