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benefits and limitations.
As mentioned previously, if the on-time
is too short, the valve may not fully open,
which can result in erratic drop volumes.
Some valves are also limited by a maximum
on-time. The designer must determine if the
valve is rated for continuous duty or has a
limited allowable on-time. Exceeding this
limitation may overheat the valve, causing
permanent damage. Running the valve for
extended on-times may also result in unwanted heating of the fluid. This could damage the
fluid or change the viscosity, again altering
the dispense volume.
Pressure is the motivating force on the
fluid. Increases in pressure will increase the
drop volume while decreases in pressure will
reduce the volume. If the pressure is too high,
it can result in high exit velocities at the noz-
zle. This can lead to splattering or splashing
when the droplet makes impact with the
surface. This can also cause problems with the
stability of the droplet.
A minimum drop velocity is needed to
ensure that the drop properly “jets” from the
nozzle. Reducing the pressure too much leads
to drops that do not cleanly break off from
the nozzle. These drops will continue to build
up, and eventually gravity will force the drops
to dispense from the nozzle. This phenomenon is often referred to as “drooling.” This
scenario causes missed dispenses and then a
sudden “large drop,” which is actually several
drops all at once.
The final factor is the diameter of the
nozzle, or discharge orifice. Typically, this
diameter is determined during the system
design phase and is not easily adjusted during
dispensing. The nozzle must not only be sized
to handle the range of required dispenses
but it must also allow for the pressure and
valve on-time to remain in their stable ranges.
There are also physical limitations that need
to be considered.
Nozzles that are too large in diameter
will not be restrictive enough to compensate
for variations in the upstream components.
Properly designed nozzles will have a high
enough restriction (typically 10X more than
the upstream system), which will make small
system variations invisible to the overall
Nozzles sized too small will be very susceptible to clogging. Even systems that are very
clean may have random wear particles that
can clog a very small diameter nozzle. Exit
velocities may also be a problem if the nozzle
diameter is too small.
A robust dispense system can be achieved if
steps are taken to ensure consistent, repeatable dispensing of droplets. Designers must
consider the properties of the fluid being
dispensed, as well as the dynamics and design
of the overall system, to ensure consistent performance and reliability.
For more information, visit www.theleeco.com.
A robust dispense system
can be achieved if steps
are taken to ensure
dispensing of droplets