7.2. How does spray equipment function ?

Let us look at how the two most common methods of paint spray application operate.

7.2.1. Low pressure spray equipment

In the case of conventional air spraying the paint is in a container on the spray-gun itself or it may be in a separate container (pressure feed tank or pot) joined to the spray-gun by a hose. The paint is forced to the gun and through the nozzle using a relatively moderate pressure. The nozzle on the spray-gun is so constructed that the paint is forced through one opening in the nozzle and air through other holes in the nozzle. This method of paint application makes use of compressed air to transport the paint towards the surface to be coated. The paint jet leaving the nozzle will be broken up by the air jet, atomised and blown towards the surface to be coated.

7.2.2. High pressure spray equipment

The term “airless spray” (without air) is commonly used to differentiate between high pressure spraying and low pressure spraying (conventional spraying). High pressure spraying (airless spraying), is a high-capacity method, suitable for spraying thick films, and for paints that usually do not require further thinning. The high pressure in the paint is generated by a pump, usually a piston pump. This pump may be driven by various means, e.g. by an electrical motor, but the most common solution is to connect an airmotor to the paint pump. A combination like this, eliminates for instance the sparks created by electrical motors. The compressed air supply associated with this type of spray equipment is thus used to drive the air motor, not to spray the paint itself.

This method pressurises the paint and forces it through a small nozzle on the spray-gun. The combination of pressure and small nozzle opening turns the paint into an aerosol. The aerosol pressure is usually around 180 - 280 bars, but certain pumps provide pressures of up to 400 bars. This gives the paint the very high speed necessary for atomisation. The sudden fall in pressure when leaving the nozzle will more or less cause the paint to explode into tiny droplets. Thus, no compressed air is required for atomisation nor for transportation of the paint to the substrate. In this way we get less dust formation than for air spraying.

The most important detail in the airless spray equipment is the nozzle. The nozzle has a tungsten carbide tip with a narrow opening, through which the paint is forced. The dimension of the opening varies for the different nozzles. Furthermore, the opening may also be varied, allowing a choice of fan angels during spraying. The result of the application is to a large extent dependent upon the correct choice of nozzle.

An air driven, high-pressure pump functions in the following way: The air is introduced into a compressed air motor at the top through a reduction valve. Then the air passes through a channel of the distribution valve at the top of the motor casing and under the piston in the compressed air motor. The piston is pushed up, and at the same time forcing the air above the piston through the valve out into the atmosphere. The piston in the compressed air motor is connected to the piston in the paint pump through a piston rod. The piston in the paint pump is now on the way up, thus closing the ball valve in the piston and opening the ball valve in the bottom of the cylinder. The paint in the cylinder above the piston is forced up through the filter and out through the high pressure hose to the spray-gun.

The air distributing valve decides in which direction the piston will move and is controlled by means of a plunger on the piston rod. When the air piston and the paint pump piston are both near the top of the cylinder the plunger receives an impact from below, making the valve change its position and forcing it down. The air below the piston will be forced out through the air distributing valve and into the atmosphere.

This results in the paint piston being forced downwards. The ball valve in the bottom of the cylinder closes while the ball valve in the piston opens. The large volume of the piston rod will partly displace the paint present in the pump cylinder. The paint is thus forced out through the filter and the high pressure hose to the spray-gun. When the piston is nearly at the bottom the air distributing valve will again be displaced and the piston will again be forced upwards. The paint is pumped out through the filter to the spray-gun both by upward and downward piston movements.

The pressure of the paint leaving the cylinder is decided by the input air pressure which can be regulated and by the gear ratio. The gear ration is the ratio between the area of the compressed air piston and the area of the piston in the paint pump. Most high pressure pumps have a gear ratio in the range 1-28 up to 60 : 1. Recommended air pressure is normally 5-7 kg/cm² (70-100 p.s.i.). Supposing an input air pressure of 7 kg/cm² and a gear ratio of 40 : 1, the paint will have a pressure of 280 kg/ cm² or 280 atmospheres (4000 p.s.i.) as it leaves the high pressure pump.

A drop in pressure must be expected as the paint travels through the filter and hose. Too long a hose will cause a considerable drop in the pressure. In the example mentioned we calculated with a pressure in the spray-gun of approx. 250 kg/ cm² (3500 p.s.i.).