Polyoxymethylene melting point, density and crystallinity data are routinely extracted from supplier datasheets without sufficient attention to the test method, thermal history, and grade composition. Reading the datasheet requires treating the reported values as method-dependent results rather than fixed material constants. Unmodified homopolymer grades typically list a second-heating melting peak between 175 °C and 181 °C under ISO 11357-3:2018 or ASTM D3418-21, whereas copolymer grades containing 1,3-dioxolane or ethylene oxide comonomer list melting peaks between 162 °C and 173 °C. The reported density for unfilled homopolymer is usually 1.41–1.43 g/cm³ by ISO 1183-1:2019, while unfilled copolymer density ranges from 1.38 g/cm³ to 1.41 g/cm³.

A datasheet that omits heating rate, first- or second-heating condition, specimen mass, and calibration standard is insufficient for specification review. The first-heating endotherm from an as-molded part includes orientation, skin-core structure, and lamellar thickening effects. The same thermal history limitation applies to density, because cooling rate and nucleating additives alter the semi-crystalline morphology and therefore the measured density. Crystallinity is more method-sensitive because density, differential scanning calorimetry, and X-ray diffraction report different phase fractions.

What Does the Endothermic Peak Actually Quantify in POM?

Under ISO 11357-3:2018, the melting temperature extracted by differential scanning calorimetry is normally the peak temperature of the second heating run, recorded at 10 K/min with specimen mass between 5 mg and 10 mg. That peak is not the equilibrium melting temperature of an ideal POM crystal; it is a distribution-weighted average of lamellar melting points that depends on the cooling history imposed before the run. The extrapolated onset temperature is typically 2–5 K below the peak value for POM, while the endset can extend 5–12 K above the peak, depending on comonomer distribution, molecular weight, and prior cooling rate.

Calibration of the DSC instrument against certified indium and zinc is required for meaningful interlaboratory comparison. Indium onset is 156.6 °C and zinc onset is 419.5 °C; a laboratory reporting only the endothermic peak without calibration metadata can introduce bias of several kelvin. For POM copolymers, comonomer units depress the melting point and broaden the endotherm. A datasheet value of 165 °C for a copolymer and 178 °C for a homopolymer do not indicate a direct thermal-stability difference unless the integration limits, heating rate, and second-heating protocol are identical.

For unfilled POM grades, density values in supplier literature are generated by liquid displacement per ISO 1183-1:2019 method A or by density gradient column per method B. The two-phase relation X_c = (ρ_s − ρ_a) / (ρ_c − ρ_a) converts measured density ρ_s into crystalline volume fraction only when the amorphous density ρ_a and crystalline density ρ_c are known. For POM, published crystalline unit-cell density ranges from 1.49 g/cm³ to 1.51 g/cm³, and amorphous density obtained by melt extrapolation is near 1.25 g/cm³. A measured unfilled homopolymer density of 1.41 g/cm³ therefore corresponds to a crystalline volume fraction of approximately 0.65–0.69. The supplier should state whether crystallinity is expressed as mass fraction or volume fraction; DSC enthalpy ratios yield mass fraction, while density and X-ray diffraction can yield either depending on correction.

DSC crystallinity requires dividing the measured enthalpy of fusion by the enthalpy of a completely crystalline POM. Published reference enthalpies commonly span 230 J/g to 250 J/g, with some reports as high as 249 J/g for the crystalline phase. A reported crystallinity of 68% is therefore only comparable to another datasheet if the same reference enthalpy and integration limits are disclosed.

When the material is glass-fiber reinforced, density cannot be used to estimate polymer crystallinity directly. A glass-reinforced POM grade may report density near 1.58 g/cm³, but the organic matrix requires ash analysis by ISO 3451-1:2019 and normalization of the DSC enthalpy to the polymer fraction. Without this correction, the filled grade appears to have lower polymer crystallinity than an unfilled grade even when the POM matrix crystallinity is unchanged.

When a POM Datasheet Cites Both Density and XRD Crystallinity as Equivalent Grade Indicators

When density and X-ray diffraction crystallinity are listed on the same datasheet, the values do not partition the polymer phase identically. Wide-angle X-ray scattering using Cu Kα radiation typically integrates crystalline reflections and the amorphous halo over a 2θ interval of 5°–70° after baseline subtraction. The result is influenced by crystallite size, orientation texture, and peak deconvolution algorithm. A density-derived volume crystallinity near 67% and an XRD-derived crystallinity near 72% are not necessarily contradictory: density is sensitive to voids and secondary phases, while XRD is sensitive to lattice order and may exclude defective crystalline domains or include highly oriented mesophase intensity.

When DSC reports mass-fraction crystallinity and density reports volume-fraction crystallinity, the two are related by X_c,m = X_c,v × ρ_c / ρ_s. A datasheet that lists only a single crystallinity value without method designations ignores this distinction. Correlation between DSC and density methods for unfilled POM can be acceptable only when the same sample thermal history, void content, and reference densities are used. Void formation during injection molding also changes density without changing polymer crystallinity; a molded specimen with 1.5% void volume can show an apparent density reduction of approximately 0.02 g/cm³, which biases the calculated crystallinity by several percent.

Heater Band Set Points, Crystallization Half-Time and Hot Runner Thermal Degradation Limits

Melting point and crystallinity data are read together to set injection molding barrel temperatures. For unfilled homopolymer, the melt pool is commonly maintained between 190 °C and 215 °C, with nozzle temperature not exceeding 220 °C because formaldehyde evolution accelerates above 230 °C. For copolymer, the lower melting peak permits barrel settings from 180 °C to 205 °C. Mold temperature is typically held between 60 °C and 100 °C; at the lower bound, rapid cooling produces thinner lamellae and lower bulk crystallinity, increasing post-mold shrinkage and reducing elevated-temperature modulus. In production-scale injection molding, a 2 mm wall thickness in a mold at 80 °C can be demolded after 10–30 s if nucleating additives and gate geometry are appropriate. Published data for specific commercial nucleated formulations is limited; direct comparison of crystallization half-time requires identical resin backbone and additive package.

Isothermal DSC crystallization studies indicate that the crystallization half-time for POM passes through a minimum near 120–150 °C. Below 100 °C the half-time increases because chain transport slows; above 160 °C it increases because thermodynamic driving force declines. The practical consequence is that mold temperature must be controlled within a narrow window to balance cycle time against crystallinity-dependent shrinkage and impact resistance. POM grades with low residual comonomer content crystallize faster than high-comonomer copolymers, but the datasheet rarely provides crystallization half-time; this parameter is often available only in technical reports or upon request.

Acid-releasing polymers must be purged thoroughly from the barrel and hot runner before POM is introduced. Polyvinyl chloride or halogenated flame-retardant residues release acidic species that catalyze formaldehyde generation and chain scission, producing splay, mold deposits, and viscosity loss. A barrel transition from PVC to POM should include a mechanical purge with polyolefin at POM processing temperatures followed by a short POM purge. Published data for this specific purging configuration is limited; direct temperature monitoring at the nozzle and vent is required to avoid local overheating above 230 °C.