Powder coating technology. Features of the powder coating process Powder coating at what temperature

There are four main powder coating processes: electrostatic spray, fluidized bed, electrostatic fluidized bed, and flame spray.

Electrostatic spraying is the most popular powder coating method today. For all application methods, surface preparation (i.e., cleaning and conversion coating) should provide a good basis for coating application. The surface must be prepared accordingly.

Features of four different powder coating methods:

1. In the process electrostatic spray dry powder particles acquire an electrical charge, while the surface to be painted is electrically neutral. The charged powder and neutral work area create an electrostatic field that attracts dry paint particles to the surface. Once on the surface to be painted, the powder coating retains its charge, which keeps the powder on the surface. The surface painted in this way is placed in a special oven, where the paint particles melt and are absorbed by the surface, gradually losing their charge.
2. Second method application involves the powder paint particles being held in suspension by air flow. Coming into contact with a preheated surface to be painted, these particles melt and are firmly held on its surface. The thickness of the powder coating depends on the temperature, the degree of heating of the surface, and also on the duration of contact with the powder particles. When applying thermoplastic coatings, post-heating is generally not required. However, in some cases, additional heating is necessary to completely cure the powder coating.
3. Electrostatic powder coating with air flow in many ways similar to the previous one, however, in this case, the air flow holding the paint particles is electrically charged. Ionized air molecules charge the paint particles as they move up in a special oven, where the surface to be painted is placed, and form a cloud of charged particles. The surface to be painted, which has a neutral charge, is covered with a layer of charged particles. In this case, pre-heating of the surface to be painted is not required. This technology is suitable for painting small and simple objects.
4. Flame dyeing method appeared relatively recently and was mainly used for thermoplastic powder coatings. The thermoplastic powder is melted under the influence of compressed air and enters a special gun, where it passes through burning propane. Melted paint particles are applied to the surface to be painted, forming a durable layer. Since this method does not require direct heating, it is suitable for most materials. With this technology it is possible to paint surfaces made of metal, wood, rubber and stone. Flame application is also suitable for large or fixed objects.

The choice of powder paint depends on the desired surface characteristics. The properties of the powders must meet the individual requirements of the client in relation to surfaces. Powder coatings are divided into different categories, depending on the specific application. Thermoplastic coatings are used to paint denser surfaces and provide durability, while thermostatic powder coating is used to paint thinner materials, mainly for decorative purposes. Powder paints use polyethylene, polyvinyl, nylon, fluoropolymers, epoxy, polyester and acrylic resins.

Material Compatibility

• Electrostatic airflow technology is best suited for painting small metal objects.
• As with all types of painting, powder coatings are applied to a clean, smooth and well-prepared surface. The surface to be painted does not need to be pre-treated, however additional surface preparation (e.g. iron phosphate treatment for steel, zinc phosphate treatment for galvanic cells, or steel and chromium phosphate treatment for aluminum surfaces) significantly improves the quality of the powder coating.
• Only materials that can be heated to high temperatures can be powder coated using electrostatic spray, airflow or electrostatic air application. Therefore, these technologies are most suitable for small metal objects.

Health and safety

• Powder paints can ignite easily near open flames. The concentration of powder in the air must be reliably controlled to ensure a safe working environment. Despite the absence of flammable solvents, any organic material such as dust or powder can form an explosive substance in the air.
• When coloring, inhalation of paint powder should be avoided, as this can damage the lungs and protective membranes of the body.

A typical powder coating process is the following sequence of operations:

1. Preparing the surface of the product for painting.
2. The application of a powder coating to the surface to be painted in the spray chamber using a sprayer in which the particles of the polymer powder are electrically charged, and which, using compressed air, transports the powder to the part. Under the action of electrostatic forces, powder particles are attracted to the surface of the part to be painted and are arranged in uniform layers on it.
3. Heating of the product in a reflow and polymerization oven at a temperature of 140-220°C (depending on the type of paint). As a result of heating, the powder melts, polymerizes and the coating acquires the necessary protective and decorative properties.

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. Melting and polymerization take place in a special furnace. 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 staining time and set the 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 powder paint layer. 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 cures. 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.

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:

  

Existing painting technologies make it possible to simplify the task and speed up the painting of metal products. So, powder painting (which partially replaced the standard technology) makes it possible not only to qualitatively paint the metal surface, but also to protect it from the influence of negative factors. The use of this method has a positive effect on the appearance of the painted products.

Scope of Powder Coating

The method under consideration is one of the most widely used and is used in various fields. This technology finds its application in the production of construction works, in machine and instrument making. Powder coating of metal is actively used in the automotive industry and in car repair: the ability of the paint to improve the performance of the surface, as well as safety and environmental friendliness, make it the best option for restoring coatings.

The properties allow the composition to be used to perform such operations as powder coating discs and a number of others. Making a choice in favor of this option if it is necessary to restore the car's paintwork, there is no doubt that it is able to restore an attractive appearance to the vehicle, reliably protect it from corrosion and adverse environmental factors. This effective method copes well with the task of painting both small parts and large elements, including the car body.

Painting work can be done using powder paint yourself, especially if it is a very small detail. But such painting requires skills and abilities, so if you need an excellent appearance of the painted element, it is recommended to contact professionals. If powder coating of metal products of a large area is required (for example, a car body), special equipment is indispensable.

Advantages of Powder Coating

Compared with traditional methods of painting, powder paint has a number of undoubted advantages, including strength and resistance to corrosion, high speed of work, durability, low material consumption, absence of solvents and safety for human health. Painting contributes to the formation of a protective film on the painted surface, which prevents scratches and other damage.

It is also significant that there is no need to create special conditions for storing the composition.

Types of powder paints

Powder paint is divided into two types - thermoplastic and thermoset. The first option, in turn, is divided into varieties depending on the underlying composition of the substance. Paint based on polyvinyl butyral is recommended for indoor painting, and compositions based on polyvinyl chloride are among the universal ones (for outdoor and indoor use). The same can be said about polyamide compounds, which are resistant to various external influences.

There are also polypropylene powder paints, but this option is more aimed at protecting the surface, and not at creating a decorative coating. Thermosetting coatings can be based on acrylates, epoxy resin and other components. This variety has a wide range of applications and can be used, including for painting cars.

What is Powder Coating?

Powder paint is a fine powder with a polymer structure. The composition of such paints may include various components (hardeners, resins), as well as substances intended to impart color to the composition. The composition of the coating in combination with the painting technology is able to reliably protect metal products from the influence of aggressive environments and give it excellent aesthetic characteristics.

Advantages, benefits, disadvantages

Along with the listed advantages (strength, durability, environmental friendliness, etc.), you need to pay attention to the fact that this type of paint provides a large selection of colors and shades, so it is always possible to choose the right option. As for the benefits, such painting is among the economical options: the material is used almost 100%, leaving no large amount of waste.

The disadvantages of the paint include the need for constant monitoring of the painting process in the chamber and the difficulty of working at low temperatures. Additional inconvenience can be created by the fact that each color must be stored in an individual container. But in any case, powder paint is an option that has a minimum of disadvantages.

Application of powder paint

To work with powder paint, it is necessary to equip a paint shop (this will require a space of 100-150 square meters). An important point: such a workshop should not be located near (at a distance of less than 5 m) possible sources of ignition. Powder coating requires careful adherence to technology: only in this case the expected result is guaranteed.

What equipment for powder coating is required by technology?

Painting work requires special equipment. In order to paint any product, you will need a spray booth and a polymerization oven, a compressor and a spray gun. Powder coating equipment includes a complex designed to prepare the surface for painting work and some other components.

What should I pay attention to when choosing equipment for powder coating?

The acquisition of special equipment requires financial costs, therefore, when choosing it, a number of important factors must be taken into account, among which the purpose of its purchase is of decisive importance. Thus, painting booths on the market can be intended both for single products and for organizing large-scale painting work.

The atomizer (depending on the chamber) is included in the chamber or purchased separately. For domestic use, it is recommended to choose manual guns. Expensive sprayer options cover a large surface area, thereby significantly reducing the painting time.

Preparing for painting

The quality and durability of the coating is largely determined by the preparation for painting. Preparatory measures include thorough cleaning and degreasing of the surface, protective treatment and phosphating (required to improve adhesion). Passivation often completes the process, which consists in surface treatment with chromium nitrates and sodium (increases corrosion resistance).

Technological process

Immediately before painting, the product to be painted must be masked, that is, those elements of it that do not need to be painted must be reliably protected. Masking is also required in the case of painting with several colors. After carrying out the preparatory measures and masking, they begin the process of applying paint.

Paint application

The coating is evenly applied to the surface with a sprayer. It is recommended to ground the product that is being painted: this helps to keep particles on the surface. Then the painted product must be placed in an oven (the layer must melt, a film will form on the coating) and cool in fresh air.

Coating quality control

After removing the product from the oven and cooling, the coating becomes solid. However, it is too early to transport or operate it: to complete the process, you must wait 24 hours (during this time, the coating will gain maximum strength).

Thus, the considered type of painting is an effective way of painting various types of products. The use of this technology allows not only to give the product an excellent appearance, but also to reliably protect it from the influence of aggressive environments.

Powder coating of metal is a modern method of painting and protecting surfaces. A liquid coating with powder particles is applied to the part to be painted. Particles are held on the surface by the force of electrostatic attraction. At high temperature, fine powder particles melt and polymerize, forming a single high-quality coating.

Characteristics and scope of powder coating

Powder paint is a liquid composition based on polymer resins with hardeners and flow modifiers. Pigments added for color. The processing temperature in the chamber is 200–250 degrees. Powder coating technology is used for products that can withstand without deformation the temperature at which the coating is baked.

The most widespread technology has received:

• in the industrial production of metal products;
• in the production of building materials.

Glass, ceramics, MDF are also painted using this method.

Powder paint covers a wide segment of goods and structures, including:

• furniture, household appliances;
• medical instruments, equipment;
• Sports Equipment;
• sheet metal, aluminum profiles.

The main advantages and disadvantages of powder coating

Powder coating protects the surface well. The paint lays down in a dense layer, 35-250 microns thick, the number of pores is less. One layer replaces 2-3 layers of conventional paint. A smooth, durable coating film is not scratched or damaged during transportation.

The production technology of works allows to collect paint sprayed in the air for reuse. Losses of the coloring composition are minimized, amounting to 1–4% of the total mass. The process of painting metal is simple, labor-intensive, and does not require a large number of workers. These factors reduce the cost of application per square meter of construction.

Corrosion of metal painted in this way is excluded. Metal products do not fade under sunlight, color, coating quality does not change in any weather conditions. A diverse palette has many shades, reproduces the complex textures of bronze, granite, silver. Gloss varies from matte to glossy.

Powder paint is supplied by the manufacturer ready for use, no solvent is used. Details for powder coating are not primed.

The disadvantages of powder coating include:

• the composition is not tinted, the choice comes from a ready-made palette of shades;
• the impossibility of applying manually, only in workshop conditions on special equipment;
• in case of a defect in the metal painting, it is impossible to fix a separate section, the part is repainted as a whole;
• the material of the metal part must withstand 200–250 degrees, which is not always possible;
• the dimensions of the parts depend on the dimensions of the polymerization chamber.

Variety of powder coating

Metal painting takes place in three stages. Powder coating is applied to the prepared surface. After spraying the coloring composition, the part is sent to the polymerization oven.

The following equipment is required for painting:

• Application chamber. Equipped with air suction for paint collection, return or disposal.
• Pneumatic spray gun. Together with the feeder, it forms a powder coating tool.
• Feeder.
• polymerization chamber. Creates sufficient temperature to complete the process.

The installation, consisting of a spray gun and a feeder, creates a mixture of coloring matter with air, forms a torch, and gives an electric charge to paint particles. The shape of the torch depends on the installed gun nozzle. Charged particles, settling on the workpiece being processed, are held by the force of electric attraction.

Existing overlay methods

Methods of imposition according to the type of obtaining a charge by particles are called electrostatic and tribostatic.

By electrostatic method, the charge is communicated by a corona electrode under a high, 20–100 thousand V, voltage. Electrostatic installations are more powerful and productive. Decreasing the electrode voltage increases the speed of the air jet.

The tribostatic effect is achieved by rubbing the particles against each other and the material of the gun body. The body of the gun to increase friction is made of fluoroplastic.

Tribostatic installations are cheaper, the performance of the units is less than that of electrostatic ones. The percentage of particles settling on the part is lower. Not all paints for metal are designed for friction charging, you need to choose special ones or use adaptive additives. Gun parts wear out and need to be replaced. The tribostatic method is more convenient to process parts of complex shape, grooves, recesses. The electrostatic method in such conditions is not effective, leaves non-painting.

According to the composition of the resins, the mixtures are divided into three categories:

• epoxy paints;
• epoxy-polyester compounds;
• polyester paints.

Epoxy Powder Coatings

Epoxy paints for metal are durable, resistant to chemicals, oil and fuel. A primer is not required under them, they themselves can be a primer layer before applying liquid powder coatings. The thickness of the applied layer is up to 500 microns.

Epoxy paint does not conduct electricity; for its insulating properties, it is in demand in the electrical and radio engineering industries when painting metal, which requires increased anti-corrosion properties. Ferrous metals, galvanized steel are phosphated, aluminum and aluminum alloys are chromated. An impact-resistant coating with good adhesion is formed.

Epoxy-polyester coatings are more decorative. Based on them, you can get complex textures for embossed leather, aged surface effects, a wide palette of metallic shades with varying degrees of sheen. The disadvantage of the epoxy-polyester coating is the reduced resistance of the paint to atmospheric phenomena and the weak resistance to metal corrosion processes.

Polyester powder paints are weather-resistant, mechanically strong, abrasion-resistant coatings. High adhesion of polyester compositions allows coating on all types of metals, including light alloys. Good electrical isolation. Reacting with alkali, the paint layer is destroyed.

Features of the technology of applying powder paint and polymerization

Powder coating is applied in three stages:

1. Surface preparation. Includes the removal of contaminants and the application of additional conversion coatings for enhanced protection and durability.
2. Application of painting in the spray booth using the installation.
3. Polymerization in an oven at high temperature.

Chemical degreasing of metal for painting is mandatory. Residual oil, chemicals, or drops of moisture can cause discoloration spots, punctures, and pitting. The workpiece is inspected for the presence of sharp edges, burrs, sagging from welds and.

It is necessary to clean the surface from rust and dust. The addition of additional properties by surface phosphating, chromating or passivation depends on the requirements of the coating.

The application chamber is equipped with a recovery system that returns the microparticles to the feeder.

The curing temperature of each type of paint is indicated by the manufacturer in the accompanying documents and, as a rule, is 180–200 degrees. Under the polymerization temperature understand the temperature of the surface of the workpiece, and not the temperature of the operating mode of the furnace.

Hardening of the paint in the polymerization chamber is recommended to be carried out at low temperatures and for long periods. This will increase the hardness and avoid coating defects such as pebbles and streaks.

Massive metal products are recommended to be heated in advance so that the period of the part in the furnace is enough for the final hardening. No dust is allowed in the room. It is forbidden to transport a metal product with hot paint.

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