US 3383208 A
Beskrivning (OCR-text kan innehålla fel)
y 1968 J. 5. CORRAL 3,383,208
COMPACTING METHOD AND MEANS Filed Feb. 3, 1966 2 Sheets-Sheet 1 FIG. I
JOSEPH S. CORRAL FIG. 2
ATTORNEY y 4, 1968 J. s. CORRAL 3,383,208
- COMPACTING METHOD AND MEANS Filed Feb. 5, 1966 2 Sheets-Sheet 2 INVENTOR.
JOSEPH S. CORRAL ATTORNEY FIG. 5
United States Patent 3,383,208 COMPACTING METHOD AND MEANS Joseph S. Corral, Costa Mesa, Calif., assignor to North American Rockwell Corporation, a corporation of Delaware Filed Feb. 3, 1966, Ser. No. 524,729 13 Claims. (Cl. 75-214) ABSTRACT OF THE DISCLOSURE A process for forming very dense metallic articles, particularly those of the refractory metals, by first isostatically compressing powders of the metal, followed by explosive forming of the compact while the compact is surrounded by a mass of material of aluminum, copper, lead, or their alloys. These materials retard the propagation of the shock waves, insuring uniform high density in the resultant product.
This invention concerns method for compacting porous masses such as powdered metals and diverse non-metallic materials to an essentially non-porous or otherwise relatively high density state.
Although the inventive teachings disclosed herein have wide applicability in forming a variety of diverse shapes from different materials, the invention will be described for the sake of illustration in connection with powdered metals and especially powdered tungsten, tantalum, columbium and molybdenum. It will be understood that the scope of the inventive concept is not limited by any of the specific details used to explain the invention, except as determined by reference to the accompanying claims. Also, the several features of the inventive concept which separately contribute to the final overall results may be used individually to advantage as well as in the combina tions claimed.
Of the various materials in widespread modern use for applications involving extremely high temperature resistance properties, high strength and relatively severe weight limitations, the mentioned materials are typical and are inevitably the most difficult to machine or otherwise form because of their extreme hardness and resistance to bending. When parts are required to be made from materials which cannot be conveniently rolled, stamped, cut or deformed, the commonest expedient in modern manufacturing technique is to make a precision casting of the part and subsequently perform shaping operations by grinding or the like. However, certain metals such as tungsten melt above 6,000 F. and cannot normally be cast in a substantially pure state due to the difiiculty of containing metal at such high temperature. To avoid the foregoing problems, attempts have been made to form tungsten parts by use of compacted powdered tungsten followed by sintering of the compact. However, to compact powdered tungsten involves enormous pressures in excess of 50,000 psi, whereby the cost of machines and short service life thereof make this approach prohibitive from an economic standpoint. Moreover, the density of the compacted powdered tungsten achieved by conventional methods results in a final densification no greater than 75% of the theoretical value for this material. In addition, sintering of tungsten involves heating the compacted billet to temperatures as high as 2,000" C. which results in degradation of grain size and molecular structure in the part, which compromises the final properties thereof in that larger grain size results in lower tensile and yield strengths, for example. Lesser sintering temperatures would result in smaller grain size and commensurately higher strength of the specimen, but higher initial densification of compacted powder is required for satisfactory results in sintering at lower temperatures. In the case of tungsten powder with an average particle of size of 8 microns, to achieve densities above of theoreti- "ice cal, compacting pressures in excess of 1,000,000 p.s.i.
would be required. Considering that die materials currently available have yield strengths under 300,000 p.s.i., it is readily apparent that densification of refractory materials of the type mentioned hereinabove is impractical and virtually impossible using methods and apparatus known to the prior art and involving dies for application of compressive force.
Another method known to the prior art for compacting powders includes the use of explosive energy to apply the compacting force, either directly to a powdered mass or through die surfaces. Among the various disadvantages of such methods in the prior art involving presses or dies is the fact that the explosive force delivered to the workpiece and to the dies causes welding of the specimen to the dies making it necessary to destroy the dies to recover the specimen. Also, it frequently happens whether dies are used or not that specimens compacted by explosive force show laminated or striated structure and cracking caused by nonuniformity of densification and by elastic recovery of the part due to sudden release of applied forces after completion of the compacting operation. Moreover, explosive methods known to the prior art involve particular sensitivity of the workpiece to the amount of explosive force, whereby insuflicient force produces incomplete compaction and excessive force cracks or otherwise destroys the integrity of the finished part.
Accordingly, it is a principal object of the invention in the case to form a mass of relatively high density and substantially uniform densification by compaction and joinder of discrete particles of material.
It is a further object in this case to provide improved method according to the above object wherein the final density of the mass may be controlled within relatively narrow marginal limits.
It is another object in this case to form a mass as set forth in the above objects wherein reliable and repeatable results may be achieved by duplication of factors alfecting density of the compact mass.
It is a further object in this case to provide method as set forth in the above object wherein the costs of the process and equipment necessary to practice it are relatively less than conventionalcompacting methods and means.
Other objects and advantages of the instant invention will become apparent upon a close reading of the following detailed description of an illustrative embodiment of the inventive method and means, reference being had to the accompanying drawings, wherein:
FIGURE 1 is a side elevational view, partly in crosssection, showing an initial mass of powdered particles in a container prior to the compacting operation.
FIGURE 2 is a side elevational view, partly in crosssection, of the powdered mass from FIGURE 1 situated within the compacting apparatus during a preliminary step in the inventive process,
FIGURE 3 shows in general a cross-sectional view of a compacted billet obtained from the process shown in FIGURE 2, and repackaged within a mold as necessary for the final compacting step,
FIGURE 4 shows a cross-sectional view through the billet and surrounding container of FIGURE 3 positioned as required before the final compacting step, and
FIGURE 5 shows a view corresponding to FIGURE 1, with changed workpiece form.
With reference to the drawings described above, and particularly to FIGURE 1, the inventive process preferably begins with containment of a quantity of powdered material 10 within a flexible, liquid-impervious bag 12 which may comprise liquid-resistant fabric, thin plastic, rubber, or the like. Bag 12 is situated within a generally cylindrical and relatively stitf mold which may take the form of paper tube 14 and which functions merely to give initial shape to the contents of flexible bag 12. Atop the powdered material is placed a separating layer which may take the form of paper disc 18, followed by a porous and flexible layer which may comprise gauze, several layers of fabric, or the like. Atop gauze 20, a rubber lug or disc 22 having a diameter substantially equal to the inside diameter of mold 14, is placed in contact with layer 20, after which bag 12 is gathered at the to as shown in FIGURE 1 and securely closed by suitable means such as wire 24 or the like.
Following completion of the above steps, the atmospheric or other gas content within bag 12 and intermixed with powdered material 10 is removed by communicating the general area occupied by the powdered material with a vacuum by suitable means such as vacuum line 26 having needle 28 mounted thereon and adapted to penetrate items 12, 18, 20 and 22. After removal of needle 28 following evacuation of atmosphere or other gases from bag 12 and powdered mass 10, the bag together with its contents is placed within a surrounding liquid 30 contained within a cavity 32 formed in a suitable high pressure resistant die or block 34. Thereafter, compression means for applying force to increase the pressure of liquid 30 within cavity 32 are actuated such as by downward movement of a plunger 36 or the like having a size and shape adapted to cooperate with the shape and size of cavity 32 in the manner of a piston within a cylinder. Application of force in the foregoing manner causes isostatic compression of liquid 30, bag 12, and the contents thereof, to form a highly porous billet, the density of which will vary with the workpiece materials in each case and the amount of pressure which can safely be applied by the apparatus. Following the foregoing compression step, as applied to powdered tungsten particles, for example, a resulting density of the compacted billet on the order of 6075% of theoretical value may be achieved.
Following the above initial compaction step, the firmly unified but highly porous billet 40 formed by isostatic compression is removed from bag 12 and placed in a smaller mold which may preferably be formed by dipping or spraying billet 40 with temperature-resistant plastic of a suitable type in a sufiicient amount to build up a coating thereabout which is then dried to form an encapsulating protective coat or layer 42. The contents within encapsulating layer 42 are thereafter evacuated of atmosphere or other gases by suitable means which may correspond with the procedure discussed above in connection with FIGURE 1. Thereafter, items 40 and 42 are centered within an elongate cavity 48 formed within a resilient, generally cylindrical jacket 44 which is preferably soft steel. A mass 46 of suitable hardenable material is placed into jacket 44 so as to completely surround items 40 and 42. Mass 46 is preferably a metal or alloy having a sufiiciently low melting point so that the temperature effects thereof on billet 40 will not be damaging to the billet. Moreover, the selection of material comprising mass 46 will depend upon the retarding effect of the material on a shock wave, for reasons which will appear from the description below.
Mass 46 completely fills cavity 48 which is required to be spaced a substantially uniform distance from the surface of billet 40 and which is therefore substantially cylindrical in shape when billet 40 is cylindrical. Cavity 48 is formed in axial alignment with the longitudinal center of jacket 44 as shown in FIGURE 3, and has access opening means at one end thereof. Jacket 44 is formed with its outer surface 50 having a discontinuous or nonuniform surface contour along that portion thereof which is radially aligned and substantially coextensive with cavity 48 proximate to billet 40 as shown by surface portion 52 in FIGURE 3. Surface portion 52 may illustratively comprise a plurality of intermittent grooves and projections circumferentially surrounding jacket 44 and which may economically be formed by cutting standard screw threads of helical form, for example, on the jacket. The upper end of jacket 44 in the position shown in FIGURE 3 is open to atmosphere and affords access for introduction of mass 46, while the lower end closure portion 54 of the jacket is integrally formed on jacket 44 and is shaped to form an inner cavity surface 56 of shallow conical form which is contiguous with the inner surface of cavity 48. Outer surface 58 of end closure portion 54 is generally planar and substantially round in shape due to edge 60 formed by intersection of surface 58 with substantially conical surface 62 in the manner shown.
Following placement of billet 40 within jacket 44 and hardening of mass 46 in the manner discussed above, the jacket and its contents are inverted and placed with its open end partially immersed or embedded in a solid supporting mass 66. Mass 66 is preferably formed of the same material comprising mass 46 or otherwise has substantially the same shock-wave retarding characteristics. Mass 66 may be poured in the molten state and then the end portion of jacket 44 immersed therein. After hardening or solidifying by cooling, mass 66 securely holds and supports in cantilevered relationship jacket 44 and its contents as shown in FIGURE 4. Mass 66 may be contained in any suitable manner such as by steel pipe 68 set within concrete mass 70, for example. Thereafter, a quantity of suitable explosive is placed around jacket 44. Thus, platform support means 72 adapted to fit closely around jacket 44 and adapted to be supported thereon such as by force-fitting, is positioned as shown in FIGURE 4. Support means 72 may comprise any suitable material such as plywood. Container means which may take the form of generally cylindrical tube 74 and adapted to rest on support means 72 is positioned as shown with jacket 44 substantially at the axial center thereof. Containing means 74, which may illustratively comprise waterproof cardboard, is thereafter filled with a suitable explosive 76 which surrounds roughencd portion 52 of jacket 44 and situated in the manner shown. Atop explosive 76, which may conveniently be of powdered or semi-liquid form, is placed a suitable explosive in sheet form having the general shape of a disc with the center thereof adapted to fit around jacket 44 in the manner shown by disc 78 in FIGURE 4. Disc 78 in turn supports an additional mass of explosive 80 which preferably corresponds with explosive 76 but is of generally conical shape and is covered by a substantially conical section of sheet explosive as shown by sheet 82 in FIG- URE 4. Explosion initiating means of suitable form such as blasting cap 84 are situated at the axial center of the cone formed by sheet 82 as shown by item 84 in FIGURE 4. Explosive 76 is preferably of substantially uniform thickness, concentration and composition so as to produce uniformity or radial symmetry of the shock-wave pattern resulting from detonation of the explosive.
The shape and general arrangement of the explosive charge resulting from items 76, '78, 80 and 82 discussed above result in application of high biaxial compressive forces of short duration against the exterior walls of jacket 44. Thus, detonation of blasting cap 84 results in virtually instantaneous detonation of the explosive charge surrounding and situated atop jacket 44. Explosive masses 76 and 80 may illustratively comprise dynamite consisting of nitroglycerin, ammonium nitrate, and gelatin, having a detonating velocity of approximately 12,000 per second. Detonation of explosive mass 80 applies a generally downward force tending to drive jacket 44 deeper into mass 66, although permanent displacement of jacket 44 in any direction is prevented by mass 66. The effect of explosive mass 76 is to produce a lateral implosion causing shock waves to penetrate jacket 44 and to travel through solidified mass 46 with a substantially uniform radial distribution of resulting force inwardly toward the longitudinal axis of billet 40. The effect of threads or the like on portion 52 is to disrupt the longitudinal shock wave pattern whereby it does not concentrate an excess of force in a localized area of the billet. The effect of solidified mass 46 is to retard the propagation rate of the shock wave. Mass 46 may comprise lead, aluminum or copper, but the alloy commercially known as Cerrobend is particularly advantageous because it offers ease of melting, pouring, and removal, primarily due to its low melting point of approximately 158 F. Also, Cerrobend possesses the desirable characteristic of increasing its volume upon solidification, whereby firm and intimate contact between masses 46 and the contacting surface of encapsulating member 42 is achieved upon cooling of the molten mass. Moreover, the foregoing increase in volume tends to apply high initial forces against the inner surfaces 48 and 46 of the cavity within jacket 44 which minimizes the effects of variations in the propagating medium surrounding the billet 40.
Upon completion of the procedure set forth above, mass 46 is again heated to the molten state whereby billet 40 may be removed from within jacket 44 and layer 42 removed from the billet. Compaction by the stated method has been found to produce substantially uniform densification of billet 40 to an amount in excess of 90% of theoretical value and in many cases in excess of 95% of theoretical density in the case of tungsten. Moreover, billet 40 is characterized by an absence of softness in the core area being that portion proximate the longitudinal axis through the center of the mass comprising billet 40. Also, the inventive method disclosed hereinabove is characterized by total absence of stratification or localized planes of weakness or cracking in billet 40.
Following the procedures set forth above, as applied in the case of compacted tungsten, billet 40 may be sintered at less than normal temperatures, such as 1600 1700 C. to achieve further increase in the density thereof and a final grain size which remains much smaller than that associated with sintering temperatures of 2000 C. and above. As a result, the tensile limit strength of billets compacted by the method and apparatus disclosed herein have been found by actual test to closely approach those obtained in rolled tungsten workpieces and which are in excess of twice the tensile strength of the relatively porous billet resulting from isostatic compression in the manner shown by FIGURE 2. Moreover, workpieces of the type corresponding to billet 40 characterized by dimensions far exceeding the largest billet sizes heretofore achievable for compacting tungsten by methods known in the prior art have been found possible with the method and means taught herein. Moreover, it will be understood by those skilled in the art that the two-step compaction process comprising an initial isostatic compaction followed by explosive compaction as described for billets and 40, respectively, results in a considerably higher and more uniform densification than that associated with explosive compaction alone, although the encapsulation of a powdered mass in a shock-wave retarding material and use of nonuniform surface continuity in jacket 44 afford separate and beneficial features of the invention in this case which would improve any explosive compaction process even in the absence of the other novel teachings disclosed herein. Thus, the advantages derived from use of jacket 44 and mass 46, and the processes related thereto, may be used in compaction of powdered materials such as powdered mass 10 in FIGURE 1 as well as solid masses such as billet 40, even in the absence of an initial isostatic compaction step. However, very significant advantages are had from the two-step compaction process as disclosed above where powder is first isostatically compacted and the resulting solid billet is then further compacted by explosive shock-waves. Just as metal if sufficiently ductile may be deep-drawn in a succession of separate steps, metal is more effectively compacted from powder into highdensity solid form in a similarly progressive manner.
Moreover, the relatively fixed mounting arrangement of jacket 44 in mass 66 has been found to contribute significantly to the improved result obtained from the novel process disclosed herein, by insuring that shock-wave energy is not dissipated or absorbed in displacing the mass comprising jacket 44 and its contents. Also, the impact pattern of shock waves on the workpiece is better controlled and more predictable by fixing the workpiece and its containing die components rather than leaving the entire mass to be violently thrown about by the explosive force of detonation.
While all of the underlying reasons for the very significant result of the method and means disclosed herein are not in all respects readily acertainable, it has been found by actual experimentation that each of the several features contributes to this result. Thus, for example, the absence of discontinuous con-tour portion 52 on jacket 44 seems to cause occasional non-uniformity in the densification of billet 40 such as to result in stratification of billet 40, or planes of incipient failure in the final part particularly near the ends thereof. Also, excessive rigidity of the jacket such as by use of extremely hard steels or excessive wall thickness reduces the amount of densification achievable by the method disclosed herein. Ill-ustratively, a jacket 44 of 10 inches total length and two inches diameter, having a cavity of about 1% inches diameter resulting in a wall thickness of about /a inch, and made either from 1020 or 4340 ASTM steel has been found highly successful in producing compacted billets 40 of tungsten having 3 inches total length and /2inch dimeter.
Moreover, use of a metal or alloy having non-shrinking properties when cooled from the molten state has been found especially advantageous in the context discussed above with regard to mass 46. Thus, it is important in the explosive compaction of porous workpieces, such as powder mass 10 or billet 40, that all voids or volumetric discontinuities surrounding the workpiece be assiduously avoided in the mounting thereof within jacket 44. The use of an alloy such as Cerrobend, for example, results in extremely close, intimate, uniform and continuous contact between the workpiece and the materials which surround the same. Cerrobend is especially suited to this purpose because it expands slightly upon cooling from the molt-en to the solid state, whereby compressive force is exerted uniformly over the entire surface of encapsulating layer 42 and is of course transmitted through the same, tending to compress billet 40 over the entire area thereof. It is another and separate feature of the inventive concept in this case that the material comprising mass 46 is selected from those materials exhibiting a marked ability to retard a propagation rate of a shockwave through the material. Cenrobend has the property of retarding the shock-wave velocity as much as 75 to The foregoing characteristic in metals and alloys is related closely to the velocity of stress waves through a body, which depends upon properties of the material as well as dimension or shape of the body. Approximate velocity values for stress waves in four illustrative materials in plate form are as follows:
Ft./sec. Lead 4,300 Steel 17,800 Tin 8,300 Zinc 11,900
The foregoing values are slightly less for the same materials in the form of rod. The foregoing values relates to longitudinal w-aves identified as tensile or compressive in all four cases, while stress wave velocities for shear or transverse waves are considerably less than the values shown in the case of each stated material. Thus, the selection of material comprising mass 46 is of particular importance in achieving the improved results of the invention in this case. In the absence of means for retarding the initial shock-wave velocity resulting from the ex- 7 plosive release of energy at the time of detonation, shockwaves converging toward the center of the workpiece resulting from the implosion pattern discussed hereinabove cause destructive interfering effects between the shockwaves which have been found to produce soft cores in the longitudinal center of billet 40.
In practicing the inventive concept, it has also been found important that plastic coat 42 be of uniform thickness and free of bubbles or voids, in addition to being temperature-resistant whereby no breakdown of composition in the plastic occurs by reason of contact with the molten metal or alloy comprising mass 46. In the absence of layer 42, the alloy used for mass 46 could enter the porous areas of billet and contaminate the same, or have other deleterious effects thereon. Layer 42 is also required to be air-tight to permit evacuation of air and other gases from porous billet 40 prior to surrounding the billet with mass 46.
The material disclosed herein as preferred for use in forming mass 46 has the general composition of about bismuth, about 27% lead, about 13-14% tin, and about 10% cadmium, and is commercially available from the Cerro Sales Corporation, 300 Park Avenue, New York 22, New York, under the trade name Cerrobend.
The inventive process disclosed herein has also been found particularly advantageous in the formation of generally disc or wafer shaped workpieces or billets. Thus, as shown in FIGURE 5, the method discussed above in connection with FIGURE 1 may be modified whereby the initial introduction of powder into bag 12 may be done in a series of separate steps whereby each added increment of powder 86 is followed by insertiOn of a separating member which may take the form of paper disc 88 within bag 12. Use of the inventive teachings set forth above to compact masses 8-6 first isostatically in the manner shown by FIGURE 2 and thereafter explosively in the manner discussed in connection with FIGURES 3 and 4, has been found to produce compacted discs of powdered material such as tungsten with the same superior results with regard to densification and minimum porosity described in connection with billet 40. Moreover, the producton of a plurality of discs 86 by use of a single explosive compacting step is vastly more economical than some production methods known heretofore. All of the above teachings related to billets 10 and 40 apply equally in the case of discs 86 regarding explosive compaction in jacket 44 and embedding in mass 46 prior to the final compaction step.
While the particular details set forth above and in the drawings are fully capable of attaining the objects and providing the advantages herein stated, the structure and method thus disclosed are merely illustrative and could be varied or modified to produce the same results without departing from the scope of the inventive concept as defined in the appended claims.
1. A process for compacting metallic powder comprising the steps of:
placing a quantity of powder in a flexible liquid-impervious container,
isostatically compressing said powder in said flexible container to form a porous billet by placing said container and powder in a surrounding liquid and increasing the pressure of said liquid, and thereafter removing said porous billet from said liquid and surrounding said billet with a mass of material selected from the group consisting of lead, aluminum, copper and their alloys adapted to retard substantially the propagation rate of a shock-wave, and
impacting said billet and said surrounding mass with at least one shock-wave to cause a substantial decrease in the porosity of said porous billet.
2. The process set forth in claim 1 above, wherein:
said material comprises a low melting point alloy of lead aluminum or copper.
3. The process set forth in claim 2 above, wherein:
8 said low melting point alloy comprises an alloy of about 50% bismuth, about 27% lead, about 1314% tin and about 10% cadmium. 4-. The process set forth in claim 1 above, wherein: said impacting step comprises the steps of surrounding a major portion of said mass of material and said billet with a layer of explosive of substantially uniform thickness and composition, and detonating said explosive to cause a plurality of shockwaves to impact said mass of material and said billet radially inwardly toward the center of said billet and substantially uniformly distributed thereabout. 5. The process set forth in claim 4 above, wherein: said step of surrounding said porous billet with said mass of material is accomplished by:
placing said billet within a rigid container, substantially at the center thereof, placing a quantity of said mate-rial in the molten state into said rigid container in an amount sufficient to surround said billet, and cooling said material sufiiciently to solidify the same. 6. The process set forth in claim 5 above, wherein: said step of surrounding said porous billet with said mass of material is preceded by:
covering said billet with an encapsulating layer of temperature-resistant liquid-impervious material adapted to prevent contact between said billet and said mass of material in the molten state. 7. The process set forth in claim 5 above, wherein: said rigid container is a steel jacket having a cavity therein for containing said billet and said mass of material in the molten state, and having access opening means for pouring said material into said cavity, and said jacket is placed with said access opening means embedded in a solid mass adapted to support said jacket prior to said detonating of said explosive. 8. The process set forth in claim 1 above, wherein: said step of placing said quantity of powder comprises: adding said quantity into said flexible container by increments, each of said incremental additions being followed byplacement of a thin separating element to separate each of said incremental additions from the other said additions of said powder. 9. A process for compacting a porous mass to increase the density thereof, comprising the steps of:
covering said porous mass with a thin layer of protective material to isolate the same from surrounding atmosphere, placing said mass and protective layer in a resilient container having a central cavity therein, surrounding said mass and protective layer with an alloy of lead, aluminum or copper adapted to retard the propagation rate of a shock-wave, said surrounding mass being substantially uniform in thickness and composition, surrounding a major portion of said container with a layer of explosive having substantially uniform thickness and composition, and de'tonating said explosive substantially instantaneously. 10. The process as set forth in claim 9 above, wherein: said thin layer is an encapsulating plastic coat applied by spraying said plastic onto said porous mass and allowing said coat to dry, and said encapsulating plastic coat is evacuated of all gases from within the same prior to being surrounded with said alloy. 11. The process set forth in claim 9 above, wherein: said resilient container has an access opening into said cavity, and further including the steps of:
embedding a minor portion of said container com- .prising said access opening in a solid supporting mass prior to said detonating step, said supporting mass being adapted to cantilever support the major unembedded remaining portion of said container by said minor embedded portion, said solid supporting mass being of sufficient strength to resist permanent displacement of said container by detonation of said explosive. 12. The process set forth in claim 11 above, wherein: said solid supporting mass comprises material of substantially the same composition as said alloy. 13. The process set forth in claim 12 above, wherein: said resilient container is of elongate form and said cavity is similarly elongate and coaxially aligned with the longitudinal axis of said container, said cavity terminating at one end thereof in end closure means integrally formed on said container, said layer of explosive being substantially co-extensive with said cavity, and a quantity of said explosive is placed on said end closure means and detonated substantially simultaneously with said layer of explosive.
References Cited UNITED STATES PATENTS 8/ 1953 McKenna et a1. 7/1960 Lenihart 75-214 8/1960 La Rocca et al. 7/1961 Barbera 75-214 X 2/19621 C-oursen et al. 12/ 1964 Quartullo 75-200 1/1965 Callender 75-226 X 10/1966 Ballard et al. 75-226 1/ 1965 Bentov 29-4205 X 9/ 1967 Hague et a1. 75-226 X FOREIGN PATENTS 1.1/ 1965 Great Britain.
OARL D. QUAR FORTH, Primary Examiner.
R. L. GRUDZI'ECKJ, Assistant Examiner.
Citat från patent
Hänvisningar finns i följande patent