Each ultrasonic operational frequency has its own unique
characteristics. These characteristics can be used to determine which
frequencies might be best suited to a given cleaning application.
The higher
the operating frequency of the ultrasonic cleaning system, the more gentle the
scrubbing action produced, and the more evenly distributed that scrubbing action
will be across the surface of the parts. Lower frequencies produce more
aggressive cleaning action. However, the scrubbing action is not evenly
distributed within the tank.
Low frequency ultrasonic cleaners operating at
25kHz produce "hotspots" approximately 1" apart starting from the transducer
mounting location, typically the bottom of the tank. However, there is a
"blast radius" produced at every cavitation site which allows cleaning of
neighboring areas of the parts. As the cleaning action commences, jets of
liquid strike the surface of the parts which spreads out across the surface to
clean neighboring areas. The amount of neighboring area which can be
cleaned will vary depending upon the effectiveness of the cleaning agent, and
contaminant-to-part bond strength. If the contaminants are lightly bonded,
the entire part may be effectively cleaned even though the system has poor
energy distribution.
Low frequency ultrasonic cleaners also have a tendency to remove larger
particle sizes more effectively than higher operational frequencies. Tests
which were performed by several companies in the disk drive industry clearly
indicated that the higher the operational frequency, the smaller the particle
size most effectively addressed.
a 40kHz ultrasonic system, perhaps the most
common single-frequency system available, produces hotspots which are roughly
1/2" apart, more evenly distributed than
25kHz,
but not as even as
80kHz.
High frequency ultrasonic systems in the range of
80kHz produce more gentle, but more evenly distributed cleaning action.
Cavitation produced in such systems do not produce as large a blast radius as
that produced in lower frequencies. However, all part areas are being
evenly attacked by ultrasonic cavitation at all times. When coupled with
an effective cleaning agent, even high frequency systems have the capability to
remove highly bonded soils.
High frequencies also produce a more penetrating scrubbing action, and have
been shown to remove particles of a smaller size more effectively.
When selecting an ultrasonic frequency, the size of the part being cleaned
must also be considered. Large and heavy items, such as a plastic
injection mold, are typically cleaned in lower frequencies, since these
frequencies have better penetration into internal gas cavities and other detail
which require cleaning. If higher frequencies were used for this
application, the outside of the part would be cleaned, but the inside of the
part would not, since high frequency ultrasonic systems do not have the energy
to cavitate the inside of these heavy objects.
Plastic parts may also require the use of lower operational frequencies since
plastics tend to absorb and deaden ultrasonic activity. At higher
frequencies, not enough energy is present to remove the contaminants in question
since the activity is buffered by the material itself. Lower frequencies
are typically more effective in these applications.
One can easily understand why Zenith developed its
CROSSFIRE Multi-frequency Ultrasonic
Cleaning System which includes 2 or more ultrasonic frequencies in the same
tank. By combining more-than-one frequency, the range of particle sizes
most effectively addressed by the system is double that of any single frequency
system. Also, power does not have to be sacrificed for even cleaning
action, since a
CROSSFIRE system
includes both power and even cleaning action.
Additional information regarding the properties of each ultrasonic frequency
can be found in the Ultrasonic College drop-down menu, under the following web
pages:
