In polyoxymethylene injection moulding, melt range, mould temperature and shrinkage are treated as a single thermal-mechanical system rather than as independent setpoints. POM is a semicrystalline acetal resin supplied in homopolymer and copolymer forms that differ in melting peak, thermal stability and crystallisation rate. The homopolymer typically exhibits a higher crystalline melting peak and requires a higher mould temperature to stabilise dimensions after ejection, while the copolymer tolerates a wider melt residence window. Processing data for unfilled injection grades are anchored to ISO 9988-2 classification and ISO 11357-3 thermal analysis. The melt temperature applied at the nozzle must remain above the crystalline melting peak to eliminate unmelted spherulitic residues, but below the threshold at which the acetal backbone begins to unzip into formaldehyde. Because solidification in the cavity occurs under pressure, the final shrinkage is governed not only by POM grade but also by gate geometry, packing pressure and cavity wall temperature.

How Does Melt Temperature Interact With POM Homopolymer and Copolymer Grades?

Melting peaks measured under ISO 11357-3 at 10 K/min heating rate typically fall between 175 °C and 183 °C for homopolymer and 162 °C and 173 °C for copolymer. Industrial supplier data sheets therefore specify the melt processing range for injection moulding at 200–230 °C for homopolymer and 190–230 °C for copolymer. The lower boundary is not an arbitrary margin; a melt temperature only 10 K above the peak can leave ordered domains that act as flow obstructions in thin-wall sections and can reduce impact resistance at knit lines. A nozzle setpoint of 210–220 °C is common for unfilled POM-C, while unfilled POM-H is often processed at 215–225 °C to control viscosity without approaching the upper degradation limit. Above 240 °C, residence time must be sharply reduced because thermal unzipping accelerates and produces formaldehyde gas that appears as splay, mould deposits and odour.

Machines with 18:1–22:1 L/D screws and compression ratios between 2.4:1 and 2.8:1 are recommended for homogeneous melt preparation; a non-return valve with low dead volume is preferred. The melt temperature is measured at the nozzle, not merely by barrel setpoints, and should be verified with a probe pyrometer during trials. For unfilled POM-C, a barrel profile of 180–200 °C in the feed zone, 190–210 °C in compression, 200–220 °C in metering, and 205–220 °C at the nozzle is reported by material suppliers. Unfilled POM-H may require a nozzle temperature 5–10 K higher because of its higher melting peak. Melt-pressure readings during injection typically range from 70 MPa to 120 MPa for medium-flow grades, although the exact value depends on flow length-to-wall-thickness ratio and gate geometry. Injection speed should be set to fill the cavity in 0.5–2.0 s for thin-wall parts; slow injection allows freeze-off of the flow front and visible flow lines. Back pressure of 0.5–1.5 MPa is applied to homogenise melt without excessive shear heating.

Melt flow rate under ISO 1133-1:2022 at 190 °C and 2.16 kg is used to differentiate injection grades. Published grades range from below 5 g/10 min to above 25 g/10 min, with high-flow grades generally reserved for thin-wall parts. The melt flow rate value is not by itself sufficient for process control; a high-flow grade may still require a high mould temperature to prevent brittle weld lines and post-mould growth.

Residence time at the upper end of the melt range is the main source of production-scale POM degradation. In hot-runner systems with high internal volume, a residence time above 10–15 min at 230 °C can produce visible silver streaking and a drop in melt viscosity. Thermal degradation is accelerated by contact with acidic species; POM should not be processed in barrels previously used for PVC unless thoroughly purged, and halogenated flame retardants or acid-containing masterbatches are incompatible. Hot-runner manifold temperatures should not exceed 230 °C and should be switched off during interruptions. A low-density polyethylene purge is recommended before shutdown because it displaces POM melt from the barrel at temperatures below the rapid degradation threshold. Accumulation of formaldehyde is a workplace exposure concern; local exhaust ventilation is required under national occupational exposure limits and machine surfaces should be cleaned to prevent mould deposits.

Although POM is not markedly hygroscopic, surface condensation at relative humidity above 60 % can produce splay on moulded surfaces. Drying at 80–90 °C for 2–4 h is therefore applied when containers have been opened in humid conditions. Clean sprue and runner regrind is generally usable at 20–30 % addition, but each heat history shifts melt viscosity and narrows the melt range; higher regrind fractions should be avoided in parts with thin ribs, living hinges or tight dimensional tolerances.

Cavity Wall Temperature Control and Weld-Line Performance

Unfilled POM-H typically requires mould temperatures of 90–120 °C; unfilled POM-C is generally processed between 60 °C and 95 °C, though close-tolerance parts often use 80–100 °C to reduce post-mould growth and improve surface finish. The cavity wall temperature must be distinguished from the setpoint on the mould temperature control unit; heat-transfer oil or pressurised water at flow velocities above 2 m/s is used for tight cavity-temperature uniformity. A temperature spread of more than ±5 °C across the cavity can shift local solidification rates enough to produce differential shrinkage, warpage and inconsistent weld lines. Low mould temperatures below 60 °C freeze the skin before packing is complete, which lowers weld-line tensile strength and increases visible sink marks over ribs and bosses. Weld-line tensile strength in POM-C moulded at 60 °C can be substantially lower than at 100 °C; validation is performed with ISO 527-2 Type 1A tensile specimens containing a knit line.

POM exhibits a sharp crystallisation exotherm during cooling; if the cavity wall is too cold, the spherulitic structure freezes with a high level of disordered amorphous content at the surface, which later densifies and produces post-mould shrinkage. Mould temperatures of 90–120 °C keep the cavity wall above the effective crystallisation temperature long enough for the packing pressure to continue feeding the part. For unfilled copolymer, the lower mould-temperature limit can be 60 °C, but parts moulded at this boundary often require post-mould annealing at 80–100 °C for 30 min per 2 mm wall thickness to stabilise dimensions. High mould temperatures above 120 °C can extend cooling time and may cause ejection deformation if the part is not sufficiently rigid; mould release and draft angles above 0.5° are then required.

Under ISO 294-4, shrinkage in POM is not a single tooling constant; it is a function of melt temperature, mould temperature, packing pressure, gate geometry and filler content. Linear mould shrinkage of unfilled POM is typically 1.8–2.5 % for copolymer and 1.8–2.6 % for homopolymer when measured on 60 mm × 60 mm × 2 mm plaques. ASTM D955-21 is sometimes used in North American tooling workflows; the two methods are not directly interchangeable because specimen geometry and measurement timing differ. The cavity should be cut oversized by the shrinkage factor, but the factor must be applied differently for flow-direction and cross-flow-direction dimensions when gate orientation is not central. For a nominal mould shrinkage of 2.0 %, a 100.00 mm part dimension requires a cavity dimension of 102.04 mm, not 102.00 mm.

Post-mould shrinkage in POM continues after ejection as crystallinity increases and residual stress relaxes; annealing at 80–120 °C can stabilise dimensions but may add a small post-anneal dimensional change. Packing pressure has a stronger influence on sink and final shrinkage than melt temperature within the acceptable melt band, so holding pressure and holding time should be documented during process validation. Shrinkage is also anisotropic: a centre-gated plaque may show lower shrinkage along flow near the gate and higher shrinkage near the end of fill. Published data for actual tooling should therefore be derived from mould-filling simulation validated with a trial shot, rather than from a single datasheet value.

When Glass-Fibre Reinforcement Lowers Bulk Shrinkage but Widens Flow-to-Cross-Flow Difference

Glass-fibre-reinforced POM grades, commonly at 25 wt% or 30 wt% glass, reduce bulk linear mould shrinkage to approximately 0.4–0.9 % when measured by ISO 294-4. The reduction is not isotropic; fibres align with flow and restrict shrinkage parallel to orientation, while transverse shrinkage remains higher because the matrix contracts without the same fibre restraint. Tooling compensation for glass-filled POM therefore requires separate flow-direction and cross-flow-direction factors. Mould temperature for reinforced POM is typically raised to 100–120 °C to reduce skin freezing and to improve fibre wetting; this partially offsets the higher thermal conductivity of glass-filled compounds. Vent depths should be kept at 0.01–0.02 mm because glass-filled POM produces more volatile organic compounds and glass fibres can block deep vents.

Glass-fibre reinforcement also increases melt viscosity and requires higher injection pressures; typical unreinforced POM-C injection pressures of 70–120 MPa may increase to 100–150 MPa for 30 % glass-filled grades. Injection units should be specified with hardened screws and barrels because glass fibre abrasion can increase screw clearance and reduce melt quality over production runs. Mould wear is accelerated; tool steels hardened to 50 HRC or equivalent are specified for glass-filled POM moulds. Gate size and location require review; restrictive gates can increase fibre breakage and generate anisotropic surface gloss. The flow-direction shrinkage for 25 % glass-filled POM can be 0.4–0.7 %, while cross-flow can be 0.8–1.2 %, producing a directional spread that must be compensated separately. Published data for a specific gating configuration should be obtained from the compounder because the shrinkage difference between flow and cross-flow directions can be greater than 2:1 in some glass-filled POM grades.