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When engineers order CNC plastic machined parts, the conversation usually starts with dimensions and tolerances. But the surface finish is what determines how the part looks in a customer's hands, how it slides against a mating component, and how well it survives cleaning chemicals, UV exposure, and daily wear. The right finish can turn a functional prototype into a presentation-ready product; the wrong one can crack a polycarbonate housing or ruin a press fit.
This guide walks through the finish options commonly offered for machined plastic parts, explains which materials each option suits, and shows how to specify a finish without blowing up your tolerances or budget.
Plastics behave very differently from metals during and after machining. Most engineering plastics conduct heat poorly, expand far more than aluminum or steel when warmed, and soften at relatively low temperatures. That means aggressive blasting, high-temperature curing, or strong solvents can warp a part, stress-crack it, or melt the very surface you are trying to improve. Many finishes that are routine on metal — anodizing, black oxide, passivation — simply do not exist for plastics, while other processes such as vapor polishing are unique to them. Choosing a finish for a plastic part always starts with the base resin.
The default option: the part comes straight off the machine with only deburring applied. With sharp single-flute tooling, correct feed rates, and a light finishing pass, a skilled shop can hold around Ra 1.6–3.2 µm directly off the cutter, and light tool marks remain visible under close inspection. For internal components, jigs, and functional prototypes, an as-machined finish is the fastest and cheapest choice, and it adds nothing to lead time. It is also the safest option for parts with tight tolerances, because no material is added or removed after machining.
Fine glass beads are propelled at the surface under controlled pressure, removing tool marks and leaving a uniform matte or satin texture. Bead blasting works well on rigid materials such as POM (acetal/Delrin), nylon, ABS, and glass-filled grades, and it is popular for housings and handheld parts where a consistent, non-reflective look matters. It should be avoided on very soft resins like UHMW-PE or PTFE, which tend to gouge rather than texture evenly, and on thin-walled parts where blasting pressure can cause distortion. Because blasting removes a small amount of material, critical bores and threads should be masked.
Progressive wet sanding followed by buffing compounds brings plastic surfaces from matte to semi-gloss or full gloss. Polishing is the standard route for visible cosmetic parts — covers, bezels, knobs, and display windows. Hard, rigid plastics such as acrylic (PMMA), polycarbonate, and PEEK respond well; softer or fibrous materials do not hold a shine. At ANOK, precision polishing on suitable materials can reach a mirror-level roughness down to Ra 0.2 µm, which is also useful when a sealing surface or low-friction sliding face is required, not just for appearance.
Vapor polishing exposes the part to a controlled solvent vapor that briefly melts the outermost micron of the surface, which then re-solidifies into a smooth, glossy, often optically clear layer. It is the go-to finish for polycarbonate and acrylic lenses, light guides, manifolds, and medical flow components where you need to see through the part or inspect fluid paths. The process only works on solvent-responsive thermoplastics — PC, PMMA, ABS, and a few others — and it requires careful fixturing, because the same solvent that polishes the surface can attack sharp internal corners or cause stress cracking if the part is not properly annealed beforehand.
A brief pass of a controlled flame melts the very top of the surface, smoothing edges and producing a clear, glossy effect — most commonly on acrylic edges after cutting or milling. It is fast and economical for flat edges and simple profiles, but it offers less control than vapor polishing on complex 3D geometry and is not recommended for precision features or thin sections.
Painting adds color, hides tool marks and minor surface variation, and provides UV and chemical protection. Most rigid plastics accept epoxy or polyurethane topcoats, provided the surface is properly prepared — typically light abrasion plus a primer or an adhesion-promoting treatment such as corona or plasma for low-surface-energy resins. Paint adds roughly 0.02–0.08 mm per coat to a surface, so press fits, threads, and mating faces need masking or undersized machining. Cure temperatures must stay below the heat-deflection limit of the resin, which rules out high-temperature powder-coat cycles for most plastics.
Nylon parts can be immersion-dyed to produce through-color finishes that will not chip the way paint can, which is useful for wear components and consumer parts that see abrasion. Tinting is also used on clear polycarbonate and acrylic for light-filtering applications. Color consistency depends on resin batch and dye time, so dyeing is best specified when approximate color matching is acceptable.
Textures for grip or aesthetics can be machined directly into the part or applied afterward by bead blasting patterns or hand methods. For branding and labeling, silk screening, pad printing, and laser engraving add logos, scales, and legends without affecting part dimensions — laser engraving is effectively permanent, while inks may need a protective clear coat in high-wear areas.
Certain plastics — ABS above all — can be electroplated with copper, nickel, and chrome after chemical etching and seeding, producing a true metallic surface for chrome-look trim and EMI shielding. Vacuum metallizing (PVD) achieves a similar metallic appearance on a wider range of resins. Both are specialty processes with significant cost and lead time, and they are generally reserved for cosmetic consumer parts rather than industrial components.
Material compatibility is the single biggest factor in finish selection. The table below summarizes what typically works on the engineering plastics most often quoted for plastic CNC machining work:
| Material | Recommended finishes | Use with caution / avoid |
|---|---|---|
| ABS | Painting, bead blasting, plating, vapor polishing | High-temperature cure cycles |
| POM (Acetal/Delrin) | Bead blasting, polishing, as-machined | Painting (poor adhesion without treatment) |
| Polycarbonate (PC) | Vapor polishing, wet sanding, hard/UV coatings | Strong solvents (stress cracking) |
| PMMA (Acrylic) | Flame polishing, vapor polishing, buffing | Aggressive blasting (edge chipping) |
| Nylon (PA6/PA66) | Dyeing, bead blasting, as-machined | High-gloss polishing; painting on unfilled grades |
| PEEK | Polishing, bead blasting, as-machined | Vapor polishing, plating |
| PTFE / UHMW-PE | As-machined | Painting, plating, high-gloss polishing |
Every finish either adds material, removes material, or adds process time — often all three. Painting and plating build up measurable thickness that can close a designed clearance or clog a fine thread, so those features should be masked or machined with compensation. Blasting and polishing remove material, which matters when tolerances approach ±0.05 mm or tighter. If a part requires both a precision fit and a cosmetic finish, the practical approach is to finish the cosmetic surfaces and mask the functional ones, then state clearly on the drawing which dimensions apply after finishing.
On cost, the ranking is predictable: as-machined is essentially free, bead blasting adds a small handling charge, polishing is labor-intensive and scales with surface area and target gloss, and vapor polishing, plating, and metallizing are specialty steps with dedicated setup. Lead time follows the same curve — from zero for as-machined to a week or more for plated parts. Specifying only the finish the application truly needs is the easiest cost saving available on a plastic parts order.
A finish callout that a machinist can act on has three parts: the process, the target result, and the boundaries. For example, "bead blast, uniform matte, all external surfaces except datum B" or "polish optical face to Ra 0.4 µm, mask bore and threads." Vague notes like "make it look nice" or "smooth finish" invite mismatched expectations. If the part is cosmetic, sending a reference sample or a photo of an approved finish removes nearly all ambiguity. And if you are unsure whether a resin will accept a finish, ask before ordering material — switching from, say, PTFE to POM can open up options that change both appearance and cost.
ANOK machines the full range of engineering plastics — ABS, nylon, POM, PEEK, PET, PC, PTFE, and PMMA — and supports parts from one-off prototypes to production volumes with tolerances down to ±0.002 mm. Beyond machining, our in-house and partner coating and surface treatment capabilities cover bead blasting, polishing to mirror-level roughness, painting, and additional treatments coordinated under one quality system, so parts arrive finished rather than needing a second supplier. If you tell us the material, the cosmetic requirement, and which surfaces are functional, we will recommend the most economical finish that meets all three.
Have a plastic part drawing ready? Send it with your finish requirements through our contact page, and our engineering team will return a quote with finishing recommendations included.