US20030015308A1

US20030015308A1 - Core and pattern manufacture for investment casting

Info

Publication number: US20030015308A1
Application number: US09/911,358
Authority: US
Inventors: Ken Fosaaen; Jeffery Smith; David Erny; Robert Shay; Nick Lirones
Original assignee: Howmet Research Corp
Current assignee: Howmet Corp
Priority date: 2001-07-23
Filing date: 2001-07-23
Publication date: 2003-01-23
Also published as: DE10231436A1; GB2379632A; GB0215816D0; FR2827533A1; JP2003053480A

Abstract

Method and apparatus for making a ceramic core or fugitive pattern for use in investment casting wherein a fluid ceramic core or pattern material is introduced into a molding cavity defined by cooperating dies, and at least a region of one or both of the dies is heated during filling of the molding cavity with the material and then cooled to a lower ejection temperature before removal of a ceramic core or pattern from the molding cavity.

Description

FIELD OF THE INVENTION

The present invention relates to manufacture of ceramic cores and fugitive patterns for use in making shell molds for investment casting of metals and alloys.

BACKGROUND OF THE INVENTION

In casting hollow gas turbine engine blades and vanes (airfoils) using conventional equiaxed and directional solidification techniques, a fired ceramic core is positioned in a ceramic investment shell mold to form internal cooling passageways in the airfoil. During service in the gas turbine engine, cooling air is directed through the passageways to maintain airfoil temperature within an acceptable range. The fired ceramic core used in investment casting of hollow turbine engine airfoils typically has an airfoil-shaped region with a thin cross-section trailing edge region.

The ceramic core typically is formed to desired core configuration by injection molding, transfer molding or pouring of an appropriate fluid ceramic core material that includes one or more ceramic powders, a binder, and optional additives into a suitably shaped core molding die. After the green molded core is removed from the die, it is subjected to firing at elevated (superambient) temperature in one or more steps to remove the fugitive binder and sinter and strengthen the core for use in casting metallic material, such as a nickel or cobalt base superalloy typically used to cast hollow gas turbine engine blades and vanes (airfoils).

The fired ceramic core then is used in manufacture of the shell mold by the well known lost wax process wherein the ceramic core is placed in a pattern molding die and a fugitive pattern is formed about the core by injecting under pressure pattern material, such as wax, thermoplastic and the like, into the die in the space between the core the inner die walls. The pattern typically has an airfoil-shaped region with a thin cross-section trailing edge region corresponding in location to trailing edge features of the core.

The fugitive pattern with the ceramic core therein is subjected to repeated steps to build up the shell mold thereon. For example, the pattern/core assembly is repeatedly dipped in ceramic slurry, drained of excess slurry, stuccoed with coarse ceramic stucco or sand, and then air dried to build up multiple ceramic layers that form the shell mold on the assembly. The resulting invested pattern/core assembly then is subjected to a pattern removal operation, such as steam autoclaving, to selectively remove the fugitive pattern, leaving the shell mold with the ceramic core located therein. The shell mold then is fired at elevated temperature to develop adequate shell mold strength for metal casting.

Certain complex features at the thin cross-section trailing edge region of the ceramic core and fugitive pattern used in investment casting of turbine airfoils (e.g. turbine blades) have presented manufacturing difficulties. In particular, the thin cross-section trailing edge region of the core includes multiple narrow spaced apart ribs that will form narrow cooling air exit openings at the trailing edge of the cast turbine blade.

The core molding die is machined to include spaced apart wall features that define therebetween narrow channels that will form the ceramic core ribs when filled with ceramic core material. These narrow channels have been difficult to completely fill during the core injection molding operations. In most cases, the ceramic core material entering the channels solidifies prematurely prior to completely filling the channels and forces remaining ceramic material to flow around the blockages through core print regions of the molding cavity and enter the channels from the opposite side where unfortunately another prematurely solidified front is formed in the channels. During the so-called high pressure packing phase of the core molding cycle following a fill cycle, the prematurely solidified fronts located in the channels are pushed and “forged” together, resulting in formation of a so-called weld or knit line where the prematurely solidified fronts are “forged” together under the packing pressure. These weld or knit lines are relatively mechanically weak areas that can easily break or fracture during normal core processing and handling, resulting in scrapping of the core. These problems have persisted even after high die fill speeds (e.g. less than 150 milliseconds fill time), high ceramic material temperatures, and high packing pressures (e.g. 2000 psi) have been employed in past attempts to overcome the problem of inadequate filling of the channels at the trailing edge regions of the core molding die maintained at 80 degrees F. plus or minus 5 degrees F. by control of press platen temperature. Moreover, such injection parameters provide an unstable pressure profile during the molding operation.

Similar problems have been experienced in filling of thin cross-section trailing edge regions and other regions of the fugitive pattern injection molding die.

An object of the present invention is to provide improved method and apparatus for making ceramic cores and fugitive patterns for use in investment casting of metals and alloys.

SUMMARY OF THE INVENTION

In an embodiment of the invention, a fluid material having a composition selected to form a ceramic core or a fugitive pattern is introduced into a molding cavity defined by cooperating dies. At least a region of one or both dies proximate a hard-to-fill region of the molding cavity is heated to a superambient temperature prior to and during filling of the molding cavity with the fluid material followed by cooling thereof to a lower ejection temperature before removal of a molded ceramic core or a molded pattern from the molding cavity.

In an embodiment of the invention to make a molded airfoil-shaped body of ceramic core material or pattern material, the heated/cooled die region is located proximate a hard-to-fill, thin cross-section trailing edge region or other region of an airfoil-shaped molding cavity configured to form the body.

In another embodiment of the invention, at least a region of one or both of the dies is heated/cooled by one or more thermoelectric elements disposed on one or both of the dies. A temperature sensor is disposed proximate the region, and an electrical power controller is connected to the thermoelectric element (s) to control electrical power thereto in response to sensed temperature.

The invention provides apparatus for molding an airfoil-shaped body for use in casting of a metallic airfoil. The apparatus comprises first and second dies defining a molding cavity having an airfoil-shape, and at least one thermoelectric element disposed on at least one of the dies proximate a hard-to-fill region of the molding cavity to heat that region during filling of the molding cavity with fluid material and to cool the region to a lower ejection temperature before a molded airfoil-shaped body is removed from the molding cavity. The apparatus can include an inlet conduit for conducting a heat exchange fluid to remove heat from each thermoelectric element and an outlet conduit for exhausting the heat exchange fluid therefrom. The molding cavity has a configuration of a ceramic core or a fugitive pattern that replicates the airfoil-shaped body.

The above objects and advantages of the present invention will become more readily apparent from the following detailed description taken with the following drawings.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a die assembly of an injection molding press. [0015]
FIG. 2 is a sectional view of upper and lower molding dies of FIG. 1. [0016]
FIG. 3 is a top elevation view of the lower molding die having thermoelectric elements thereon pursuant to an embodiment of the invention. [0017]
FIG. 3A is a top elevation view similar to FIG. 3 of a lower molding die having thermoelectric elements thereon pursuant to another embodiment of the invention. [0018]
FIG. 3B is a top elevation view similar to FIG. 3 of a lower molding die having fluid passages pursuant to another embodiment of the invention. [0019]
FIG. 4 is a perspective view of a ceramic core manufactured in accordance with an embodiment of the invention.[0020]

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to manufacture of molded ceramic cores and fugitive patterns for use in making shell molds for investment casting of metals and alloys. The invention is especially useful in making ceramic cores and fugitive patterns for use in the casting of nickel based and cobalt based superalloys to form hollow gas turbine engine airfoils such as turbine blades and vanes using conventional equiaxed to produce equiaxed grain airfoils and directional solidification techniques to produce columnar grain and single crystal airfoils. However, the invention is not so limited and can be practiced to make ceramic core and fugitive patterns for use in casting of other metallic components. [0021]
The invention is useful to provide filling of one or more hard-to-fill regions of the molding cavity where fluid ceramic core material or fugitive pattern material has difficulty in filling the region(s). A hard-to-fill region of the molding cavity can be difficult to fill with fluid material by virtue of its thin cross-sectional dimension or other dimension(s), complex configuration, remoteness from the inlet of the fluid material into the molding cavity, local rapid heat loss through dies, flow characteristics of the fluid material proximate the region, and combinations of these factors. The invention can be practiced to fill such a hard-to-fill region(s) of the molding cavity with fluid ceramic core material or fluid pattern material in the molding of ceramic cores and fugitive patterns for use in investment casting. Although the invention is described below for purposes of illustration with respect to filling of a thin cross-section trailing edge region of an airfoil-shaped core molding cavity, the invention is not so limited and can be practiced to fill any hard-to-fill region of the mold cavity, regardless of location in the molding cavity. For example, the invention can be practiced to improve filling of one or more hard-to-fill region(s) at the leading edge of the ceramic core or fugitive pattern. [0022]
For purposes of illustration and not limitation, FIG. 1 illustrates first and second cooperating [0023] dies 10, 12, which are maintained at 82-83 degrees F. by water cooled aluminum plates 23 a, 23 b disposed between fixed press platen 25 a and die base 27 a and movable press platen 25 b and die base 27 b of the injection molding press. The die base 27 a holds die 10 while the die base 27 b holds die 12. The water cooled plates 23 a, 23 b each includes a cooling water inlet I to supply cooling water to a water passage (not shown) in each plate 23 a, 23 b and then out a water outlet L. In lieu of, or in addition, to cooling plates 23 a, 23 b, the die bases 27 a, 27 b can be similarly water cooled to maintain overall die temperature except at local die region 30.
FIGS. [0024] 1-3 illustrate cooperating core molding dies 10, 12 as defining a main core molding cavity 14 therebetween having a general airfoil shape. The dies 10, 12 typically are made of steel although other suitable die materials can be used. The molding cavity 14 typically includes complex die surface features such as turbulators, channels, pedestal recesses and the like, as dictated by a particular core design, to be molded onto the core but which are omitted in FIG. 3 as they form no part of the invention. The airfoil-shaped mold cavity 14 includes a leading edge region 14 a and a trailing edge region 14 b that tapers to a relatively thin cross-section as compared to the cross-section of the main core cavity 14. For example only, the trailing edge region 14 b can taper down to a thickness of less than 0.014 inch. This compares to a maximum thickness of 0.500 inch of core molding cavity 14 near the leading edge region. The leading region and trailing edge regions 14 a, 14 b of the molding cavity 14 form respective leading and trailing edges LE, TE on the molded core C.
Wall features [0025] 14 c are machined on the lower die 10 and upper die 12 to cooperate when the dies are closed to define narrow channels 14 d therebetween. The narrow channels 14 d form narrow ceramic ribs R separated by open spaces OP on the ceramic core C, FIG. 4, that is molded in the molding cavity 14. The ribs R will form exit openings for cooling air from the trailing edge of the cast superalloy turbine airfoil when the core is removed therefrom in a manner known in the airfoil casting art.
The [0026] die 10 includes a molding surface 14 e to form a convex airfoil-shaped core surface S1, and the die 12 includes a molding surface 14 f to form a concave airfoil-shaped core surface S2 on the molded core C, FIG. 4. The molding cavity 14 includes secondary regions 14 g, 14 h that are adapted to form core print regions P1, P2 on the molded core C. A plurality (3 shown) of air vent passages 14 p are disposed between the trailing edge region 14 d and grooves 14 g to vent air from molding cavity 14 as ceramic core material is introduced into the cavity 14.
A plurality of ejector pins EP are disposed in the [0027] die 10 and movable in a manner to eject the molded core C from the molding cavity 14. Ejector pins EP proximate the trailing edge region 14 b are shown in FIG. 3. Other ejector pins located at various other locations in the molding cavity 14 to effect removal of the molded core therefrom in conventional manner are not shown.
An inlet opening [0028] 10 a is formed in the lower die 10 or upper die 12, or both, and communicated to a pump P of a conventional core injection molding press (not shown). Ceramic core material, such as a fluid ceramic compound, is injected under pressure (e.g. only 500 to 2000 psi) into the molding cavity 14 via opening 10 a. The dies 10, 12 and pump P can be part of a conventional hydraulic ceramic core injection molding press available as model DCS-2 from Howmet Tempcraft Inc., Cleveland, Ohio. The injection molding press is operated to provide a fill stage during which ceramic core material is injected under pressure at constant injection ram speed or volumetric rate into the molding cavity 14, a pack stage during which the ceramic core material pressure is increased and stabilized to fully fill the mold cavity 14, a hold stage during which pressure on the ceramic core material is maintained until core solidification is complete, and a core ejection stage when the dies are opened to allow removal of the molded core. Solidification occurs as a result of heat loss from the ceramic core material into the dies 10, 12.
The fluid ceramic core compound injected into the [0029] molding cavity 14 comprises a mixture of one or more suitable ceramic powders (flours), a fugitive binder and other constituents such as one or more fugitive filler materials, dispersants, plasticizers, lubricants and other constituents. The binder can be a thermoplastic wax-based binder, a thermoplastic resin, or an organometallic liquid, such as prehydrolized ethyl silicate, mixed with the ceramic powder(s) in appropriate proportions to form a ceramic powder/binder mixture for molding to shape. The ceramic powders can be blended using a conventional V-cone blender, pneumatic blender, or other such blending equipment. The binder can be added using conventional high-shear mixing equipment at room temperature or elevated temperature. The ceramic powders may comprise alumina, silica, zirconia, zircon, yttria, and other powders and mixtures thereof suitable for casting a particular metal or alloy. U.S. Pat. No. 4,837,187 describes an alumina based ceramic core made from alumina and yttria flours. The particular ceramic powders, fugitive binder and other constituents of the ceramic powder/binder mixture form no part of the invention as conventional ceramic powder and binder systems can be used to form the ceramic core.
As described above in the Background Of The Invention section, the [0030] channels 14 d defined between wall features 14 c at the thin cross-section trailing edge region 14 b of the molding cavity 14 have presented manufacturing difficulties in that channels 14 d have been difficult to completely fill with the fluid ceramic compound during the core injection molding operation. In particular, the fluid ceramic compound (binder material such as thermoplastic wax) solidifies prematurely in the channels 14 d at their entrances to the main molding cavity 14 and forces the ceramic compound to flow around the blockages through the core print region 14 f to enter the channels 14 d (see arrows A) from the opposite outermost side of the trailing edge region 14 b. During the so-called pack phase of the injection cycle, the prematurely solidified ceramic compound fronts located in the channels 14 d are pushed and “forged” together trapping air between the fronts, resulting in formation of so-called weld or knit lines that are relatively mechanically weak areas that can easily break or fracture during normal core handling and processing, resulting in scrapping of the core. The problem of weak knit line formation has persisted for certain core designs even though high die fill speeds (e.g. less than 150 milliseconds), high ceramic compound temperatures (e.g. 290 degrees F.), and high pack pressures (e.g. 2000 psi) have been used in past attempts to overcome the problem.
Pursuant to an embodiment of the invention, a [0031] local region 30 of die 10 or die 12, or both, proximate the trailing edge channels 14 d is heated prior to and during introduction of the fluid ceramic material or compound followed by cooling of the local region(s) 30 before removal of a ceramic core from the molding cavity 14. In an illustrative embodiment of the invention offered for purposes of illustrating and not limiting the invention, the local region 30 of one or both of the dies is heated/cooled by one or more Peltier thermoelectric elements 40 disposed on In FIG. 2, thermoelectric element 40 is shown in detail while a similar thermoelectric element 40′ is shown schematically by dashed lines. The invention can be practiced with one more thermoelectric elements 40 on die 10 or on die 12, or on both dies 10, 12, proximate to the respective local region(s) 30 including the trailing edge channels 14 d. If thermoelectric elements 40 are provided on both dies 10, 12, they typically will be of the same in type. Only the thermoelectric elements 40 on die 10 will be described below for sake of convenience, it being understood the thermoelectric element(s) on die 12 would be similar.
Referring to FIGS. [0032] 2-3, die 10 is machined or otherwise formed to include one or more grooves 14 j (two grooves shown in FIG. 3) configured to receive a respective Peltier semiconductor thermoelectric element 40 that can provide a heating effect or cooling effect depending the direction of electrical current flow through the element 40 from an electrical power controller S, such as a voltage controller that can provide a desired selected voltage magnitude and polarity. The grooves 14 j are oriented at an angle to the parting surface PS of the die 10 such that surface 14 s 1 of the grooves 14 j is generally parallel to and spaced about {fraction (5/32)} inch from the trailing edge region 14 b of the molding cavity for purposes of illustration and not limitation.
Each Peltier thermoelectric (PTE) [0033] element 40 comprises a commercially available PTE element and includes a thermally conductive dielectric plate 40 a in thermal contact with the adjacent die groove-forming surface 14 s 1, a thermally conductive dielectric plate 40 b in thermal contact with a heat exchanger 42, and a plurality of semiconductors 40 c therebetween as is known. Suitable PTE elements 40 for practicing the invention are commercially available from Melcor Corporation, Trenton, N.J. A thermally conductive boron nitride paste, aluminum nitride foil, or other thermally conductive material preferably is placed between plate 40 a and die surface 14 s 1 and plate 40 b and heat exchanger 42. The boron nitride paste is available commercially from Advanced Ceramics Corporation, Cleveland, Ohio. The aluminum nitride foil is available commercially from Melcor Corporation, Trenton, N.J.
Each [0034] heat exchanger 42 comprises a hollow metal (e.g. copper) or other thermally conductive material manifold communicated to cooling fluid inlet conduit 42 a and cooling fluid outlet conduit 42 b. The heat exchanger 42 can include internal serpentine passages (not shown) for flow of the cooling fluid therethrough. The conduits 42 a, 42 b are received in a primary groove 10 b and secondary grooves 10 c of the die 10 extending perpendicular to primary groove 10 b. The vent passages 14 p communicate to grooves 10 b, 10 c to vent air from mold cavity 14 during filling thereof with ceramic core material. The cooling fluid can comprise compressed air, water, or other fluid such that the inlet conduit 42 a is connected to a source of cooling water (e.g. shop water) or compressed air (e.g. compressed shop air) or other close or open loop fluid system. The outlet conduit 42 b for compressed air can be communicated to ambient air. If a liquid (e.g. water) is used, the outlet conduit 42 b is connected to a conventional sewer water drain or provided in a closed loop recirculation system.
An alternative embodiment of the invention shown in FIG. 3A omits the [0035] heat exchanger 42 and places thermally conductive material 45′, such as for example only boron nitride paste or aluminum nitride pad, between the plate 40 b, and the adjacent die groove-forming surface 14 s 2 in FIG. 2 to provide thermal contact therebetween. The boron nitride paste is available commercially from Advanced Ceramics Corporation, Cleveland, Ohio. The aluminum nitride pad or foil is available commercially from Melcor Corporation, Trenton, N.J. In FIG. 3A, like features of FIGS. 1-3 bear like reference numeral primed.
A [0036] thermocouple 50 is positioned in a bore in the die 10 to monitor the temperature of the local die region 30. The thermocouple 50 is connected to power controller S, such as the voltage controller described above, to provide feedback signals representative of sensed die temperature at the local die region 30 to the voltage controller. The thermocouple can be positioned at any position between surface 14 s 1 and region 14 b in die 10 and/or 12 to this end. The voltage controller can be connected to the injection machine microprocessor controller MC so that thermal cycling (heating/cooling) of the local die region 30 pursuant to the invention can be coordinated with the fill stage and hold stage of press operation. The voltage controller S is connected by lead wires W1, W2 to PTE elements 40 to provide a voltage magnitude and polarity to the PTE elements 40 to heat or cool the local die region 30 depending upon the stage of operation of the injection molding press.
For example, pursuant to an embodiment of the invention, [0037] local region 30 of die 10 (and/or die 12) is heated prior to and during the filling stage until the fluid ceramic core material fills the molding cavity 14. Heating of the local region 30 by the PTE elements 40 is controlled by power controller S to provide an elevated superambient temperature at the local die region 30 that will substantially prevent premature solidification of the liquid ceramic slurry before it fills the trailing edge channels 14 d. That is, the ceramic material remains fluid until the channels 14 d are filled. The amount of heat energy that must be supplied to the local regions 30 by the PTE elements 40 will vary in dependence on the composition and temperature of the ceramic core material being introduced, ambient air temperature, die temperature, thermal conductivity of the die material, and core/die geometric factors and can be determined empirically for given core molding parameters.
For purposes of illustration and not limitation, the [0038] local die region 30 can be heated to a temperature of 160 degrees F. and above for a ceramic compound of the type described in U.S. Pat. No. 4,837,187 having a solidification temperature in the range of 155 to 90 degrees F. and at a injection temperature of 290 degrees F. injected into steel dies at a flow rate of 11.5 cubic inches/second to achieve filling of trailing edge channels 14 d of the type illustrated in FIGS. 1-2 with ceramic compound such that the aforementioned mechanically weak weld or knit lines are completely eliminated at the trailing edge ceramic ribs R, FIG. 4.
At an empirically determined point after the fill stage, the [0039] PTE elements 40 are controlled to provide a cooling effect, rather than a heating effect, at the local die region 30 to cool that region to a suitable lower core ejection temperature that will permit removal of the molded core C from molding cavity 14 by movement of ejector pins EP without sticking of the trailing edge region 14 b to the surfaces of the molding cavity and without damage to the green core. To this end, the voltage provided to the PTE elements 40 is reversed in direction and controlled by power controller S to provide a lower ejection temperature at die region 30. A suitable ejection temperature to avoid sticking of the molded core (or molded pattern) to the molding cavity and core damage, such as breaking, cracking, and/or distortion of the molded core, can be determined empirically for a given core (or pattern) molding operation. A typical second lower ejection temperature of the die region 30 can be 85 degrees F. for the above ceramic core compound and molding parameters described above to mold core C.
The above described cycling of the local die region between the first superambient temperature and second lower ejection temperature enables molding of ceramic cores C of the type shown in FIG. 4 as well as other cores with adequate green core strength and reduced scrapped cores due to the presence of weakened weld or knit lines at the ribs R and without sticking problems when the core is removed from the [0040] molding cavity 14. Moreover, reduced die fill speeds (e.g. greater than 150 milliseconds), reduced core material temperatures, and reduced pack pressures (e.g. less than 2000 psi) may be used. Reduced fill speeds are advantageous to reduce wear of dies 10, 12 and reduce entrapped air in the molded core or pattern.
After the green (unfired) core C is removed from the dies [0041] 10, 12 it is sintered at elevated temperature in conventional manner to achieve consolidation of the ceramic powder particles by heating to impart strength to the core for use in the investment casting process. Sintering of the green ceramic core is achieved by means of heat treatment to an elevated temperature based on the requirements of the ceramic powders employed. Above U.S. Pat. No. 4,837,187 describes thermal processing of an alumina based ceramic core. The particular thermal processing technique forms no part of the invention as conventional thermal processing techniques can be used to make the fired, porous ceramic core C, FIG. 4.
The invention has been described above with respect to use of [0042] PTE elements 40 to heat and then cool the local die region 30 since these elements are durable, compact and relatively rapidly heat and cool the local die region 30 to accommodate short cycle times of the injection molding machine. The invention is not so limited as other heating and cooling devices or techniques may be used depending upon cycle times of the molding machine employed. For example, hot fluid heating and cold fluid cooling of the die region 30 using proximate oil or water passages in the dies 10 and/or 12 or mold bases 27 a, 27 b may be used in the event that longer machine cycle times are acceptable. Referring to FIG. 3B where like features of previous figures are represented by like reference numerals double primed, lower die 10″, is shown including a water, oil or other fluid passage P1″ to heat and cool die region 30″ proximate the trailing edge region 14 b″ of molding cavity 14″ and a similar water, oil, or other fluid passage P2 ″ to heat and cool die region 31″ proximate a leading edge region 14 a″ of molding cavity 14″. The locations of the passages P1″, P2″ in the die 10″ and/or die 12″ proximate to regions 30″ and 31″ can be selected empirically to provide heating and cooling thereof as described above to achieve the benefits of the invention. A manifold M″ can supply the water, oil or other fluid to passages P1″, P2″. The manifold M″ is communicated by lines or conduits L1″, L2″ alternately to a fluid heater H″ (e.g. a 18 kilowatt electrical hot water heater) to provide hot fluid at a suitable temperature (e.g. hot water at 140 to 160 degrees F.) to passages P1″, P2″ to heat die regions 30″, 31″. The manifold M″ then is communicated to a fluid chiller CH″ (e.g. conventional water chiller) to provide cooled fluid at a suitable temperature (e.g. cold water at 45 degrees F.) to passages P1″, P2″ to cool die regions 30″, 31″ to the ejection temperature as described above. After the regions 30″, 31″ are cooled down to the core ejection temperature, fluid flow through passages P1″, P2″ can be terminated. The manifold M″ and passages P1″, P2″ are connected in closed loop manner by return lines LR″ with the fluid heater H″ and fluid chiller CH″ with conventional valves V″ provided and controlled in a manner to alternately communicate the manifold M″ to the heater H″ or chiller CH″ as needed to heat and then cool die regions 30″, 31″. The fluid supply lines or conduits L″, L2″ can include conventional check valves (not shown) to prevent reverse flow. The heater H″ and chiller CH″ are controlled in response to temperature sensed by thermocouple 50″. Although not shown, the upper die (not shown) can include similar fluid passages as passages P1″, P2″ to this same. Either one or both of dies 10″, 12″ can includes such fluid passages.
Those skilled in the art will appreciate that the invention can be practiced using a combination of the [0043] thermoelectric elements 40 and one or more fluid passages described above to heat and cool one or more hard-to-fill regions of the molding cavity 14 in practice of the invention.
Moreover, although the invention has been described above with respect to heating and cooling of one or more hard-to-fill regions of the [0044] molding cavity 14, it is not limited in this manner in that the dies 10 and/or 12 can be generally, rather than locally, heat and cooled to improve filling of the molding cavity. For example, oil, water or other fluid passages can be provided throughout one or both dies 10, 12 in a configuration necessary to provide general heating and cooling of the die(s) to heat and cool one or more hard-to-fill regions of molding cavity 14 pursuant to another embodiment of the invention to improve filling thereof.
In addition, the invention can be practiced in manufacture of a solid fugitive pattern, or a fugitive pattern injected about the ceramic core C. For example, the ceramic core C of FIG. 4 typically is placed between pattern molding dies (not shown) forming a pattern molding cavity, and then molten pattern material, such as wax, is injected under pressure about the core in the pattern molding cavity. Such a procedure is described in U.S. Pat. No. 5,296,308 and a filled pattern wax material employed to form a pattern is described in U.S. Pat. No. 5,983,982, the teachings of both of which patents are incorporated herein by reference. Pattern materials can be selected from pattern wax materials, pattern polymer materials (e.g. polyurethane, polystyrene, and others) and others known in the lost wax investment casting art to produce a fugitive pattern that is invested in a ceramic shell mold and then subsequently removed thermally or by other means from the shell mold. The trailing edge of the fugitive pattern is molded to fill the spaces between the ceramic ribs R of the core C, FIG. 4, and form a thin cross-section trailing edge region on the pattern. [0045]
The invention envisions heating and cooling at least the trailing edge region of the pattern molding cavity in a manner similar to that described above for the [0046] core molding cavity 14 to insure complete filling of the spaces between the ceramic ribs R and other details of the trailing edge region without damaging the core while the pattern material (e.g. wax) remains molten and then cooling to a second lower pattern ejection temperature to permit removal of the fugitive pattern without sticking to the die surfaces. The invention envisions similarly heating/cooling other regions of the pattern molding cavity, such as the leading edge region, as necessary to fill one or more hard-to-fill regions thereof. The invention also envisions generally, rather than locally, heating and cooling of one or both pattern molding dies to this same end.
Although the invention has been described with respect to certain embodiments thereof, those skilled in the art will appreciate that the invention is not limited to these embodiments and changes, modifications, and the like can be made therein within the scope of the invention as set forth in the appended claims. [0047]

Claims

We claim:

1. A method of making a molded body for use in investment casting, comprising filling a molding cavity disposed between cooperating dies with a fluid material selected to form a ceramic core or a fugitive pattern, heating at least a region of at least one of said dies to a superambient temperature during filling of said molding cavity, and cooling said local region to a lower temperature before removal of a molded body from said molding cavity.

2. The method of claim 1 wherein said region of said at least one of said dies is heated and cooled by at least one thermoelectric element disposed thereon.

3. The method of claim 2 including sensing temperature of said region and controlling said at least one thermoelectric element in response to sensed temperature.

4. The method of claim 2 wherein said region comprises a trailing edge-shaped region of an airfoil, said trailing edge-shaped region being heated prior to and during filling of said molding cavity and cooled before removal of said molded body from said molding cavity.

5. The method of claim 4 wherein said trailing edge-shaped region includes relatively narrow channels that are heated by said at least one thermoelectric element to permit filling thereof with said fluid material.

6. The method of claim 1 wherein said region of said at least one of said dies is heated and cooled by at least one fluid passage disposed therein.

7. The method of claim 2 including removing heat from said at least one thermoelectric element using a heat exchange fluid.

8. The method of claim 7 wherein said heat exchange fluid is selected from the group consisting of a liquid and gas.

9. The method of claim 1 wherein said fluid material comprises a ceramic core material comprising ceramic flour and a fluid binder.

10. The method of claim 1 wherein said fluid material comprises a pattern material selected from the group consisting of a wax and a polymer.

11. Apparatus for molding an airfoil-shaped body for use in casting a metallic airfoil, comprising first and second dies defining a molding cavity having an airfoil-shaped cavity, and at least one thermoelectric element disposed on at least one of said dies to heat at least a hard-to-fill region of said cavity to a superambient temperature during filling of said molding cavity with a fluid material, and to cool said region to a lower temperature before an airfoil-shaped body is removed from said molding cavity.

12. The apparatus of claim 11 including a temperature sensor proximate said region and an electrical power controller connected to said at least one thermoelectric element to control electrical power thereto in response to sensed temperature.

13. The apparatus of claim 11 including a heat exchanger in thermal contact with said thermoelectric element.

14. The apparatus of claim 13 including an inlet conduit conducting a heat exchange fluid to said heat exchanger to remove heat therefrom and an outlet conduit for exhausting said heat exchange fluid.

15. The apparatus of claim 11 including a thermally conductive material disposed between said at least one thermoelectric element and said at least one of said dies for conducting heat from said at least one thermoelectric element.

16. The apparatus of claim 11 wherein said molding cavity has a configuration of a ceramic core.

17. The apparatus of claim 11 wherein said molding cavity has a configuration corresponding to a pattern that replicates said airfoil-shaped body.

18. Apparatus for molding an airfoil-shaped body for use in casting a metallic airfoil, comprising first and second dies defining a molding cavity having an airfoil-shaped cavity, and at least one fluid passage on at least one of said dies to provide a fluid to heat at least a hard-to-fill region of said cavity to a superambient temperature during filling of said molding cavity with a fluid material, and to cool said region to a lower temperature before an airfoil-shaped body is removed from said molding cavity.

19. The apparatus of claim 18 wherein said molding cavity has a configuration of a ceramic core.

20. The apparatus of claim 18 wherein said molding cavity has a configuration corresponding to a pattern that replicates said airfoil-shaped body.