Finishing Delrin and Acetal (Why Anodizing and Paint Both Fail)

Anodized, no, that's a metal process and acetal is a polymer with no oxide to grow. Painted or plated, technically possible but genuinely difficult and usually inadvisable, because acetal (POM) is one of the hardest plastics to coat or bond. Its very low surface energy, high crystallinity, and chemical inertness, the same properties that make it a superb low-friction bearing and chemically resistant material, cause paints, inks, adhesives, and plating to refuse to wet or grip the surface. Even with aggressive surface preparation (flame, plasma, or chromic-acid etching to raise surface energy), adhesion on acetal is marginal and unreliable compared to easier-to-bond plastics like ABS. So in practice, Delrin and acetal parts are not painted or plated; color comes from the compounded resin (natural white, black, or other colors molded all the way through), and identification is done by laser marking or engraving rather than ink, because those don't depend on surface adhesion. If your design truly requires a painted, plated, or reliably bonded surface, acetal is often the wrong material choice, and a more bondable plastic should be considered, or you accept the limitation and use mechanical fastening and molded-in color instead. The takeaway: acetal is chosen because nothing sticks to it, which is excellent for bearings and bad for coatings.

Delrin 150 is a homopolymer acetal (DuPont) and acetal copolymer is the alternative chemistry, and the differences matter for machined-from-stock parts. Homopolymer (Delrin) has slightly higher stiffness, tensile strength, and surface hardness, giving a marginally better mechanical performance and a hard, durable machined surface, which is why it's favored for high-load gears and bearings. Its drawback is centerline porosity: homopolymer rod and thick stock can have a less-dense core, so if you machine a part that exposes the centerline (thin walls, parts hogged from large stock), you may find porosity or voids at the core that affect appearance, sealing, or strength. Acetal copolymer has a more uniform structure with no centerline porosity, so it machines soundly even from thick stock, and it offers somewhat better resistance to hot water, hydrolysis, and certain chemicals, making it preferred for parts in hot/wet or chemically aggressive service and for thick-sectioned machined components. For finishing and surface quality both machine excellently and take a fine low-friction surface off the tool. The choice usually comes down to: homopolymer for maximum stiffness/strength and hard wear surfaces where the centerline won't be exposed, copolymer for thick machined parts, hot-water/chemical exposure, and where porosity-free cores are required. Both are colored through the resin and neither accepts coatings well, so the grade choice is about mechanical and structural soundness, not surface finish options.

Yes, for tight-tolerance and critical acetal parts, stress-relief annealing is recommended, though acetal is somewhat more forgiving than some engineering plastics. Machining (and molding) introduces residual stresses that can cause the part to warp or drift out of tolerance over time, especially in parts machined from thick stock or with uneven wall sections, and especially if the part will see elevated temperature in service. Annealing relieves those stresses and stabilizes dimensions and crystallinity. The process is a controlled heat cycle, acetal is typically annealed in the rough range of about 150-160°C (well below its roughly 165-175°C melting point, so the schedule must be careful), held for a time based on section thickness, then slowly cooled to avoid reintroducing stress. A common best practice for precision parts is rough machine, anneal, then finish machine, so the part is dimensionally stable at final size. Annealing can be done in air ovens or in inert/oil media depending on the shop. If you skip it on a close-tolerance gear or valve part, the typical consequences are dimensional drift, warping, and reduced flatness after machining or in service. For non-critical or loosely toleranced acetal parts, annealing is often unnecessary. A good acetal machining source will advise on and perform annealing for precision work and can provide the cycle used. Combined with proper deburring, annealing is one of the few genuine finishing operations acetal actually needs, since applied coatings are largely off the table.

Because acetal's low surface energy makes inks, paints, and many adhesive labels unreliable, the preferred way to mark Delrin and acetal parts is laser marking or mechanical engraving, neither of which depends on surface adhesion. Laser marking uses a focused laser to locally alter or ablate the surface, producing a permanent, high-contrast mark (often a light or dark mark depending on the laser and the acetal color and grade) that won't rub off, smear, or peel, which is ideal for part numbers, logos, serial numbers, and medical-device UDI marking on acetal. Mechanical engraving or hot stamping (under the right conditions) are alternatives for larger or deeper marks. CO2 and fiber lasers are both used on acetal, with parameters tuned to the grade and color to get good contrast without melting or charring excessively. Pad printing and ink marking can be done if the surface is first treated (flame or plasma) to improve adhesion, but the result is less durable than laser marking and is generally avoided for parts that see handling, wear, or chemical exposure. Adhesive labels are also unreliable on bare acetal for the same surface-energy reason. So the practical answer for identifying acetal parts is: laser mark for permanent, durable, no-adhesion-needed identification, engrave for deep mechanical marks, and reserve ink/printing for cosmetic, low-wear cases with surface treatment, this is the polymer-world reality that follows from acetal's non-stick nature, the very property that also rules out anodizing and most coatings.