Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/08—Shaking, vibrating, or turning of moulds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D25/00—Special casting characterised by the nature of the product
  • B22D25/06—Special casting characterised by the nature of the product by its physical properties
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/04—Influencing the temperature of the metal, e.g. by heating or cooling the mould
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D27/00—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting
  • B22D27/15—Treating the metal in the mould while it is molten or ductile ; Pressure or vacuum casting by using vacuum

Definitions

• Collimators are devices designed to absorb scattered rays (e.g., from x-ray and gamma ray sources). Methods and materials for producing collimators are continuously evolving to improve strength, fineness of grid details, and absorptive characteristics while reducing manufacturing costs. Cast collimators can be made by filling a mold with dry materials (e.g., metal powder) and applying a binding agent that encapsulates the dry materials when the binding agent hardens. Cast collimators can also be made by filing a mold with one or more dry materials and heating the mold to melt that materials in-situ. [0002] Where strength and durability of the collimator are of particular concern, it is desirable to produce the cast collimator by filing a mold with molten materials.
• dry materials e.g., metal powder
• a binding agent that encapsulates the dry materials when the binding agent hardens.
• Cast collimators can also be made by filing a mold with one or more dry materials and heating the mold to melt that materials in-situ
• a method of manufacturing a cast collimator includes pre-heating a mold, filling the mold with flowable material (e.g., in a liquid, molten, solid, semi-solid, and granulate, or powder), simultaneously app ying vi ra ion, vacuum an ea o e mo i e ma erial reacnes a substantially solid phase.
• flowable material e.g., in a liquid, molten, solid, semi-solid, and granulate, or powder
• the step of pre-heating the mold includes heating the mold until it reaches a temperature that is within approximately about 30°C to about 80°C of the material to be poured into the mold. In one embodiment, where ambient temperature is within about 30°C to about 80°C of the molten material, no preheating may be necessary.
• the mold is then heated to melt the material in the mold.
• the material includes gold and tin.
• the material used is selected from a group including antimony, arsenic, barium, beryllium, bismuth, cadmium, gold, indium, lead, mercury, osmium, palladium, platinum, thallium, tin, tungsten, zinc and mixtures thereof.
• the material includes bismuth, tin, antimony and zinc and mixtures thereof.
• FIG. 1 is a flow chart depicting an exemplary method of the present invention.
• Fig. 2 is a perspective view of an exemplary mold for use in the method of the present invention.
• FIG. 3 is a side view of the exemplary mold shown in Fig. 2 for use in a method according to the present invention.
• FIG. 4a is a perspective view of an exemplary spherical collimator of the present invention.
• _ ig. is a p an view o e exemp ary sp erica o ima or s own in rig.
• FIG. 5 is a side view of an exemplary vacuum chamber according to the present invention.
• Fig. 6a is a perspective view of a cast collimator produced according to the present invention.
• Fig. 6b is a partial side view of the cast collimator in Fig. 6a.
• Fig. 1 illustrates an exemplary method of the present invention.
• a mold is pre-heated using any method selected by those skilled in the art, such as hot plates or ovens.
• pre-heating is meant heating a mold prior to placement of material in the mold.
• the mold is pre-heated approximately to the temperature of the material to be placed in the mold.
• the temperature of the pre-heated mold is the temperature of the material to be placed in the mold +/- approximately 0°C to approximately 100°C, preferably +/- approximately 30°C to approximately 80°C.
• the temperature of the mold is any temperature below the temperature of the material to be .
• m mold is pre-heated to a temperature less than 180°C, preferably between approximately 100°C and approximately 150°C.
• the mold may be constructed of silicone (e.g., RTV silicone), fluorosilicone, Teflon, ceramic, metal or any other material those skilled in art may select for this application.
• the mold in one embodiment is a flexible mold.
• the mold in another embodiment is a rigid or a semi-rigid mold. Molds may be formed by any method, including but not limited to the methods described in PCT Application PCT/US02/17936, U.S. Provisional Applications 60/295,564 and 60/339,773 and U.S. Patent Applications 10/282,441 and 10/282,402, each of which are hereby incorporated by reference as if set forth in their entirety herein.
• FIGs. 2 and 3 show a side view of an exemplary mold 210 for use in the present invention.
• Mold 210 includes a casting chamber 220.
• Casting chamber 220 may further include product chamber 230 and reservoir 240 above product chamber 230.
• product chamber 230 houses precision components such as openings or chambers (e.g., micro-chambers) 250 and posts 260.
• microchamber 250 and post 260 are arranged to form a series of openings 420 as shown in Figs. 4a and 4b.
• Microchambers 250 are openings of a predetermined shape and width, for example, as small as approximately 0.004 inches between posts 220.
• reservoir 240 is an upper portion of casting chamber 220 and above product chamber 230.
• any type or geometry of collimator including, for example, spherical, rectangular, focused, unfocused collimators and combinations thereof may be constructed.
• spherical collimators can be constructed in a spherical mold such that x-rays coming from an x-ray tube that is radiating out in a spherical front can be collimated to parallel.
• Figs. 4a and 4b show perspective and plan views, respectively, of a sp erica co ima or pro uce y e me o o e presen inven ion, n rig.
• ⁇ ta, coinmaior 410 includes openings 420 formed from microchambers 250 and posts 260, as shown in Figs. 2 and 3.
• the collimator is a high resolution collimator.
• a vacuum is applied to a mold cavity.
• a vacuum of 28 inches of mercury or higher is applied (e.g., by a simple rotational vacuum pump).
• the vacuum pump 560 is preferably connected to mold 510 or product chamber 230.
• the vacuum is applied in vacuum chamber 520, e.g., a bell-jar type vacuum chamber.
• Vent valve 530 is connected to vacuum chamber 520 so as to return vacuum chamber 520 to atmospheric pressure according to the present invention.
• the vacuum removes air from product chamber 230 and from casting material 540.
• vacuum can be applied to remove air selectively from vacuum chamber 520 or product chamber 230.
• mold 510 can be selectively isolated and placed under vacuum.
• the degree of vacuum applied to mold 510 or product chamber 230 is selectively varied.
• atmospheric pressure e.g., 14.7 pounds per square inch
• mold 510 is pre-heated (e.g., as described herein) while it is under vacuum.
• mold 510 may be pre-heated prior to exposing it to vacuum.
• mold 510 is pre-heated after it is exposed to vacuum.
• the mold is exposed to gravitational forces.
• the mold is vibrated, such as by vibration source or vibration table 550, as shown in Fig. 5.
• An exemplary vibration source 550 is a Syntron Model V-2-B.
• the mold is vibrated in a vertical direction between 30 and 60 cycles per second.
• one em o imen , . e vi -ra.ion requency is e we en a,ou. c ycles per secon ⁇ and about 80 cycles per second, preferably between approximately 30 cycles per second to approximately 60 cycles per second. Vibration at this rate can impart large momentary forces to the material when placed in the mold.
• centrifugal casting imparts the gravitational force.
• the fixtures for centrifugal casting may be any fixture known to those skilled in the art. Using centrifugal casting, many multiples of gravity can be applied to a casting material in a mold, causing the material to flow more readily into the mold and mold details, than it would under normal gravity. In some embodiments, vibration is a more convenient method of accelerating material into mold details.
• the mold may be placed on a table 550, to impart the necessary gravitational force as opposed to mechanically slinging the mold through an arc.
• vibration is preferred for its ability to cause powders or pastes of solid particles in a liquid to fluidize. In some embodiments, these particles are fluidized suddenly during application of a vibrational force.
• the pastes may undergo a collapse in viscosity that can be often dramatic under vibration.
• a mound of such a paste upon application of vibration, can instantly collapses to a free-flowing liquid, thus allowing highly- filled compositions, or metal alloys in a paste-like phase to be cast.
• a mixture of gold powder in an epoxy resin with a very high amount of gold by weight, such as greater than 90%, is so stiff in viscosity that it cannot be poured at all under normal circumstances.
• a very high amount of gold by weight such as greater than 90%
• uc composi ions wo normally hot be pouraoie or castable, or would be very difficult to pour or cast.
• the vibration can be applied to a metal filled resin, wherein the filler can include a very dense metal such as tungsten, gold, or other heavy metal, or a mixture of two or more heavy metals.
• the metal particles preferably are driven downward into the mold details, and as vibration is continued, excess resin is expelled or expressed from the mixture.
• the resulting casting is nearly all metal filler.
• the casting is equal to or greater than 90%, preferably 92% metal filler. The surplus expressed resin it trimmed off the top of the mold reservoir area after curing and discarded. Accordingly, vibrational compacting allows dense, accurate castings to be made from materials that heretofore were not castable or were difficult to cast.
• Exposure of the mold to gravitational force in one embodiment is commenced prior to placement of material in the mold.
• gravitational force e.g., vibration, centrifugation
• the mold is exposed to gravitational forces during material placement or after material placement in the mold.
• application of gravity forces may take place either before or after pre-heating and/or vacuum application.
• the mold simultaneously is heated, exposed to vacuum and exposed to gravitational forces. This simultaneous application of such external stimuli may take place prior to, during, or after placement of material in the mold.
• step 108 material is placed in the mold.
• Methods of placing the material include, without limitation, pouring, injecting and any other methods known to those skilled in the art for filling a mold.
• the material placed into the mold can be a solid, a semi-solid, a solid powder, a granular material, liquid, molten or any other suitable form.
• the material is liquid and/or fluid material, preferably a liquid and/or fluid high ensi y ma eria , in one em o imen , e ig ensi y a e ia a prior ⁇ piacmg u in the mold to improve pourability of the material.
• step 110 sufficient material is poured into the mold to create a head (e.g., a hydraulic head) of material above the mold.
• external stimuli e.g., one or more of heated mold, vacuum, vibration
• one or more of the external stimuli are applied continuously until the material in the mold becomes substantially solid (e.g., cools to substantially form a solid).
• one or more of the external stimuli are applied intermittently as the material in the mold cures.
• the material being cured has a relatively sharp transition from liquid to solid phase (e.g., a gold/tin eutectic).
• the material being cured in the mold does not have a sharp liquid to solid transition.
• multiple component mixtures such as commercially available bismuth lead/tin alloys (e.g., Cerroshield (52.5% bismuth, 32% lead and 15.5%Tin), autobody or plumbing solders may be mixed so as to provide wide plastic ranges for extended working times (e.g., for modeling, trowlling or wiping the cooling metal).
• a tin/lead eutectic material of 63% tin and 37% lead posses such sharp liquid solid transitions.
• Other alloys e.g., 50-50 solder
• the one or more external stimulus can be applied as the molten metal transitions from liquid phase, to the solid phase.
• the material may exhibit slush characteristics wherein crystals form (e.g., grow) and are suspended within the liquid phase.
• this phase is analogous to resin/metal casting composites as described herein.
• n one em o imen e ma eria is e-gasse w i e is i i ⁇ pnase e.g., oy vibrating the mold). Vibrating the material during curing further ensures that the material stays in the liquid phase longer and therefore facilitates the migration of molten material into mold details.
• the external stimuli e.g., one or more of heated mold, vacuum, vibration
• external stimuli are removed in stages.
• the vibration force is removed after the heat is removed from the mold.
• the mold is vibrated until all other stimuli are removed.
• vacuum is removed prior to the removal of the vibration force and/or heat.
• vacuum is removed after the removal of the vibration force and/or heat.
• a collimator produced from this method is a heavy metal cast collimator, as shown in Figs. 6a and 6b.
• the casting materials are of low melting point and high strength.
• heavy metal is used to cast collimators.
• the materials are flowable. Suitable materials achieve a high stiffness and hardness to prevent (or at least reduce) deformation, and that are radiographically opaque.
• heavy metal materials include, but are not limited to, antimony, barium, beryllium, bismuth, cadmium, gold, indium, lead, mercury, osmium, palladium, platinum, thallium, tin, tungsten, zinc, hafnium, ruthenium, tantalum, or any other metal or alloy having good mechanical strength, dimensional stability, resistance to atmospheric corrosion, and density and alloys and mixtures thereof.
• densities of over 10 g/cc are achieved according to the present invention. Of course, lower and higher densities may result depending on the starting materials.
• ery ium may e se ec e ecausen as v ra ie"naraness; sminess and elasticity characteristics.
• Beryllium has a radiographic opacity that may not be desirable for some applications.
• the collimator is a eutectic mixture (e.g., gold and tin eutectic).
• a gold/tin eutectic has favorable melting point, hardness, stiffness and opacity qualities.
• the ratio of gold to tin is 80:20.
• gold is selected because it is a dense material and has a resistance to tarnishing.
• non- eutectic mixtures are used to form a collimator.
• the collimator is an alloy containing bismuth.
• Bismuth has the desirable characteristic of a low melting point making it suited for use with molds that cannot withstand high temperatures (e.g., RTV silicon molds). This is desirable in one embodiment where pre-heating a mold poses concerns of dimensional instability of the mold details.
• the collimator is derived from metallic powder exposed to the inventive process described herein.
• metallic powders are used in the present invention in combination with the process disclosed in European Patent Application No. 98119778.3, which is hereby incorporated by reference as if disclosed in its entirety herein.
• bismuth concentrations in a useable finished product preferably range from approximately 50% to approximately 90% or more, preferably 60% to 75%, more preferably 65% to 75%. In some instances, alloys with bismuth concentrations above 90% become impractical to use where mechanical strength is a concern in the finished product. In one embodiment, a bismuth alloy containing about 68% ismu is preterre .
• tin concentrations range from approximately 5 to approximately 50%, preferably approximately 10% to 30%, more preferably approximately 20%.
• the alloy preferably contains antimony in ranges from approximately 0.5 to approximately 5%.
• antimony comprises approximately 1% to approximately 3%, preferably about 1.5% of the alloy.
• zinc comprises preferably between approximately 1% and approximately 20% zinc. In preferred embodiments, zinc comprises approximately 5% to approximately 15%, preferably about 0.5% of the alloy.
• a preferred alloy comprises 68% bismuth, 20% tin, 1.5% antimony and 10.5% zinc. At these ratios, there has been achieved a satisfactory combination of low melting point and strength. This alloy is particularly useful because it possess characteristics of semi-liquid combined with semi-solid (e.g., "slush") over a relatively large range of temperatures.
• the alloy simultaneously possess crystals of an antimony/tin intermetalic compound, a zinc/bismuth intermetalic compound, a zinc/antimony intermetalic compound and a liquid quaternary eutectic of antimony, tin, zinc, and bismuth which are simultaneously exposed to outside stimuli (e.g., vibration, vacuum, heated mold) as the crystalline portion grows and the liquid quaternary eutectic solidifies.
• outside stimuli e.g., vibration, vacuum, heated mold
• arsenic or tellurium can be substituted for antimony
• cadmium can be substituted for zinc
• indium can be substituted for tin.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Moulds For Moulding Plastics Or The Like (AREA)
• Powder Metallurgy (AREA)

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• 2003
  • 2003-10-07 EP EP03759754A patent/EP1562717A4/de not_active Withdrawn
  • 2003-10-07 CA CA002501350A patent/CA2501350A1/en not_active Abandoned
  • 2003-10-07 AU AU2003275476A patent/AU2003275476A1/en not_active Abandoned
  • 2003-10-07 WO PCT/US2003/031781 patent/WO2004033132A1/en not_active Ceased
• 2005
  • 2005-04-07 US US11/100,700 patent/US20050269052A1/en not_active Abandoned

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