Polyoxymethylene (POM) is specified for hollow machined components when stiffness, dimensional stability in humid air, and low friction are required without fibre reinforcement. The material is supplied as extruded tube, extruded solid bar, or compression-moulded billet; hollow parts are produced either by free extrusion of the tube or by drilling and reaming a solid bar. Each route imposes different constraints on material yield, internal stress, and the geometrical tolerance chain from stock shape to finished part. Material classification for POM is defined in ISO 9988-1 and ASTM D6778.

Extruded Polyoxymethylene Tube or Bored Bar? Cost and Tolerance Compared

At the centre of the comparison is the waste fraction. Extruded POM tube is a near-net shape; the bore is generated during the extrusion process before the polymer quenches. Bored bar is a subtractive route; the bore is generated by removing a cylindrical core from a solid cross-section. The cost implications are therefore nonlinear in the ratio of inside diameter to outside diameter.

Stock removal economics in extruded POM tube versus bored bar

Production extrusion of POM tube is carried out on single-screw machines with a grooved feed zone and an L/D ratio of 30:1. The melt is maintained at 190–210 °C, filtered, passed through a spider or spiral die, and then cooled in a vacuum calibration sleeve. The haul-off speed and internal air pressure establish the final ID and wall thickness. Material yield is high once the line is stable; start-up purge and saw kerf can keep losses below 5% on long runs. The main economic barrier to extrusion is the die and calibrator tooling amortised over the batch; for non-standard OD/ID combinations, the order length must be large enough to absorb that fixed cost.

Bored bar economics follow the removed cross-sectional area. For a solid POM rod of outside diameter D bored to inside diameter d, the removed volume fraction is (d/D)². A thick-walled configuration of 50 mm OD and 25 mm ID removes 25% of the stock volume, while a thin-walled configuration of 50 mm OD and 40 mm ID removes 64%. The resulting chip stream is not routinely re-pelletised in the same production environment because reprocessing acetal resin without dedicated venting and stabiliser adjustment can generate formaldehyde; therefore, the removed material represents a disposal or low-value regrind stream.

Machining of POM is performed on automatic lathes or machining centres with sharp carbide or polycrystalline diamond-tipped tools, high surface speeds, and flood coolant or external air blast. The heat deflection temperature of POM under a load of 1.8 MPa is approximately 95–110 °C; uncontrolled temperature rise at the tool interface leads to local softening, smear, or dimensional overshoot. Machine-hour rates, setup time, and coolant maintenance dominate small-batch cost. When only a small number of hollow parts is required, the bored bar usually avoids the die amortisation and minimum-order quantity of extrusion.

Tolerance capability differs by location on the part. An extruded POM tube has an as-formed OD and ID controlled by the external calibrator and internal air or mandrel support. Wall thickness is the difference of two independently controlled surfaces, so wall variation is not the same as OD tolerance. Suppliers often state OD tolerances as a function of diameter; a typical band for small tubing may be ±0.05 mm on diameters up to 6 mm, widening to ±0.30 mm above 50 mm, though the exact values are supplier-specific and are not a universal ISO tolerance class. ID control is generally looser than OD control when the tube is free-form extruded and internally air-calibrated, because the ID is not positively sized by a fixed mandrel. Boring reverses this hierarchy: the reamed bore is the controlled feature, while the stock OD remains in the as-extruded condition unless a subsequent turning pass is specified.

On a CNC lathe with flood coolant and a reamer intended for plastics, a POM bore can be held to a total diameter tolerance of ±0.02 mm on diameters below 20 mm, and approximately ±0.03 mm to ±0.05 mm on diameters between 20 mm and 80 mm. These values correspond approximately to IT7 to IT8 per ISO 286-1, but thermal effects can shift the actual size after the part returns to room temperature. Because the coefficient of linear thermal expansion of POM is approximately 1.1×10^-4 K^-1, a temperature rise of 5 °C in a 50 mm bore changes the dimension by 0.0275 mm. Consequently, in-process measurement must reference a defined stabilisation temperature, or the machined geometry must be compensated for the difference between cutting temperature and inspection temperature.

When solid-bar core stress outweighs extrusion calibration cost

For a component with an OD above 80 mm and an ID below 20 mm, the removed volume fraction is 6.25%, so boring from a stock bar is economically rational even if the bar contains a central core anomaly; the boring operation removes that zone. Extrusion of a thick-walled tube with such a small ID relative to OD may be impractical because the internal diameter may close during calibration or the die gap required for that wall section may produce high pressure and melt fracture. Conversely, for a thin-walled component with 60 mm OD and 50 mm ID, the removed volume fraction from boring is 69.4%, and extrusion is the lower-cost route at production volumes.

Residual stress state is the most frequently overlooked differentiator. A solid POM bar greater than approximately 60 mm in diameter can develop a central zone of microvoids or tensile stress because the outer layers quench first and constrain the slower-cooling core. When a hollow component is bored from the bar, the boring operation removes the highest-stress central region, which often improves long-term dimensional stability. However, the act of boring also unbalances the residual stress field, and out-of-round distortion may appear within hours or days if the bar was not stress-relieved before machining. Extruded tube, by contrast, cools from both the outer surface and the inner surface, reducing the maximum wall thickness and producing a more uniform through-thickness stress distribution. Tube is therefore preferred for thin-walled hollow parts with long unsupported sections, while bored bar is preferred when the bore must be concentric to a machined OD and the inherent stock-roundness of the bar is acceptable.

Extrusion and machining both have processing boundaries. In extrusion, increasing the melt temperature above 220–230 °C accelerates formaldehyde generation; below approximately 180 °C, POM homopolymer may exhibit melt fracture and rough internal surfaces. The processing window is therefore narrow, and barrel residence time must be limited. In boring, feed rates that are too low may generate frictional heat and cause bore expansion during cutting; feed rates that are too high may produce torn surfaces or chip wrapping around the reamer. Sharp tools with positive cutting geometry and adequate clearance are required because POM is notch-sensitive and can present brittle chip formation if the tool edge breaks down. No single cutting parameter set applies to all diameters; published data for this specific configuration is limited to general machining guidelines.