A Look at the Factors
Regardless of the material or the application, achieving a consistent flow of drops from a dispensing system
can be much more challenging than designers may realize. There are a number of factors to consider as
well as changing variables that can occur during the process. This article will offer considerations for
engineers in need of a dispensing solution.
Ralph Buck, Product Manager,
EFS Division, The Lee Co.
Dispensing a droplet of liquid is a simple task. Conversely, repeatedly dispensing a consistent drop of liquid
is not an easy task. Droplet formation is dependent upon a number of factors, including
fluid properties, system design, and system
dynamics. System designers must pay close
attention to these factors to maximize the
consistency and repeatability of the droplet.
Dispensed volume = ƒ(pressure, system
restriction, fluid properties, time)
Fluid properties, such as viscosity and sur-
face tension, are usually dictated by the ap-
plication. The designer must consider how
the viscosity will change over the operating
temperature range of the fluid. As the
temperature increases and the fluid thins
out, the droplet volume will increase unless
something else in the system compensates
for this change in viscosity.
The surface tension of the fluid will affect
drop quality. If this is not considered carefully, drops may “wick” up onto the sides
of the nozzle. Some of these drops may not
break off, and some may have double volumes. This can result in a dispense stream
that “drools” instead of drops.
A basic dispensing system consists of a fluid
reservoir, a pressure source, a means to
control the pressure (valve), and an outlet
restriction (nozzle). The pressure source
may be gravity, a pump, or a pressurized
vessel. The key here is to keep the pres-
sure constant. Any
change in pres-
sure will result in
a change in the
drop volume. The
designer also needs
to consider eleva-
tion changes with
gravity systems, and
recovery times on
The accuracy of the
pressure source or
regulator must also
restriction, or noz-
zle, should act as the governing restriction
in the system. It needs to be more restric-
tive than the rest of the system to “filter”
out dynamic changes upstream. A
common problem is designing a system
where the restriction from the tubing is a
significantly high percentage of the overall
restriction. This will not be an issue on a
single unit but if multiple systems are made,
the tubing tolerance on the ID may affect
the dispense volume.
The final consideration is the control valve
of the system. This is the dynamic portion
of the dispensing system. The open time, or
on-time, of the valve is the final determining factor of dispensed volume. The longer
the valve stays open, the larger the volume
dispensed. To decrease the fluid volume,
the valve should be opened for a shorter
period of time.
When a valve is actuated, there is a
delay between the electrical signal and the
actuation of the valve. This is referred to as
response time. When the valve is opened
for very short periods of time, the response
time becomes a significant portion of the
This leads to inaccurate dispenses with
drops that do not have very repeatable volumes. When the valve is opening, it is operating on a very steep portion of the curve.
Very small changes in on-time can have a
significant effect on the plunger position.
An on-time that is too short may actually
prevent the valve from fully opening. This
is a very unstable situation. A well-designed
system will have a minimum operating on-time that is long enough to ensure the valve
has fully opened, thus allowing a repeatable
volume for that given time period.
Controlling Drop Volume
There is a triad of factors that can be used
to control the drop volume. Each has its
Well-designed systems will have a minimum operating on-time that is long
enough to ensure the valve has fully opened, thus allowing a repeatable
volume for that given time period.