Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD OF CONTROLLING THE THICKNESS OF AN
ELECTRODEPOSITED COATING
Back~round of the Invention
The present invention relates to a method of con-
trolling the thickness of an electrodeposited coating pro-
duced by electrophoresis whereby a desired coating thickness
can be easily achieved.
The thickness of an electrodeposited coating pro-
duced by electrophoresis is dependent on the electro-
deposition voltage, the length of time for which this
voltage is applied, and the characteristics (the p~, the
resistivity, and the temperature~ of the electrodeposition
paint in which the object to be coated by electrodepostion
is immersed. In mass production, it is important that every
object to be coated have the same coating thickness. The
usual method of attaining a uniform coating is to regulate
all of the .:bv-ie~Lactors so that they do not vary from
object to object.
This method has the advantage that the electro-
deposition operation can always be carried out at maximum
efficiency by setting the pH, the resistivity, and the
temperature at optimal values. However, it has the over-
riding disadvantage that it requires great capital invest-
ment; a pH regulator is necessary to regulate the pH, a
dialysis apparatus is necessary to regulate the resistivity,
and a temperature regulator is necessary to regulate the
temperature of the electrodeposition paint.
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There is -therefore need of a different method of
thickness control which does not make use of such regulat;ng
devices.
It is an object of the present invention to provide
a method of controlling the thickness of an electrodeposi-ted
coating which does not require strict control over the
temperature, the pll, or the resistivity o-f the electrodeposition
pa.in-t being used.
According to the present invention there is provided
a method of con-trolling the thickness of an electrodeposited
coating comprising: previously determining the relationship
between electrodeposited charge, electrodeposition paint
resistivity, electrodeposition paint pH, coating thickness
and surface area of the object to be coated; measuring the
presen-t pH and the resistivity of the electrodeposition paint
at the time of electrodeposition; calculating the electro-
deposited charge required to produce a given thickness and
surface area of coating given said present resistivity and
pH using said previously determined relationship; and regulating
the electrodeposition current level of the electrodeposition
apparatus being used and the length of time for which said
current- flows so as to achieve said calculated electrodeposited
charge.
In the present method, the pH, the resistivity,
and the temperature of the electrodeposition paint are per
mitted to vary freely within allowable limits, and the desired
thickness of electrodeposited coating is obtained by regulating
the electrodeposited charge.
s~
The invention will now he described in more detail,
by way of example only, with reference to the accompanying
drawings, in which:-
Figure 1 is a graph showing the relationship betweenthe weight per square cm of an electrodeposited coating and
the resistivity of the electrodeposition paint used to produce
this coating with pH and temperature held constant.
Figure 2 is a graph of the relationship between
coating weight per square cm and pH of the electrodeposition
paint with resistivity and temperature held constant.
Figure 3 is a graph showing the relationship
between coating weight per square cm and temperature of the
electrodeposition paint with resistivity and pH held constant.
Figure 4 is a graph showing the relationship between
electrodeposition efficiency (weight per square cm per coulomb)
and temperature, with resistivity, pH, and coating thickness
held constant.
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Figure 5 is a graph showing the relationship
between electrodeposited charge and surface area of the
object to be coated for 4 different coating thic~nesses with
the resistivity and pH of the electrodeposition paint held
constant,
Figure 6 is a schematic diagram of an electro-
deposition apparatus employing the coating thickness control
method according to the present invention.
DESCRIPTION OF THE PREFERRED EMBODIME~T
The electrodeposited charge necessary to produce a
given thickness of coating using a given electrodeposition
paint is dependent on the pH and the resistivity of the
paint but is independent of the temperature. This is shown
in Figures 1 through 4. The first 3 figures are the results
of e~perimental electrodeposition carried out on identical
samples at a constant voltage for a set length of time, at
the end of which time the weight of coating per square cm
was measured.
The data Eor Figure 1 was obtained by varying the
resistivity of the electrodeposition paint while maintaining
the pH and temperature constant. For Fiaure 2, the p~ was
varied and the resistivity and temperature were held
constant. For Figure 3, the temperature of the electro-
deposition paint was varied while the resisitivity and the
pH were held constant.
Figure 4 shows electrodeposition efficiency (the
weiyht of coating in mg per square cm divided by the electro-
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deposited charge required to produce this weight) plotted asa function of temperature with resistivity, pH, and coating
thickness held constant. Whereas in the first three qraphs
the electrodeposition voltage and time were held constant,
for Figure 4, they were varied so as to achieve the same
coating thickness for each electrodeposition.
Since the coating thickness and the weight per
square cm were held constant, the horizontal slope of the
curve clearly shows that the electrodeposited charge in
coulombs necessary to produce a given thickness of coating
is unaffected by temperature.
The present method takes advantage of that fact
and does not involve measuring the temperature. It involves
measuring only the pH and the resistivity of the
electrodeposition paint being used, determining the
electrodeposited charge required to produce a desired
thickness of coating given that pH and resistivity, and
regulati.ng the electrodeposition current of the
electrodeposition apparatus being used so that the required
electrodeposited charge is produced.
The preferred method for controlling the thickness
of an electrodeposited coating according to the present
invention is as follows. First, it is necessary to deter-
mine by experiment the relationship between electrodeposited
charge, resistivity, pH, coating thickness, and surface area
of the object to be coated. This is accomplished by carry-
ing out electrodeposition numerous times on samples of
various surface area under various values of resistivity and
pH, measuring the electrodeposited charge required to pro-
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duce a given coating thickness. Sample results are plotted
in Figure 5 with surface area as the abscissa and electro--
deposited charge in coulombs as the ordinate. Each curve in
the figure shows the relationship between charge and surface
area for one of 4 coating thicknesses (t) with pH and
resisitivity held constant for all the curves. A great many
graphs similar to Figure 5 are prepared using other values
of resisitivity and pH, which result in different curves.
Enough graphs are prepared so that it is possible to deter-
mine the electrodeposited charge in coulombs necessary to
produce a desired coating thickness and surface area for any
given resistivity and pH, either by direct reference to the
appropriate graph for that resistivity and pH or by interpo-
lation.
After completing the above preliminary procedure
of determining the relationship between electrodeposited
charge, resisitivity, pH, coating thickness, and surface
area, actual e]ectrodeposition is carried out.
~ nlike electrodeposition according to the conven-
tional method, the pH, the resistivity, and the temperature
of the electrodeposition paint do not need to be strictly
regulated but can vary freely within certain limits. The pH
and the resisitivity of the electrodeoosition p~int are
measured, and the electrodeposited charge necessary to pro-
duce a desired coating thickness given the present
resistivity and pH is calculated using the previously deter-
mined relationship. Since the desired coating thickness and
surface area of the object to be coated are known, the
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electrodeposited charge can be easily read or interpolated
from a graph like the one shown in Figure 5.
Electrodeposition is then carried out until the
electrodeposited charge equals the charge calculated in the
previous step. Since charge = current x time, the desired
electrodeposited charge can be produced by regulating the
level of the electrodeposition current, the length of time
for which this current flows, or both.
Many of the steps in the method described above
can be performed automatically by a computer, which ma~es
the thic~ness control method according to the present
invention particularly suitable for an automated electro-
deposition system.
Figure 2 shows a schematic of an apparatus for
automatically controlling the thickness of an
electrode?osited coating using the method according to the
present invention. 2 is the object to be coated by
electrodeposition. The electrodeposition paint 3 comprises
~ater-dispersed varnish and mica powder. ~ is a pH sensor
for measuring the pH of the paint 3 and 5 is a resistivity
sensor for measuring the paint's resistivity. 6 is a shunt
used to measure the electrodeposition current flowing
throuah the ob~ect 2. The sensors 4,5 and the shunt 6 are
electrically connected to a microcomputer 7 having an input
portion 8, a calculation portion 9, and an output portion
10. 11 is a DC po~er supply controlled by a voltage
regulator 12 which is electrically connected to and
controlled by the output portion 10. 13 is a switch
electrically connected to and controlled by the output
portion 10. By closing and opening
the switch 13, the electrodeposition process is commenced or
terminated.
The use and operation of the app~ratus shown in
Figure 2 are as follows. First the relationship between
electrodeposited charge, resistivity, pH, coating thickness,
and surface area is determined by experiment, as described
earlier. This relationship is then programmed into the
microcomputer 7. In addition, the desired thickness of
coating and the surface area of the object to be coated are
input to the microcomputer 7.
Electrodeposition is then carried out. The pH
sensor 4 and the resistivity sensor 5 measure the pH and the
resistivity, respectively, of the electrodeposition paint
and input corresponding voltages to the input portion 8 of
the microcomputer 7. sased on the input from the sensor 4
and S, the calculation portion 9 of the microcomputer 7
calculates the electrodeposited charge required to produce
the desired thic~ness of coating using the previously deter-
mined relationship between electrodeposited charge,
resistivity, pH, coating thickness, and surface area.
Electrical signals from the output portion 10 of the micro-
computer 7 close the switch 13 to commence electro-
deposition, and other signals from the output portion 10 con-
trol the voltage regulator 12 and thus the voltage produced
by the DC power supply 11 so that an appropriate electro-
deposition current flows through the shunt 6 and the object
2. When the microcomputer 7 determines that the product of
the electrodeposition current flowing through the shunt 6
and the length of time for which the current has flowed
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equals the calculated electrodeposited charge, an electrical
signal from the output portion 10 causes the switch 13 to ,
open, and the electrodeposition process is completed.
It should be clear that an electrodeposition coat-
ing apparatus employing the control method according to the
present invention is much cheaper than an apparatus employ-
ing the conventional control method, since the present
method does not require pH, resistivity, or temperature
regulators. Moreover, a control apparatus employing the
present invention is more versatile in that the size of the
control apparatus is not dependent on the size of the
electrodeposition bath with which it is used. In contrast,
the size of a control apparatus using the conventional
control method is very dependent on the size of the electro-
deposition bath, since a large electrodeposition bath
requires much larger pH, resistivity, and temperature regula-
tors than does a small bath.
The present method was described for use with the
automa~c corltrol apparatus pictured in Figure 2, but it is
not restricted to use with that particular apparatus and may
be effectively used with any other appropriate control
apparatus.
Further, although the present method was described
for use wlth an electrodeoosition paint comprising
water-disoersed varnish and mica powder, thi~s method may be
used with any other electrodeposition paint for which the
electrodeposition efficiency is independent of temperature.
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