Method for making molds and mold components for casting single crystal metallic articles

Disclosed is an improved method for making molds and mold components for use in casting single crystal articles. According to the improved method, a preformed and reusable replica of a mold growth cavity and crystal selector passage is provided, the replica including a cylinder in the form of the desired growth cavity and a helical strand in the form of the desired crystal selector passage, one end of the strand being attached to an end of the cylinder such that their axes are coincident. In one embodiment, the replica is coated with a thin layer of wax or other removable material and then utilized in a variety of mold making processes, including investment molding, to form a growth zone and helical crystal selector in the mold. After mold making, the embedded replica is removed from the mold by first removing the thin layer of wax to provide a clearance space between the replica and mold and then rotating the cylinder to unscrew the helical strand from the mold in the same manner that a threaded member is removed from a threaded hole. The replica is then reused to produce additional molds or mold components having a reproducible growth zone and helical crystal selector therein.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is related to methods for making molds and mold 
components for use in casting single crystal metallic articles. 
2. Description of the Prior Art 
In casting single crystal metallic articles, a mold having a crystalline 
growth zone or cavity at the lower end and a narrow passage above and in 
communication with the growth zone is widely used as, for example, in the 
Piearcey patent, U.S. Pat. No. 3,494,709 of common assignee herewith. 
Generally, the mold growth zone has an open bottom so that when the mold 
is placed on a chill plate during casting, the molten metal therein will 
be unidirectionally solidified to cause columnar grain growth toward the 
passage. The narrow passage functions to select a single crystal from 
among the numerous crystals growing upwardly through the mold. In the 
past, it has been common for the passage to take the form of a helix of 
small cross-section as compared to the growth cavity, such a passage being 
illustrated in U.S. Pat. Nos. 3,625,275; 3,627,015; 3,667,533; 3,690,368; 
3,700,023 and 3,712,368, all of common assignee with the present 
invention. 
In one prior art practice, the molds used in single crystal casting are 
made by the well known shell molding or "lost wax" process in which layers 
of ceramic material are deposited on a disposable pattern such as a wax 
pattern. The wax pattern generally comprises an article portion, a helix 
portion and a growth zone portion, the helix portion being wax welded by 
hand between the others. It is the assembled wax portions which are then 
covered with the ceramic layers to form a shell mold therearound. After 
the desired thickness of ceramic shell is deposited, the wax pattern is 
melted out, leaving behind a ceramic shell mold having an upper article 
cavity connected to a lower growth cavity by a helical passage of small 
cross-section. 
As a result of the small cross-section and consequent structural weakness 
of the wax helix, problems have arisen during assembly of the wax pattern 
portions and during shell molding in the form of distortion or breakage of 
the helix. These problems require operating personnel to use utmost care 
in these operations; however, notwithstanding such care, distortion and 
breakage of the helix occur all too frequently and result in lower 
production and increased casting costs. 
In another prior art practice, molds for casting single crystals are 
provided by assembling a precast central mold element on strongback 
between precast outer mold elements as shown in detail in the Hayes and 
Phipps patent, U.S. 3,965,963 of common assignee herewith. In this method, 
the mold elements are preformed or precast by injecting ceramic slurry 
into a suitably configured molding cavity. 
SUMMARY OF THE INVENTION 
The present invention provides an improved method for making molds and mold 
components for casting single crystal metallic articles. 
An important feature of the invention is a preformed and reusable replica 
of a mold growth cavity and a crystal selector passage which replica can 
be used in a variety of processes to produce molds and mold components 
having a reproducible growth zone - crystal selector configuration 
therein. Typically, the replica includes a cylinder of predetermined size 
for the mold growth zone and a helical strand suitably sized and 
configured to form a helical crystal selector passage in the mold or mold 
component, the strand having a small cross-section as compared to that of 
the cylinder and having one end attached to an end of the cylinder such 
that the axis of the helix is substantially coincident with that of the 
cylinder. As explained below, it is important that the axes of the helical 
strand and cylinder be substantially coincident to insure easy removal of 
the replica from the formed mold. Preferably, to provide optimum 
structural strength and service life, the replica is fabricated from 
metallic members such as, for example, an aluminum cylinder and an 
aluminum helical strand welded in the prescribed axial relationship to one 
end of the cylinder. 
The replica can be used in a wide variety of mold making processes. 
According to one embodiment of the invention, the replica is used in 
conjunction with the investment molding process to produce an open ended 
mold for single crystal casting, the mold including a growth zone or 
cavity at its lower end and a helical crystal selector passage leading 
upwardly from the growth zone to an article cavity. As is usual in the 
prior art, the mold growth zone is open at the bottom to expose the molten 
metal therein to the chill plate during casting. In this embodiment, the 
replica is first coated with a thin layer of removable material such as 
wax and the coated replica is then joined to the lower end of a disposable 
pattern of the article to be cast. Preferably, joining is effected by 
placing the wax coated replica in a suitably shaped mold and injecting a 
wax pattern of the article thereagainst, the pattern being formed and 
attached to the replica in the same operation. Thereafter, ceramic 
material, such as ceramic slurry, is deposited on the structure of replica 
and wax pattern until the desired thickness for a mold wall is obtained. 
Since the mold is to have an open bottom, it is preferable that no ceramic 
be deposited on the end of the cylinder opposite the helical strand. The 
embedded replica is then removed from the formed mold in essentially two 
steps. First, the thin layer of wax or other disposable material is 
removed from the embedded replica to provide a clearance space between the 
replica and ceramic mold. Then, the exposed end of the cylinder is rotated 
to unscrew the helical strand from the mold in the same manner that a 
threaded member is removed from a threaded hole. To insure satisfactory 
unscrewing of the helix from the shell mold, it is important that the 
prescribed axial relationship between the helical strand and cylindrical 
body be provided. After removal, the replica can be reused in making other 
molds. 
In another embodiment of the invention, the preformed replica can be used 
in somewhat similar fashion in ceramic injection processes to make a 
ceramic crystal selector insert which, in conjunction with other preformed 
ceramic mold components, provides a complete mold for casting single 
crystal articles. 
Or, alternately, the replica may be constructed such that the helical 
strand is tapered to provide a gradually increasing cross-section toward 
the cylinder and this replica used in ceramic injection processes without 
the need for initially wax coating the replica. Removal of such replica is 
effected by rotating the exposed end of the cylinder as above, the taper 
or draft on the helical strand permitting easy unscrewing of the strand 
from the formed insert. 
These and other advantages and objects of the invention will become more 
fully apparent from the following drawings and detailed description of the 
preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now in detail to the present preferred and illustrative 
embodiments of the invention, a preformed and reusable replica of a mold 
growth zone and helical crystal selector for use in making molds and mold 
components for casting single crystal metallic articles is shown in FIG. 
1. The replica includes right cylinder 2 of predetermined size to form a 
suitable growth zone or cavity in the mold and a helical strand 4 suitably 
configured and sized to form a helical crystal selector passage in the 
mold, one end of the strand 4 being attached to the top end of the 
cylinder in such a manner that the axis of the helix substantially 
coincides with the axis of the cylinder. As illustrated, the cross-section 
of the strand is preferably circular and of small diameter as compared to 
that of the cylinder. It is important that the radius of curvature of the 
helix be greater than the radius of the strand itself so that there is no 
possibility for direct vertical growth of a crystal from the growth zone 
through the helical passage. Although not essential to the invention, it 
is preferred that the replica be constructed of metallic members, for 
example, an aluminum cylinder and aluminum helical strand welded to the 
end of the cylinder, as shown in FIG. 1, have proved very durable and free 
of significant distortion after extensive use in making molds and mold 
components. Minimization of distortion of the replica, especially the 
helical strand, is highly important since selection and growth of a single 
metallic crystal can be adversely affected by minor dimensional changes in 
the helix. Those skilled in the art will recognize, however, that other 
materials may find use in fabricating such replicas including, but not 
limited to, plastic, ceramic, wood and the like. Since the replica can be 
readily made to very close tolerance, it is apparent that the growth zone 
and helical crystal selector in the mold can likewise be provided to very 
close tolerance. This precise control ensures that crystal selection is 
optimized from one mold to the next during casting. 
The preformed and reusable replica can be used in a variety of mold making 
processes such as investment molding including the "lost wax" process, 
high and low pressure ceramic injection processes, and the like. 
Regardless of the mold making process utilized, the first step in one 
embodiment of the invention is to coat the replica with a thin layer 5 of 
removable material such as low melting point wax. If wax is used, a layer 
of 1/8 inch thickness has been found satisfactory. Since the bottom of the 
mold growth zone is to be open, the end 6 of the right cylinder is 
preferably covered by a flat plate which can be easily removed after 
investment molding. 
In investment molding processes, the wax coated replica is then joined to a 
disposable pattern or patterns of the article to be cast. The pattern is 
typically wax and can be united with the replica in a number of ways, 
including manual assembly. A preferred technique, however, is to expose 
the end 7 of the strand opposite the cylinder to a suitable pattern 
molding cavity and inject molten wax therein against end 7 to not only 
form the pattern but also unite it with the replica as the wax solidifies. 
A typical injected structure is shown in FIG. 2 and comprises the wax 
coated replica, including cylinder 2 and helical strand 4, and pattern 8 
which, by way of illustration, is shown as a turbine blade having a pour 
cup 10 attached to the top portion. Thereafter, the united structure of 
replica and pattern is invested in ceramic to form a mold therearound. In 
the "lost wax" or shell investment process, the structure is repeatedly 
dipped in ceramic slurry and dusted with ceramic particulate to form a 
ceramic shell 12 thereon, FIG. 3, the mold being shown with a base or 
flange portion 14 which rests on the chill plate (not shown) during 
casting. Since the mold is to have a growth zone with an open bottom, 
preferably no ceramic is deposited on the end 6 of cylinder 2 opposite the 
helical strand. Deposition is prevented if the end is covered by the flat 
plate already mentioned. In the solid investment molding process, the 
united structure is surrounded by a flask or other suitable container and 
ceramic mold slurry is poured into the flask around all but the end 6 of 
the cylinder. In either investment process, after the ceramic material 
hardens, the replica is removed to provide the desired growth zone and 
helical crystal selector passage in the mold. 
Removal of the embedded replica is effected in two steps including first 
removing the thin layer of wax or other disposable material on the replica 
to provide a clearance space between the replica and ceramic mold and then 
rotating the exposed end of the cylinder to unscrew the helical strand 
from the mold in the same manner that a screw or other threaded member is 
removed from a threaded hole. It is essential for easy removal without 
damage to either the mold or the replica that the axis of the helix be 
substantially coincident with the axis of the right cylinder. During or 
after replica removal, the pattern 8 having pour cup 10 is removed to 
provide a ceramic mold for casting single crystal metallic articles. 
Removal of the wax layer and pattern is accomplished by conventional means 
such as heating, microwave dewaxing, dissolution and the like. Of course, 
the replica can thereafter be used in making additional molds for casting. 
It is also possible to use the preformed replica to make a mold component 
such as a ceramic crystal selector insert which, in conjunction with other 
preformed mold mold components, provides a complete mold for casting 
single crystal objects. Such an insert can be made by placing the wax 
coated replica in a ceramic injection mold of suitable shape and at high 
pressure or low pressure injecting ceramic slurry into the mold around the 
replica, except for end 6. Thereafter when the ceramic hardens, the 
replica is removed by the two step process described hereinabove. The 
resultant ceramic crystal selector insert 16 of FIG. 4 can then be 
combined with mold elements like those discussed in the Hayes and Phipps 
patent, U.S. Pat. No. 3,963,965, to provide a complete mold for casting 
single crystal articles. In such a case, the thickness of the ceramic 
insert in the area of the helix and growth zone can be increased to 
provide an insulating effect to improve the thermal gradient and minimize 
spurious grain nucleation. Alternately, the ceramic crystal selector 
insert can be united in a pattern mold with a wax or other disposable 
pattern of the article to be cast and the structure of insert and pattern 
then invested in ceramic to form a mold therearound, such a technique 
being described in the copending application entitled "Single Crystal 
Casting Mold and Method for Making Same" by Douglas R. Hayes. Again, 
removal of the replica from the formed mold is as described hereinabove. 
In another embodiment of the invention, the replica is constructed such 
that the helical strand exhibits a gradually increasing cross-section from 
end 7 toward the cylinder 2. This taper or draft permits the replica to be 
utilized in mold making processes, especially ceramic injection processes, 
without the need for first coating the replica with the thin layer of wax. 
After the mold component is formed, the replica is removed as described 
above by rotating the exposed end 6 of the cylinder, the draft on the 
helical strand permitting ready unscrewing of the strand from the 
component. The formed mold component can then be used in conjunction with 
other preformed components or united with a wax pattern as described 
above. 
It is now apparent that the present invention is capable of providing large 
numbers of molds and mold components having a reproducible growth zone and 
helical crystal selector therein for casting single crystal articles. This 
feature is highly advantageous in producing large numbers of single 
crystal castings with automated foundry equipment and techniques since 
overall quality is improved while, at the same time, manual operations and 
mold and casting rejections are reduced to lower casting costs. 
Although the invention has been shown and described with respect to 
illustrative embodiments thereof, it will be understood by those skilled 
in the art that changes and additions in the form and detail thereof may 
be made without departing from the spirit and the scope of the invention.