In airless spraying the paint is fed by high pressure, through a pressure resistant hose, to the spray gun, where the paint is forced through a narrow, hard, metal nozzle. The paint is atomised due to the air resistance and arising pressure difference, and the shape of the nozzle orifice.The paint pressure is achieved by a piston or membrane pump. The power can be pneumatic, electric, internal combustion engine or a hydraulic piston pump. Picture 11 shows an airless spraying outfit powered by a pneumatic piston pump. Because no compressed air is used in the atomising of the paint, as with conventional spray, the method is called ‘airless’.A pneumatic airless spray gun consists of an air motor and a feed pump for the paint. The relation between the cross-sectional areas of the plungers of the air motor and the paint pump describes the pressure relation of the spraying system. E.g. a pump with the pressure ratio of 40:1 gives a pressure of 200 bar at the spray nozzle when the pressure of the compressed air is 5 bar. The final pressure in the nozzle depends also on the length and diameter of the hose, the number and position of the filters (to remove any bits from the paint which may block the very fine spray nozzle), the size of the nozzle, the type of paint and its viscosity and temperature.
When choosing the airless spray gun, attention should also be paid to the required throughput, measured in litres per minute. Too low a throughput limits the use of bigger nozzles even if the pressure relation is big enough. This is noticed as cyclic changes in the width of the spray fan. Also, if the litre capacity of the pump is small, it will wear out faster.When adjusting the pressure you should always try to find the lowest pressure which is sufficient to atomise the paint. Too high a pressure stresses the gun and hoses and increases paint wasted as ‘over-spray’. Thixotropic epoxy, coal-tar and chlorinated rubber paints also need higher pressures. Solvent-free epoxy, polyurethane and polyester coatings need very high nozzle pressures up to 200–300 bar in order to get good atomisation. Too low a pressure, or a too thick paint in relation to the pressure, will be clearly noticed by the spray fan having a stripy appearance, known as ‘fingering’. By heating or thinning in accordance with the paint suppliers recommendations, the need for high pressures can be reduced to a more workable level.
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