Metal finishing is a topic that can’t be left untreated, as almost everything around us is made of some type of metal or contains a metal part. Metal has played a key role in our lives for thousands of years, and its benefits affect every technological innovation. Life would have been very hard and boring without metals, starting with automobiles, jewelry, planes, doors, windows, bolts, nuts, etc. made with metals.
Because metals are used to produce decorative items, their final appearance must be appealing. In some situations, a finishing process must be performed to ensure the final appearance of an item made of metal.
In addition to metal finishes, metal finishing plays an important role in improving the properties of a metal by covering the surface with a thin layer. In this reading, I’ll discuss metal finishing, its processes, applications, types, and benefits.
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What is Metal Finishing?
The metal finishing process is the process of placing or applying some type of metal coating on the surface of a metallic part. It is used to treat the exterior of metal products by applying a thin complementary layer to its surface. Metal finishing can also be said to be the implementation of a process for cleaning, polishing, or enhancing metal surfaces.
The industrial applications of metal finishing are automotive, electronics, aerospace, hardware, heavy equipment, appliances, and telecommunications There are a few things to consider when selecting metal finishing techniques for your project. The following stated below are the factors:
- Production speed: how fast will the finishing technique be performed?
- Cost-effectiveness: some finishing machines like vibratory tumblers are very expensive but will offer faster cycle rates.
- The potential for vulcanization is also one factor to be considered.
- Metal properties: if the material is hard metals an intense finishing method like grinding may be required.
Related: What is Surface Finish? its Properties, Symbols Parameters
Types of Metal Finishes
The various kinds of metal finishing are electroplating, electroless plating, vibratory finishing, passivation, cladding, electrocoating, hot blackening, powder coating, electropolishing, abrsive blasting, buff polishing, mass finishing, shot peening, soda blasting, case hardening, tumble slurry parts, parts cleaning.
1. Electroplating
This metal finishing process is also known as electrodeposition, which is achieved by depositing metal or metal alloy onto the surface of a substrate. An electric current is passed through a liquid that contains dissolved ions and the substrate.
This object serves as a cathode and attracts the metal ions in the plating bath. A pretreatment process such as cleaning, rinsing, and stripping is performed by the electroplating process.
Electroless Plating
Electroless plating is similar to the electroplating finishing method in that the substrate is placed in a liquid solution containing the desired metal, often copper or nickel.
Their major difference is the way in which the deposition occurs: the metal coating is applied via chemical reaction instead of electricity. The plating bath normally includes a complexing agent to maintain the metal solution and various which helps to maintain stability. It also increases the bath life.
2. Vibratory Finishing
The finishing process is effective for removing rough edges and some other surface imperfections from metal parts. The parts are placed inside of a bowl or tub which contains a pellet-shaped media.
The activation of the bowl aids the vibration action that causes the media and the parts to rub against each other. The vibratory finishing process is ideal for smaller parts with applications such as edge-breaking, cleaning, descaling, burnishing, or polishing.
3. Passivation
Passivation is a post-treatment plating process helping to provide additional corrosion protection. It is achieved by using a metal oxide coating that can lengthen the time in which the onset of rust occurs. It also enhances the appearance of the finished part as it is available in a variety of colors.
4. Cladding
Cladding is a mechanical process that uses heat and pressure to create a bond between the substrate and the metal coating. it requires the use of specialized equipment which makes it more costly than other metal finishing processes.
However, cladding provides a thicker metal coating than either electroplating or electroless plating.
5. Electrocoating
Electrocoating which is also known as e-coating also uses the electrodeposition principle like the electroplating process. The parts are soaked in a solution containing the coating before the introduction of an electric current.
But instead of metal ions, the solution contains electrically charged paint or epoxy particles that are deposited on the substrate surfaces. E-coating is often used as a primer coat on metal surfaces before painting in order to promote paint adhesion.
It is sometimes used to enhance the appearance of metal parts as it works well on any type of metal.
6. Hot blackening
this is used to produce a black matte finish mostly on automotive parts and firearms, as well as military applications where a protective dark coating is required. it is done by applying a thin black oxide coating on the surface of a substrate which helps to increase the abrasion resistance of the metal part.
This metal part is allowed to travel through a series of tanks containing various cleaning agents, caustic materials, and coolants. The hot blackening finishing process is generally performed in large batches, making it perfect for smaller parts.
7. Powder Coating
This is the application of dry powder coating on metal parts instead of using liquid coating. A powder coating provides greater thickness than a liquid coating. this metal finishing process is achieved by combining some list of ingredients that includes pigments, curative, flow modifiers, leveling agents, and other additives to create the powder.
The powder is then electrostatically deposited onto the surface of the substrate which is then cured in a specially designed oven. The oven produces a chemical reaction that results in high-link density. This finishing process is also applied to some plastics and other non-metallic substrates.
8. Electropolishing
Electropolishing is an electrochemical metal finishing process that is the opposite way of electroplating. As electroplating involves the deposition of metal irons onto the surface of a substrate, electropolishing is the removal of metal ions from the substrate.
It offers a smooth, streamlined surface texture to the workpiece. It is accomplished by dipping the objects into an electrolyte bath which serves as the anode. The object is also exposed to a terminal that provides a DC power supply that serves as the cathode.
This process is ideal for removing burrs or small amounts of debris from part surface and eliminating peaks and valleys.
9. Abrasive Blasting
This is also known as sandblasting or bead blasting which combines the cleaning and finishing process together. Abrasive blasting saves time and money. The cleaning, polishing, and finishing action is achieved when high-pressure compressed air is used to project a medium onto the surface of the parts.
A wide variety of media such as sand, silicon carbide, glass beads, aluminum steel grit or shot, or organic materials like walnut shells or corn cobs.
Typically, harder media is used when more aggressive cleaning action is required while soft materials are used on surfaces that are more susceptible to damage. Pressure control can also prevail in order to adjust and accommodate specific cleaning or finishing requirements.
10. Buff Polishing
Buff polishing is performed to clean and smooth the surface of a substrate just as electropolishing. But it does not involve the use of any type of electrochemical reaction instead, a machine equipped with a cloth wheel is used to buff the surface.
The process is similar to the buffing done in automobile parts after waxing. The finishing process is applied to provide a glossy, decorative finish to metallic parts.
11. Mass Finishing
Mass finishing is also known as mechanical surface finishing, offering a cost-effective way to polish parts in bulk. This finishing process is employed in a wide range of processes in order to provide uniform cleaning, edge-breaking, smoothing, and polishing of smaller components at once.
This helps to decrease the production time and also obtain a uniform surface finish on the components. The process also includes tumble finishing and vibratory finishing.
12. Shot Peening
This finishing process is done to reduce metal fatigue and stress, preventing failure caused by corrosion and increasing the durability and lifespan of the parts. The process is achieved by using the ball-peen hammer on the metal object which will dimple the surface.
The resulting compression stress will actually strengthen the surface of the material. a piece of special equipment is used to blast small projectiles which is known as a shot. This projectile can be made of ceramic, metal, glass, or other materials.
13. Soda Blasting
This finishing process works in a similar way to abrasive blasting. The major difference is that baking soda is applied onto the surface of the material instead of abrasive media. This process is effective in removing grease, oil, rust, paint, and other surface contaminants.
Soda blasting also offers a gentler cleaning action than abrasive blasting. For these reasons, it is usually employed on surfaces that are vulnerable to scratching.
14. Case Hardening
This process helps to protect a metal surface and makes it harder. It is a metallurgical process that adds protection through surface modification which can be achieved by one of many techniques such as carburizing, nitriding, and micro-casting.
All of these processes can offer desired hard case to metal while the underlying metal remains relatively soft. Case hardening helps to increase the durability and wear resistance of the underlying workpiece.
15. Tumble Slurry Blasting
Tumble slurry blasting is also called wet blasting due to the fact that it uses hot water to provide the desired cleaning or surface preparation outcome. It is a three-part process that involves the application of alkali hot water to prepare the surface, and the blasting of an abrasive slurry onto the surface.
Finally using another alkali hot water application to rinse the part. This process is totally different from other blasting processes as a high-speed-blast wheel is used to drive the slurry instead of compressed air.
16. Parts Cleaning
Parts cleaning is a very important process necessary for removing contaminants before the implementation of other industrial metal finishing processes. ultrasonic washing is one of the most common cleaning methods where high-frequency sound waves are used to produce an intensive scrubbing action. This action helps to remove stubborn contaminants and debris.
Metal Finishing Tools
The common types of metal finishing tools include cartridge rolls, cross pads, convolute deburring wheels, flap disc, and lap wheels.
Most common tools utilized for metal surface preparation are color-coded non-woven wheels which are ideal tools for finishing metals like stainless steel and non-ferrous metals. These tools are widely used for blending, polishing, cleaning, and other industrial applications, making them easy to use with water or solvents.
Cartridge Rolls are used for surface finishing and deburring on ferrous metals, blending, cutting, and light grinding. They come in square and tapered shapes, making them ideal for cleaning hard-to-reach areas. Cross pads have a flexible square shape and are used for finishing tubes, internal threads, and tight spaces. Available in different grit sizes, they are easy to attach with mandrels and power tools.
Convolute deburring wheels are made of non-woven material with bonded abrasive grains, used for stock removal and metal surface finishing. These wheels provide a rigid structure for rapid stock removal and a uniform finish.
Flap discs and wheels are essential tools for aggressive cutting and finishing surfaces on various materials. Flap discs are made with premium bonded grains and R backing, allowing for easy and quick change. Flap wheels are made of aluminum oxide abrasive material, and are designed for grinding and finishing corners and tubes, shaping, cleaning, and finishing contours. These wheels are suitable for deburring, cleaning, blending metals, and other tasks.
Cotton buffing wheels and interleaf flap disc are two common metal surface preparation tools. Cotton wheels are lightweight and designed for easy cleaning, polishing, and finishing of metal surfaces. They are sewn tightly for a stronger structure during the buffing process.
Interleaf flap discs are used for grinding and finishing simultaneously, producing a consistent, uniform finish on various surfaces like aluminum, stainless steel, and sheet metal. They provide a mirror-like finish on most surfaces.
How to Choose The Right Metal Finishes?
When selecting an industrial metal finishing process, factors you should consider include production speed, hardness of the metal, and cost. A process that is compatible with tight deadlines and is not overly abrasive or gentle may damage the part.
Consider checking for all available pricing alternatives can help save money. For example, electroplating with a precious metal may not fit budget constraints, but substituting a less expensive metal like palladium may achieve similar results at a lower overall cost. By considering these factors, you can ensure a successful industrial metal finishing process that meets your specific needs and budget constraints.
Related: What is Sheet Metal? its Types, Sizes & Forming Processes
Advantages of Metal Finishes
Below are the benefits of applying finishing treatment to metal products:
- Cleaning, polishing, and removing surface defects
- Increased durability
- Reduce the impact of corrosion
- Improved decorative appeal
- Enhanced electrical conductivity
- Increasing solderability
- Higher electrical resistance
- increasing wear resistance
- Higher chemical resistance
- Reducing friction effects
- Higher tarnish resistance
- Potential for vulcanization
- Strengthening the substrate