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US20050281701A1 - Densified particulate/binder composites - Google Patents

Densified particulate/binder composites Download PDF

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Publication number
US20050281701A1
US20050281701A1 US11/148,479 US14847905A US2005281701A1 US 20050281701 A1 US20050281701 A1 US 20050281701A1 US 14847905 A US14847905 A US 14847905A US 2005281701 A1 US2005281701 A1 US 2005281701A1
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Prior art keywords
particulate material
binder
composite
mold
collimator
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US11/148,479
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English (en)
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Robert Lynch
Jonathan Rolfe
Mark Jasmin
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Individual
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Priority to US11/148,479 priority Critical patent/US20050281701A1/en
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Abandoned legal-status Critical Current

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    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21K—TECHNIQUES FOR HANDLING PARTICLES OR IONISING RADIATION NOT OTHERWISE PROVIDED FOR; IRRADIATION DEVICES; GAMMA RAY OR X-RAY MICROSCOPES
    • G21K1/00—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating
    • G21K1/02—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators
    • G21K1/025—Arrangements for handling particles or ionising radiation, e.g. focusing or moderating using diaphragms, collimators using multiple collimators, e.g. Bucky screens; other devices for eliminating undesired or dispersed radiation
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F3/00—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces
    • B22F3/22—Manufacture of workpieces or articles from metallic powder characterised by the manner of compacting or sintering; Apparatus specially adapted therefor ; Presses and furnaces for producing castings from a slip
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C32/00—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ
    • C22C32/0094—Non-ferrous alloys containing at least 5% by weight but less than 50% by weight of oxides, carbides, borides, nitrides, silicides or other metal compounds, e.g. oxynitrides, sulfides, whether added as such or formed in situ with organic materials as the main non-metallic constituent, e.g. resin
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2999/00—Aspects linked to processes or compositions used in powder metallurgy

Definitions

  • High Density precision devices may be produced by a stack lamination processes wherein a multiplicity of thin etched foil sheets are stacked to form the high density device.
  • High density precision devices may also be manufactured by a casting process.
  • one type of high density precision device a radiation collimator—may be produced using a lead casting process.
  • One method of using a lead casting process employs square wires assembled into a mold. Upon casting with lead or its alloys, the wire cores are pulled from the casting. However, because of the pulling forces needed to strip the core from the molds, the desirable thinness of the walls between the grid voids is limited.
  • a composite may be poured or injected into a mold.
  • nylon/tungsten powder material can also be used to form high density devices.
  • tungsten powder filled nylon thermoplastic originally developed for non-lead-containing bullets, was adapted to the use of making injection-molded X-Ray collimators.
  • lead-free thermoplastic composite materials include Ecomass® manufactured and sold by PolyOne Corp of Avon Lake, Ohio and as described in U.S. Pat. Nos. 6,048,379 and 6,517,774 each of which is herein incorporated by reference in their entirety.
  • the present invention addresses a desire to create very dense objects having very precise details (e.g., thin septa and narrow openings in a radiation collimator).
  • the present invention includes a particulate material (e.g., metal powder) and binder (e.g., epoxy).
  • a particulate material e.g., metal powder
  • binder e.g., epoxy
  • One embodiment of the present invention is directed to a method of manufacturing a collimator.
  • the method includes placing a particulate material in a mold; placing a binder in the mold; and forming a composite including about 90-99% by weight of the particulate material, and about 1-10% by weight of the binder.
  • the forming step includes curing the composite in the mold.
  • the forming step includes removing a portion of the binder from the composite.
  • the method includes applying a vacuum to the particulate material.
  • the method includes applying vibration to the particulate material.
  • the method includes applying sound waves to the particulate material. In another embodiment, the method includes applying gravitational force to the mold. In one embodiment, the removing step occurs prior to a curing step. In another embodiment, the removing step includes mechanically removing the binder from at least a portion of the composite.
  • the particulate material includes a heavy metal. In one embodiment the heavy metal is tungsten. In other embodiments, the heavy metal is tungsten, osmium, platinum, gold, rhenium, tantalum and/or mixtures thereof. In one embodiment, the particulate material includes spheroidal particles. In one embodiment, the particulate material includes more than one size of particles. In one embodiment, the binder includes an epoxy resin. In one embodiment, the particulate material has a specific gravity SG p and the binder has specific gravity SG B wherein the ratio of SG P :SG B is approximately 8.
  • the present invention is also directed to a method of manufacturing a collimator. That method includes combining 90-99% by weight of a particulate material and 1-10% by weight of a binder; mixing the particulate material and binder into a composite; placing the composite in a mold; and curing the composite. In one embodiment, the method also includes removing a portion of the binder from the composite. In one embodiment of the method, the combining and mixing steps take place in the mold.
  • the method in another embodiment includes applying a vacuum to the composite. In another embodiment, the method includes applying vibration to the composite. In one embodiment, the method includes applying sound waves to the particulate material. In another embodiment, the method includes applying gravitational force to the mold.
  • the removing step occurs prior to the curing step.
  • the method includes mechanically removing the binder from at least a portion of the composite.
  • the particulate material includes a heavy metal such as tungsten, osmium, platinum, gold, rhenium, tantalum and/or mixtures thereof.
  • the particulate material in one embodiment includes spheroidal particles.
  • the particulate material in another embodiment includes more than one size of particles.
  • the binder comprises an epoxy resin.
  • the particulate material has a specific gravity SG P and the binder has specific gravity SG B wherein the ratio of SG P :SG B is approximately 8.
  • the method in one embodiment includes adding a surfactant to the composite.
  • the present invention also includes collimator having about 90-99% by weight of a particulate material; and about 1-10% by weight of a binder.
  • the collimator includes a heavy metal.
  • the heavy metal in one embodiment includes tungsten, osmium, platinum, gold, rhenium, tantalum and/or mixtures thereof.
  • the collimator in one embodiment contains spheroidal particles.
  • the particulate material includes more than one size of particles.
  • the collimator in one embodiment includes an epoxy resin.
  • the collimator includes particulate material having a specific gravity SG P and a binder having specific gravity SG B wherein the ratio of SG P :SG B is approximately 8.
  • there is a surfactant in one embodiment, the density of the collimator is at least about 11.4 grams per cubic centimeters.
  • FIGS. 1 ( a )-( f ) illustrate a method of making a densified composite according to the present invention.
  • FIG. 2 ( a ) is a cross-sectional illustration of a mold used in a preferred embodiment for making a densified composite according to the present invention.
  • FIG. 2 b is an illustration of a mold used for making a densified composite according to the present invention.
  • FIG. 3 illustrates a system for making a densified composite according to the present invention.
  • FIG. 4 illustrates a method for making a densified composite according to the present invention.
  • FIG. 5 illustrates a method for making a densified composite according to the present invention.
  • FIG. 6 illustrates an apparatus for making a densified composite according to the present invention.
  • FIG. 7 illustrates an apparatus for making a densified composite according to the present invention.
  • FIG. 8 illustrates one embodiment of a densified composite product according to the present invention.
  • FIG. 9 a illustrates one embodiment of a positive master according to the present invention.
  • FIG. 9 b is a cross-sectional diagram illustrating one embodiment of a positive master according to the present invention.
  • System 100 for making a high density precision-molded device (e.g., a radiation collimator).
  • System 100 includes mold 110 , optional force applying device 120 , optional vacuum source 130 , optional reservoir 140 , and a composite including a particulate material 150 and binder 160 .
  • system 100 is operated to produce a precision-molded composite device 800 , for example, as shown in FIG. 8 .
  • a composite device according to the present invention is a homogeneous mixture of particulate material 150 and binder 160 .
  • the exemplary composite includes particulate material 150 and binder 160 .
  • Composite device 800 is preferably constructed from densely packed particulate material 150 in combination with a quantity of binder, for example, a quantity of binder that is necessary to fill the voids between the densely packed particles of particulate material 150 and to securely bind the particles to form composite device 800 .
  • the ratio on a weight basis of binder 160 to particulate matter 150 is in the range of approximately 1:99 and 20:80.
  • the ratio on a weight basis of binder 160 to particulate matter 150 is approximately 10:90. In one embodiment, the range of binder 160 on a weight basis to particulate material 150 on a weight basis is in the range of approximately 4:96. It is preferable that the remaining volume includes degassed binder though in some embodiments, the remaining volume is not entirely filled with degassed binder. In one embodiment, for example, employing a highly detailed but low-energy application where a relatively fast production rate is a priority, one may select a higher binder content by weight. As density decreases, in one embodiment, benefits of the vibration/acceleration aspects of the invention are less pronounced. In an embodiment in which high energy radiation performance is prioritized, one may selected a higher particulate material content.
  • the density of a composite device can be increased by approximately 10-20 percent over previous composite devices including a particulate material and a binder.
  • particulate material 150 is tungsten powder
  • metal powders that have densities greater than tungsten powder e.g., osmium, platinum, or gold powder
  • the density of the composite is greater than the tap density (e.g., the apparent density of a volume of particulate material obtained when its receptacle is tapped or vibrated) of the particulate material.
  • the difference in density between the compacted dry particulate material and the particulate material/binder composite is substantially the result of the additional of binder.
  • the presence of binder in the composite does not substantially alter the dry density (e.g., the density of the particles) relative to that of dry particulate material.
  • the presence of binder e.g., with a specific gravity of 1.6 significantly contributes to the higher density of the composite in comparison to the tap density of the particulate material.
  • particulate material 150 may be any material that one desires to incorporate into a finished composite molded device.
  • particulate material 150 preferably is heavy metal (e.g., in the form of a powder, pellets, granules, sprayed powder, tumbled spherodized filings, precipitate nodules, or other types of particles) such as tungsten, osmium, platinum, gold, rhenium, tantalum or mixtures thereof.
  • the present invention also includes other particulate material 150 including metals such as niobium, silver, uranium, vanadium, or any metals in groups IIA, IIIB, IVB, VB, VIB, VIIB, VIIIB, IB, IIB, IIIA, IVA, VA and VIA in the Periodic Table.
  • materials are selected with consideration given to properties such as radiation absorption, density, toxicity, ease of handling or manufacture, and/or commercial availability.
  • particulate material 150 preferably is chosen so as to be free-flowing and readily compactible.
  • Particulate material 150 may have regularly-shaped particles, irregularly-shaped particles and/or a mixture of particles having regular and irregular shapes.
  • a particulate material with particles of any regular or irregular shape can be used according to the present invention, depending upon the desired composite device to be prepared.
  • regularly shaped particles facilitate a densely packed structure.
  • the use of particulate material 150 having regularly shaped particles may form a less viscous composite than particulate material 150 having irregular shapes.
  • a lower viscosity is further found to promote favorable moldability qualities.
  • higher loading rates e.g., a high dose of metal powder in the composite
  • particle shape e.g., regular shaped particles.
  • composites with loading rates in the rage of approximately 90 to 99% by weight and preferably between 96% to 99% by weight can be used to achieve the desired molded products.
  • a further benefit of the present invention is to permit the molding of viscous materials (e.g., materials that are not readily poured or injected) into molds (e.g., precision molds having small details).
  • Particulate material 150 may also include particles of any shape.
  • particles of particulate material 150 are spherical. In practice, however, truly spherical particles are rarely achieved. Thus, in many embodiments, particles are rounded and not perfectly spherical.
  • particles may be any irregular species of a true sphere (e.g., spheroidal) such as ovoids and/or ellipsoids.
  • Particles may also be any three-dimensional shape including shapes that are substantially trianguloid, rectanguloid, pyramid, cylindroid, or any other geometric shape, such as polygonal solids (e.g., dodecahedrons). In a preferred embodiment, spheroidal particles are chosen.
  • spheroidal particles make up 75% or more of the particles. In one embodiment, substantially all the particles are spherical and/or spheroidal. Other embodiments may employ particulate material 150 having less than 10% or even substantially no spherical or spheroidal particles.
  • Particulate material 150 may also include particles having any texture including, smooth, rough and mixtures of smooth and rough textured particles.
  • particles are mirror smooth.
  • the particles are less than mirror smooth (e.g., having one or more portions having at least a slight texture).
  • Particulate material 150 may be produced by any method including vacuum spraying, fluidized bed methods, and pulverization.
  • An example of particulate material 150 useful in the present invention is TECHNONTM tungsten powder, sold by Tungsten Heavy Powder, Inc. of San Diego, Calif. which is created by vacuum spraying.
  • Particulate material 150 may have any size distribution.
  • the metal particles in particulate material 150 are substantially approximately the same size.
  • the particles have a less uniform particle size distribution.
  • the particle size distribution is a well graded distribution including multiple particle sizes in predetermined proportions.
  • the particle size distribution is a bimodal distribution including two predetermined particle sizes.
  • the bi-modal particle size distribution is characterized by two peaks at predetermined particle size dimensions.
  • the bimodal distribution includes peaks at approximately 50 microns and approximately 8 microns, respectively.
  • particulate material 150 has a bimodal distribution including peaks at approximately 45 microns and 5 microns.
  • particulate material 150 may be modified using any known technique.
  • particulate material 150 may be modified by screening through one or more sieves (e.g., a 45 micron sieve) to form an engineered particulate material blend.
  • the screening process may be used to refine the particulate material to achieve, for example, a preferable packing configuration.
  • particulate material 150 preferably is well-graded (e.g., having a predetermined particle size distribution).
  • particulate material 150 may be characterized by particles having predetermined varying sizes and shapes such that smaller particles fill the interstices between the larger particles.
  • particulate material 150 may have a narrow particle size distribution.
  • a monomodally dispersed powder i.e., a powder have particles substantially of one size
  • a narrow distribution of particle sizes will have a lower tap density than a powder having particles including several size ranges because, where a range of sizes are present, the smaller particles pack into the interstices between the larger particles.
  • Particulate material density can be approximated using any method known to those skilled in the art (e.g., the methods described in the Ph.D. thesis of R. Wildman, Loughboro University which is hereby incorporated by reference herein).
  • Binder 160 is any binder material that when placed in contact with particulate material 150 will form a composite device having the desired physical and chemical characteristics.
  • binder 160 is a polymer and may include plastic (e.g., acrylic reactive mixture, fluorosilicone, polyester, urethane, and mixtures thereof) and/or adhesive (e.g., epoxy resin).
  • binder 160 is a thermoset material or a thermoplastic material.
  • binder 160 is chosen for its low viscosity. In one embodiment, binder 160 has a viscosity in the range of approximately 400 to 100 centipoise, and preferably 100 centipoise or less.
  • Binder 160 preferably is selected with consideration given to the method by which composite device 800 will be formed.
  • binder 160 is mixed with particulate material 150 before introduction into mold 110 (discussed in more detail below).
  • binder 160 is mixed with particulate material 150 during the introduction of particulate material 150 to mold 110 .
  • Binder 160 may also be mixed with particulate material 150 after introduction of particulate material 150 to mold 110 .
  • binder 160 is selected with viscosity ranging from approximately 100 centipoise to 400 centipoise. With care, binder 160 with viscosity as high as 300-400 centipoise may be successfully used. Higher viscosity binders may be effective in some applications (e.g., those applications which are tolerant to lower particulate material loading).
  • phenolic epoxy having a viscosity of 350 centipoise is selected. Phenolic epoxy is selected for it exceptionally high radiation resistance.
  • phenolic resins are selected for high energy gamma ray applications such as where long product life is prioritized over maximum density.
  • binder 160 with a relatively low viscosity is used (e.g., in applications tolerant to less detail and lower densities).
  • the binder 150 e.g., a resin
  • the specific gravity of particulate material 150 is approximately four (4) to twelve (12) and preferably eight (8) times that of binder 160 .
  • binder 160 has a specific gravity of not greater than approximately 2 grams per cubic centimeter while particulate material 150 has a specific gravity of approximately 16 grams per cubic centimeter (e.g., tungsten). In a preferred embodiment, while particulate material 150 has a specific gravity of 16 it has a tap density that is approximately 12 grams per cubic centimeter. In a preferred embodiment, metals with tap densities exceeding 9 grams per cubic centimeter and solid metal density of 11 grams per cubic centimeter are selected.
  • Binder 160 may be modified with surfactants to improve the wetting and capillary infusion effects of the binder.
  • Surfactant modification may be particularly useful to enhance permeation of binder 160 throughout particulate material 150 (described in more detail below).
  • resins made for optical bonding or fiber optic applications have surfactant additions or modification that suits them to use in the present invention.
  • Such resins promote wetting of the fine metal particles.
  • Tra-Con F-110 resin made by Tra-Con corporation of Danvers, Mass.
  • Epo-Tek 301 resin made by Epoxy Technologies of Billerica, Mass.
  • binder 160 is modified to enhance performance in the present invention.
  • binder 160 is combined with an antifoaming material.
  • the antifoaming material is mineral oil.
  • Antifoam 88 made by Emerson Cummings of Canton Mass. is used.
  • mineral oil may be combined with binder 160 to prevent binder 160 from foaming.
  • Mold 110 may be formed from any material appropriate for the desired use.
  • molds can be made of any material that will withstand casting forces and curing temperatures.
  • mold 110 is used many times (e.g., repeatedly removing product after curing)
  • the mold is silicone rubber.
  • Mold may also be constructed from polymers such as Teflon or polypropylene (e.g., by machining the mold from solid stock).
  • mold 110 is a polymeric resin (e.g., RTV silicone rubber) that is chosen in some embodiments for its dimensional stability.
  • mold 110 is formed from a positive master.
  • the mold 110 and a positive master may be constructed from a stack laminated process such as that disclosed is PCT application WO02098624 which is hereby incorporated by reference herein in its entirety.
  • Creating masters using stack lamination is an effective means for producing the present invention.
  • the use of a stack laminated master has its limitations. As those skilled in the art will appreciate, no matter how much care is taken in stacking and laminating the foils, perfect registration is rarely achieved. As the foil sheets are stacked cumulative errors are bound to result even if those errors are quite small. And, even the smallest of those cumulative errors may propagate defects that are not tolerable in some applications.
  • CAD 3-Dimensional prototyping methods are used.
  • Recent advances in ink-jet and polymer technologies have enabled computer-driven devices to assemble from polymers or other materials accurate three-dimensional parts from liquid or powder starting materials to greater and greater accuracy.
  • Exemplary ink jet printing techniques useful according to the present invention include: techniques using Sanders ModelMakerTM distributed by Sanders Prototype, Inc. of Wilton, N.H., USA, Multi-Jet ModelingTM, Z402 Ink Jet SystemTM and Three-Dimensional Printing.
  • SLA Stereolithography
  • SLS® Selective Laser Sintering
  • direct powder microassembly may also be used to produce a master.
  • direct powder microassembly methods a design of a complex object, that exists as a computer file, can be readily produced by directing patterns of droplets of a polymer that hardens to “grow” a part.
  • U.S. Pat. No. 6,610,429 hereby incorporated by referenced in its entirety herein, is directed to a three dimensional printing material system and method.
  • complex objects may also be produced by milling sheets of material that are later stacked and bonded.
  • U.S. Pat. No. 6,627,835 hereby incorporated by reference in its entirety herein, is directed to three dimensional object fabrication techniques which can be combined, in one embodiment, with the methods described herein (e.g., stacking and bonding the objects) to produce masters and/or molds.
  • Parts can also be generated by selectively assembling small powder particles by writing a pattern of adhesive droplets on a bed of powder so as to bond the particles. The loose excess powder is shaken or otherwise removed from the finished part.
  • Z Corporation in Massachusetts of Burlington, Mass., USA. uses the latter method in its 3DPTM process disclosed in U.S. Pat. No. 6,610,429.
  • stereolithography may be used to produce the master.
  • a model may be generated using ultraviolet lasers to selectively harden tiny areas in a bath of pre-polymer.
  • Sterolithography is a process in which high powered, ultra-violet lasers are directed into a basin of liquid polymer resin solidifying certain areas of the polymer to form the master.
  • the part e.g., the master
  • the part is created in layers from the bottom up. In one embodiment, each layer is measured in thousandths of an inch thick.
  • FineLine Prototyping, Inc. of Raleigh, N.C. produces small parts by stereolithography using 3D Systems' manufactured equipment.
  • Stereolithograph is one of several known solid free-form fabrication (“SFF”) techniques.
  • SFF solid free-form fabrication
  • SLA stereolithography apparatus
  • An ultraviolet laser beam selectively scans a reservoir of a photosensitive liquid lying in the beam's path, the exposed portions of the liquid cure or solidify through polymerization.
  • An example of stereolithographic methods and equipment are disclosed in U.S. Pat. No. 5,256,340, which is hereby incorporated by reference in its entirety herein.
  • Precision objects such as positive masters, may also be produced using commercial stacked and sintered technology.
  • Miniature chemical process devices (a.k.a., a “lab on a chip”) containing two input ports, a pressure-circulated mixing area, a buffer section, a reactor area, serpentine cooling section, and output buffer and connection, may all be placed in a single-piece hermetically sealed package.
  • the package size is 32 ⁇ 16 ⁇ 2.4 mm.
  • the resulting alumina ceramic device may be inert to most chemicals and is very strong and durable.
  • micro thermal and chemical systems reactors, fuel cells, chemical sensors and analyzers, meso-scale packaging, and more versatile alternatives to micro-machined silicon (MEMS) devices.
  • MEMS micro-machined silicon
  • Both metal and ceramic assemblies are routinely made.
  • Cam-Lem Incorporated Cleveland Ohio, practice this technology.
  • individual slices of sheet stock engineering material such as “green” ceramic tape) are laser-cut per the computed contours.
  • the resulting part-slice regions are extracted from the sheet stock and stacked to assemble a physical 3-D realization of the original CAD description.
  • the assembly operation includes a tacking procedure that fixes the position of each sheet relative to the pre-existing stack.
  • the layers are laminated together to achieve intimate inter-layer contact, promoting high-integrity bonding in the subsequent sintering operation.
  • the laminated “green” object is then fired to densify the object into a monolithic structure.
  • the laminated history is completely erased and components offer material properties equivalent to those of standard manufacturing methods.
  • U.S. Pat. No. 6,136,132 hereby incorporated by reference in its entirety herein, further describes such a process.
  • masters may be produced in accordance with the methods described U.S. Pat. Nos. 5,231,654; 5,581,592; 6,175,615; 6,370,227; 6,377,661; 6,415,017; and 6,470,072 each of which is herein incorporated by reference in their entirety.
  • a single-step solid electrical discharge machining (“EDM”) process has been developed to produce positive masters.
  • positive master 910 illustrated in FIG. 9 ( a ), is directly machined.
  • Master 910 may be machined from any number of materials including metals such as aluminum and stainless steel.
  • master 910 is machined from 440 stainless steel which has been found to produce the most accurate dimensions.
  • Master 910 is shown with a multiplicity of accurately formed cells 920 .
  • Cells 920 are bounded by walls 930 .
  • walls 930 are tapered such that they are two times thicker at the top than at the bottom. Walls 930 may also be focused. For example, in one focused master, when lines bisecting each cell are extended beyond the surface of master 910 , all lines meet a focal point located a certain distance from the surface of master 910 .
  • the direct machining of master 910 has advantages over other methods of producing a master that include: lower labor costs, more rapid prototyping, comparable accuracy, and a lack of joints or stacking impressions of the laminations. Because master 910 is produced without joints or stacking impressions, there may be produced a superior surface finish of the insides of cells 920 than, for example, those produced in laminated photomechanical foil processes. In one embodiment, this results in easier stripping of molds (e.g., silicone or other polymeric molds) from the master, and/or in easier removal of the molded pieces from the polymeric mold. Thus, the molds may have longer service lives and can produce more parts before they become worn out, or when the “cores” fail in tension during stripping.
  • molds e.g., silicone or other polymeric molds
  • mold 110 may contain relatively long cavities in which protrusion or “fingers” are formed. As the length of the finger increases, there is an increase in the difficulty of removing the cast piece from the mold (e.g., without destroying the mold or the finger).
  • molds that are produced from masters formed by, for example, an EDM methods described herein are smoother and facilitate remove of pieces having longer fingers in comparison to molds made from stack laminated masters which may have propagated alignment errors that promote resistance of the part to removal from the mold.
  • the use of a laminated master mold will limit the casting thickness (e.g., thickness of wall 220 ) to approximately 10 mm. In one embodiment, the use of an EDM master will permit the casting thickness to improve to 15 mm.
  • master 910 preferably is produced by first drilling pilot holes.
  • the pilot holes are an array of round holes in a piece (e.g., 440 stainless steel plate) using computer numerically controlled (“CNC”) methods.
  • An EDM machine preferably threads a wire through the pilot hole, collects the loose end of the wire, and then begins to feed the wire under appropriate tension as an electrical discharge is maintained.
  • the computer directed EDM machine then cuts holes in the piece to the specified shape.
  • a radiation collimator one may desire square or rectangular openings.
  • An auto-splicing wire EDM machine e.g., Fanuc I1A
  • the desired shape e.g., square or rectangular
  • Any shape can be produced using this method such as sound, polygonal or other geometric shapes.
  • 0.006 inch diameter wire is used to produce corners with a 0.003 inch radius.
  • narrower or wider diameter wires are used to produce smaller or larger radii with correspondingly slower or faster cutting speeds. In practice, even with multilayer laminated constructions made with photomachined sheets or foils, true 90 degree zero radius corners are not achievable due to etch effects and optical limitations in the process.
  • mold 110 includes a casting chamber 230 .
  • Casting chamber 230 may further include product chamber 240 and reservoir 140 .
  • product chamber 240 houses precision components such as chambers (e.g., microchambers) 210 and posts 220 .
  • Chambers 210 are openings of a predetermined shape and width, for example, as small as approximately 0.004 inches between posts 220 .
  • reservoir 140 is an upper portion of casting chamber 230 and above product chamber 240 .
  • cell sizes ranging from greater than 0.250 inches to approximately 0.040 inches have been generated.
  • wall thickness e.g., septa bounding a collimator cell
  • septa bounding a collimator cell have ranged from approximately 0.100 inches to 0.004 inches.
  • force applying device 120 is any device that will apply a gravitational-type force (e.g., a force expressed in Gravities, (G), whether by vibration, impact acceleration, centrifugal force, or by some other means) to the mold and its contents to assist in urging, for example, the particulate material 150 to enter the details of mold 110 .
  • a gravitational-type force e.g., a force expressed in Gravities, (G), whether by vibration, impact acceleration, centrifugal force, or by some other means
  • G Gravities
  • force applying device 120 is centrifuge system 700 .
  • force applying device 120 is a vibration device such as a Syntron Model V-2-B, industrial vibrator from FMC Technologies, Homer City, Pa., USA. Such a vibration device preferably delivers a vertical vibrating action, though other vibrating actions (e.g., elliptical, orbital) are within the scope of the invention as well.
  • force applying device 120 is a vibrating table fixture. In other embodiments, force applying device 120 may be any device that will achieve a compaction, consolidation and/or a packing of particulate material within mold 110 .
  • mold 110 engages force applying device 120 .
  • mold 110 is secured to force applying device 120 .
  • mold 110 is not secured to force applying device 120 .
  • mold 110 may be attached to an orientation fixture that facilitates the desired orientation of mold 110 with respect to force applying device 120 while maintaining an engagement with force applying device 120 .
  • vacuum source 130 is used in the present invention.
  • Vacuum source 130 may be any device that will apply a vacuum to mold 110 .
  • vacuum source 130 is any vacuum pump that achieves 29 inches of vacuum or higher.
  • a Welch Model 1399 pump is used (Welch Rietschle Thomas, 7301 N. Central Ave. P.O. Box 183 Skokie, Ill. 60076-0183 USA) in conjunction with several ordinarily available bell jars.
  • vacuum source 130 is any vacuum source that will degas particulate material 150 , binder 160 , and put the void space of particulate material 150 under sufficient negative pressure to induce the flow of binder 160 into the void space upon exposure to positive (e.g., atmospheric) pressure.
  • Vacuum source 130 may also be in communication with product chamber 240 and reservoir 140 such that when vacuum is applied to reservoir 140 , the contents of product chamber 240 and reservoir 140 are degassed. Referring to FIG. 3 , particulate material 150 is loaded in product chamber 240 , binder 160 is loaded in reservoir 140 , and vacuum 130 is applied to mold 110 such that binder 160 and particulate material 150 are degassed.
  • the present invention is useful for the manufacture of castings in general and precision cast devices in particular.
  • high density composites are cured within a mold having very small mold details thus forming a precision cast device.
  • the cast device contains a maximum quantity of densely compacted particulate material 150 and the minimum quantity of binder 160 that can be used to adequately maintain the compacted particulate material 150 in a dimensionally stable condition.
  • the composite material forms the septa in an anti-scatter radiation collimator.
  • the composite is formed within the mold details by introducing a dry particulate material into a mold and adding the binder to the mold in-situ.
  • the particulate material and binder are mixed together ex-situ (i.e., outside the mold) prior to placement in the mold and then the raw, uncured, composite is placed in the mold to cure.
  • these techniques may be combined in a hybrid approach to making such precision molded devices.
  • binder 160 is separated from particulate material 150 in-situ. This may be accomplished by the continued consolidation of the composite (e.g., through the application of an external force) to maximizes the density of the composite in the mold details. Because the continued consolidation is preferably accomplished in a manner which can be described as gradual, as opposed to the application of a shocking force, particulate material migrates into mold details without compromising dimensional stability.
  • the limit to the size of the details (e.g., openings) that can be molded with high-density materials approaches the size of the particles (e.g., particle sizes of particulate material 150 ).
  • the range in size of small openings is from 8 to 50 microns.
  • FIG. 1 One method of using system 100 to produce a precision molded device is illustrated in FIG. 1 .
  • the process preferably begins by introducing particulate material 150 (e.g., metal powder) into mold 110 ( FIG. 1 a ).
  • Particulate material 150 is added to mold 110 , for example, by pouring, injecting, shaking, sifting or any other means.
  • particulate material 150 readily flows into the details of the mold 110 . Because dry particulate material 150 readily flows to the details of mold 110 , concerns of the dimensional stability of mold 110 , that may be caused by the application of direct force, are mitigated.
  • the composite is formed within the mold, concerns that the final product will be weakened by, for example, manipulation during the molding and de-molding processes or will retain undesirable strains, are mitigated.
  • particulate material 150 is introduced to mold 110 in a substantially dry condition.
  • particulate material 150 is treated with heat (e.g., by oven drying) to remove substantially all moisture prior to placement in the mold.
  • particulate material 150 is dried to a constant weight before placement in mold 110 .
  • Particulate material 150 is preferably used in this state because, for example, it flows more readily into the mold details than if contained moisture which would tend to cause particles to bind together and flow less easily.
  • particulate material 150 is introduced into mold 110 , in one embodiment, particulate material 150 is urged in the details of mold 110 , by some external force (e.g., vibration, centrifugation, vacuum, sound waves, and combinations of various external forces) (see e.g., FIGS. 1 b and 1 c ).
  • the external force may further compact or consolidate the particulate material 150 within the mold detail.
  • external forces are applied to mold 110 during, and for some period of time after, the introduction of particulate material 150 to mold 110 . The application of external force has been found to enhance the flow of particulate material 150 to the details of mold 110 .
  • particulate material 150 is compacted into mold 110 and the void space within particulate material 150 is put under a negative pressure (e.g., by exposing the mold to vacuum).
  • the external force is produced with sound waves.
  • sonic energy is applied using a sonic compactor (e.g., as produced by Krell Engineering, Baltimore Md., USA).
  • sub-sonic waves are used to compact particulate material 150 (e.g., using the ULTRAKERAMIC CCD693 manufactured by Diagram, Forli, Italy).
  • mold 110 preferably is engaged with vibrating table 120 ( FIG. 3 ).
  • mold 110 is manipulated (e.g., by tilting or rocking) to different angles while the mold is being filled with particulate material 110 to allow particulate material 150 to fill the details of mold 110 .
  • the manipulation of mold 110 occurs while the external force is being applied.
  • Manipulating the orientation of mold 110 may facilitate the filling of mold 110 and the compaction of particulate material 150 into mold 110 .
  • binder 160 is applied to the densely-packed particulate material 150 .
  • binder 160 is applied to the surface of particulate material 150 ( FIG. 1 d ) until binder 160 covers the entire surface of particulate material 150 and fills at least a portion of reservoir 140 . Binder 160 is then allowed to permeate particulate matter 150 via gravity, wicking, capillary action, or percolation.
  • particulate material 150 remains in a highly densified state during the introduction of binder 160 , and the volume of binder 160 that is used to form the composite is limited to substantially the volume of void space within the dry particulate material 150 packed in mold 110 .
  • binder 160 is de-aired (e.g., through the application of vacuum) prior to being introduced to mold 110 .
  • vacuum is applied to the mold to degas the binder and the void space within compacted particulate material 150 ( FIG. 1 e ).
  • system 100 is then exposed to atmospheric pressure thereby inducing the binder 160 to infiltrate the void spaces (that had been under negative pressure) of the compacted particulate material 150 .
  • an external force is maintained throughout the compaction of particulate material 150 only.
  • the external force may be applied until after binder 160 has fully permeated particulate material 150 .
  • Other embodiments may vary the temporal application of the external force; either by applying the external force on an intermittent basis or by extending or limiting the duration of the applied external force.
  • One objective in maintaining the external force is, for example, to further consolidate the particulate material into the details of mold 110 and consolidate the composite before and during its curing phase.
  • binder 160 preferably is displaced from the mold as the particles are densified and the pore pressure is dissipated.
  • the displaced binder forms a binder supernatant to be removed in post molding activities.
  • substantially all the interstitial air in particulate matter 150 may be removed.
  • the negative pressure created may induce the infiltration of binder 160 into the void space of particulate matter 150 (e.g., as described above).
  • binder 160 substantially fills all the void space of particulate matter 150 . It is furthermore preferred that particulate material 150 is as densely packed as possible and that the quantity of binder 160 within the composite is the minimum amount sufficient to fill the voids in the densely packed particulate material 150 . It is also preferable that all air be removed from the composite. To achieve these objectives, preferably binder 160 and particulate material 150 are degassed prior to and/or during application of binder 160 to particulate material 150 . The preferable means of degassing binder 160 and particulate material 150 is by the application of vacuum (such as describe above), though other methods may be used.
  • FIG. 6 An alternative embodiment for applying binder is illustrated in FIG. 6 .
  • a container 620 holding binder 160 is mounted or suspended above the powder-filled mold 110 in vacuum chamber 610 .
  • the placement of the binder 160 within vacuum chamber 160 allows the simultaneous degassing of binder 160 and mold 110 prior to placement of binder 160 into mold 110 .
  • valve 630 e.g., a remotely controlled valve
  • binder 160 is introduced while mold 110 is under vacuum to insure the absence of air bubbles.
  • binder 160 e.g., degassed binder
  • binder 160 is introduced (e.g., by injection) into the consolidated powder.
  • a further benefit to using external forces, such as vibration, to achieve the desired compaction of metal powder 150 includes inducing a desired alignment of mold details (e.g., posts, cores).
  • mold details e.g., posts, cores.
  • FIG. 5 a there is shown an ideal resilient mold 110 with no distorted mold details ( 210 , 220 ).
  • FIG. 5 b illustrates mold 110 with distorted (e.g., sagging) mold details ( 210 , 220 ).
  • the external densifying forces 120 further act on particulate material 150 to prop mold details ( 210 , 220 ) into alignment.
  • a heavily-filled composite including particulate material 150 and binder 160 (e.g., a metal/polymer composite) is thus formed throughout casting chamber 230 .
  • the composite devices achieved from the methods according to the present invention may have a higher metal loading and higher densities than can be achieved with a premixed composite.
  • the methods according to the present invention therefore, are directed to avoid the problems posed by viscosity, shear and/or flow characteristics of composites with heavy loading (e.g., the difficulty in pouring a viscous material into the small spaces in a mold).
  • Reservoir 140 may also be employed to contain an ex-situ mixed composite (e.g., a stiff composite with a high particulate material loading rate) of particulate material 150 and binder 160 .
  • an ex-situ mixed composite e.g., a stiff composite with a high particulate material loading rate
  • an uncured blended composite including particulate material 150 and binder 160 is subjected to the application of force (e.g., vibrational fluidization and compaction) that causes the blended composite to migrate into product chamber 240 .
  • force e.g., vibrational fluidization and compaction
  • mold 110 is filled with the premixed composite 410 that includes particulate material 150 and binder 160 .
  • composite 410 has a stiff-consistency which is achieved by pre-mixing 8 grams of binder (e.g., an epoxy such as Tra-Con F-110 and its hardener) to every 130 grams of particulate material 150 (e.g., a heavy metal such as tungsten powder).
  • binder e.g., an epoxy such as Tra-Con F-110 and its hardener
  • particulate material 150 e.g., a heavy metal such as tungsten powder.
  • the maximum quantity of particulate material 150 that can be mixed into composite 410 is affected by a number of factors (e.g., including the size and size distribution of particles, shape of particles, texture of particles, and uniformity of particles).
  • the particulate material 150 is a spheroidal metal powder that is carefully graded and blended in size distribution, higher metal loadings (e.g., 92% to 96%) and packing densities of the powder have been observed. Accordingly, in one embodiment, the free-flowing nature of the spheroidal particles reduces the viscosity-limited concentration amounts.
  • One of the challenges in creating devices with a high viscosity composite is getting the composite into the details in the mold.
  • One method of ensuring that a pre-mixed high viscosity composite (whether pourable or not pourable) fills mold details includes heating the mold and/or the composite to reduce the viscosity of the composite while the mold is being filled.
  • care is taken to avoid temperatures that create dimensional instability (e.g., thermal expansion, contraction or shrinkage) in the mold which may affect the certain characteristics of finished products.
  • One embodiment of the current invention enables casting of a stiff composite 410 at ambient temperature to facilitate producing the precise dimensions of the finished product sought at ambient temperature. Since the process is conducted at room temperature, the need for an allowance for thermal expansion, contraction and shrinkage is avoided. In practice, casting may occur in a controlled temperature environment slightly above room temperature (e.g., 60 to 90 degrees Fahrenheit, preferably approximately 70-85 degrees Fahrenheit, and preferably 80 degrees Fahrenheit).
  • composite 410 upon placement of composite 410 into mold 110 at ambient temperature, composite 410 is urged into the mold through the application of an external force (e.g., vibration, vacuum, centrifugal force, sonic vibration, combinations of one or more external forces).
  • an external force e.g., vibration, vacuum, centrifugal force, sonic vibration, combinations of one or more external forces.
  • the external force may be applied in the presence of an elevated temperature (preferably a temperature that will not require allowance for degradation of the mold).
  • the application of external force may be performed at discrete times throughout the mold filling process or continually throughout the process. External force may also be applied intermittently or until mold 110 is completely filled and composite 410 is consolidated in the mold.
  • the applications of external forces may transform a stiff mixture into a free-flowing material (e.g., via liquification).
  • further vibration may be used to achieve densification of the composite within the mold. That densification process may also combine vibration with centrifugation (discussed below).
  • vacuum may be applied to the composite loaded mold 110 to remove air bubbles and create negative pressure in mold 110 .
  • the application of vacuum may be applied at many times or throughout the casting process.
  • vacuum is briefly applied to the filled mold 110 to remove air, and to insure the mixture 410 is pressed into the fine mold details.
  • FIG. 7 An exemplary casting system 700 incorporating centrifugal force is illustrated in FIG. 7 .
  • System 700 includes mold 110 and centrifugal casting fixture 780 .
  • mold 110 is filled with composite 410 and placed into centrifugal casting fixture 780 .
  • bottom 750 of the mold 110 is circumferentially aligned with fixture 780 .
  • counterweight 740 is also attached to fixture 780 .
  • mold 110 is rotated for ten to fifteen minutes at 100 to 1000 rpm, preferably at 600 rpm, to compact composite 410 to densities of at least 11.4 grams per cubic centimeters.
  • particulate material 150 e.g., metal particles
  • excess binder 160 e.g., a metal-depleted epoxy sprue
  • the excess binder 160 is removed during centrifugation.
  • Supernatant alternatively may be removed prior to or after the composite is cured.
  • the supernatant is removed in the course of post-molding manufacturing (e.g., by mechanical removal such as by machining or milling).
  • Centrifugal methods may also be used to form composites in-situ.
  • particulate material 150 is placed in mold 100 and rotated to urge particulate material 150 into the mold details, as shown in FIG. 7 .
  • Binder 160 may then be applied to the surface of particulate material 150 and the rotation continued to form a composite according to the present invention.
  • binder 160 is placed in mold 110 and optionally rotated. Then, particulate material 150 is added to binder 160 and then rotated to form a composite according to the present invention.
  • a sprued mold is used (e.g., to cast the part in a net-shape finished condition). Excess binder 160 from a sprued mold preferably is removed in the normal course of the molding operation.
  • the molded device generally is cured after mold 510 is removed from fixture 580 and the molded device is placed in a horizontal position on the bottom.
  • centrifugal densification may be used with or without other techniques described herein. For example, one may combine centrifugal densification with the above described vibratory compacting. One may also combine centrifugal densification with the application of vacuum (with or without vibration). It may also be beneficial in some instances to apply one of centrifugation, vibration, or vacuum processes to achieve the desired finished product.
  • the above particulate material 150 and binder 160 compositions may be used to form segments of an air-cross grid type of radiation collimator for X-Ray or Gamma-Ray imaging or detection. Segments of any dimension may be produced by this method.
  • each of the cast segments are approximately 1 ⁇ 4 inches thick.
  • the segments in one embodiment are lapped flat and assembled in one or more layers to produce a collimator structure of any desired height.
  • the collimator of FIG. 8 is produced. In one embodiment the collimator of FIG. 8 is 21 ⁇ 2 inches tall.
  • a casting method may now be employed to achieve faithful reproductions of master prototypes in a metal-reinforced silicone rubber mold.
  • the novel fixturing methods described herein will allow rapid production of cast air-cross grid elements.
  • One embodiment of the present invention is a method of manufacturing a high density molded device that includes introducing a composite including a particulate material 150 and a binder 160 (e.g., a metal powder and an epoxy) into a mold; and densifying the composite.
  • the densifying includes vibrating the composite.
  • the vibrating includes using sound waves (i.e., sonic vibration) to vibrating the composite.
  • the densifying includes applying a vacuum to the composite.
  • the composite is densified during introduction of the composite to the mold.
  • the densifying is performed during the introducing of the metal powder.
  • the method includes curing the composite.
  • the method includes removing a supernatant fraction from the mold prior to a curing of the composite. In one embodiment, the method includes removing the molded composite device from the mold; and removing a supernatant fraction from the molded device.
  • the metal powder includes a heavy metal. In one embodiment, the heavy metal is selected from the group consisting of tungsten, platinum, lead, tantalum, osmium, platinum, and gold. In one embodiment, the metal powder contains spheroidal particles. In one embodiment, substantially all of the metal powder consists of spheroidal particles. In one embodiment, the mold is a metal reinforced silicone mold.
  • a method of manufacturing a high density device in a mold that includes, creating a composite including a metal powder having a specific gravity SG MP and a binder having a specific gravity SG B such that SG B is low relative to SG MP , and with the metal powder and the binder combination in the mold, densifying the composite.
  • the creating the composite occurs prior to placing the composite in the mold.
  • the method includes pouring the composite into the mold.
  • the method includes drawing the composite into the mold through the application of vacuum.
  • the method includes injecting the composite into the mold.
  • the creating the composite includes placing the metal powder in the mold; and applying the binder to a surface of the metal powder in the mold.
  • the densifying includes applying a vibration to the composite. In one embodiment, the densifying includes applying vacuum to the composite. In one embodiment, the densifying includes applying a gravitational force to the mold. In one embodiment, the applying a gravitational force includes vibrating the mold. In one embodiment, the applying a gravitational force includes centrifuging the mold. In one embodiment, the metal powder includes a multiplicity of rounded particles. In one embodiment, the metal powder includes a first plurality of particles; a plurality of interstices among the first particles; and a second plurality of particles within the plurality of interstices. In one embodiment, the metal powder further includes a multiplicity of particles that are well-graded. In one embodiment, the binder is a polymer. In one embodiment, the polymer is a surfactant-modified reactive polymer.
  • a molded device including a densified composite having a metal powder and a binder.
  • an x-ray collimator that includes a densified composite having a metal powder and a binder.
  • the system includes a particulate material 150 have a specific gravity greater than lead.
  • the system further includes a binder 160 with a viscosity of up to 300 centipoise, preferably 100 centipoise.
  • the system still further includes a mold that is engaged with a force applying device, a reservoir containing at least one component of the composite. The mold and the reservoir in the present invention are both subjected to vacuum.
  • the mold is created from a stack laminated master. In another embodiment, the mold is created from a machined master. In one embodiment, the machined master is an electronic discharge machined master.
  • a blank composite including particulate material 150 and binder 160 is formed according to the methods described herein, and subsequent to formation is further processed (e.g., by any physical or mechanical means such as machining, etching, etc.) to form a desired product.
  • one or more composites according to the present invention can then be assembled to form a finished product.

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