Abstract:
An apparatus for removing surface coverings comprises a feeder device capable of providing a supply of dry ice pieces, a pellet-size reducer operatively associated with the feeder device to receive dry ice pieces from said feeder device and reduce the size of the dry ice pieces to nominal diameters less than 1 mm, and a blast gun connected to the pellet-size reducer and adapted for operative communication with an associated supply of flowing gas. The blast gun incorporates a venturi to entrain the reduced-sized dry ice pieces into the flowing gas and directs the flowing gas and dry ice pieces to an associated surface covering to be removed. Preferably, the pellet-size reducer is a knife- or disc-type grinding mill. In one embodiment, the grinding mill is adjustable for controlling the size of the dry ice particles supplied to the blast gun. The apparatus preferably includes a humidity controller to suppress static charges caused by the interaction of the dry ice particles with the surface being blasted.

Description:
This application claims priority from U.S. Provisional Patent Application Ser. No. 60/037,033, filed Feb. 5, 1997. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and method for removing surface coverings by blasting with dry ice pellets. In particular, the present invention relates to an apparatus for removing surface coverings using reduced-size dry ice pellets that is especially suitable for use in the production of electronic devices. 
     The use of dry ice pellets for blast cleaning is well known in the art. Examples of conventional pneumatically powered blast cleaning systems are described in U.S. Pat. No. 4,389,820 and U.S. Pat. No. 5,365,699, which are incorporated herein by reference. 
     In such systems, pellets of dry ice (solidified CO 2 ) are drawn into a flowing gas stream (typically compressed air) by the action of a venturi in a blast gun, where they are entrained into the gas stream and propelled out of the gun to impinge against the surface to be cleaned. After the pellets collide with the surface, removing unwanted surface coverings by their impact, they sublimate into gaseous CO 2  and become part of the ambient atmosphere. The only residue from this process is the removed surface covering. 
     The dry ice pellets typically are produced by machines called pelletizers. Conventional examples of these machines are disclosed in U.S. Pat. No. 4,780,119 and U.S. Pat. No. 5,475,981, which are incorporated herein by reference. In these machines, liquid CO 2  is injected into a cylinder, where it solidifies in the form of snow-like solid CO 2  particles. A piston within the cylinder then compresses the solidified CO 2  and extrudes rod-shaped dry ice through orifices in a die at one end of the cylinder. The dry ice rods are either sliced during extrusion or further broken down to form pellets. The pellets used for typical dry ice blasting vary in size depending on the surface conditions to be treated. Rice-sized pellets (about 3-3.5 mm in diameter) are useful for cleaning paint and other surface coverings that might otherwise be cleaned by sand blasting. The smallest pellets produced by conventional means are about 1-2 mm in diameter. 
     In the manufacture of semiconductors, printed circuit boards, flat-panel displays, and other products in the electronics industry, extremely thin layers of material often must be removed in precisely defined, narrow strips to achieve the desired thin-film configuration or circuitry. Traditionally, etching by acid solutions or by photo-chemical mechanisms has been used in manufacturing these products. For example, in producing a printed circuit board, a thin conductive film is deposited on a substrate, a patterned deposition of “resist” material is made, and the acid or other etching medium is applied. Because the resist material is resistant to the etching process, only the exposed conductor is removed, leaving the resist-covered conductor to serve as the desired circuit. 
     The electronics industry also has used sand-blasting techniques to produce the desired circuit patterns for some applications, such as flat panel displays. Because of the small dimensions of the circuits, however, sand having a diameter on the order of one micron (0.001 mm) must be used. Using sand blasting techniques to manufacture such products, however, has several drawbacks. For example, the process results in considerable sand residue. In addition, the special micron-size sand is expensive. Furthermore, static charges created by the interaction of the sand with the resist material or the material being removed can damage the resulting electronic circuit. 
     Although dry ice pellets might be applicable to electronic circuit manufacture without the inherent drawbacks of sand blasting, it is not possible to use conventional dry ice blasting techniques. The basic problem is that conventional pelletizers cannot be used to produce the small-diameter pellets required to remove such finely defined circuit patterns. Solidified CO 2  cannot readily be extruded through sub-millimeter-sized orifices in a die. 
     Another application in which conventional sand blasting and CO 2  pellet blasting techniques do not work well is in the cleaning of thin layers of metal, for example, in preparation for surface plating of copper layers. The high kinetic energy of these conventional blasting media unacceptably deforms or damages the thin layers during cleaning. Dry ice pellets of smaller mass are required for this application. 
     A third application in which conventional techniques do not work well is in the cleaning of flux and solder from printed circuit boards. The primary problem with conventional blasting systems is the static charges that are created from the interaction of the relatively dry cleaning media with the circuit boards. These static charges cause severe damage to the electronic components mounted on the boards. 
     This invention is intended, therefore, to provide a method and apparatus for producing dry ice pellets having a diameter less than 1000 microns (1 mm). The invention also is intended to provide a method and apparatus for removing small-dimensioned surface areas using such sub-millimeter-sized dry ice pellets. Furthermore, this invention is intended to provide an apparatus and method for controlling pellet size so that a single blasting machine can be used for a variety of applications in which optimum pellet size varies. In addition, this invention is intended to provide an apparatus and method for limiting the static charges created during cleaning with dry ice pellets. 
     Additional advantages of the present invention will be set forth in part in the description that follows, and in part will be obvious from that description or can be learned by practice of the invention. The advantages of the invention can be realized and obtained by the apparatus and method particularly pointed out in the appended claims. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the problems of prior art surface removal devices by utilizing a size-reducing mill in a dry ice blasting apparatus to reduce the size of the dry ice particles before they reach the blast gun. In one embodiment of the invention, this mill provides for adjusting the size reduction at the blasting site. Further reduction in pellet size can be accomplished, if desired, by the configuration used in the blast nozzle. This invention also contemplates controlling the humidity, temperature, and pressure of the propellant gas to optimize conditions where the pellets impact the surface to remove the desired layers of material. 
     To overcome the problems of the prior art surface-removal processes, and in accordance with the purpose of the invention, as embodied and broadly described herein, the apparatus of this invention for removing surface coverings comprises a feeder device capable of providing a supply of dry ice pieces, a pellet-size reducer operatively associated with the feeder device to receive dry ice pieces from the feeder device and reduce the size of the dry ice pieces to nominal diameters less than 1 mm, and a blast gun connected to the pellet-size reducer and adapted for operative communication with an associated supply of flowing gas. The blast gun entrains the reduced-size dry ice pieces into the flowing gas and directs the flowing gas and dry ice pieces to an associated surface covering to be removed. 
     Preferably, the pellet-size reducer includes a knife- or disc-type grinding mill. In addition, the preferred embodiment of the apparatus includes a humidity controller operatively associated with the blast gun to control the moisture content of the flowing gas directed by the blast gun. 
     In one preferred embodiment of the invention, the apparatus comprises a supply of flowing gas; a feeder device capable of providing a supply of dry ice pieces; a pellet-size reducer operatively associated with the feeder device to receive dry ice pieces from the feeder device and reduce the size of the dry ice pieces to nominal diameters less than 1 mm; and a blast gun having a primary flow path defined by an inlet connected to the flowing-gas supply, an outlet, a venturi intermediate the first inlet and the outlet, and at least one dry ice port connected to the pellet-size reducer and opening into the primary flow path intermediate the venturi and the outlet. In operation, the pressure of gas flowing from the inlet to the outlet decreases proximate the position where the dry ice port opens into the primary flow path and causes the reduced-size dry ice pieces to be drawn through the dry ice port into the primary flow path and to exit the outlet entrained in the flowing gas. 
     The accompanying drawings, which are incorporated in and which constitute a part of this specification, illustrate at least one embodiment of the invention and, together with the description, explain the principles of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic overall view of the apparatus of the present invention; 
     FIGS.  2 ( a ) and  2 ( b ) are cross-sectional views of one embodiment of the pellet-size reducer used in the apparatus of the invention, taken along lines A—A and B—B, respectively, in FIG. 1; 
     FIG. 3 is a cross-sectional view of the blast gun used with the present invention; 
     FIG. 4 is a schematic view of a portion of a second embodiment of the apparatus of the invention; and 
     FIG. 5 is a cross-sectional view of a second embodiment of the pellet-size reducer used in the apparatus of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference now will be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. 
     An overall layout of the system of this invention is shown in FIG.  1 . With specific reference to the right-hand side of FIG. 1, the system includes feeder device, designated generally by reference numeral  10 , capable of providing a supply of dry ice pieces. Feeder device  10  includes a hopper  12  having an open top portion  14  supporting a hinged cover  15 , a bottom portion  16 , two side walls (not shown), and end walls  18 ,  20 . Hopper  12  is of conventional design, with the opposite side walls and the end walls  18 ,  20  being inclined toward each other from top portion  14  to bottom portion  16 . Feeder device  10  also includes a feed cylinder  22  connected to bottom portion  16  of hopper  12  and in communication with the interior of hopper  12 . Feed cylinder  22  preferably is vented to the atmosphere and contains an augur  24  that is rotated by a motor  26 , which may be electrically, hydraulically, or pneumatically powered. A supply of dry ice pellets produced by a conventional pelletizer is held in hopper  12 . The sizes of these pellets may range from about 1 mm to about 20 mm in diameter. 
     As stated above, hopper  12  is in communication with feed cylinder  22 . Consequently, pellets placed in hopper  12  are fed by gravity into feed cylinder  22 . When augur  24  is rotated, the pellets in cylinder  22  are moved by the augur blades to one end of cylinder  22  (the left-hand end in FIG.  1 ), where they drop out of the cylinder  22  through an output duct  28 . An aperture (not shown) with a diameter slightly greater than the outer diameter of blades of augur  24  is provided in forward end wall  20  to enable the pellets to move past wall  20  to output duct  28  in a controlled fashion. 
     In accordance with the invention, the lower end of output duct  28  is connected to the input side of a size-reducing mill  30 . In the embodiment shown in FIG. 1, mill  30  is a conventional knife mill normally used to reduce the size of various materials, such as minerals, ores, building materials, plastics, cellulose, paper, leather, animal feed, and cloth. An example of this type of size-reducing mill that has proved suitable for use with dry ice pellets is the Cross Beater Mill marketed by Glen Mills Inc. of Clifton, N.J. 
     Mill  30  includes a generally cylindrical housing  31  that, as shown in greater detail in FIGS.  2 ( a ) and  2 ( b ), is comprised of housing portions  32  and  33 , which preferably are connected by a hinge mechanism. An input chute  34  is incorporated into housing portion  32  and communicates with the interior of housing  31  via opening  35  Input chute  34  is connected to output duct  28  so that dry ice pellets dropping out of feed cylinder  22  through output duct  28  are fed into the interior of housing  31  via input chute  34  and opening  35 . Preferably, the interior of housing  31  is vented to the atmosphere. 
     A first series of serrations  36  are formed on the interior face of housing portion  32 , and a second series of serrations  37  are formed on the inner circumferential surface of housing portion  33 . A rotor  38  disposed within housing portion  32  is connected to and rotated by motor  40 , which may be electrically, hydraulically, or pneumatically powered. Mounted on each arm of rotor  38  is a knife blade  42 . Rotation of rotor  38  by motor  40  causes the dry ice pellets fed to the interior of housing  31  to be sliced and crushed into smaller sizes by the interaction of knife blades  42  with serrations  36  and  37 . The reduced-size dry ice pellets then exit the interior of housing  31  through an opening  43  provided in the housing portion  33  and connecting to exit chute  44 . Preferably, an operator selectable sizing screen (not shown) is positioned across opening  43  to limit the particle size of the material produced by the mill. For applications in the electronic circuit industry, as an example, the sizing screen preferably is selected from a set of screens having web densities that permit only pellets of a diameter of about 100 microns or less to pass through the screen into exit chute  44 . 
     The exit chute  44  of mill  30  in turn is connected by a flexible hose  48  to the blast nozzle or blast gun  50 . Preferably, hose  48  is insulated to limit sublimation of the dry ice pellets before they reach the blast gun and has a smooth bore to minimize flow resistance between the mill and the blast gun. The exit chute of mill  30  also can be provided with a magnet (not shown) to retain metal particles that might result from the grinding process and prevent them from being conducted to the blast gun. 
     Gun  50  also is connected to a gas stream source by a flexible hose  52 . The gun is operable when coupled to a gas stream to dispense dry ice particles according to the well known venturi principle in a manner described in more detail below. The gas stream preferably is compressed air supplied by a conventional air compressor (not shown). As an alternative, especially in locations where access to electrical power is a problem or in clean-room applications where possible contamination from air-borne particles is a concern, the gas stream supplied through flexible hose  52  can be nitrogen or carbon dioxide gas from a reservoir of liquid nitrogen or liquid CO 2 . 
     The preferred form of gun  50  is shown in FIG.  3 . The gas stream from hose  52  enters the input end  54  of gun  50  and is constricted by venturi  56 . As a result of passing through the venturi, the pressure of the gas stream is substantially decreased in mixing section  58  of the gun. The shape of the venturi and the transition to mixing section  58  should be selected to result in a pressure less than atmospheric pressure in section  58 . 
     Communicating with section  58  are a pair of ports  60 , which are connected to hose  48 . Because the dry ice supplied by the feed cylinder  22  and grinding mill  30  is at atmospheric pressure, the lowered pressure in section  58  draws the dry ice particles produced by mill  30  through hose  48  and into the gas stream passing through section  58  of gun  50 . The CO 2  particles become entrained in the gas stream and exit end  62  of the gun, where they impact on the surface to be cleaned. 
     Ports  60  preferably are in line with each other so that the dry ice particles entering section  58  from opposite sides collide with each other, possibly further reducing the size of the dry ice particles exiting the gun at end  62 . Generally, this additional reduction in size will apply only to the larger particles, as the probability of particles colliding with each other decreases with decreased size. The dry ice particles exiting gun  50  under this system are substantially smaller than the pellets fed into hopper  12 . Consequently, these particles are far better adapted to the layer-removal needs of the electronics industry than conventional dry ice pellet blasting systems. 
     It also is important to minimize the incidence of electrostatic charges on the electronic circuitry being processed by the above-described system. Accordingly, the system preferably includes means for selectively controlling the humidity of the gas stream connected to gun  50 . Humidity control is accomplished by a water injector  80  (see FIG. 1) connected to the conduit supplying the gas stream to gun  50 . A variety of commercial atomized water-injection systems are available for use in the system of the invention. In the alternative, a supply of water in a finely porous plastic or stainless steel pod can be located in the gas stream, with the flow of air or other propellant gas over the surface of the pod extracting moisture through the porous surface. The addition of water vapor to the gas stream provides a path by which electrostatic charges can be grounded. 
     In many applications, it also is desirable to control the temperature of the gas stream by heating the (,as stream. This is particularly important when water is being added to the gas stream to control electrostatic charges. In the absence of gas heating, the dry ice pieces will cool the gas stream and cause unwanted condensation of the water vapor. In addition, certain electronic components can be damaged by exposure to extremely low temperatures, and CO 2  pellets can drop the temperature of a compressed air stream to as low as −100° F. Accordingly, in the preferred embodiment the gas-stream temperature is selectively controlled by heater  90 , which is connected to the gas stream downstream of water injector  80 . Injector  80  preferably monitors the humidity of the gas stream at point  92  downstream of heater  90 . Heater  90  preferably is an electric trim heater such as the Model TT-9 heater sold by Thermax Inc. 
     The amount of humidity and/or heat that should be added to the gas stream depends on the particular conditions that exist, particularly the composition of the gas stream and the nature of the work piece being cleaned or blasted. Most electronic components, for example, can safely withstand temperatures down to about −55° F., so the gas stream preferably is heated so that it is maintained above that temperature at the workpiece surface. Of course, the temperature level necessary to avoid condensation of water vapor when combined in the gun with dry ice pieces will depend on the humidity level of the gas stream. Under most conditions, the gas stream temperature need not be raised above about 150° F. at point  92  to avoid excess condensation. For inhibiting electrostatic discharge in typical situations, the dew point of the gas stream following heating (e.g., at point  92  of FIG. 1) preferably should be in the range of 40-80° F. The amount of water that must be added to achieve that dew point range will heavily depend on whether the gas stream supply is compressed air or a gas provided from a supply of cryogenic liquid. The dew point readings for typical unconditioned gas streams range from −40° F. for compressed air to −100° F. for nitrogen gas. 
     The volume flow of the gas stream also should be controlled, preferably by sensing and controlling the gas stream pressure. The gas stream pressure affects a number of parameters, such as the speed of the dry ice particles entering the gun at section  58  as well as the speed of the particles exiting the gun. Different pressures must be used for different layer-removal conditions, including the type of material being removed and the thickness of the material. The apparatus shown in FIG. 1 is operable within a pressure range as wide as 10-350 psi, but in normal blasting operations gas stream pressure usually is 30-150 psi. Pressure is controlled by ball valve  94  regulator, which automatically controls gas stream flow to result in a preset pressure. 
     The above-noted preferred values for gas-stream humidity, temperature, and pressure, of course, will not apply to all situations. The more fragile or sensitive the workpiece is, the more care must be taken in optimizing these gas-stream parameters to effectuate material removal while avoiding damage to the workpiece. 
     In certain clean-room applications of this invention, it also might be desirable to isolate the surface being cleaned from air-borne contaminants. In such applications, nitrogen or carbon dioxide gas is used to provide the propellant stream, and the top of hopper  12  is subjected to an inert-gas blanket, using the same gas as for the propellant stream, at an elevated pressure to prevent air from entering the system. In these applications, a zero-pressure regulator  98  is connected to hopper  12 . When regulator  98  senses that the inert-gas pressure within the hopper drops to a predetermined minimum, the system supplying the inert-gas blanket (not shown) is actuated to increase the supply of inert gas to prevent the entrance of ambient-pressure air into the gas system. The blast gun operates in the same manner as described above, with venturi  56  creating a low-pressure region at  58  to draw dry ice particles from the elevated-pressure hopper. In this application, mill  30  preferably is sealed to prevent contamination from the ambient atmosphere. 
     Finally, when this system is employed in the context of removing layers of materials from printed circuit boards and the like, gun  50  preferably is mounted on a precision X-Y table or on a movable gantry or robot mechanism so that movement of the gun relative to the workpiece and, consequently, the material removal process can be numerically controlled. 
     A second embodiment of the system of the invention is shown in FIG.  4 . This embodiment of the invention is similar to that shown in FIG. 1, with a hopper  12  feeding dry ice particles by gravity into feed cylinder  22 . In this embodiment of the invention, however, the systems is adapted to receive “nuggets” of dry ice having dimensions substantially larger than the dimensions normally associated with CO 2  pellets, yet it can produce particles at the associated blast gun that are on the order of 100 microns in size or smaller. For example, hopper  12  in this second embodiment of the invention can be filled with rods of dry ice having diameters in the range of 20 mm and lengths up to about 10 cm. Such rods are readily produced by extrusion with a pelletizer having a die with 20 mm diameter orifices. 
     In accordance with the invention, an initial reduction in the size of the CO 2  particles takes place in cylinder  22 . In the embodiment shown in FIG. 4, the augur  124  mounted within cylinder  22  has a predetermined radial clearance “r” between its root diameter and the wall of cylinder  22  and a predetermined axial clearance “a” between adjacent augur blades that prevents pieces of dry ice larger than about 20 mm from being conveyed past the aperture (not shown) in the forward end wall  20  of hopper  12  as the augur turns. Instead, pieces of dry ice larger than 20 mm are broken into smaller pieces by their interaction with augur  124 , the hopper wall  20 , and other dry ice nuggets. In the presently preferred embodiment, “r” is about 14 mm and “a” is about 19 mm. 
     The dry ice pieces that are conveyed (to the right in FIG. 4) by augur  124  past end wall  20  of hopper  12  then drop out of cylinder  22  into output duct  128 , which is shorter than the duct  28  shown in FIG. 1 to provide for a more compact blasting apparatus. Mounted at the end of duct  128  is a three-way valve  129 , which selectively operable to connect duct  128  to grinding mill  130  via conduit  132  or directly to outlet  134 . 
     A schematic cross-sectional view of grinding mill  130  is shown in FIG.  5 . The mill includes a main housing  144  suitably connected to a stationary disc support  146 , preferably by threads  148 . Stationary disc support  146  includes a first portion  146 A that rotates relative to housing  144  and a second portion  146 B that moves only linearly, being restrained from rotating by a key (not shown) that engages with a slot (not shown) in housing  144 . The two portions  146 A and  146 B are connected by a bearing (not shown). Affixed to disc support  146  is stationary grinding disc  150 . An L-shaped feed chamber  152  is defined within disc support  146 . Chamber  152  is in communication with conduit  132 , from which it receives dry ice nuggets falling out of the end of cylinder  22 . 
     A shaft  154  is rotationally supported by housing  144  and is driven by a motor (not shown), preferably a pneumatic motor. One end of shaft  154  is disposed within chamber  152  and includes one or more flutes  156  that convey the dry ice nuggets out of chamber  152  as shaft  154  rotates. The dry ice nuggets are conveyed by flutes  156  toward the left in FIG.  4  and between stationary grinding disc  150  and a complementary grinding disc  158 . Disc  158  is supported by rotational disc support  160  and rotates with shaft  154 . Grinding discs  150  and  158  preferably are formed from steel and include on their opposing faces a series of helical grinding blades (not shown). 
     The threaded connection between housing  144  and stationary disc support  146  enables adjustment of the clearance between grinding discs  150  and  158 . Dry ice nuggets that enter the space between the two discs from chamber  152  are ground into smaller pieces by the discs and are conveyed through outlet  162  into a flexible conduit and, ultimately, into a blast gun such as that shown in FIG.  2 . The low pressure formed in the blast gun ensures that the air stream or other gas medium conducts the ground CO 2  pellets out of the mill and toward the blast gun. 
     Grinding mill  130  preferably is an adjustable disc mill such as the Model No. S.500 disc mill sold by Glen Mills Inc. that can produce particles with a preselected nominal diameter between 250 and 1000 microns. The adjustability of mill  130  enables the user to control the size of the dry ice pellets being fed to the blast gun at the cleaning site without regard to the size of the dry ice pieces in the hopper. Either relatively large CO 2  nuggets or conventional rice-sized pellets can be used as the starter material. This adjustability permits increased flexibility of use. The user no longer needs to transport a supply of specifically sized CO 2  pellets to the work site; control of pellet size can be accomplished in the field. The ability of disc mill  130  to adjust CO 2  particle size by varying the distance between the grinding discs also enables the elimination of the sizing screen used with the knife mill  30  described with reference to FIG.  1 . 
     If the cleaning application requires use of substantial quantities of conventional, rice-sized dry ice pellets, the apparatus of FIG. 3 also can be used. In such an application, the pellets are fed by hopper  12  into cylinder  22 , and augur  124  conveys them past wall  121 , where they drop down outlet duct  128 . Ball valve  129  is used to connect output duct  128  to outlet  134 , which in turn is connected to a blast gun. Previously sized dry ice pellets thus are fed to the blast gun without passing through the grinding mill. Thus, the same apparatus can be used for conventional blasting applications as well as for those requiring reduced-size dry ice pellets. 
     It will be apparent to those skilled in the art that other modifications and variations can be made in the method of and apparatus of the invention without departing from the scope of the invention. For example, to the extent that disc mill grinders or other grinders are adjustable to produce dry ice particles with nominal diameters less than the 1000-250 micron range for the Model No. S.500 mill, such mills could be used in the apparatus and method of the present invention. The invention in its broader aspects is, therefore, not limited to the specific details and illustrated examples shown and described. Accordingly, it is intended that the present invention cover such modifications and variations provided that they fall within the scope of the appended claims and their equivalents.