Metal finishing of CNC machined parts is used to change the appearance or improve the performance of machined metal parts through physical, chemical, or mechanical processes. Different finishing methods may have different effects, but each option will apply a decorative or functional layer onto the surface to achieve the required look or protective exterior. Some metal finishing options are more suitable for specific materials, such as hard metals usually requiring higher strength polishing techniques or harder abrasives.
Chromate conversion coating (chem film) or its brand name, Alodine, is a chemical coating that passivates and protects the aluminum from corrosion and is used as a base layer before priming and painting parts. The standard most specified in engineering applications is MIL-DTL-5541F, which refers specifically to the coating of aluminum alloys.
This protective layer is much thinner than an anodizing layer, and while both are created by immersion of parts into a bath, Alodine is a simple chemical coating, and its application process does not use electrical current.
Alodine is more prone to scratches, wear, and cosmetic damage than anodizing. The coating’s most common color is an iridescent greenish-gold, and so may be used for cosmetic purposes. However, its unique color comes from hexavalent chromium, which is toxic. So, for RoHS compliance needs, there are clear versions of the coating available as well.
Anodizing is an electrolytic passivation process that grows the natural oxide layer on aluminum parts for protection from wear and corrosion, as well as for cosmetic effects. During the process, the surface of the aluminum recedes dimensionally before the protective oxide layer is built up which means that the oxide layer is integral to the aluminum substrate below, which means it won’t chip or flake.
There are a few main types of anodizing. Type II, or sulfuric acid anodizing, leaves a film that can be 0.0001″ - 0.001″ thick. Type III, hard anodize, is much thicker and denser and provides better wear resistance. Once sealed, the coating will be 0.0002 - 0.0012” in thickness, in accordance with the common engineering spec MIL-A-8625 Type II.
Black oxide coating is the process of coating ferrous materials, stainless steel, copper, copper-based alloys, or zinc materials with a chemical coating process that coats them in iron oxide. This provides many benefits including reducing reduces light reflection, adding thickness to steel, and helping reduce corrosion and friction due to the oil or wax present.
Electroless nickel plating (ENP) uses a purely chemical reaction to induce plating and does not leverage electricity to deposit a nickel-phosphorus alloy onto the surface of a metal. Though this process takes much longer than electroplating, it achieves more uniform thickness over even the most complex surfaces because it doesn’t rely on varying electric fields. It also provides superior wear and corrosion resistance. The standard specification for ENP in North America is MIL-C-26074E, with various grades specifying thicknesses from 0.0003 - 0.002”.
Electropolishing is an electrochemical process used to improve the surface finish of a part by removing material to level microscopic peaks and valleys. This process can polish, passivate, and deburr parts. It’s the inverse of plating processes, as the part acts as the anode in the reaction which makes it useful for polishing irregular parts with hard-to-reach surfaces.
Media blasting uses a pressurized jet of abrasive media to apply a matte, uniform finish to parts. The most common media used are varying sizes of glass beads or sand, though, for other levels of abrasion, plastic beads are also used.
Electroplating or surface treatment involves applying a thin layer of metal or metal alloy. The purpose is to change the technical or physical properties of the surface of the items being plated, such as their electrical conductivity, or to improve their durability, appearance, and performance. Nickel plating provides a unique combination of corrosion and wear resistance. It can add brightness, luster, and appeal. It also provides excellent adhesion properties for subsequent coating layers.
Passivation is a chemical reaction that increases metals’ resistance to corrosion and other environmental factors. The passivation process is a method of improving the corrosion resistance of stainless steel parts by removing ferrous contaminants like free iron from their surface, restoring them to their original corrosion specifications.
Powder coating is a process in which a dry powder paint (either a thermoplastic or thermoset polymer) is applied to a metal surface using electrostatic application and then melted and hardened into an even coating. This finishing process is suitable for various materials, including metals and plastics, and can provide both functional and decorative surface coatings in a wide range of colors, finishes, and textures that are not readily achievable by conventional liquid coating methods. This allows for the application of thicker coatings without running or sagging, and the coatings are also more scratch and corrosion resistant as a result.
Tumbling is a finishing process used to clean, deburr, and slightly smooth smaller parts. Tumbling uses a horizontal drum filled with an abrasive substance, such as sand or ceramic chips. The drum rotates slowly, causing the media to abrade the parts, breaking any sharp edges, and smoothing the surface.
Galvanizing, or zinc plating, is a manufacturing process where a coating of zinc is applied to steel or iron to offer protection and prevent rusting. There are several galvanizing processes available, but the most commonly offered and used method is called hot-dip galvanizing.