Applications on pneumatic:Paint spraying

Paint spraying

There are three basic methods of spray painting: low pressure (the conventional method), high pressure and electrostatic painting. Ail three can be used for cold or hot spraying. They each have their own particular characteristics, and in order to decide which is the appropriate one for a particular application a number of factors have to be taken into account, such as economy in the use of paint, finish and capacity. Table 1rates the various methods for suitability. Compressed air is used in each method, and particular care must be taken to ensure that the air is supplied dry and oil-free, if high quality work is required. Air may also be used for preparation of the surface prior to painting. Cleaning off the rust, old paint etc.,is done by blast cleaning using shot, steel sand or some other kind of abrasive particle.

Low pressure paint spraying

In this technique compressed air is used both for atomizing the paint and for carrying it to the spray gun. The air pressure used is about 6 bar. Paint is supplied to the gun by one of the three methods iiiustrated in Figure 1. The choice of method depends on the type and quantity of the paint to be used. For low viscosity paint, suction feed is adequate; gravity and pressure feed are better for higher viscosity paints. The pressure feed method is suitable for large capacity spraying. With suction or gravity feed, the capacity is up to litre/min; pressure feed ailows up to 2 litre/min. The air consumption varies from 2 litre/s to 10 litre/s according to capacity.

The flow rate and spray pattern is controlled by the needle valve in the spray gun. The paint is atomized by the air passing through the nozzle with the result that a mist of paint particles is present in the spray booth, which can be wasteful in the use of paint, particularly when painting open structures such as bicycle frames. The method is used for automobiles, kitchen machines and furniture.

High pressure paint spraying

This method relies on a high pressure pump to supply the paint to the gun. No air is employed to atomize the paint nor does any air issue from the gun, so the method is also known as "airless" spray painting. Air at 6 bar is supplied to the pump which generates a spray pressure up to 360 bar. The air consumption per litre of paint is lower than with low pressure painting.

The paint passes through a tungsten carbide nozzle with a small orifice and is atomized by the high pressure. Because no air is used at the nozzle, there is little or no mist created. Thick coats of high viscosity paint can be applied, so it is suitable for high capacity applications on large structures, ships and buildings. Up to 5 litres/min can be applied.

Electrostatic spraying

A drawback of both high and low pressure spraying is the wastage of paint when spraying open structures and small components. In electrostatic spraying, the method of atomizing and delivering the paint is the same as with either of the two methods described but, in addition, an electrostatic field of 50 to 100kV is created between the paint and the sprayed object. The paint droplets follow the lines of the charge field, so it is possible to achieve "wrap-around", as shown in Figure 2, and obtain adequate coverage by spraying from one side only.

This method is useful for car bodies, bicycles and small objects; the coat thickness is even and of high quality, which is difficult to achieve on small objects with the other methods.

It is economical in the use of paint, and there are other savings such as lower cleaning costs.

The operator, spray equipment and workpiece are earthed and the paint is electrostati­ cally charged in the gun. Safety is important with this method, so all the safety precautions must be observed; conductive footwear must be worn.

Cold or hot spray

Hot spraying allows a paint to be sprayed in a more concentrated form, flows better and produces a more homogeneous coating; it also results in better economy . Hot spraying is particularly suited to the application of thicker coats so is often preferred for priming coats. Low pressure hot spraying is the preferred choice for high quality ,gloss finish coats.

Recent developments in the design of spray nozzles has, however, improved the quality oflow pressure spraying so that the quality is claimed to be as good as with high pressure systems. The nozzles are designed to work with comparatively low pressure (up to 2.5 bar) and improve the atomization of the paint to produce not only a better finish but less wastage and consequently lower emission of fumes. The design of the new nozzle is shown in Figure 3.

The operating principle for hot spraying is i11ustrated in Figure 4. Paint is held in a heated

container, using either direct or indirect heating. When spraying is not in progress, it circulates in a closed circuit through the pump, heater, and spray gun back to the suction

1.Paint heater

2. High pressure pump 3 .Spray gun

4. .Back pressure regulator

5. From the paint container or paint circulation system

side of the pump. A back pressure regulator is used to adjust the paint quality returned to the main circuit.

Paint transport and feed

The paint feed alternatives shown in Figure I can be considered as batch feed systems: the paint pot has to be refilled when empty, thus the process is discontinuous. A continuous paint feed system is illustrated in Figure 5. A pressure-operated circulation pump supplies paint to several guns through a ring main. There can be different spray systems on the same line, or the main can incorporate a heater.