POM Rod Stock Comes with Core Stress: Why Boring Beats Buying Oversize

Polyoxymethylene (POM) round bar stock is supplied as extruded solid rod in diameters typically from 6 mm to 300 mm and is designated under ISO 9988-1 or ASTM D6778. The extrusion operation produces a radial thermal gradient while the semicrystalline acetal resin cools from melt temperature to handling temperature. The outer skin solidifies quickly in the cooling water or calibrating sleeve, while the core remains above the crystallization range. This mismatch creates a triaxial residual stress field consisting of compressive hoop and axial stress near the surface and tensile stress in the centre. The core stress is a bulk mechanical condition, not a surface defect. It remains in the bar until material is removed or the stock is annealed. For a component with a central bore, that condition changes the stock-selection logic: starting from an oversize solid bar and boring out the high-tensile core is more stable than buying an even larger rod and machining only the outside diameter. Consequently, boring beats buying oversize when a sleeve, gear blank, bushing, or sealing element is required.

Residual Stress Distribution in Extruded POM Round Bar

Commercial POM rod is often run on a single-screw extruder with an L/D ratio between 24:1 and 30:1, followed by a gear pump and a calibration/cooling section. For a 60 mm diameter copolymer rod, melt temperature of 195–210°C and cooling water temperature of 30–50°C are representative process settings. The skin layer, which contacts the cooled calibrator, solidifies within seconds. The core continues to cool slowly because POM has low thermal conductivity, approximately 0.23 W/(m·K) at ambient temperature. Differential solidification and crystallization shrinkage generate the characteristic stress profile. The outer layer is placed in compression as the inner material contracts later. The central region carries tensile stress. Published technical data for exact stress magnitudes in specific POM rod grades is limited, but polymer processing literature reports tensile core stresses in the range of 2–6 MPa for large-diameter acetal extrusions when measured by layer removal or hole drilling. The stress magnitude increases with rod diameter and cooling rate differential. A high-molecular-weight homopolymer grade may retain a slightly wider stress range than an easy-flow copolymer because the higher melt viscosity resists stress relaxation during the short cooling section.

The stress profile also influences apparent mechanical properties. Specimens cut from the skin of an as-extruded POM bar can show higher tensile modulus and lower elongation at break than specimens cut from the core when tested in accordance with ISO 527-2. Differential scanning calorimetry per ISO 11357-3 frequently reveals a different lamellar thickness distribution between skin and core. These gradients are not defects in the material specification; they are consequences of non-isothermal crystallization during bar production.

Residual stress also depends on line speed and cooling capacity. A 100 mm diameter rod cannot be cooled like a 20 mm diameter rod without retaining a greater tensile core gradient because the thermal path length is longer. Suppliers therefore use cooled calibration sleeves or water baths matched to diameter and throughput. Incoming inspection of bar stock rarely includes a published residual stress value; a first-article trial cut or layer-removal test is used to characterise the lot. Without this check, the workshop treats the core as an unknown mechanical preload that will be released during machining.

When a production cell turns only the outside diameter of an oversize acetal rod, the compressive skin layer is stripped while the tensile core remains largely intact. The resulting imbalance does not resolve immediately. POM is viscoelastic at ambient temperature, and residual stress redistribution occurs over hours and days. Roundness measured immediately after OD turning can pass an in-process check and then fail final inspection after 24 h. The condition is more severe above 50 mm diameter because the core volume is large enough to drive radial runout and axial bow after the part is released from the chuck. Published data for exact runout values in this non-standardised machining condition is limited, but trade literature for acetal round stock describes post-machining deviations in the range of 0.05–0.20 mm for large-diameter solid parts machined without core removal.

What Happens When an Oversize POM Rod Is Machined Without Boring?

When the central bore is not present in the finished part, machining an oversize rod down to final diameter leaves the tensile core as an internal spring. After the compressive outer shell is removed, the core strain energy is no longer constrained. The part can relax by ovalisation, lobing, or axial bow. A gear blank machined from an 80 mm diameter POM round to a 76 mm finished OD may show roundness error that exceeds the tolerance band specified for fine machining under ISO 2768-1 f if the work is completed in one clamping without core removal or intermediate stress relief. The central region of large POM rod can also contain microvoids from solidification shrinkage, which locally reduce density and promote asymmetric movement when the surrounding material is cut away.

POM rod stock is not mechanically isotropic in the as-extruded condition. Tensile specimens machined from the longitudinal direction may show yield stress in the 60–70 MPa range and elongation at break in the 15–35% range under ISO 527-2 for homopolymer, but the same grade can show lower elongation in the transverse direction because of orientation and residual stress. This anisotropy becomes important when the part is machined from a large diameter solid. If the core remains, the central zone may have higher apparent modulus along the extrusion axis than the surrounding material. Differential stiffness acts as a constrained beam, and any asymmetry in OD turning releases it unevenly.

The problem is compounded by cutting-induced stress. Even with sharp tools, single-point turning introduces a shallow surface layer of compressive stress and local heat. In a solid bar, this skin effect adds to the existing residual stress gradient. The resulting part may meet dimensional specifications immediately after unclamping and then move as the core stress relaxes. Stress relaxation is time-dependent. In an uncontrolled environment, a solid machined POM blank can continue to move for 48–72 h after machining before reaching dimensional stability. The largest dimensional change is frequently observed within the first 12 h after the compressive shell is removed, followed by slower relaxation. For this reason, a simple OD-turning operation on an oversized POM solid round should not be treated as stable unless the part is stress-relieved before finishing or the high-tensile centre is removed by boring.

When the Bore Is Opened Before the OD Is Finished, Core Stress Does Not Return

The stock-selection strategy changes when the final component requires a through hole. An oversize solid POM rod is selected with enough diameter allowance to permit rough boring of the centre before final OD and ID machining. The preferred sequence is to rough turn the OD leaving 1.0 mm oversize, then rough bore the ID leaving 0.5 mm oversize, then release the part from the chuck and allow it to relax for 12–24 h at ambient temperature. The rough boring operation removes the tensile core. The remaining annular wall has a more balanced residual stress state because both the original outer skin and the new inner surface are near-compressive. The final OD and ID cuts are then taken with small, symmetric passes. This sequence is particularly effective for sleeves, bushings, seal carriers, and gear blanks with bore-to-wall ratios above 0.5. From a stock-selection standpoint, boring beats buying oversize because the material removed from the centre is not merely scrap but the highest-risk residual stress zone.

Tooling for rough boring should use a rigid boring bar with a polished carbide insert. A worn or flexible bar produces vibration and uneven surface temperature. The bore should be opened in at least two passes for diameters above 25 mm to prevent the boring bar from drifting along the central stress gradient. The resulting swarf should be free-flowing; fine powder or melted ribbon indicates excessive heat generation, which can alter the stress state of the remaining wall.

As-extruded POM stock with a wall thickness above 25 mm often carries enough residual stress to warrant pre-annealing before finish machining. A stress-relief annealing cycle for acetal copolymer is commonly performed at 150–160°C for 1 h per 25 mm of cross-section, followed by controlled cooling to 80°C before the part is removed from the forced-air oven. Homopolymer grades may require the lower end of that temperature range to limit formaldehyde generation. Annealing near or above 170°C should be avoided for many POM grades because surface discolouration and thermal degradation can occur. Moisture conditioning per ISO 291 at 23°C and 50% relative humidity for 48 h is applied after machining because POM absorbs only about 0.2 wt% water at equilibrium. Moisture-related dimensional change is therefore minor compared with the movement caused by residual stress release.

Coolant, Feed Rate, and the Remaining Annular Wall

Boring introduces cutting heat and mechanical pressure into the remaining annular wall. If the wall is thin after core removal, the thermal gradient from machining can mimic the original extrusion gradient on a smaller scale. Internal flood coolant through the boring bar is recommended to maintain a stable bore temperature and to evacuate chips. For POM, carbide inserts are usually run at 180–250 m/min cutting speed, with a finishing feed rate of 0.15–0.25 mm/rev and a depth of cut of 0.5–1.0 mm. High-speed steel tooling should remain below 100 m/min to avoid premature edge wear. The cutting geometry should include a positive rake angle of 10–15° and a clearance angle of 7–10°. Dull or zero-rake tooling can raise the bore surface temperature above 90°C, producing uneven localized annealing and a new residual stress layer at the ID. Coolant compatibility should be checked against the POM grade; prolonged contact with strong acids or oxidising agents is not recommended. Compressed air can be adequate for small diameters and light cuts, but flood coolant is preferred when the bore length exceeds 2× the bore diameter.

Clamping forces also interact with residual stress. A bored POM sleeve with a wall thickness of 5 mm and an outside diameter of 100 mm requires soft jaws and low chuck pressure to prevent elastic clamping deformation during final boring. Standard three-jaw chucks set at high pressure can produce three-lobed deformation that remains visible after unclamping. Pneumatic or piezo-assisted chucking with reduced pressure, or a split fixture with full-circumference support, is preferred. The clamping correction must be verified by removing the part from the fixture and re-measuring roundness after 30 min because POM shows delayed recovery from clamping strain.

Dimensional verification of a bored POM part is performed after the part has been unclamped and conditioned for 24 h in the reference atmosphere of ISO 291. Roundness is checked according to ISO 12181-1 and cylindricity according to ISO 12180-1. Acceptance limits for a given part depend on the drawing, but precision sleeves in acetal are often controlled to 0.03 mm roundness after stress relief. The table below lists the verification points applied to POM rod stock and machined components in high-tolerance production.