US 20050281701 A1
A system and method to produce high density precision composite devices. A system and method to consolidate a high density composite within details of a mold. One method includes creating a densified composite within the mold. Another method includes densifying a pre-mixed composite within a mold. A very high density precision molded composite device.
1. A method of manufacturing a collimator comprising:
placing a particulate material in a mold;
placing a binder in the mold;
forming a composite comprising about 90-99% by weight of the particulate material, and about 1-10% by weight of the binder.
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21. A method of manufacturing a collimator comprising:
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.
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38. A collimator comprising:
a composite of about 90-99% by weight of a particulate material and about 1-10% by weight of a binder.
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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. For example, 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.
Other high density precision devices are molded by manipulating a metal powder/epoxy composite. Examples of those methods are disclosed in U.S. Provisional Patent Application 60/295,564 and U.S. patent application Ser. No. 09/964,348 each of which is hereby incorporated by reference herein in their entirety. Other examples of those methods are described in U.S. Pat. No. 4,588,433, U.S. Pat. No. 5,581,592, U.S. Pat. No. 6,415,017; U.S. Pat. No. 6,519,313, and European Pat. No. 042935B2 each of which is hereby incorporated by reference herein in their entirety.
In other examples, a composite may be poured or injected into a mold. For example, nylon/tungsten powder material can also be used to form high density devices. Recently, a tungsten powder filled nylon thermoplastic, originally developed for non-lead-containing bullets, was adapted to the use of making injection-molded X-Ray collimators. Examples of 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). However, attempts to increase particulate material loading while retaining moldability of precision objects have resulted in a prohibitively weak, or dimensionally strained (e.g., having molded-in stresses that resolve to warp or distort a device), or otherwise compromised, material. There has thus heretofore been a practical limit to particulate material loading in precision devices.
It is, accordingly, understood that maximum achievable densities have been limited to the amount of any high modulus material (e.g., a metal) that may be compounded into a mold (e.g., a thermoplastic or thermoset mold) without seriously degrading the molds physical properties. For example, in tungsten composites having a tungsten powder loading density that is greater than approximately 11 grams per cubic centimeter, serious viscosity and shear effects have prevented adequate flow for mold cavity filling. In the instance of higher tungsten powder loading in nylon or other thermoplastic material, shear damage to the binder (e.g., polymer) has also been observed. Higher loadings are also known to promote abrasive scouring in the course of filing or injecting thus causing erosion of molds and equipment (e.g., injection molder nozzles, screws, pistons, and barrels).
Other known techniques for casting high density precision devices (e.g., collimators, x-ray grids, scatter reduction grids, detector array grids or any device having very small features) limit the precision that can be achieved in the devices. It is known, for example, that where a device is cast from a composite metal having a melting point of 400° F., the mold might have to be heated to at least 200° F. to keep the molten mass from “freezing” as soon as it hits the mold. When such a device cools to room temperature, it may have dimensions that vary by 10% or more from the dimensions of the mold.
There is therefore a need for improved high density precision molded device.
In one embodiment, the present invention includes a particulate material (e.g., metal powder) and binder (e.g., epoxy). One embodiment of the present invention is directed to a method of manufacturing a collimator. In one embodiment, 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. In one embodiment, the forming step includes curing the composite in the mold. In another embodiment, the forming step includes removing a portion of the binder from the composite. In another embodiment, the method includes applying a vacuum to the particulate material. In another embodiment, the method includes applying vibration to the particulate material. 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. 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. In one embodiment, 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 SGp and the binder has specific gravity SGB wherein the ratio of SGP:SGB 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. In one embodiment of the method, the removing step occurs prior to the curing step. In another embodiment, the method includes mechanically removing the binder from at least a portion of the composite. In a further embodiment, 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. In one embodiment, the binder comprises an epoxy resin. In another embodiment, the particulate material has a specific gravity SGP and the binder has specific gravity SGB wherein the ratio of SGP:SGB 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. In one embodiment, 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. In another embodiment, the particulate material includes more than one size of particles. The collimator in one embodiment includes an epoxy resin. In one embodiment, the collimator includes particulate material having a specific gravity SGP and a binder having specific gravity SGB wherein the ratio of SGP:SGB is approximately 8. In one embodiment of the collimator, there is a surfactant. In one embodiment, the density of the collimator is at least about 11.4 grams per cubic centimeters.
The accompanying drawings, which are included to provide further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
In the drawings:
FIGS. 1(a)-(f) illustrate a method of making a densified composite according to the present invention.
Reference will now be made in detail to preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. To provide a thorough understanding of the present invention, numerous specific details of preferred embodiments are set forth including material types, dimensions, and procedures. Practitioners having ordinary skill in the art, will understand that the embodiments of the invention may be practiced without many of these details. In other instances, well-known devices, methods, and processes have not been described in detail to avoid obscuring the invention.
In a preferred embodiment, system 100 is operated to produce a precision-molded composite device 800, for example, as shown in
As described below, various types of particulate material are useful in the preferred composite device. In one embodiment according to the present invention, 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. For example, in one embodiment in which particulate material 150 is tungsten powder, it is possible to produce devices having densities of approximately at least 11.4 gram per cubic centimeter to 13.0 grams per cubic centimeter; an improvement over known densities. Embodiments using metal powders that have densities greater than tungsten powder (e.g., osmium, platinum, or gold powder) would achieve even better results. In one embodiment, 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. In one embodiment, 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. In one embodiment, 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. Thus, in that embodiment, 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.
In one embodiment, 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. In one embodiment, 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.
In one embodiment, it has been found that 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. Thus, while higher loading rates (e.g., a high dose of metal powder in the composite) are generally considered, in some embodiments, to negatively impact viscosity, the effect of those higher loading rates on moldability may be mitigated by appropriate selection of particle shape (e.g., regular shaped particles). Thus, in one embodiment of the present invention, 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. Indeed, 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. In a preferred embodiment 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. For example 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. In one embodiment, 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. In a preferred embodiment, particles are mirror smooth. In still other embodiments 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 TECHNON™ 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. In one embodiment, the metal particles in particulate material 150 are substantially approximately the same size. In another embodiment, the particles have a less uniform particle size distribution. For example, in one preferred embodiment, the particle size distribution is a well graded distribution including multiple particle sizes in predetermined proportions. In another embodiment, the particle size distribution is a bimodal distribution including two predetermined particle sizes. In one embodiment, the bi-modal particle size distribution is characterized by two peaks at predetermined particle size dimensions. For example, in one embodiment, the bimodal distribution includes peaks at approximately 50 microns and approximately 8 microns, respectively. In another embodiment, particulate material 150 has a bimodal distribution including peaks at approximately 45 microns and 5 microns.
The particle size distribution of particulate material 150 may be modified using any known technique. For example, in one embodiment, 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.
In one embodiment, particulate material 150 preferably is well-graded (e.g., having a predetermined particle size distribution). For example, in one embodiment, 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.
In one embodiment, particulate material 150 may have a narrow particle size distribution. Generally, a monomodally dispersed powder (i.e., a powder have particles substantially of one size) with 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. In one embodiment 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). In one embodiment, binder 160 is a thermoset material or a thermoplastic material. In one embodiment, 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. For example, in one embodiment, binder 160 is mixed with particulate material 150 before introduction into mold 110 (discussed in more detail below). In another embodiment, 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.
In some embodiments, 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). In one embodiment, phenolic epoxy having a viscosity of 350 centipoise is selected. Phenolic epoxy is selected for it exceptionally high radiation resistance. In one embodiment, phenolic resins are selected for high energy gamma ray applications such as where long product life is prioritized over maximum density. In a preferred embodiment, binder 160 with a relatively low viscosity is used (e.g., in applications tolerant to less detail and lower densities). In a preferred embodiment (e.g., where binder 160 is mixed with particulate material 150 after particulate material 150 is placed in mold 110), the binder 150 (e.g., a resin) has a specific gravity that is lower than the metal powder with which it is combined. In one embodiment, the specific gravity of particulate material 150 is approximately four (4) to twelve (12) and preferably eight (8) times that of binder 160. In one embodiment, 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). In one embodiment, it has been found that 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. For example, Tra-Con F-110 resin, made by Tra-Con corporation of Danvers, Mass., and Epo-Tek 301 resin, made by Epoxy Technologies of Billerica, Mass., have the viscosity and wetting properties to prepare highly loaded, strong, and dense castings according to the present invention.
In some embodiments, binder 160 is modified to enhance performance in the present invention. In one embodiment, binder 160 is combined with an antifoaming material. In one embodiment the antifoaming material is mineral oil. In one embodiment, Antifoam 88 made by Emerson Cummings of Canton Mass. is used. Thus, for example, where binder 160 will be degassed using vacuum processes, mineral oil may be combined with binder 160 to prevent binder 160 from foaming.
Mold and Master
Mold 110 may be formed from any material appropriate for the desired use. For example, in one embodiment, molds can be made of any material that will withstand casting forces and curing temperatures. In a preferred embodiment, mold 110 is used many times (e.g., repeatedly removing product after curing) In one embodiment, 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). In a preferred embodiment, mold 110 is a polymeric resin (e.g., RTV silicone rubber) that is chosen in some embodiments for its dimensional stability.
In one embodiment, 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, however, 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.
Other methods of producing a master have been developed to avoid those defects (e.g., in applications that require a higher level of precision). In one embodiment, direct 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 ModelMaker™ distributed by Sanders Prototype, Inc. of Wilton, N.H., USA, Multi-Jet Modeling™, Z402 Ink Jet System™ and Three-Dimensional Printing. Additional techniques such as the Stereolithography (SLA) or Selective Laser Sintering (SLS®) systems are also useful in the present invention. U.S. Pat. No. 6,402,403, hereby incorporated by reference in its entirety herein, is directed to 3D printing and forming of structures that are useful in the present invention.
In one embodiment, direct powder microassembly may also be used to produce a master. In 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. For example, 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.
In another embodiment, complex objects may also be produced by milling sheets of material that are later stacked and bonded. For example, 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 3DP™ process disclosed in U.S. Pat. No. 6,610,429.
In another embodiment, stereolithography may be used to produce the master. In stereo lithographic methods, 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) is created in layers from the bottom up. In one embodiment, each layer is measured in thousandths of an inch thick. The result can be a precise physical copy of the virtual computer model significantly lower in price than conventional prototyping and delivered in a fraction of the time. 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. In practicing this process using equipment commonly known as stereolithography apparatus (“SLA”), 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. For example, in one embodiment, 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. Applications for this technology have included, for example, 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. Both metal and ceramic assemblies are routinely made. Cam-Lem Incorporated, Cleveland Ohio, practice this technology. In Cam-Lem's process, 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.
After assembly, 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. During this step 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.
Additionally, 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.
In a preferred embodiment, a single-step solid electrical discharge machining (“EDM”) process has been developed to produce positive masters. In one embodiment, positive master 910, illustrated in
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. In one embodiment, lower stripping forces which result from smoother walls facilitate the preparation of molds and masters that are thicker or higher than can be made with the stacked-foil process. In one embodiment, 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). In one embodiment, 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. In one embodiment, 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.
In one embodiment, master 910 preferably is produced by first drilling pilot holes. In one embodiment, 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. In one embodiment (e.g., a radiation collimator), one may desire square or rectangular openings. An auto-splicing wire EDM machine (e.g., Fanuc I1A) then cuts the desired shape (e.g., square or rectangular) opening (e.g., to produce cells 920). Any shape can be produced using this method such as sound, polygonal or other geometric shapes. In the preferred embodiment, 0.006 inch diameter wire is used to produce corners with a 0.003 inch radius. In alternative embodiments, 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.
A master produced from any method may be used to produce mold 110 for casting precision devices of the present invention. Referring to
For example, using the described methods, and materials, cell sizes ranging from greater than 0.250 inches to approximately 0.040 inches have been generated. Furthermore, wall thickness (e.g., septa bounding a collimator cell) have ranged from approximately 0.100 inches to 0.004 inches.
Force Applying Device
In one embodiment, 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.
In one embodiment, illustrated in
In the preferred embodiment, mold 110 engages force applying device 120. In one embodiment, mold 110 is secured to force applying device 120. In an alternative embodiment, mold 110 is not secured to force applying device 120. In one embodiment, it is advantageous to manipulate the orientation of mold 110 with respect to force applying device 120. In that embodiment, 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.
In one embodiment, vacuum source 130 is used in the present invention. Vacuum source 130 may be any device that will apply a vacuum to mold 110. In the preferred embodiment, vacuum source 130 is any vacuum pump that achieves 29 inches of vacuum or higher. In one embodiment, 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. In another embodiment 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
Methods of Operation
The present invention is useful for the manufacture of castings in general and precision cast devices in particular. In one embodiment, high density composites are cured within a mold having very small mold details thus forming a precision cast device. In preferred embodiments, 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. Thus, for example, the composite material forms the septa in an anti-scatter radiation collimator. In one embodiment of the present invention, 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. In an alternative embodiment, 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. In other embodiments, these techniques may be combined in a hybrid approach to making such precision molded devices. In the preferred methods 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.
Using the methods disclosed herein, 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). In one exemplary embodiment of a preferred mold 110, the range in size of small openings is from 8 to 50 microns.
In-Situ Formation of Composite
One method of using system 100 to produce a precision molded device is illustrated in
In one embodiment, particulate material 150 is introduced to mold 110 in a substantially dry condition. In one embodiment, particulate material 150 is treated with heat (e.g., by oven drying) to remove substantially all moisture prior to placement in the mold. In a preferred embodiment, 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.
As 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.,
Where the preferred external force is vibration, mold 110 preferably is engaged with vibrating table 120 (
Preferably binder 160 is de-aired (e.g., through the application of vacuum) prior to being introduced to mold 110. In one embodiment, vacuum is applied to the mold to degas the binder and the void space within compacted particulate material 150 (
In one embodiment, an external force is maintained throughout the compaction of particulate material 150 only. In another embodiment, 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. In some embodiments, it is also preferable to maintain an external force to mold 110 throughout the application of binder. 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.
While the composite consolidates, 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.
In embodiments using vacuum as an external force, substantially all the interstitial air in particulate matter 150 may be removed. Thus, in addition to urging particulate matter 150 into the details of mold 110, the negative pressure created may induce the infiltration of binder 160 into the void space of particulate matter 150 (e.g., as described above).
In a preferred embodiment, 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.
An alternative embodiment for applying binder is illustrated in
Proper alignment of mold features is another important consideration in casting precision devices that can be achieved using a preferred embodiment of the present invention. Thus, 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). Referring to
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).
Ex-Situ Particulate Material/Resin Combination
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.
In a preferred ex-situ method, 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.
In system 400, rather than introducing dry particulate material 150 and binder 160 to mold 110 in separate steps, mold 110 is filled with the premixed composite 410 that includes particulate material 150 and binder 160.
In one embodiment, 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). When making such a composite of binder 160 and particulate material 150, there is a point at which a further addition of particulate material 150 makes the composite resist pouring absent some type of external force (e.g., vibration or vacuum). 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). For example, where 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.
As discussed above, 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. In a preferred embodiment, 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).
The applications of external forces such as vibration or vacuum, may transform a stiff mixture into a free-flowing material (e.g., via liquification). In another embodiment, 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).
Since highly loaded composites including a particulate material 150 and binder 160 will generally contain more binder than is necessary during production to achieve and maintain the dimensional stability desired in the finished product, in one embodiment, it is desirable to remove a portion of the binder while densifying the composite so that a maximum amount of particulate material 150 with respect the overall volume of composite in the finished produce is achieved. This is contrary to the usual objective in forming composites in which great lengths are sought to maintain the compositional ratios of the composite. Methods similar to those described above may be used to achieve this separation and promote the consolidation of the composite.
As illustrated in
Another method of consolidating the composite containing particulate material 150 and binder 160 is through centrifugal force. An exemplary casting system 700 incorporating centrifugal force is illustrated in
In one embodiment as shown in
Centrifugal methods may also be used to form composites in-situ. Thus, in one embodiment, particulate material 150 is placed in mold 100 and rotated to urge particulate material 150 into the mold details, as shown in
In an embodiment using an open top mold, supernatant will form at the top of the mold. In other embodiments, 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.
In another embodiment, 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.
Application of Finished Products
In one embodiment, the above particulate material 150 and binder 160 compositions (e.g., metal/polymer composite) and processes 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. In one embodiment, using the methods described herein, each of the cast segments are approximately ¼ 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. In one embodiment, the collimator of
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. In one embodiment, the densifying includes vibrating the composite. In one embodiment, the vibrating includes using sound waves (i.e., sonic vibration) to vibrating the composite. In one embodiment, the densifying includes applying a vacuum to the composite. In one embodiment, the composite is densified during introduction of the composite to the mold. In one embodiment, the densifying is performed during the introducing of the metal powder. In one embodiment, the method includes curing the composite. In one embodiment, 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. In one embodiment, 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.
In one embodiment, there is a method of manufacturing a high density device in a mold that includes, creating a composite including a metal powder having a specific gravity SGMP and a binder having a specific gravity SGB such that SGB is low relative to SGMP, and with the metal powder and the binder combination in the mold, densifying the composite. In one embodiment, the creating the composite occurs prior to placing the composite in the mold. In one embodiment, the method includes pouring the composite into the mold. In one embodiment, the method includes drawing the composite into the mold through the application of vacuum. In one embodiment, the method includes injecting the composite into the mold. In one embodiment, 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. In one embodiment, 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.
In one embodiment, there is a molded device including a densified composite having a metal powder and a binder.
In one embodiment, there is an x-ray collimator that includes a densified composite having a metal powder and a binder.
In one embodiment there is a system for producing a high density precision molded device. 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.
In one embodiment, 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.
In one embodiment of the present invention, 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. In one embodiment, one or more composites according to the present invention, can then be assembled to form a finished product.
Although the foregoing description is directed to the preferred embodiments of the invention, it is noted that other, variations and modifications in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the preferred embodiment of the invention, will be apparent to those skilled in the art, and may be made without departing from the spirit or scope of the invention.
All references referred to herein are hereby incorporated by reference in their entirety.