The basics of liquid and powder coatings
Taking on finishing in-house? Get to know the coating technologies first
Metal fabricators know that once a metal product leaves its doors that it has lost its ability to guarantee delivery. The fab shop is at the mercy of its subcontractors. That scenario has more fabricators considering in-house finishing capabilities. This guide is designed to help that decision-making
Almost all metal fabricators consider on-time part delivery a key metric. In today’s just-in-time manufacturing environment, the parts need to be there so the customers don’t miss a beat in their own manufacturing facilities.
To maintain their ability to get parts where they need to be and when they need to be there, more fabricators are investigating in-house finishing capabilities. They know that once metal parts leave the shop to receive a liquid or a powder coating, they have lost their ability to guarantee a delivery date. It is quite literally out of their hands.
Now, a finishing contractor may be a good supply chain partner, but a metal fabricator is one of its many customers—perhaps one of the smaller ones. If the custom coater needs to clear the production schedule for one of its largest customers, guess where that leaves the small batch from the metal fabricator?
Metal fabricators looking to take on finishing should know about two of the most common finishing alternatives—liquid and powder coating—and the requirements involved for a company hoping to apply one or both.
What Makes up the Coating?
Liquid. The basic raw materials comprising a liquid coating are additives, carriers, pigments, and resins. Additives make up the smallest portion of any liquid coating composition, but they impart special characteristics on the overall finish; for example, they might assist with rust prevention or UV protection. The carrier is the main liquid used to formulate the paint. The carrier can be water, solvent, or a combination of the two. Liquid coatings heavy in solvents traditionally have been the dominant form of liquid finish applied to metal parts over the years, but more interest has been directed to waterborne and high-solids coatings, which release a minimal amount of volatile organic compounds (VOCs) during application when compared to traditional solventborne coatings. Pigments play a role in final appearance and performance to some extent. As a rule, the volume of pigments influences the gloss of the film. The more pigment present, the lower the coating’s gloss. Resins act as the base of the liquid coating. They primarily govern the overall performance of the coating, helping the paint to excel for particular applications.
Powder. Powder coatings don’t require a carrier. The additives, pigments, and resins are formulated in a powder form. To apply the material to parts, the powder is electrostatically charged and conveyed via compressed air. The charged powder is attracted to a grounded part. The part is then taken through an oven, where the heat changes the powder from a solid to a liquid and then to a solid coating. Generally, powders do not have any or extremely low VOCs.
How Does the Coating Affect the Environment?
Liquid. Solventborne coatings are often specified for a finish because of their dependable performance and ability to air-dry in a matter of minutes. Unfortunately, most of the traditional liquid paint formulations from yesteryear no longer are around because of the need to reduce VOCs emitted during application.
That has led to the development of more environmentally friendly coatings, such as new waterborne formulations and high-solids coatings, that emit low VOCs. The waterbornes, which have come a long way from the early versions used 15 years ago, are slowly growing in acceptance. However, some manufacturers still have reservations about applying a water-based product directly to metal. High-solids are liquid coatings that have a solids content of at least 65 percent, which means minimal solvents are present. But fewer solvents also means that the coating is more viscous. That has led to the development of multipart application systems (referred to as 2K systems if two parts are mixed, 3K if three parts are mixed, etc.) that are formulated to be mixed only seconds before application.
All finishers that apply liquid coatings spray to waste. The overspray can’t be reclaimed. If filters are used to capture the overspray, the finisher has to dispose of the filters according to regulations established by local authorities.
Powder. The powder booth does not require an exhaust. As stated previously, if any VOCs are emitted during the powder coating process, they are typically very low.
Powder coatings can be recycled with the right reclamation equipment. Companies looking for Class A finishes have reclamation systems that depend on thorough cleaning and maintenance because any cross-contamination of reclaimed material ruins the original material’s ability to deliver a specific color.
Again, disposal of unreclaimed powder coating material is governed by local regulations. In some instances, local law may require disposal in sealed containers or require that the powder coating material be baked into the form of a brick for disposal in a local landfill.
What Are the Characteristics of the Final Coating?
Liquid. When someone talks about a Class A finish, typically used to define the coating quality on a new automobile, most people think of liquid coatings. Specialty finishes, such as those incorporating metallic flakes, also are possible in liquid coatings.
Durability has improved over the years as well, especially with the emergence of 2K coatings. These are typically used on large metal parts that can’t be powder-coated because they can’t fit into a typical booth and oven setup.
Liquid coating finishes can be applied in various thicknesses. Obviously, the more mils applied, the better the protection. In many instances, a manufacturer, such as an automaker, will seek to balance maximum protection with the minimum amount of paint mil thickness.
Powder. Powder coatings offer the same characteristics that come with liquid coatings.
Properly cured powder coatings can offer superior protection against chipping, scratching, UV rays, and corrosive elements. This is why powder coatings are often specified to coat metal products destined for outdoor use.
Material development has progressed to the point where powder coatings can deliver a Class A finish. In fact, a major European automaker and a U.S. motorcycle manufacturer are using powder coatings for their clear coating. However, many manufacturers, including automakers, still prefer liquid coatings for that topnotch finish.
Standard powder coating finishes are applied in the thickness range of 2 to 4 mils. Specialty finishes like a hammertone or a texture are usually 3 to 5 mils thick. Functional coatings can be 10 to 40 mils thick.
What Type of Cleaning Is Necessary Before Parts Are Coated?
Liquid. A lot of fabricators simply wipe the part clean with a rag soaked in solvent. Others rely on a wash of some kind with pretreatment chemicals.
Solvent helps to prepare the metal surface because it has aggressive cleaning action and actually prepares the surface for adhering to the paint. New coatings that have less solvent content may require much more formalized pretreatment processes to achieve a quality finish.
Powder. Pretreatment is critical when it comes to powder coating. If a powder coating is going to last, the part needs to be thoroughly cleaned.
Pretreatment can range from a simple abrasive media blasting chamber to a multistage pretreatment system with several chemical application and rinse stations and an oven. (Parts have to be dried and cooled before any application of powder takes place.) Some of the pretreatment chemicals, such as zinc phosphate, have to be treated before disposal, but newer, environmentally friendly pretreatment chemicals have emerged in recent years to ease the disposal hassles. The Environmental Protection Agency, however, likely will seek changes in the near future that will call for treatment of all wastewater prior to discharge.
What Are the Basic Booth Requirements?
Liquid. Booths or stations used in liquid coating are typically made of metal. Whether in a manual or an automated setup, the paint is often sprayed to waste.
Fumes are exhausted outside during the painting process to keep the work environment in and around the paint booth free from the strong odors.
Powder. In some instances, particularly if batch finishing is occurring, a shop might use the same booth for both liquid and powder coating. But fabricators need to keep in mind that if they choose to powder coat in a liquid application booth and exhaust overspray outside the building, employees could be walking out to cars with all new finishes at the end of the shift on a very hot day—depending on where that powder overspray landed. For the most part, however, filters and correct powder coating technique should prevent most powder overspray from going outside.
If a company is doing any kind of high-volume powder coating, it should consider a reclamation system. Single-color reclamation systems are cartridge-based and typically made of stainless steel. Multicolor systems, which are designed for fast color changeout, are plastic, making them easier to clean. Sophisticated fan setups keep the overspray in the booth, and the polymer-based interior walls prevent the powder from adhering to them. The overspray is collected and recycled for another application.
How Do the Application Guns Work?
Liquid. Conventional spray guns for liquid coatings rely on highly pressurized compressed air (2 to 3.5 bar) to propel atomized coating material to the surface. These guns have a low transfer efficiency when compared to more modern paint application guns and may not be approved for use in some areas. However, they are relatively inexpensive and easy to maintain.
High-volume, low-pressure (HVLP) spray guns also atomize the paint like a conventional spray gun, but use low-pressure air, usually less than 0.7 bar, to propel the paint onto the object. The lower velocity of the air results in less paint jetting through the spray gun’s air nozzle and allows for a more controlled application. Higher application rates are then possible.
Other spray gun technologies are airless spray guns that force the paint through a smaller nozzle and electrostatic-based guns that rely on paint being “attracted” to the object to be coated. (In electrostatic application, a charge is applied to the liquid coating while it is being atomized. In turn, the coating is attracted to any surface that is grounded, which happens to be the workpiece. Obviously, this approach works very well with metal workpieces.) For new users of paint technology, conventional or HVLP spray guns are typically the choice for paint application.
Powder. Powder coatings rely on the charging of material for application as well. Today’s market primarily uses corona guns to do this.
These guns impart a strong electrostatic charge on the powder material as it leaves the spray gun via compressed air. As the powder coating is discharged, it is attracted to the grounded metal part hanging from a metal rack. It is necessary for the rack to have some area of exposed metal to ensure a solid grounding for good electrostatic powder application.
Tribo guns also are used for powder application. With this method, the powder material picks up a positive charge while rubbing against the gun’s interior Teflon® walls.
An operator needs less experience to apply powder coatings in an efficient and effective manner than someone applying liquid coatings.
What Are the General Oven Requirements?
Liquid. Because most liquid coatings can air-day, ovens aren’t necessarily needed. However, if a manufacturer wants to speed up drying times, it needs an oven that is capable of heating between 130 and 170 degrees F.
Powder. Powder coatings need a much hotter oven to melt the particles so that they can flow and react chemically to form a smooth finish on the workpiece. Most powder coatings reach this stage in an oven heated to 350 to 400 degrees F. For a proper curing, the substrate must be at this temperature for at least 10 minutes.
On some occasions a manufacturer can use the same oven for drying liquid coatings and curing powder coatings (as long as both coating chemistries are compatible with each other). The key is scheduling parts headed through the oven so that the temperature can be adjusted accordingly for specific coating jobs.
Why Use a Liquid Coating?
This coating technology is prevalent for many reasons:
- It can be cured quickly, resulting in faster production cycles.
- It is cost-effective in the sense that the initial investment for the equipment is much less than for powder coating equipment (although liquid material is more expensive than powder and can’t be reclaimed).
- The coating can be used to finish parts containing sensitive materials, such as a metal cylinder with a rubber seal, because it does not require dramatically high temperatures to dry.
- It can be used to finish very large parts that are not able to fit into an oven or can’t be moved easily.
- A thin coating is achievable. It can routinely be applied as thin as 0.5 mil.
- It provides an automotive-quality finish (although this performance advantage over powder coatings has been narrowed greatly over the years).
Why Use a Powder Coating?
Users of powder coatings turn to this technology for a few specific reasons:
- As soon as the workpiece has cooled after curing, the part doesn’t require overly protective handling and immediately can be sent to downstream processes such as assembly or packaging.
- It is very durable. It is commonly used for outdoor applications ranging from outdoor furniture to agricultural.
- The process does not emit VOCs, which means local air quality regulations aren’t likely to be an issue for the manufacturer undertaking powder coating. Additionally, the material can be reclaimed, if the right equipment is installed, which limits the amount of waste material that has to be placed into the waste stream.
- Although it requires a large upfront investment, the long-term costs of applying powder are less than those for a liquid system. In some instances, the price of liquid material may be four times more.
The FABRICATOR is North America's leading magazine for the metal forming and fabricating industry. The magazine delivers the news, technical articles, and case histories that enable fabricators to do their jobs more efficiently. The FABRICATOR has served the industry since 1971.