Abstract:
A method for cleaning objects ( 4 ) in a pressure tank ( 2 ) using a compressed clean fluid, which contains a gas and which is compressed and decompressed one or more times. According to the present invention, the cleaning fluid is decompressed to a pressure at which the gas has a volume that is a multiple of the volume of the compressed cleaning fluid in the pressure tank. In this manner, it is possible to remove particle-sized and other impurities from recesses, blind holes, or open cavities ( 6 ) in the objects.

Description:
[0001]    This application claims priority of German Application No. 10055127, filed Nov. 7, 2000, which is hereby incorporated by reference herein.  
         BACKGROUND INFORMATION  
         [0002]    The present invention relates to method and a device for the cleaning of objects in a pressure tank using a compressed cleaning fluid, which contains a gas and which is compressed and decompressed one or more times in succession.  
           [0003]    U.S. Pat. No. 5,514,229 describes a method of this type for cleaning using a cleaning fluid, which is in a near- or supercritical state, i.e., in a state in which no distinction is possible between liquid and gas. Between a near- or supercritical state, on the one hand, and a supercritical state, on the other hand, periodic pressure changes occur, altering the solubility of the fluid for specific impurities. The impurities that are precipitated out in a decompression phase can be separated. This means that using this method only soluble impurities can be removed.  
           [0004]    Insoluble impurities of machined parts, for example, manufacturing residues such as molding sand or shavings, processing residues such as coverings or bore dust, or accidental contamination such as dust, are conventionally removed in mechanical fashion, for example, through the intensive relative motion of a cleaning fluid and the objects to be cleaned, it being possible to add mechanical scouring agents to the cleaning fluid. However, cleaning methods of this type are less effective, the more complicated the shapes are of the objects to be cleaned. It is particularly difficult to remove impurities which are located in recesses, for example, blind holes or open cavities in the objects. In cleaning using a conventional cleaning fluid, it is necessary to assure a simultaneous supply and removal of the fluid; otherwise the result is a blockage without an exchange of the cleaning fluid. The more complex, the deeper, and the larger the recesses are, the more difficult this process becomes.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a method for the cleaning of objects in a pressure tank using a compressed cleaning fluid, which contains a gas and which is compressed and decompressed one or more times, wherein the cleaning fluid is decompressed to a pressure at which the gas has a volume that is a multiple of the volume of the compressed cleaning fluid in the pressure tank ( 2 ;  20 ).  
           [0006]    The present invention also provides a device for the cleaning of objects using a cleaning fluid, having a pressure tank for receiving the objects to be cleaned and having a compressor for the cleaning fluid. The device is characterized by a lifting piston device ( 22 ) having a lifting piston ( 24 ), which is coupled in a drive relationship to the compressor ( 14 ) and which subdivides the lifting piston device into two chambers ( 28 ,  30 ), a first chamber ( 28 ) of the lifting piston device being connected via a first valve ( 32 ) to a pressure reservoir ( 16 ), which is connected to the outlet of the compressor ( 14 ) and, via a second valve ( 18 ), to the pressure tank ( 20 ), and a second chamber ( 30 ) of the lifting piston device being connected via a third valve ( 38 ) to the pressure tank and being connected via a fourth valve ( 40 ) to a separator ( 42 ) for impurities.  
           [0007]    According to the present invention, the cleaning fluid is decompressed to the point that, in the event that the cleaning fluid is a gas, the latter expands to a multiple of the volume of the compressed gas, preferably to a volume in the order of magnitude of 100 times the volume of the compressed gas. Alternatively, the cleaning fluid can be a liquid, in which the gas is soluble in the compressed state. In this case, the decompression is carried out so that here too a multiple of the volume of the compressed cleaning fluid is released as gas.  
           [0008]    If the gas expands or is released, there arise in recesses in the objects to be cleaned currents directed outwards which effectively carry with them the impurities. If the compression and decompression are carried out repeatedly, the impurities again and again being precipitated out from the cleaning fluid, then components having complex shapes can be cleaned very carefully.  
           [0009]    In one refinement of the method, a non-gaseous material, in which the compressed cleaning fluid is soluble and which has the tendency to bind itself to impurities, is applied to an object to be cleaned and/or is introduced into any open cavities in the object, before the object is placed into the pressure tank. The nongaseous material, which is advantageously liquid, plastic, or pasty, in order to assure an effective binding to the impurities, forms so-called removal aids. As a result of the solubility in the compressed cleaning fluid, the removal aids in response to decompression are removed from the recesses particularly effectively and, in the process, take the impurities with them. In this manner, it is possible to reliably remove very heavy, very small, or very inaccessible impurities. If the cleaning fluid is composed of carbon dioxide, suitable removal aids are commercial alcohols, oils, fats, or waxes on a hydrocarbon base, in which carbon dioxide is soluble.  
           [0010]    Both in the basic form of the method as well as in the refinement using removal aids, the cleaning fluid in the compression phase of the cleaning process can attain a supercritical state. But during the entire cleaning phase, the fluid can also remain in a subcritical state, because the change in the gas volume as a function of pressure is in any case greatest in the subcritical range, as is desirable for generating intensive, effective expansion currents from the recesses.  
           [0011]    If removal aids are used, it is still possible that residues from them remain adhering to the objects. Removal-aid residues of this type are preferably removed by compressing the cleaning fluid at the end of the method, for example, to a near- or supercritical state. In this state, the appropriate removal aids are particularly soluble in the cleaning fluid and are flushed away along with it.  
           [0012]    In another refinement of the method, the pressure tank, before the cleaning process, is essentially completely filled by one or a multiplicity of objects to be cleaned as well as by a multiplicity of solid filling bodies. In this case, the pressure tank must be filled with significantly less cleaning fluid, so that compression work is saved.  
           [0013]    A further saving on compression work is made possible by the device according to the present invention, which contains a lifting piston, which is coupled in a drive relationship to a compressor for the cleaning fluid, so that the work that is released in the decompression is partially recovered for the compression work of the compressor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]    Further features and advantages of the present invention are yielded from the following description of exemplary embodiments on the basis of the drawing. The following are the contents:  
         [0015]    [0015]FIGS. 1 a  and  1   b  depict block diagrams for illustrating the method for cleaning complex machined parts,  
         [0016]    [0016]FIGS. 2 a  and  2   b  depict block diagrams for illustrating one variant of the method for cleaning complex machined parts, and  
         [0017]    [0017]FIGS. 3 a  and  3   b  depict block diagrams for illustrating a device for cleaning complex machined parts. 
     
    
     DETAILED DESCRIPTION  
       [0018]    [0018]FIGS. 1 a  and  1   b  both depict a closed pressure tank  2 , in which a complex machined part  4  is situated. Machined part  4  contains a cavity  6 , which is connected to the outside by a narrow opening  8 . Machined part  4  is any product which is soiled, for example, by manufacturing residues such as molding sand, shavings or cooling lubricants, processing residues such as coverings or bore dust, or accidental contamination such as dust. Machined part  4  as sketched can be, for example, a casting, which is soiled by residues of molding sand which are located especially in cavity  6 . Machined part  4 , however, can also be any other product which contains any areas that are hard to access, for example, recesses, undercuts, holes, blind holes, or channels, which in each case constitute a cavity  6 .  
         [0019]    To remove the impurities from machined part  4 , pressure tank  2  is opened, machined part  4  is placed in it, and pressure tank  2  is securely closed. Via an inlet  10 , a highly compressed gas such as carbon dioxide is introduced, or is generated by pumps (FIG. 1 a ). As soon as a desired pressure is achieved, decompression via inlet  10  occurs spontaneously (FIG. 2). In this context, the volume of the gas increases, and the gas exits from opening  8 . This gas flow takes particles and other impurities in cavity  6  with it. In order that the gas flow be sufficiently intensive, the decompression should occur as rapidly as possible. This means that the pressure equalization between the interior of pressure tank  2  and, for example, the atmosphere should take place essentially more rapidly than the pressure equalization between cavity  6  and the interior of pressure tank  2 .  
         [0020]    The pressures to which the gas is alternately compressed and decompressed are selected so that, in decompression, the volume of the gas increases by a multiple, for example, by 200 times. At a volume increase of this magnitude, the expansion current from cavity  6  is intensive enough for a powerful cleaning effect. To remove impurities to the greatest extent possible, the compression and decompression are carried out repeatedly, the gas again and again being filtered so that no impurities are recycled into cavity  6 .  
         [0021]    Volume changes in the above-mentioned order of magnitude require a significant amount of compression work, which constitutes a large part of the operating costs. The energy level of pressure tank  2  is the product of pressure and residual volume (the volume of pressure tank  2  minus the volume of machined part  4 ). To reduce the residual volume, in addition to machined part  4  and any further objects to be cleaned, it is possible to fill pressure tank  2  with a multiplicity of compact filling bodies  12 , as is depicted in FIGS. 2 a  and  2   b . Filling bodies  12  are, for example, solid spheres made of a material that stands up to the compression pressure without changing in volume. Minimizing the residual volume results in proportionate savings in the compression work to be exerted.  
         [0022]    In a further exemplary embodiment, machined part  4  is first provided with removal aids, before the method is carried out as described above. The removal aids are substances that at the working temperature are liquid, plastic, or pasty, and in which the gas is soluble. In the event that the gas is carbon dioxide, the appropriate removal aids are commercial alcohols, oils, fats, or waxes made on a hydrocarbon base. Machined part  4  to be cleaned is covered or filled with removal aids, the removal aids surrounding the impurities and binding to them physically or chemically. In the compression phase, the gas dissolves in the removal aids, and in response to the spontaneous expansion, the gas that is released takes the removal aids and therefore the impurities bound to them with it. The removal aids are driven out together with the impurities. In practice, however, it is possible that residues of the removal aids can remain adhering to the component. In this case, the component must be cleaned using a subsequent supercritical extraction of the remaining removal-aid residues. For example, a wax as the removal aid is very soluble in carbon dioxide which is in a supercritical state.  
         [0023]    Furthermore, in certain types of impurities, it is possible to use the impurities themselves as removal aids. If carbon dioxide is used as the cleaning fluid, then impurities themselves act as removal aids, for example, in the form of cooling lubricants or coverings on a hydrocarbon base.  
         [0024]    [0024]FIGS. 3 a  and  3   b  are block diagrams for illustrating the exemplary embodiments for a device for carrying out the method described above. The device contains a compressor  14 , whose outlet is connected to a pressure reservoir  16 . Pressure reservoir  16  is connected via a valve  18  to a pressure tank  20  as a receptacle for the objects to be cleaned. In addition, the device contains a lifting piston device  22 , which is a hollow cylinder that is closed on both ends so as to be gas-tight and in which an axially movable piston  24  is located. Piston  24  is coupled in a drive relationship to compressor  14 , for example, by a joint piston rod or by a connecting rod and a crank, as is indicated by a dotted line  26 . In the event compressor  14  is a lifting piston compressor, the piston of compressor  14  and the piston of lifting piston device  22  can also be arranged in a common hollow cylinder and can be coupled to each other via a piston rod, which extends in a gas-tight manner through a separating wall between compressor  14  and lifting piston device  22 .  
         [0025]    Piston  24  divides lifting piston device  22  into a first chamber  28  and a second chamber  30 . First chamber  28  is connected via a valve  32  to a pressure reservoir  16  and via a valve  34  to a reserve tank  36  for the cleaning fluid. Second chamber  30  is connected via a valve  38  to pressure tank  20  and via a valve  40  to a separator  42  for impurities, whose outlet is connected to reserve tank  36 . Reserve tank  36  is also connected to the inlet of compressor  14 .  
         [0026]    [0026]FIG. 3 a  depicts the decompression phase in which valves  32  and  38  are opened and valves  18 ,  34 , and  40  are closed. Piston  24  moves upwards in the direction indicated by the arrow, to decompress pressure tank  20  and in the process to clean the objects contained therein. The gas emerging from pressure tank  20  partially directly supports the expulsion of the gas from first chamber  28  into pressure reservoir  16 , and it partially supports, via coupling  26 , compressor  14 , which also fills pressure reservoir  16  with gas.  
         [0027]    [0027]FIG. 3 b  depicts the compression phase, in which valves  32  and  38  are closed and valves  18 ,  34 , and  40  are opened. While pressure tank  20  is filled via valve  18  with compressed air from pressure reservoir  16 , piston  24  moves downwards in the direction indicated by the arrow, to drive the gas, which has accumulated during the decompression phase in second chamber  30 , through separator  42  and reserve tank  36  into compressor  14  and first chamber  28 . Reserve tank  36  acts here as a buffer for the gas that has been purified in separator  42 . However, the gas can also be conveyed from separator  42  directly into compressor  14  and first chamber  28 . Reserve tank  36  is then required only for supplying fresh gas at the beginning of the method or for adjusting for leakage losses.  
         [0028]    Expelling gas in second chamber  30  and drawing in gas in first chamber  28  during the compression phase can be supported or carried out by storing the work achieved during the decompression phase in piston  24 , e.g., in a driven plate such as a disk flywheel, which is connected to piston  24  via a crank and a connecting rod, and which in lifting piston device  22  is used for the expelling and the drawing-in processes.  
         [0029]    “A multiple of” as defined herein means many times over, i.e. at least twice, and need not be an exact integer.