Basic knowledge

What is powder coating and how does it work?

| Editor: Theresa Knell

Surface finishing by powder coating is suitable for all electrically conductive materials such as steel or aluminum.
Gallery: 1 image
Surface finishing by powder coating is suitable for all electrically conductive materials such as steel or aluminum. (Source: Ebbinghaus)

Powder coating can both protect and optically enhance the surfaces of sheet metal parts.

Windows and doors, sunblinds, railings and even entire facades – many modern everyday products, machines and vehicles are enhanced with powder coatings.

Powder coating protects against corrosion and scratches and also produces wear-resistant surfaces. At the same time, powder coating optically enhance these objects. For technical reasons powder coatings were only used for metallic surfaces for a long time. In the meantime, innovative technologies have made it possible to use this process also for the finishing ofnon-metallic surfaces. Powder coating is particularly suitable for bulky, heavy components.

What is powder coating?In classic powder coating, the surfaces of electrically conductive workpieces are coated with a powder paint. The original powder coating process is therefore only suitable for metallic objects. Prior to the actual powder coating, the workpieces must be cleaned and pre-treated. Then they are forwarded to an application system. At this point, the powder is applied to the workpieces and baked into their surfaces at temperatures of up to 250°. The technically correct term for this process is cross-linking, and constitutes the core of powder coating process. The effect of heat causes cross-links between the structures of the paint powder and the surface of the workpiece. The workpieces are forwarded by a conveyor system to the various powder coating stations.

Certifications and standards

Several standards apply to powder coating:

  • DIN 55633 regulates the requirements for corrosion protection by means of powder coating as well as the
  • evaluation of steel structures with powder-coated surfaces.
  • EN 15773 contains the technical specifications for the powder coating of fire resistant materials or
  • diffusion galvanized steel workpieces.
  • EN 12981 regulates the safety requirements for the coating booths in which the powder coating is carried out.

Many companies that offer powder coating as a service have independent certificates for their production technology and the quality of their products. The coatings must also meet strict quality requirements and, for example, withstand adverse weather conditions for several years.

The history of powder coating

The basics of powder coating were developed in the late 1940s and early 1950s. Initially, organic, pulverized polymers were applied to metallic surfaces by means of flame-spraying. The first standard process for powder coating wasthe whirl sintering process developed by the German scientist Dr. Erwin Gemmer. The disadvantage of this type of powder coating was, among other things, its very high layer thickness, which was not practicable for many applications. In the mid-1960s, this problem was solved by the emergence of theelectrostatic spray guns which allowed for a much finer powder coating to be applied. The corresponding technology was mainly developed in the USA, but the pistols for theelectrostatic powder coating were launched on the European market right from the start. Roughly at the same time - between 1966 and 1973 - thebasic types of plastics used to this day were developed.

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In the late 1960s, the firstcoating booths were introduced. Initially, these were conventional paint shops that had been converted accordingly and which were equipped with a separate high-voltage cable for the power supply for instance. The turnaround came with the development of the so-calledcorona pistol in the early 1970s, when the high voltage supply was integrated into the pistol. It became the prototype of today's pistols used for powder coating.

Application of the powder coating in a powder booth.
Application of the powder coating in a powder booth. (Source: Meeh)

The technology for the coating booths and spray pistols, as well as the coatings themselves, have been continuously optimized since the 1970s. Today, more than 90 % of the powder coating systems are equipped with so-called flat jet nozzles. Since the early 1990s there have been Powder coating conveyors on the market.

In the course of these technological developments, powder coating has prevailed worldwide. Since the end of the 1980s, the market for technical equipment and paints for powder coating has shown a verydynamic growth. A few years ago, around 10% of all paint jobs in Europe were done by powder coating. The main areas of application are general metal coating, powder coating of household appliances ("white goods"), facade coatings as well as furniture and automobile painting. Since then, the powder coating market has probably grown further.

Today, innovative processes make it possible to use powder coating for temperature-sensitive materials such as plastics. Digitization and industry 4.0 will also revolutionize powder coating technologically.

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The process: How does powder coating work?

1. Surface pretreatment

Powder coating starts with thepretreatment of the workpiece to be painted. It consists of several steps. First, the workpieces are fastened to corresponding holders. On these racks they are conveyed through the entire coating process. It is important that these racks ensure the electrical conductivity of the workpieces by grounding them.

Then the residues such as dust, scale, rust, dirt and grease are removed from the workpieces. Under industrial conditions, they are cleaned in a fully automatic washing line. Cleaning agents based on acetone or ethanol, which evaporate at room temperature, leave no residue on the surface of the workpieces. Solvents or aqueous cleaning agents are also used to remove grease residues. The workpieces are usually sprayed or dipped for cleaning. Mechanically, the cleaning process is supported through brushing, blasting or grinding as required.

Subsequently, several powder coating processes are carried out to prepare the powder coating for the application of several conversion layers. These layers increase the active surface area and thus also the adhesion of the coating. Typical processes involve the phosphating of steel, the etching of galvanized steel and the chromating or anodizing of aluminum. Due to corresponding legal regulations, chromium-based conversion processes are becoming less important. If the workpiece is also to be coated with corrosion protectiont prior to coating, it is also applied in the form of a conversion layer.

At the end of the pre-treatment, the workpieces are dried Adhesive water dryers are typically used for this purpose. Before the actual powder coating begins, the surface of the workpieces must be absolutely dry. A thorough surface pretreatment is indispensable for a proper powder coating, to exclude paint detachment or craters in the paint layer.

2. Application of the paint

After the pre-treatment has been completed, thepaint particles are applied. The application of the paint also consists of several steps:

In-feed and preparation of the paint

As a rule, the paint is forwarded directly from the batch into a container. If recovered powder coating is re-used, fresh powder is mixed with the recycled one. Prior to the transport of the paint particle through the plant, they are usually fluidized (mechanically shaken and swirled). This improves the flow properties.

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The transport processes of the paint particles are distinguished between mass conveying and precision conveying. Mass conveying is used to transport the paint between different containers. More than 5 kg of paint powder are conveyed per minute. In precision conveying, the powder coating reaches the spray gun in quantities of 50 to 100 g/min. In order to avoid irregular powder coating layers, this process requires the use of an exact, uniform and pulsation-free dosing of the paint powder. This process is carried out with special injectors. A very modern funding method in the field of powder coating is the digital dense phase conveying which can be used for both mass and precision conveying. It is carried out according to the principle of counter pressure conveying, in which paint powder is alternately sucked into two chambers or a vacuum is generated. The process enables exact dosing of the paint particles, fluidization of the paint powder is not required.

Charging and application of the paint

The actual powder coating is carried out by means of powder spray pistols. The pistols are basically also suitable for manual coating applications and coating plants. The rule today is, at least in larger companies, the fully automatic and digitally controlled powder coating process. So-called powder spray booths, which are completely closed except for the openings for the transport of the workpieces, are also frequently used. They are made of glass, metal or plastic. The highest quality requirements for a powder coating are fulfilled by so-called cabins, which are operated under overpressure. The entire powder cycle takes place inside them, so that dust and dirt ingress are excluded.

View inside a powder coating plant.
View inside a powder coating plant. (Source: MS Oberflächentechnik)

The Application of the paint is done electrostatically by electrically charging of the paint particles. The powder spray pistol generates an electrically charged powder cloud, whose particles are deposited on the surface of the workpieces, adhere electrostatically on it and form the powder coating. There are various methods for charging the paint particles, depending on the technical equipment of the spray pistol used:

Ionization (corona application)

For charging by ionization the paint particles are guided along an electrode with a voltage of 30 to 100 kV. The high voltage ionizes the ambient air of the particles. When the paint particles pass the electric field between the grounded workpiece and the high voltage electrode of the spray pistol, air ions accumulate on its surface. However, this is only 1 to 3 % of the air ions present, the remaining free ions are called space charge. In the so-called low-ion corona charging process these ions are absorbed by an additional electrode at the tip of the powder spray pistol where the charge is dissipated. This process is named after the corona - a blue-white light at the tip of the high-voltage electrode.

The most important advantage of the corona application lies in its universal application possibilities. Many coatings and especially the majority of effect powder coatings are unsuitable for other charging processes. Above all, the low-ion corona charging process supports the formation of a very smooth coating. The air consumption and wear of the paint spray pistols is lower than with other charging variants.

Triboelectric or electrokinetic charging

For triboelectric charging, the electrostatic charge is generated by the contact of the particles with the walls of the spray pistols. In a frictional process, electrons are released from the coating materials of these walls. In order to create the largest possible contact surfaces, the spray pistols for triboelectric charging are fitted with an annular gap or spiral channel equipped with a Teflon coating. Since the paint particles are detached very quickly from the spray pistol, their electrostatic charge is maintained even during the atomization process.

Triboelectric charging is ideal for powder coating consisting of several layers of paint. Further advantages of the process are lower investment costs and facilitated automation. With manual powder coating, it is possible to position the pistol as close as desired to the surface to be painted. However, this process can only be used for a few effect paints, as the desired effect image cannot produced with it.

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Alternative charging methods

Alternative charging processes for powder coating are vortex sintering, coil coating and the application of powder coatings in the form of aqueous suspensions. The latter is mainly used in the automotive industry. Whirl sintering was used for the application of thermoplastic, non-bonding powder coatings and is no longer in use today. Thanks to the innovative coil coating, powder coating of steel strappings (coils) is carried out directly in the rolling mill. Since these strappings pass through the rolling mill at a very high speed, one challenge of this process is to achieve a sufficiently fast cross-linking between steel and powder coating.

Curing (cross-linking) and drying

The penultimate operation of powder coating is the curing or cross-linking of the powder coating onto the workpiece surface. It takes place in an oven or dryer. The baking temperatures for powder coating are usually between 140 and 200°C. Theoretically temperatures between 110 and 250°C are possible. Coatings that cross-link at temperatures of less than 140°C are referred to as low-temperature coatings. Some of them require an extension of the stoving time.

The bonding process consists of the holding time - the time the coating is kept at the baking temperature - and the heating time, which depends on the thickness of the coating layer. The sum of the holding and heating times results in the dwell time of the workpiece in the curing oven. The dwell time and the exact oven temperature depend on the baking temperatures, the workpiece throughput and dryer-specific factors. The holding time for a powder coating ranges between 5 and 30 minutes.

Materials suitable for powder coating

Practically all metals can be powder-coated. However, in reality, especially stainless steel and aluminum are powder coated. The only exceptions are cold-brazed metals, pure zinc workpieces and objects with silicone residues on their surfaces. It is important that all elements of a workpiece that is to be powder coated are heat-resistant in order to withstand the baking process.

Powder coating makes it possible to produce a wide range of color cards, including special shades and different degrees of gloss. Effect powder coatings, for example, produce metallic or micaceous gloss effects. It is also possible to provide a powder coating with antimicrobiological, antistatic or anti-graffiti properties or to make it particularly robust. Only fluorescent and neon shades of the RAL card are no longer used for powder coating due to their comparatively short durability.

It is also important to avoid mistakes in powder coating by knowing the overall process.

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Layer thickness and particle size of powder coatings

The coating layer of a powder coating is typically 60 to 120 μm, but layer thicknesses deviating from these values are also possible.

The powder coatings consist of granular, dry paint particles with a particle size ranging between one and 100 μm. They are usually produced on the basis of epoxy or polyester resins. Hybrid systems are able to use both substances as binders. In certain application areas, PVC, polyurethane, polyamide or acrylic are also used in powder coating. The mechanical properties of a paint during powder coating - such as its flowability and particle size - decide its behavior. In contrast, their chemical composition plays only a secondary role. Since the paint particles are temperature-sensitive and their melting point is already at around 50°C, they often tend to sinter during their application.

Powder coating recoveryFor the recovery, paint powder that has not been applied to the workpieces is collected, processed and returned to the production cycle. However, this method is only worthwhile if no frequent color changes are planned and powder coating is to be carried out in large quantities. The particle size of recovered powder coatings is generally smaller than that of “fresh” coatings. Recovered paint and new powder are therefore mixed together in a fixed ratio and prepared before its application. In order to ensure a constant quality of the coating powder, this process is often controlled automatically by so-called fresh powder systems. Every two years the Paintexpo Trade Fair for Industrial Paint Finishing Technology is taking place. In Karlsruhe, the industry is showing the new possibilities for the economical surface technology of functional parts and assemblies.

This article was first published by blechnet.com.

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