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Description  |
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TECHNICAL FIELD
This invention relates to an apparatus for dispensing a measured amount of
solvent into a painting apparatus to push out paint from the painting
apparatus between color changes of the paint.
BACKGROUND OF THE INVENTION
Paints can be applied to a substrate in a number of ways, e.g., by dipping,
brushing, or spraying. For painting large surfaces rapidly, however, the
most efficient method for applying paint is by spraying. In automobile
production, an automatic paint spraying apparatus is generally employed to
paint car bodies on the assembly line. This apparatus includes a color
change means which allows the apparatus to spray more than one color of
paint. When changing from spraying one paint color to another, the paint
flow in the apparatus is stopped and the excess paint in the apparatus is
driven out. Afterwards, the spray apparatus is flushed with solvent and
air. It is then ready to be used to spray a different color of paint. Many
automotive paint spraying assembly line operations change from one color
of paint to another every minute, and the cumulative waste of paint that
is flushed out of the apparatus over a period of time can be quite costly.
It would be advantageous if only a slight excess of paint, over that
required to paint the object, would be left in the paint spraying
apparatus between color changes. Then, only a small amount of paint would
be wasted during the paint cleanout process. This could be accomplished by
shutting off the paint prior to the end of the painting cycle and using
air or solvent to push the remaining paint through the apparatus to
within, for example, a foot of the spray gun. In this way, almost all of
the paint in the apparatus could be used in spraying the substrate, and
only a small amount of paint would be left in the apparatus between color
changes. Consequently, only a small amount of paint, would need to be
discharged and disposed of. While both air and solvent can be employed to
push the paint out between color changes, their use results in an
increased rate of flow of the paint due to a decrease in the friction of
the fluid within the paint line. One might attempt to employ metering
pumps to control the flow of the paint as it is pushed out by, e.g.,
solvent. However, currently available gear type, metering pumps are not
able to control the speed at which the solvent pushes the paint through
the spray apparatus. The difference in viscosity between the paint and the
less viscous solvent presents a problem to currently available flushable,
metering pumps. These pumps are able to precisely meter materials whose
viscosity is similar to that of the paint materials, but are not able to
control the metering of the much less viscous solvent to the degree
necessary in such painting operations. Since the solvent and paint
presents a varied pressure to the pump as solvent travels through the
pump, the pump is susceptible to slip. This is particularly a problem in
spraying two component paints. In two component paints, the pigmented
resin and clear crosslinking agent are fed separately into the spray gun,
i.e., they are only combined as they are being sprayed on the substrate.
This is in contrast to one component paints which are fed into the spray
gun already mixed, i.e., as one component. In two component paint systems,
it is critically necessary to maintain a particular ratio of pigmented
resin to crosslinking agent in the coating composition. Thus the flow of
the two components into the spray gun is carefully metered in a prescribed
ratio by means of metering pumps and an electronic control system. If, a
solvent were to be used to push the resin out of the apparatus between
color changes, the solvent would need to be driven through the apparatus
at the speed necessary to maintain the proper ratio of pigmented resin to
crosslinking agent in the sprayed composition. If for example, the
pigmented resin were to be pushed too fast through the apparatus by
solvent, the amount of pigmented resin in the sprayed composition would be
more than that necessray for the paint composition. Additionally, the
paint could be forced out of the spray gun before the automobile was
completely painted, with the result of that solvent would be sprayed on
the automobile. Conversely, if the pigmented resin was being pushed by the
solvent through the apparatus at too slow a rate, the amount of pigmented
resin in the sprayed composition would be less then that necessary for the
paint composition, and more pigmented resin would be left in the apparatus
between color changes resulting in an excess of paint again being wasted
during the flushing. What is necessary is a method for pushing the
pigmented resin through the painting apparatus by solvent at a precise
rate in order that it may be properly mixed with the crosslinking agent in
the spraying apparatus. Additionally, by pushing the resin through the
spray apparatus at a precise rate, the resin can be pushed so as to be
within a given distance of the spray gun at the time that the substrate
has been fully painted and thus minimize the amount of paint which will be
flushed out. It is not, however, possible to control with any accuracy the
speed at which the pigmented resin would be pushed through the system by
solvent if one uses currently available flushable metering pumps to meter
the solvent.
BRIEF DESCRIPTION OF THE INVENTION
This invention is directed to an apparatus for dispensing a measured amount
of solvent into a painting apparatus to push out paint from the painting
apparatus between color changes. The apparatus of this invention
comprises:
(A) a chamber separated into variable volume first and second compartments
by separating means which is free to move within the chamber so as to vary
the volume of the first and second compartments;
(B) means for dispensing solvent into the first compartment;
(C) means for dispensing solvent from the first compartment into the
painting apparatus;
(D) metering means for dispensing viscous fluid into the second
compartment; and
(E) means for dispensing the viscous fluid from the second compartment,
wherein during a solvent fill cycle solvent flows into the first
compartment while at the same time an equal amount of viscous fluid flows
out of the second compartment, and wherein during a solvent dispensing
cycle viscous liquid flows into the second compartment while at the same
time an equal amount of solvent flows into the painting apparatus.
The invention in this application is also directed to the apparatus
described above in combination with a paint spray apparatus comprising a
color change apparatus.
This invention minimizes the problems described above relating to paint
pushout between color changes by providing an apparatus which may be
employed to push paint out of the painting apparatus at a very critically
controlled flow rate and in a metered amount.
One advantage of the apparatus of this invention is that it can effectively
control the rate of flow of the paint being pushed out of the painting
apparatus between color changes so as to maintain the proper proportion of
pigmented resin and crosslinking agent in the composition applied to the
substrate.
Another advantage of the apparatus of this invention is that it can
effectively control the amount of paint pushed out of the paint spray
apparatus in a given time, and thus in essence, it can control the amount
of paint left in the paint spray apparatus at the completion of the
painting operation.
BRIEF DESCRIPTION OF THE DRAWINGS
The FIGURE is a cross-sectional view of one embodiment of an apparatus
according to this invention for dispensing a measured amount of solvent
into the color change means of a spray painting apparatus for applying two
component paints.
DETAILED DESCRIPTION OF THE INVENTION
Other features and advantages of this invention will be apparent from the
succeeding, detailed description thereof. The invention is best understood
by referring to the drawing.
The drawing depicts a spray apparatus, including the paint pushout
apparatus of this invention, for applying a two-component paint. The color
resin is fed through color valve 7, through metering pump 4, through line
2 into spray gun 1, while at the same time the clear crosslinking agent is
fed from crosslinking agent supply line 6, metered by positive
displacement pump 5, and fed through line 3 into spray gun 1. At the
beginning of the paint pushout cycle of the apparatus (i.e., the cycle
whereby solvent is dispensed into the paint apparatus to pushout the
paint), color valve 7 of the color resin being sprayed is closed. At this
time, control valves 9 and 15 are closed, the solvent port control valve
25 is opened, viscous material supply control valve 14 is opened, and the
viscous material positive displacement pump 18 is started. With control
valves 9 and 15 closed, the viscous material 21 flows from the viscous
material supply tank 20 through supply line 19, check valve 17, positive
displacement pump 18, and control valve 14 into the second compartment 27
of chamber 12. This forces separating means 13 to the left and displaces a
controlled flow of solvent out of the prefilled first compartment 26 of
chamber 12. The controlled flow of the solvent through supply line 11,
control valve 25, through solvent port 24 into the common color change
manifold 28 of color change apparatus 23, through master color
displacement pump 4, color supply line 2 and three-way dump valve 30,
which is open to the spray gun and line 2, forces a controlled flow of the
color component ahead of it and out through the spray gun 1. At the end of
the spray cycle, the spray gun is shut off, the viscous material positive
displacement pump 18 is stopped and viscous material supply valve 14 is
closed. Subsequently, solvent control valve 9 is opened, viscous material
return valve 15 is opened, and dump valve 30 is opened to line 2 and
solvent recovery tank 29. Solvent from high pressure supply line 10 flows
through valve 9, line 11, valve 25, solvent port 24, through color changer
28, through line 2, out through dump valve 30, into solvent recovery tank
29. At this time, flush solvent under the high pressure of the solvent
supply line 10 forces the separating means 13 in chamber 12 to the right,
filling the first compartment 26 of the chamber with solvent and pushing
the viscous material in the second compartment 27 of the chamber past
control valve 15 through line 16. In the preferred embodiment of this
invention, the apparatus would include a means for returning the viscous
material from line 16 back to supply tank 20. However such viscous
material can instead be disposed of, if such is desired.
At the end of the solvent fill cycle, valve 25 is closed and subsequently
valve 9 is closed. Three way valve 30 is opened to the spray gun and line
2, then valve 22 is opened, allowing high pressure air from air supply
line 8 to blow out the color change head, color line and clear the gun of
solvent. Valve 30 is then opened to solvent recovery tank 29, valve 22 is
closed and a new color line is opened to prefill the color change head and
color supply line 2, color pump 4, while dumping a small amount of new
color to the solvent recovery tank 29. Thereafter, valve 30 is opened to
spray gun 1 and a new paint cycle is started.
Pumps 4 and 18 are positive displacement gear pumps. While pump 4 must be
flushable pump, pump 18 need not be flushable. The flushable pumps, as
compared to non-flushable pumps, include a by-pass valve around the gears
within the pump. During color change operation, this valve is opened so
that high velocity air or solvent is permitted to by-pass the gears and
pass from the pump inlet to outlet port, as well as through the pump
(gears). This allows a rapid scrubbing action to take place in (through)
pump 5, color supply line 2, dump valve 30 to recovery tank 29 on spray
gun 1. Exemplary of such a flushable pump is that taught in U.S. patent
application Ser. No. 601,110, filed Apr. 18, 1984. During the solvent
pushout cycle, pump 4 and pump 18 are set to operate at the same rate of
material delivery. The pump can be driven at the proper RPM by step
motors, which may be manually controlled or be automatically controlled by
a microprocessor or programable controller.
The viscous material 21 may be a material, such as dioctyl phthlate, which
is compatible with the paint and solvent and which has a viscosity similar
to that of the paint being pushed out. This assures that in the event that
any viscous fluid leaks past the separating means in the chamber, it will
not adversely affect the system or painting operation. Generally a
viscosity of about 14 to 26 seconds measured with a #4 Ford cup at
80.degree. F. would be suitable for the viscous material employed in the
paint pushout process of this invention.
The amount of solvent ahead of the separation means in the chamber must be
sufficient to fill the spray apparatus to within a given distance of
about, e.g., one foot of spray head 1, during the solvent dispensing
cycle. For example, in a standard paint assembly operation, the length of
hose 2 to spray gun 1 is about 10 feet long, and the amount of solvent
required to be dispensed into the spray apparatus during the solvent
dispensing cycle to push the paint to within about a foot of spray gun 1
is between about 150-200 cc.
Separating means 13 between the first and second compartments of the
chamber may be a solid movable separating means. Preferably, chamber 12 is
a cylindrical chamber and as is depicted in the drawing, the separating
means 13 therein comprises two circular, disc shaped pistons arranged
perpendicular to the longitudinal axis of the chamber and a stabilizing
means for the pistons, e.g., a bar, whereby the pistons are affixed apart
from each other. The separating means should form a liquid tight seal with
the walls of the chamber so as to prevent the viscous material and solvent
from leaking past the separating means.
While the apparatus of this invention has been taught as having particular
usefulness with a two component paint spraying apparatus, the apparatus of
this invention is not limited to such a system. It may, for example, be
used to push paint out of a one component paint spraying apparatus between
color changes. While particular embodiments of this invention, e.g.,
relative the viscosity of the material, amount of solvent to be dispersed
into the paint spraying apparatus, separating means, etc., have been
discussed above, they are not meant to be limiting to the apparatus of
this invention. Selection of the optimal characteristic of such variables
of the invention would be well within the skill of those in the art.
In view of the disclosure, many modifications of this invention will be
apparent to those skilled in the art. It is intended that all such
modifications which falls within the true scope of this invention be
included within the terms of the appended claims.
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