Method of casting a complex metal part

A flexible and resilient positive pattern is made for a desired high temperature metal or ceramic part such as a gas turbine impeller having a complex geometry with walls defining undercut spaces, and the pattern with flexible walls is dipped into a ceramic molding media capable of drying and hardening. The pattern is removed from the media to form a ceramic layer on the flexible pattern, and the layer is coated with sand and air dried to form a ceramic layer. The dipping, sanding and drying operations are repeated several times to form a multiple layer ceramic shell. The flexible wall pattern is removed from the shell, by partially collapsing with suction if necessary, to form a first ceramic shell mold with a negative cavity defining the part and to provide for reusing the pattern. A second ceramic shell mold is formed on the first shell mold to define the back of the part and a pour passage, and the combined shell molds are fired in a kiln. A high temperature casting material is poured into the shell molds, and after the casting material solidifies, the shell molds are removed by breaking the molds.

BACKGROUND OF THE INVENTION 
In the investment casting of high temperature casting materials to form 
parts of complex geometry with walls defining undercut recesses or spaces, 
such as, for example, complex ferrous impellers for use in gas turbines, 
it is common to first construct an aluminum hub mold and an aluminum 
single blade mold by machining solid aluminum. The molds are injected with 
melted wax to form a wax hub pattern and a wax pattern for each blade 
which has compound curves. Each of the wax blade patterns is positioned on 
the wax hub pattern using a fixture, and the wax blades are welded or 
fused by heat to the wax hub which also receives a preformed ceramic pour 
funnel. 
The combined wax patterns and the pour funnel are then dipped into a 
ceramic slurry, removed from the slurry and coated with sand or 
vermiculite to form on the wax patterns a ceramic layer having 
permeability. The layer is dried, and the dipping, sanding and drying 
operations are repeated several times to create a multiple layer ceramic 
shell mold enclosing or encapsulating the combined wax patterns. The shell 
mold and wax patterns and the pour funnel are then placed within a kiln 
and fired to remove the wax and harden the ceramic shell mold and pour 
funnel. 
A molten ferrous metal or high temperature casting material is poured into 
the shell mold, and after the material hardens, the shell mold is removed 
by destroying the mold and to form an impeller capable of withstanding 
high temperatures. Since all of the wax patterns and the shell mold are 
destroyed during the production of the impeller, there is substantial cost 
to produce a number of impellers, with the result that complex impellers 
produced by the above described investment casting method are not commonly 
used in the high volume production of gas turbines and turbochargers. 
Instead, such gas turbines and turbochargers use ferrous impellers with 
less efficient and simple blade geometry which may be simply cast without 
using the above investment casting method. 
It is also known to cast or produce non-ferrous parts or impellers with 
either simple or complex blade geometry using a solid mold casting 
process. In this process, a flexible and resilient positive pattern is 
made of the part or impeller by placing a solid positive master pattern of 
the impeller into a suitable flask and then pouring a flexible and 
resilient material, such as silastic or platinum rubber material, over the 
master pattern. After the flexible material has cured, the solid master 
pattern of the impeller is removed from the flexible material to form a 
flexible mold with a reverse or negative cavity of the master pattern. A 
closed flask is then placed around the flexible mold of the master 
pattern, and a flexible and resilient curable material is poured into the 
cavity of the reverse mold. After the flexible and resilient material 
cures to form a positive flexible pattern of the impeller, the positive 
flexible pattern is removed from the flexible negative mold to form a 
positive flexible pattern of the impeller. 
The flexible pattern is then placed in an open top metal flask, and foundry 
plaster is poured into the flask. After the plaster has set up, the 
positive flexible pattern is removed from the plaster, leaving a negative 
plaster mold. The flask is removed from the plaster mold which is dried to 
remove moisture, and a non-ferrous molten material is poured into the 
plaster mold. After the non-ferrous molten material solidifies and cools, 
the plaster is removed and destroyed to produce a positive non-ferrous 
reproduction of the original part or impeller. This casting process is 
used for non-ferrous or lower temperature casting materials and cannot be 
used for producing parts of high temperature casting materials such as 
ferrous metals and titanium. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved investment casting process 
or method which is ideally suited for high volume production of parts of 
complex geometry and of a high temperature resisting material such as a 
ferrous metal or titanium impeller having a complex geometry with walls or 
blades defining undercut spaces. The investment casting method of the 
invention may also be used for casting or producing non-ferrous parts 
having simple or complex geometry, especially when it is desirable for the 
part to have precision dimensions and surfaces as provided by a ceramic 
investment casting process. 
In accordance with one embodiment of the invention for producing, or 
example, a gas turbine impeller, a positive flexible and resilient pattern 
is made of the part or impeller using the method steps as described above 
for casting a non-ferrous part and using a flexible and resilient material 
such as silastic or platinum rubber material. The positive flexible 
impeller is mounted on a metal disk which is attached to a back plate with 
a projecting handle. With a larger impeller, the flexible pattern may be 
provided with a cavity which is adapted to receive a vacuum. The flexible 
pattern is dipped into a ceramic molding media multiple times, such as 5-9 
dips, and after each dip, sand is coated on the ceramic molding media, and 
the dip layer is allowed to air dry. The repetitive dipping, sanding and 
drying operations produce a multiple layer ceramic shell mold of 
sufficient thickness to support the ferrous casting material. 
The flexible pattern of the Impeller is removed from the shell mold by 
pulling the pattern out of the mold, which results in flexing the walls of 
the pattern. With a flexible pattern for a large impeller, a partial 
vacuum may be pulled within the center cavity of the flexible pattern to 
aid in collapsing the pattern inwardly for withdrawing the pattern from 
the shell mold cavity. 
A machined or injection molded wax pattern having an outer surface 
conforming to a sprue opening and the back plate for the impeller, is 
attached by adhesive to the ceramic shell mold, and the wax pattern and 
shell mold are again dipped and redipped with sanding and air drying 
between each dip. The combined ceramic shell molds are then placed in a 
kiln and fired to remove the wax and for drying and hardening the shell 
molds. A molten ferrous metal or titanium is then poured into the combined 
shell molds, and after the metal solidifies, the combined ceramic molds 
are removed, leaving a cast ferrous or titanium reproduction of the 
original impeller. After the flexible pattern is removed from the ceramic 
shell mold, the pattern may be reused to produce many additional shell 
molds and thus many cast reproductions of the impeller. 
Other features and advantages of the invention will be apparent from the 
following description, the accompanying drawings and the appended claims.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
For purpose of illustrating the method steps of the invention, FIG. 1 
illustrates a gas turbine impeller 10 which has a complex blade geometry 
and produced in a ferrous material by the investment casting method of the 
invention. The impeller 10 includes an annular hub 12 which extends to 
form a circular base or back plate 13 having a curved upper surface 14. A 
series of peripherally spaced thin walled blades 16 are cast as an 
integral part of the impeller 10, and each blade has a compound curve and 
defines an undercut recess or space between the outer portion of the blade 
and the surface 14. It is understood that the impeller 10 is shown and 
described herein for purpose of illustrating a typical part of complex 
geometry and which cannot be produced in a ferrous material by a simple 
casting process. However, the method may be used for producing many other 
parts, especially complex parts of high temperature materials such as 
titanium and ceramic. 
Referring to FIGS. 2-4 the investment casting method of the invention 
includes the steps of first making a flexible and resilient rubber-like 
positive pattern 20 of the part or impeller 10, and the positive flexible 
pattern 20 is constructed as mentioned above by first either machining a 
positive solid master impeller or producing a solid master impeller by 3D 
stereolithography. The master impeller is placed within a suitable flask, 
and a silastic, platinum rubber, urethane or other similar material is 
poured into the flask to cover the solid master impeller. After the 
flexible material has cured, the master impeller is removed from the 
flexible mold leaving a reverse or negative cavity corresponding to the 
solid master impeller. The flexible negative mold is placed within a 
flask, and the flexible material is poured into the cavity of the flexible 
negative mold and allowed to cure. The flexible positive pattern 20 is 
then removed from the flexible negative mold. 
Referring to FIG. 2, the flexible pattern 20 may be provided with a central 
vacuum cavity (not shown) and is preferably formed around a circular 
mounting plate 23 having peripherally spaced threaded holes. The flexible 
pattern 20 is mounted on a larger parting plate 24 and secured by a set of 
screws 27, and a handle member 28 projects upwardly from the parting plate 
24. The flexible pattern 20 has thin flexible walls 29 corresponding to 
the blades 16 which project from a circular base 32. The base 32 projects 
downwardly from the parting plate 24 by a distance corresponding to a 
desired thickness of the impeller base plate 13 plus the thickness of the 
additional metal desired for machining the bottom surface of the impeller. 
By gripping the handle 28, the flexible pattern 20 is manually or 
mechanically dipped into a slurry S of ceramic material which is 
circulated within a container C by means of a motor driven agitator 34. 
The pattern 20 is dipped until the parting plate 24 contacts the surface 
of the ceramic slurry S. The pattern 20 is then lifted from the slurry 
which results in a first ceramic layer 36 forming on the outer surfaces of 
the pattern 20. After the first dipping operation, sand is coated onto the 
slurry layer 36, and the layer is allowed to air dry to form a thin gas 
permeable ceramic shell layer on the flexible pattern 20. The dipping, 
sanding and drying operations are repeated to form a second ceramic shell 
mold layer 38 and a third ceramic shell mold layer 39. Preferably, the 
dipping, sanding and drying operations continue for five to nine dips to 
form a multiple layer ceramic shell mold 45 on the flexible pattern 20. 
After the last ceramic layer of the shell mold 45 is dry, the parting plate 
24 is cleaned off around the shell mold 45, and then the flexible pattern 
20 is physically pulled out of the shell mold 45, causing the curved walls 
29 to flex. With larger parts or impellers, a vacuum may be applied to the 
cavity within the flexible pattern 20 to cause slight inward collapsing of 
the pattern and thereby permit easier removal of the flexible pattern from 
the ceramic shell mold 45. After the flexible pattern 20 is removed from 
the ceramic shell mold 45 (FIG. 4), the mold 45 defines a negative mold 
cavity 47 which corresponds into the precise shape of the flexible pattern 
20. The pattern 20 may then be used to form another ceramic shell mold. 
Referring to FIGS. 5-7, a pouring spout and base plate mold 55 is formed 
from a wax material by machining or injection molding the wax into a metal 
mold cavity, and the bottom flat surface of the mold 55 is attached to the 
upper surface of the ceramic shell mold 45 at the interface 57 by a 
suitable adhesive such as a latex adhesive. The mold 55 has an outer 
surface 59 which defines a metal pouring passage and a base 61 which 
defines a back plate for the impeller to be cast within the shell mold 45. 
The wax mold 55 and the attached shell mold 45 are then dipped into the 
ceramic slurry S, removed from the slurry, coated with sand and allowed to 
air dry to form a first ceramic coating or layer 66 on the assembly of the 
molds 45 and 55. The dipping, sanding and drying operations are repeated, 
preferably from three to five times, to add additional coatings and layers 
68 and 69 over the first layer 66, thereby forming a multiple layer 
ceramic mold 70 overlying the multiple layer shell mold 45 and the wax 
mold 55. The mold 70 is then placed within a kiln and fired to remove the 
wax mold 55 and to dry and harden the multiple layer ceramic mold 70. 
After the mold 70 is removed from the kiln, it is ready to receive a molten 
ferrous metal or other high temperature casting material which is poured 
into the cavity defined by the shell molds 45 and 70 through a passage 71 
as defined by the outer surface 59 of the wax mold 55. Since the shell 
molds 45 and 70 are ceramic, the molten material may be a high temperature 
material such as titanium or a ferrous metal or a ceramic material, all of 
which are cast at a temperature substantially higher than the temperature 
for casting a non-ferrous metal. The ceramic shell mold 45 defines the 
blades and front surface of the impeller, and the lower inner surface of 
the ceramic shell mold 70 defines the base or back plate for the impeller. 
After the casting material solidifies and hardens, the shell molds 70 and 
45 are removed by breaking the molds. The cast impeller is then machined 
to form the impeller 10 with the desired back plate 13 and to remove the 
sprue formed by the pour passage 71. 
From the drawings and the above description, it is apparent that the 
investment casting process of the invention provides desirable features 
and advantages. For example, the casting method provides for casting a 
non-draftable part or impeller of complex geometry of a high temperature 
casting material and with a reusable flexible pattern. As a result, the 
cost for producing such a high temperature resisting part or impeller is 
substantially reduced. This permits, for example, a cast ferrous impeller 
with more efficient complex blade geometry to be used to produce lower 
priced and more efficient gas turbines and turbochargers where such 
impellers are highly desirable in the gas compressor section. 
While the investment casting method herein described constitutes a 
preferred embodiment of the invention, it is to be understood that the 
invention is not limited to the precise method described, and that changes 
may be made therein without departing from the scope and spirit of the 
invention as defined in the appended claims.