Inspecting POM Tube and Hollow Bar on Receipt: Bore, Ovality and Colour

Incoming inspection of polyoxymethylene tube and hollow bar on receipt functions as a process-control gate rather than a simple administrative check. The three measured attributes—bore, ovality and colour—are linked through continuous extrusion history: a drifting melt-temperature profile, uneven cooling in the sizing tank or a change in regrind ratio can alter internal diameter, out-of-roundness and optical appearance without necessarily raising a batch-level dimensional flag. A useful receiving protocol therefore records the measured values, the instrument identification, the conditioning state, the supplier lot number and the certificate reference before the stock enters the cutting cell. Material identified only as “acetal” without a grade symbol is not accepted; the markings and certificate are checked against ISO 1043-1 and ISO 9988-1 for POM-H or POM-C designation, viscosity number and intended processing method.

The receiving inspection of polyoxymethylene tube and hollow bar follows the same dimensional metrology principles used for metal bar stock, but the procedure must be modified for a semi-crystalline polymer. POM is supplied as homopolymer or copolymer grades under ISO 9988-1, with extruded tube and hollow bar typically produced on single-screw extruders with L/D ratios of 20:1 to 30:1 and melt temperatures controlled between 180 °C and 220 °C. Because the extruded form retains a frozen-in thermal history, the first inspection step is not measurement but conditioning. Dimensional checks performed on cold, unaged material from a truck dock will differ from checks after 24 h at 23 °C due to thermal contraction and stress relaxation. The receiving area therefore has a quarantine shelf separated from the main warehouse, where POM tube and hollow bar is placed in a single layer on flat supports until temperature equilibration is complete.

Does Bore Measurement Detect Processing History or Only Machining Allowance?

Bore inspection on POM hollow bar is performed for two distinct purposes: to confirm that the supplied internal diameter leaves sufficient reaming or boring allowance, and to detect processing instability that would not be visible on the outside diameter. POM is a semi-crystalline engineering plastic with a linear coefficient of thermal expansion of approximately 1.0 × 10⁻⁴ K⁻¹ to 1.2 × 10⁻⁴ K⁻¹ measured by ISO 11359-2. Consequently, measurement temperature is not a secondary concern. A 5 K difference during hand measurement of a 40 mm bore introduces an apparent dimensional change of about 0.022 mm, which is close to a commercial H9 tolerance band for smaller diameters. The incoming stock is therefore conditioned at 23 °C ± 2 °C and 50 % ± 10 % RH for at least 24 h in accordance with ISO 291 before any variable measurement is recorded. Calipers and spring-loaded bore gauges are not acceptable for final acceptance of precision bore; a three-point internal micrometer with anvil sizes matched to the expected diameter and a resolution of 0.001 mm is used for bores up to 40 mm. Above 40 mm, a calibrated internal micrometer or bore gauge with a setting ring is employed. Because POM has a Shore D hardness of approximately 80–85 per ISO 868, uncontrolled anvil pressure from a heavy bore gauge can deform the surface by several micrometres and lead to false undersize readings. The inspector records the gauge load or uses a controlled-force instrument when the measured values fall within the tolerance boundary.

For dimensional acceptance, the bore callout must specify whether it is as-extruded or machined. A finished bore of 25.0 mm on a machined POM bushing is commonly toleranced per ISO 286-1 as H9, which gives a lower deviation of 0.000 mm and an upper deviation of +0.052 mm. An as-extruded hollow bar bore is usually specified to a wider class such as H11 or H12 because the melt yields at the calibrating mandrel and subsequent shrink vary along the bar. ISO 3126 provides the dimensional measurement framework for plastics piping components; the same cross-sectional logic is applied to hollow bar even when the material is not intended for fluid transport. The receiving inspector checks internal diameter at a minimum of three axial positions, typically one at each end and one at mid-length, and at two orthogonal angular positions per plane. When a high-volume inspection is required, a go/no-go plug gauge set is used to sort bore quickly, with variable measurement reserved for borderline lots. A no-go gauge must not be forced, because POM is not an elastomer and repeated compressive deformation can mask an undersize condition.

The bore measurement system is calibrated before each shift using a setting ring of known diameter. A calibrated setting ring has a certified roundness and diameter traceable to national standards. The difference between the setting ring and the reading of the internal micrometer is recorded as the instrument offset. For a 25 mm H9 bore with a tolerance band of 0.052 mm, the measurement uncertainty from a hand-held three-point micrometer can be 0.005–0.010 mm if the operator is trained. This consumes a substantial portion of the tolerance band, so go/no-go gauges are preferred for high-volume decisions. The receiving protocol disallows the use of a two-jaw vernier caliper for internal diameter acceptance because the method cannot reliably separate bore from ovality.

Examination of the bore surface should occur at the same time as dimensional measurement. Chatter marks, heavy longitudinal scoring or concentric drag lines on the as-extruded bore can indicate a fouled mandrel or inadequate melt conditioning during extrusion. These features are not necessarily dimensional faults, but they are recorded because they can create stress concentrations and complicate subsequent machining. For a 25 mm bore with a 5 mm wall, a bore error of 0.5 mm is equivalent to 10 % of the nominal wall lost on one side, which is sufficient to turn a balanced machining job into a thin-wall instability in a lathe collet. Bore inspection therefore cannot be reduced to average diameter; the worst-side wall thickness must be estimated when ovality is present.

The actual bore of POM hollow bar is produced either by a fixed mandrel during extrusion or by a secondary drilling or reaming operation. Extruded bores are less consistent than reamed bores because they depend on melt drawdown, calibrator vacuum and shrink around the mandrel. The purchasing specification should state whether the incoming stock is as-extruded or pre-machined. If it is pre-machined, the bore tolerance is tighter and the inspector expects the absence of extrusion draw marks. In a production-scale failure mode, a worn sizing mandrel can produce a bore that slowly drifts from the lower to the upper tolerance over a run, while the outside diameter remains stable. Receiving inspection cannot see the middle of the production run, but a three-point axial check will detect that the ends differ. A longitudinal bore difference greater than 0.05 mm per 300 mm on a 25 mm nominal bore is treated as non-uniform extrusion taper. The stock is then checked with a plug gauge before release to CNC lathes.

Ovality measurement on POM hollow bar is not simply a two-point diameter subtraction. The internal profile of an extruded hollow bar can be elliptical, lobed or distorted under residual stress. Ovality is calculated as 100 × (Dmax − Dmin) / Dnominal at a single cross-section. The inspector obtains internal diameters at 0°, 45°, 90° and 135° with a two-point internal micrometer; if a three-point micrometer is used alone, odd-lobed conditions can be under-reported. The maximum and minimum values are compared at the same axial plane. For critical components, a coordinate measuring machine with a scanning probe records 200–500 points per circumference and computes roundness according to ISO 12181-1. On the shop floor, a simple stress-relief indicator is to measure the bore before and after a 25 mm ring is parted off. A change greater than 0.05 mm on a 50 mm bore indicates that machining may release residual stress asymmetrically, even when the stock meets ovality tolerance as delivered.

Acceptance limits for ovality depend on whether the bore is as-extruded or finished. As-extruded hollow bar is commonly accepted at ≤ 1.0 % of nominal internal diameter, while machined or reamed stock is typically held to ≤ 0.5 %. Published data for this specific configuration is limited; many supply contracts set ovality limits in the purchase specification because no single general standard defines it for all POM semi-finished products. For rotating parts produced directly from hollow bar, ovality above 0.3 % may create dynamic imbalance and collet-clamping unevenness. The inspector should not compensate for ovality by averaging two diameters; averaging conceals the stress state and the machinability risk. In vacuum-sizing extrusion of POM, ovality can arise from uneven water-spray cooling, eccentric mandrel position, asymmetric haul-off force or feed-temperature variations. A lot that shows consistent ovality within tolerance but high side-to-side wall variation should be quarantined because later facing cuts may produce non-uniform relaxation.

For thin-walled POM tube, outside-diameter ovality and inside-diameter ovality do not always follow the same orientation. A tube may have a round outside surface and an eccentrically oval bore because of mandrel drift. The receiving inspector therefore measures both outside-diameter and inside-diameter ovality and compares the wall-thickness map. Wall thickness is measured with an ultrasonic thickness gauge or by differential bore-OD measurement at matched angles. Ultrasonic gauges for POM should be calibrated on the same grade and at the same temperature because the sound velocity changes with filler content and temperature. A thickness variation of more than 0.1 mm on a nominal 5 mm wall is flagged for machining review. This is highly relevant for bearing and roller applications where bore runout after machining is a function of as-received stress symmetry, not just initial roundness.

Colour Shift, Degradation Pathways and Acceptance Limits

Colour inspection on receipt of POM tube and hollow bar starts with a visual observation under a controlled light booth at D65 illumination and not under mixed factory lighting. Visual inspection alone is insufficient because minor yellowing of natural POM and gloss differences can appear as colour variation. Instrumental measurement is performed with a benchtop spectrophotometer using d/8° geometry, specular included and UV excluded, reporting CIE L*a*b* coordinates according to ISO 11664-4. The acceptance window is based on ΔE*ab from an approved master plaque or cut reference; a common incoming limit is ΔE*ab ≤ 1.5 for natural and black stock, tightened to ≤ 1.0 where adjacent parts are visible. A positive Δb* shift greater than 0.8 is treated as a suspect indicator, because POM yellows during oxidative degradation. Yellowness index is evaluated according to ASTM E313; the protocol records the lot-to-lot shift rather than relying only on a pass/fail threshold.

Natural POM homopolymer is milky white and POM copolymer may have a faint blue-white cast; black POM is usually coloured with carbon black for ultraviolet stability and electrical conductivity. The failure modes associated with colour shift in POM are not cosmetic only. Excessive melt temperature above 230 °C, prolonged residence time, poor venting or aggressive regrind can initiate chain unzipping and formaldehyde evolution, producing yellow or brown oxidation products. If yellowing is combined with a sharp formaldehyde odour, the lot is quarantined and sampled for melt flow-rate verification. Conversely, a batch of black POM that is lighter than the master can indicate insufficient carbon black dispersion or a changed pigment grade; the inspector records L* reflectance and surface streaking. A streaked black bar may still meet ΔE*ab because integrating-sphere instruments average surfaces; therefore visual inspection for streaks remains a separate pass/fail criterion. The minimum visual inspection condition is 800 lux illumination at the surface, with the bar rotated to reveal swirls and flow lines. Colour inspection of curved POM tube should be performed on flat polished coupons cut from both ends or with a cylindrical port adaptor if a non-destructive measurement is required; talc, oil and fingerprint contaminants are cleaned off with a dry lint-free cloth before measurement.

Instrumental colour measurement is conducted to ASTM D2244 for the calculation of colour differences from the CIE coordinates, and the yellowness index is reported using ASTM E313. The spectrophotometer is configured with an aperture diameter of at least 10 mm to avoid local streaks dominating the reading. If the POM stock is translucent in thin sections, the backing material behind the sample influences the measurement; a white backing plate is specified. The protocol states whether UV is included or excluded, because optical brighteners and stabilizers may show differences under UV-containing lamps that are not visible under D65 alone. Published data for this specific configuration is limited; therefore the laboratory maintains a reference set of three acceptable lots and one rejected lot to anchor the acceptance boundary.

POM is also sensitive to acids and oxidising conditions. Incoming colour shift does not distinguish between thermal oxidative degradation and contamination by a foreign polymer such as PVC, which can release hydrogen chloride and attack POM at processing temperatures. If contamination is suspected, a Fourier-transform infrared spectroscopy scan of a cut surface may reveal absorptions outside POM. However, FT-IR is not part of routine receiving because it is too slow for high-volume inspection. The escalation protocol is: yellowing plus odour → melt flow-rate; white specks or gels → microscopic contamination check; grey or brown streaks → FT-IR or differential scanning calorimetry. This sequence prevents an entire mill order from being accepted or rejected on a single colour datum.

When Acceptance Sampling Should Trigger Additional Melt-Flow Verification

Acceptance sampling for POM tube and hollow bar is normally performed according to ISO 2859-1, General Inspection Level II, with an AQL of 1.0 for bore and ovality and an AQL of 2.5 for colour depending on the application. When dimensional results are marginal or colour shift is observed, a representative sample is cut from the suspect hollow bar and submitted for melt flow-rate measurement according to ISO 1133-1:2022 at 190 °C under a 2.16 kg load. The measured MFR is compared with the supplier’s certificate for the same POM grade. A deviation of more than 20 % from the certified lot value is grounds for rejection, because POM that has undergone thermal or mechanical chain scission may still appear dimensionally acceptable. This verification is not part of routine dimensional inspection; it is an escalation step for lots that fail one or more visual or optical criteria. The receiving record includes the sampling plan, the measured values, the instrument calibration status and the certificate reference so that the lot is traceable if a later machining failure occurs.

The measured values of bore, ovality and colour should be reviewed by the production engineer before the stock is released to the machining cell. POM hollow bar is stored on flat racks or V-supports at 20 °C ± 2 °C, away from direct sunlight, radiators and local heat sources. Because POM is harder and less forgiving than polyethylene, stacked bars can dent or deform if the rack contact points are too narrow. The inspector confirms that the received stock has not been exposed to rainwater or condensation; although POM moisture uptake is low, surface water can falsify optical readings and cause micrometre contact variation. All incoming values are transferred to a central SPC chart by diameter and wall thickness; a sustained drift of bore toward the upper tolerance limit across multiple deliveries can indicate a supplier process shift even when each single lot is acceptable.