Accurate coverage

Getting a coating spray precisely on target is of vital importance.  With over-spraying potentially expensive coatings are wasted or worse may contaminate areas where they are not needed.  With under-spraying the coating will not be complete and this will affect product quality.  So ensuring the emerging spray is of the correct angle and that it remains consistent along a spray line is clearly of the upmost importance.

On the face of it ensuring spray remains on target is a matter of simple trigonometry.  The spray angle of the nozzle can be used to calculate where the spray will be at a given distance from the target.  Then by using basic trigonometry the correct height of the bar can be determined to ensure coverage.  There are, however, several points to consider over and above a simplistic basic trigonometry calculation.

1- Sprays are not even.  Most sprays do not deliver their fluid evenly along the pattern line.  In fan spray nozzles there will tend to be a tapering of fluid distribution towards each end of the spray.  As such, sprays may need to be overlapped to ensure a consistent spray.

2- Spray angles vary a bit with pressure.  For many nozzles this variation is quite minor but the spray angle shown in a nozzle datasheet is never 100% constant for all pressures.  As pressure increases internal 'whirl chamber' full cone type nozzles will see their spray angle decrease.  Conversely, though most flat fan nozzles will see their spray angle increase with increase pressure.  Air atomising nozzles will have a more complex relationship between spray angle and pressure as both fluid and air pressure will affect the spray shape.

3- Fluid properties will affect the spray angle.  The spray angles given in nozzle datasheets will be for water.  If spraying fluids with different specific gravities, viscosities or surface tensions to water then the spray angle delivered may vary.  For more viscous fluids the spray angle tends to decrease but it is hard to give hard and fast rules as to how much by.  The increased viscosity will also affect the spray distribution as the ability to form fine droplets is impaired.

4- Gravity.  Most nozzles will only maintain an approximate geometric shaped pattern at a distance of 300mm from the nozzle.  After this point, particularly for finely atomised sprays, the pattern tends to be drawn in by the effects of gravity.  For many spraying applications the target will be close enough to the nozzle to ignore this effect but for some this many not be possible.  As such, the coverage calculation and the spacing of nozzles on the spray bar may need to take this effect into account.

Coating engineering considerations

Coating nozzles