Cleaning was one of the earliest industrial applications of
ultrasonics. Objects to be cleaned are placed in a bath of fluid
which is violently agitated by a number of ultrasonic transducers.
The fluid may be water or solvent based, depending on the
application. Traditionally the transducers were fitted around the
walls of the cleaning bath, but some modern equipment uses an
external transducer attached to a resonant probe which transmits
the vibrations to the fluid.
The ultrasonics may affect the cleaning process in several ways.
Rapid movement in the fluid can help to de-wet surfaces, overcoming
surface tension, and may also help to dislodge dirt particles and
carry them away from the surface. Cavitation is probably the most
interesting (and potent) effect - the shock waves generated by tiny
implosions of vapour bubbles can be devastating at close range. The
bubbles are so tiny that they can penetrate even the smallest
crevices, making the process ideal for parts which could not be
cleaned by other methods. Note also that the process must be well
controlled to minimise erosion of the surfaces of the parts being
cleaned. The standard test of ultrasonic intensity in a cleaning
bath is to immerse a standard foil strip for a set time, then
remove it and count the number of holes.
Imagine a knife which moves itself backwards and forwards in a
sawing action, thirty thousand times a second. True the distance
moved is very small but the acceleration is so high that nothing
can move with the blade or stick to it. Ultrasonic scalpels are
used by surgeons where they want to cut without exerting any
pressure. In industry ultrasonic cutting tools are used for
products that are difficult to cut by other means.
The heat generated by the ultrasonic vibrations can also be
useful. Some man-made fabrics are cut and simultaneously sealed
using ultrasonic knives to prevent fraying.
Ultrasonics have been used in several ways for machining metals.
Lathe tools may benefit from deliberately-induced vibrations to
prevent "chatter" which compromises the surface finish of the
finished component. Ultrasonic drills, used on very hard ceramics,
work by grinding or eroding material away - a liquid slurry around
the drill bit contains loose hard particles which are smashed into
the surface by the vibrations, eroding material away and creating
more loose hard particles
CarnaudMetalbox R&D (now a part of Crown Cork and Seal - the
biggest packaging company in the world) and Loughborough University
developed a new aerosol can using a number of novel metal-forming
processes, starting with ultrasonic necking (i.e. reducing the
diameter of the can at one end). The advantage of using ultrasonics
in this case was to minimise friction between the can and the die,
thus reducing the forming force. Without ultrasonics the force was
so high that the can body would buckle and collapse during the
necking process. With ultrasonics a 30% reduction in can diameter
could be achieved in a single operation (in conventional necking
processes the maximum is typically about 5%).
The ultrasonic forming process went into production making
small-diameter aerosol cans in a UK factory. The production line
still runs intermittently, making promotional packaging for several
prominent customers. One of its products ("Fleurs de Paris" parfum
deospray can) won a silver in the 1997 Metal Packaging
Manufacturers Association awards.
Ultrasonics can be used to weld different metals together,
without solder and flux or special preparation. The process is
different to plastic welding in that the two components are
vibrated parallel to the interface. This is a more intuitively
logical method of generating friction between them, but frictional
heating is not thought to be the prime mechanism of the process -
the temperature needed to melt (or even soften) most metals would
be very difficult to achieve. Instead the mechanism is thought to
be diffusion-bonding: atoms of each part diffuse into the other
when the two surfaces are brought together in close contact. The
ultrasonics promotes this close contact by breaking down the
surface oxide layers, allowing the "raw" metals to make
Plastic welding is used for a huge variety of products ranging
from blister packs, cartons and small consumer goods up to car fuel
tanks and dashboards. It works by generating heat exactly where it
is needed - at the interface between the components to be joined.
The components are clamped between a vibrating sonotrode and a
fixed mounting. Strangely, the vibrations are usually applied
perpendicular to the contact surface, although much of this
vibration may be converted to in-plane movement. This also has the
advantage that the clamping pressure will keep the sonotrode in
contact with the component - serrated surfaces are generally not
required. Best results are achieved when the components are clamped
close to the interface ("near-field" welding) but if this is not
possible then the process can still work at a distance
Staking, or insertion, is a variation of this process in which a
metal part (generally a threaded bush) is driven into a hole in a
plastic component, which then solidifies around it to form a
permanent join. This is a convenient method of producing strong
tapped holes in a plastic part.
Industrial sieves are normally agitated at low frequency to help
the product to distribute itself evenly over the surface and to
help the small particles go through. Vibrating the mesh at
ultrasonic frequencies (in addition to this low-frequency
oscillation) can improve the rate of flow dramatically, preventing
the product from blocking the holes in the mesh and helping to
separate the small particles from the large.
The powder-metallurgy process is used to manufacture top-quality
steels and other metals. The powder must be packed as closely as
possible before the sintering process begins to prevent the
formation of voids or other weaknesses in the finished product.
Published research papers indicate that a significant increase in
the packing density can be achieved using ultrasonics. Can anyone
confirm that this process is in production?
Ultrasonic waves are projected through a medium (water) and a
object (carbon fiber) and the internal structure can be analized
for foriegn objects because the sound waves are bounced at
different rates through the foriegn material than expected.