Note: Descriptions are shown in the official language in which they were submitted.
` wos1~1s336 P~/US91/02332
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IMPROVEMENT IN BLASTING APPARATUS
- This invention relates to improved apparatus for
directing fine particles in a compressed air stream
toward a workpiece.
BACKGROUND OF THE INVENTION
- Standard sand blasting equipment consists of a
pressure vessel or blast pot to hold particles of a
blasting medium such as sand, connected to a source
of compressed air by means of a hose and having a
means of metering the blasting medium from the blast
pot, which operates at a pressure that is the same
or slightly higher than the conveying hose
pressure. The sand/compressed air mixture is
transported to a nozzle where the sand particles are
accelerated and directed toward a workpiece. Flow
rates of the sand or other blast media are
determined by the size of the equipment.
Commercially available sand blasting apparatus
typically employ media flow rates of 20-30 pounds
per minute. About l.2 pounds of sand are used
typically with about l.0 pound of air, thus yielding
a ration of l.20.
When it is required to remove coatings such as
paint or to clean surfaces such as aluminum,
magnesium, plastic composites and the like, less
aggressive abrasives, including inorganic salts such
as sodium chloride and sodium bicarbonate, can be
used in conventional sand blasting equipment. The
medium flow rates required for the less aggressive
abrasives is substantially less than that used for
sand blasting, and has been determined to be from
about 0.5 to about l0.0 pounds per minute, using
similar equipment. This requires a much lower
medium to air ratio, in the range of about 0.05 to
0.25.
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However, difficulties are encountered in
maintaining continuous flow at these low flow rates
when conventional sand blasting equipment is
employed. The fine particles of a medium such as
sodium bicarbonate are difficult to convey by
pneumatic systems by their very nature. Further,
they tend to agglomerate upon exposure to a
moisture-containing atmosphere, as is typical of the
compressed air used in sand blasting. Flow aids
such as hydrophobic silica have been added to the
bicarbonate in an effort to improve the flow, but a
substantially uniform flow of bicarbonate material
to the nozzle has not been possible up til now.
Sporadic flow of the blasting media leads to erratic
performance, which in turn results in increased
cleaning time and even to damage of somewhat
delicate surfaces.
Thus it is desired to have a blasting apparatus
that can deliver the blast media at a uniform rate
that can be controlled in a predictable manner, at
flow rates yielding a medium-to-air ratio of between
about 0.05 and 0.25 oy weight, using a configuration
similar to conventional commercially available sand
blasting equipment.
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SUMMARY OF THE INVENTION
A conventional blasting apparatus is
modified to provide a separate source of line air to
- a blast pot through a pressure regulator to provide
a greater pressure in the blast pot than is provided
to the conveying hose. This differential pressure
is maintained by an orifice having a predetermined
area situate between the blast pot and the conveying ~-
hose. This orifice provides an exit for the blast
medium and a relatively small quantity of air from
the blast pot to the conveying hose, and ultimately
to the nozzle and finally the workpiece. The
differential air pressure, typically operating
between l.0 and 5.0 psi with an orifice having an
- appropriate area, yields acceptable media flow rates
~ in a controlled manner.
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: BRIEF DESCRIPTION OF THE DRAWING
Figure l illustrates a blasting apparatus
modified in accordance with the present invention.
; Figures 2 and 3 are graphs of media flow
rate versus pressure.
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DETA I LED DE SCR I PT I ON OF THE I NVENT I ON
In order to feed fine particles of a
material such as a bicarbonate having a mean
particle size of from 50 to 1000 microns, preferably
from about 250 to 300 microns, at a uniform rate,
pressures within the blast pot, including the blast
hose pressure, must be positive with respect to the
nozzle. Pressures are typically in the range of
about 20-125 psig.
Since the blast pot and the conveying hose
operate at about the same pressure, the flow of
blast media in conventional sand blasting equipment
is controlled by gravity feed and a metering valve.
We found that the blast pot was under a small
differential pressure with respect to the blast
delivery hose pressure, which fluctuated between
positive and negative; the result was that the flow
rates of the blast media fluctuated also in response
to the differential pressure changes. Further
according to the invention, a differential pressure
gauge is installed between the delivery hose and the
blast pot to monitor the differential pressure
directly. The pressure can be closely controlled by
means of a pressure regulator at any hose pressure
from 10 to 125 psig or higher, depending on the
supply air pressure. The present invention
eliminates this source of flow rate variation and
also modifies conventional equipment to handle blast
media at low flow rates of about 0.5 to 10 pounds
per minute, preferably up to about 5 pounds per
minute.
The invention will be described by
reference to Figure 1. Although the blast media
illustrated is sodium bicarbonate, other blast media
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such as potassium bicarbonate, ammonium bicarbonate,
sodium chloride and other water-soluble salts are
meant to be included herein.
Referring to Figure 1, blast apparatus 8
includes a blast pot 10, partially filled with blast
media 12. The blast pot 10, suitably having a
cavity of about 6 feet, terminates in a media
exit line 14 governed by a valve 16. The medium
control area, typically but not limited to an
orifice plate 18, further restricts the flow of the
media 12 to the desired flow rate. A line 20 is
connected to a source or pressurized air (not shown)
which is monitored with an inlet of monitor 22. Air
valve 24 is a remotely operated on/off valve that
activates the air flow to the nozzle and the opening
and closing of the media cut off valve. Nozzle
pressure regulator valve 26 regulates the nozzle
pressure by means of a monitor 28 when the system is
in operation. Nozzle pressure regulator valve 26
can maintain the desired nozzle pressure. The
nozzle pressure monitor 28 enables a controlled
pressure to be applied to the nozzle 30, suitably
having a throat diameter of about 0.5 inch. The
differential pressure gauge 32 monitors the pressure
between the blast pot 10 and the conveying hose 34.
The pot pressure regulator 36, measured by gauge 38,
is used to provide a pressure higher than the
pressure in the conveying hose 34, thus allowing the
differential pressure to be monitored by
differential pressure gauge 32. Optional equipment
for protection of and cooling of the workpiece and
the control of dust is provided by a water injection
line 40, which injects water to the nozzle 30.
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In operation, the blast media 12 is fed
through media exit line 14 and the valve 16 to an
orifice plate 18, which regulates the flow of media
to the compressed air line 20. The orifice openings
can vary from about 0.063 to about 0.156 inch
diameter, or openings corresponding to the area
provided by circular orifices of 0.063 to 0.156 inch
diameter. Preferably the openings correspond to
about a 0.125 inch opening for sodium bicarbonate
media having a mean particle size of about 70
microns, and 0.156 inch opening for a media having a
mean particle size from about 250 to about 300
microns. A positive pressure of between about 1 to
5 psig, preferably about 2 to 4 psig, between the
media e~it line 14 and the conveying hose 34 is
maintained at all times. A source of compressed air
is also fed to the air line 20, regulated by the
valves 24 and 26 to the desired air pressure and
nozzle pressure, respectively, which preferably is
between about 1~ to about 125 psig. The pot
pressure regulator 36 controls the pressure to the
top of the blast pot 10, further ensuring a
controlled and uniform flow of blast media 12. The
manometer or other differential pressure gauge 32
measures the differential pressure, which is
proportional to the amount of media flowing through
orifice 18. The blast media, compressed air and
water are delivered to the nozzle 30 and ejected
toward the workpiece (not shown) at a uniform and
controllable rate.
A stream of sodium ~icarbonate media at a
pressure of 64 psig and feed rate of about 2 pounds
per minute, nozzle pressures of psig and water
pressure of 200 psi, was directed at painted
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WO91/15336 PCT/US91/02332
~à~ù'mlnum panels 2 feet by 2 feet by 0.032 inch thick
situate 18 inches from the orifice of the nozzle.
The panels were depainted and all corrosion products
removed in four minutes, with no damage to the
aluminum panels.
Figure 2 is a graph of media flow rate of
from 1 to 5 pounds per minute versus different
pressures in psi varying from 1 to 5 psi. The data
points were made using a sodium bicarbonate medium
having a mean particle size of about 65 microns, a
- nozzle pressure of 60 psi and an orifice opening of
- 5/32 inch. It is apparent that the media flow
varies linearly with pressure.
Figure 3 is a graph of media flow rate in
pounds/min versus different pressure in psi using a
sodium bicarbonate media having a mean particle size
of 250 microns. Again, the media flow varies
? linearly with different pressures.
The present apparatus has an added benefit
in that surface corrosion is removed at the same
time as the coating, eliminating separate hand
sanding or solvent dissolution techniques. Further,
the present apparatus removed paint and other
coatings efficiently and effectively from the
surface of delicate metal parts, including areas
around seams, rivets, screws, and the like, that
heretofore required separate, special techniques.
The system can be used efficiently and controllably
with robotics.
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