Method of producing a metal mold

A method of producing a metal mold is disclosed. A heat resistant sheet is forcedly brought into close contact with a product configuration surface of a matrix such as a wooden pattern, a resin model, or the like by making use of negative pressure. The matrix is brought into contact with a melt of a low melting-point alloy while that state of close contact is being maintained and the melt is allowed to cool as it is, thereby casting one part of the mold which makes up a pair. The matrix is then removed, and by using the one part of the mold thus cast as a new matrix, this new matrix is brought into contact with the melt of a low melting-point alloy via the heat resistant rubber sheet and is allowed to cool as it is, thereby casting a counterpart of the mold that makes up the pair.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method of producing a metal mold easily 
by using a low melting-point alloy. 
2. Description of the Related Art 
Conventionally, to produce a metal mold easily, a method has been generally 
adopted in which a gypsum pattern and a sand mold are fabricated 
consecutively from a matrix by means of transfer, and a low melting-point 
alloy (mold material) such as a zinc alloy is cast into the sand mold 
("Metal Press" November 1976, pp. 16 and 17). According to this method, 
however, there is a problem that transfer processes are numerous and a 
long period is required for fabrication of the mold, resulting in a high 
cost burden. 
Accordingly, a dual forming method has been proposed in which a press 
apparatus and a melting apparatus are formed integrally, and a forming 
model (matrix) having a product thickness of such as a sheet metal model 
and provided with bores is immersed in a melt of a low melting-point alloy 
in a melting tank and is allowed to solidify as it is, thereby 
simultaneously obtaining a punch and a die (Japanese Patent Publication 
No. 7576/1973). As a modification of the dual forming method, a method is 
also known in which, by using a forming model having no bores, an outer 
surface of the forming model is brought into contact with a melt of a low 
melting-point alloy, and an alloy of a type different from the 
aforementioned alloy is poured into its inner surface and is allowed to 
solidify as it is, thereby simultaneously obtaining a punch and a die of 
different types of alloy (Japanese Patent Publication No. 15969/1978, 
Japanese Patent Laid-Open No. 55733/1976, etc.). These methods have 
already been established and put to practical use. According to these 
methods, it is possible to obtain a punch and a die simultaneously via a 
forming model. The efficiency with which metal molds are produced can be 
improved substantially over the widely practiced method using the 
aforementioned sand mold. 
However, with these methods using forming models, it is necessary to 
fabricate the forming models with high accuracy in order to ensure a 
clearance between the punch and the die. Hence, there have been drawbacks 
that the fabrication of the forming models is very troublesome, and that 
it is impossible to readily cope with design changes. 
Meanwhile, Japanese Patent Publication No. 6014/1980 discloses a method 
wherein a heat resistant resin sheet is attached to a matrix such as a 
wooden pattern, and this assembly is accommodated in a casting frame, into 
which a melt of a low melting-point alloy is subsequently cast to produce 
a female die. The resin sheet is transferred onto and attached to the 
female die by the heat generated at that time, the casting frame is 
inverted, and the melt of a low melting-point alloy is poured into the 
female die, thereby casting a male die. According to this method, since it 
is possible to ensure a die clearance by means of the resin sheet, the 
aforementioned forming model becomes unnecessary, so that various problems 
encountered in the use of the forming models can be overcome. 
However, since the aforementioned heat resistant resin sheet is an adhesive 
sheet which thermosets afterwards, if an attempt is made to bring the 
sheet into close contact with the product configuration surface of the 
matrix, a local elongation unavoidably occurs, and the occurrence of 
wrinkles and the trapping of air cannot be avoided. For this reason, 
so-called patching work must be performed by dividing the sheet into a 
multiplicity of small pieces. Consequently, much labor and skill are 
required in the same way as in the attachment of sheet wax, with the 
result that a decline in the work efficiency becomes unavoidable. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a method of 
producing a metal die which is capable of obtaining a metal model with 
ease and high accuracy without using a forming model, thereby overcoming 
the above-described drawbacks of the conventional art. 
To this end, in accordance with the present invention, there is provided a 
method of producing a metal mold, comprising the steps of: causing a heat 
resistant and elastic sheet to be forcedly brought into close contact with 
a product configuration surface of a matrix such as a wooden pattern, a 
resin model, or the like, by making use of negative pressure; bringing the 
matrix into contact with a melt of a low melting-point alloy while 
maintaining that state of close contact; cooling the melt as it is, 
thereby casting one part of the mold which makes up a pair; removing the 
matrix; bringing the one part of the mold into contact with the melt of a 
low melting-point alloy while maintaining the state of close contact 
between the heat resistant rubber sheet and the one part of the mold cast; 
and allowing the melt to cool as it is, thereby casting a counterpart of 
the mold that makes up the pair. 
In the present invention, the type of the aforementioned low melting-point 
alloy is not particularly restricted if its solidification shrinkage rate 
is small, for instance, it is possible to select the low melting-point in 
the range of 50.degree.-250.degree. C. from a Bi-Sn alloy, Sn-Pb alloy, 
Bi-Sn-Pb alloy, Bi-Sn-Pb-Cd alloy, and Bi-Sn-Pb-Sb alloy. 
In addition, in the present invention, the type of the aforementioned heat 
resistant rubber sheet is not particularly restricted insofar as it is 
capable of withstanding the melting temperature of the low melting-point 
alloy and maintaining resiliency. For instance, the heat resistant rubber 
sheet may be formed of silicone rubber, fluororubber, ethylene propylene 
rubber, acrylic rubber, chloroprene rubber, butadiene rubber, or the like. 
In cases where an alloy having a relatively high melting point is used as 
the low melting-point alloy, such as a Bi-Sn alloy or Sn-Pb alloy, it is 
preferable to use silicone rubber or fluororubber which excels in heat 
resistance. In addition, the thickness of the rubber sheet is selected to 
be capable of securing a clearance corresponding to a product thickness 
between the same and a die component that makes up a pair. 
In the present invention, as described above, the heat resistant rubber 
sheet is brought into close contact with a matrix by making use of 
negative pressure. However, since there is the possibility of the 
thickness of the rubber sheet when in close contact becoming nonuniform 
due to variations in the elongation of the rubber sheet caused by 
friction, preferably a substance which has a low coefficient of friction 
and is not modified at the melting temperature of the low melting-point 
alloy is applied in advance to the surface of the rubber sheet and/or the 
matrix which are adhered with each other. Such a substance may be selected 
from, for instance, molybdenum disulfide (MoS.sub.2), boron nitride (BN), 
and graphite (C). 
In addition, in the present invention, after one part of the mold that 
makes up a pair is cast, the matrix is removed, and the melt of a low 
melting-point alloy is brought into contact with the one part of the mold 
via the heat resistant rubber sheet, thereby to cast the counterpart of 
the mold. The rubber sheet is thermally bonded to the one part of the mold 
by means of the heat of the melt at the time of the initial casting. 
Accordingly, it is unnecessary to take into special consideration the 
close contact of the rubber sheet during the subsequent casting; however, 
where there is a large unevenness in the mold surface, there is the 
possibility of the rubber sheet partially floating up. Hence it is 
preferred that the close contact of the rubber sheet is supported by 
making use of negative pressure, or that an adhesive constituted by a 
thermosetting resin is applied in advance to the surface of the rubber 
sheet to be brought into contact with the melt, thereby allowing the 
rubber sheet to be bonded to the one part of the mold upon completion of 
solidification by making use of the setting of the adhesive. 
In casting, two casting frames are used, and are assembled as a unit with a 
matrix accommodated in one of them. At this time, the heat resistant 
rubber sheet is interposed between the two casting frames, and is clamped 
therebetween. The matrix may preferably be provided with a plurality of 
air vent holes, and if negative pressure is supplied through these holes 
via the casting frame, the rubber sheet can be brought into close contact 
with the matrix positively. The two casting frames may be joined 
vertically or horizontally, and in each case the one casting frame opposed 
to the other casting frame with the matrix accommodated therein provides a 
casting space. The melt of the low melting-point alloy is supplied to this 
casting space and is brought into contact with the matrix. A method of 
supplying the melt into this casting space is arbitrary and, for instance, 
the melt may be poured simply from above, or is pushed upward from below 
in the manner of low-pressure casting. When the two casting frames are 
joined vertically, since a casting space can be formed in the lower 
casting frame, a heater may be incorporated in this lower casting frame so 
as to hold in advance the melt in the casting space. 
Further, when two casting frames are joined vertically, in order to make 
the air venting easier, the casting may be carried out in such a manner 
that the casting in the upper casting frame is carried out in advance by 
accommodating the matrix in the lower casting frame, subsequently the 
casting frame is turned upside down and the casting in the casting frame 
located on the top and from which the matrix is removed. 
The type of the metal mold used in the present invention is not 
particularly restricted, but may preferably be applied to a press die for 
a sheet metal processing and a mold for resin molding. In the case of the 
press die, if one part of the mold that makes up a pair is set as a die, 
and a counterpart thereof as a punch, as a matrix it suffices to prepare 
one for the die alone, and the fabrication of the matrix can be effected 
readily. In addition, the aforementioned matrix may be of a split type. In 
cases where a split type matrix is used, if the entire matrix is first 
accommodated in a casting frame to cast the counterpart die, the split 
components of the matrix are then removed consecutively or together, and 
casting is effected consecutively by making use of the evacuated space. 
Thus, it is possible to obtain a press die having, for instance, a die, a 
punch, and a blank holder. 
According to the method of producing a metal mold of the present invention, 
it is possible to produce a metal mold easily by adhering a heat resistant 
rubber sheet to a matrix or one casting frame and also enables design 
changes to be made easily. Further, since it is possible to adhere the 
heat resistant rubber sheet having an excellent property of expansion and 
contraction to the surface of a product configuration consisting of the 
matrix evenly and rapidly by making use of the negative pressure, not only 
the productivity but also the precision of the product can be increased. 
In addition, the rubber sheet having heat resistance can be used 
repeatedly to attain a cost reduction through saving articles of 
consumption.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the accompanying drawings, a description will be given of 
the preferred embodiments of the invention. 
FIGS. 1 and 2 illustrate a first embodiment of the present invention. The 
first embodiment mainly comprises a first step (see FIG. 1) for casting a 
die D and a second step (see FIG. 2) for casting a punch P. In FIGS. 1 and 
2, a die casting frame 1 communicates with a reservoir tank 3 via a 
partition plate 2. The die casting frame 1 and the reservoir tank 3 are 
covered with a cover plate 4 so as to maintain a hermetically closed 
state. A melt 5 of a low melting-point alloy is held in the interiors 
thereof. The cover plate 4 is provided with a large-diameter opening 6 in 
correspondence with the die casting frame 1 and a small-diameter opening 7 
in correspondence with the reservoir tank 3. A heat resistant rubber sheet 
8 formed of, for instance, silicone rubber or fluororubber is provided on 
the cover plate 4 in such a manner as to cover the opening 6 and is 
pressed by an outer plate 9. In addition, a pipe 11 having a pressure 
gauge 10 midway thereof is connected to the opening 7 on the reservoir 
tank 3 side, compressed air being supplied to the pipe 11. 
Meanwhile, a matrix 12, which comprises a wooden pattern, resin model or 
the like, and a punch casting frame 13 for accommodating the matrix 12, 
are prepared. The matrix 12 and the punch casting frame 13 are mounted 
integrally on the outer plate 9 and are detachably fixed to the die 
casting frame 1. The matrix 12 has an air vent hole 12b which is open in 
its product configuration surface 12a and a rear surface thereof. The 
arrangement is such that, in the aforementioned fixed state, the product 
configuration surface 12a faces the interior of the die casting tank 1 via 
the rubber sheet 8. In addition, the punch casting frame 13 has a hole 14 
in a side wall on its bottom side, and a pipe 16 having a vacuum pump 15 
midway thereof is connected to the hole 14. An air tank 17, one end of 
which is closed, is provided on the bottom of the die casting frame 1. The 
other end of the air tank 17 is connected to the cover plate 4 so as to 
communicate with the outside, a pipe 19 having a vacuum pump 18 midway 
thereof being connected to said other end (see FIG. 2). 
In the first step, the melt 5 of the low melting-point alloy is held in 
advance in the die casting frame 1, or the melt 5 of the low melting-point 
alloy melted separately is supplied to the die casting frame 1. First, the 
rubber sheet 8 is placed on the cover plate 4, and is secured by the outer 
plate 9. At this time, the rubber sheet 8 is stretched with appropriate 
tension. Subsequently, the matrix 12, together with the punch casting 
frame 13, is placed on and fixed to the outer plate 9. By means of this 
fixing, the product configuration surface 12a of the matrix 12 projects 
substantially into the die casting frame 1 while expanding the rubber 
sheet 8. In this state, the vacuum pump 15 provided midway in the pipe 16 
is actuated to introduce negative pressure into the punch casting frame 
13. This negative pressure is supplied into the gap between the matrix 12 
and the rubber sheet 8 via the air vent hole 12b, so that the rubber sheet 
8 is brought into close contact with the product configuration surface 12a 
of the matrix 12. At this time, by virtue of its resiliency, the rubber 
sheet 8 stretches and shrinks in the direction in which its thickness 
becomes uniform, and the rubber sheet 8 becomes attached to the product 
configuration surface 12a, thereby securing a clearance necessary for a 
press. 
Subsequently, compressed air is supplied to the interior of the reservoir 
tank 3 so as to push up the melt 5 of the low melting-point alloy to the 
upper end of the die casting frame 1. As a result, the product 
configuration surface 12a of the matrix 12 is immersed in the melt 5 of 
the low melting-point alloy via the rubber sheet 8, and the die casting 
frame 1 is then cooled while this immersed state is being maintained. 
Then, the melt 5 of the low melting-point alloy solidifies, with the 
result that the die D (see FIG. 2) to which the product configuration 
surface 12a of the matrix 12 is transferred, is obtained. 
After the casting of the die D, a second step for casting the punch P is 
commenced. At the time of commencing this second step, the matrix 12 is 
removed after removing the punch casting frame 13, and the rubber sheet 8 
is also removed. Subsequently, air vent holes 17' are bored in the die D 
in such a manner as to extend vertically therethrough and communicate with 
the air tank 17, as shown in FIG. 2. The rubber sheet 8 is then placed 
again on the die casting frame 1, the punch casting frame 13 with its 
bottom removed is set thereon, and the vacuum pump 18 provided midway in 
the pipe 19 is actuated to introduce negative pressure into the interior 
of the die casting frame 1. This negative pressure is supplied into the 
gap between the die D and the rubber sheet 8 via the air vent holes 17', 
causing the rubber sheet 8 to be brought into close contact with the 
punch-receiving surface of the die D. Subsequently, the punch casting 
frame 13 with its bottom removed is set on the die casting frame 1, and if 
the melt 5 of the low melting-point alloy is poured into the punch casting 
frame 13 in the same manner as described above and is allowed to solidify, 
the punch P can be obtained. 
Hence, in the above-described steps, since the rubber sheet 8 is interposed 
between the punch P and the die D, a predetermined mold clearance can be 
secured between the punch P and the die D, making it possible to use these 
components as they are, as a press die. When used as a die press, the die 
casting frame 1 and the punch casting frame 13 are mounted on a press so 
as to be employed as holding frames for the die D and the punch P as they 
are, which simplifies the setting of the die D and the punch P on the 
press. Since in the first embodiment, in particular, the punch casting 
frame 13 also serves as a negative pressure chamber, it is possible to 
attain a reduction in equipment investment. 
In the above-described first embodiment, at the time of casting the punch 
P, the rubber sheet 8 is brought into close contact with the die D by the 
use of negative pressure. However, since this rubber sheet 8 is slightly 
heat bonded to the die D by the heat of the melt 5, and since its state of 
close contact is maintained by the action of the external pressure 
(atmospheric pressure) unless air enters the gap between the rubber sheet 
8 and the die D, it is possible to omit the use of negative pressure. In 
this case, the operation of inserting the air tank 17 and the operation of 
providing the die D with the air vent holes 17' can be dispensed with. 
FIGS. 3 and 4 illustrate a second embodiment of the present invention which 
mainly comprises a first step (see FIG. 3) for casting the die D and a 
second step (see FIG. 4) for casting the punch P in the same manner as the 
first embodiment. In FIGS. 3 and 4, a die casting frame 21 and a punch 
casting frame 22 are respectively composed of a base plate 23 and side 
plates 24. The upper side plates 24 are provided with pouring ports 25, 
26, respectively, and the die casting frame 21 and the punch casting frame 
22 are formed integrally and are disposed on the left and the right with 
the pouring ports 25, 26 facing upward. A casting space inside the 
integrally formed die casting frame 21 and the punch casting frame 22 is 
divided into two by a heat resistant rubber sheet 27 formed of, for 
instance, silicone rubber or fluororubber. In the first step, a matrix 28 
consisting of a wooden pattern, a resin model or the like is prepared, and 
the matrix 28 is accommodated in the punch casting frame 22 in such a 
manner that a product configuration surface 28a thereof is set vertically. 
A plurality of pegs 29 are provided on an inner surface of the base plate 
27 of each casting frame 21, 22 in such a manner as to project therefrom, 
the matrix 28 being accommodated with its rear surface abutting against 
the pegs 29. An air vent hole 30 communicating with the product 
configuration surface 28a and the rear surface of the matrix 28 is 
provided in the matrix 28. The base plate 23 and the side plates 24 which 
comprise the casting frame 21, 22 are arranged to be capable of being 
disassembled. 
A detailed description will be given hereinunder of the second embodiment 
in the order of steps. 
First, in a state in which the die casting frame 21 and the punch casting 
frame 22 are separated from each other, the matrix 28 is accommodated in 
the punch casting frame 22, and the two casting frames 21, 22 are 
assembled as a unit with the rubber sheet 27 clamped therebetween, as 
shown in FIG. 3. At this time, the rubber sheet 27 is stretched with 
appropriate tension so that wrinkles will not be produced. Then, the 
pouring port 26 on the punch casting frame 22 side is connected to a 
vacuum pump (not shown), thereby causing the rubber sheet 27 to be 
forcedly brought into contact with the matrix 28. By virtue of its 
excellent configuration following characteristics, the rubber sheet 27 
stretches or shrinks in the direction in which its thickness becomes 
uniform, and the rubber sheet 27 becomes attached to the product 
configuration surface 28a of the matrix 28 with a thickness corresponding 
to the thickness of a press product. 
Then, while the state of close contact between the rubber sheet 27 and the 
matrix 28 is being maintained, a melt 31 of a low melting-point alloy is 
poured into the die casting frame 21 through the pouring port 25. The 
level of the melt 31 gradually rises from the bottom toward the top and, 
in the meantime, the air inside the die casting frame 21 is discharged to 
the outside through gaps between the base plate 23 and the side plates 24 
in the upper portions thereof. Meanwhile, the air between the rubber sheet 
27 and the matrix 28 moves upward through the gap therebetween and the air 
vent hole 30 and is discharged to the outside through the pouring port 26 
on the punch casting frame 22 side. Thus, the melt 31 of the low 
melting-point alloy is poured, and pouring is completed by leaving an air 
chamber 32 marginally between the upper side plate 24 and the melt 31. In 
this state, the melt 31 is allowed to cool and solidify, thereby 
completing the casting operation of the die D. 
In the above-described first step, an arrangement may be provided such 
that, upon completion of the pouring of the melt 31 of the low 
melting-point alloy, a pipe 33 is connected to the pouring port 25, and 
compressed air is supplied to the air chamber 32 so as to press the melt 
31. This allows the rubber sheet 27 to be brought into close contact with 
the matrix 28 more positively. 
Subsequently, the base plate 23 on the punch casting frame 22 side is 
removed, the matrix 28 placed inside is also removed, and the base plate 
23 is then reinstalled, as shown in FIG. 4. During this operation, 
negative pressure is applied to the die casting frame 21 side by means of 
a vacuum pump (not shown) through the pouring port 25 on the die casting 
frame 21 side. This causes the air between the rubber sheet 27 and the die 
D to be exhausted, allowing the rubber sheet 27 to maintain its state of 
close contact with the die D. Subsequently, the melt of the low 
melting-point alloy is poured into the punch casting frame 22 through the 
pouring port 26 and, in this state, the melt 31 is allowed to cool and 
solidify, thereby obtaining the punch P. At this time, an arrangement may 
be provided such that, in the same manner as the above-described first 
step, a pipe 34 is connected to the pouring port 26 (see FIG. 4), and the 
melt 31 is pressed by supplying compressed air into the interior of the 
punch casting frame 22. This allows the rubber sheet 27 to be brought into 
close contact with the die D more positively. 
In the above second embodiment, since the die molding frame 21 and the 
punch molding frame 22 are joined from the left side and the right side to 
make the product configuration surface 28a of the matrix 28 or the product 
configuration surface of a die D along the vertical plane, the air is 
easily exhausted upward in a casting process to avoid the occurrence of a 
defective casting by involving air therein. 
After the casting of the punch P, the casting frames 21, 22 are separated 
from each other, the side plates 24 are removed from the casting frames 
21, 22, and the rubber sheet 27 is also removed. Then, as shown in FIG. 5, 
a press die having the die D and the punch P respectively provided on the 
base plates 23 is completed. Since the die D and the punch P are securely 
connected to the respective base plates 23 by means of the pegs 29, it is 
possible to effect a stable stamping operation without any occurrence of 
rattling during use. Incidentally, the side plates 24 may not be removed 
and may be set on a stamping machine as they are as holding frames. 
Further, as a modification from the above second embodiment, it is also 
possible to use the above die molding frame 21 and the punch molding frame 
22 by joining them vertically. In this case, in order to make the air 
ventilation easier it is preferable first to cast the die D by pouring the 
melt of the low melting point alloy in the upper molding frame 21, 
subsequently to turn the die molding frame 21 and the punch molding frame 
22 upside down, to remove the matrix 28 from the upper punch molding frame 
22 and pour the melt to cast the punch P. 
FIGS. 6 and 7 illustrate a third embodiment of the present invention, and 
its arrangement is identical with that of the above-described second 
embodiment, in that it mainly comprises a first step (see FIG. 6) for 
casting the die D and a second step (see FIG. 7) for casting the punch P. 
Components that are identical with those shown in FIGS. 3 and 4 are 
denoted by the same reference numerals, and a description thereof will be 
omitted. In this third embodiment, a pair of fixing members 41, 42 are 
provided respectively on the upper and lower side plates 24 of the die 
casting frame 21 in such a manner as to oppose each other vertically, so 
that opposite ends of a metal pipe 43 accommodated in the casting frame 21 
can be supported and made watertight by the fixing members 41, 42. In 
addition, the lower side plates 24 of the casting frames 21, 22 are 
respectively provided with lower pouring ports 44, 45 separately from the 
aforementioned pouring ports 25, 26. An arrangement is provided such that 
vacuum pumps 46, 47 can be respectively provided in the aforementioned 
pipes 33, 34 that are connected to the pouring ports 25, 26 of the casting 
frames 21, 22, while the metal pipe 43 can be connected to the vacuum pump 
46 on the die casting frame 21 side. 
In the first step of the third embodiment, the metal pipe 43 is 
accommodated in advance in the die casting frame 21, and its ends are 
secured by the fixing members 41, 42. The matrix 28 is then accommodated 
in the punch casting frame 22. After the two casting frames 21, 22 are 
assembled as a unit with the rubber sheet 27 clamped therebetween, 
negative pressure is supplied to the punch casting frame 22 by the 
operation of the vacuum pump 47 on the punch casting frame 22 side, and 
causing the rubber sheet 27 to be brought into close contact with the 
product configuration surface 28a of the matrix 28. Then, as shown in FIG. 
6, this assembly is positioned on a holding furnace 52 of a low-pressure 
casting apparatus 51. A stalk with its end portion immersed in the melt 31 
of the low melting-point alloy inside the holding furnace 52 is connected 
to the lower pouring port 44 on the die casting frame 21 side. The holding 
furnace 52 is hermetically closed by a cover 54, and compressed air is 
supplied to the holding furnace 52 through an air supply port 55 provided 
in the cover 54. Then, the melt 31 is pushed upward inside the die casting 
frame 21 via the stalk 53. When the melt level has risen up to a point 
where the air chamber 32 remains marginally in the upper portion of the 
holding furnace 52, a valve 56 provided in the stalk 53 is closed. At the 
same time, compressed air is supplied from the pouring port 25 to the die 
casting frame 21 so as to press the melt 31, which, in turn, causes the 
rubber sheet 27 to be brought into close contact with the product 
configuration surface 28a of the matrix 28. In this state, the melt 31 is 
then allowed to solidify, thereby producing the die D with the metal pipe 
43 inserted therein. It should be noted that the pressing of the melt 31 
by compressed air through the pouring port 25 may be omitted. 
Next, the die casting frame 21 and the punch casting frame 22 are separated 
from each other, the matrix 28 is removed from the punch casting frame 22, 
and the rubber sheet 27 is also removed. Then, air vent holes 48 are bored 
in the die D in such a manner as to extend horizontally, as viewed in FIG. 
7, and communicate with the metal pipe 43. Subsequently, the casting 
frames 21, 22 are reinstalled with the rubber sheet 27 clamped 
therebetween, and negative pressure is supplied to the interiors of the 
die casting frame 21 and the metal pipe 43 by the operation of the vacuum 
pump 46 on the die casting frame 21 side (at this time, the lower fixing 
member 42 is replaced with a hermetic plug 42'), allowing the rubber sheet 
27 to be brought into close contact with the punch-receiving surface of the 
die D. In addition, the stalk 53 is connected to the lower pouring port 45 
on the punch casting frame 22 side, and the melt 31 of the low 
melting-point alloy in the holding furnace 52 is then pushed upward in the 
punch casting frame 22 in the same manner as the above-described first step 
and is allowed to solidify, thereby casting the punch P. 
In the second step of the above-described third embodiment, the melt 31 may 
be pushed by compressed air through the pouring port 26 on the punch 
casting frame 22 side so as to assist the close contact between the rubber 
sheet 27 and the die D. In addition, an arrangement may be provided such 
that, as shown in FIG. 7, a metal pipe 49 is accommodated in advance in 
the punch casting frame 22 and fixed by a pair of fixing members 50 
provided on the side plates 24. In this case, as shown in FIG. 8, since 
the metal pipes 43, 49 are respectively inserted in the die D and punch P 
thus obtained, these metal pipes 43, 49 can be utilized in the setting of 
the die assembly on a stamping machine. 
FIGS. 9 to 11 illustrate a fourth embodiment of the present invention. This 
fourth embodiment of the present invention mainly comprises a first step 
(see FIG. 9) for casting the die D, a second step (see FIG. 10) for 
casting the punch P, and a third step (see FIG. 11) for casting a blank 
holder B. Since the basic arrangement of the apparatus is identical with 
that of the above-described second embodiment, components that are 
identical with those shown in FIGS. 3 and 4 are denoted by the same 
reference numerals, and a description thereof will be omitted. In this 
fourth embodiment, the matrix 28 is arranged to be split into the two 
parts of a matrix 61 for a punch and a matrix 62 for a blank holder. 
First, as shown in FIG. 9, the matrix 61 for the punch and the matrix 62 
for the blank holder are accommodated in the punch casting frame 22 
together. The punch casting frame 22 is provided with a pipe 64 in 
correspondence with an air vent hole 63 provided in the matrix 61 for the 
punch, and a vacuum pump (not shown) is connected to the pipe 64. In 
addition, the matrix 61 for the punch is fixed to the punch casting frame 
22 by means of bolts 65. After the die casting frame 21 and the punch 
casting frame 22 are arranged integrally with the rubber sheet 27 clamped 
therebetween, negative pressure is immediately introduced into the punch 
casting frame 22 via the pipe 64, thereby causing the rubber sheet 27 to 
be brought into close contact with the matrix 61 for the punch and the 
matrix 62 for the blank holder. While this state of close contact is being 
maintained, a predetermined amount of the melt 31 of the low melting-point 
alloy is poured into the die casting frame 21 through the pouring port 25 
and, in this state, the melt 31 is allowed to cool and solidify, thereby 
casting the die D. 
Then, as shown in FIG. 10, the base plate 23 of the punch casting frame 22 
is removed, and the matrix 61 for the punch is removed from the punch 
casting frame 22. At this juncture, negative pressure is supplied to the 
interior of the die casting frame 21, allowing the rubber sheet 27 to be 
brought into close contact with the die D. Subsequently, after the base 
plate 23 is reinstalled, the melt 31 is poured into the punch casting 
frame 22 through a pouring pipe 66 provided in the base plate 23, and the 
melt 31 is filled into the space where the matrix 61 for the punch has 
been evacuated. Then, the melt is allowed to cool and solidify as it is, 
thereby producing the punch P. 
Subsequently, as shown in FIG. 11, the base plate 23 of the punch casting 
frame 22 is removed, the matrix 62 for the blank holder is also removed 
from within the punch casting frame 22, and a second heat resistant rubber 
sheet 67 formed of, for instance, silicone rubber or fluororubber is 
attached to a side surface of the punch P by the use of a heat resistant 
adhesive. Then, the base plate 23 is reinstalled, the melt 31 is poured 
into the punch casting frame 22 and into the evacuated space of the matrix 
62 for the blank holder through the pouring port 26 and, in this state, the 
melt 31 is allowed to cool and solidify, thereby casting the blank holder 
B. 
The die D, punch P, and blank holder B obtained as described above are set 
on a stamping machine together with the die casting frame 21 and the punch 
casting frame 22, as shown in FIG. 12. In FIG. 12, reference numeral 71 
denotes an upper ram; 72, a bolster; and 73, a cushion. The die D is 
secured on the upper ram 71 via the die casting frame 21, and the punch P 
is secured to the bolster 72 via the punch casting frame 22. Meanwhile, 
the blank holder B is supported by the cushions 73 extending through the 
base plate 23 of the punch casting frame 22. As such, predetermined 
clearances 74, 75 are respectively formed between the die D on the one 
hand and the punch P and the blank holder B on the other, as well as 
between the punch P and the blank holder B, thereby making it possible to 
perform a stamping operation effectively. At this juncture, if the die 
casting frame 21 is provided with guide holes 76, and the punch casting 
frame 22 is provided with guide pins 77, and the pins 77 are inserted into 
the respective holes 76, the die casting frame 21 and the punch casting 
frame 22 can be accurately positioned with respect to each other during a 
stamping operation, making it possible to enhance processing accuracy. 
In the above-described fourth embodiment, the casting of the punch P is 
effected prior to the casting of the blank holder B, but the casting of 
the blank holder B may be effected before the casting of the punch P. In 
this case, the order of removing the matrix 61 for the punch and the 
matrix 62 for the blank holder is the reverse of the aforementioned order. 
Further, as a modification from the fourth embodiment, it is possible to 
cast the punch P and the blank holder B simultaneously. In this case, it 
is cast in such a manner that the die molding frame 21 and the punch 
molding frame 22 are joined integrally in a vertical direction and first 
the die D is-cast by pouring the melt alloy of the lower melting point in 
the upper die molding frame 21 , then both of the above molding frames are 
turned upside down and the punch matrix 61 and the blank holder 62 are 
removed from the punch molding frame 22, and in the punch molding frame 22 
a punch profile (instead of the rubber sheet 67 of FIG. 11) is disposed. 
Subsequently, by pouring in the punch molding frame 22, the punch P and 
the blank holder B are simultaneously cast. In this case, the above punch 
matrix and the blank holder may be formed integrally without being 
divided. 
FIG. 13 illustrates a fifth embodiment of the present invention. A 
characteristic feature of this fifth embodiment lies in that, in a step 
corresponding to the first step of the above-described second embodiment, 
after the heat resistant rubber sheet 27 is brought into close contact 
with the matrix 28 by making use of negative pressure, an adhesive 81 
comprising a thermosetting resin is applied to the surface of the rubber 
sheet 27. As a result of application of the adhesive 81 to the rubber 
sheet 27, the adhesive 81 hardens by the heat of the melt 31 of the low 
melting-point alloy poured into the die casting frame 21, and the rubber 
sheet 27 is bonded to the die D undergoing a solidifying process by means 
of the hardened adhesive, thereby maintaining the state of close contact 
between the rubber sheet 27 and the surface of the die D. Accordingly, 
through the second step (see FIG. 4) for casting the punch P, the rubber 
sheet 27 is prevented from floating above the die surface, and even if 
negative pressure is not introduced into the die casting frame 21, it is 
possible to obtain an excellent punch in terms of accuracy. 
FIGS. 14 and 15 illustrate a sixth embodiment of the present invention. A 
characteristic feature of this sixth embodiment lies in that a silicone 
resin is sprayed in advance onto the surface of the matrix 28, and the 
silicone resin is allowed to sufficiently permeate the matrix 28 so as to 
form a silicone resin impregnated layer 91 on a surface layer portion of 
the matrix 28. Then, molybdenum disulfide (MoS.sub.2), i.e., a substance 
having a low coefficient of friction, is sprayed onto the surface of the 
matrix 28 so as to form a film 92 of molybdenum disulfide on that surface. 
At this juncture, it is preferred that, after the spraying of molybdenum 
disulfide, the film 92 is slightly rubbed to improve its sliding 
characteristic. If this arrangement is adopted, when the rubber sheet 27 
is brought into close contact with the matrix 28, since the frictional 
resistance between the rubber sheet 27 and the matrix 28 is reduced due to 
the presence of the film 92 of molybdenum disulfide, the rubber sheet 27 
stretches or shrinks freely in the direction in which a difference in 
strain in the horizontal direction is overcome, with the result that the 
rubber sheet 27 is attached to the matrix 28 with a substantially fixed 
thickness corresponding to the thickness of a product to be stamped. It 
should be noted that a coefficient of friction .mu. between the rubber 
sheet 27 and the matrix 28 is 0.2 in a case where the film 92 of 
molybdenum disulfide is formed as in the case of this embodiment, whereas 
the coefficient of friction .mu. is 0.9 in a case where the film 92 is not 
provided. Thus it can be appreciated that this large difference in the 
coefficient of friction .mu. contributes to overcoming the aforementioned 
difference in strain. In addition, in this sixth embodiment, since the 
silicone resin impregnated layer 91 is formed underneath the film 92 of 
molybdenum disulfide, the effect of the surface roughness of the matrix 28 
is minimized, which allows the rubber sheet 27 to stretch and shrink more 
smoothly, thereby making its thickness more uniform. By forming the 
silicone resin impregnated layer 91, it is possible to restrain generation 
of gas for the matrix 28, and in addition the harsh surface of the cast 
caused by the penetrating of the gas through the rubber sheet 27 is also 
restrained. Further, this silicone resin is replaceable with other heat 
resistant materials such as epoxy resin, fluorine resin or ceramics 
coating agent. 
Accordingly, if the melt 31 of the low melting-point alloy is subsequently 
poured into the die casting frame 21 so as to be brought into contact with 
the matrix 28 and is allowed to cool and solidify in that state, it is 
possible to obtain the die D having a high degree of accuracy.