Abstract:
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 situated 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 15.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner.

Description:
This application is a continuation in part which discloses and claims subject matter disclosed in earlier filed pending application Ser. No. 07/730,514, filed Jul. 12, 1991 now U.S. Pat. No. 5,081,799, which is a continuation of application Ser. No. 07/505,918, filed Apr. 6, 1990 and which is now abandoned; and Ser. No. 07/680,337, filed Apr. 4, 1991 now U.S. Pat. No. 5,083,402 which is a continuation of application Ser. No. 07/505,918, filed Apr. 6, 1990 and which is now abandoned. 
    
    
     This invention relates to improved apparatus for directing fine particles in a compressed air stream toward a workpiece and to methods of blasting utilizing such apparatus. 
     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 ratio 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 and are safer than chemical stripping. 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 0.40. 
     Recent developments have indicated the effectiveness of another stripping media which is also quicker and safer than chemical stripping. This media is a granular media consisting of numerous particles of a plastic material which are also accelerated to high speed and directed against the surface to be cleaned. The media particles can be of various sizes, depending on the application, and can be accelerated to produce a continuous media flow using conventional sand blasting equipment. This system has also been shown to be highly effective in removing paint and other coatings from harder surfaces, such as metal, and also for deburring and other finishing processes and the like. It is far safer than chemical stripping, presents little hazardous waste disposal problems, and greatly reduces the man-hours and expense of surface cleaning. Blast cleaning with plastic media has been shown to be effective on the metal parts of aircraft, as well as suitable for stripping composites. 
     Here, too, the medium flow rates required for the less aggressive plastic abrasive media is substantially less than that used for sand blasting, and has been determined to be from about 0.5 to about 12.0 pounds per minute, using similar equipment. This, again, requires a much lower medium to air ratio, in the range of about 0.05 to 0.80. 
     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 inorganic salts or plastic particles are difficult to convey by pneumatic systems by their very nature. Further, they tend to agglomerate when utilized in apparatus as is typically 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 abrasive material to the nozzle has not been possible up till 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 0.80 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 situated 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 15.0 psi with an orifice having an appropriate area, yields acceptable media flow rates in a controlled manner. 
     Accordingly, the present invention also provides surface cleaning methods using less aggressive abrasives than sand wherein such abrasives have a mean particle size of from about 50 to 2000 microns. Such abrasives include inorganic salts such as sodium bicarbonate, sodium sulfate and the like, either alone or mixed and, if desired, admixed with small amounts of more aggressive materials to provide desired profiles and patterns. The plastic media contemplated include granular media substantially composed of particles of a plastic material which has a Mohs scale hardness number preferably lower than about 3.5. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a blasting apparatus modified in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to feed fine particles of a material such as an inorganic salt, such as a bicarbonate, having a mean particle size of from about 50 to 1500 microns, preferably, from about 250 to 850 microns, or a plastic media having a mean particle size of from about 200 to 2000 microns, preferably, from about 500 to 1000 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 and, preferably, about 20-60 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. It has been 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 12.0 pounds per minute, preferably up from about 0.5 to 5.0 pounds per minute. 
     While a preferred blast media is sodium bicarbonate, other blast media such as potassium bicarbonate, ammonium bicarbonate, sodium chloride, sodium sulfate and other water-soluble salts or mixtures thereof, are meant to be included herein. Also included are mixtures of such water-soluble salts with more aggressive materials, such as, aluminum oxide, which is insoluble, especially where precise flow control is necessary. 
     Another effective media contemplated herein and which has been found to be effective for use on soft substrates such as composites and which avoids surface damage when properly applied, is a plastic media with very specific properties. The media is composed of particles of a material having a Mohs scale hardness number of approximately 2.5 to 4.0. Particle hardness preferably should not exceed a Mohs hardness of about 3.5, as this has been found to damage soft substrates, e.g., composite surfaces. Plastic has been found to be the most suitable material for the media. Urea formaldehyde, thermoplastic or thermoset acrylics, melamine formaldehyde, polyester or other thermoplastic or thermoset plastics can readily be formed into granular particles for this purpose. A Mohs hardness of 3.0 is substantially softer than other blast media, such as sand, which has a Mohs hardness of 7. It is the relative softness of the media, in combination with the method described below, which prevents damage to composite surfaces. A suitable commercially available media which can be used with the present invention is POLYEXTRA® Blast Cleaning Media, manufactured by the U.S. Plastic and Chemical Corporation or SOLIDSTRIP® Blast Media manufactured by E. I. duPont de Nemours &amp; Co. (Inc.). 
     Plastic blast media are generally classified as to particle size by U.S. standard sieve sizes. While it is not believed to be critical, media with a sieve size of 20-30 i.e., about 600 to 850 microns, is known to be suitable for use with the present invention. It is anticipated that media having sieve sizes ranging from 12-16 to 60-80, i.e., about 200 to about 2000 microns, can be used, with the selection of the size being based on the particular application. 
     Referring to FIG. 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 cubic 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. When necessary 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 18, which regulates the flow of media to the compressed air line 20. The orifice openings can vary from about 0.063 to about 0.875 inch diameter, or openings corresponding to the area provided by circular orifices of 0.063 to 0.875 inch diameter depending on the media employed. 
     When using a plastic media the openings correspond to about a 0.50 inch opening for media having a mean particle size of about 2.50 to 420 microns, and 0.875 inch opening for a media having a mean particle size from about 600 to 850 microns. A positive pressure of between about 1 to 15 psig, preferably about 1 to 5 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. 
     A stream of plastic composite media (POLYEXTRA Blast Cleaning media) having a mean particle size of 500 microns, at a pressure of 40 psig and feed rate of about 10 pounds per minute, nozzle pressure 40 psi, was directed at aluminum panels 2 feet by 2 feet by 0.032 inch thick situated 12 inches from the orifice of the nozzle. The panels were depainted removed in 1.6 minutes, with no damage to the aluminum panels. 
     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, as well as from articles fabricated from soft substrates such as composite materials that heretofore required separate, special techniques. The system can be used efficiently and controllably with robotics. 
     It will be understood that the specific parameters of the preferred embodiments described hereinabove may be varied without departing from the scope of this invention. Accordingly, the preceding description should be construed as illustrative and not in a limiting sense.