Note: Descriptions are shown in the official language in which they were submitted.
METHOD AND APPARATUS FOR SPRAYING POWDER
The present invention relates to a method and
apparatus for spraying powder. During various steel
finishing processes wherein steel is shaped or formed
while it is hot, it is often desirable to mark the hot
steel for, for example, identification or marking
defects. Since the steel is hot, marking materials
such as paint cannot be used as these would quickly
burn off. Hence the practice has been to apply dry
powder pigments or metallic powders onto the hot steel
surface, preferably by pneumatically spraying such
powders.
Although there is a selection of commercially
available equipment for spraying dry powders, such
prior art equipment has not been completely suitable
for spraying the types of powders used to mark hot
steel. That is to say, the powders most commonly
preferred for marking hot steel, for example, titanium
dioxide or zinc oxide, tend to agglomerate when
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pneumatically pumped, and thus frequently plug and
clog the powder spraying equipment.
United States Patent Specification No.
3,899,131 discloses a method and apparatus for
spraying agglomerating powders such as titanium
dioxide wherein the agglomeration problems are
somewhat reduced. Although that method and apparatus
are currently being used successfully in some
commercial operations, they have certain limitations
which render the practice unsuitable under certain
conditions such as the marking of such hot products as
bars, billets or slabs moving at high line speeds,
that is, greater than 1000 feet per minute.
Specifically, the impact of the marking powder on fast
moving products has not been sufficient to ensure
permanent adhesion to the product, and/or the volume
of the marking powder reaching the product does not
provide a marking of adequate contrast for visability
at remote distances. Indeed, the system requires
close proximity of the marking apparatus to a
stationary, or slow moving product in order to assure
adequate contrast, but this is not always possible on
some production lines as the spraying apparatus soon
becomes damaged, or it is not possible to stop or slow
the production line.
According to the present invention, there is
provided a method of pneumatically spraying powder
through an apparatus comprising a fluidized bed powder
reservoir, a pump housing attached within said
reservoir and having a chamber therein communicating
with a barrel assembly and an opening in a wall
thereof communicating with the interior of said
reservoir, a valve for selectively opening and closing
said opening, a gas-tight seal between said valve and
said housing for sealing said chamber with respect to
: the interior of said reservoir when the valve is
: closed, and means for blowing a gas through said
housing and said barrel assembly; the method
comprising activating said fluidized bed powder
reservoir to fluidize powder therein, opening said
valve to admit a quantity of fluidized powder into
said chamber, closing said valve to trap said quantity
of fluidized powder within said chamber, and
activating said blowing means to blow said quantity of
fluidized powder out through said barrel assembly.
The invention also provides an apparatus for
carrying out the method.
The invention is further described, by way of
example, with reference to the accompanying drawings,
in which:-
Figure 1 is an elevation view in partialsection showing an apparatus for spraying ary powder
in accordance with this invention,
Figure 2 is a plan view in partial section of
the apparatus shown in Figure 1 taken on lines II-II
of Figure 1,
Figure 3 is a schematic diagram of the
pneumatic control circuit for operating the apparatus
shown in Figures 1 and 2,
Figure 4 is a sectional view of an improved
- nozzle design pursuant to one embodiment of this
invention, and
Figure 5 is a sectional view of another
embodiment of the improved nozzle design.
With reference to the drawings, the apparatus
comprises a fluidized bed powder reservoir 10, a pump
assembly 12, an air cylinder 14 and a barrel assembly
16. The pump 12 is supported as a single unit by a
framework consisting of two side bars 20, a front
plate 22 and a rear plate 2~. The frame~ork is
attached within reservoir 10 by bolts 26 and 28. Such
an arrangement isolates the pump housing from the
outside atmosphere. The pump itself consists of a
tubular housing 30 welded to front plate 22, and is
provided with a pair of elongate, opposed openings 34
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and 36. A pump cylinder 38, consisting of a hollow
metal barrel coated with polytetrafluoroethylene is
slidably mounted within a polytetrafluoroethylene
sleeve 40 which is retained within housing 30 by a
keeper plate 42. Cylinder 3~ thus acts as a valve
closure member for selectively opening and closing
openings 34 and 36. The forward end of cylinder 38 is
fitted with a replaceable nozzle 44. In the closed
position as shown, nozzle 44 seats against a hollow
cylindrical muzzle 46 which is fitted into the forward
end of housing 30. Muzzle 46 is also made of
polytetrafluoroethylene.
Attached to the forward end of the housing 30
by means of threaded nut 48, is the replaceable barrel
assembly 16, which consists of a metal tube 50 having
an adapter ring 52 soldered to its inside end. Tube
50 contains a polytetrafluoroethylene sleeve 54
retained therein by a metal cap 56 soldered to the
forward end of tube 50.
The rearward end of cylinder 38 is fitted
with an angled adapter 60 for accommodating an air
inlet supply hose 62, and for attachment of a push xod
64. Push rod 64 extends through the rear wall of
reservoir 10 with its outer end connected to air
cylinder rod 66 of the air cylinder 14 by coupling
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68. A bellows 70 is supported on rod 64 and attached
to the rear plate 24. Accordingly, reciprocal
activation of air cylinder 14 will cause cylinder 38
to reciprocate between the closed position as shown,
and an open position wherein the inside of housing 30
is exposed to the atmosphere within reservoir 10
through openings 34 and 36.
For operation, cylinder 14 is actuated to the
closed position and the powder ~o be sprayed through
barrel assembly 16 is placed into reservoir 10 and
fluidized by air passing through a porous membrane
72. Thereafter, purging air or other gas is admitted
at low velocity through hose 62. This low velocity
purging gas flows through cylinder 38, muzzle 46 and
barrel assembly 16. The appara~us is thus activated
and ready to spray powders on demand. To spray
powders pursuant to prior art practices, the velocity
of air or gas admitted through hose 62 would be
increased as cylinder 38 is opened by air cylinder 14
20 to admit fluidized powder within housing 30. Thus
admitted, the powder would be pneumatically ejected
through muzzle 46 and barrel assembly 16. When it is
desired that powder spraying be discontinued, air
cylinder 14 would be activated to close cylinder 38 as
the air admitted therethrough is reduced back to a low
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velocity purging blow to keep the passageway clear of
accumulated powders.
Pursuant to the prior art practice therefore,
cylinder 38 would remain in the open position
throughout the entire period that of spraying. In
contrast thereto, the present invention requires that
cylinder 38 be closed when spraying. That is,
cylinder 38 is opened just prior to the blow in order
to admit the desired quantity of fluidized powder
within housing 30. Thereafter, cylinder 38 is closed,
trapping a small quantity of fluidized powder within
cylinder 38. When cylinder 38 is completely closed,
the small quantity of powder within cylinder 38 is
blown through barrel assembly 16 using a relatively
low pressure, for e~ample, 40 psig, to effect a
substantially higher rate of speed than is possible
with an open cylinder 38. Although a significantly
smaller volume of powder is so blown, its greater
speed provides a greater impact force on a more
concentrated area of impact. The powder as so
projected has a stronger adhesion on the target area
and its more concen~rated nature renders a more
visible area. In addition, the powder can be sprayed
over a greater distance and still leave a readily
visible mark, even on products traveling at line
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speeds in excess of 1000 feet per minute.
In order to achieve a high velocity of powder
through barrel assembly 16, it is necessary that the
seal between nozzle 44 and muzzle 46 be very
effective. Any leakage therebetween will cause
turbulence in muzzle 46 and reduce velocity.
Referring to Figure 4, the sealing face of nozzle 44
is tapered to provide a contact angle ~ of from ~0
to 50. In the alternative, a spherical contact
surface may be provided as shown in Figure 5 where the
tangent ~o radius R at the contact point with nozzle
44 is between 40 and 50.
The interior surfaces of muzzle 46 and barrel
assembly 16 should be streamlined to minimize
turbulence, that is, sharp corners on any orifices
should be removed to avoid turbulence which would tend
to reduce air velocityO The ideal velocity should
approach but should not exceed sonic velocity.
The pneumatic control circuitry of the
apparatus is shown in Figure 3. A main air supply
line 76 from a manifold 74 connects to a four-way
valve 78 which supplies air to the air cylinder 14
either at its forward end 80 or rearward end 82
depending on the position of valve 78. Solenoid 84,
which activates valve 78, is energized by any suitable
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power supply 86 through a spring return switch 88. A
second solenoid valve 90 is provided on air line 76
which supplies air supply to pu~p inlet supply hose
62. Solenoid 90 is energized by any sui~able power
supply 92 through a spring return switch 94. Air is
continuously supplied to the fluidized bed reservoir
10 via line 96 through regulator 98.
In operation, when reservoir 10 is fluidized,
the operator may depress switch 88 which sends air to
inlet 80 on air cylinder 14. This causes cylinder 38
to be retracted, thereby admitting fluidized powder
into housing 30. Release of switch 88 causes air
cylinder to reverse its direction, thus closing
cylinder 38 trapping a quantity of powder in housing
30. Switch 94 is then depressed which opens solenoid
valve 90 and permits full air supply pressure to
suddenly surge through hose 62 and cylinder 38
projecting the fixed quantity of powder through barrel
assembly 16. In an actual production line
application, switches 88 and 94 are usually operated
automatically in sequence in response to a signal for
spraying powder.
