Note: Descriptions are shown in the official language in which they were submitted.
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D~SCRIPIlON
FABRlCATiO~ OF TOOLING BY T~F~MA~ SPRAYING
BACKGROUND OF THE INVENTION
The ~esen~ invention relates to the art of fabricating tools by
thermal spraying of metals.
Industrial processes such as molding and layup of composite
materials require tools having shapes specific to the article to be made. For
example, a composite article can be formed in a mold having an internal
15 shape corresponding to the shape of the desired article. Tools for
fabrication of small parts are often m~chin~A from solid metals, using
conventional machining techniques. These techniques are impractical in
the case of very large molds, having dimensions of a meter or more. There
has been an increasing need for large molds in many industries, and
20 panicularly in the aerospace industries, with the increasing prevalence of
composite structural materials in airframes. Composite parts used in the
airframes must meet exacting standards for fit and finish and often
incorporate complex curved surfaces. Moreover, certain particularly useful
composites, such as carbon-fiber reinforced graphite composites, must be
25 molded at relatively high temperatures. Molds formed from nickel alloys
having low coefficients of therrnal expansion are preferred for molding these
materials. The cost of machining very large molds from solid blocks of
these metals is prohibitive.
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Various proposals have been advanced for forming tools by
providing a matrix having a shape co~ o,lding to that of the part and then
depositing metal on the matrix to forrn the tool. For c~a.~.~le, tools can be
made by a ~r~ccss known as electrofo~ g, in which the metal forming the
S tool is electroplated onto the matrix until a sufficient thickness of metal has
been built up. Elecl~fo~ning is slow and e~ si.~e and impractical for
very large tools. Moreover, many metals which are uscful for tooling, such
as st~inl~ss steel and cenain nickel-co,~tail"ng alloys cannot be
elecll ~fol ",ed readily.
It has also been proposed to fabricate tools by thermal spraying
of metals onto the matrix. As used in this disclosure, the term "thermal
spraying" refers to a l~r~cess in which a metal is heated to convert it to
droplets of molten metal; the droplets are sprayed onto a solid matrix and
the dc~osiled droplets solidify. Examples of thermal spraying include flame
15 spraying in which the metal is melted to form the droplets by passing it
through a flame; arc spraying, in which the metal is melted by passage
through an electrical arc and plasma spraying, in which the metal is melted
by contact with a hot plasma. The article Spray Metal Composite Tooling by
Thorp et al., 26th National SAMPE Symposium, April 28-30, 1981 (pp.
20 37~387) and Progress ~eport: Large Sprayed Metal Composite Tooling
also by Thorp et al., 27th National SAMPE Symposium, 1982, (pp. 85
864) describe forrnation of tools for making large fiber glass-epoxy
composite articles by spraying a relatively low-melting metal known as
kirksite onto a plastic-coated plaster mandrel and then applying a
25 honeycomb composite backing on the outside of the sprayed kirksite and
removing the mandrel. Martyniak, Prototype and Limited Production,
Spray Metal Tools for Injection and Blow Molding, Technical Paper PE 93-
165, Society of Manufacturing Engineers, 1993 describes forrnation of
piastics molds by spraying zinc onto a matrix and backing the sprayed zinc
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shell with an epoxy for structural reinforcement. Garner, U.S. Patent
3,784,451 discloses a process for m~kin,~ a sprayed metal mold such as a
spr~yed zinc mold in which the sprayed metal shell such as a zinc or
~luminum shell is impregnated with a polymeric resin and eleclro~lated to
S provide a smooth surface. U.S. Patent 5,189,781 discioses fabrication of a
matrix for thermal spraying by co.l,~-ltcr~riven solid modeling techniques.
Here again, however, the sprayed metal is a relatively low-melting material
such as an alloyed zinc composition. Putz, U.S. Patent 4,777,002 discloses
a further process for making a mold by spraying steel onto a mold core
10 which has first been coated with a water soluble separating layer; applying abacking to the sprayed layer of steel and then separating the backing and
steel layer by dissolving the water soluble layer.
Milovich ct al., U.S. Patent 5,073,589 describes a backing
resin for a sprayed metal tool to be used at temperatures up to about 750~C.
15 The backing resin composition is placed on the rear surface of a metal shell.The metal shell is formed by spraying a matrix or pattern with metals such
as tin, zinc, aluminum or steel alloys. Milovich, Metal-Faced Composite
Tooling, paper TE-89-507, Society of Manufacturing Engineers, 1989 notes
that "spray meta~ tooling" has been tried in one forrn or another ~by
20 virtually every aerospace manufacturer over the last forty years" and furthernotes that "the lack of success, and consequent industry non-acceptance has
generally been attributed to poor basic material l)lo~lLies rather than the
insufficient material and processing development that has characterized this
little known field." The paper goes on to report measurements of the
25 coefficients of thermal expansion of various metals ranging from a tin/zinc
alloy (melt temperature of 455 ~F), to nickel alloys (melt temperatures
approximately 2700~F), and further reports fabrication of demonstration
tools from aluminum bronze. Milovich, Fabrication and Analysis of
lnvar/Faced Composites for Tooling Applications, SME Conference:
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Tooling for Composites 93, January 18-19, 1993, P~Aerl~, California,
rc~l~ the fabrication of thin test plates of a nickel alloy rcf~l,~ to as Invar
by thernal spraying of the alloy onto a mono1ithic gla~hite ma~ix. The
l~f~ence notes that the Invar must be made thin to reduce dimensional
5 changes caused by shrinkage of the Invar after thcrmal spraying.
Dcspite these and other efforts in the art, however, prior to the
~s~-lt invention, it has not been practical to provide tools suitable for
fabrication of large, composite articles at high molding te,-,~,alul~s by
thermal spraying of metals. In particular, no truly satisfactory method of
10 tool fabrication has been found for fabrication of large, relatively thick
shells having the strcngth characteristics required for fol...alion of large
composite parts such as those employed in the acl~oayace industry.
Moreover, it has been particularly difficult to form such large, thick shells
in metals having relatively high melting points such as stainless steel and
nickel alloys Thus, prior to the present invention there have been
significant, unmet needs for further improvement in techniques of
Fabricating tools by therrnal spraying of metals and there have been
corresponding needs for improvements in the tools themselves.
SUMMARY OF THE INVENTION
One aspect of the present invention provides methods of
making molds. A preferred method in accordance with this aspect of the
invention includes the step of providing a matrix having a shape to be
molded. The matrix may be formed from any convenient material, such as
a readily machinable polymeric composition or a castable, multicomponent
polymeric composition. Methods according to this aspect of the invention
further include the step of depositing a metal on the matrix by spraying the
metal in molten form from a spray gun while moving the spray gun relative
to the matrix so that the spray gun passes back and forth over the matrix in a
movement direction. The spray gun shifts in a step direction transverse to
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S_
the movement direction between passes. The molten metal is ejected from
the spray gun in a spray direction. Most preferably, the method includes the
step of turning the gun bet~,veen passes so as to change the spray direction.
The motion of the spray gun relative to the matri~ is controlled so that metal
5 is deposited in overlapping regions during succ~-sc;~e passes. Thus, during
at least some of the sl~c~ssi~e passes, metal is deposited in the same region
of the matrix from two spray directions in a crisscloss pattern. The
del~osilcd metal forms a shell on the matrix. After deposition, the shell is
removed from the matrix.
Preferred processes according to this aspect of the plescllt
invention can form robust, thick metal shells of essentially any desired size
which accurately reproduce the shape of the matrix. These shells can be
used as molds for making a wide variety of articles. Although the present
invention can be used to make essentially any size or thickness of shell
lS desired, it is particularly useful in forming relatively thick, self-supporting
shells, desirably more than about 3mm thiclc and more preferably about
8mm thick or more. Also, the preferred processes according to the present
invention are particularly useful in forming large shells having length and
width dimensions of about 0.5mm or more and encompassing an area of at
20 least about one square meter. Preferred methods according to this aspect of
the present invention can be used to fabricate molds from essentially any
metal. However, they are particularly useful in fabricsting molds from
metals having melting points above about 400~C and metals having
substantiaIly higher melting points, such as above about 1000~C can also be
25 formed into molds using the process. Metals including one or more
constituent metals selected from the group consisting of iron, nickel, zinc,
aluminum and copper can be employed.
Particularly preferred applications for the process include
fabrication of molds from stainless steels and from ruckel alloys such as
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nickel-iron alloys. For example, a nickel-iron alloy cont~inin~ about 30%
to about 55%, and more preferably about 36~Zo to about ~0% nickel, have
low co~ iç~s of thennal e~pansion and are particularly desirable
mst~ lc for large molds to be used at high t~ ~es. It has not been
S practical heretofore to fabricate large, thick, robust molds from these
materials heretofore by thermal spraying. Although the l.lcse..t invention is
not limited by any theory of operation, it is believed that the particular
techniques used in the spraying step, including the crisscross overlapping
spray pattern provide enll~nced resistance to warpage and distortion of the
10 sprayed metal shell.
The process may include application of the sprayed metal in
two or more stages, most ~ ,felably three stages. In the first and third
stages, the sprayed metal is applied relatively slowly with relatively low
power di~sir~tion in the therrnal spray gun and relatively low speed of
15 motion of the spray gun over the matrix. In the second stage, the sprayed
metal is applied at a higher rate, with high power llissip~lion in the spray
gun and more rapid motion of the spray gun over the surface. The
conditions used during the first stage provide for fine replication of the
surface contours and formation of a dense metal layer adjacent the matrix
20 without overheating the matrix. The conditions used in the second stage
provide for rapid buildup of metal to form the thiclc, robust shell. The shell
can be made into a "wickable" or "non-wickable" mold as desired. As used
in this disclosure with reference to a mold component, the term "wickable"
means that the component has sufficient permeability to gases to dissipate
25 volatiles evolved in a molding operation, whereas the terrn "non-wickable"
means that the shell lacks this degree of gas permeability. To render the
shells non-wickable, the shells may be impregnated with polymeric materials
or with metals such as nickel alloys which can be applied by electroless
plating or by electroplating. Alterna~vely or additionally, the shells can be
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--7--
impregnated with polymeric materials such as the homopolymers and
copolymers of monomers selected from the group con~ic!ing of
tet;rafluoroethylene,fluorinated ethylene propylene, perfluoroalkoxyethylene,
acrylics, vinylidene fluorides and ~mides.
A further aspect of the l,lesent invention provides metallic
shells fo~l-,ed from spray deposited metals having combinations of
~.o~clLies not heretofore obtainable in spray deposited metal shells.
Preferred shells acco~.li,lg to tnis aspect of the invention include those
having length and width dimensions of at least about 0.5 meters; having
thickness of at least about 3mm and being formed from a spray deposited
metal having a melting te~ ature of at least about ~00~C and more
preferably at least about 1000~C.
BRIEF DESCRI~IION OF THE DRAWINGS
Fig. 1 is a diagrammatic sectional view depicting a matrix and
1~ shell used in one embodiment of the invention.
Fig. 2 is a diagram depicting spray gun motion snd sprsy
coverage in the embodiment of Fig. 1.
Fig. 3 is a diagrammatic view of the shell formed in
accordance with the embodiments of Figs. l-2.
Fig. 4 is diagrammatic view depicting the shell of Fig. 3 in a
molding operation.
DETAILED DESCRIYI ION OF THE PREFERRED EMBODIMENTS
A process in accordance with one embodiment of the invention
utilizes a matrix l0 having a cavity region 12 depicting the shape of the part
to be molded. Matrix 10 further includes edge regions 14 bounding cavity
region 12 and side wal}s 16 projecting from edge regions 14 and cavity
region 12. Side walls 16 have draft angles~ most desirably about 3~-5~ or
more. Preferably, edge regions 14 are about two inches (5 cm) wide or
more. Matrix l 0 desirably is formed from a material having useful
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structural s~ength up to about 220~F (104~C) or more. High temperatureepoxy composite tooling compounds, such as those sold under the
dc~ignstion EL-325~ epo~cy composite tooling co~.~u-~s by Adtech
Corporation of Charlotte, Michigan can be employed. Such cpoxies are
5 provided commercially as two component systems which can be mixed and
thcn cast to shape and curcd. Alternatively, thc mi~ture can be cast into a
bloc~ and then m~chine~ to the desircd shape. Other suitable materials are
sold under the trademark REN SHAP~ by the Ciba-Geigy Co.~lation of
Ardsley, New York and by Ren Plastics, Inc. of T ~ncing, Michigan. The
10 REN SHAPE material commonly is provided as boards or prefabricated
shapes which can be rnaehinerl to the desired shape for the mold. Other
readily formable or machinable materials including metals such as alumin~
or brass or graphite may be employed. The desired shape for the cavity
region may be provided as computer data and the matrix may be machined
15 to shape using a conventional computer controlled machine tools.
Polymeric materials such as the epo~y materials mentioned above can be
reinforced with conventional fibrous reinforcements.
A temperature sensitive probe 18 such as a therrnocouple is
embedded in matrix l0 in the cavity region, adjacent the surface of the
20 mold. The leln~lature sensitive probe is linked to a conventional readout
device 20 so that the operator can monitor the temperature of the matrix
surface during the process to follow. Matrix l0 desirably is held and
stabilized on a supporting structure such as a stainless steel hble 21.
A therrnal spray gun 22 linked to a robot 24 (Fig. 2) is used to
25 apply the sprayed metal. Thermal spray gun 22 may be a conventional
plasma spray gun or arc spray gun. For example, a spray gun of the type
sold under the designation model BP 400 Arc Spray System by Miller
Therrnal, Inc. of Appleton, W~sconsin. The BP 400 Gun is arranged to
advance a pair of wires con~inually while applying an electrical poten~ial to
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strike an arc between the wires. The wires continually melt and the molten
metal is stomized and blown onto the sul~s~ate by a stream of co.~ essed
gas such as a stream of co~ "essed air. The spray gun is equipped with a
high velocity air cap arranged to dircct the spr~y of metal dr~l,lcts and gas
5 at a high velocity in a relatively narrow, cone-shaped pattern directed
generally along a spray direction 24 extending from the front of ~e gun.
Spray gun 22 is mounted on a convcntional industrial robot 26
which is programmable using conventional techniques. Robot 26 is
programmed to move the gun in the manner ~iccusse~ below. Gun 22 is
10 also connçct-~ to a source of the wire to be employed in the process (not
shown) as well as a source of con.yl~ssed gas, such as co-"~,lessed air, for
ejecting the droplets. The wire used by gun 22 has the same composition as
the desired metallic shell. Among the metals which can be used are
aluminum alloys such as those in the 2000; 3000; 5000; 6000 or 7000 series
15 of aluminum alloys. A wide range of ferrous metals including stainless
steels can also be used. Among the stainless steel alloys which can be
successfully employed are types 303, 304, 415, and 15.5. As mentioned
above, certain iron-nickel alloys are particularly useful to provide finished
molds with very low thermal expansion coefficients. Alloys of iron and
20 nickel having between about 30% and about 55% nickel, and between about
45% and about 70% iron are particularly preferred for this purpose. The
most preferred alloys include alloys containing about 36% nickel. Among
these alloys are those sold under the designations " nickel 36 " by the
aforementioned Miller Thermal, Inc. and the material referred to as Invar
25 36. Nickel containing alloy of the type sold under the designa~ion Nilo 36
Filler material, by INCO Alloys International of Huntington, West Virginia
can also be employed. The Nilo filler material contains approximately
about 36.2970 nickel, 61.47% Fe; 1.14% Co; 0.26% Ti; 0.09% Si; 0.52%
Mn and 0.23 % C. The alloy referred to as Nilo 36 metal core, containing
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about 40.72% Ni and 58% Fe, available from the aforementioned INCO
Alloys International may also be employed. Also, the 50% nickel - 50
Invar alloy may be employed.
Prior to application of the metal, the surfaces of matrix 10 may
be coated with a suitable sealant such as an cpoxy hi-tcn.~latulre surface
coat available from the afo~ c~ oncd AdTech Co.~ul~l~on. In the metal
depositing y~'~ss~ robot 26 sweeps gun 22 over the surface of matlix 10 so
that tne gun moves in a movement direction inAic~t~A. by double headed
arrow 28 (Fig. 2) during each pass and shifts in a step direction 30 between
10 passes. Robot 26 n~int~ins the spray outlet of the gun at a preselecteA
standoff distance from the surface of matrix 10. The robot also turns gun
22 so as to turn the spray direction relative to the movement direction in
alternate passes. Thus, in one set of passes, the gun is directed as indicated
in solid lines at 22 in Fig. 2, and the spray direction is the first spray
15 direction 24. In another set of passes interspersed with the first set, the gun
is positioned as indicated in broken lines at 22' in Fig. 2 and the spray
direction is the second spray direction 24'. All of these motions are
coordinated with one another so that the sprayed metal impinging on the
surface of matrix 10 is deposited in a series of overlapping strips. For
20 example, in a first pass, with gun 2' oriented as indicated in solid lines, and
with the sprayed metal directed in the first spray direction 24, the metal
impinges on matrix 10 in a strip 32a as the gun sweeps in the movement
direction towards the top of the drawing in Fig. 2. In the next pass, the
spray direction is the second spray direction 24', the gun moves in the
25 opposite movement direction, towards the bottom of the drawing as seen in
Fig. 2, and the sprayed metal impinges on the matrix in a strip 32b
overlapping strip 32a. This process continues, so that the next pass uses the
first spray direction 24 and covers strip 32c overlapping strip 32b and so on.
In each pass, the deposited metal covers about one half of the width of the
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strip covered in the preceding pass. The speed of the gun movement
typically is between 1200 and 2600 inches per minute.
As this process continues, metal is deposited in the two spray
directions 24 and 24' in a crisscross pattern. 'rhis pattern of operation is
S repeated over the entire surface of cavity region 12 and edge region 14 and
e~ct~n-lc onto side walls so that the deposited metal extends onto all of these
regions. The deposited metal forms a shell 37 with a cavity surface 35
facing toward the matrix and duplicating the shape of the matrix. The shell
also has ribs 33 extending oYer the side walls of the matrix and projecting
10 from surface 35. During the spray process, tc.nyclature monitoring sensor
18 and indicator 20 are used to check the te-~ ature of the matrix. The
spray conditions are controlled so that the tclllyclature of the matrix does
not exceed the service temperature of the matrix and preferably remains
below about 200~F (95~C).
The spraying process is performed in three stages. In the first
stage, a first set of spray conditions are employed. The first spray
conditions include a first, relatively low level of power dissipation in the arcof the spray gun and hence a relatively low rate of metal deposition as well
as a first, relatively low linear speed of in movement direction 28 and low
20 rate of wire feed to the gun. These first-stage conditions are maintained
until a predetermined thickness of metal has been built up on the matrix,
typically about .030-.060 inches (about 0.75mm to about l.Smm). The
second stage uses a higher power dissipation, higher wire feed rate and
greater movement speed than the first stage. The conditions used in the third
25 stage desirably are similar to those used in the first stage but with relatively
high power dissipation. The first-stage conditions produce a fine, dense
metal coating on the matrix surface whereas the second stage conditions
promote rapid buildup of deposited metal. A~though particular spray
conditions will vary somewhat with the particular spray gun used and also
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with the metal being deposited, the following conditions have been found
satisfactory for deposition of 36% nickel - 64~ iron alloys:
STAGE ARC ARC STAND- MOVE WIRE
VOLTAGE AMPERES OFF DIST. MENT FEED
SPEED RATE
PER
M~TE
FIRST 25 15 ~6 inches1200 inches .31 Ft/min
(10-15cm) (30m) (.lOm/min)
SECOND 35 40 ~6 inches2100 inches >31Ft/min
(10-lScm) (53m) (> lOmlmin)
THIRD 34 90 4 6 inches1800 inches ~31Pt/min
(l~lScm) (46m) (.lOm/min)
Deposition under the second stage conditions is continued until
5 almost all of the desired thickness has been built up. Shells of essentially
any thic3cness can be made by continuing the deposition for the desired time.
Preferably, the thickness of the shell is built up to about at least 3mm, more
preferably at least about 8mm, and most preferably up to about 13mm (1/2
inch) during the deposition process.
Aher completion of the deposition, the deposited metal shell is
allowed to cool gradually, desirably over a period of at least several hours
and more preferably over a longer time before being removed from the
matrix. For example, cooling from about 150~C to about 20~C over a
period of two to six weeks in a control-temperature environment, followed
15 by several days additional cooling at room tem~clature is particularly
preferred in the case of very large molds. It is believed that the gradual
cooling tends to stabilize the metal shell and prevent warpage when the shell
is removed. Also, although the present invention is not limited by any
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theory of operation, it is believed that the presence of the integral metal 33
l~ojccting from the cavity surface 3~ of the mold along the wall surfaces 16
of the matrix rultl-cr stabilizes the metal shell ~ nct warpage during and
imm~i~ly after cooling. After the shell is removed from the matrix, ribs
5 33 are removed to leave the shell in the form it will have as a final mold.
The cavity surface 35 of the shell, which was originally in contact with
matrix 10, faithfully replicates the shape of the matrix and has a good
surface finish. P~.,fclably, the surface finish is enh~nced by polishing,
typically to a finish of about 125 microinches surface roughness or
10 smoother. After polishing, the molding surfaces may be impregnated with a
polymer or with a metal such as nickel by electroplating or by electroless
plating. Suitable polymeric coatings include homopolymers and copolymers
of monomers selected from the group consisting of tetrafluoroethylene,
fluorinated ethylene propylene, perfluoroalkoxyethylene, acrylics vinylidene
15 fluorides and amides. These can be applied by conventional coating and
impregnation techniques. Among the polymeric materials which can be
employed are the synergistic coatings sold under the registered trademark
LECTROFLUOR, by General Magnaplate Corporation of Linden, New
Jersey. Where a non-wickable mold is desired, the completed, impregnated
20 shell should pass a vacuum integrity test.
The completed shell typically has sufficient structural strength
to serve as a mold. However, for additional structural strength, the shell
can be reinforced, as by casting or laying up a polymeric reinforcement 39
on the surface of the shell 37 opposite from the cavity surface 35.
2~ Reinforcement 39 may incorporate additional metallic elements, or else may
incorporate a honeycomb or other reinforcing structure within the cast or
laid up polymer. The reinforcing structure may also ;nclude elements such
as cooling and heating tubes for cooling or heating the shell in surface. In a
variant of the process, cooling and heating tubes, solid metal reinforcing
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elements and other metallic inclusions may be embedded in the shell during
the metal de~oaiLion process. Thus, such inclusions may be placed on the
partially compl~ted shell so that additional layers of metal are built up over
the inclusions.
The completed mold may be used to forrn a c~ osite articles
such as the articlc 41. The mold may be used with a m~tchin~ mold 43
formed by a similar process. Molds fabricated according to the foregoing
p.occss can be used to form essenti~lly any m~teri~l. However, they are
particularly useful in formation of carbon fiber and graphite composites.
Such composites are used in the aerospace industry to form airframe
structures. Such composites commonly are cured at temperatures of about
350~F (177~C). Nickel alloy molds fabricated as described above can be
used to form these materials even in very large pieces, having dimensions of
several meters or even tens of meters.
The metallic shells formed in accordance with the procedures
discussed above can be used for purposes other than molding.
As these and other variations and combinations of the features
described above can be utilized without departing from the present
invention, the foregoing description of the preferred embodiments should be
taken by way of illustration rather than by way of limitation of the invention
as defined by the claims.
