Powder Coating for Machined Parts: A Guide

This article is part of the CNC Surface Finishing Guide: Anodizing, Plating, Coatings & Ra on CNX Precision.

Powder coating protects CNC machined parts with a tough, decorative finish. Unlike liquid paint, it applies dry powder that cures into a hard film. Therefore, it resists chips, corrosion, and chemicals. This guide explains the finishing process, adhesion requirements, and what to expect on machined parts. Machined aluminum and steel both accept powder coating well. In addition, the process suits batch production with low waste.

The Powder Coating Process

The process follows a clear sequence. First, the part goes through pre-treatment. Cleaning removes oil, coolant residue, and dust. Degreasing uses alkaline baths or aqueous cleaners. Then surface preparation roughens the metal for adhesion. Grit blasting creates a uniform anchor profile. Next, masking protects threads, bores, and sealing surfaces. Operators apply masking tape, plugs, and high-temperature caps. After that, electrostatic spraying applies the powder. Charged powder particles stick to the grounded part. Finally, curing fuses the powder into a continuous film. Ovens run around 200 degrees Celsius for 10 to 20 minutes. The exact time depends on part mass and powder type.

The curing cycle matters as much as the application. Under-cured powder looks dull and chips easily. Over-cured powder yellows or blisters. Therefore, oven temperature and part mass must match. Heavy parts take longer to reach full temperature. Consequently, coating shops use temperature recorders to verify the cycle.

Color choice is nearly unlimited. Powder suppliers match RAL and custom colors. Textured, gloss, and matte finishes change the appearance. In addition, metallic powders add sparkle for consumer products. However, metallic powders may need a clear top coat for durability. Therefore, discuss the finish with your coating partner before production.

Small parts often run on conveyor racks. Operators hang parts so powder reaches every face. In addition, hooks leave small contact marks that designers should tolerate. Therefore, define the visible face before quoting.

Recovery systems capture overspray. Therefore, material waste stays near zero. In addition, unused powder recycles into the next run. Consequently, the process meets modern environmental standards.

Powder Types

• Epoxy powders offer excellent adhesion and chemical resistance.
• Polyester powders resist weather and UV light.
• Hybrid powders balance cost and performance.
• Thermoset powders cure permanently, while thermoplastic powders can re-melt.

Therefore, the environment decides the resin. Outdoor products need polyester. Chemical environments need epoxy. Meanwhile, interior parts often use hybrid powders.

Powder Coating Adhesion and Surface Preparation

Adhesion makes or breaks the finish. Powder bonds mechanically and chemically to the surface. Therefore, preparation quality decides long-term durability. Grit blasting produces a surface profile of about 25 to 75 microns. This roughness gives the powder something to grip. In addition, chemical pretreatment improves corrosion resistance. Iron phosphate suits steel, while zinc phosphate gives better protection. Aluminum parts benefit from conversion coating. Consequently, a coated aluminum part resists salt spray far better than an untreated one. Machined parts carry coolant residue from the mill. Therefore, thorough degreasing is non-negotiable. Any residual oil causes fish-eyes and peeling. As a result, coating shops inspect cleanliness before spraying.

Blast media changes the anchor profile. Aluminum oxide gives a sharp profile for good grip. Glass beads give a smoother surface for thin coatings. Steel grit works on heavy steel parts. Therefore, match the media to the part and finish.

Rack contact points leave tiny marks on the part. These marks matter on visible faces. Therefore, ask your coater about rack design. In addition, hooks can be re-tipped to reduce marking.

Coating thickness must be checked after curing. A magnetic gauge measures thickness on steel. Eddy current gauges work on aluminum. In addition, adhesion tests like the cross-hatch test verify bond quality. These checks catch preparation errors early. Consequently, a good coating partner provides a certificate of conformity.

Key Prep Steps

1. Degrease with alkaline or aqueous cleaner.
2. Rinse and dry completely.
3. Blast with aluminum oxide or steel grit.
4. Apply phosphate or conversion coating.
5. Mask threads, bores, and datum surfaces.

Powder Coating on Machined Parts: Design Considerations

Coating thickness changes part dimensions. Typical powder coating thickness runs from 60 to 120 microns per side. On threaded holes, this thickness reduces the effective pitch diameter. Therefore, mask threads that must accept fasteners. Press-fit bores also need protection. In addition, coating hides fine machining marks but not deep scratches. The film follows the surface contour. Consequently, ground surfaces stay visible under a thin clear coat. For critical tolerances, machine after coating or allow for coating thickness in the drawing. The table below shows typical thickness ranges.

Three coating strategies exist. First, coat the finished part with masked features. Second, machine critical surfaces after coating. Third, apply coating only to non-functional faces. Each strategy trades cost against protection. For example, coating after machining is simplest. However, machining after coating keeps tight tolerances. Therefore, decide the strategy before quoting.

Powder tends to thin at sharp edges. Therefore, rounding edges with a small radius improves coverage. In addition, designers can specify a heavier coating for edge-heavy parts. Consequently, edge coverage becomes predictable.

Curing temperature limits part selection. Zinc die castings and thin plastic parts cannot survive the oven. Therefore, confirm the material temperature rating before scheduling coating.

Masking Rules

• Mask threaded holes and tapped features.
• Mask precision bores and sliding surfaces.
• Leave reference datums uncoated.
• Use high-temperature tape rated above curing temperature.

Powder Coating FAQ

How thick is a powder coating?

Most production powder coatings measure 60 to 120 microns. Heavy-duty outdoor finishes reach 200 microns. The thickness affects both durability and dimensional fit. Thicker coatings improve edge coverage but may clog fine threads. Therefore, measure thickness after curing and before shipping.

Does powder coating fill machining marks?

No. The film follows the surface profile. It hides slight marks but cannot fill deep scratches or tool marks. Therefore, grind or polish surfaces that must look smooth after coating. A light glass-bead blast before coating also helps adhesion on smooth surfaces.

How long does powder coating last?

With proper preparation, the finish lasts years outdoors. Polyester systems resist UV degradation. Consequently, most manufacturers warranty powder coatings for ten years or more.

Powder coating adds value when preparation and masking are done right. Choose the resin for the environment, then protect every functional feature. In short, powder coating transforms a machined blank into a finished product. CNX Precision coordinates coating at qualified partners for machined parts. Therefore, your parts arrive with finish and function intact. We will review the part, resin, color, and masking plan with you. Request a coating quote with your next machining inquiry.

For related information, see our guide to cnc machining service and 5-axis cnc machining and cnc milling vs turning and cnc machining tolerances.