Powder painting method. Metal powder coating and its features

Modern technologies for painting metal products with powder paints are rapidly developing. The use of liquid paints and varnishes in production conditions is gradually fading into the background. Most manufacturers of metal products opt for powder paints, as they provide a high-quality and durable decorative and protective coating.

What is Powder Coatings

This high-tech coloring material has unique properties that liquid paints do not have. They consist of coloring pigments, film-forming resins and catalysts that ensure the hardening of the material. There is no solvent in their composition, and air performs the function of the dispersion medium. This makes powder coatings less toxic and cheaper to manufacture.

What is dry paint

The powder coating method is not suitable for all surfaces. It is used when additional corrosion protection, durability and strength are needed. In some cases, powder paint is able to provide electrical insulation.

Powder coating is used mainly in industrial production for:

• forged products, aluminum profiles and galvanized metal;
• laboratory and medical equipment;
• furniture;
• household appliances;
• sports equipment.

The benefits of powder coating

1. Minimum amount of waste. Coloring on high-quality equipment gives an efficiency of up to 98%.

Sanitary and hygienic conditions there are changing for the better. This is an environmentally friendly technology, in which even in the furnace the concentration of volatile substances does not reach the maximum permissible standards.

2. Solvents are not used, which gives less shrinkage and virtually no pores on the surface of the product.
3. More economical use of material when painting. The powder coating cures within half an hour and gives a thicker single coat. The economy also lies in the absence of the need to maintain large production areas for drying the product in the air. During transportation, the harder powder coating is not damaged, which makes it possible to reduce packaging costs.
4. The powder coated surface is UV resistant, electrically insulating and anti-corrosive.
5. Powder paint allows you to create a palette of more than 5000 colors.
6. Reduced degree of explosion and fire hazard in production.

Disadvantages of Powder Coating

1. The melting of the powder is carried out at a temperature above 150 0C, which makes it impossible to paint wood and plastic.
2. It is difficult to apply a thin layer of paint.
3. Equipment for dry dyeing is narrowly focused. In large ovens, it is inefficient to paint small parts, and in a small oven, it is not possible to paint a large surface area.
4. A separate container must be used for each color.
5. It is difficult to paint irregularly shaped objects or prefabricated structures.
6. Equipping a painting line requires a lot of investment.
7. If defects appear on the surface, they cannot be eliminated locally, and the entire product will have to be repainted.
8. There is no possibility to do tinting, you can use only factory paints.

Types of powder paints

According to the type of film formation, dry paints are usually divided into:

• thermosetting. The finished film is formed after chemical transformations;
• thermoplastic. Coloring occurs under the influence of high temperature without chemical reactions.

Thermoset paints are more common. For their preparation, acrylic, epoxy or polyester resins are used. Their advantage is that the surface will not deform after reheating. Thermoset paints can be used to paint products that will be used in harsh conditions.

Thermoplastic paints can use polyesters, vinyls, or nylons as resins. A hard coating is formed without a chemical reaction only by cooling and solidification. The composition of the cured paint is similar to that of the original material. This allows repeated heating and melting of the powder.

Methods for applying powder paint

The technology of dyeing with a dry material allows the use of several options for spraying the powder.

Application of paint with directed air flow. The product is heated and with the help of an airbrush the powder particles are distributed over the surface. A high-quality coating is obtained only after the most accurate determination of the heating temperature of the metal. The disadvantage of this method is the need for additional heat treatment after polymerization.

electrostatic spraying. This staining method is the most common. The adhesion of particles is provided by electrostatic voltage. After polymerization, the product cools down in natural conditions. Non-adhering powder can be reused, special chambers are provided for its collection. This method is best suited for products of simple shape and small size.

1. The use of flame. For this staining method, guns with a built-in propane torch are used. Powder particles are melted, passing through the flame, and fall on the surface of the product in a semi-liquid state. The surface of the product is not exposed to heat. The paint layer is thinner and more durable. This method is mainly used for staining large objects.

Dry dyeing equipment

In powder coating, applying paint is not the final step. In order for the polymer to fix on the surface, it is heated in furnaces. The powder coating line consists of:

• powder application chambers. In this sealed chamber, a coloring agent is applied to the metal;
• electrostatic spray gun for powder application. Thanks to the static electricity generated by a high voltage source, the paint is evenly applied to structures of any shape;
• polymerization chambers. It provides a constant temperature and is equipped with a ventilation system. In it, the process of polymerization of the paint and its uniform distribution over the product takes place;
• compressor. It is designed to create a certain pressure in the staining chamber;
• devices for transporting metal products. Heavy and large painted products must be carefully transported so that the powder does not crumble from them. This is provided by special carts moving along the monorail.

Powder Coating Technology

It is possible to obtain a high-quality decorative coating on a metal product using powder paint only by strictly observing the painting technology. The technique lies in the fact that dry paint particles are sprayed onto a cleaned and degreased surface. An even, uniform layer of powder on the product is ensured by the fact that paint particles with a positive charge easily adhere to a negatively charged metal surface. In order for these particles to turn into a layer of paint, they are baked in an oven at a temperature of 150-250 0C.

Powder coating technology consists of three stages:

• preparation;
• staining;
• polymerization.

Preparing the surface of the product for painting

This stage is the longest and most difficult. The further quality of the coating will depend on the preliminary preparation of the metal surface: strength, elasticity. The preliminary stage includes:

• cleaning from pollution;
• degreasing;
• phosphating.

Rust, oxides, dirt are removed from the metal surface. If the old coating is left, the paint will not adhere well to the surface and the coating will not last long.

The most effective method for removing rust and oxides is shot blasting. For this, sand, steel or cast iron granules are used. Small particles under strong pressure or under the influence of centrifugal force are fed to the metal and knock off dirt from it.

Chemical cleaning or pickling can be used. For this, hydrochloric, sulfuric, nitric or phosphoric acids are suitable. This is an easier method that allows you to process a larger number of products than shot blasting. But it requires subsequent washing of the product from acids, which leads to additional time and financial costs.

Phosphating the product is similar to priming. The surface is treated with a compound that creates a phosphate film that improves adhesion.

Paint application

Painting is done by electrostatic spraying in special chambers with an air suction system that prevents the paint from getting out. Pass-through cameras are used for painting large objects, and dead-end cameras are used for small parts. There are chambers in which the paint is applied by automatic manipulator guns.

Spraying is done with a pneumatic gun. Positively charged paint particles wrap around a grounded part and stick to it. The whole process goes like this:

• powder paint in a special hopper is mixed with air. Proportions are regulated by means of valves;
• a mixture of paint and air passes through a spray gun with a high voltage source, where the particles receive the necessary positive charge;
• paint is sprayed onto the product and fixed on it;
• exhaust ventilation carries away particles that have not received the desired charge. There they are collected in a special bunker, and then reused or disposed of.

polymerization or baking

The metal product with the applied paint is placed in the oven. In it, under the influence of a constant temperature, the part is heated and the paint polymerizes. The particles fuse to form a film, then solidify and cool. The whole process takes about 15-30 minutes. Curing time depends on product size and oven type.

The temperature in the polymerization chamber is kept within 150-200 0C and depends on the type of paint. The molten powder is able to fill all microroughnesses, which gives good adhesion to the metal surface.

The paint receives all the necessary properties at the curing stage - strength, appearance, protection. After that, the product should cool for 15 minutes. Otherwise, the coating may be damaged, dust and dirt will adhere to it.

Outcome

Powder coating- this is the most economical, fast and environmentally friendly way to obtain a reliable protective surface on the metal. The service life of the product is significantly increased, and the decorative coating can be varied not only in color, but also in structure.

The complexity of the technology lies in the strict observance of all stages. This requires a special production line. Problems may arise when:

• coloring of large-sized objects;
• products of complex shape;
• structures made of mixed materials.

The dry method has undeniable advantages over other types of staining:

• non-waste;
• a variety of colors in terms of cost and properties;
• high physical and mechanical properties of the painted metal surface.

For these reasons, powder coating has become one of the most popular modern methods of protecting metal from damage.

Surface preparation:

The first step in any painting process is surface pre-treatment. This is the most time-consuming and time-consuming process, which is often overlooked, but which is a necessary condition for obtaining a quality coating.

Surface preparation predetermines:

• quality,
• resilience,
• elasticity and durability of the coating,
• promotes optimal adhesion of powder paint to the painted surface
• and improve its anti-corrosion properties.

When removing contaminants from the surface, it is important to choose the most appropriate treatment method and the composition used for this purpose. Their choice depends on the material of the treated surface, type, degree of contamination, as well as the requirements for conditions and terms of operation. For preliminary surface treatment before painting, methods of degreasing, removing oxide films (abrasive cleaning, etching) and applying a conversion layer (phosphating, chromating) are used.

Of these, only the first method is mandatory, and the rest are applied depending on specific conditions.

The surface preparation process includes several stages:

• Surface cleaning and degreasing;
• Phosphating (with iron or zinc phosphates);
• Rinsing and fixing;
• Coating drying.

At the first stage, degreasing and cleaning of the treated surface takes place. It can be produced mechanically or chemically.

For mechanical cleaning, steel brushes or grinding discs are used, and depending on the size of the surface, it can also be lapped with a clean cloth soaked in solvent. Chemical cleaning is carried out using alkaline, acidic or neutral substances, as well as solvents used depending on the type and degree of contamination, type, material and size of the treated surface, etc.

When processing with a chemical composition, parts can be immersed in a bath with a solution or blasted (the solution is supplied under pressure through special holes). In the latter case, the processing efficiency is significantly increased, since the surface is also subjected to mechanical stress, in addition, the pure solution is continuously supplied to the surface.

The application of a conversion sublayer prevents moisture and contaminants from getting under the coating, causing peeling and further destruction of the coating.

Phosphating and chromating of the treated surface with the application of a thin layer of inorganic paint improves the adhesion ("adhesion") of the surface with the paint and protects it from rust, increasing its anti-corrosion properties. Usually the surface is treated with iron phosphate (for steel surfaces), zinc (for galvanic cells), chromium (for aluminum materials) or manganese, as well as chromic anhydride. For aluminum and its alloys, chromating or anodizing methods are often used. Zinc phosphate treatment provides the best protection against corrosion, but this process is more complex than others. Phosphating can increase the adhesion of the paint to the surface by 2-3 times.

To remove oxides (these include scale, rust and oxide films), abrasive cleaning is used (shot blasting, shot blasting, mechanical) and chemical cleaning (etching).

Abrasive cleaning is carried out using abrasive particles (sand, shot), steel or cast iron granules, as well as walnut shells, which are fed to the surface at high speed using compressed air or centrifugal force. Abrasive particles hit the surface, breaking off pieces of metal with rust or scale and other contaminants. This cleaning improves the adhesion of the coating.

It should be remembered that abrasive cleaning can only be applied to materials whose thickness is more than 3 mm. The correct choice of material plays an important role, since too coarse shot can lead to a large surface roughness, and the coating will lie unevenly.

Pickling is the removal of dirt, oxides and rust by using pickling solutions based on sulfuric, hydrochloric, phosphoric, nitric acid or caustic soda. Solutions contain inhibitors that slow down the dissolution of already cleaned surface areas.

Chemical cleaning is more productive and easier to use than abrasive, but after it it is necessary to wash the surface from solutions, which necessitates the use of additional treatment facilities.

At the final stage of surface preparation, surface passivation is used, that is, its treatment with chromium compounds and sodium nitrate. Passivation prevents secondary corrosion. It can be used both after degreasing the surface, and after phosphating or chromating the surface.

After rinsing and drying, the surface is ready for powder coating.

After the parts leave the pre-treatment area, they are rinsed and dried. Parts are dried in a separate oven or in a special curing oven section. By using a curing oven for drying, the dimensions of the system are reduced and there is no need for additional equipment.

Powder coating application:

When the parts are completely dry, they are cooled at air temperature. After that, they are placed in the spray chamber, where powder paint is applied to them. The main purpose of the chamber is to trap powder particles that have not settled on the product, dispose of the paint and prevent it from entering the room. It is equipped with a filter system and built-in cleaning facilities (eg hoppers, vibrating screen, etc.) as well as suction systems. Chambers are divided into dead-end and walk-through. Usually, small-sized products are painted in dead-end chambers, and long-length products are painted in checkpoints.

There are also automatic spray booths, in which paint is applied with the help of manipulator guns in a matter of seconds. The most common method of applying powder coatings is electrostatic spraying. It involves the application of an electrostatically charged powder to a grounded item using a pneumatic spray gun (also called spray guns, guns, and applicators).

Any sprayer combines a number of different modes of operation:

• tension can spread both up and down;
• the flow force (pressure, jet flow) of the paint, as well as the speed of the powder output, can be regulated;
• the distance from the nozzle outlet to the part can change, as well as the size of the paint particles.

First, powder paint is poured into the feeder. Through the porous partition of the feeder air is supplied under pressure, which brings the powder into suspension, forming the so-called "fluidized bed" of paint. Compressed air can also be supplied by a compressor, thus creating a local "fluidized bed" area. Further, the air suspension is taken from the container using an air pump (ejector), diluted with air to a lower concentration and fed into the sprayer, where the powder paint acquires an electrostatic charge due to friction (friction). It happens in the following way. A high voltage is applied to the charging electrode located in the main gun, thereby generating an electrical gradient. This creates an electric field near the electrons. Particles that carry a charge opposite to that of the electrode are attracted to it. When paint particles are driven through this space, air particles give them an electrical charge.

With the help of compressed air, charged powder paint falls on a neutrally charged surface, settles and is held on it due to electrostatic attraction.

There are two types of electrostatic spraying:

• electrostatic with particle charging in the coronal charge field
• and tribostatic spraying.

With the electrostatic method of spraying, the particles receive a charge from an external source of electricity (for example, a corona electrode), and with the tribostatic method, as a result of their friction against the walls of the sprayer turbine.

In the first method of applying paint, high-voltage equipment is used.

Powder paint acquires an electrical charge through ionized air in the area of ​​the corona discharge between the electrodes of the charging head and the surface to be painted. The corona discharge is supported by a high voltage source built into the nebulizer. The disadvantage of this method is that when using it, it may be difficult to apply paint on surfaces with blind holes and recesses. Since the ink particles are deposited on the raised areas of the surface first, it may not be painted over evenly.

In tribostatic spraying, the paint is applied using compressed air and held on to the surface by the charge acquired as a result of friction on the dielectric. "Tribo" means "friction". Fluoroplastic is used as a dielectric, from which individual parts of the paint sprayer are made. Tribo spraying does not require a power source, so this method is much cheaper. It is used for painting parts that have a complex shape. The disadvantages of the tribostatic method include a low degree of electrization, which significantly reduces its productivity by 1.5–2 times compared to the electrostatic method.

The quality of the coating can be affected by the volume and resistance of the paint, the shape and size of the particles. The efficiency of the process also depends on the size and shape of the part, the configuration of the equipment, and the time spent on painting.

Unlike traditional painting methods, powder paint is not lost forever, but enters the spray booth regeneration system and can be reused. A reduced pressure is maintained in the chamber, which prevents the release of powder particles from it, so the need for workers to use respirators is practically eliminated.

Polymerization:

At the final stage of dyeing, the powder paint applied to the product is melted and polymerized in the polymerization chamber.

After applying powder paint, the product is sent to the stage of coating formation. It involves the melting of the paint layer, the subsequent production of the coating film, its curing and cooling. The reflow process takes place in a special reflow and polymerization oven. There are many varieties of polymerization chambers, their design may vary depending on the conditions and characteristics of production at a particular enterprise. In appearance, the oven is a drying cabinet with electronic "stuffing". Using the control unit, you can control the temperature of the oven, the time of staining and set a timer to automatically turn off the oven at the end of the process. Energy sources for polymerization ovens can be electricity, natural gas and even fuel oil.

Furnaces are divided into continuous and dead-end, horizontal and vertical, single and multi-pass. For dead-end furnaces, the rate of temperature rise is an important point. This requirement is best met by furnaces with air recirculation. Coating chambers made from electrically conductive dielectric coatings ensure uniform distribution of powder paint on the surface of the part, however, if used incorrectly, they can accumulate electrical charges and be dangerous.

Melting and polymerization occurs at a temperature of 150-220 ° C for 15-30 minutes, after which the powder paint forms a film (polymerizes). The main requirement for polymerization chambers is to maintain a constant set temperature (temperature variation of at least 5°C is allowed in different parts of the furnace) for uniform heating of the product.

When heated in an oven, a product with a layer of powder paint applied, the paint particles melt, go into a viscous state and merge into a continuous film, while displacing the air that was in the layer of powder paint. Part of the air may still remain in the film, forming pores that degrade the quality of the coating. To avoid the appearance of pores, painting should be carried out at a temperature above the melting point of the paint, and the coating should be applied in a thin layer.

With further heating of the product, the paint penetrates deeply into the surface and then hardens. At this stage, a coating is formed with the specified characteristics of the structure, appearance, strength, protective properties, etc.

When painting large metal parts, their surface temperature rises much more slowly than for thin-walled products, so the coating does not have time to fully harden, resulting in a decrease in its strength and adhesion. In this case, the part is preheated or its curing time is increased.

Curing is recommended at lower temperatures and for a longer period of time. This mode reduces the likelihood of defects and improves the mechanical properties of the coating.

The time of obtaining the required temperature on the surface of the product is affected by the mass of the product and the properties of the material from which the part is made.

After curing, the surface is subjected to cooling, which is provided by lengthening the conveyor chain. Also for this purpose, special cooling chambers are used, which can be part of the curing oven.

The appropriate mode for coating formation must be selected taking into account the type of powder paint, the characteristics of the product to be painted, the type of oven, etc. It must be remembered that temperature plays a decisive role in powder coating, especially when coating heat-resistant plastics or wood products.

At the end of polymerization, the product is cooled in air. After the product has cooled, the coating is ready.

Types of Powder Coatings

Epoxy powder paints:

Epoxy resin powders are used which provide a high degree of gloss, smoothness, excellent adhesion, flexibility and hardness, as well as resistance to chemicals and solvents.

The main disadvantages are low heat resistance and light resistance, as well as a pronounced tendency to turn yellow with increasing temperature and under the influence of diffuse daylight. Acrylic Powder Coatings: Widely used in surface coating applications; have a good degree of retention of characteristics such as gloss and color under the influence of external stimuli, and are also resistant to heat and alkaline environments.

Polyester Powder Coatings:

The general characteristics are the same as those of epoxy and acrylic resin powders. Such powders have high strength and high resistance to yellowing when exposed to ultraviolet light. Most of the coatings currently available on buildings are based on linear polyesters.

Hybrid powder coatings containing epoxy and polyester resins:

They contain as a component a large part (sometimes more than 50%) of a special polyester resin. The properties of these hybrids resemble those of epoxy resin powders, however, they have the added benefit of increased resistance to yellowing due to drying out and improved weather tolerance. Hybrid powders are now considered the backbone of the powder coating industry.

Polyurethane Powder Coatings: have an even set of good physical and chemical characteristics and also provide good exterior strength.

Powder coating is becoming more and more popular today. What is it? This is a modern technology designed to produce high quality decorative and protective coatings. Polymer powders are used in the work (hence the name - "powder"). They turn into a coating due to exposure to high temperatures. Because of this feature of the procedure, powder coating of metal and glass is most common.

Advantages

This process has a number of positive aspects. These include:

Profitability. The fact is that such paint can be reused if it does not settle when sprayed.
on the treated surface. Thus, the loss of material is no more than 5%. By the way, this indicator for ordinary paints will be 8 times higher - about 40%. Also in this case there is no need for solvents.

Ease of use. Materials for this type of work are produced completely ready. This guarantees consistently high quality coatings. In addition, it is very easy to clean the equipment after work, because the powder is easily removed from the parts.

Speed. Powder coating does not require drying products before placing them in the oven. If the surfaces covered with ordinary paint need to be dried for quite a long time, then in this case the process is reduced significantly.

Durability. The technology of these works involves the polymerization of a layer of elastic plastic, which has a fairly high adhesion, directly on the surface that is being painted. The result is a durable coating that boasts excellent electrical insulating and anti-corrosion properties, as well as resistance to various substances.

Environmental friendliness. As already mentioned, solvents are not used in this case, which has a positive effect on the environment. The wastelessness of production also plays a role.

Decorative. Powder paints make it possible to obtain a surface of any shade. The palette of presented materials today has more than 5 thousand colors and shades with various textures. If desired, you can get a glossy or matte surface, as well as under granite, moire, etc.

Powder coating as a business idea

Given all the advantages of this type of work, it becomes obvious that such a business will be quite profitable. If you do not have the opportunity to immediately invest a large amount in your business, it is advisable to at least just learn how do-it-yourself powder coating is done.

But it is worth considering that you still have to spend money. First of all, you will need to take care of the availability of special equipment and a separate room. As the latter, a simple garage is quite suitable, but on the condition that it has enough space to accommodate all the tools and direct work. What equipment is needed for powder coating?

Camera

Work will be impossible without a special camera. It is in it that most of the whole process is performed. The powder coating chamber is needed for air purification (recovery process), in addition, it is due to it that it remains possible to reuse the material. Here, the paint that does not fall on the surface to be treated is sent to the filters, and then dumped.

Such equipment can be of various sizes. Which one to choose - you need to decide in each case individually, having previously determined which products you plan to work with.

Furnace and gun

You will also need a reflow oven. This is a prefabricated structure consisting of panels (their thickness is 100 mm). Thermal insulation material - basalt fiber. If you are just trying yourself in this type of work, it is not necessary to immediately buy a special oven. For this purpose, it is quite possible to use a conventional oven. However, to build a business, it is still recommended to purchase professional equipment.

Powder coating technology also requires a spray gun that allows the use of compressed air. It can also be used as a compressor. If you have made a choice in favor of the latter, then please note that a filter for high pressure must be installed on it.

Recuperator and transport system

Powder paint residues are collected using a recuperator. Instead, at first, you can use a cyclone type vacuum cleaner. In this case, you must first check the power supply in the room and make sure that grounding is present.

If you plan to work with large-sized products, then you should also consider purchasing a transport system. In it, the workpieces are moved on special trolleys that move along rails. Thus, a powder coating line is built. Such equipment improves the productivity of the process, ensuring its continuity.

Powder Coating Technology

The process of doing the work itself is divided, as it was already possible to understand, into several stages:

1. Let's talk about each stage separately. Preparation of the product, or rather its surface, for processing.
2. Apply paint in powder form.
3. Polymerization, i.e. heating the product in an oven.

Preparatory stage: cleaning, degreasing

We can say that this stage is the most time-consuming. And it depends on him how high-quality and resistant the coating will turn out. In the process of preparing the surface, it is necessary to remove all contaminants from it, degrease it.

Cleaning is carried out either mechanically or chemically. The first option involves the use of steel brushes or a grinding disc. You can also perform lapping with a clean cloth, after wetting it in a solvent.

The second cleaning option involves the use of an alkaline, neutral or acidic composition, as well as solvents. Their choice depends on how dirty the surface is, what material the product is made of, what type it is and what dimensions it has.

Phosphating and chromating

Next, a conversion underlayer can be applied to the product, which will prevent moisture and dirt from getting under the coating. Phosphating and chromating procedures provide better adhesion and protect the surface from rust. For this purpose, iron phosphate (for steel), zinc (when working with galvanic cells), chromium (for aluminum) or manganese and chromic anhydride are most often used.

Then it will be necessary to remove the oxides, which is carried out using abrasive and dry cleaning. The first is produced by means of abrasive particles (shot, sand), walnut shells. These substances are supplied with compressed air at a fairly high speed. As a result, particles "crash" into the surface of the product and bounce off it along with pollution.

Pickling (chemical cleaning) is the removal of various contaminants using special pickling solutions, the main components of which are sulfuric, hydrochloric, nitric, phosphoric acid or caustic soda. This method is considered more productive, however, after such processing, the product must be washed from solutions.

Passivation

This is the final step in the surface preparation phase. It is necessary to process the part with sodium and chromium nitrate compounds. This is done in order to prevent the recurrence of corrosion.

After all the preparatory work has been done, the product is rinsed and dried in an oven. Now powder coating of the surface can be performed directly.

Paint application

What is powder coating technology itself? The prepared product must be placed in the chamber. Here powder (paint) will be applied to it. If you have a dead end box, then only small details can be painted in it. Large items can only be processed in long chambers.

Most often, the method of electrostatic spraying is used to apply paint. In this case, powder coating guns are used. Such instruments are also called spray guns or applicators. This device is a pneumatic spray gun that applies an electrostatically charged substance to a pre-grounded part.

Coating formation

Let's move on to the next stage of work. The paint has been applied, now you need to form a coating. First of all, the product is sent to the polymerization oven. Such cameras can be different: vertical, horizontal, again, dead-end or walk-through, single- and multi-pass.

The aforementioned equipment for powder coating provides heating of the surface to a certain temperature - 150-220 ° C. The treatment lasts about half an hour, resulting in the formation of a film. At this stage, it is important that the part is heated evenly, which is possible only if the temperature in the chamber is stable.

Which mode to choose for processing a particular part depends on it itself, on the type of paint and equipment. After polymerization is completed, the product must be cooled in air. All work done.

Applications

As you can see, powder coating is a rather time-consuming job that requires certain investments. What products are subject to it? The considered method of painting is ideal for processing aluminum or forged products, as well as galvanized surfaces.

Powder paints in our time are finding more and more "fans". Now they are used in instrument making, and in construction, and in the automotive industry, as well as in other areas. With their help, paint medical equipment, roofing materials, household appliances, ceramics, gypsum and glass items, furniture. Among car enthusiasts, powder coating of disks is gaining more and more popularity.

Business organization

These works in specialized centers today are quite expensive. If you want to try yourself in this business, then if you have the financial means, you can quite start. Of course, not everyone can afford a powder coating line (automated system), but thanks to our recommendations, you can replace some elements with other tools at first.

Start with small items. It can be plaster figurines, ceramic dishes and much more. Try painting something in your house first (start with something you don't mind ruining). Gradually, you will have the necessary skills and dexterity, then you will be able to take orders from friends. However, you should not expect a large income if you interrupt only with one-time orders from individuals.

The best scenario for the development of events involves the presence of a large start-up capital. In this case, you can immediately purchase the necessary equipment and hire workers. Customers should be sought among enterprises engaged in the production of metal products. Only the presence of such customers will allow your business to exist and develop.

Powder paints were developed in the 60s of the XX century due to the need to protect the painted surfaces, give them an attractive appearance, reduce painting costs, and also in order to reduce environmental damage. At the same time, an electrostatic coating method and an anodizing system arose. Coatings with a metallic effect and paints resistant to adverse external factors began to appear.

Polymer powder coating is first sprayed onto the product, and then polymerized in a special oven and at a certain temperature. Powder coating technology involves the following steps:

• Surface preparation
• Application of powder paint
• Polymerization

Surface pretreatment

Pre-treatment of the product is the longest and most laborious process, which is sometimes not given the necessary attention, while the durability, quality and elasticity of the coating depend on it. Surface preparation for the painting process includes the removal of any contaminants, degreasing and phosphating in order to increase adhesion, as well as protect the metal from corrosion.

Cleaning of the treated surface can be carried out mechanically or chemically. In the case of mechanical cleaning, steel brushes or grinding discs are used; lapping with a clean cloth soaked in solvent is possible. As for chemical treatment, it is carried out using acidic, alkaline or neutral substances and solvents, which are selected depending on the degree of contamination, material, size and type of surface to be treated and other factors.

The application of a conversion sublayer helps prevent various kinds of contaminants and moisture from getting under the coating, which cause peeling and subsequent destruction of the coating. Phosphating the surface with the application of a layer of inorganic paint makes it possible to increase adhesion, that is, the adhesion of the surface to the paint by 2-3 times, and protect it from rust. When removing oxides (scale, rust and oxide films), abrasive (mechanical, shot blasting, shot blasting) and chemical cleaning, that is, pickling, are very effective.

• Abrasive cleaning is carried out with the help of small particles (shot, sand), iron or steel granules, walnut shells, which are fed to the surface at high speed by means of compressed air or centrifugal force. These particles break off pieces of metal with scale, rust or other contaminants, which significantly increases the adhesion of the coating.
• Etching is the removal of oxides, rust and other contaminants using solutions based on hydrochloric, sulfuric, nitric, phosphoric acids or caustic soda. They contain inhibitors that slow down the dissolution of the cleaned surface. The advantages of chemical cleaning over abrasive cleaning are greater productivity and ease of use. But after it, it is necessary to rinse the cleaned surface from solutions, and this, in turn, necessitates the additional use of cleaning agents.
• The final stage of surface preparation is passivation. In other words, body treatment with sodium and chromium nitrate compounds. Passivation is carried out in order to prevent the occurrence of secondary corrosion at any stage of surface preparation - after degreasing, phosphating or chromating.

After rinsing and drying the part in the oven (curing section), the surface can be considered ready for powder coating.

Applying powder paint to the surface of the product

When pre-treatment is completed, the object to be painted is placed in the spray chamber, where powder paint is directly applied to it.

The main purpose of this box is to trap powder particles that have not settled on the product to be painted, to dispose of the paint, and to prevent it from entering the room. Such a chamber is equipped with a filter system, cleaning means (vibrating screen, hoppers, etc.) and suction systems.

There are dead-end and walk-through types of boxes. In dead-end chambers, small items are usually painted, while large-sized items are painted in long ones. There are also automatic models, where in a matter of seconds the powder coating is applied using pistol manipulators.

The most common way to apply powder paint is electrostatic spraying, that is, applying an electrostatically charged powder to a grounded product using a pneumatic spray gun, also called a gun, applicator or spray gun.

Coating formation

When the paint has already been applied to the product, it is sent to the next stage - coating formation, which includes melting the paint layer, obtaining a coating film, curing it and cooling.

The reflow process is carried out in a special furnace or chamber. There are many types of polymerization chambers, depending on the specifics of production, their design may vary. In simple terms, such an oven is a kind of drying cabinet with electronic “stuffing”. With the help of the control unit, it is possible to control the temperature regime of the chamber and the staining time, set up automatic shutdown at the end of the process. The energy source for the polymerization oven can be electricity, natural gas or even oil.

Separate horizontal and vertical, continuous and dead-end, single and multi-pass furnaces. Melting and polymerization occur at a temperature of 150-220 ° C for 15-30 minutes, resulting in a film, that is, powder paint polymerizes.

The main requirement for polymerization chambers is to constantly maintain the set temperature for uniform heating of the product to be painted. The required mode for forming the coating is selected taking into account the characteristics of this product, the type of powder paint, the type of oven, etc.

At the end of polymerization, the part to be painted is cooled in air, and after it has cooled down, we can assume that the coating is ready.

When processing large-sized parts or large volumes of production, a transport system is used. Thanks to it, the painted products are easily moved from one stage of painting to another. The principle of operation is that the painted objects are fed on a special suspension or trolleys moving along rails. Such a transport system makes it possible to continuously carry out the painting process, which, in turn, can significantly increase work productivity.

Benefits of Powder Coatings

Metal powder coating technology has many advantages:

• Excellent physico-chemical and decorative properties of coatings that cannot be achieved by other methods of painting, including a rich palette of possible colors.
• Good performance properties of coatings
• Durability of products painted with powder paints
• Single layer coating due to 100% dry matter content, which means economical use of powder coatings
• Small porosity
• Improved impact and corrosion resistance compared to other paints
• No need to control viscosity, as powder paints are delivered to the direct consumer in a ready-to-use form
• Losses when painting with powder paints are 1-4%, and, for example, when using liquid paints - about 40%
• Hardening of the coating within 30 minutes
• No need for large storage areas for powder coatings
• Minimize damage to painted parts during transportation and reduce the cost of their packaging
• Ecological safety of painting with powder paints

In view of all the above advantages of this method of painting metal, most industrialists today give their preference to it.

The use of the powder coating method is associated with the likelihood of some defects, the appearance of which is quite simple to prevent.

Mechanical inclusions and "weediness"

These defects can be caused by the following reasons

Mechanical inclusions and "weediness". Click on the photo to enlarge.

• Use of poor quality powder paint.
• Contamination of paint with various foreign inclusions directly in the installation.

In the first case, it is recommended to check the purity of powder paint by sifting through a special sieve or a detailed study of its composition under a microscope. You can also apply a layer of paint from the container used and examine it for foreign impurities. If they are found, it is necessary to replace the paint.

If the paint is contaminated with foreign particles, the quality of the powder paint should be checked both in the plant feeder and in the recovery system. The presence of foreign impurities indicates the need for cleaning the installation and sifting the paint. Checking for the absence of impurities is also recommended when preparing the surface to be painted during the paint application process.

The appearance of “shagreen leather” when using the powder coating method is influenced by a number of potential reasons:

• Exceeding the shelf life of powder paint.
• Exceeding the maximum allowable coating thickness.
• Insufficient curing time and temperature.
• The presence of coarse fractions in the paint.

Paintwork defect - shagreen leather. Click on the photo to enlarge.

Defects that occur during powder coating as a result of the reasons described above are quite simple to eliminate. Checking the date of manufacture of the paint will allow you to check the excess of the regulated shelf life, and the thickness of the coating can be adjusted by reducing or increasing the powder supply, voltage or paint application time.

The study of the relevant recommendations on compliance with the required curing regime and the measurement of the main parameters (time and temperature in the polymerization chamber) will help to avoid the appearance of "shagreen" on the painted surface. The dispersion of powder paint is easily checked using a sieve equipped with mesh No. 01 (the residue on this mesh exceeds the standard values ​​by 0.5% - 1.0%).

Insufficient thickness or complete absence of coating in some places

These paint defects can occur due to various factors:

Insufficient thickness or complete absence of coating in some places. Click on the photo to enlarge.

• complex configuration of painted products;
• increased voltage;
• the close location of the painted products ("screening");
• poor surface preparation (insufficient degreasing).
• Insufficient "covering" of the paint.

When painting products with a rather complex configuration, it is necessary to pay special attention to insufficiently painted areas and check the thickness of the coating. Paint defects due to insufficient coating thickness can be eliminated by lowering the voltage. Adjustment of the position of the sprayers, preheating of the products to be painted and the use of tribostatics also contribute to a better application of powder paint on complex surfaces with a complex configuration.

In cases where products that are close to each other are “shielded”, it is enough to simply increase the distance between them on the suspension. In the event of "covering" of the paint, it is recommended to replace the powder composition in the case when the coating thickness corresponds to the standard values. The process of degreasing should be given increased attention, since the service life of the coating applied to the product depends on the quality of degreasing. Degreasing must be carried out until characteristic traces of an oil film remain on the surface of the product.

Paintwork defect - punctures. Click on the photo to enlarge.

Punctures are among the most common defects that occur when using powder paint. Below is a list of the alleged causes of punctures and measures to prevent their occurrence. High humidity caused by improper transport, storage or poor packaging. This problem is prevented by a simple moisture test, carried out by drying a sample of 1 gram of paint at 50°C for two hours.

If the moisture index exceeds 1%, it is necessary to replace the paint or dry it in a special feeder. Humid air supply to the feeder. To avoid this phenomenon, as in the previous case, check the moisture content of the powder paint from the feeder. In the case when the humidity index exceeds 1%, a number of special measures should be taken: cleaning the compressed air, replacing the absorbent, installing a filter on the line. Insufficient drying time of the product after washing with water (during surface preparation). Applying paint on a dry surface, achieved by ensuring the necessary quality of drying before applying paint in the paint booth, avoids pinholes.

The formation of oxides during prolonged interaction with the air. The appearance of traces of rust on the surface of a painted product after prolonged contact with air indicates that the surface preparation was not carried out at the proper level. Reducing the time interval between preparatory operations avoids the occurrence of punctures. Gas emission characteristic of thick-walled and cast products. In order to obtain a normal coating after applying the control powder coating, it is necessary to preheat cast and thick-walled products.

The following factors can influence the occurrence of craters when painting with powder paint:

• insufficient air purification from oil drops;
• non-compliance of paint with specifications;
• insufficient cleaning of the installation or accidental contamination.

Preventing craters is quite simple. In the first case, it is enough to ensure normal air purification by timely replacement of the absorbent and installation of a filter on the line. If the powder paint does not meet the specifications, it must be replaced. Thorough cleaning of the installation also helps to avoid the formation of craters.

The appearance of bubbles in the surface layer and on the surface can be caused by a number of factors:

• applying a thick layer of paint, eliminated by reducing the thickness of the powder spray;
• insufficient degreasing of the surface in hard-to-reach places (crevices, welds, holes). High-quality surface preparation avoids the appearance of bubbles;
• some defects of the painted product (traces of old paint, outgassing from the casting), which can be eliminated by preheating and removing the old paint.

  

  

  

  Color change

  A change in the color of the powder paint can cause an uneven or increased temperature distribution in the polarization oven (chamber) or an increased period of time required for the complete curing of the coating. These defects can be avoided by taking control measurements and then adjusting the temperature in the polarization chamber, and by checking and setting (if necessary) the normal polarization time.

  Leaks can occur due to the following factors:

  • increasing the air pressure value for the paint supply (increased concentration of the "torch");
  • increase staining time and tension;
  • increasing the curing temperature;
  • increased ability of paint to spill.

  In order to avoid negative consequences in the first two cases, it is enough to adjust the main parameters of painting: paint supply, voltage and spraying time. Compliance of the selected temperature regime with the recommended one contributes to better curing, and the method of control painting in the recommended regime will allow avoiding smudges. If after this procedure the smudges do not disappear, then the paint should be replaced.

  fine mesh cracks

  There are only two potential causes of this defect:

  • uncured coating;
  • unaccounted heat capacity of the product.

  An undercured coating is the result of a mismatch between the selected curing mode and recommendations. This defect is quite simple to prevent by ordinary adjustment. The heat capacity of the product must be taken into account when carrying out control spraying of powder paint on sheet steel. If the surface condition is satisfactory, it is necessary to increase the curing time of the product surface in the polymerization chamber (taking into account the heating of the product).

  

  Waviness and uneven coating thickness. Click on the photo to enlarge.

  Defects that occur when using the powder coating method can have a pronounced appearance - uneven coating thickness or waviness. Such defects can be caused by the wrong relative position of the spray guns in relation to each other, the wrong choice of nozzle and the application of a thin coating. The thickness of the applied coating is influenced by the adjustment of painting parameters such as the supply of powder paint and spraying time.

  The consequence of incorrect selection of the nozzle and the location of the spray guns can be a waviness of the surface or a large variation in the thickness of the coating (unevenness). These defects can be avoided by checking the thickness of the coating, qualitatively adjusting the location of the nozzles, choosing the right nozzle and selecting the optimal location of the product on the suspension in the chamber.

  blurry drawing

  The main reason for the appearance of this defect is a large variation in the thickness of the coating. In order to prevent the appearance of a smeared pattern, it is necessary to adjust the location of the nozzles and find the optimal location of the product on the suspension.

  gas sinks

  High temperatures and excessive curing times can cause gas pockets in the powder coating, which can be avoided by checking the selected modes for compliance with the recommendations, control measurements of curing time and temperature in the oven, and checking the quality of degreasing.

  The following factors underlie the occurrence of such a variety of powder coating defects as low adhesion: