Powder coating of metal was invented back in the 60s of the last century and very quickly became widespread. This is due to the many advantages of this technology, such as efficiency, environmental friendliness, attractive appearance coverings.
General information
So, the meaning of this technology is that a polymer powder dye is sprayed onto the surface to be painted. That is why this method got its name. After applying the dye, the surface is exposed heat treatment, as a result of which the powder melts and forms a continuous uniform film.
The coating obtained by this method has the following properties:
- Corrosion protection;
- Good adhesion to the base;
- Resistant to temperature changes;
- Resistance to mechanical damage, including impact resistance;
- Moisture resistance;
- Resistant to chemical influences;
- Excellent decorative properties;
- Durability.
Advice!
Thanks to good adhesion, this method is the best option for painting stainless steel.
Separately, it should be said about the decorative properties of such a coating, which is distinguished by a variety of colors and textures, which is achieved through the use of various additives.
In particular, powder coating of metal allows you to obtain the following types of surface:
- Matte;
- Glossy;
- Flat or voluminous;
- Imitating gold;
- Imitating wood texture;
- Marbled;
- For silver, etc.
Advantages of powder painting technology
In addition to the possibility of obtaining a coating with high performance qualities, this technology also has a number of other advantages, such as:
- Possibility of applying the coloring composition in one layer, which is unacceptable when painting with liquid paints and varnishes.
- No need to use solvent and control the viscosity of the material.
- High dye efficiency, since the powder that has not settled on the surface to be painted can be reused. To do this, spraying is performed in a special chamber, which allows you to collect all unspent powder. As a result, the cost of powder painting of metal is lower than applying paintwork using other methods.
- The painting process takes a little time, and after applying the paint, you don’t have to wait for it to dry.
- Environmental Safety, since the dye does not contain toxic organic compounds. As a result, there is no need to use powerful ventilation systems.
- Highly automated dye application technology, which simplifies the process of learning how to operate the equipment.
Flaws
Like any other technology, powder coating metal has some disadvantages:
- It is impossible to eliminate local coating defects - if they occur, it is necessary to completely repaint the surface.
- It is impossible to do the painting yourself, as this requires special equipment and workshop conditions.
- The dimensions of painted surfaces are limited.
- – it is allowed to use only powder paints for metal from manufacturers.
- It is impossible to paint parts that will later be welded, since burnt areas of the coating cannot be restored.
Powder painting technology
Preparing the base
Pre-treatment is the most time-consuming and labor-intensive stage of painting. However, special attention must be paid to it, since the elasticity, durability and quality of the coating depend on preparation.
Preparing a part for painting involves removing any contaminants, degreasing the surface, and phosphating to improve adhesion and protect the metal from corrosion. The surface to be treated is cleaned mechanically or chemically.
To remove oxides, rust and scale, an effective cleaning method is shot blasting. They are realized using sand, steel or cast iron granules.
Under the influence of compressed air or centrifugal force, these particles are fed at high speed onto the surface being treated and crush it. As a result, scale, rust and other types of contamination break off from the metal, which significantly improves adhesion.
The chemical cleaning method is called etching.
In this case, the removal of rust, oxides and other contaminants is carried out using compositions based on the following types of acids:
- Solyanoy;
- Nitrogen;
- Sulfur;
- Phosphoric.
The advantage of etching over abrasive cleaning is greater productivity and ease of use. However, after this procedure it is necessary to rinse the surface thoroughly. Accordingly, costs arise for the use of additional cleaning products.
In the photo - painting a small part
Applying paint
After completing the pre-treatment of the metal, the part is placed in a special chamber where coloring powder is sprayed. As mentioned above, the camera is needed to catch unused material. In addition, it prevents paint particles from entering the room.
Such chambers are equipped with cleaning means such as hoppers and vibrating screens, as well as suction systems.
It must be said that there are two types of cameras:
- Walkthroughs - for painting large-sized products;
- Dead-end – for painting small objects.
In addition, there are automatic models in which the coating is applied with automatic manipulator guns. Of course, the price of such equipment is the highest, however, its productivity is also much higher - in this case, powder coating is applied literally in a matter of seconds.
As a rule, paint is applied electrostatically, i.e. Electrostatically charged powder is sprayed, which envelops the grounded part and adheres to it. The spraying itself occurs using a pneumatic sprayer, which is either simply a pistol.
After spraying the powder, the product is moved into the oven chamber, where it is subjected to heat treatment. Under influence high temperature the powder goes into a viscous-fluid state, after which the melted particles form a monolithic layer.
Note!
To obtain a high-quality coating result, the operating instructions for the equipment must be strictly followed.
Therefore, this work should be carried out by a specialist.
Conclusion
Powder coating of metal surfaces is in many ways more advanced than painting liquid paints. However, in some cases its use is limited. In addition, it can only be performed using expensive professional equipment, so it is not applicable at home.
For more information on this topic, watch the video in this article.
Surface preparation:
The initial stage of any painting process involves pre-treatment of the surface. This is the most labor-intensive and time-consuming process, which is often not given due attention, but which is a necessary condition obtaining high-quality coverage.
Surface preparation determines:
- quality,
- durability,
- elasticity and durability of the coating,
- promotes optimal adhesion of powder paint to the surface being painted
- and improving its anti-corrosion properties.
When removing contaminants from a surface, it is important to choose the most correct treatment method and composition used for this purpose. Their choice depends on the material of the surface being treated, the type, degree of contamination, as well as the requirements for operating conditions and service life. To pre-treat the surface before painting, methods of degreasing, removing oxide films (abrasive cleaning, etching) and applying a conversion layer (phosphating, chromate plating) are used.
Of these, only the first method is required, and the rest are applied depending on specific conditions.
The surface preparation process includes several stages:
- Cleaning and degreasing the surface;
- Phosphating (iron or zinc phosphates);
- Rinsing and fixing;
- Drying the coating.
At the first stage, the surface to be treated is degreased and cleaned. It can be produced mechanically or chemically.
For mechanical cleaning, steel brushes or grinding discs, also, depending on the size of the surface, it can be ground in with a clean cloth soaked in a 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 surface being treated, etc.
During processing chemical composition parts can be immersed in a bath of solution or subjected to blasting (the solution is supplied under pressure through special holes). In the latter case, the processing efficiency increases significantly, since the surface is also subjected to mechanical stress, and in addition, there is a continuous flow of clean solution to the surface.
Applying a conversion sublayer prevents moisture and contaminants from getting under the coating, causing peeling and further destruction of the coating.
Phosphating and chromating the treated surface with the application of a thin layer of inorganic paint helps improve the adhesion (“adhesion”) of the surface to the paint and protects it from rust, increasing its anti-corrosion properties. Typically the surface is treated with iron phosphate (for steel surfaces), zinc (for galvanic cells), chromium (for aluminum materials) or manganese, and chromic anhydride. Chromating or anodizing methods are often used for aluminum and its alloys. Treatment with zinc phosphate provides the best protection from corrosion, but this process is more complex than the others. Phosphating can increase the adhesion of paint to the surface by 2-3 times.
To remove oxides (this includes scale, rust and oxide films), abrasive cleaning (shot blasting, shot blasting, mechanical) and chemical cleaning (pickling) are used.
Abrasive cleaning is carried out using abrasive particles (sand, shot), steel or cast iron granules, as well as nut shells, fed to the surface at high speed using compressed air or using 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. Plays a big role right choice material, since too large a shot can lead to a large surface roughness, and the coating will lie unevenly.
Pickling is the removal of contaminants, oxides and rust by using pickling solutions based on sulfuric, hydrochloric, phosphoric, nitric acid or caustic soda. The solutions contain inhibitors that slow down the dissolution of already cleaned surface areas.
Chemical cleaning is more productive and easier to use than abrasive cleaning, but after it it is necessary to wash the surface of solutions, which necessitates the use of additional treatment facilities.
At the final stage of surface preparation, surface passivation is used, that is, it is treated 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. Drying of parts is carried out in a separate oven or in a special section of the curing oven. By using a curing oven for drying, the size of the system is reduced and the need for additional equipment is eliminated.
Powder paint application:
When the parts are completely dry, they are cooled at air temperature. After this, they are placed in a spray chamber, where powder paint is applied to them. The main purpose of the chamber is to capture powder particles that have not settled on the product, recycle paint and prevent it from entering the room. It is equipped with a filter system and built-in cleaning facilities (e.g. hoppers, vibrating screens, etc.), as well as suction systems. Cells are divided into dead-end and passage cells. Typically, small-sized products are painted in dead-end chambers, and long ones in walk-through chambers.
There are also automatic spray booths in which paint is applied using manipulator guns in a matter of seconds. The most common method of applying powder coatings is electrostatic spraying. It involves applying an electrostatically charged powder to a grounded product using a pneumatic sprayer (also called atomizers, guns, or applicators).
Any sprayer combines a number of different operating modes:
- tension can spread both upward and downward;
- the strength of the flow (pressure, flow of the jet) of paint, as well as the speed of powder release, can be adjusted;
- The distance from the sprayer outlet to the part may change, as well as the size of the paint particles.
First, powder paint is poured into the feeder. Pressurized air is supplied through the porous partition of the feeder, which suspends the powder, forming a so-called “fluidized layer” of paint. Compressed air can also be supplied by a compressor, thereby creating a local fluidized bed region. Next, 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 goes like this. A high voltage is applied to the charging electrode located in the main gun, thereby generating an electrical gradient. This creates electric field near electrons. Particles carrying a charge opposite to the charge of the electrode are attracted to it. When paint particles are forced through this space, air particles impart an electrical charge to them.
Using compressed air, charged powder paint hits a neutrally charged surface, settles and is held on it due to electrostatic attraction.
There are two types of electrostatic spray:
- electrostatic with particle charging in the coronal charge field
- and tribostatic spraying.
With the electrostatic spraying method, the particles receive a charge from external source electricity (for example, a corona electrode), and with tribostatic - as a result of their friction against the walls of the spray turbine.
The first method of applying paint uses high-voltage equipment.
Powder paint acquires an electrical charge through ionized air in the corona discharge region between the electrodes of the charging head and the surface being painted. The corona discharge is maintained by a high voltage source built into the atomizer. The disadvantage of this method is that when using it, difficulties may arise in applying paint to surfaces with blind holes and recesses. Since paint particles are first deposited on the raised areas of the surface, it may be painted unevenly.
With tribostatic spraying, paint is applied using compressed air and held on the surface due to the charge acquired as a result of friction with the dielectric. "Tribo" means "friction". PTFE is used as a dielectric, from which individual parts of the spray gun are made. With tribostatic spraying, no power source is required, so this method is much cheaper. It is used for painting parts with complex shapes. The disadvantages of the tribostatic method include the low degree of electrification, which noticeably 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, equipment configuration, and the time spent on painting.
Unlike traditional painting methods, powder paint is not lost forever, but enters the spray chamber regeneration system and can be reused. A reduced pressure is maintained in the chamber, which prevents powder particles from escaping from it, so there is virtually no need for workers to use respirators.
Polymerization:
At the final stage of painting, melting and polymerization of the powder paint applied to the product occurs in the polymerization chamber.
After applying powder paint, the product is sent to the coating formation stage. It involves melting a layer of paint, subsequent production of a coating film, its curing and cooling. The reflow process takes place in a special reflow and polymerization oven. There are many types 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 temperature regime oven, dyeing time and set a timer to automatically turn off the oven when the process is completed. Energy sources for polymerization furnaces 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, an important point is the rate of temperature rise. This requirement is best met by ovens with air recirculation. Application chambers made of dielectrics with an electrically conductive coating ensure uniform distribution of powder paint on the surface of the part, however, if used incorrectly, they can accumulate electric charges and pose a danger.
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 (in different parts In the oven, a temperature variation of at least 5°C is allowed) for uniform heating of the product.
When a product coated with a layer of powder paint is heated in an oven, the paint particles melt, become viscous and merge into a continuous film, displacing the air that was in the layer of powder paint. Some 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 structure, appearance, strength, protective properties, etc.
When painting large metal parts their surface temperature rises much more slowly than that of thin-walled products, so the coating does not have time to completely harden, resulting in a decrease in its strength and adhesion. In this case, the part is preheated or the curing time is increased.
It is recommended to cure at lower temperatures and for a longer period of time. This mode reduces the likelihood of defects occurring and improves mechanical properties coverings.
The time it takes to obtain the required temperature on the surface of the product is influenced 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 achieved by extending the conveyor chain. Also for this purpose, special cooling chambers are used, which may be part of a curing oven.
The appropriate mode for forming the coating must be selected taking into account the type of powder paint, the characteristics of the product being painted, the type of oven, etc. It is important to remember that temperature plays a critical role in powder coating, especially when coating heat-resistant plastics or wood products.
After polymerization is completed, the product is cooled in air. After the product has cooled, the coating is ready.
Types of powder coatings
Epoxy resin powder paints:
Epoxy resin powders are used which provide high degree glossy smoothness of the coating, excellent characteristics of adhesion, flexibility and hardness, as well as resistance to chemicals and solvents.
The main disadvantages are low heat and light resistance, as well as a pronounced tendency to turn yellow when the temperature rises and when exposed to diffuse daylight. Acrylic powder paints: widely used in surface coating; have a good degree of retention of characteristics such as gloss and color under the influence of external irritants, and are also resistant to thermal influences and alkaline environments.
Polyester powder paints:
The general characteristics coincide with those of epoxy and acrylic resin. Such powders are highly durable and highly resistant to yellowing when exposed to ultraviolet light. Most coatings available on buildings today are based on linear polyesters.
Hybrid powder paints containing epoxy and polyester resins:
Contain as a component a large part (sometimes more than 50%) of special polyester resin. The properties of such hybrids resemble those of epoxy resin powders, however, their additional benefit is increased resistance to yellowing as a result of over-drying and improved ability to tolerate weather. Hybrid powders are now considered the backbone of the powder coating industry.
Polyurethane powder paints: have a uniform range of good physical and chemical properties, and also provide good external strength.
Great demand in mechanical engineering, construction and other industries has led to the fact that polymer powder paint began to be used for finishing metal parts, as the most effective method and durable material. Application is mainly carried out by manual or automatic sprayers using tribostatic or electrostatic charging in feed-through or dead-end type chambers.
We will not be able to tell you in detail all the production technology. But you'll get the basics here to understand how it's done. In addition, you can watch the thematic video in this article as additional material.
Features of application in serial and small-scale production
Application technology
- The production process is environmentally friendly and harmless to others. This produces excellent decorative and protective-decorative coatings. The composition is distributed over the surface of the part, which is then placed in a powder paint polymerization oven. There the heat treatment process takes place at a certain temperature.
- The principle of coating in general terms is as follows: the workpiece is grounded and charged paint particles are attracted to it.
In general, the whole process is divided into three stages, these are:
- Part preparation (surface treatment);
- Spraying powder from a spray bottle;
- Melting of the applied powder or polymerization.
- The quality of finishing of parts will, first of all, depend on careful adherence to technology at each stage. In addition, the instructions require that there be a complete absence of burrs, protruding welds, splashes and burns, as well as oil and other stains.
Preparation
Note. To remove old coatings, scale and rust from the surface, chemical and mechanical methods are used to a greater extent.
Among mechanical methods There is blasting and abrasive processing using sandblasting, shot blasting and shot blasting machines. And aqueous alkaline and acidic washing solutions, as well as organic solvents, are used as degreasers.
Considering that organic solvents such as White Spirit, 646 are harmful to health, the production instructions limit wiping when degreasing manually with a cotton rag and this is used only for small batches.
Large batches are degreased not with your own hands, but with detergent compounds at a temperature of 40⁰C-60⁰C. The process itself occurs by dipping the part into liquid for 5-15 minutes or spraying for 1-5 minutes, followed by washing and drying.
Powder application
The application process, as can be seen in the top photo, is carried out in powder spraying chambers, where air suction and aspiration systems operate to prevent particles from entering the workshop room ().
In the dead-end chambers, the product is suspended and painted through a special window or on the side of KN-2, KN-5, and in the walk-through chamber the part is transported past the painter, through work area spraying KN-3, KN-6. For long parts, there are two-station pass-through chambers KN-3-2, KN-6-2 (two single-station cameras are turned opposite each other by 180⁰).
Since the dye for polymers is the powder itself and no mixing is necessary, a manual spraying installation (URN-2) was developed for small-scale production. It has the advantage that the powder is supplied there from the original box in which it was packaged at the factory, that is, it does not need to be poured into any container.
The suction pipe is equipped with a fluidization device, which, together with an injector and a vibrating table, allows you to process powders with high moisture content.
URN-2 can be equipped with an electrostatic and/or tribostatic spray gun. In it, the combined unit was designed for different types paints and surfaces of varying complexity. The combination allows you to almost instantly switch from electrostatic mode to tribo mode. This increases production efficiency and at the same time, naturally, the price of the products falls.
Note. The electrostatic method involves forced charging of particles using a high-voltage discharge electrode. The tribostatic method involves charging particles as they pass through the triboelectrifying unit of the tribo-barrel (triboeffect).
Reflow
After the powder has been applied to the product (this is not polymer floor paint at all), it is sent to a PP-16 type oven to form a coating by melting the layer.
Furnaces are also either dead-end or walk-through and consist of heat-insulating panels, one (dead-end) or two (pass-through) door blocks, as well as from one to eight heating blocks with an air recirculation system. The thermal insulation panel is made of 100 mm thick basalt slabs, which are sandwiched between galvanized profiled panels.
In most cases, the polymerization temperature of powder paint is 150⁰C-180⁰C with an accuracy of +-5⁰C and a holding time of 10 to 20 minutes, although this depends on the instructions of the powder manufacturer. These requirements are best met by ovens with air recirculation.
Conclusion
It should be noted that electrically conductive paint Zinga, as well as fire-retardant metal paints Polistil, have nothing to do with polymerization in an oven. The reflow process goes through exclusively powder dyes (
Existing painting technologies make it possible to simplify the task and speed up the painting of metal products. Thus, powder painting (which partially replaced standard technology) makes it possible not only to paint with high quality metal surface, but also protect it from influence negative factors. The use of this method also has a positive effect on the appearance of the painted products.
Powder coating application area
The method in question is one of the most widespread and is used in various fields. This technology finds its application in production construction work, in mechanical and instrument engineering. Powder coating of metal is actively used in the automotive industry and car repair: the paint’s ability to increase performance characteristics surfaces, as well as safety and environmental friendliness make it the best option for restoring coatings.
The properties allow the composition to be used for such operations as painting discs with powder paint and a number of others. Choosing this option if you need to restore paint coating car, there is no doubt that it is capable of returning the vehicle to an attractive appearance, reliably protecting it from corrosion and exposure to adverse factors external environment. 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 part. But such painting requires skills and abilities, so if you need an excellent appearance of the painted element, it is recommended to turn to professionals. If powder painting of metal products of a large area (for example, a car body) is required, you cannot do without special equipment.
Advantages of powder paint
Compared with traditional ways painting, powder paint has a number of undoubted advantages, including strength and resistance to corrosion, high speed performance, durability, low material consumption, absence of solvents and safety for human health. Painting promotes the formation of protective film 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 thermosetting. 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 universal (for external and internal work). 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 rather than 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 paint?
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 coating composition in combination with painting technology can reliably protect hardware from influence aggressive environments and give it excellent aesthetic characteristics.
Advantages, benefits, disadvantages
Along with the listed advantages (strength, durability, environmental safety, etc.), you need to pay attention to the fact that this type of paint provides big choice colors and shades, so it is always possible to choose suitable option. As for the benefits, such painting is one of the economical options: the material is used almost 100%, leaving no large quantity waste.
The disadvantages of paint include the need for constant monitoring of the painting process in the chamber and the difficulty of carrying out work 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.
Powder paint application
To work with powder paint, it is necessary to set up a paint shop (this will require a space of 100-150 square meters). Important point: such a workshop should not be located near (at a distance of less than 5 m) possible sources of ignition. Powder painting requires careful adherence to technology: only in this case is the expected result guaranteed.
What equipment is required for powder coating technology?
Painting work requires special equipment. In order to paint any product, you will need a paint booth and a polymerization oven, a compressor and a sprayer. Equipment for powder painting includes a complex designed to prepare the surface for painting work and some other components.
What should you pay attention to when choosing powder coating equipment?
Purchasing special equipment requires financial costs, so when choosing it, you need to take into account a number of important factors, among which the purpose of its purchase is of decisive importance. Thus, the painting booths on the market can be intended both for single products and for organizing large-scale painting works.
The atomizer (depending on the camera) is contained in the camera or purchased additionally. For household use It is recommended to choose hand guns. Expensive atomizer options cover large area surfaces, due to which painting time is significantly reduced.
Preparing for painting
The quality and durability of the coating are 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). The process is often completed by passivation, which involves treating the surface with chromium nitrates and sodium (increases corrosion resistance).
Technological process
Immediately before painting, the product to be painted must be masked, i.e., those elements that do not need painting must be reliably protected. Masking is also required when painting with several colors. After preparatory activities and masking, begin the process of applying paint.
Applying paint
The coating is evenly applied to the surface using a sprayer. It is recommended to ground the product that is being painted: this helps to retain particles on the surface. Then the painted product must be placed in an oven (the layer should melt and a film will form on the coating) and cooled in the fresh air.
Coating quality control
After removing the product from the oven and cooling, the coating becomes hard. However, it is too early to transport or use it: to complete the process, you need to wait 24 hours (during this time the coating will gain maximum strength).
Thus, the type of painting in question is an effective method of painting various types 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.