POM / Acetal
CNC Machining

POM (Polyoxymethylene, also called Acetal or Delrin — Delrin being DuPont's trade name for homopolymer acetal) is a semi-crystalline engineering thermoplastic known for its exceptional combination of stiffness, low friction, excellent machinability, and dimensional stability. It is one of the most widely machined plastics because it produces clean chips, holds tight tolerances, and requires no secondary finishing for most applications. POM is used for gears, bushings, sliders, automation components, jigs, and guide parts where metal would be too heavy or corrode, and softer plastics lack the required stiffness and wear resistance.

POM-H (homopolymer, trade name Delrin) has slightly higher stiffness, hardness, and fatigue resistance — preferred for gears and mechanical parts. However, POM-H rod stock above approximately 50 mm diameter has centerline porosity (a void at the geometric center) that can be exposed when machining parts with large center bores. POM-C (copolymer) is more homogeneous — no center porosity — making it preferred for precision machined parts with bores, and for parts requiring good chemical resistance. POM-C is also available in FDA-compliant and metal-detectable food-grade colors. For most automation components, bushings, and guide parts, either grade is suitable.

Standard CNC tolerance for POM is ±0.05 mm. For precision bore fits, shaft diameters, and slider guide dimensions, we routinely hold ±0.02–0.03 mm. Note that POM has a higher thermal expansion coefficient than metals (approximately 10× steel): for parts operating across a wide temperature range, thermal expansion must be factored into clearance calculations. A 50 mm POM bushing will expand by approximately 0.3 mm in diameter per 10°C temperature rise — significant for tight clearance fits against steel shafts.

Natural (white/ivory) POM-C and POM-H grades are available in FDA-compliant formulations suitable for direct food contact per FDA 21 CFR 177.2470. For food processing environments, blue metal-detectable POM-C is particularly useful — if any plastic fragment breaks off, it triggers metal detectors on the processing line. For medical device applications, medical-grade POM with full material traceability and biocompatibility documentation per ISO 10993 is available. Always request the FDA certificate from your supplier for regulated applications.

POM has better dimensional stability than nylon (much lower moisture absorption — nylon absorbs 1.5–3% water causing significant dimensional change and swelling), better stiffness, lower coefficient of friction, and better machinability for tight tolerances. Nylon has better impact resistance, higher fatigue strength, and better performance at elevated temperatures. For applications requiring consistent dimensions across humidity changes — precision gears, guide components, and measurement fixtures — POM is strongly preferred over nylon. For high-impact or high-temperature applications, nylon may be more appropriate.

POM is one of the easiest engineering plastics to machine, but several considerations are important: (1) Thermal expansion — POM expands significantly with heat; air or mist cooling is preferred over flood coolant. (2) Stress relief — large POM parts should be annealed before final machining to release residual stresses. (3) Centerline porosity in POM-H rod — for parts with large center bores, specify POM-C to avoid the center void. (4) Sharp tooling — dull tools cause smearing; sharp carbide or HSS gives best results. (5) Chip management — spiral flute tooling prevents re-cutting from long chips in deep features.

POM is available in natural (white/ivory), black, blue, red, and other colors. Natural POM is the most common and has the highest purity for food contact and medical applications. Black POM contains carbon black pigment which provides UV resistance for outdoor applications. Blue and other bright colors are used in food processing equipment for visual identification — some blue grades are also metal-detectable. Color does not significantly affect the base mechanical properties (stiffness, hardness, wear resistance) but may affect UV stability and chemical resistance.

POM has limited bonding options due to its low surface energy. Adhesive bonding is possible with cyanoacrylate after surface activation by UV/ozone or plasma treatment, but bond strength is moderate. Mechanical fastening (screws, press-fit inserts, snap-fits) is the preferred joining method. Threaded inserts can be installed ultrasonically for high-strength fastener points. POM can be welded by ultrasonic welding, hot-plate welding, or spin welding for joining two POM parts. Surface coating is generally not applied as POM has inherent low friction and chemical resistance.