Medical Design Technology - November/December 2013

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Made in the USA since 1960. 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 dispense volume.

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.

Conclusion

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.