Process for producing hollow cast article

A process for producing a hollow metal cast product having a desired interior contour is provided. The process comprises the steps of preparing a first lost model having an outer contour substantially corresponding to a desired interior contour of the finished product, depositing a metallic or ceramic material or a mixture thereof over the surface of said first lost model by spraying to form a layer defining a hollow core block, placing said hollow core block in a first mold, pouring or injecting a material for forming a second lost mold into said first mold to form a second lost model, coating a refractory material over said second lost model to form a second mold for casting, removing second lost mold to form a cavity, casting a molten metal or alloy into said cavity, and staving said second mold to take out a finished product.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a process for producing a cast 
article having a hollow cavity, and more particularly to a process for 
producing a hollow cast article having a desired interior contour defined 
by a very smooth inner wall surface. 
2. Related Art Statement 
A core mold has conventionally used in the prior art casting process for 
producing a hollow cast article having an interior cavity surrounded by a 
smooth inner wall surface, particularly when the interior cavity has a 
complicated contour. In case where it is desired that the cast product has 
an extremely smooth inner wall surface defining the interior cavity, a 
ceramic mold has been hitherto been used as the core mold. In general, 
such a core mold is prepared initially by molding a green core mold from a 
mixture of an aggregate, such as alumina or zirconia, and a binder, such 
as ethyl silicate, and then baking the green core mold. However, it is 
difficult to prepare a baked core mold of precise dimension, since the 
green material therefor tends to shrink or expand during the baking step 
to loose its dimensional stability. Other problems of the conventional 
ceramic core mold are that the quality thereof becomes often irregular and 
that the production efficiency thereof is inferior leading to high 
production cost. 
OBJECT AND SUMMARY OF THE INVENTION 
Accordingly, the principal object of this invention is to provide a process 
for producing a hollow metal cast product having an inner cavity 
surrounded by a smooth wall surface at a high production efficiency and at 
high dimensional stabilty. 
Another object of this invention is to provide a hollow metal cast product 
without the use of a ceramic core mold which requires preliminary baking 
step. 
Other objects and advantages of this invention will become apparent from 
the following detailed description. 
According to the present invention, there is provided a process for 
producing a hollow metal cast product comprising the steps of preparing a 
first lost model having an outer contour substantially corresponding to a 
desired interior contour of the finished product, depositing a molten 
metal or alloy over the surface of said first lost model by spraying to 
form a metal layer defining a hollow core block, placing said hollow core 
block in a first mold, pouring or injecting a material for forming a 
second lost model into said first mold to form a second lost model layer, 
coating a refractory material over said second lost model layer to form a 
second mold for casting, removing said second lost model to form a cavity, 
casting a molten metal or alloy into said cavity, and staving said second 
mold to take out a finished product.

DESCRIPTION OF THE INVENTION 
The present invention will now be described in detail with reference to 
preferred embodiments thereof. 
Referring to FIGS. 1 and 2, prepared at the first step (step 100) is a 
first lost wax model 10 having an outer contour which is agreed with the 
desired interior contour of the finished cast product. This first lost wax 
model 10 may be molded by any known methods, such as an injection molding 
process, and may be made of a plastic material which may be melted by 
heating. Preferable examples of the material for the first lost wax model 
include natural and synthetic waxes such as paraffin wax and water-soluble 
waxes, and moldable synthetic resins such as polystyrene, urea resin and 
foamed polystyrene. The thus prepared first lost wax model is carried by a 
proper support to be ready for the next step. 
The next step (step 102, see FIG. 2(A)) is the step of depositing a molten 
metallic material or ceramic material over the surface of the first lost 
wax model by spraying to form a metallic or ceramic layer 12. The metallic 
material used in this step is not particularly limited, and various metals 
and alloys may be used singly or in combination. Alternatively, the layer 
12 may be made of a mixture of one or more metallic materials with one or 
more ceramic materials. When it is desired to form the layer 12 by two or 
more metallic and/or ceramic materials, a mixture may be sprayed onto the 
surface of the first lost wax model 10, or one of them is first sprayed 
onto the surface of the first lost wax model followed by successive 
spraying of the remaining metallic or ceramic materials to form a 
multi-plied coating. 
The metallic or ceramic material may be deposited through a variety of 
spray-depositing methods. For example, when the layer 12 is made of a 
metallic material, a selected metal wire is melted by electric spark and 
blown by compressed air to be sprayed onto the surface of the first lost 
wax model. Alternatively, a selected metal wire is melted by high 
temperature flame and blown by compressed air to be sprayed onto the 
surface of the first lost wax model. In a further modified method, a metal 
powder or a mixture of metal powders is melted by high temperature flame 
and blown to be sprayed onto the surface of the first lost wax model. A 
so-called plasma metal spraying method may also be used in this step 102. 
It is desirous that either one of the molten metal spray gun or the first 
lost wax model is continuously moved to prevent a certain portion of the 
outer periphery of the lost wax model from being heated excessively by the 
applied molten metal to a temperature at which the lost wax model is 
melted. 
This step 102 may also be carried out in an electric field to improve the 
effective use of the sprayed metallic material. In such a method, the 
first lost wax model 10 is made of an electrically conductive material or 
the surface thereof is coated with a conductive material. The first lost 
wax model may be electrically conductive, for example, by using a wax 
mixture containing a surfactant or emulsifier and water uniformly 
dispersed in a commercially available wax. The surface of the first lost 
wax model may be electrically conductive, for example, by coating a 
conductive material, such as a mixture of a surfactant and water. Then, a 
molten metallic material is sprayed out of the nozzle of a spray gun while 
a DC potential is applied between the gun and the surface of the first 
lost wax model. Whereupon, the fine particles or mists of molten metallic 
material are charged with electricity of one polarity and thus attracted 
by the first lost wax model 10 charged with electricity of the other 
polarity, so that the ratio or part of the blown metallic material 
particles wastefully consumed without depositing on the surface of the 
model 10 can be decreased, or in other words, to increase the ratio of the 
metallic material which has been effectively used to form the metallic 
coating or layer 12. 
It is desirous that the molten metal spraying is implemented in an inert 
gas atmosphere when an easily oxidizable metal or alloy is used. For 
example, when a molten magnesium or titanium alloy is sprayed onto the 
surface of the first lost wax model, the molten alloy is sprayed in an 
inert gas atmosphere, such as argon gas atmosphere. It is preferred that 
the same inert gas as that used to form the inert gas atmosphere is used 
as the gas for blowing the molten metallic material from the spray gun. 
Although a single metallic or ceramic material may be sprayed by a single 
step or a mixture of metallic and/or ceramic materials may be sprayed by a 
single step until a layer 12 having a desired thickness is formed, two or 
more different metallic or ceramic materials may be deposited through 
plural separate spraying steps so that a multi-plied coating or layer 12 
having a desired thickness is formed. 
In the next step (step 104, see FIG. 2(B)), the first lost wax model 10 
applied with the coating or layer 12 is placed in position in a first mold 
14. Then, a material for forming a second lost wax model is injected or 
otherwise filled in the cavity 16 defined between the layer 12 and the 
inner periphery of the first mold 14 (step 106). At this step 106, a 
second lost wax model 18 is molded so that it surrounds the outer surface 
of the spray-deposited layer 12. When the second lost wax model 18 is 
molded by an injection molding process, it is desired that a material for 
the second lost wax model 18 has a melting point which is lower than that 
of the wax material used for forming the first lost wax model 10 in order 
that the first lost wax model 10 is not melted during this step 106. 
The composite model including the first and second lost wax models 10, 18 
and the spray-deposited layer 12 is then coated with a refractory 
material, such as a ceramic material (step 108, see FIG. 2(C)). In detail, 
the composite model is dipped in a slurry container and then the thus 
wetted composited model is applied with stacco particles. The cycle of 
dipping in the slurry container and applying with stacco particles is 
repeated until the ceramic shell mold 20 has a desired thickness. 
The next step (step 110) is a dewaxing step of heating to melt and remove 
the first and second lost wax models 10, 18. Then, the ceramic shell mold 
20 is baked to obtain a baked ceramic shell mold (step 112, see FIG. 2(D)) 
in which the spray-deposited layer 12 is fixed in position. 
In the next step (step 114), a molten metal is cast in the cavity 16A 
corresponding to the cavity 16 in the mold 14 and now defined by the inner 
periphery of the ceramic shell mold 20 and the spray-deposited layer 12. 
After cooling to solidify the cast metal, the shell mold 20 is removed 
(step 116). 
Unnecessary portions are then cut away by machining to obtain a finished 
product 22, as denoted by 120 in FIG. 1. Since the ends of the 
spray-deposited layer 12 are projecting from the end faces of the finished 
product in the illustrated embodiment, the projecting end portions of the 
layer 12 are cut away by shearing or milling operation (see FIG. 2(E)). 
It is preferable that the finished product is subjected to a post-heating 
treatment in order to increase the bonding force between the 
spray-deposited layer 12 and the cast metal layer. In case where the 
spray-deposited layer 12 is a multi-plied layer made of plural metallic 
materials, the physical and chemical properties of the combined metal 
layer can be improved by alloying the different metals at the interface 
thereof by this post-heating operation. 
A portion of a product produced in accordance with the process of the 
invention is shown in FIG. 3. The product shown in FIG. 3 is a casing 50 
for an electronic device having therein a spray-deposited layer 52 serving 
as a wave guide tube. This casing 50 is produced, generally following to 
the sequential steps shown in FIG. 1, by initially spray-depositing an 
electrically conductive metal to form the layer 52 having a generally 
rectangular and stepped section and then casting an aluminium alloy over 
the layer 52 by a lost wax casting process. The casing 52 having a 
relatively complex shape and a portion made of a different material as 
that forming the remaining portion may be easily produced by the 
application of the process of this invention.