The powder coating or the powder coating is a coating method in which an electrically conductive material with a powder coating is coated. A typical coating system consists of surface pretreatment (cleaning and / or application of a conversion layer ), intermediate drying , electrostatic coating zone and dryer . The workpieces are transported by a conveyor system .
In 2006, around 1,100,000 tons of powder coating were used worldwide for coating . One third each is in Europe and Asia, the remaining third is divided equally between North America and the rest of the world. In Europe, powder coatings account for around 10% of the overall coatings market.
Common substrates for powder coating are steel , galvanized steel and aluminum . The main area of application is general metal coating with a 35% share, followed by household appliances (so-called white goods , 21%), facade coatings (20%), furniture painting (13%) and automotive painting (8%). Nowadays, automobiles like the Smart or temperature-sensitive substrates like MDF panels are powder coated.
The powder coating films produced typically have layer thicknesses microns between 60 and 120. However, depending on the application and surface characteristics, the layer thickness can also be above or below this range.
|title||Coating materials - Corrosion protection of steel structures by powder coating systems - Evaluation of powder coating systems and execution of the coating|
The powder coating used for powder coatings generally consist of dry granular particles between 1 and 100 microns in size. Chemically, these are mostly based on epoxy or polyester resins , for certain applications they are also based on polyamide (nylon), polyurethane, PVC or acrylic. In addition, hybrid systems are common that contain both epoxy and polyester resins as binders . How a powder coating behaves when it is coated is mainly determined by its mechanical properties such as particle size and flowability . To a lesser extent, the chemical composition of the powder coating used also plays a role. Depending on the composition, the powder coating particles tend to sinter in the coating system. They are temperature sensitive and begin to melt and stick when the temperature exceeds 50 ° C.
There are several standards for powder coating . DIN 55633 relates to corrosion protection and the evaluation of coated steel structures, one of the main areas of application for powder coating. For thin-walled load-bearing components (usually with material thicknesses ≤ 3 mm), Part 1 of DIN 55634 "Coating materials and coatings - Corrosion protection of load-bearing thin-walled steel components" also specifies the requirements and test methods for powder coatings. EN 15773 relates to the powder coating of hot-dip galvanized and sherardized steel objects. EN 16985 defines safety requirements for spray booths .
Also common is the certification by quality associations that can be performed for both coated companies as well as for powder coatings. Coating companies must meet requirements with regard to production facilities, laboratory equipment , self-monitoring and the quality of the surface pretreatment and the finished coating. Manufacturers of coating powders must prove that the powders they produce meet the relevant requirements, such as maintaining the color and shine of the painted object when stored outdoors for several years.
After the development of suitable coating equipment, the first coating booths were built in the late 1960s. Initially, it was often a matter of converting classic painting systems that were originally designed for coating with liquid paint.
In the early 1970s, the types of pistols in use today were developed. The development of the corona gun made it possible to supply high voltage within the spray gun , which made an additional high-voltage cable, which had previously been the norm, superfluous. This technology is the most widely used today. The tribo pistol was developed in 1972, but it was not until the late 1990s that it became established. Only then powder coatings have been developed that allow the used therein charging by friction could be fully exploited.
In 1976, coating booths with belt filters were introduced, which enabled color change times of less than 20 minutes. In 1978, coating booths with rounded corners that were easier to clean were introduced. Cabins made of plastic , which made cleaning even easier due to the difficult adhesion of powder particles, was introduced in 1986.
The flat jet nozzle, which is now used in 80% of powder coating systems, was invented in 1985 and subsequently replaced the baffle plate nozzle that had been common up until then. The so-called plate-less nozzles, in which the powder cloud is generated by air nozzles, were introduced shortly afterwards. However, they could not establish themselves in the industry. The development of the so-called powder bell, a rotary atomizer for powder coatings, is also of lesser importance .
From 1990, powder coating conveying devices are offered which enable conveyance directly from containers. This technology is in widespread use today.
→ Main article: surface pretreatment
The term surface pretreatment or pretreatment summarizes the steps that are carried out before coating with powder paint. In addition to removing surface residues (paint, grease), attaching suitable brackets, cleaning and applying several conversion layers . These serve to achieve a better match between the substrate and the paint. Cleaning agents based on ethanol or acetone that evaporate at room temperature are used for cleaning and leave no residue on the surface. With some raw materials, a pre-treatment for corrosion protection may have to follow. Inadequately carried out surface preparation can lead to a reduction in adhesion, spontaneous detachment or to craters in the paint film.
The mechanical pretreatment removes coarse impurities such as rust or scale . Common methods used in powder coating are grinding , brushing and blasting. The degreasing is usually done using solvents or aqueous cleaners. When cleaning with aqueous cleaners, cleaning the surface is often combined with phosphating . In terms of process technology, cleaning is usually carried out by spraying or dipping .
This is followed by the application of a conversion layer. Degreasing is usually sufficient for a clean surface, but conversion layers also increase the active surface due to their roughness . This improves the paint connection. The processes in the field of powder coating include phosphating on steel, pickling on galvanized steel, chromating , and anodizing or anodizing on aluminum. Due to the increasing legal restrictions on chromium-containing layers, chromium-free pretreatment methods are becoming more important.
Before the powder coating is applied, the surface of the workpiece must be absolutely dry. The adhesive water dryer used for this is similar to the later powder coating dryer, but is usually more simple. Depending on the quality requirements, blowing off with compressed air may be sufficient.
The term application refers to the application of the powder coating and the process steps directly related to it. So it describes the actual coating process.
Preparation and conveyance of the powder
In order for the powder to be coated to be applied, it must first be transported to the spray gun . Typically, the powder is conveyed from the fresh powder container into a container. There, if recovery is used, it is mixed with prepared recovery powder and from there it is sent to the gun. The powder that is not transferred to the workpiece is transported to the processing unit and from there returned to the cycle. It is important that the delivery is gentle so that the properties of the powder are not significantly affected.
In most conveying methods, the powder coating particles are first fluidized so that the powder coating can be conveyed. In some cases, agitators or vibrating elements are also used. If the powder is conveyed directly from the original container, only local fluidization takes place.
If recovery is used, the powder fed back into the circuit must first be cleaned of fibers , coarse particles and dirt. Various types of sieve are used for this, such as vibrating, rotating, tumbling or ultrasonic sieves. The recovery powder is added to the fresh powder in a ratio to be determined.
When conveying the powder, a distinction is made between precision conveying (50 to 500 g / min) and mass conveying (usually more than 5 kg / min).
The precision conveying is used to supply the powder coating to the spray gun. In order to avoid defects and irregularities in the coating, this requires dosing that is as uniform, precise and pulsation-free as possible. Devices that are frequently used for precision conveyance are precision and rod injectors . Precision injectors convey a defined amount of powder from the container to the gun and, by adding metering air, ensure that the total amount of powder air is kept constant. In the case of rod injectors as well as precision injectors, fluidization is necessary, since the suction takes place at the bottom of the container and without the addition of fluid air it does not produce a constant delivery. Ideally, powder coating is made more transportable by means of fluidization and, in some cases, additional vibration on the container or box.
The bulk conveyance is used to transport the powder coating between two containers, which requires a conveyance that is both economical and gentle on the powder coating. Common methods for bulk conveying are push conveying (also known as plug conveying) and suction conveying. The suction conveyor works with a vacuum created by a lot of air , which carries the powder with it. The necessary separation of air and powder is carried out using mini or multi-cyclones or filter separators . In the case of push conveying, a pressure chamber with two valves is switched in such a way that the powder is pushed forward by alternating air. A separation of air and powder is not necessary.
One method that can be used for both precision and bulk conveying is the so-called digital dense phase conveying (DDF), which works on the principle of counter-pressure conveying. Counter-pressure conveying means the alternating suction of vacuum and powder coating in two chambers. No fluidization is necessary with this process. At the same time, little conveying air is required for precise dosing.
Modern powder coatings are applied electrostatically . With electrostatic powder coating, an electrically charged powder cloud is first generated. The particles charged with the same name are transported to the workpiece surface. There they precipitate, adhere electrostatically and form the powder coating. Charging by high voltage ( corona charging or ionization ) or friction ( triboelectric ( tribo for short ) or electrokinetic charging ) is possible.
With triboelectric charging, the charging occurs when the powder particles come into contact with the wall of the spray gun, which releases electrons from the coating material. So that the contact surface is as large as possible, the channel is usually designed in the manner of an annular gap or spiral and is coated on the inside with Teflon . The powder coating particles are separated from the spray gun faster than the charge can be redistributed. This keeps the powder particles charged. Finally, the powder is atomized at the nozzle.
During ionization charging, the powder coating particles are guided past an electrode to which a voltage of 30 to 100 kV is applied. This high voltage ionizes the air surrounding the powder coating particles. The electrode tip shows a blue-white light appearance, the eponymous corona . When the electrical field between the electrode and the grounded workpiece passes through the paint particles, air ions are attached to the particle surface. However, only about 1 to 3% of the air ions hit powder particles, i.e. a very small proportion. The rest is called the space charge. With so-called low-ion corona charging, there is an additional, ring-shaped electrode at the tip of the spray gun. This absorbs the excess air ions and dissipates their charge.
The main advantages of the tribo-coating are the ease with which several layers can be applied and the tendency towards better layer thickness distribution. Since Faraday cages are of almost no importance here, a better penetration depth is achieved. Furthermore, there is a better ability to automate and often lower acquisition costs. In addition, the gun can be brought as close as desired to the surface during manual coating. Since there are no unbound ions here, the surface often looks more relaxed. Effect lacquers , on the other hand, can only rarely be applied using the tribo technique, as the desired effect image is not achieved.
The advantages of the Corona application are the lower wear of the spray gun, the lower air consumption and the universal suitability (many powder coatings, including most effect powder coatings, are unsuitable for tribocharging). In addition, due to the higher powder throughput, the number of guns required is often lower. The wraparound, a measure of the formation of a layer on the back of the workpiece, is also usually better with corona application. With classic corona charging, free air ions reach the object. Due to the repulsion of charges of the same name, the undisturbed build-up of an even layer is disturbed, which is what is known as orange peel, a very wavy paint surface. This fact can be limited by using low-ion charging.
Due to the charging of the powder coating particles of the same name or the air ions attached to them, these repel each other and form a uniform powder coating cloud. This follows the field lines of the electric field. Powder coating particles thus get onto the back of the workpiece, which also causes a coating to take place there. Cavities and recessed edges, on the other hand, are not or only weakly coated in accordance with the Faraday cage principle . Very small particles are drawn into the exhaust air and fed into the recovery process, which makes the recovery powder finer than the fresh powder. Very coarse particles fall down due to gravity and are therefore not available for coating.
Every ionized particle hitting the workpiece generates an opposing charge at the moment of impact on the workpiece. Due to the attraction between the two charges, the particles adhere to the workpiece. Due to the charges of the same name, the layer formation is very even. To prevent the powder from falling, it is necessary that the electrical attraction ( Coulomb force ) between the particle charge and the counter-charge is greater than the force of gravity . This requires the coating powder to have a high electrical resistance , since otherwise the discharge occurs too quickly. The coating remains adhesive for up to a few hours before the powder falls off as a result of gradual charge equalization.
The layer formation itself is initially linear. As the layer thickness increases, the field strength within the powder layer increases , so that from a certain layer thickness the dielectric strength of the air is exceeded. There is a voltage breakdown and thus a countercurrent of charged air ions. In the saturation phase, the following particles are discharged so far by the countercurrent that they can no longer adhere or fall out of the field due to gravity. At this point there is no more powder application; instead, so-called back spray craters, a coating fault, appear due to the countercurrent. Because of this self-limitation of the layer thickness , work is usually carried out with a layer thickness that is significantly lower than the maximum achievable layer thickness. For commercially available powder coatings, this is around 150 µm.
Since the powder particles follow the field lines and their density is higher at the edges, the layer thickness is usually higher at the edges. This so-called picture frame effect is an advantage in terms of corrosion protection , but a disadvantage with regard to the accuracy of fit of the coated workpieces.
The nozzle on the spray gun is used to atomize the powder coating and thus to form a homogeneous powder coating cloud. Depending on the charging variant, baffle plate nozzles, flat jet nozzles, finger nozzles or rotating bells are used.
The oldest technology is the baffle plate, which is more rarely referred to as a baffle plate. The strongly bundled powder jet hits the plate and is torn apart there. This creates a slow, only limited controllable powder cloud with little penetration. The baffle plate is therefore mostly used for flat, large-area parts. Flat jet nozzle refers to a mouthpiece with a slot. The emerging cloud has an elliptical cross-section that is easy to align. This type of nozzle is often used for complex parts with dimples. The finger nozzle is used for workpieces with complex geometry and shallow depth . When using this type of nozzle, short booths and thus easier cleaning can be achieved.
With rotary atomization , i.e. application using bells, which is one of the standard processes for liquid painting, the powder is charged via the loading edge of the rotating bell plate. In this way, a very even layer application is achieved with a high application efficiency. The throughput is very high at 600 to 700 g / min (compared to up to 400 g / min with the baffle plate nozzle).
Arrangement of the spray guns
The correct arrangement of the spray guns is used to achieve an even layer thickness. Which arrangement is the right one depends essentially on the workpiece geometry and the nozzle used. The use of rigidly attached pistols or lifting devices is variable . The task of lifting devices is to move the guns (individually or in groups). The reciprocators usually move the guns vertically, but multi-axis designs are possible. Vertical, horizontal, diagonal or diamond-shaped arrangements of the guns are common.
The minimum number of control units results from the sum of guns and reciprocators if one control unit is used. The higher the degree of automation , the more additional modules are required for coordination. This starts with simple light barriers for switching on the guns and can go up to a complete system control.
Powder spray booths
A closed coating booth is a booth that is closed on all sides and only has openings for the entry and exit of the workpieces. A partially closed coating booth also has side openings for the spray device or manual coating systems.
Powder spray booths are made of metal , glass or plastic, whereby the latter type must be made of a flame-retardant material and must comply with special regulations regarding earthing . Plastic booths are powder-repellent, so the tendency to soiling is lower and the initial application efficiency is higher.
Powder spray booths can also be equipped with an automatic cleaning machine for interior cleaning, a discharge belt or exhaust air duct on the booth floor and a suction device on the outlet side. Powder-repellent walls and doctor blade systems can also be used. Regarding the recovery, a filter belt recovery or multi-cyclone recovery can be used in powder booths. Special requirements require round booths (easy cleaning) or quick color change booths.
To meet the highest quality requirements, it is possible to convert the entire powder circuit with a surrounding cabin operated under overpressure . In order to prevent the entry of dust in this case , the outer cabin often has air conditioning .
Whether or not recovery makes sense depends essentially on the ratio of the cost of a color change to the cost of the otherwise wasted portion of the powder coating applied.
With very frequent color changes and small quantities, recovery is not lucrative, since the costs for cleaning are higher than the costs for the wasted powder. Systems with this requirement profile therefore often do without the possibility of recovery. In the case of large numbers of items or the use of very few colors (in the extreme case of only one color), a recovery device is useful. In these systems, powder separation rates of up to 99% are achieved, which means that only 1% of the processed powder is waste. The transfer efficiency without recovery, i.e. the proportion of paint that reaches the workpiece surface after a single coating, is usually only 30 to 50%. That is lower than with a liquid painting system.
Filter belt systems (in combination with a doctor blade , additional post-air filter or cyclone ) are used in single-color operation. The variant with cyclone is the only one that allows a color change per day. Pure filter recovery systems can only be used for single-color operation.
When using multi-cyclones (with or without a doctor blade), color changes are possible to a limited extent (several times a day). A monocyclone is required for frequent color changes. In quick color change booths this is combined with a vibrating sieve, an outlet cone and a push conveyor. In this way, even with frequent color changes, a powder separation efficiency of 95% is achieved.
Influence of the particle size
The particle size of recovered powder is usually smaller than that of fresh powder. Usually both are therefore mixed in a fixed ratio and prepared for further use. Fresh powder systems for precise control of this ratio are necessary when powder consumption is high and at the same time constant admixing of fresh powder is necessary to achieve the required quality.
Special features when processing effect powder coatings
The recovery is particularly demanding when effect pigments are used in the powder coating formulation. Depending on the type of production, these are added to the powder coating afterwards and are therefore not incorporated into the particles. There are therefore several types of particles in the material, which have different particle shapes and sizes. If a recovery unit is used, this can lead to a depletion of effect pigments in the entire mixture. This changes the appearance of the paints during the coating of a series. In the case of effect powder coatings that have been produced using the dry blend process , i.e. a simple mixture of powder coating and effect pigments, the effect is enhanced by better charging of the effect pigments. This application - related weakness is significantly reduced by the bonding process , in which powder coating and effect pigment particles are mechanically connected to one another.
Due to the electrical charge and the powder cloud generated at the same time, there is a risk of an explosion . The high voltage used can cause electric shocks . Therefore, various safety precautions must be taken when powder coating.
The powder concentration in the air must either be less than 50% of the lower explosion limit or less than 10 g / m³. The cabin must be made of non-fire-supporting materials; with plastic cabin, very high-energy electrostatic discharges must be prevented. The earth leakage resistance of the workpiece suspension must be less than 1 MOhm, alternatively the possible discharge energy of the workpiece can be less than 5 MJ. Automatic powder spray booths must have an automatic fire alarm system, closed powder recovery systems must have an explosion protection system.
To protect employees, live system components should be arranged in closed coating booths, and access should be secured by switching off and immediately earthing when entering. In the case of manual coating systems, the current strength or the discharge energy is also limited. A flame arrester (CO 2 extinguishing system) in front of the cyclone is also necessary.
Alternative application techniques
→ Main article: Vortex sintering
An alternative application technique is vortex sintering . This is the common application technique for the original, thermoplastic powder coatings that do not crosslink. It can also be used for crosslinking powder coatings. A heated workpiece is immersed for a short time in a plastic powder fluidized with the aid of compressed air . The powder melts due to the high surface temperature of the workpiece and forms a plastic layer there. If necessary, crosslinking takes place in a dryer. Fluidized bed sintering is used in particular when a high layer thickness is desired.
In the case of clearcoats for automotive painting , the use of powder coating as an aqueous suspension , which is referred to as powder coating slurry , is known. The powder coating is suspended in water and applied like a liquid coating. In this process, drying takes place in two steps. First, the water is removed from the film in an evaporation step. In the second step, the powder coating is cured as usual.
→ Main article: Coil Coating
A relatively new technique is the coating of powder paint using the coil coating process , a type of coating that is common for liquid paints. With coil coating, the steel strip is coated directly during manufacture in the rolling mill . Since the steel strips (coils) have very high speeds, the main difficulty is to crosslink the powder coating quickly enough.
The cross-linking process, the so-called baking, begins with the melting of the powder coating in the dryer . The viscosity of the system initially decreases and passes through a minimum . The further the crosslinking process proceeds, the higher the viscosity again. It has been shown that the best course is achieved when this viscosity minimum is reached quickly. The minimum in this case is more pronounced and the surface of the paint becomes smoother. If the optimal stoving conditions of the paint system are exceeded, it begins to decompose.
From an economic point of view, the baking process is of decisive importance for energy costs. Savings at this point are dangerous, however, since the technical properties of the paint system may not be achieved if the crosslinking is incomplete.
Theoretically, baking temperatures for powder coatings are between 110 and 250 ° C. In the case of stoving enamels used industrially, the stoving temperatures are usually between 140 and 200 ° C. Systems that crosslink at 140 ° C are already available as low-temperature powder coatings. With a correspondingly longer baking time, powder coatings can be crosslinked at temperatures below 120 ° C.
The holding time is 5 to 30 minutes. It indicates the period of time during which the powder coating is kept at the baking temperature and essentially depends on the powder coating material. The heating time, on the other hand, depends essentially on the thickness of the substrate. The sum of both times is the dwell time. The exact setting of the oven temperature and dwell time depends on the workpiece throughput and the stoving window of the powder coating. In addition, there are dryer-specific influences such as the heating behavior of the air, heat losses and the heating speed of the conveyor.
Powder coating plants are mainly equipped with continuous dryers that can be cycled or fed continuously. In clock driven systems lend themselves to larger workpieces and low flow rates because the doors can be closed between the workpieces. Continuously loaded dryers are often equipped with so-called A-locks against heat loss, where the inlet and outlet are lower than the actual drying unit. This minimizes the loss of heated air as it rises and cannot leave the dryer.
Chamber dryers can be loaded in batches and are not tied to cycle times. It is used under varying stoving conditions, which may be necessary due to different material thicknesses, different stoving times or the use of different types of powder coating. Chamber dryers are therefore common in the laboratory and pilot plant area. The heating time can be extended with this type of oven, as the temperature drops each time the dryer door is opened. Modern types catch the rising air so that the temperature loss can be reduced.
Usually, the dryer is heated by convection . This refers to the transfer of energy through a stream of warm air that cools down on the workpiece and thus transfers the heat to it. Due to the relatively uniform heating, such dryers are often used when different workpiece shapes are to be painted at the same time. The heating takes place indirectly via heat exchangers or directly by adding heating gases (gas stoves) . The latter, however, places additional demands on the paint system (gas oven stability) and the degree of purity of the heating gas. The reason for this is the possible reaction of nitrogen oxides from the heating gas with powder coating components, which can lead to intense yellowing. The powder coating is therefore usually stabilized with antioxidants .
The heat transfer through IR radiation can be used when coating thin-walled, flat objects. It achieves faster energy transfer and is therefore easier to control, ready for operation more quickly and enables significant space savings. In the case of different, simultaneously burned-in objects or complex shapes, on the other hand, a high temperature difference is generated at different points on the object, which is due to the uneven distribution of radiation (shadow).
The conveyor technology includes the workpiece conveyor itself and the type of suspension for the workpieces. For the reusability of this device, the paint stripping of the conveyor and hanger is crucial.
The hanger connects the workpiece and the conveyor during the entire process and is therefore usually also coated. The more difficult it is to paint the hangers, the slower the layer of paint builds up on the hangers. This reduces paint stripping costs, as the individual hangers can be used more frequently before paint stripping is necessary. Additional costs arise if the weight (increased heating energy during crosslinking) or the surface (increased powder coating consumption) of the hanger is too high. Good grounding and the prevention of a coating of the suspension points are of particular importance with powder coating, since the workpiece is grounded via the suspension points. A good adjustment is particularly important with tribo-charging , since otherwise incorrect coatings will occur.
The conveyor transports the workpieces connected to the hanger through the painting line. Manual sliding tracks are common in small and pilot plants. Powder coating systems with Power & Free conveyors are mostly used here, as they allow storage and buffers to be easily implemented and the system becomes more flexible. In contrast, circular conveyors are rarely used.
In addition to the obvious application, the reuse of workpieces, paint stripping also plays a major role in the cleaning of hangers and conveyor parts. The co-coating of the hanger causes layers to build up over time, which impair the coating of other workpieces. Since powder coatings usually have a thicker layer than liquid coatings, stripping is particularly important. The processes used are the nitrogen refrigeration process (also called cryo-clean process), thermal or chemical paint stripping.
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