What surface finishing options are available for CNC machined metal and plastic parts?

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Surface finishing for CNC machined parts ranges from simple to advanced. For metals: as-machined (32-63 Ra), bead blasting (matte finish), anodizing (Type I, II, III for aluminum), powder coating (any RAL color), plating (chrome, nickel, zinc, gold), electropolishing (bright, clean surface), passivation (corrosion protection for stainless), and media tumbling (smoothing edges). For plastics: vapor smoothing (for ABS/PC), bead blasting, polishing, and painting. SOMI offers all these finishes in-house or through trusted partners.

Aluminum Finishing

Anodizing Type II: Electrochemical process that creates a decorative, corrosion-resistant surface. Available in clear and colors (black, red, blue, gold). Thickness: 5-25 microns. Cost: $. Hard coat anodizing Type III: Thicker (25-75 microns), harder (RC 60+), excellent wear resistance. Cost: $$. Powder coating: Durable color finish, any RAL color. Thickness: 60-120 microns. Best for enclosures and structural parts. Cost: $. Bead blasting: Uniform matte finish, 4-8 Ra. Cost-effective. Cost: $.

Steel and Stainless Finishing

Electropolishing: Reverses the electroplating process, removing a thin layer to create a bright, smooth, passivated surface. Ra improves from 32 to 8-16. Improves corrosion resistance. FDA-compliant for food contact. Cost: $$. Passivation: Chemical treatment that removes free iron from stainless steel surface, enhancing corrosion resistance. Required for medical and food-grade parts. Cost: $. Zinc plating: Corrosion protection for carbon steel. Clear, yellow, or black finish. Cost: $. Nickel plating: Decorative, corrosion-resistant. Electroless nickel provides uniform coating on complex geometries.

Plastic Finishing

As-machined: Visible tool marks, 32-63 Ra. Acceptable for many internal parts. Bead blasting: Uniform matte finish, hides tool marks. Vapor smoothing: Exposes plastic to solvent vapor, melting the surface to create a glossy, smooth finish. Excellent for ABS and PC. Polishing: Mechanical polishing to high gloss. Time-consuming but produces optical-quality transparency for acrylic and polycarbonate. Painting: Any color, texture, or gloss level. Primer + paint + clear coat for durability.

Why Choose SOMI Custom Parts

At SOMI Custom Parts, we offer a complete range of surface finishing services integrated with our CNC machining. Parts go straight from the machine to finishing without intermediate shipping or handling, reducing lead time and eliminating quality risks. Our engineers help you select the optimal finish for your application, considering appearance, durability, regulatory compliance, and budget.

Case Study

A consumer electronics company needed aluminum enclosures with a premium black anodized finish and laser-engraved logos. SOMI CNC machined the enclosures, applied Type II black anodizing (20 microns), then laser-engraved the logos through the anodized layer. The final surface had a consistent matte black finish with sharp white logos. The client's branding team approved the finish after evaluating 5 anodizing samples.

Industry Data

Anodizing is the most common finish for CNC-machined aluminum, specified on approximately 55% of all aluminum parts. Powder coating is the fastest-growing finishing method at 7% annual growth, driven by its durability and environmental advantages (Surface Finishing Association, 2025). Proper surface finishing can extend part service life by 2-5x compared to untreated parts.