SPRAY COATING APPLICATIONS
Spray coaters can be classified into three major groups according to the spray method: Air spray, ultrasonic spray, and electrostatic spray systems. We will address herein air spray systems. Air spray systems use compressed air to change the coating fluid into a fine mist that is sprayed onto the substrate. A typical example of an air spray nozzle:
Air spray systems disperse sizable coating fluid, causing relatively excessive loss of coating. There are many spraying systems that use a nozzle that can change the coating fluid into a fine mist that allows uniform coating of uneven surfaces, while others can maintain high-speed spraying that enables stable coating coverage. Robotic systems are capable of advanced movement control and automation of the stage and spraying nozzle. A variety of spray coating systems are available according to part design, required efficiency and coating purpose.
The impact or impingement (impact per square inch) of a spray should be taken into consideration and it depends on the spray pattern distribution and spray angle. Pressure is the most influential factor affecting flow rate and the flow rate varies with the square of the pressure. The coating’s specific gravity affects the flow rate. The lower the specific gravity the higher will be the liquid velocity through a constant surface area nozzle. Thus, coatings with lower specific gravity the flow rate is larger than for liquid with a higher specific gravity, at the same pressure. The coating liquid viscosity is another major variable, sensitive to temperature, effecting spray characteristics. Surface tension is a property affecting surface formation, as it influences spray angle, droplet size, as well as spray distribution. In general, increases in operating pressure will improve the pattern quality, capacity, spray angle, reduce drop size, increase velocity, impact and nozzle wear.
HVLP stands for high-volume, low-pressure spray guns. A high volume of air is used to propel coating at relatively lower pressures. The low pressure limits the amount of bounce back so that more coating sticks to the surface that you’re coating, limiting waste. There are two basic types of HVLP guns. The first uses an air compressor capable of producing about 25 pounds of pressure per square inch to propel coating. These types of guns are effective, but not as efficient. The second type of HVLP gun uses a turbine system instead of an air compressor to produce high volumes of air, but lower pressure – about six pounds per square inch. These types of sprayers are more expensive, but also more efficient, and since they do not require purchase of an air compressor, they often make financial sense.
High pressure compressed air mixes with low-speed coating fluid discharged from the nozzle, causing the fluid to become atomized. The friction between the liquid and accelerating air disrupting the fluid stream causes atomization. The energy source for air atomization is thus air pressure for a given capacity. The coating fluid is split up and slowed down at that moment due to air resistance, and then changes into a fine mist before reaching the coated article. When velocity decreases, fast evaporating solvents may evaporate and spray drop size may be diminished, thus affecting flow and leveling, depending on exposure time, relative humidity and other ambient conditions.
Air pressure differs greatly between a conventional and an HVLP spray gun. Conventional spray guns use high pressure to produce a relatively low volume of coating, with the same pressure at inlet and the outlet. HVLP spray guns are the opposite. All HVLP spray guns should operate at air pressures between 0.1 and 10 psi (at the air nozzle) and consume air volumes of 6-30 cfm to be considered true HVLP spray guns. The high pressures of conventional spray guns turn coating into a finely atomized mist, giving a thinner, finer application. HVLP spray guns can produce a thicker dry film thickness due to their lower pressure, producing large droplets.
HVLP guns tend to have higher transfer efficiency. More coating sticks to the surface, resulting in less waste and less cleanup. HVLP transfer efficiency of about 65% can be achieved.
Conventional compressed air sprayers tend to be less efficient. To keep waste to a minimum, make sure that your fan size is optimized, and fluid and air pressures are as low as possible. Conventional spray guns aren’t great for the environment. Because of the high levels of overspray, they release more volatile organic compounds that must be caught with filters to avoid air pollution. These filters must be regularly cleaned. HVLP spray guns generate less overspray, meaning that they release fewer pollutants to the environment.
HVLP spray guns are popular due to their high transfer efficiencies. You should consider what type of materials you’ll be spraying. Both HVLP and conventional spray guns can deal with a wide variety of materials, but HVLP guns are preferable for lower viscosity optically clear coatings. However, different spray technologies and nozzles are used for thin film optical clear and anti-glare coatings. No matter what type of sprayer you choose, preparation is key. Make sure that the spray area is well-ventilated w/ HEPA filters and that you use the proper safety equipment and coating are properly filtered. When choosing your finishes, try to choose coatings, rheology modifiers and additives that are not flammable or bad for the environment.
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