Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention:
This invention relates to a process and apparatus for
applying chilled Einely divided particles to the surFace of a
]arge metal object having crevices or pores therein. More
specifically~ it relates to a method for chilling and applying
electrostatically charged particles to such an objectO Still
more speciEically it relates to a method of rotating and
maintaining the large object at the desired raised temperature
during the application of the chilled particles. Still more
speciically it relates to apparatus for effecting this process.
Description o~ the Prior ~rt:
U.S. Patent No. 3,279,936, issued to Clarence WO Forestek on
October 18, 196~, is directed to the treating of metal surfaces
having crevices or pores therein with perfluorocarbon polymer
particles so as to deposit such particles into the pores. By
having the metal at a raised temperature to enlarge the pores and
the particles chilled to contract them, the particles are
interlocked in the pores when the particles and the metal come to
an equilibrium temperature whereby the particles are expanded and
the pores contracted.
U.S. Patent ~oO 4,051,275 issued on September 27, 1977 was
also granted to Clarence W. Forestek and is directed to applying
such chilled particles in a fluid stream carrying finely divided
particles applied at a pressure of 30-120 psi to compact
previously deposited particles and thereafter depositing
additional particles in the space freed by the compaction, said
additional particles being a temperature at least 100F below the
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temperature oE khe metal surFace so that these additional
particles will also be locked into the fissures upon reaching
e~uilibrium temperature.
OBJECTIVES OF TH~ INVENTION
It is an objective of this invention to aid in the
deposition of such finely divided particles into crevices and
pores in the surface of metal objects, particularly larqe
objects, by placing an electrostatic charge on these particles.
It is also an objective oE this invention to place an
opposite charge on the object to be impregnated with the
particles.
It is also an objective of this invention to apply the
electrostatically charged particles in a blast of Eluid medium
in which the electrostatically charged particles are directed
against the metal surFace and into its crevices and pores.
It is also an objective of this invention whereby the
blasting of the electrostatically charged particles effects a
compaction of the particles already in the crevices and pores so
as ~o increase the free space therein thereby increasing space
for additional particles to be deposited therein.
It is also an objective of this invention that the object,
particularly large objects, can be rotated on an axis so as to
expose the deposited particles to repeated compaction upon
repeated revolutions so as to effect additional free space for
additional deposition of chilled particlesL
It is also an objective of this invention to design an
apparatus for the rotation of large objects while said
electrostatically charged chilled particles are blasted thereon.
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;,UMMARY OF THE INVENTION
In accordance with the present invention it has been found
that the application of finely divided chilled particles of
perfluorocarbon polymer or graphite, molybdenum sulfate, boron
nitride or other suitable electrostatically charged particles to
the surface of a metal object, particularly large objects, having
crevices or pores in said surEace can be Eacilitated and more
adequately eEfected by rotating the object on it linear axis and
maintaining theobjec~ at a raised temperature while applying
chilled finely divided electrostatically charged particles oE
perfluorocarbon, etc., onto thesurface thereof. Advantageously
the object is substantially enclosed in a hood or shield which
protects heat loss from the object. While the object is
maintained at the desired temperature, a stream of the chilled
electrostatically charged, finely divided particles suspended in
dry air or in an inert gas such as helium, nitrogren, argon~
etc., is directed against the metal surface so as to deposit and
impinge such particles into the crevices orpores and to compact
particles which had been previously deposited in the crevices or
pores.
The electrostatic charge applied to the particles is
advantageously in the range of 50-150 kilovolts, preferably about
lOOkv. Preferablyr the particles are given a positive charge and
the object is given a minus potential. However, while this
system is preferred, the revers system of charging may be used.
Any appropriate means may be used for applying this charge, but a
particularly advantageous device is described in another
application being fi~ed by this inventor on the same date
herewith. This device comprises a spray gun, such as used for
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?lectrostatically spraying paint which has an interior channel
through which is p3ssed the sprayinq medium, in this case the
suspension of particles dispersed in dry air or inert gas, is
mi~ed with a chilled gas. A cooling medium such as liguid
nitrogen or solid carhon dio~ide is maintained in an adjacent
vessel so that a chil.ling effect can be imparted to the gaseous
suspension medium by introducing a chilled gas, preferably the
same as used in the original suspension, into the suspensin oE
electrostatically charged particles during its passage through a
passageway of the chilling apparatus. Prior to the chilling of
the gaseous suspending medium, electrostatic charges are applied
to the particles by passing khe suspension oF particles through
an electrical field effected by electrodes positioned in the
chamber oE the spray gun. Appropriate electrical charge for the
electrodes is maintained by a high voltage transformer and
condensersO
The chilled gas introduced into the suspension is desirably
maintained at -20C (-4F) to -130C (-~02~), preferably -50C
(-58F) to -120C (-184F). While the method of chilling the
suspending gas and the suspended particles may comprise any
: suitable means for effecting this chilling condition, a
particularly effeckive means is the chilling apparatus shown
hereinafter for attachment to the front of a spray gun. This
preferred chilling apparatus is considered patentable per se and
is claimed in a companion application filed concurrently
herewith.
The spray gun may be any one oE a number of commercial spray
guns suitable for spraying electrostatically charged particlesO
A particularly suitable spray gun is the one marketed under the
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trademark "GEM~" which has a Eour electrode and high voltage
generating system including a high voltage transEormer and a high
voltage ~et of condensers. The cooling attachment is fitted onto
the Eront or spraying end of the spray gun so that as ~he spray
of suspended electrostatically charged particles emanate ~rom the
spray gun they pa~s through a passageway in the chilling
apparatus and out over impact spreading cone.
The polymer suspension stream is advantageously ejected ~rom
the spray gun at a pressure of 15-150 psi (1-10 atmosphere). A
suspension of particles in the suspension medium is
advantageously kept in a storage tank in which the particles are
stirred up by Eeeding in nitrogen or other gas. When the stream
is to be applied, the appropriate valve is opened to allow the
gas to escape with particles suspended therein and carried to the
electrostatically charging chamber o~ the spray gun prior to
mixing with the chilling gas medium. Various other means may be
used to feed polymer particles into the gas stream such as
Venturi, screw feed~ blower, pump, etc.
Although large objects of various shapes may be treated by
the proces of this invention, it is particularly suitable ~or
cylindrical objects, including large rolls 1800 millimeters
(70.86 inches) or more in diameter and 2500 millimeters (980~
inches) or more in length. If the object does not have end walls
or support at the the two ends o the cylinder or other shape,
these may be aEfixed for the purpose of making the cylinder or
other object rotatable on its linear axis. A supporting axle may
be af~ixed concentric with the linear axis and this axle driven
by a motor at a controlled rate o~ rotation, advantageously at
about 0~1 to 10 revolutions per minute~ with the axle desirably
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resting on roller bearings.
As previously indicated a hood or other partial cover may be
used to shield the object against loss of heat, and also during
the spraying operation an e~hausting suction may be applied under
this hood to remove the gas carrying the particles as well as the
remaining particles still suspended therein. The particles which
fall below the object may be collected and reapplied if desiredO
The desired temperature of the object to be treated may be
attained by applying a stream of hot air under the hood and
against the object as it is rotated. Infrared or other
appropriate means of heating may be used if desired. The hood
has a slot or open area running the length of the object and
parallel to the axis of the object through which the
electrostatic spraying is to be eEfected. If desired there may
be a sliding or hinged door to cover this open area which
spraying is not being effected. ~hile the object may be
maintained at a temperature as low as 38C (100 4F), it is
advantageously maintained at a temperature of about 150-180C
(302-356F). The hot air or other means of applying heat is shut
off during the spraying operation after which the hood is removed
and the object is allowed to cool. During the spraying operation
the chilled particles have a temperature at least 55~C ~130.5F)
preferably at least 165C (329F1 below ~he temperature oE the
object.
For a roller or cyllnder one inch in thickness, 1800
mlllimeters (70.86 inches) in outer diameter and 2500 millimeters
(98.4 inches) long, it generally takes about 12 hours for the
temperature to drop from 150C to 60C~ For the spraying
operation, such a roll may be satisfactorily sprayed in about 1/2
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hour with three operators applying the spray manually with hand-
held spray guns. If desired, a plurality o~ spray guns may be
held on a rack which is mo~ed, either manually or mechanically,
in a path parallel to the axis oE the ohject, -the rate of such
parallel movement being adjusted in accordance with the rate oE
rotation of the object to give complete coverage of the object's
sur~ace by the sprayed particles.
A stain appearance on the object's surface indicates
in~uEficient impregnation of the crevices and pores with the
particiles. ~praying is continued until the stain appearance
disappears. In some cases satisfactory results may be obtained
with 5 complete revolutionsO Preferably 20 or more revolutions
produce the desired results and generally there is no additional
advantage in exceeding 120 revolutions.
A felt strip or brush with soft rollers extending down From
the hood, at a point beyond the spraying area, to the surface of
the object and extending along the length o~ the object effects a
wiping operation to brush particles into crevices and pores and
also to wipe away particles on the surEace of the object between
the crevices and pores. This wiping may be effected continuously
after the spraying operation or may be done periodically. It is
generally satisfactory to perform this wiping operation at least
4 or 5 or more times.
During the spraying operation the spray gun or guns are
advantageously held about 300-500 mm (12-20 inches) from the
object being sprayed. It is generally sufficient to effect the
spraying during 20-100 revolutions of the object. As indicated
above, the spraying is continued or repeated until the stained
appearance no longer appears after wiping.
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The speed oE the particles toward the object is determined
by the pressure of the suspending gas and the voltage of the
electro~tatic field. While it is preferred to feed the particles
into the gas prior to or during passage of the gas through the
spray gun, it is also contemplated that the particles may be fed
into the gas stream aEter the gas emanates from the gun in which
case the particles will pick up the electrostatic charge on the
way to the object's surfaceO However, it is preerred to apply
the electrostatic charge to the particles before chilling the
particles and to effect the chilling of the particles by mixing a
chilled gas with the suspension of charged particles.
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For particles of 4.7 gm or 5 microns in size and a voltage
of about 80-100 kilovolts applied between the gun and the object
i-t is estimated that the particles have an initial velocity of
about 11 meters per second and about 0.09-0.10 meters per second
in the vicinity of the workpiece~ Smaller sized particles, e.g.,
of less than 1 micron are more favorable since the ultimate speed
due to -the e]ectrical feed will he higher than for larger
particles.
In applying the electrostatic charge it may be advantageous
to have a series of electrodes spaced in pairs along the pa~h of
the suspended particles with a voltage of approximately 20
kilovolts applied between each pair of electrodes and applied
transversely to the direction of flow of the gas.
SPECIFIC EMBODIMENT OF THE INVENTION
The description of the process and apparatus of this
invention are Eacili~ated by reerence to the drawings.
Fig. 1 is an elevatlonal front vie~ of a roll mounted in a
hood for spraying.
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Fig. 2 is an elevational end view of the roll and hood of
FigO 1.
Fig. 3 is a side elevational view of a spray gun adapted to
efEect cooling of a gas stream with suspended particles and to
supply electrostatic charge to the particles.
Fig. 4 is a top view of the front portion of spray gun shown
in Fig. 2.
Fig~ 5 is a side cross-sectional view of the front portion
of the spray gun shown in Figs, 2 and 3 taken at line 5-5 of Fig.
3.
Fig. 6 is an end elevational view of a roll mounted in a
hood extending a greater distance around the roll with an opening
on one side for exposure to a portable heater and a portable
spray unit~
In Fig. 1 roll 1 is supported by axle 2 which in turn rests
onbearings 3 which rotate on supporting pins 4 fastened to
supporting standard 5 which has legs 7', crossbars 6 and base
plates 7. Axle 2 is rotated by a belt or chain 8 which is driven
by a motor means (not shown) to give the desired rate of rotation
to roll 1. hood 9 is positioned above roll 1 and has exhaust
outlet 10. If desired the hood may e~tend down further to
embrace a larger portion of roll 1.
Fig. Z is an elevational end view of the apparatus of Fig.
1. Heating means 11 is shown positioned on one side (or in back)
of roll 1 with supporting means 11'. The spray gun (not shown in
this view) is positioned on the opposite side (or in front) of
roll 1. Felt or soft bristles 13 are positioned next to the roll
1 above the spray are and are held by support means 14.
Fig. 3 is a side elevational view of spray gun attachment 19
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designed to effect cooling of a gas stream aFter applying
electrostatic charges to particles suspended in the gas stream.
The cooling nozzle 19 is preferably made of
polytetrafluoroethylene (PTFE). Metal ring or band 16 slides
over the exterior of a portion oE the spray gun attachment 19 to
hold gas entrance duct 20 in position. Cold gas such as nitrogen
is fed into the spray gun Erom cooling tank 21 which preferably
consists o~ a rigid, expanded polyurethane foam in which a heat
exchange coil 22 is positioned in cooling chamber 23. The coil
is advantageously made of copper tubing having a diameter of
about 10 mm. Cover 24 closes the camber to reduce the admission
of heat. The coolant contained in the cooling chamber is
advantageously either dry ice (solid carbon dioxide) or liquid
nitrogen. When the gas inlet line 25 is opened, gaseous
nitrogen, or other preferred gas, flows through cooling coil 22
and the cooled gas (preferably at -70 - -140C) flows through
hose connector 26 to gas entrance duct 20 attached to and
communicating with the recessed portion 20' running around the
circumference of attachment 19 and feeding cold gas through
channels 27 into the interior of the chamber 15 of cooling
attachment 19.
Pig. 4 is a top view o the cooling attachment 19 and spray
gun 18 shown in Fig. 2.
Fig. 5 shows the interior structure of the cooling
attachment and spray gun taken at line 5-5 of Fig. 4. Nozzle 19
has a gas duct 20 which is fixed in position and is sealed by
means of a metal ring 16. Hose 26 (shown in Fig. 3) feeds
chilled gas into gas duct 20. From duct 20 the chilled gas is
passed through channels 27 into mixing channel 15 where the
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electrostatically charge~ partic3.es are cooled down to the
desired low temperature, preEerably approximately -120C (-184F)
and then sprayed onto the roll or other large object being
rotated in ~ront of the spray gun, which object is grounded so as
to attract and hold the electrostatically charged particles.
Conical impact plate 29 i5 positioned at the outlet end 30 of
mixing channel 15 and can adjust the cloud of powder to the
desired size by appropriate positioning of the impact plate 29
bylongitudinal movement o:E rod 31. High voltage preferably in
the range of 70-120 kv is developed in generator 32 and
transmitted through lines 33 into the interior of the spray gun
through switch 34 to high voltage condenser cascade 35 and then
through high voltage transformer 36 to electrodes 37. Meanwhile
particles of the desired size and type are stored in reservoir 38
maintained in suspended state by gas admitted by line 39 at the
bottom of the reservoir from which the gas stream conveys the
suspended particles through hose 40 into the spray gun where they
are electrostatically charged as they pass between electrodes 37
into mixing channel 28 where the particles are chilled to- the
desired low temperature by mixture with the cold gas being
admitted through duct 20.
Fig. 6 is an end view of the roll arrangement as shown in
Fig. 2 except that the hood extends further around the roll by
the addition of hood extension 9'. In place oE the heater
arrangement shown in Fig. 2, the heater 11 and heater support 11
are made portable by means of wheels 41 fastened to the heater
support 11' by supports 42. The heat source is not shown but may
be electrical, eqg~, hotwire, or heat lamps, or hot air fed by
flexible ducts leading into heater 11. When the roll is properly
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locatedr the portable spray gun (not shown) is moved into the
position vacated by the heater and soEt brush 13 is moved in a
position above the spray gun. With the rotatable spray gun
positioned in the opposite side of the roll it may be desirable
to rotate the roll in the opposite direction from that shown in
Fig. 2.
While certain features of this invention have been described
in detail with respect to various embodiments thereof, it will of
course be apparent that other modifications can be made within
the spirit and scope of this invention, and it is not intended
to limit the invention to the exact details shown above except
insofar as they are defined in the following claims.
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SUP PLEMENTARY D L SCLOSURE
In accordance with the present invention apparatus for the
application and compaction of chilled Einely divided particles
into the pores and crevices of the metal surface of a large
object comprises the combination of:
(a) a means for supporting and rotating said large object
about its linear axis at a controlled rate of rotation;
(b) a heating means for applying heat to said large obiect
until the temperature of said metal surface is sufficient to give
a desired increase in the size of said pores and crevices fo~
receiving finely divided particles;
(c) a charging means for applying electrostatic charges to
finely divided particles while suspended in a gaseous medium;
(d) a second charging means for applying an opposite charge
to said large object from that applied on said particles;
(e) a cooling means for chilling said electrostatically
charged particles; and
(f) a means for spraying said chilled, electrostatically
charged, gas-suspended particLes onto the said heated surface of
said large object and depositing said particles into the pores
and crevices in the metal surface of said large object, said
rotatable large object being surrounde.d for a substantial portion
of the upper area of said object by a hood adapted to reduce heat
loss from said object and being surrounded for a substantial
portion of the undèrside~ area of said object by a catch basin
adapted to catch particles falling from the surface of said
object and also adapted to further shield said object from heat
loss; said heating means and said spraying means both being
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portable and each means being adapted to be moved to a position
adjacent to the said obiect and to be moved away from said object
after performance of its function following which the other means
may be moved into a position adiacent to the said object for
performance of its function.
Preferably the said heating means comprises a means for
directing a heated gas against the surface of said object and the
rotatable large ob-ect may be surrounded for a substantial
portion of the upper area of said object by a hood adapted to
reduce heat loss from said object. The hood may have an exhaust
duct connected thereto through which said heater gas may be
exited after eit has flowed over a portion of said large object.
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