Summary of Surface Treatment Methods for CNC Machined Parts

After the parts are processed, they are usually not put into use immediately. Generally, they need to be surface treated to enhance some of their characteristics, such as functionality, corrosion resistance, or colorful appearance. This article will review the surface treatment methods used for CNC machined parts.

Anodizing
Anodizing is to immerse aluminum or aluminum alloy parts in an acidic electrolyte, and form an anti-corrosion oxide film layer that is firmly bonded to the substrate on the surface of the parts as an anode under the action of an external current. Before anodizing, polishing, degreasing, cleaning and other pretreatments are required, followed by rinsing, coloring and sealing.

The anodized oxide film can be dyed black, red, blue, green, gold, etc.

Electroplating
Electroplating is an electrochemical and redox process. Take copper plating as an example; the metal parts are immersed in a metal salt solution as the cathode, and the metal copper plate is used as the anode. After the DC power supply is turned on, a metal copper plating layer will be deposited on the parts. Electroplating is divided into ordinary electroplating (tank plating) and special electroplating.

1. Ordinary electroplating

Galvanizing: A surface treatment technology that coats a layer of zinc on the surface of steel alloy materials to achieve aesthetic and rust-proof effects. The surface zinc layer is an electrochemical protective layer that can prevent metal corrosion. The main methods used are hot-dip galvanizing and electroplating. Suitable for surface treatment of steel and iron.

Tin plating: The corrosion products of tin are harmless to humans and are easy to braze. Tin plating is widely used in food canning packaging products, drinking utensils, tableware and many parts that need brazing in the electronics industry.

Copper plating: Copper plating is often used as an intermediate layer of other plating layers to improve the bonding strength between the surface plating layer and the base metal. In the power industry, iron wire can also be plated with thick copper to replace pure copper wire to reduce the consumption of copper.

Nickel plating: Nickel plating has a wide range of applications and can be used for protective decorative and functional purposes. The former is mainly used for protective decorative plating of parts such as bicycles, household appliances, hardware products, automobiles, cameras, etc. The latter is mainly used for restorative electroplating of products that are prone to wear.

Chromium plating: Chromium can maintain its luster for a long time in the atmosphere, and does not react in alkali solution, nitric acid, sulfuric acid and many organic acids. The chromium plating layer has high hardness, excellent wear resistance and low friction coefficient. Therefore, chromium plating is often used to protect decorative plating, prevent the base metal from rusting and beautify the appearance. It is also often used to improve the wear resistance of products or repair wear.

2. Special electroplating

Brush plating

Brush plating does not require a plating tank. The rotating workpiece is connected to the negative pole of the DC power supply, and the plating pen is connected to the positive pole. The front end of the plating pen is wrapped with absorbent cotton to immerse the poured plating solution in the absorbent cotton sleeve. The metal positive ions in the plating solution obtain electrons on the surface of the workpiece (cathode) under the action of the electric field force and are deposited on the surface to form an electroplating layer.

The plating thickness can reach 0.01~0.5mm. The plating quality is good, the bonding strength is high, and the speed is faster than tank plating.
Brush plating equipment is simple and flexible to operate. It can be used for local electroplating and outdoor operations. For decorative or dimensional electroplating of large quantities of small and medium-sized parts or large-area plated workpieces, brush plating is not as good as tank plating.
Brush plating is mainly used for local electroplating of large parts or local repair of large and medium-sized parts.
Bluing/blackening
Bluing is an oxidation treatment of steel parts to form a very thin layer of ferroferric oxide film on their surface. Alkaline chemical solution oxidation method is commonly used. Use an aqueous solution of sodium hydroxide and sodium nitrite for 60 to 90 minutes at 135 to 145°C, then soak in soap solution for 3 to 5 minutes, and finally wash, dry and soak in oil. After bluing, the parts are blue-black and dark black, which can improve the corrosion resistance and lubricity of the parts and improve the appearance.

Phosphating
Phosphating is the treatment of steel parts in phosphating solution to deposit a layer of water-insoluble crystalline phosphate film on the surface. Commonly used phosphating solutions are acidic dilute aqueous solutions composed of zinc dihydrogen phosphate or ferrous dihydrogen phosphate and manganese dihydrogen phosphate. Treat at 90~98°C for 8~20 minutes. After phosphating, it is gray or gray-black, and its corrosion resistance is worse than bluing, but its appearance is not as good as bluing. Phosphating is mainly used for corrosion protection of steel parts and pre-painting treatment to increase the adhesion and protection of the paint film to the steel workpiece.

Passivation
Passivation is the process of treating the metal in a nitrite, nitrate, chromate or dichromate solution to form a chromate passivation film on the surface of the workpiece. It is also called chromate treatment. The passivation film is composed of trivalent chromium and hexavalent chromium compounds and chromate of the base metal. The appearance varies with the alloy composition and film thickness, from colorless to rainbow or brown-yellow. The film layer has corrosion resistance and decorative properties, and is closely combined with the substrate to play an isolation and protection role.

Physical vapor deposition (PVD)
Physical vapor deposition refers to the technology of vaporizing materials into atoms, molecules or ionizing them into ions under vacuum conditions by physical methods, and depositing a thin film on the surface of the material through a gas phase process. Physical deposition technology mainly includes three basic methods: vacuum evaporation, sputtering, and ion plating.

Vacuum evaporation is a method of evaporating film-forming materials to vaporize or sublimate them and deposit them on the surface of the workpiece to form a thin film.
Sputtering is a method of ionizing argon gas through glow discharge in a vacuum, and the argon ions are accelerated under the action of the electric field to bombard the cathode and the sputtered particles are deposited on the surface of the workpiece to form a film.
Ion plating is a method of using gas discharge technology in a vacuum to ionize the evaporated atoms into ions, and deposit them on the surface of the workpiece together with a large number of high-energy neutral particles generated at the same time to form a film.
Physical vapor deposition has a wide range of applicable substrate materials and film materials; simple process, material saving, no pollution; the obtained film has the advantages of strong adhesion to the film base, uniform film thickness, dense film, and few pinholes.

Sandblasting
Using compressed air as the power, a high-speed jet beam is formed to spray the material (quartz sand, corundum, iron sand, etc.) at high speed onto the surface of the workpiece to be processed, so that the appearance or shape of the outer surface of the workpiece changes. Due to the impact and cutting action of the abrasive on the workpiece surface, the surface of the workpiece obtains a certain degree of cleanliness and different roughness, so that the mechanical properties of the workpiece surface are improved, thereby improving the fatigue resistance of the workpiece and increasing the adhesion between it and the coating.

Powder spraying
Use powder spraying equipment (electrostatic spraying machine) to spray powder coating onto the surface of the workpiece. Under the action of static electricity, the powder will be evenly adsorbed on the surface of the workpiece to form a powder coating; the powder coating is baked at high temperature, leveled and solidified, and becomes a final coating with different effects.

Polishing
Polishing is a finishing method for modifying the surface of parts. Generally, it can only obtain a smooth surface, and cannot improve or even maintain the original processing accuracy. Depending on the pre-processing conditions, the Ra value after polishing can reach 1.6~0.008 μm.

Polishing can be divided into three types: mechanical polishing, chemical polishing, and electrochemical polishing.

1. Mechanical polishing

Wheel polishing: Polishing is achieved by rolling and micro-cutting the workpiece surface with a high-speed rotating flexible polishing wheel and extremely fine abrasives. The polishing wheel is made of multiple layers of canvas, felt or leather, and is used for polishing larger parts.

Drum polishing and vibration polishing: The workpiece, abrasive and polishing liquid are loaded into a drum or a vibration box. The drum rolls slowly or the vibration box vibrates, so that the workpiece and the workpiece, and the workpiece and the abrasive rub against each other. Combined with the chemical action of the polishing liquid, the oil and rust on the surface of the workpiece are removed and the peaks are polished to obtain a smooth surface. It is used for polishing small and large parts. The latter has higher productivity and better polishing effect than the former.

2. Chemical polishing

Immerse the metal parts in a special chemical solution, and use the phenomenon that the convex parts of the metal surface dissolve faster than the concave parts to achieve the polishing of the part surface.

3. Electrolytic polishing

Electropolishing is an electrochemical process in which the atoms of the workpiece immersed in the electrolyte are converted into ions and removed from the surface due to the passage of electric current. Unlike chemical polishing, this process requires direct current, and the workpiece is connected to the anode to dissolve. The raised parts of the metal surface will dissolve faster than the concave parts. Most metals can be electrolytically polished, and the most common one is for surface polishing of stainless steel.

Silkscreen
The ink is transferred to the substrate through the mesh of the image part through the extrusion of the scraper, forming the same image as the original. Almost all materials can be screen printed, including plastics, metals, ceramics and glass.

The pattern to be printed is first made into a steel plate gravure by film etching, and then a curved pad head made of silicone rubber material is used to dip the ink on the gravure onto the surface of the pad head, and then press it on the surface of the printing object to print text, patterns, etc. Almost all materials can use pad printing technology, except for materials softer than silicone pads, such as PTFE.

GIF cover
Laser marking
Use laser beams to carve patterns, text or marks on the surface of a material or inside a transparent material.

Wire drawing
A surface treatment method that forms lines on the surface of a workpiece by grinding to achieve a decorative effect. According to the different lines after drawing, it can be divided into: straight drawing, random drawing, corrugation, and spiral. Almost all metal materials can use the drawing process.

Baking paint
Baking paint means that after spraying or painting, the workpiece is not allowed to cure naturally, but the workpiece is sent to the paint room and heated by electric heating or far infrared rays to cure the paint layer. The paint layer after baking paint is tighter and not easy to fall off, and the paint film is uniform and the color is full.

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