Note: Descriptions are shown in the official language in which they were submitted.
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CAST COLLIMATOR AND METHOD FOR MAKING SAME
BACKGROUND OF THE INVENTION
[0001] 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.
Manufacturers of cast collimators made from pouring molten metals into a mold
are
challenged to ensure that the molten materials flow into and cast very small
mold details on a
consistent basis. This is particularly challenging due to the high surface
tension of molten
materials. However, high surface tension molten materials happen to have other
desirable
characteristics that make them preferable materials for casting collimators,
as described
further below. There is thus a need for dense, durable, finely detailed cast
collimators and
methods of making same.
SUMMARY OF THE PREFERRED EMBODIMENTS
[0003] In one embodiment, a method of manufacturing a cast collimator, such as
a high
resolution 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
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[OOU4] applying mbration, vacuum and heat to the mol"d ixfftil the material
reacnes a
substantially solid phase.
[0005] In a further embodiment of the inventive method, 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 pre-
heating may be necessary.
[0006] In other embodiments, the mold is then heated to melt the material in
the mold.
[0007] In another embodiment of the inventive method, the material includes
gold and tin.
[0008] In an exemplary method, 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.
[0009] In another exemplary method, the material includes bismuth, tin,
antimony and zinc
and mixtures thereof.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] Reference is made to the accompanying drawings in which are shown
illustrative
embodiments of the invention, from which its novel features and advantages
will be apparent.
[0011] In the drawings:
[0012] FIG. 1 is a flow chart depicting an exemplary method of the present
invention.
[0013] Fig. 2 is a perspective view of an exemplary mold for use in the method
of the present
invention.
[0014] FIG. 3 is a side view of the exemplary mold shown in Fig. 2 for use in
a method
according to the present invention.
[0015] Fig. 4a is a perspective view of an exemplary spherical collimator of
the present
invention.
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WO 2004/033132 PCT/US2003/031781
[OU16] rug. 4b is a plan W ew of the exemplary sphemc~l ~Ollriiator-shown m
rig. 4a.
[0017] Fig. 5 is a side view of an exemplary vacuum chamber according to the
present
invention.
[0018] Fig. 6a is a perspective view of a cast collimator produced according
to the present
invention.
[0019] Fig. 6b is a partial side view of the cast collimator in Fig. 6a.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] 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.
[0021] Fig. 1 illustrates an exemplary method of the present invention. In
step 102 a mold is
pre-heated using any method selected by those skilled in the art, such as hot
plates or ovens.
By "pre-heating" is meant heating a mold prior to placement of material in the
mold. In one
embodiment, the mold is pre-heated approximately to the temperature of the
material to be
placed in the mold. In one embodiment, 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. In one embodiment, the
temperature of the mold is any temperature below the temperature of the
material to be
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placea m me m~u. For example, where material is placed"iri the uiiold atPd~~m
~ o~ .., m~~
mold is pre-heated to a temperature less than 180°C, preferably between
approximately
100°C and approximately 150°C.
[0022] In one embodiment, 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/LTS02/17936,
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.
[0023] 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. In
Fig. 2,
product chamber 230 houses precision components such as openings or chambers
(e.g.,
micro-chambers) 250 and posts 260. In one embodiment, 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. In Figs. 2 and 3, reservoir 240 is an upper
portion of casting
chamber 220 and above product chamber 230.
[0024] Using the methods described herein, any type or geometry of collimator
including, for
example, spherical, rectangular, focused, unfocused collimators and
combinations thereof
may be constructed. For example, 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
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WO 2004/033132 PCT/US2003/031781
spherical collimator produced by the method of the present invention. 1n r ig.
4a, commator
410 includes openings 420 formed from microchambers 250 and posts 260, as
shown in Figs.
2 and 3. In one embodiment, the collimator is a high resolution collimator.
[0025] In step 104, a vacuum is applied to a mold cavity. In one embodiment, a
vacuum of
28 inches of mercury or higher is applied (e.g., by a simple rotational vacuum
pump). As
shown in Fig. 5, the vacuum pump 560 is preferably connected to mold 510 or
product
chamber 230. In one embodiment, 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. In
one embodiment, the vacuum removes air from product chamber 230 and from
casting
material 540.
(0026] In alternative embodiments, vacuum can be applied to remove air
selectively from
vacuum chamber 520 or product chamber 230. In the latter embodiment, mold 510
can be
selectively isolated and placed under vacuum. In one embodiment, the degree of
vacuum
applied to mold 510 or product chamber 230 is selectively varied. In one
embodiment, when
vent valve 530 is opened, atmospheric pressure (e.g., 14.7 pounds per square
inch) aids in
forcing the casting material into the recesses and details of product chamber
230.
[0027] In one embodiment, mold 510 is pre-heated (e.g., as described herein)
while it is
under vacuum. In another embodiment, mold 510 may be pre-heated prior to
exposing it to
vacuum. In yet another embodiment, mold 510 is pre-heated after it is exposed
to vacuum.
[0028] In step 106, the mold is exposed to gravitational forces. In one
embodiment, 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. Preferably, the mold
is vibrated
in a vertical direction between 30 and 60 cycles per second.
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[OOZ j ln~one embodiment, the vibration frequency is-betweeii abbut 1U yc es
pe ~secolrial
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. Vibration at this rate imparts large
momentary
forces to the material when placed in the mold (e.g., approximately up to 30
or more
gravitational forces) thus achieving a high degree of compaction and/or
consolidation into the
intimate details of the mold by, for example, overcoming the surface tension
of the material.
[0030 In another embodiment, 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. For example, in one embodiment, 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. In another embodiment, 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. In
embodiments
where pastes are applied to a mold, the pastes may undergo a collapse in
viscosity that can be
often dramatic under vibration. In one embodiment, 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. For
example, 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.
Upon application of vibration, such stiff mixtures collapses into a free
flowing liquid that
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WO 2004/033132 PCT/US2003/031781
readily fills the mold cavities. Such compositions ~oitt'd i~orrriafiy not-be
pourame or
castable, or would be very difficult to pour or cast.
[0031] In another embodiment, 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. In such an embodiment, 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. In one embodiment, the resulting
casting is nearly
all metal filler. In some embodiments, 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.
[0032] Exposure of the mold to gravitational force (e.g., vibration,
centrifugation) in one
embodiment is commenced prior to placement of material in the mold. In
alternative
embodiments, the mold is exposed to gravitational forces during material
placement or after
material placement in the mold. In some embodiments, application of gravity
forces may
take place either before or after pre-heating and/or vacuum application. In
the preferred
embodiment, at some point in the process according to one embodiment of the
present
invention, 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.
[0033] In 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. In a
preferred
embodiment, the material is liquid and/or fluid material, preferably a liquid
and/or fluid high
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den y2ma a iai 3 in one embodiment, the high density'h'iatei~iau'15'ne~tea p
o~U u2pia~~i ~ gi m
the mold to improve pourability of the material. In one embodiment, sufficient
material is
poured into the mold to create a head (e.g., a hydraulic head) of material
above the mold.
[0034] In step 110, external stimuli (e.g., one or more of heated mold,
vacuum, vibration) are
applied to the mold during the curing (e.g., cooling) process. In one
embodiment, 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). In another
embodiment, one or
more of the external stimuli are applied intermittently as the material in the
mold cures.
[0035] In one embodiment, the material being cured has a relatively sharp
transition from
liquid to solid phase (e.g., a gold/tin eutectic). In another embodiment, the
material being
cured in the mold does not have a sharp liquid to solid transition. For
example, 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). In one embodiment, a tin/lead eutectic material of
63% tin and
37% lead posses such sharp liquid solid transitions. Other alloys (e.g., 50-50
solder) have a
wide range of solidification temperatures where they are past-like in a range
of temperatures
as the melt approaches solidification.
[0036] In an embodiment, where the poured material is molten metal, the one or
more
external stimulus can be applied as the molten metal transitions from liquid
phase, to the solid
phase. Where the transition is more gradual from liquid to solid, the material
may exhibit
slush characteristics wherein crystals form (e.g., grow) and are suspended
within the liquid
phase. In one embodiment, this phase is analogous to resin/metal casting
composites as
described herein.
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0037 In one embodiment, the material is de-gassed while a is m its uqma 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.
[0038] The external stimuli (e.g., one or more of heated mold, vacuum,
vibration) are
removed in step 112. In one embodiment, external stimuli are removed in
stages. For
example, in one embodiment, the vibration force is removed after the heat is
removed from
the mold. In another embodiment the mold is vibrated until all other stimuli
are removed. In
one embodiment, vacuum is removed prior to the removal of the vibration force
and/or heat.
In another embodiment, vacuum is removed after the removal of the vibration
force and/or
heat.
[0039] One example of a collimator produced from this method is a heavy metal
cast
collimator, as shown in Figs. 6a and 6b. Preferably, the casting materials are
of low melting
point and high strength. In one embodiment, heavy metal is used to cast
collimators.
Preferably the materials are flowable. Suitable materials achieve a high
stiffness and
hardness to prevent (or at least reduce) deformation, and that are
radiographically opaque.
Examples of such 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. Preferably, 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.
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WO 2004/033132 PCT/US2003/031781
0040 In one example, beryllmm may be selected beca~us~wt~nas
t~hvrir~~~e~~narane~s; srrrraess
and elasticity characteristics. Beryllium, however, has a radiographic opacity
that may not be
desirable for some applications.
[0041] In one embodiment, 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. In
one embodiment, the ratio of gold to tin is 80:20. In one embodiment, gold is
selected
because it is a dense material and has a resistance to tarnishing. In one
embodiment, non-
eutectic mixtures are used to form a collimator.
[0042] In another embodiment, 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.
[0043] In another embodiment the collimator is derived from metallic powder
exposed to the
inventive process described herein. In one embodiment, such 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.
[0044] However, some products containing alloys with high concentrations of
bismuth or
other heavy metals can pose dimensional stability problems in the finished
products (e.g., the
product may have a tendency to crumble). It has been found that 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%
CA 02501350 2005-04-06
bism uh io pre (erred. The substantial balance of the ally<<~nay ~cot~a~tn ~~-
~ri ~ aniU~o~roy ~a~e~~~c.
In one embodiment, tin concentrations range from approximately S to
approximately 50%,
preferably approximately 10% to 30%, more preferably approximately 20%. In one
embodiment, the alloy preferably contains antimony in ranges from
approximately 0.5 to
approximately 5%. In preferred embodiments, antimony comprises approximately
1% to
approximately 3%, preferably about 1.5% of the alloy. In another embodiment,
zinc
comprises preferably between approximately 1% and approximately 20% zinc. In
preferred
embodiments, zinc comprises approximately 5% to approximately IS%, preferably
about
0.5% of the alloy.
[0045] Experimentation has shown that 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. Thus, in one embodiment, over a 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. In one embodiment,
arsenic or tellurium
can be substituted for antimony, cadmium can be substituted for zinc, and/or
indium can be
substituted for tin.
[0046] 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
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the nature of the preferred embodiment of the mvenriort, v~i'11 'be app~~ent
LO LIlUSC SK111C(1 lIl
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 as if
recited herein.
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