I PROVEMENT 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 1.2 pounds of sand are used typically with about 1.0 pound of air, thus yielding a ration of 1.20.
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 10.0 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. 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 by weight, using a configuration similar to conventional commercially available sand blasting equipment.
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 1.0 and 5.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner.
BRIEF DESCRIPTION OF THE DRAWING Figure 1 illustrates a blasting apparatus modified in accordance with the present invention. Figures 2 and 3 are graphs of media flow rate versus pressure.
DETAILED DESCRIPTION OF THE INVENTION 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 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
3 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. 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 exit 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 15 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 bicarbonate 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 aluminum 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.