Elastically deformable die and method of die forming using the die

An elastically deformable die includes two or more die members formed of materials exhibiting different degrees of elastic deformation under the same stress which are combined and arranged such that the elastic deformation thereof under the pressure received from a workpiece during a forming operation causes the surface of the die in contact with the workpiece to be formed to a prescribed configuration. In die forming the composite die is brought in contact with a surface of the workpiece, the die is elastically deformed at the surface of contact by the pressure received from the workpiece during the forming process, and the workpiece is formed to the prescribed configuration by this deformation.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to die forming by forming a material using a die, 
and more specifically to a composite die constituted of a plurality of die 
members formed of materials exhibiting different degrees of elastic 
deformation and to a method of die forming using the die in which a 
workpiece is formed to a desired shape by utilizing the different degrees 
of elastic deformation of the materials constituting the die members for 
controlling the elastic deformation of the die at the time of the forming 
operation. 
2. Description of the Prior Art 
In the field of die forming it is nowadays frequently necessary to subject 
a workpiece to a minute amount of forming on the micron order. This is 
true, for example, in the production of optical products, which often have 
to be imparted with minute changes in shape with high precision and good 
reproducibility. 
In carrying out such forming by conventional die forming, the die is 
constituted and supported so as to resist elastic deformation. In 
producing a flat surface having minute irregularities (hills and valleys), 
therefore, it has been necessary to form the surface of the die to have a 
shape corresponding to the minute irregularities to be formed. With such a 
method, however, fabrication of the mold becomes increasingly difficult as 
the size of the irregularities becomes smaller. 
In their earlier U.S. patent application Ser. No. 611,045, the inventors 
proposed a die forming method which overcomes this problem by using an 
elastically deformable die whose elastic deformation is controlled by the 
thickness distribution of the material constituting the die. While this 
earlier proposed method greatly simplifies the formation of minute 
irregularities, the fact that it utilizes the elastic deformation of a die 
having irregularities formed on its rear surface makes it necessary to use 
a thin die so that the application of the method to the processing of 
relatively hard materials such as metals is not possible unless special 
measures are adopted. 
The object of this invention is to provide a die and a method of die 
forming using the die which, with a simple structural arrangement and 
without need for precision machining of the die surface, make it possible 
to impart minute changes in shape with high reproducibility to materials 
of both ordinary and high hardness. 
SUMMARY OF THE INVENTION 
For achieving the aforesaid purpose, the present invention provides an 
elastically deformable die comprising two or more die members formed of 
materials exhibiting different degrees of elastic deformation under the 
same stress which are combined and arranged such that the deformation 
caused by the pressure received from the workpiece during forming 
operation forms prescribed irregularities (hills and valleys) in the flat 
surface of the die and further provides a method of die forming in which a 
composite die constituted of a plurality of die members in the foregoing 
manner is brought in contact with a surface of a workpiece, the die is 
elastically deformed at the surface of contact by the pressure received 
from the workpiece during the forming process, and the workpiece is formed 
to a prescribed shape by this deformation. 
Thus in accordance with the present invention if at the time of fabricating 
the die from the plurality of die members exhibiting different degrees of 
elastic deformation the type, size arrangement and the like of the die 
members are appropriately selected, it becomes possible to regulate the 
size and distribution of the irregularities that will be formed on the 
initially flat surface of the die under the pressure received thereby from 
the workpiece during the forming process and thus become possible to 
impart the desired configuration to the workpiece. In the specific case of 
forming a hard material, irregularities of the desired size and 
distribution can be easily formed by fabricating the die from materials 
exhibiting an appropriately small degree of elastic deformation. Since 
this means that there is no need to use thin materials, the formation of 
such hard materials can be readily conducted with good reproducibility. 
The above and other objects and features of the invention will become 
apparent from the following detailed description with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
In die forming techniques such as forging, casting, extrusion molding and 
the like, the die invariably receives pressure from the workpiece, 
although the degree of this pressure may differ between the different 
techniques. It is thus probable that the die will elastically deform, 
causing dimensional inaccuracies in the product. Since conventional 
thinking was that die deformation has to be avoided, die materials 
exhibiting small degrees of elastic deformation were selected and efforts 
were made to support the die so as to prevent it from deforming. In actual 
practice, however, there are cases in which deformation of the die cannot 
be avoided. This problem has been coped with by analyzing the deformation 
of the die (during forging, for example), and then modifying the die 
surface configuration based on the results of the analysis so as to 
improve the dimensional accuracy of the product. 
In contrast, the approach of the present inventors was to accept die 
deformation as an intrinsic factor which can better be positively and 
effectively taken advantage of than be prevented or be compensated for by 
modifying the die surface shape according to the predicted deformation 
and, based on this logic, they accomplished the present invention. 
More specifically, the elastically deformable die according to this 
invention comprises two or more die members formed of materials exhibiting 
different degrees of elastic deformation under the same stress which are 
combined and arranged such that the deformation caused by the pressure 
received from the workpiece during forming operation forms prescribed 
irregularities (hills and valleys) in the flat surface of the die, while 
the method of die forming according to the invention comprises the steps 
of bringing a composite die constituted of a plurality of materials in the 
foregoing manner in contact with a surface of the workpiece, allowing the 
die to be elastically deformed at the surface of contact under the 
pressure received from the workpiece during the forming process, and 
allowing the workpiece to be formed to a prescribed shape by this 
deformation. 
The die forming using a composite die according to the invention can be 
applied to forging, casting, extrusion molding or any other type of 
forming insofar as it uses a die to form a workpiece. 
FIG. 1 is a schematic representation showing the arrangement of a die 
forming device for implementing the die forming method employing a 
composite die according to this invention. As shown, a disk-shaped 
composite die 1 consisting of two die members 1a, 1b made of different 
materials is mounted on a fixed table 4. The workpiece 5 is placed on top 
of the composite die 1 and a cylindrical ram 6 disposed above the fixed 
table 4 is lowered onto the workpiece 5 so as to press it onto the 
composite die 1. As a result, the surface of the composite die 1 in 
contact with the workpiece 5 is elastically deformed and the workpiece 5 
is formed in the shape resulting from this deformation. 
In the embodiment under discussion, the composite die 1 mounted on the 
fixed table 4 consists of a first die member 1a having on its bottom 
surface two concentric annular ridges 2 and a second die member 1b having 
on its top surface two annular grooves 3 with which the ridges 2 mesh. The 
die member 1a and die member 1b are fabricated from materials exhibiting 
different degrees of elastic deformation and are bonded together at the 
junctions between the ridges 2 and the grooves 3 so as to constitute the 
composite die 1. 
Where a large deformation of the surface of the composite die is desired 
during the die forming operation, a thin first die member 1a is used in 
combination with a thick second die member 1b formed of a material having 
a degree of elastic deformation that is larger than that of the first die 
member 1a. Where slight local deformation is desired, a material having a 
small degree of elastic deformation is disposed to a greater thickness at 
the associated portion of the first die member 1a. In other words, the 
ridges 2 are formed. Varying the height of the ridges 2 varies the degree 
of deformation of the die surface. 
When the composite die 1 fabricated in the aforesaid manner receives 
pressure from the workpiece 5 being formed during the forming operation, 
its flat surface is elastically deformed. The amount of deformation at 
this time is determined primarily by the magnitude of the aforesaid 
pressure, the Young's moduli of the die members and the thickness of the 
die members. Thick portions constituted of a material exhibiting a high 
degree of elastic deformation experience large elastic deformation, while, 
as mentioned earlier, thick portions constituted of a material exhibiting 
a low degree of elastic deformation experience small elastic deformation. 
Therefore, when a material exhibiting a high degree of elastic deformation 
and provided with local thickness variations is distributed within a 
material exhibiting a low degree of elastic deformation, the pressure 
applied during the forming operation reduces the thickness variations of 
the high deformation material and thus causes irregularities (hill and 
valleys) to appear on the surface of the die. Depending on the die support 
conditions, the irregularities will be either of the compressive 
deformation type or of the bending type. 
The changes in the shape of the die surface produced by the differences in 
the amount of elastic deformation are very much smaller than the changes 
in shape occurring at the boundary between the high deformation material 
and the low deformation material. Because of this, it is possible to 
obtain a formed product having high configurational precision even when 
the machining precision used for fabricating the die is lower than the 
configurational precision required of the formed product. Although it is 
of course necessary for the configurational precision of the die surface 
to be equal to that required by the product, it is immeasurably easier to 
obtain a high-precision die surface configuration by the method of this 
invention than it is by directly imparting configuration changes to the 
die surface. 
As high deformation materials usable for fabrication of the composite die 
there can be mentioned rubber, synthetic resins, aluminum alloys, copper 
alloys and the like. Usable low deformation materials include stainless 
steel and copper. 
The composite die and die forming method of the invention can be used for 
the formation not only of synthetic resins and plastics that have been 
softened by heating but also of lead and other such high hardness metal 
materials. 
The pressure received by the composite die from the workpiece is made to be 
lower than the yield point of the materials constituting the composite 
die. After the pressure is removed, therefore, the surface of the die in 
contact with the workpiece is restored to its initial flat condition. As a 
result, it is possible to impart minute shape changes to a large number of 
workpieces with high reproducibility. 
FIG. 2(a) shows another embodiment of the composite die 1 according to the 
invention. The composite die 1 is fabricated by forming holes or grooves 3 
in the top surface of a second die member 1b, closing these holes or 
grooves 3 by inserting or casting into the holes or grooves 3 die members 
1c constituted of material having a different degree of elastic 
deformation from the second die member 1b, grinding and/or polishing the 
common top surface of the second die member 1b and die members 1c until it 
becomes flat, and then bonding onto the flat surface a disk-shaped first 
die member 1a whose top surface is polished to a mirror finish. 
A die for producing a complex configuration can be fabricated relatively 
easily in accordance with this embodiment of the composite die by 
appropriate determination of the position, shape and depth of the holes or 
grooves 3 and selection of the material of the die members. 
FIG. 2(b) shows another embodiment of the composite die 1 according to the 
invention. This die is fabricated by superposing a first die member 1a 
having ridges 2 on its bottom surface onto a second die member 1b such 
that a cavity remains between the two die members, drilling a hole 7 in 
the second die member 1b so as to communicate the cavity with the 
exterior, injecting a molten metal or resin having a different degree of 
elastic deformation from either the first die member 1a or the second die 
member 1b into the cavity to serve as a die member 1d. 
With this embodiment of the composite die 1, the shaped assumed by the 
surface of the die during the forming operation can be freely determined 
by appropriate selection of the shape of the cavity and the material of 
the die member 1d. 
The composite die shown in FIG. 2(c) is a modification on that shown in 
FIG. 2(b). In this embodiment, the bottom surface of the first die member 
1a is not provided with ridges 2 but has wires or small pieces 1e attached 
thereto. Again, the composite die 1 is completed by injecting molten 
material for the die member 1d into the cavity between the first die 
member 1a and the second die member 1b. The degree of deformation 
experienced by the surface of this composite die during the forming 
operation will be large if the degree of elastic deformation of the die 
member 1d injected into the cavity is larger than that of the wires or 
small pieces 1e attached to the bottom surface of the first die member 1a 
and will be small if it is smaller. 
As can be understood from the foregoing, die forming with the composite die 
according to this invention makes it possible to effectively utilize the 
elastic deformability of the die for imparting a desired configuration to 
the workpiece, by a simple means and without need for precision machining 
of the die surface. Moreover, since the desired die surface configuration 
can be obtained during the molding operation even when the die members of 
the composite die are made thick, the strength of the die can be increased 
for enabling it to form hard materials. 
Examples of using the composite design of the invention for forging lead 
will now be explained. 
As shown in FIG. 3, the composite die 1 used in this example consisted of a 
first die member 1a having two concentric annular ridges projecting from 
its bottom surface, one near the periphery and the other inward thereof, 
and a second die member 1b having annular grooves for receiving the ridges 
of the first die member 1a. The two die members were bonded together with 
the ridges fitted into the grooves. The first die member 1a was made from 
S45C carbon steel and the second die member 1b from duralumin. The 
dimensions of the composite die were as shown in FIG. 3. 
The height of the region of this composite die in which the two die members 
are meshed with each other is 2.5 mm. Taking the Young's modulus of S45C 
carbon steel to be 21.times.10.sup.10 PA and that of duralumin to be 
7.times.10.sup.10 PA, the difference in the amount of elastic deformation 
within this 2.5 mm height region can be found by calculation to be 2.0 
.mu.m. 
The surface of the composite die 1 to be brought into contact with the 
workpiece was polished to a mirror finish using a rotary polisher and then 
mounted in the die forming device. As the die forming device there was 
used a 200-ton universal tester manufactured by Shimadzu Seisakusho, Ltd. 
of Japan. Before conducting the test, a parallel light beam was projected 
onto the surface of the composite die 1 mounted in the die forming device 
and the reflected image was observed for determining whether or not any 
irregularities corresponding to the internal ridges and grooves of the die 
were present on the die surface. The result of the observation was 
negative. A lead billet measuring 40 mm in diameter and 5.5 mm in height 
was placed on the composite die and an upper die formed of S45C carbon 
steel and measuring 50 mm in diameter and 12 mm in thickness was brought 
into contact with the top surface of the lead billet. The lead billet was 
then forged by using the tester to apply a 16-ton load (corresponding to a 
stress of 80 MPa) to the lead billet. 
The forged lead billet had a diameter of about 50 mm. The configuration of 
the surface of the lead billet pressed by the composite die was examined 
by projecting a parallel laser beam onto the surface and observing the 
reflected image projected onto a screen located 1 m away. The image was 
found to include two bright rings at positions corresponding to those of 
the two annular ridges formed on the first die member. It was thus 
confirmed that the surface of the lead billet had been formed with two 
annular recesses which caused the reflected light to converge at the 
bright regions. A parallel light beam was similarly reflected off the 
surface of the die which had been pressed onto the lead billet. 
Observation of the reflected image showed that no irregularities had been 
formed on the die surface by yield deformation of the die members. 
Next, a composite die was fabricated in the manner shown in FIG. 2(a). As 
the second die member 1b there was used a copper disk measuring 50 mm in 
diameter and 20 mm in thickness and provided in its surface with two 
concentric grooves measuring 5 mm in width and 3 mm in depth. A molten 
zinc-aluminum alloy was cast into the grooves and allowed to solidify, 
whereafter the surface of the die member was ground and polished until it 
becomes flat. The composite die was then completed by bonding to the flat 
surface a disk of roll-hardened SUS 304 stainless steel whose surface had 
been polished to a mirror finish. Examination of the surface of the 
so-fabricated composite die conducted by observing the reflected image 
obtained when a parallel light beam was projected thereon showed it to 
have a high degree of flatness. 
The composite die was mounted in the die forming device used in the 
preceding example and a lead disk measuring 40 mm in diameter and 3.5 mm 
in thickness was placed thereon for die forming. A copper upper die 
measuring 50 mm in diameter and 20 mm in thickness was lowered onto the 
lead disk and forging was conducted by applying a 30-ton load. 
The surface of the forged lead disk that had been in contact with the 
composite die retained its mirror finish. A parallel beam was projected 
onto this surface and the reflected image formed on a screen located 50 cm 
away was observed. The image was dark at the portions corresponding to the 
grooves of the second die member filled with zinc-aluminum alloy and 
bright at other portions. It was thus confirmed that minute irregularities 
had been successfully imparted to the surface that was to be die formed. 
From the foregoing detailed description, it will understood that the 
invention provides a simple means for enabling the elastic deformation of 
a die to be effectively and easily utilized for forming a product with 
configurational irregularities of a microscopic magnitude virtually 
impossible to realize with a die forming method using a conventional die 
with no deformability. Moreover, since the composite die according to the 
invention does not have to be thin, its thickness can be increased as 
required for attaining the required degree of strength. As a result, the 
invention can be applied to the forming of hard materials exhibiting a 
high resistance to deformation. 
While the present invention can be effectively applied in the manufacture 
of optical elements, ornaments that utilize light reflection and the like 
for imparting these articles with minute variations in configuration with 
high precision, it is of course not limited to these applications.