In essence, an HVLP gun is still an air spray gun as it uses
air as its primary atomization force. Both HVLP and conventional air spray guns
use the same two components of compressed air, pressure and volume but in different
quantities. The pressure, which is normally noted in terms of pounds per square
inch or PSI and volume, which is noted in terms of cubic feet per minute, are
both necessary for either gun to work.
Originally HVLP guns where designed to use extremely high
volumes of CFM that were generated by turbines rather than compressors. These
turbines delivered, in some cases, hundreds of CFM but very little pressure. In
order to remain competitive with the turbine HVLP guns, the manufacturers of
the traditional air spray guns soon figured out how to convert their guns to
HVLP using compressed air rather than turbine air. Today HVLP guns are as
common as the old conventional guns.
HVLP technology proved that if you used a lot of volume of
air instead of a lot of pressure like conventional guns did, you could not only
atomize coatings but also do so more efficiently. The efficiency improvement
was mostly related to the fact that the atomized paint particles traveled at a
slower speed than paint particles that were atomized with high air pressure. We
refer to this characteristic as spray particle velocity. The higher the
velocity of the paint particle the lower the efficiency would be.
I like to use the analogy of a tennis ball and a cement
wall. The paint particle is representative of the atomized paint particle and
the wall represents the substrate being coated. The harder you throw the ball
against the wall, the further the ball bounces from the wall. During air spray
atomization the paint particle can travel at speeds greater than 30 feet per
second or FPS. The slower the particle velocity, the less bounce back and over
spray, the more efficient the technology will be.
In addition to spray particle velocity, another
characteristic that affects guns that use a lot of volume of air such as HVLP
is air damming. In this scenario the box is the inside of a cabinet and the
blocks represent a cubic foot of air from your gun. Try to imagine these blocks
of air as they exit the spray gun and enter the box. When you spray into the
box you are filling it with the blocks of air. The inside area of the box can
only hold 8 cubic feet or 8 blocks, but the gun you are using is putting out 20
cubic feet per minute or 20t blocks per minute.
The result is that within seconds you are putting too many
blocks into the box and those extra blocks are preventing you from putting in
more blocks and they are spilling out all over the place. The volume of air has
to go somewhere because it usually can't pass through the item you are trying
to spray. The high volume of air from the gun is preventing more air and most
importantly, the coating from getting into the box. This is an example of air
damming and the higher the volume of air the more damming you will get.
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