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Spray Gun

Spray painting is a painting technique where a device sprays a coating (paint, ink, varnish etc.) through the air onto a surface. The most common types employ compressed gas usually air compressed by an air compressor to atomize and direct the paint particles. Spray guns developed from airbrushes and the two are usually distinguished by their size and the size of the spray pattern they produce. Airbrushes are hand held and used instead of a brush for detailed work such as photo retouching, painting nails or fine art. Air gun spraying uses equipment that is generally larger. It is typically used for covering large surfaces with an even coating of liquid. Spray guns can be either automated or hand-held and have interchangeable heads to allow for different spray patterns.

Air Gun Spraying
This process occurs when paint is applied to an object through the use of an air-pressurized spray gun. The air gun has a nozzle, paint basin and an air compressor. When the trigger is pressed the paint mixes with the compressed air stream and is released in a fine spray.[1]

Due to a wide range of nozzle shapes and sizes the consistency of the paint can be varied. The shape of the workpiece and the desired paint consistency and pattern should be important factors when choosing a nozzle. The three most common nozzles are the full cone, hollow cone, and flat stream.

There are two types of air gun spraying processes. In a manual operation method the air gun sprayer is hand held by a skilled operator and moved back and forth over the surface, each stroke overlapping the previous to ensure a continuous coat. In an automatic process the gun head is attached to a mounting block and delivers the stream of paint from that position. Whenever possible, the object being painted should be placed on rollers or a turntable to ensure overall equal coverage of all sides.
During manual spraying the gun should be about 200 300 mm  away from the object.

When air gun spraying is used as a manual process, the air gun is manipulated back and forth over the surface to be coated so that the pattern of each stroke overlaps that of the previous one and forms a continuous coating.

HVLP (High Volume Low Pressure)
This is similar to a conventional spray gun using a compressor to supply the air, but the spray gun itself requires a lower pressure (LP). A higher volume (HV) of air is used to aerosols and propel the paint at lower air pressure. The result is a higher proportion of paint reaching the target surface with reduced over-spray, materials consumption and air pollution. A regulator is often required so that the air pressure from a conventional compressor can be lowered for the HVLP spray gun. Alternatively a turbine unit (commonly containing a vacuum cleaner derived motor) can be used to propel the air without the need for an airline.

As a rule of thumb puts 2/3 of the coating on the substrate and 1/3 in the air. True HVLP guns use 8  20 cfm and a minimum 5 hp industrial compressor is required. HVLP spray systems are used in the automotive, marine, architectural coating, furniture finishing, and cosmetic industries.

 LVLP (Low Volume Low Pressure)
Like HVLP, these spray guns also operate at a lower pressure (LP), but they apply a low volume (LV) of coating (paint). This is a further effort at increasing the transfer efficiency (amount of coating that ends up on the target surface) of spray guns.

Electrostatic spray painting
When the powdered paint is forced through the barrel of the paint gun it rubs against the side of the barrel and builds up an electrostatic charge. This means the paint particles repel each other, and spread themselves evenly as the exit the spray nozzle. The object being painted is charged oppositely, or grounded. The paint is then attracted to the object giving a more even coat than wet spray painting, and also increasing the percentage of paint that actually sticks to the object. This method also means that paint covers hard to reach areas. The whole is then baked to properly attach paint. Car body panels and bike frames are two examples of where electrostatic spray painting is often used.

Rotational bell
With this method the paint is flung into the air by a spinning metal disc ("bell"). The metal disc also imparts an electrical charge to the coating particle.

Electric fan
There are a variety of hand-held paint sprayers that either combine the paint with air, or convert the paint to tiny droplets and accelerate these out a nozzle.

Air Assisted Airless spray guns
These use air pressure and fluid pressure (300  3000 psi) to achieve atomization of the coating. This equipment provides high transfer and increase application speed but is best used with flat line applications. Commonly found in factory finish shops.

Airless spray guns
These operate connected to a high pressure pump commonly found using 300  7500 psi pressure to atomize the coating using different tip sizes to achieve desired atomization and spray pattern size. This type of system is used by contract painters to paint heavy duty industrial,chemical and marine coatings and linings.

Advantages of airless spray are:

Coating penetrates better into pits and crevices.

A uniform thick coating is produced, reducing the number of coats required.

A very "wet" coating is applied, ensuring good adhesion and flow-out.

Most coatings can be sprayed with very little thinner added, thereby reducing drying time and decreasing the solvent release into the environment.

Automated Linear Spray Systems
Manufacturers who mass produce wood products use a Automated Spray system, that allows them to paint materials at a very high rate as well as at a minimum of personnel. Automated Spray Systems usually incorporate a paint saving system, this eliminates waste. Commonly, linear spray systems are when products are on are laying flat on a conveyor belt and then enters into a linear spray system, where automated spray guns are stationed above, and when the material is directly below the guns, the guns then begin to paint the material. Materials consist of window frames and any other material that is simple in design.

Automated Flatline Spray Systems
Mass produced material is loaded on a conveyor belt where it is fed into one of these flatline machines. Flatline machines are designed to specifically paint material that is complex in shape. For example a kitchen cabinet. Spray guns are aligned above the material and the guns are in motion in order to hit all the grooves of the material. The guns can be moved in a cycle or can be moved back and forth in order to apply paint evenly across the material. Flatline systems are typically large and can paint doors, kitchen cabinets, and other plastic or wooden products.

Related pages varnish lacquer aerosol Electrostatic spray painting
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