Polyoxymethylene (POM) is the generic polymer class behind the trade designation Delrin. In procurement specifications, the phrase “Delrin Is a Brand, Not a Plastic: What You Are Actually Buying” identifies a recurring substitution risk: the material purchased is not a single composition but a family of oxymethylene polymers with different chain architectures, stabilizer packages, and processing windows. Commercial POM is supplied as either a homopolymer, designated POM-H, or a copolymer, designated POM-C. The homopolymer is produced by anionic polymerization of formaldehyde and is end-capped with acetate or methoxy groups to suppress thermal unzipping. The copolymer is produced by cationic copolymerization of trioxane with a minor fraction of 1,3-dioxolane or ethylene oxide; the comonomer inserts carbon-carbon bonds into the oxymethylene sequence. These structural differences are not semantic because they change molecular weight retention, crystallinity, and failure mode under thermal loading.

The generic terms “acetal” and “polyacetal” add further ambiguity. In polymer science, an acetal is a molecule with two alkoxy groups attached to a single carbon; a POM chain is therefore a polyacetal, but the reverse is not automatically true for every acetal trade name. ISO 9988-1 classifies POM-H and POM-C by chain type, while ASTM D6778 provides a cell classification system for acetal molding and extrusion materials. A specification that records only “Delrin” without a base-polymer designation may allow unintended substitution. The current Delrin brand covers multiple melt-flow rates, filled grades, and additive packages, but it does not represent the entire POM market. For equivalent parts, the purchaser should record the actual classification and melt-flow rate, for example POM-H with a melt flow rate of 2.5 g/10 min at 190 °C and 2.16 kg per ISO 1133-1.

Does the Homopolymer Backbone Alone Explain the Delrin Performance Legacy?

The homopolymer has a higher concentration of oxymethylene repeat units than the copolymer, which increases crystallinity and yields a room-temperature tensile stress at yield of approximately 70 MPa when tested per ISO 527-2. Typical unfilled homopolymer grades also exhibit a flexural modulus near 3000 MPa per ISO 178, while equivalent unfilled copolymer grades are usually 10–15 % lower in flexural modulus. The difference is not a defect; it is a consequence of comonomer units reducing chain regularity. Crystallinity values derived from density measurements per ISO 1183-1 may approach 75–85 % for well-annealed POM-H samples, while POM-C grades are typically several percentage points lower. The higher crystallinity lowers impact energy absorption but improves stiffness and creep resistance at room temperature.

The same structural regularity creates a sharper melting point. Differential scanning calorimetry under ISO 11357-3 places homopolymer melting at around 178 °C and copolymer melting at around 166 °C. However, the fully oxymethylene chain is more vulnerable to thermal depolymerization if end-cap integrity is lost. For this reason, melt-temperature control and residence-time management are more critical for POM-H than for POM-C. Notched impact resistance is often lower than that of many engineering thermoplastics. An unfilled POM-H grade tested per ISO 179-1/1eA typically records a notched Charpy impact of 7 kJ/m² at 23 °C, but values drop below 4 kJ/m² at −30 °C. The notch sensitivity must be part of design review when a metal component is replaced by acetal.

Molecular weight is not normally used as a purchase specification. Instead, melt flow rate per ISO 1133-1 at 190 °C and 2.16 kg separates general-purpose, high-flow, and extrusion grades. A 2.5 g/10 min homopolymer is suited to thick-section machined stock, while a 27 g/10 min grade fills thin-wall multicavity molds. The melt-flow selection changes shear heating, pressure drop, gate freeze-off, and the tendency to form centerline porosity in thick sections. Published datasheets for individual grades may differ from these representative values, and design stress calculations should not use a single number without reference to the actual grade test certificate.

Before melt processing, POM requires closed-loop desiccant drying. A desiccant dryer set to 80 °C for 3–4 h with a dew point of −30 °C or lower reduces surface moisture below 0.1 %. Insufficient drying causes splay, voids, and surface defects during melt processing. Molding operations typically use a melt temperature of 205–215 °C for POM-H and 190–210 °C for POM-C. Mold temperatures are maintained at 80–110 °C to develop crystallinity and dimensional stability. At melt temperatures above 230 °C, formaldehyde evolution becomes measurable, and prolonged hold times create a risk of gas defects. Injection molding machines with general-purpose screws of 20:1 to 25:1 L/D and compression ratios of 2:1 to 2.5:1 are commonly used. Hot-runner systems should be externally heated and free of dead spots because stagnation zones generate formaldehyde and may cause localized black specks.

Compounding lines using co-rotating twin-screw extruders with L/D ratios of 32:1 to 40:1 are used for glass-fiber, mineral, or internally lubricated POM compounds. The shear profile is managed so that melt temperature remains below the depolymerization threshold while dispersing additives. Vacuum venting removes residual formaldehyde. If a vent port is blocked or vacuum is lost, surface splay and odor increase. During injection molding start-up or interruption, POM-H should not be left idling at melt temperature for more than 10–15 min; the barrel should be purged with an acrylic or polyethylene purge before restart. Unfilled homopolymer mold shrinkage is commonly 1.8–2.2 %, while unfilled copolymer shrinkage is 1.9–2.1 % per ISO 294-4. The shrinkage range must be compensated by tool design because asymmetrical gate location produces differential shrinkage and warpage in semi-crystalline POM.

Mechanical Property Benchmarks for POM-H and POM-C

The table values are representative of unfilled general-purpose grades and are not specification limits. Published datasheets for specific commercial grades may differ by melt-flow rate and additive package. Creep resistance under sustained load differentiates POM-H from POM-C. Homopolymer grades often display lower creep at room temperature when measured per ISO 899-1, but the advantage narrows as temperature approaches 60 °C. For spring-like geometries and snap fits, the lower copolymer modulus may increase retention while reducing initial engagement force. Substitution without requalification can therefore alter assembled force, even when both resins carry the generic label “acetal.”

POM resists repeated exposure to aliphatic hydrocarbons, diesel, ethers, and many neutral liquids. However, hot aqueous acids at pH below 4 cause acid-catalyzed hydrolysis of oxymethylene units. Oxidizing agents, chlorinated solvents, and strong bases also degrade the surface. Supplier data and ASTM D543 immersion tests should be reviewed for each chemical environment. Continuous exposure to chlorinated potable water must be assessed separately because chlorine attack can roughen the surface and reduce molecular weight at the exposed layer. Unmodified POM-H and POM-C typically contain no intentionally added lead, cadmium, mercury, hexavalent chromium, polybrominated biphenyls, or polybrominated diphenyl ethers. This supports alignment with RoHS Directive 2011/65/EU Annex II restrictions, but a supplier declaration must confirm the particular lot. Food-contact suitability is formulation-specific; for certain acetal copolymers and homopolymers, 21 CFR 177.2470 and 21 CFR 177.2480 may be referenced, but a general POM certificate is insufficient without confirmation of the exact grade and additive system.

When Dimensional Stability in Humid Environments Dictates Grade Selection

POM absorbs less water than polyamides. After 24 h immersion in water at 23 °C per ASTM D570, unfilled POM-H typically gains 0.25 % and unfilled POM-C approximately 0.22 % by weight. The associated dimensional change is small but not zero. In a precision gear with a bore tolerance of ±0.02 mm, moisture cycling can shift the effective clearance if the part was machined dry and measured immediately after molding. Post-molding shrinkage continues for up to 48 h. If assembled before crystallization is complete, a POM hub may lose interference fit. Conditioning at 23 °C and 50 % RH for 24 h before final measurement is a common industrial practice based on ISO 291.

The coefficient of linear thermal expansion is approximately 110 × 10⁻⁶ 1/K for unfilled POM-H and 110–130 × 10⁻⁶ 1/K for unfilled POM-C. Mating a POM gear to a steel shaft produces a large differential expansion; bearing clearances must be evaluated at the upper service temperature of the assembly. Glass-fiber-reinforced POM lowers thermal expansion substantially, but the filled grade also changes mold shrinkage, weld-line strength, and surface appearance. Dimensional checks after machining should allow for thermal relaxation because machined POM parts can relax and shift dimensions after the first heating cycle above 80 °C.

In unlubricated gear, cam, and conveyor link service, POM is selected for a hard, low-friction surface. Dynamic coefficient of friction against polished steel is typically 0.20–0.35 when measured per ISO 8295, with homopolymer grades often near the lower end of that range. However, coefficient of friction is not an intrinsic material constant; it depends on steel surface finish, load, and velocity. Plain bearing PV limits are grade-specific. Published manufacturer data for unfilled POM-H frequently list continuous dry-running PV limits below 0.1 MPa·m/s for long service, while filled or internally lubricated grades extend the permissible envelope. Above the PV limit, frictional heat softens the surface, increases wear, and may generate formaldehyde vapor.

Wear testing per ASTM D3702 on a thrust washer configuration is used to rank materials under controlled load and sliding velocity. Published data for specific unfilled POM-H and POM-C comparisons in this configuration is limited; ranking should be based on the actual grade, counterface roughness, and test protocol rather than generic acetal data. In snap-fit and pressed-metal insert applications, POM stress relaxation is measurable. Retention of initial preload at elevated temperature should be confirmed by creep or stress-relaxation testing per ISO 899-1 before substitution. A production lot change from homopolymer to copolymer may alter engagement force, and vice versa, even if both materials are called “acetal.”