This thumb-actuated device accepts 5- and 10-mL syringes. Each click of the thumb-switch pushes down the piston a set amount. Photo courtesy Tridak LLC
beauty of a benchtop time-pressure dispensing system is its simplicity. An
adhesive or other fluid is packaged in a disposable syringe. An end cap
connected to a compressed air line fits over the end of the syringe. An
electronic controller applies a measured amount of air pressure, for a measured
amount of time, to the back of the syringe, forcing the material out the tip.
Inexpensive, fast, flexible and easy to operate, these systems can be handheld
or mounted to a Cartesian robot. Syringes are available in sizes ranging from 3
to 55 cc. When the syringe is empty, it’s thrown away. No cleaning is required.
“By changing the time and pressure, you get different quantities of adhesive,”
says Tom Bray, applications design engineer at Hernon Manufacturing Inc. “You
can dispense a wide range of materials, from very low-viscosity liquids to very
Finding the right combination of time and pressure for a particular material
requires some experimentation. “You want to start with shortest time, lowest
pressure and smallest tip you can,” says Bray. “From there, you work your way
up: more time, more pressure, bigger needle.”
Although time-pressure systems have been around for decades, the technology has
not remained static. Indeed, equipment suppliers have recently introduced a number
of improvements to the technology.
One of those improvements has been digital control. In the past, when operators
wanted to adjust time or pressure, they turned dials. Now, operators enter in
the exact numerical settings.
“Instead of a potentiometer or an internal timer, you have digital readouts,”
says Vladimir Siroky, director of operations at Fisnar Inc.
Digital control also gives engineers the ability to store and recall time-
pressure settings for specific materials. It also permits them to lock settings
to prevent operator adjustment, and it enables them to program automatic
adjustments to those parameters over time.
This digital dispenser automatically adjusts time and pressure settings
at set intervals of time or amounts of deposits. Photo courtesy EFD Inc.
Problems and Solutions
the most part, time-pressure dispensers are very repeatable, says Bray. Two
common sources of variability in shot size are fluctuations in air pressure and
material temperature. The former can be addressed with a precision pressure
regulator. The latter can be addressed by warming or insulating the material supply.
One problem with time-pressure systems is that their performance can decline as
the syringe empties. Less time and pressure are needed to dispense a given
volume from a syringe with 25 cc of adhesive and 5 cc of air than from a
syringe with 5 cc of adhesive and 25 cc of air. As a result, if time and
pressure settings remain constant, the shot size can diminish over time.
Assemblers can overcome that problem by dispensing from smaller syringes or
starting with syringes that are less than full. “That way, there’s less of a
[volume difference] between a full syringe and an empty one,” says Mike Vidal,
engineering manager at EFD Inc.
EFD has come up with its own solution to the problem that doesn’t require any
sort of adjustments or volume compromises. It’s called the Optimeter. The
central component of the Optimeter is a cone-shaped pin. The base of the cone
is connected to the piston inside the syringe.
When the syringe is full, the narrow end of the cone passes through the air
hole of the end cap, limiting how much air pressure can be applied to the
piston, explains Vidal. As the syringe empties, the cone moves down with the
piston, gradually retracting from the air hole and allowing more pressure to
act on the piston.
“What’s nice about the Optimeter is that it’s passive,” says Vidal. “You don’t
have any fancy electronics or feedback loops. It’s like a spigot. As the barrel
empties, it opens up the flow more. You get the same pressure pulse with a full
barrel as you get with an empty barrel.”
The Optimeter is available for 10- and 30-cc syringe barrels. “The taper on the
pin is optimized for each barrel size,” says Vidal.
Another common issue with time-
pressure dispensers lies with reactive materials, such as epoxies, that change
in viscosity over time. As these materials thicken, more pressure, time or both
are needed to maintain a consistent deposit size. If time and pressure settings
remain constant, the shot size will decrease over time.
Here, too, suppliers are rising to the challenge. For example, the Ultimus V
digital dispenser from EFD can be programmed to match the viscosity changes of
the material, automatically adjusting time and pressure settings at set
intervals of time or amounts of deposits.
For instance, say an epoxy becomes markedly thicker 10 minutes after it’s
mixed, and thicker still 20 minutes later. An engineer can program the unit to
dispense at a certain time and pressure for the first 10 minutes, another time
and pressure for the next 20 minutes, and a third time and pressure for the
remaining material. Alternatively, the unit could be programmed to dispense at
a certain time and pressure for the first 200 shots, another time and pressure
for the next 400 shots, and a third time and pressure for the remaining
“There is some up-front work that has to be done to understand the viscosity
profile of the material,” notes Vidal. “However, once you develop the profile,
the material is generally going to behave in the same way.”
For high-volume dispensing with a time-pressure system, the syringe can be replaced with a pressurized reservoir. Photo courtesy Hernon Manufacturing Inc.
Time-pressure dispensing systems can be handheld or mounted to a
Cartesian robot. Photo courtesy Fisnar Inc.
For precise applications, another type of benchtop dispenser
uses a leadscrew driven by a stepper motor to push the piston, instead of
compressed air. Each turn of the motor pushes the piston forward a set amount,
explains Nancy Gleason, director of business development at Fishman Corp. Based
on the volume of the syringe and the pitch of the screw, the system calculates
how far the piston must travel to dispense the desired amount of material. The
amount dispensed remains constant, regardless of the viscosity of the material
or the volume remaining in the syringe.
An interference fit between the piston and the syringe prevents pressure loss
and keeps material from slipping past the piston, says Gleason. In addition,
the piston can be programmed to pull back after each dot is dispensed, which
keeps material from oozing out of the tip.
With a 3-cc syringe, a leadscrew for large volumes can dispense dots as small
as 0.0009 cc with a standard deviation of three sigma. A leadscrew for small
volumes can put down dots as small as 0.00023 cc.
Another motor-driven alternative to the time-pressure unit is a peristaltic
pump dispenser. As with a pinch-tube system, the adhesive travels from a
reservoir to the dispense tip via flexible tubing. Instead of using compressed
air to force out the adhesive, a rotor turns a series of rollers that press
against the tubing. With each turn of the rotor, the rollers push a set amount
of adhesive forward in the tubing.
“It works like your esophagus,” says Siroky. “It’s a positive-displacement
technology, and it’s very easy to control.”
Because it’s a closed system, the device is ideal for dispensing volatile or
hard-to-work-with materials. Safe for use in clean rooms, the peristaltic pump
can operate in manual or timed modes. Reversing the rollers slightly produces a
suck-back to prevent drips from the dispense tip. Equipped with a Teflon tube,
it can output 0.01 to 6 mL/min. With a silicone tube, it can put out 0.1 to 20
On the opposite end of the spectrum, assemblers can avoid any electromechanical
control at all with a device like the Microdot from Tridak. This thumb-actuated
device accepts 5- and 10-mL syringes. Each click of the thumb switch pushes the
piston down a set amount. Shot size is adjusted using a knurled knob near the
top of the unit, and a safety lock prevents accidental actuation.
“The Microdot allows you to move around freely, and you get a very repeatable
shot with each click,” says Cole. “We have customers who use it for doing
rework in warehouses. They can take all the adhesives they need with them and
just use the one dispenser.”
When dispensing a material with a viscosity of 400 centipoise, the device can
put down a shot as small as 0.003 mL with a 25-gauge taper tip or as large as
0.17 mL with a 14-gauge taper tip.
This story was originally published in Assembly magazine, January 2010.