The procurement calculation captured by the phrase Near-Net POM Gear Blanks and Bushing Stock: When Pre-Shaping Pays is fundamentally a comparison of material utilization, machining sequence length, and post-machining dimensional drift. Polyoxymethylene (POM), supplied as homopolymer or copolymer, has a density of 1.41–1.42 g/cm³ per ISO 1183-1:2019 and a semi-crystalline structure that releases frozen-in stress as material is removed. Pre-shaped gear blanks and bushing stock shift the cost from general machining to first-article tooth cutting or finish boring. The break point normally appears when a gear rim or flange cannot be injection molded without sink marks, or when the annual volume is too low to amortize a hardened cavity mold. For bushing stock, near-net tube forms eliminate trepanning and kerf loss; for gear blanks, pre-shaped discs allow the hob or gear shaper to cut full-form teeth without first removing a thick annulus from plate.

Injection-molded POM gears are net-shape but require a mold temperature of 80–120 °C and a melt temperature of 190–210 °C. Gate blush and weld lines can produce radial runout above 0.05 mm in unfilled POM-C, and hard tooling is not economically attractive below roughly 5,000 pieces per year for a single cavity. In this volume band, pre-shaped stock is machined on standard CNC lathes and hobbing centers, with no mold qualification. Published data for the exact break-even volume is mold-specific, but the presence of a pre-machined pilot bore and turned OD reduces setup and roughing time.

Thermal Degradation Pathways and Cylindrical Stock Cooling Stresses

Extruded POM-C round stock is produced on a single-screw extruder with L/D 24:1–30:1, typically fitted with a pineapple or Maddock mixing section and a calibrating die. Barrel temperature profile from feed to metering is 160–205 °C; homopolymer grades require the upper limit held below 210 °C to avoid rapid depolymerization. At melt temperatures above 210 °C, formaldehyde generation accelerates and creates gas bubbles in thick sections. Cooling of a 100-mm diameter rod is non-uniform: the skin solidifies first, while the core remains above the crystallization temperature. A spherulite size gradient results, and residual tensile stress in the core can reach 3–5 MPa when measured by hole-drilling strain gauges in machined rings. Near-net pre-shaping at the stock-shape producer allows the billet to be cut into discs or bushings before machining, so subsequent material removal is distributed along the original axial symmetry. This reduces asymmetric stress release that commonly causes bore ovality after roughing.

Conditioning of POM blanks before machining does not require the aggressive drying used for polyamide 6. Water absorption after 24 h immersion at 23 °C per ISO 62:2008 is typically 0.2% or less. However, stock that has been stored in an unheated warehouse with relative humidity above 60% should be brought to 23 °C ±2 °C for 24 h before final dimensioning. The concern is not hydrolysis but thermal expansion: the coefficient of linear thermal expansion is approximately 1.1 × 10⁻⁴ K⁻¹ between 23 °C and 60 °C per ISO 11359-2:1999, so a 40-mm bushing OD measured at 8 °C differs by 0.05 mm from the same part at 23 °C. Machined POM-C components should not be exposed to pH below 4 or to strong oxidizing acids. Acetal resins depolymerize in the presence of chlorine dioxide, sodium hypochlorite, or hot formic acid. This incompatibility boundary is documented in material datasheets and in ISO 9988-2:2006.

What Void Content at the Core of a 100-mm Rod Does to Gear Tooth Form Stability?

Core porosity in thick POM-H stock is screened by immersion ultrasonic C-scan using a 5-MHz focused transducer. In rods above 80 mm diameter, void content in the central 20 mm zone can exceed 0.5% by volume when extruder screw speed is above 80 min⁻¹. If a gear blank is machined from that zone, teeth cut into the affected core exhibit micro-voids at the pitch circle that reduce flank load-bearing area. For a module 1.5 spur gear with 30 teeth, a 40-mm face width and a nominal tooth load of 120 N, the flank contact stress can approach 25 MPa; voids act as stress raisers. Tooth-to-tooth pitch error after hobbing has been observed to increase when the core void fraction exceeds 0.5%, and ISO 1328-1:2013 class Q7 may not be met. Pre-shaped stock sold specifically for gear blanks is ultrasonically screened and is often cut from the outer radial zone of a larger billet, away from the extruded core. That is one reason a near-net blank may carry a higher price per kilogram than generic rod but lower total cost per qualified gear.

Bushings with wall thickness below 4 mm are poor candidates for machining from solid round bar because internal stress release produces ovality. In those cases, pre-formed tubular stock with an OD of 32 mm and an ID of 24 mm is specified so that finish boring depth of cut remains 0.2–0.3 mm. A bushing blank supplied with 0.15 mm machining allowance on the ID and 0.25 mm on the OD can be finished in one chucking, whereas a solid rod requires drilling, rough boring, and two setups. Fewer clamping cycles reduce distortion caused by jaw pressure. Three-jaw chucks with soft jaws bored to the stock OD are specified; clamping pressure should not exceed 1.5 N/mm² on the jaw contact area. Hard jaws produce indentations that act as stress concentrators when the bushing is pressed into a housing.

When Annealing at 150 °C Reveals Internal Stress in Thick-Walled Hollow Stock

Annealing of POM-H hollow stock at 150 °C for 1 h per 10 mm of wall thickness is used before finish boring of bushings and before gear hobbing of blanks. A forced-air annealing oven with ±3 °C control is loaded so that blanks do not touch and air can circulate. Heating rate is kept to 20–30 °C/h; cooling rate in the oven should not exceed 15 °C/h to prevent re-introduction of thermal stress. The annealing temperature for POM-C is 130–140 °C, below the melting temperature of 165 °C and above the expected service temperature of the finished part. If annealing is omitted, a 50-mm thick blank can exhibit radial runout of 0.08 mm after rough turning due to release of as-extruded stress. This is a processing failure observed when a machine shop switches from metal bar to POM stock without altering the roughing sequence. The pre-shaped blank is not a stress-free guarantee; it is a geometry that reduces the amount of unbalanced material removal.

Documentation for incoming blanks should include resin type, lot number, and ultrasonic screening report. For food-contact applications, the stock-shape producer must certify compliance with FDA 21 CFR 177.2470 and EU Regulation 10/2011. RoHS compliance is evaluated under 2011/65/EU Annex II; unfilled POM does not typically contain restricted heavy metals or flame retardants. Dimensional acceptance of pre-shaped blanks is checked against ISO 2768-1 class m for general tolerances and ISO 1101 for circular runout of the pre-machined OD. A lot that passes these checks still requires first-article inspection after hobbing because the tooth form is not controlled by the blank supplier.

Pre-Shaped Tube Stock Removes Trepanning from the Machining Sequence

When gear blanks are cut from square plate, the blank preparation sequence is saw, face, rough bore, turn OD, face to thickness, deburr, and then hob. A pre-shaped POM-C blank with an as-supplied OD of 55.0 mm, a pilot bore of 12.0 mm, and a thickness of 16.0 mm eliminates the bandsaw and reduces rough-turning cuts. On a CNC lathe with a 42-mm spindle bore, a pre-shaped blank can be finished with one pass at a depth of cut of 0.5 mm and a feed rate of 0.1 mm/rev. The hobbing operation then proceeds with a climb-hobbing strategy, cutting fluid-free or with dry compressed air. Cutting data used for acetal are a hob speed of 180–300 m/min and a feed per workpiece revolution of 0.5–1.0 mm/rev for module 1.0–1.5 spur gears. These parameters reduce heat buildup because POM softens above 85 °C under load. The use of pre-shaped blanks ensures that the hob enters a clean tooth space rather than a saw-cut surface with localized smearing or microcracks.

Gear hobbing of POM requires a tooth thickness allowance of 0.05–0.10 mm per flank because the polymer recovers elastically after cutting. Measuring over pins immediately after cutting can give values 0.02–0.04 mm larger than after 24 h at 23 °C. This is due to viscoelastic recovery and is more pronounced in homopolymer than copolymer. Pre-shaped blanks that have been annealed show less post-hobbing drift than blanks machined from unannealed plate. The gear cutter should record the over-pin measurement after 24 h before accepting the setup.

Dry-running POM-C bushings on mild steel shafts are evaluated under ASTM D3702-94(2019) thrust washer conditions; the dynamic coefficient of friction is typically 0.15–0.30 at a sliding speed of 0.25 m/s and a unit pressure of 0.25 MPa. The limiting PV for continuous dry service of unfilled POM-C is commonly cited as 0.10–0.15 MPa·m/s. Exceeding that value raises surface temperature above the 85 °C maximum continuous operating temperature under load and can produce stick-slip noise. Near-net bushing stock with a pre-machined OD and pilot bore reduces radial wall thickness variation to 0.10 mm TIR before boring; this is critical because wall thickness variation above 0.20 mm causes asymmetric thermal expansion and an effective clearance loss of 0.03–0.05 mm on a 30-mm bore at 70 °C. The stock is not a finished bushing; it is a controlled starting blank that shifts the machining burden from bulk removal to final boring and grooving.