Setting Up a POM Moulding Cell: Barrel, Screw and Residence Time

Commissioning a polyoxymethylene (POM) moulding cell is not a single-temperature exercise; barrel zoning, screw dimensions and molten residence time are coupled variables that determine whether formaldehyde is generated faster than the cell can vent it. Under ISO 1043-1, acetal homopolymer is designated POM-H and acetal copolymer POM-C. Melt volume-flow rate is measured at 190 °C under 2.16 kg per ISO 1133-1:2022. The homopolymer melting endotherm is approximately 175 °C, while copolymer grades melt near 165 °C; production melt temperatures for general-purpose unfilled grades therefore sit between 190 °C and 215 °C. Above 220 °C, chain-end unzipping releases formaldehyde, and the formaldehyde can be oxidised to formic acid, which catalyses further degradation. That degradation pathway makes the barrel, screw and residence time specification an interlocking problem: a hotter front zone reduces melt viscosity and improves flow, but it also shortens the safe time window before gas generation, yellowing and surface defects appear.

Pre-drying is specified only when surface moisture exceeds approximately 0.2 wt%. POM at 23 °C and 50 % relative humidity typically absorbs no more than 0.2 wt% moisture, but condensation on cold pellets can create local hydrolysis. If drying is required, desiccant drying at 80 °C for 3–4 h with a dew point of ≤−40 °C is applied. Hopper drying should not be continued indefinitely; granule residence at 120 °C for several hours can initiate surface degradation and release formaldehyde into the feed zone.

Moulded property retention is validated using plaques or bars produced per ISO 294-1:2017 and tensile properties evaluated per ISO 527-2:2012. This anchors the set-up to a reference specimen geometry and prevents a moulding cell from being accepted solely on visual appearance.

Which Barrel Zone Set-Points and Material Limits Control Formaldehyde Build-Up?

The barrel profile for unfilled POM is set as a flat-to-rising sequence from rear to nozzle. A POM-H grade with MVR ≤4.0 cm³/10 min is run with rear zone 160–175 °C, centre zone 175–190 °C, front zone 190–210 °C and nozzle 190–205 °C. POM-C grades with MVR between 4.1 cm³/10 min and 12.0 cm³/10 min are processed 5–10 °C lower in the rear and centre zones. No zone is set above 230 °C. This ceiling is fixed by degradation rate, not merely melt temperature; formaldehyde evolution accelerates sharply above 220 °C, and the resulting acidic carbonyl residues attack the barrel surface when residence time is also extended.

Heater-band control is a critical equipment variable. A heater band with oscillation of ±3 °C can push the front-zone surface toward 213 °C when the set point is 210 °C, while local adiabatic heating at the screw tip may add another 5–10 °C. Production cells require PID autotuning, thermocouples seated in the band groove with positive contact, and a verified nozzle heater circuit. The nozzle is controlled independently because a cold nozzle freezes the sprue and raises injection pressure, while an overheated nozzle generates drool and exposes melt to additional air oxidation. A reverse-taper nozzle or shut-off nozzle is permitted only if internal flow channels are polished and free of dead spots.

The following set-point ranges apply to a general-purpose POM-C; POM-H grades are operated in the upper half of each band because their higher melting point and lower comonomer content require more energy input.

Barrel and screw surfaces must tolerate acidic carbonyl degradation products. Plain carbon steel is not sufficient for continuous production; bimetallic barrel liners and chrome-plated screw surfaces reduce iron-catalysed surface degradation and progressive pitting. Production experience demonstrates that rear-zone heater failure creates a cold slug that can stall the screw and overtorque the drive, while a failed centre band can send unmelted POM to the non-return valve and block forward flow. Melt temperature is verified by an air shot at low screw speed; a needle thermocouple inserted into the purged melt reads 195–215 °C depending on grade. If the air-shot melt exceeds 220 °C, the front-zone set point or screw speed is reduced before the cycle is started.

A low-compression, chrome-plated screw with an L/D ratio of 18:1 to 22:1 and a compression ratio of 2.5:1 to 3.5:1 is specified for unfilled POM. The screw is a general-purpose, gradual-transition design rather than a high-shear polyolefin screw, because excessive viscous heating can raise local melt temperature into the formaldehyde-generation range. The feed-section flight depth is sufficient to convey rigid granules without grinding, and the metering section is polished to a roughness below 0.2 µm Ra to prevent stagnant melt adhesion. For a 30 mm screw, the metering depth is checked against the machine manufacturer’s plastication chart; a depth that is too shallow creates shear heating, while a depth that is too deep reduces melting uniformity.

A ring-type non-return valve with a hardened seat is preferred over a ball check because ball valves create recirculation zones that hold melt longer than the mean barrel residence time. The check ring must close with a full circumferential seal; a worn ring leaks during injection, reducing shot weight and shifting the residence distribution of the next shot. Screw surface hard-chrome thickness is normally 0.025–0.05 mm, and the base metal is protected from acidic carbonyl exposure at any exposed land or flight edge. For glass-filled POM, the screw and barrel are upgraded to a bimetallic barrel liner and surface-hardened screw flights because short glass fibre abrasion of the check ring rapidly changes shot weight.

Screw speed for a 25–40 mm screw is normally set between 40 rpm and 80 rpm, with back pressure held at 0.2–0.5 MPa hydraulic. High screw speed combined with high back pressure introduces unnecessary shear heating; low back pressure can leave gas absorbed in the melt and produce splay. Decompression after recovery is limited to 2–5 mm to avoid pulling air into the melt stream. Injection is run at moderate to high velocity to prevent premature freezing in thin walls. Mould filling pressure is usually 70–100 MPa, with hold pressure 50–80 MPa depending on gate geometry. If the gate is too small, high melt velocity through the restriction produces shear heating above 220 °C and causes gas streaks at the gate.

The mould surface is held at 60–100 °C; mould temperatures at the upper end of this range improve crystallinity and reduce post-mould shrinkage but extend cycle time. Close-tolerance gear and bearing parts are typically run at 90–100 °C to stabilise dimensions. These mould settings interact with residence time because a longer cooling time increases cycle time and, for a fixed shot mass, increases the average time that material remains in the barrel.

The feed throat is water-cooled to 35–50 °C to prevent pellet bridging and premature melting. Cooling must not lower the throat below the local dew point; condensation on the throat wall introduces water directly into the feed zone and promotes hydrolysis. The screw return stroke is set so that cushion remains at 2–5 mm, preventing the screw tip from contacting the nozzle and minimising hold-up at the non-return valve.

Residence Time Thresholds, Shot Utilization and Purging Sequences

Average molten residence time is calculated as the mass of POM held in the barrel divided by shot mass, multiplied by cycle time. Barrel capacity ratings are usually expressed in grams of general-purpose polystyrene; POM melt density is approximately 1.15 g/cm³ compared with 0.94 g/cm³ for GPPS at processing temperature, so the actual POM inventory is roughly 20–25 % higher for the same volumetric capacity. If this correction is ignored, the calculated residence time is understated and the process may be started closer to the degradation threshold than intended. For a barrel rated at 150 g GPPS, the POM mass inventory is approximately 180 g. With a shot mass of 75 g and a cycle of 30 s, the corrected average residence time is 72 s. If the same calculation incorrectly uses the GPPS rating, the result is 60 s, a 17 % underestimation.

Unfilled POM-C is typically kept below 15 min total residence at melt temperature; POM-H is held below 10 min at 210 °C. If cycle interruption exceeds 5 min, the process is not simply restarted. Molten POM held in a hot barrel without plastication generates formaldehyde gas, which causes nozzle leakage, splay on the next several shots, and an acid residue that can corrode the check ring. Production experience on 1200 kN toggle machines shows that shot masses below 25 % of rated barrel capacity produce intermittent splay within 10–20 cycles even when the average residence calculation is within limit, because the hot boundary layer remains in the barrel across many cycles. The shot mass is therefore targeted at 40–70 % of the rated GPPS barrel capacity, with 50–60 % preferred for unfilled grades.

Direct residence-time verification is performed by introducing a colour concentrate tracer at 0.5 wt% at the feed throat and counting the number of shots until the colour clears. The measured maximum residence time is used for set-up confirmation. If it exceeds 10 min for POM-H or 15 min for POM-C, shot mass is increased, cycle time is shortened, or a smaller barrel module is installed. Melt viscosity before and after a production interruption is compared by MVR per ISO 1133-1:2022; a shift greater than 10 % from the virgin pellet value indicates chain scission, and the barrel is purged before restart.

Before shutdown, the screw is retracted and POM is purged out with HDPE having a melt flow rate of 0.3–2.0 g/10 min at 190 °C/2.16 kg. The purge is continued until the material exiting the nozzle is uniformly white and free of POM streaks. The barrel temperatures are then reduced to 140–150 °C and the screw stopped. The barrel is never left filled with POM at production melt temperature because gas pressure generated during thermal decomposition can push molten material from the nozzle and deposit carbonised residue on the screw and non-return valve. HDPE is used because it is thermally stable at the reduced temperature and leaves no acidic residue. Chlorine-containing purge compounds and PVC are incompatible with POM because acidic chlorine species catalyse POM depolymerization.

Because POM processing continuously vents low levels of formaldehyde, the moulding cell is equipped with local exhaust capture at the nozzle and mould parting line. Capture velocities at the parting line are maintained at 0.5–1.0 m/s, with occupational exposure limits referenced to the 0.75 ppm 8-hour time-weighted average and 2 ppm short-term exposure limit. Cavity vents for unfilled POM use land lengths of 0.8–1.2 mm and depths of 0.02–0.04 mm; blocked vents replicate the effects of excessive residence time by trapping gas in the cavity and causing splay or short shots. Vent condition and residence time are therefore inspected together before any temperature adjustment is made in response to gas defects.