Ultrasonic cleaning has
a wide range of applications. In general, one can say that it is
indispensable in any cleaning operation that requires speed, cleaning
of any type of cavity (whether it’s blind or not), economy
of cleaning fluid and the elimination of individual manual work.
Ultrasonic cleaning consists of a combined microscopic mechanical
mixing effect coupled with an enhancement of chemical activity.
The main characteristic of ultrasonic cleaning is that the mechanical
energy in the form high frequency and high intensity waves is transferred
to the cleaning fluid. This energy can be seen in the phenomenon
known in fluid mechanics as cavitation.
Cavitation is produced when ultrasound
waves are emitted into the liquid. These waves propagate as
high and low pressure waves that compress and depress giving
rise to the formation of bubbles in the heart of the fluid.
Upon reaching a certain size that depends on the wave frequency,
characteristics of the liquid and temperature among other variables,
these bubbles disappear due to a violent implosion. This is
what cavitation is about.
When the waves are applied to the liquid at high intensity,
zones of low and high pressure are produced. During the low
pressure period, small bubbles are formed as a consequence of
the evaporation of liquid from the periphery of the bubble.
When the compression period takes place, the bubbles return
to liquid state and generate a local impact due to the implosion.
When this occurs on the inelastic surface of the item being
cleaned, it’s as if a microscopic brush were at work.
The
sum total of these impacts is known as cavitation and the effect
is dependant on the pulsating way in which the piece is hit
all over its surface as if it were being scrubbed uniformly.
Since this phenomenon occurs throughout the liquid, this microscopic
scrubbing effect takes place on the entire submersed item whether
the part being cleaned is visible or not.
The cleaning solution used is made of water and different chemical
products that vary in composition according to the material
from which the piece is made and the type of grime to be removed.
Cavitation greatly accelerates the cleaning process thus allowing
the use of chemicals that are not harmful to our health and
are biodegradable.
Cavitation removes gas from the liquid, this is necessary for
the cleaning process.
The intensity of the ultrasound waves to be used is determined
by a series of factors such as: the type of cleaning to be done,
characteristics of the cleaning solution, concentration and
temperature of the fluid, volume of liquid used and time required
to accomplish the process.
A simple way of demonstrating the effects of cavitation is to
take a thin sheet of aluminum and to place it in water. A few
moments later cavitation begins to erode the thin sheet of aluminum
according to the intensity of the waves applied to it.
Components involved in ultrasonic cleaning
Basically, there are three main components:
Tank: It can be made of metal (stainless steel) or plastic.
In case of using submersible transducers, it must be capable of
resisting both cavitation and the chemicals that will be used
in each case.
Transducer: This is the component that transforms the electrical
energy into mechanical energy which is transferred to the liquid
by direct coupling or submersible systems.
Types of cleaning fluids: Each type of grime is different therefore different chemical
products are required according to the type of material to be
removed. Nowadays, the great majority of cleaning fluids are biodegradable.
Ultrasonic Cleaning: The Process
The items
to be cleaned are completely submersed in the fluid and should
be placed in such a way so that if the grime removed has solid
particles, they may fall to the bottom due to gravity. It must
be kept in mind that brusque mixing will decrease cavitation notably.
After cleaning for the required amount of time, the item is removed
and rinsed in the following bath in order to eliminate traces
of the liquid or to neutralize the acid or alkaline action of
the fluid. This is usually done with tap water. In some cases,
drying is necessary and it is done with high pressure air jets.
The process described is very generic. However, it must be kept
in mind that the items may be transferred from one area to the
other manually or by an automated system that will vary in its
degree of automation according to the case.
In order to determine a specific cleaning routine, a trial will
be carried out during which the most adequate chemical, temperature,
time and repetition of each cycle will be chosen.
Preparing for and starting the cleaning operation
El lavado
por ultrasonido requiere de una serie de pasos previos para comenzar
a operar, una vez determinado el tipo de producto químico
que se utilizara se seguirán las instrucciones del fabricante
en cuanto a su utilización, se procederá a llenar
la cuba de lavado teniendo en cuenta el volumen de la carga de
las piezas a limpiar. Luego se procede a calentar el baño
hasta la temperatura de trabajo teniendo en cuenta que no deberá
excederse de los 60° C.
Antes de comenzar el lavado es necesario desgasificar el liquido,
etapa que es el mecanismo que se utiliza para eliminar el aire
que se haya disuelto en el baño, esta etapa es imprescindible
para un optimo lavado y se lleva a cabo mediante la sencilla operación
de hacer funcionar la maquina, ya alistada para lavar, sin piezas,
por espacio de 15” el no hacerlo va en contra de la calidad
y uniformidad de los sucesivos lavados, puesto que con el uso
a través del tiempo se lograra el objetivo de la desgasificación
pero no el de la uniformidad del mismo, dicho de otro modo es
posible que las piezas procesadas durante el periodo de desgasificación
no tengan todas el mismo grado de limpieza y se observen variantes
en su terminación, lo cual puede originar inconvenientes
posteriores en la secuencia de fabricación de la pieza.
Si se desea reducir los tiempos de preparación se puede
aplicar ultrasonido durante el periodo de calentamiento del agua,
de este modo cuando el liquido tenga la temperatura adecuada la
etapa de desgasificación seguramente estará completada.
Otro aspecto a tener en cuenta es el grado de saturación
o degradación del liquido por el propio uso, en esta situación
se procederá a reemplazar total o parcialmente el producto
de acuerdo a las especificaciones del fabricante.
When the waves are applied to the liquid at high intensity,
zones of low and high pressure are produced. During the low
pressure period, small bubbles are formed as a consequence of
the evaporation of liquid from the periphery of the bubble.
The
sum total of these impacts is known as cavitation and the effect
is dependant on the pulsating way in which the piece is hit
all over its surface as if it were being scrubbed uniformly.
Since this phenomenon occurs throughout the liquid, this microscopic
scrubbing effect takes place on the entire submersed item whether
the part being cleaned is visible or not.
The intensity of the ultrasound waves to be used is determined
by a series of factors such as: the type of cleaning to be done,
characteristics of the cleaning solution, concentration and
temperature of the fluid, volume of liquid used and time required
to accomplish the process.
