Metal finishing methods are used to alter the surfaces of metals and alloys to make them more resistant to rust and other corrosion, along with providing other properties that prolong the life of the part or product. Additionally, metal finishing methods enhance the appearance of a product through the removal of burrs and other exterior production-related flaws. Metal surfaces can also be anodized, passivated, plated, powder coated or otherwise finished to improve both the durability and aesthetics of a product.
No matter which of the many metal finishing methods a manufacturer might choose, the primary purpose of such processes is to extend the lifespan of the part or product. A properly finished part’s surface has greater resistance to environmental factors that cause deterioration. When properly applied, metal finishing methods provide multiple industries with more durable components that keep their machinery from failing prematurely while also making them prettier.
Comparing Metal Finishing Methods
Techniques for finishing are the final step in the manufacturing process for many components and products made from metals or alloys. By improving durability and enhancing the aesthetics of surfaces, metal finishing methods ensure a longer-lasting and more attractive product. Yet metal finishing methods each have unique advantages and disadvantages, with some better for certain applications and industries than others.
Essentially, metal finishing methods can be categorized in one of two ways:
- Adding to or altering the surface
- Removing from or reshaping the surface
These two categories are used by manufacturers in different industries to improve durability and augment the appearance of parts or products made from metal. Finishing methods also depend on the application for which these products are to be used. Metal finishing methods will differ on how long they take and how much they cost, along with the labor involved for each.
Metal Finishing Methods That Add To or Alter Surfaces
Various metal finishing methods that add to or alter the surfaces of parts or products improve not only durability and aesthetics but also allow easier soldering. Adding to or altering the surfaces of products or parts can also enhance other properties of the metal. Finishing methods like anodization, passivation, plating and powder coating are a few of the more commonly used techniques in this category.
This may require a pre-Anodize or pre-Plate surface finish. Before coating (anodizing, plating, powder coating} all burrs, edgebreak,and surface flaws should be addressed. This can be done by tumble, bead blast, or by hand.
Anodizing
Generally, anodizing involves adding a coating to aluminum or alloys containing the metal. Finishing methods for anodizing aluminum use both a chemical solution and an electric current that causes a layer of aluminum oxide to develop on the metal’s surface. The current also allows the protective layer to form and thicken more quickly than if only chemical reactions were used. This oxidized metal layer makes the component stronger and more durable, while also protecting it against ultraviolet radiation and corrosion.
These finishing methods for metal can also be applied to parts made from nonferrous metals like magnesium, niobium, tantalum and titanium. For such applications, anodization creates a thin film of aluminum that acts as a shield for other nonferrous metals, altering the molecular structure to give the surface additional protective properties. Additionally, though anodized metals typically aren’t colored, this metal finishing process leaves tiny pores that can accept dyes, allowing surfaces that can be made into a wide range of colors.
Applications for anodized metals include:
- Architectural and structural features
- Artwork
- Boating components
- Equipment used in food preparation
- Furnishings
- Home appliances
- Jewelry
- Sporting goods
- Vehicle parts
Like many metal finishing methods, anodization has its drawbacks. The complexity of the process makes it expensive and time-consuming, while it also uses significant amounts of energy that add to production costs and make the process less environmentally sound.
Passivation
Passivation is a chemical treatment that keeps metal components from corroding and otherwise degrading due to environmental factors, including making medical devices more biocompatible. There are a few different types of metal finishing methods that involve passivation, though the process is primarily carried out with citric or nitric acid. As passivation doesn’t change a component’s size, passivated parts retain their tolerances after having gone through the process.
Passivation is usually used for stainless steel and other alloys of ferrous metal. Finishing methods like plating often first use passivation techniques to create a thick protective coating of metal oxide. This helps prevent premature corrosion that doesn’t noticeably affect surface appearance. The acid solutions used for these metal finishing methods also remove any residual contamination from the production process, leaving passivated parts with exceptionally clean surfaces.
Applications for passivated metals include:
- Aerospace components
- Food processing equipment (using citric acid passivation)
- Passivated nickel for sewage and water treatment
- Surgical instruments
- Ball bearings
- Fasteners
- Semiconductors
- Solar cells
- Medical implants
- Orthopedic prosthetics
- Packaging for canned foods
- Electronic sensors
- Jewelry
However, not all metals can be passivated. With certain types of metal, finishing methods that include passivation can make component surfaces flake. Additionally, some industries aren’t able to use even properly passivated metals, as the chemicals used in the process can cause problems with certain electrochemical applications.
Citric vs. Nitric Passivation Methods
Many different grades of stainless steel can be passivated with either citric or nitric acid. However, it’s become apparent within the industry that there are environmental and safety hazards resulting from nitric acid. For this reason, citric acid is seen as the more environmentally friendly and safer option. Citric acid passivation has become increasingly common since its use in the industry began in the 1990s. Passivation metal finishing methods using nitric acid are thus falling out of favor, as nitrogen oxide produced through the process contributes to smog and acid rain, while in turn requiring more regulatory oversight. On the other hand, citric acid passivation requires neither compliance with regulations nor special equipment and handling.
Plating
Though plating requires several steps, this finishing technique involves coating a product or part with a layer of conductive metal. Finishing methods like electroplating and electroless plating are two of the more common types of metal plating, though there are other metal plating methods. These include carburizing, coating with plasma spray, immersion and physical vapor deposition (PVD).
These metal finishing methods both augment appearance and improve the functionality of components. Plating hardens surfaces to heighten strength, inhibits corrosion and provides parts with a better friction coefficient, while also improving how well paint sticks to metal surfaces. Generally, plating adds a layer of tougher metal to help parts resist abrasion and provide greater stability, with plated finishes made from metals that include aluminum, chrome, copper, gold, nickel, tin and zinc.
Applications for plated metals include:
- Connectors for electronic equipment
- Electrodes for diagnostic equipment like ECGs (electrocardiograms) and EEGs (electroencephalograms)
- Medical implants and devices
- Parts like bolts, nuts, pins, screws and washers
- Surgical instruments
There are, of course, a few downsides for manufacturers who use plated metal. Finishing methods like plating are often overly time-consuming and sometimes also require manual polishing to achieve the desired quality finish. Plated metals can also chip and crack more easily under stressful conditions, especially when using less expensive metals or alloys. Additionally, surface imperfections like scratches or pitting can become more apparent if the process isn’t done right.
Electroless Plating vs. Electroplating
Two commonly used metal plating methods are electroless plating and electroplating. Also known as chemical or auto-catalytic plating, electroless metal finishing methods use a chemical process to deposit metals like copper, gold, nickel or silver onto a part’s surface. Electroplating uses a chemical bath combined with electricity to improve the conductivity and strength of components. The main difference between the two metal finishing methods is that electroless plating requires no special equipment, whereas electroplating does.
Powder Coating
Unlike painting, powder coating finishes use a dry powder rather than a liquid. Powder coating processes use various dry ingredients and additives to electrostatically deposit the powder onto the surface of a metal. Finishing methods involving powder coating offer metal surfaces better resistance to chemicals, impact, water and weather conditions, while also improving aesthetics. Generally, powder coating is one of the more popular metal finishing methods for aluminum, as it makes components extremely durable while also offering nearly unlimited color and texture options. Powder coating is also more environmentally friendly than painting, as it doesn’t involve the use of solvents.
Applications for powder-coated metals include:
- Architectural applications like frames for doors and windows
- Automotive engine parts
- Beds of pickup trucks
- Bicycle frames and parts
- Car radiators
- Exercise equipment
- Exterior components on vehicles, such as bumpers, decorative trim, door handles and hubcaps
- Household appliances
- Modular furnishings
- Sporting equipment like golf clubs and ski poles
Though powder coating can be used for all manner of metal surfaces, it does have its disadvantages. Powder-coated metal finishes are generally more costly to produce, requiring specialized equipment like a spray gun, oven and booth, while also being difficult to touch up. Unlike painting, powder coating metal finishing methods also allow less control over surface smoothness, along with the amount of coating applied and the speed at which it’s applied. Too much exposure to ultraviolet radiation can also cause the powder coating to break down.
Metal Finishing Methods That Remove From or Replace Surfaces
Though these types of metal finishing methods are used primarily to augment aesthetics, they can also provide additional protection for component surfaces. Metal finishing methods that involve removing and reshaping material on the surface of a product or part include abrasive blasting, buffing and polishing, deburring and tumble finishing. Among these are also subcategories of these metal finishing techniques.
Abrasive Blasting
These metal finishing methods propel abrasive materials in a stream against the part’s surface. Abrasive blasting combines surface finishing with cleaning, so saves manufacturers both time and money. The process involves removing surface contaminants that can prepare surfaces for other metal finishing methods like coating, painting or plating. Abrasive blasting techniques generate cleaner and smoother finishes, while also increasing the surface area and altering the shape of a component.
There are numerous variations in abrasive blasting techniques, based mainly on the type of abrasives and tools that are used for metal finishing. The most common method is known as shot blasting, which uses shot made from materials like aluminum oxide or carbon grit. This is then combined with sand to sandblast the surface. Another common media involves glass beads, a technique known as bead blasting, though many other materials like garnets, silicon carbide or walnut shells can also be used.
Applications for abrasive blasted metals include:
- Cleans rust or oil from surfaces
- Making dies or molds
- Precision components
- Preparation for other metal finishing methods like painting, plating or powder coating
- Reinforcing metal surfaces
Facilities that use abrasive blasting tend to be noisy and dusty, while the technique may also release toxic materials into the surrounding environment that present a hazard. For these metal finishing methods, proper PPE (personal protective equipment) is often required.
Micro Abrasive Blasting
Sometimes referred to as “pencil blasting” due to the tiny nozzles this type of abrasive blasting uses, the process suits the production of smaller precision parts that require tight tolerances. It allows manufacturers to precisely shape surface texture and deburr components. Unlike tumble blasting, only one surface or a portion of the surface is blasted.
Tumble Blasting
This metal finishing technique applies blast media to components that slowly rotate within a basket. It can be done with either batches or a single part. It’s one of the more economical metal finishing methods for texturing or deburring the surfaces of smaller components.
Buffing & Polishing
Metal finishing methods like buffing and polishing are used across many industries, as they prepare part surfaces for final use or as a precursor to another finishing process. In short, both buffing and polishing are metal finishing methods that use abrasive materials to create smooth surfaces. It enhances the appearance of a part or product, with buffing resulting in brighter and smoother finishes.
In addition to their ability to improve the aesthetics of a surface, buffing and polishing provides a means to halt corrosion, while also removing oxidation. This in turn significantly prolongs a component’s lifecycle, increasing the durability of surfaces made from metal. Finishing methods involving buffing and polishing further augment conductivity, decrease chemical-related corrosion and reduce electrical resistance.
Applications for buffed or polished metals include:
- Architectural metal
- Cooking utensils and kitchenware
- Electronic devices
- Exterior parts on boats, ships and yachts
- Handrails
- Jewelry
- Metal sculptures
- Musical instruments
- Prevents corrosion and kills microbes on plumbing implements used in dairy and pharmaceutical processing
- Vehicle exteriors and interiors
However, as there’s a certain level of toxicity in the substances used to buff and polish metal, these finishing methods can pose a health hazard to workers and environmental issues for manufacturers. Industrial buffing and polishing also require specialized and expensive machinery, not to mention the considerable time it takes to achieve the right level of buffing or polishing. Additionally, both metal finishing methods are temporary when used on thinner surfaces.
Buffing vs. Polishing
Though laypeople often use the terms buffing and polishing interchangeably, the two techniques have their distinctions. Buffing is used primarily to create a smooth surface to achieve a lustrous finish and eliminate defects. Polishing involves removing imperfections like scratches and smoothing the surface as well but also focuses on improving surface reflectivity and clarity.
Deburring
Burrs are those ridges or rough edges that result from machining or stamping operations during the initial stages of fabrication. Burrs can occur in difficult-to-access areas that can compromise structural aspects or tolerances of a component, so removing burrs is an integral part of manufacturing any precision components. Deburring reduces friction between metal parts while improving appearance and lengthening lifespans. Additionally, metal finishing methods used to deburr components protect tooling used in other processes, make products safer, decrease wear on machinery and cut assembly time.
The purpose for deburring metals includes:
- Ensuring components are standard shapes and sizes
- Helping paint adhere better to surfaces
- Smoothing edges of parts to make them safer
Myriad methods for deburring metal parts can be used, but typically deburring metal parts is done either manually or en masse. These mechanical processes must also maintain the components’ structural integrity, so the different metal deburring methods have various advantages and disadvantages.
Manual Deburring
Deburring by hand is a comparatively expensive and time-consuming process that involves several steps. Manual deburring can also be done with tools like buffing jacks and wheels that use non-woven abrasives. While technically it’s a manual method, many finishing services utilize machines to make the deburring process quicker and more accurate. Additionally, the use of human labor in the process will vary according to a worker’s experience, while it’s also easy to miss areas or deburr them incompletely.
Deburring in Batches
Mass metal finishing methods can be used to deburr parts more efficiently than deburring done by hand. Sometimes just referred to as mass finishing, batch deburring is not only more economical but also offers greater component uniformity. However, there are downsides, as it requires a separate system for deburring, which requires additional space for manufacturers wanting to deburr parts directly after fabrication. Generally, batch deburring uses a tumbler, which is why mass deburring is closely related to the tumbling finishing process.
Tumble Finishing
Tumble finishing is a type of mass metal finishing method that serves to deburr and polish rough parts in batches. The process can take several hours to just a few minutes, contingent on the requirements for the components being finished. Certain types of these finishing techniques are referred to as barrel finishing, though tumble finishing generally seeks to clean, deburr, de-flash, descale, make radii, polish and/or smooth surfaces. Two commonly used mass metal finishing methods that involve tumbling processes are high-energy tumbling and vibratory tumbling, both of which have their own advantages and disadvantages.
High-Energy Tumbling
High-energy tumbling uses a lidded barrel, into which components, media, water and a wetting agent are enclosed all together. This barrel is then placed on a carousel with other barrels and rotated. While the barrel spins one way, the carousel spins the other way, generating centrifugal force that provides the desired finish. It’s one of the more efficient metal finishing methods, which can be used for aluminum, different types of steel and titanium.
High-energy tumbling equipment is less expensive and used to finish harder metals that require more robust action to remove blemishes on components. The process causes less wear to the abrasive media used, though it’s more appropriate for smaller batches. Producing coarser finishes, it works well for components with substantial radii. However, the rapidity of the process increases the risk of indenting surfaces as components collide.
Vibratory Tumbling
In contrast, vibratory tumbling uses bowls or troughs rather than barrels. The process smooths parts using abrasive media, gravity and water. Used in the metal finishing sector since the 1940s, this metal finishing method produces incredibly smooth surfaces, while it’s also safer for more fragile components. Vibratory finishing is quieter and more versatile, allowing both batch and continuous processing, while also offering a smoother finish. Additionally, the process can be automated, making it even more efficient. However, media wears out more quickly with vibratory finishing, while the process is rather complicated and equipment is costly.
To learn more about which of the metal finishing methods best fits your needs, contact the finishing specialists at RP Abrasives for a quote on your project.