Abstract:
An apparatus for compressing powders and the like including a head assembly with a distensible elastic platen mounted in a chambered header plate containing a pressurizing fluid. The elastic platen distends in response to the pressurizing fluid. Further, a base assembly includes a rigid platen mounted in a base plate. The rigid platen includes a face with at least one cavity, into which is added powder to be compressed. The elastic platen is aligned with the rigid platen, and during compression, the two platens may be held firmly in contact. The pressurizing fluid pumped into the head assembly causes the elastic platen to deform forming a single distention per cavity. The distensions compress the powder to an optimal density. The apparatus safely and easily compact multiple small samples of explosives and the like into miniature charges.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for Governmental purposes without the payment of any royalties thereon or therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to compaction presses, and in particular to an apparatus for compacting explosive materials easily and safely to form miniature charges of optimized density. 
     2. Description of the Conventional Technology 
     With the miniaturization of munitions and their components, there is a growing need for technology for reliably fabricating primary charges of increasingly smaller diameters. There exist several methods, for example ink-jet printing and femtosecond laser cutting, by which explosive materials can be formed in small volumes, however there are some drawbacks. Laser cutting methods cannot be used with most materials, including primary explosives, such as, lead azide and lead styphnate, which detonate if irradiated with a laser sufficient for ablating material. (Roos, E., Benterou, J., Lee, R., Roeske, F., &amp; Stuart, B. (2002).  Femtosecond laser interaction with energetic materials  (Preprint UCRL-JC-145670. pp. 11) Taos, N M: SPIE: International Symposium High-Power Laser Ablation). In addition, ink-jet and other methods leave a charge with less than optimal density. 
     Miniature rams have been used to compact charges in rigid cavities, however, as the dimensions of the cavity decrease, the rams become more prone to breakage; and variations in diameter and alignment become a greater concern. Also, the technology is difficult to scale as multiple needle-like rams across a broad area are susceptible to skew, bending and breakage due to otherwise small variations in alignment, flatness, and load height of the explosive in the cavities. Tooling of the cavities and rams becomes more difficult and expensive. 
     A schematic of a prior art ram press  100  is shown in  FIG. 1 . The press  100  includes a rigid platen  42  with a plurality of cavities  44 . The cavities are filled with powdered charge material. The powdered charge material is compacted by a ram  132 , one ram per cavity where each ram is typically rigid and not deformable, forming the charge  70 . The schematic illustrates the position of the rams after compression and retraction of the ram. The multiple rams depend from the press head plate  130  driven by cylinder  140 . 
     In the related art, miniature charges are now on the order of 1 millimeter in diameter and 0.5 millimeters thick. Therefore, the rams used in the prior art apparatus  100  are just under 1 millimeter in diameter, and their margin of error (e.g. tolerance) must be correspondingly small. 
     Needed is an apparatus to compress small samples of powders, such as primary explosives. The samples can be formed by ink-jet printing and the like, and then compacted easily and safely to form miniature charges of optimized density. 
     SUMMARY OF THE INVENTION 
     The invention is an apparatus for compressing powders, and more particularly an apparatus for making miniature explosive powder charges. The apparatus comprises a head assembly with a distensible elastic platen mounted in a chambered header plate containing a pressurizing fluid. The elastic platen distends, where possible, in response to the pressure of the pressurizing fluid. Alternatively stated, when there is an increase in the pressure in the chambered header plate, the distensible elastic platen will deform under adequate pressure forming one or more distensions, and as the elastic platen distends there will be an increase in volume of pressurizing fluid in the head assembly. Conversely, when there is a reduction in the pressure, the distension will retract, and the volume in the chambered header plate decreases. Pressure is required to cause the distension. The apparatus has a base assembly with a rigid platen mounted in a base plate. The rigid platen has a face with at least one cavity. Each cavity has an opening that opens to the face of the rigid platen. The elastic platen is aligned with the rigid platen. Compression occurs when the platens are in contact. During compression, the elastic platen deforms forming multiple distensions, one distention per cavity that extends into the cavity. The elastic platen cannot distend against the face of the rigid platen, as the platens are held firmly in contact. The shape of the cavity, such as the diameter, will largely determine the diameter of the distension. Operationally, the distension acts as a ram having variable length and diameter. The length of the distension is a function of several factors; amongst them are the depth of the cavity, the amount of material in the cavity, and the density of the material. In the case of a cavity filled with a fluffy loose powder having a low density, the distension pushes the powder toward the bottom (or the rear, depending on the orientation of the cavity) of the cavity to a point where the material is compacted to a density that resists further densification. Higher pressure may result in greater compaction and densification with coincident extension of the distension. If the starting powder is more granular, then generally there will be less densification because the stating material is denser. The apparatus is especially suitable for safely and easily compacting a plurality of small samples of explosives and the like, and in particular primary and high explosives, therein forming miniature charges of optimized density. The small samples are generally created using ink-jet technology, and several of these small samples are combined to form miniature charges. 
     The elastic platen includes a non-tacky polymer having substantially no adherence to the cavity or the various powders or plurality of small samples. The polymer is deformable when pressurized, and retracts cleanly and readily when the pressure is released. 
     A cavity can have one or more air outlets that provide an expulsion route for air entrained in the cavity. Alternatively, the cavity can be evacuated. 
     The naming convention employed in this disclosure utilizes the accepted notation that articles “a” and “an” can denote one or more, and are not limited to a single number. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing invention will become readily apparent by referring to the following detailed description and the appended drawings in which: 
         FIG. 1  is a schematic illustration of a conventional art ram press; 
         FIG. 2  is a schematic of the head and base assembly of an illustrated embodiment of the invented apparatus; 
         FIG. 3   a  is a schematic illustration of the apparatus loaded with powder or small samples not yet compacted; 
         FIG. 3   b  is a schematic illustration of the apparatus during compression where the distensions have densified the loaded powder or small samples to an optimized density; 
         FIG. 4  is a schematic of an illustrated embodiment of a sealed apparatus where the cavities can be evacuated or otherwise expelled of entrained air through micro-channels, the apparatus having a diffusible membrane and, optionally, other traps to collect potentially explosive fugitive vapors and particulate matter; 
         FIG. 5  is a schematic of another illustrated embodiment of the sealed apparatus where the cavities can be evacuated or otherwise expelled of entrained air through micro-channels in the rigid platen and through an etched silicon wafer that is bonded to the back of the rigid platen, where the silicon wafer is etched with micro-channels that are in right angle fluid communication with the rigid platen micro-channels, the orthogonal orientation reducing the escape of potentially explosive particles from the base assembly; 
         FIG. 6  is a schematic of another illustrated embodiment of the sealed apparatus, wherein the rigid platen is fabricated using a Silicon-On-Insulator (SOI) wafer, where the cavities are formed in the first silicon layer (top), there are micro-channels are in the first silicon layer, micro-channels in the insulator layer (the first silicon dioxide layer), and micro-channels in the second silicon layer (bottom); and 
         FIG. 7  is a schematic illustration of an array of thirty six distensions that are formed when the distensible elastic platen is aligned and held firmly in contact with the rigid platen having thirty six cavities, and the pressurized fluid has caused the elastic polymer to distend into the cavities. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The apparatus is a means for compacting materials to a desired density, and is particularly suitable for safely and easily compacting a plurality of small samples of explosives and the like, and in particular primary and high explosives, therein forming miniature charges of optimized density. The apparatus  10 , as shown in  FIGS. 2 and 3 , includes a head assembly  20  and base assembly  40 . The head assembly  20  includes a distensible elastic platen  22  mounted in a chambered header plate  24  containing a pressurizing fluid  26  that is conveyed through a fluid line  30 . In the schematic illustration the exemplary chambered header plate  24  has only one chamber  28 , but in other exemplary embodiments, additional chambers may be employed if necessary. The arrows are representative of the pressurizing fluid  26 , which applies a uniform pressure to the back of the distensible elastic platen  22 . The base assembly  40  includes a rigid platen  42  mounted in a base plate  50 . The rigid platen  42  has a face  46  with at least one cavity  44 , each cavity  44  has an opening  48  that extends and opens to the face  46  of the rigid platen  42 . In an exemplary embodiment, five cavities are illustrated in the schematic; however the rigid platen  42  could have many more cavities of various dimensions, such that many powder samples could be simultaneously compressed to the same density. 
     Referring to  FIG. 3   a , a material, such as, loose powder, and/or inkjet samples  60  are preloaded into the cavities and the distensible elastic platen  22  is held firmly in contact with the surface  46  of the rigid platen  42 . Referring to  FIG. 3   b , pressurizing fluid  26  is pumped into the chamber through fluid line  30 . The uniform pressure causes the distensible elastic platen  22  to expand. The base assembly and the head assembly are held firmly together such that the only available expansion is a deformation of the distensible elastic platen  22 . The distensible elastic platen  22  deforms producing a single distension  32  per cavity. The distension  32  extends through the opening  48  compressing the loose powder and/or small samples  60 . The compression densifies the loose powder and/or small samples  60  forming miniature charges or miniature samples  70  of optimized density. 
     The distensible elastic platen  22  is composed of a non-tacky elastomeric polymer, having substantially no adherence to the cavity or the various powders. The polymer is deformable when pressurized, and retracts cleanly and readily when the pressure is released. The elastomeric polymer is selected from the group consisting of a silicone rubber, a polyurethane rubber, a polyacrylate rubber, a natural rubber, and a combination thereof. Various grades of these polymers have excellent elongation and recovery (retraction). To facilitate a clean release the distensible elastic platen can be coated with a release agent, such as a silicone release, a fluorinated compound or polyvinyl N-octadecyl carbamate (pvodc). 
     In an exemplary embodiment, the elastomeric polymer is intrinsically of low tack or is compounded to have low tack, and retracts cleanly and readily, not adhering to the cavity or to the preloaded powder or the compressed powder. In an exemplary embodiment, polydimethylsiloxane (i.e. silicone rubber or PDMS) has good release properties, it is substantially inert, and it has good recovery(retraction). In an exemplary embodiment PDMS is used as the elastomeric polymer. 
     The apparatus may further include a means for expelling air entrained in the cavity or in the powder/small samples being compressed. Air outlets in the cavity are a possibility, but when compressing powders of primary explosive, a major concern is where fractious particles of the explosive may stray. Therefore, in an exemplary embodiment, the apparatus employs an evacuation system, as an evacuation system maintains a level of control over where the particles are collected, and the vacuum facilitates the compression and the uniformity of the charge. The vacuum system may also cause out-gassing and this situation is addressed. Referring to  FIG. 4 , the cavities have an exhaust port  62  at the rear of the cavity, which is the deepest point in the cavity in relation to the face  46 . The exhaust port  62  leads to, in an exemplary embodiment, a micro-channel  58 , a diffusion membrane  52  and a vacuum line  54 . The base assembly is fitted with seals  56  to keep out the ambient air. The vacuum line  54  generally leads to an additional trap selected from the group consisting of a filter, a centrifugal filter, a cryogenically cooled trap, an absorbent and/or dissolving liquid bath, a semi-permeable membrane, a diffusion membrane or a combination thereof. The trap  64  prevents vapors from reaching the vacuum pump. A cryogenically cooled trap removes both water vapor and organic vapors, and both of these shorten the life and reduce the performance of the pump. 
     The exhaust port  62  is sized small, but sufficiently large that a vacuum is achieved. Entrapped air results in non-uniform charges. Diffusion membrane  52  prevents particles from the loose powder and/or inkjet samples  60  from being drawn into the vacuum line  54 . The diffusion membrane  52  may be contaminated to some degree after each pressing, but the level of contamination is small enough to permit repeated use of the diffusion membrane  52 . The diffusion membrane  52  is ultimately replaced and disposed of after a significant number of cycles. 
     Another embodiment of the sealed apparatus  10  employing a vacuum system is shown in  FIG. 5 . An etched silicon wafer  68  is bonded to the back of the rigid platen  42 . A photoresist layer  72  of the silicon wafer  68  is etched with micro-channels that are in right angle fluid communication with the rigid platen micro-channels  58 . The orthogonal relationship reduces the escape of potentially explosive particles from the cavities  44 . This exemplary embodiment lends itself to fabrication using the advantages of photolithography and the developed processes for MicroElectroMechanical Systems (MEMS) that exist for fabricating devices. 
     The exemplary embodiment of the apparatus  10  illustrated in  FIG. 6  expands on the technology disclosed in  FIG. 5 . Referring to  FIG. 6 , the base assembly  40  has a rigid platen  42  that is sealedly mounted in a base plate  50 . The rigid platen  42  is generally composed of a silicon on insulator (SOI) wafer. The rigid platen  42  includes a face with at least one cavity etched in a first silicon layer  74  where each cavity has an opening that opens to the face of the rigid platen  42 . Further, at least one cavity  60  includes an exhaust port  62  that is substantially near a deepest point of the cavity in relation to the face. The exhaust port  62  is in fluid communication through a first micro-channel  76  located in the first silicon layer  74 . The first micro-channel  76  is in fluid communication with a second micro-channel  80  etched in a first silicon oxide layer  78 . The fluid communication extends through a third micro-channel  84  etched in a second silicon layer  82 . The fluid communication further extending to the vacuum line  54  connected to the base plate. In the illustration the channels in the oxide layer  80  are exaggerated in scale. In practice, they would only be 1 to 2 microns deep (the thickness of a general oxide layer in SOI for MEMS). The micro-channels are offset, and have a circuitous route, and this route reduces particulate explosive material from entering the vacuum pump or other areas of the base assembly  40 . 
       FIG. 7  is a schematic illustration of an array of thirty six distensions  32  are formed when the distensible elastic platen  22  is aligned and held firmly in contact with the rigid platen  42  having thirty six cavities  44 , and the pressurized fluid has caused the elastic polymer to distend into the cavities. Of course, the number and diameter of the distensions  32  are determined by the number and diameter of cavities. 
     The apparatus and methodology are particularly applicable to MEMS safety and arming devices for military ordnance, including cheap and practical “salvage-fuzing” or autodestruct features for submunitions. The apparatus could also have commercial applications in the manufacture of such devices as sophisticated automobile airbag inflation systems, fire extinguisher cartridges, and aircrew escape devices. Other applications in the security arena would include micro-miniature and “stealth” destruct devices for microelectronics devices and systems, and micro-sized single-shot power sources for surveillance systems. 
     It is to be understood that the foregoing description and specific embodiments are merely illustrative of the best mode of the invention and the principles thereof, and that various modifications and additions may be made to the invention by those skilled in the art, without departing from the spirit and scope of this invention, which is therefore understood to be limited only by the scope of the appended claims. 
     Finally, any numerical parameters set forth in the specification and attached claims are approximations (for example, by using the term “about”) that may vary depending upon the desired properties sought to be obtained by the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should at least be construed in light of the number of significant digits and by applying ordinary rounding.