Moving a Polyoxymethylene Part from PA66 or PBT: Tooling and Process Changes is primarily a dimensional and thermal-management task. The three polymers are semicrystalline, but they diverge in mold shrinkage, moisture uptake, melt temperature, and degradation behavior. POM copolymer, for example, typically shows a mold shrinkage of 1.8%–2.2% measured after 24 h at 23 °C in accordance with ISO 294-4, while PBT and PA66 datasheets often list higher or lower bands depending on wall thickness, glass fiber content, and flow orientation. The change from PA66 or PBT to POM therefore affects cavity steel dimensions, gate freeze behavior, vent depth, drying parameters, mold temperature, screw selection, and post-mold inspection. A PA66 component conditioned to equilibrium at 50% RH loses tensile modulus and gains toughness relative to its dry as-molded state under ISO 527-1/-2; POM does not require moisture conditioning and delivers more stable dimensions in humid service. These differences mean that reusing the same mold without engineering review produces a part that may be dimensionally nonconforming to the original drawing, even if the part fills and ejects.

What Shrinkage Difference Should a Toolmaker Expect When Reusing PA66 or PBT Cavity Steel?

Mold shrinkage measured after 24 h per ISO 294-4 is the first tooling variable to address. Unfilled PA66 typically shows a mold shrinkage range of 0.8%–2.0%; unfilled PBT typically shows 1.5%–2.5%; unfilled POM homopolymer and copolymer fall near 1.8%–2.5% and 1.8%–2.2% respectively. These broad ranges depend on wall thickness, gate location, melt temperature, mold temperature, and hold pressure. If a cavity for a nominal 20.000 mm PA66 feature was cut using a 1.5% shrinkage allowance, the cavity dimension is approximately 20.305 mm. Running POM at 2.2% shrinkage in the same cavity yields a part feature near 19.86 mm. This is not a toolmaking error; it is a material substitution consequence that requires cavity steel adjustment for any drawing tolerance tighter than about 0.05 mm. For broader tolerances, process shifts in mold temperature and hold pressure may compensate, but only after dimensional capability studies on the actual production press.

These are not specification limits; they are typical datasheet ranges for unfilled injection-molding grades. Grade-specific values must be obtained from the resin manufacturer and revalidated on the production tool.

Gate Land Thickness, Runner Diameter, and Shear Heating in POM

POM solidifies rapidly because of high crystallinity; gates that worked in PA66 or PBT may freeze too early or generate excessive shear heating. A POM gate land length is typically 0.5–1.0 mm for wall thicknesses near 2.0 mm, whereas PA66/PBT tools often have land lengths above 1.2 mm. The shorter land reduces flow resistance and avoids premature gate freeze before the hold-pressure phase is complete. Undersized gates, however, are not a universal remedy. POM is shear thinning, and a gate diameter reduced without mold-filling analysis can push shear rates above 100,000 s⁻¹, increasing melt temperature locally and initiating degradation. Gate shear stress should be checked with simulation using melt viscosity data at 190–210 °C and shear rates from 100 s⁻¹ to 10,000 s⁻¹. Cold runner systems should be full-round rather than trapezoidal, with a cold slug well at each runner terminus. Hot runner drops that contain stagnant zones, internal dead pockets, or unheated tips are high-risk because POM degradation products can accumulate and generate black specks or formaldehyde odor. If a hot runner used on PA66/PBT is retained, it must be purged completely and mapped with thermocouples to confirm that no zone exceeds 215 °C during steady cycling.

When Barrel Residence Time Exceeds Five Minutes at Melt Temperature

Barrel residence time is a critical process boundary in POM conversion because the polymer degrades by formaldehyde evolution when retained at melt temperature too long. A practical operating rule for unfilled POM is to keep total residence time below 5 min at melt temperatures above 190 °C, and preferably below 3 min for fast cycles. The barrel profile should be reduced from PA66/PBT levels to a rear zone of 175–195 °C, center zone of 185–205 °C, front zone of 190–210 °C, and nozzle of 190–210 °C. Melt temperature measured with an invasive pyrometer should not exceed 220 °C for most homopolymer grades and 210 °C for some low-melt copolymer grades. A thermocouple calibration error of only +5 °C can push the hot-runner or nozzle boundary above 230 °C, where degradation accelerates. The screw should have a compression ratio of 2.0:1–2.5:1 for POM, compared with the 2.5:1–3.5:1 general-purpose screws often used for PA66/PBT. Lower compression reduces shear heating. Check ring and nozzle assemblies must be free of sharp corners and dead volumes; any holdup volume can retain material through multiple cycles and cause intermittent discolored product. On a closed-loop injection molding machine with a 20:1 L/D barrier screw, shot size should remain between 20% and 80% of barrel capacity. If the shot size is below 20% of barrel capacity, residence time is usually too long for POM; a smaller injection unit should be used. When transitioning from PA66 or PBT, the barrel and hot runner should be purged with a thermally stable purging compound or a low-melt polyolefin before introducing POM. Do not use PA66 or PBT purge leftovers as a transition if they require temperatures above 240 °C.

Drying requirements change substantially because POM is not hygroscopic to the same degree as PA66. Water absorption of POM after 24 h immersion is about 0.2–0.3% per ISO 62, compared with 1.0–1.5% for PA66 and 0.06–0.1% for PBT. POM normally does not require pre-drying when stored at ambient conditions below 60% RH. If condensation is present or if the material was exposed to outdoor humidity, a desiccant dryer set at 80 °C for 2–4 h with a dew point of no more than −40 °C is sufficient. PA66 cannot be transferred directly to POM settings because PA66 requires drying to below 0.20% moisture content and PBT to below 0.05%. The existing hopper dryer, if left at PBT settings above 110 °C, can bridge or melt POM granules and create feed starvation. Reset the dryer thermostat and verify dew point before starting production.

Venting Capacity Must Be Reworked to Prevent Flash and Formaldehyde Trap

Venting must be reworked because POM has lower melt viscosity at high shear than many filled PA66/PBT grades and can flash through vents that were safe in the original material. A typical vent depth for PA66/PBT molds ranges from 0.010–0.030 mm; for POM the depth is usually limited to 0.005–0.015 mm, with a land length of 0.5–1.5 mm before the atmosphere. Vent locations at the last fill point, ribs, bosses, and weld lines should be added or widened if the mold originally used minimal venting. POM degradation produces formaldehyde, and localized gas trapping can create burn marks or surface splay. The molding cell should have local exhaust ventilation at the mold open position and around the purging station. In high-volume production, a mold vacuum system connected to the runner and cavity vents may be required to evacuate degradation products and reduce deposit formation on the parting line. Formaldehyde oxidation can form formic acid in the presence of moisture; vent surfaces and parting lines may show staining over long runs. Corrosion-resistant tool steel or hard chrome plating is used for cavitation layers when running POM at mold temperatures above 90 °C. Published data for the exact vacuum flow rate required in a given tool is limited; it must be evaluated on the production machine with the actual grade and cycle.

Mold temperature control is often the largest process change when moving from PBT or PA66 to POM. PA66 molds typically operate from 40–80 °C, and PBT molds from 25–80 °C; POM requires 60–120 °C, with 80–100 °C preferred for dimensional stability and low warpage. The higher mold temperature increases crystallization and can improve crack resistance, but it also increases cooling time. If the tool was connected to a water temperature control unit rated only to 90 °C, a pressurized water unit or oil heater rated to 120 °C may be required. Parallel cooling circuits with independent control for the cavity and core are preferable; the core may run 5–10 °C cooler than the cavity to encourage part retention on the core. Insulation plates between the mold and machine platens can reduce heat loss and stabilize the thermal condition. Cooling channels that are too far from the cavity surface or that are placed unevenly will create warping because POM crystallizes rapidly. The tool should be evaluated with a thermal camera or mold-mounted thermocouples to confirm a surface temperature variation below ±5 °C across the forming surfaces after 20 min of steady cycling.

Back pressure and screw speed must also be reset. POM melts under lower compression than PA66/PBT; excessive back pressure above 2 MPa or screw speed above 100 rpm can shear-heat the melt and produce formaldehyde odor. Injection velocity should be set from a short-shot study to fill 90% of the cavity in 0.5–2 s for thin walls below 2.0 mm. Hold pressure must be high enough to maintain a gate seal time greater than the freeze time; gate seal can be confirmed by weighing molded parts after increasing hold time in increments of 0.5 s until part weight stabilizes. If a PA66/PBT process used long hold times and low injection speeds, POM may require a faster injection and a shorter, more intense holding profile. These conditions are machine-specific and should be derived by design of experiments rather than copied from a previous resin.

Snap-Fit and Gear Load Cases Under POM’s Notch Sensitivity

Mechanical loading after conversion cannot be assumed from PA66 or PBT data. POM has a lower notched impact strength than conditioned PA66 and is notch sensitive; snap-fits that survived in PA66 may fracture in POM if the retention hook is sharp. Redesign the snap-fit root radius and reduce strain at the hook to stay below the POM grade’s allowable strain in an ISO 527-1/-2 tensile test. For gear and sliding applications, POM offers low friction and good dimensional stability in humid environments because it absorbs little water compared with PA66. However, POM is incompatible with strong acids, concentrated alkaline solutions, and oxidizing media, including some chlorine-based cleaning agents. If the predecessor PA66 or PBT part was exposed to such media, chemical resistance must be revalidated per ISO 22088-3 environmental stress cracking or by immersion testing under representative service conditions. Published data for the specific interaction of every cleaning fluid with POM is limited; a grade-specific chemical resistance table from the resin supplier is required.

Post-mold conditioning and inspection differ. PA66 parts are often moisture conditioned to reach equilibrium at 50% RH before measuring mechanical properties; PBT parts may need little conditioning. POM parts can be measured for dimensional conformity after 24 h per ISO 294-4, but close-tolerance parts may require annealing at 80–100 °C for 1–2 h in an air-convection oven to relax molded-in stress and stabilize dimensions. Annealing can increase shrinkage slightly; parts should be measured before and after annealing to establish the actual post-anneal dimension. Existing inspection fixtures and gages created for PA66 or PBT parts may not match POM parts because thermal expansion coefficient differs. Unfilled POM CLTE is about 110–130 ×10⁻⁶ K⁻¹ when measured by ISO 11359-2, while PA66 and PBT are often in the 60–110 ×10⁻⁶ K⁻¹ range. Clearance fits and bearing journals must be recalculated for the maximum and minimum service temperatures, not for the injection-molding shop floor temperature alone.