Method for molding optical element

There is disclosed a method for molding an optical element, by preparing an upper mold member and a lower mold member for press molding a glass material, heating the mold members together or individually to a predetermined temperature, deforming the glass material according to molding faces of the mold members by a pressing force applied to the mold members, then transferring the mold members and the molded glass to a cooling step and subsequently taking out the molded glass by opening the mold members, thereby transferring the optically functional faces corresponding to the molding faces of the mold members, to the glass material. The cooling step includes a step of opening the mold members to separate the glass material from the molding faces of the mold members to release the adhesion state upon press molding between the molding faces and the glass material, and a step of closing the mold members again so as to maintain the molded state of the glass material and cooling the molded glass in this state further to a taking-out temperature thereof.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical element molding method for 
molding an optical element such as a highly precise glass lens by pressure 
molding a softened glass material. 
2. Related Background Art 
With the reduction in size and weight of optical equipment, aspherical 
glass lenses are desired for use in the optical system. Since it is 
difficult to produce such lenses by ordinary polishing operation, there is 
being developed a method for producing an optical element without a 
post-working step such as polishing, by placing a glass material for 
producing the element such as a glass blank preliminarily molded to 
certain shape and surface precision between upper and lower mold members 
of a predetermined surface precision, and press molding the glass material 
under heating. 
In such method for producing an optical element by press molding, as 
already disclosed in the Japanese Patent Publication No. 62-292636, a 
glass material is sandwiched between mold members for molding the optical 
element, then is heated, together with the mold members, to a temperature 
above the yield point of the glass and is pressed to the predetermined 
shape. Subsequently, the glass material is cooled to a temperature below 
the glass transition point while it is pressurized by the mold members, 
and is then taken out from the mold members to obtain a highly precise 
optical element. In this process, the mold members together with a 
cylindrical mold constitute a molding block, and the optical element is 
molded during transporting of the block to the stages of heating, pressing 
and cooling while the glass material is held in the block. 
However, when the glass material is cooled to the temperature below the 
glass transition point while it is pressurized by the mold members, the 
mold members and the glass material are maintained in close contact, and, 
due to the difference in the contractions of the mold members and the 
glass material resulting from the difference in the thermal expansion 
coefficients thereof, a thermal stress is generated in the glass material. 
After the mold members are opened, the thermal stress is released, whereby 
a deformation is generated in the molded optical element. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, the object of the present invention is 
to provide a method for molding an optical element, capable of avoiding 
unnecessary thermal stress or strain in the cooling step and maintaining 
the optically functional face of the final molded product in a highly 
precise state of molding. 
The above-mentioned object can be attained, according to the present 
invention, by a method for molding an optical element by preparing an 
upper mold and a lower mold for press molding a glass material, heating 
the mold members together or individually to a predetermined temperature, 
deforming the glass material along the molding faces of the mold members 
by a pressing force applied to said mold members, then effecting a cooling 
step on the mold members and molded glass after the molding thereof, then 
opening the mold members and taking out the molded glass thereby 
transferring the optically functional faces corresponding to the molding 
faces of the mold members to said glass material, wherein the cooling step 
includes a step of opening the mold members to separate said glass 
material from the molding faces of said mold members, thereby releasing 
the close contact state between the molding faces and the glass material 
during the press molding, and a step of closing the mold members again in 
order to maintain the molded state of the molded glass to cool the molded 
glass to the taking-out temperature thereof in such closed state of the 
mold members. 
Other objects of the present invention, and the features thereof, will 
become fully apparent from the following description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be explained in detail by an embodiment 
thereof shown in the attached drawings. In the following there will be 
shown a case of molding a biconvex lens from a spherical glass blank, 
composed of heavy crown glass SK12, having specified values of thermal 
characteristic temperatures as shown in Table 1. The molded convex lens 
has, for example, a shape and dimensions as shown in FIG. 7. 
TABLE 1 
______________________________________ 
Strain Anneal Transition 
Yield Softening 
point point point temp. point 
Name StP AP Tg AT SP 
______________________________________ 
Viscosity 
14.5 13 12-13 11 7.65 
log .eta. 
Temp. (.degree.C.) 
503 534 550 588 672 
______________________________________ 
The yield point is defined as follows: When a specimen of glass bar (for 
example, length of 50 mm diameter of 4 mm) fully annealed is heated 
uniformly at constant rate of 4.degree. C./min. under loading of 50 g 
thereto in axial direction, the elongation and temperature are accurately 
measured, thereby obtaining a glass thermal expansion curve. In the 
thermal expansion curve, there is an inflexion point that the specimen 
apparently stops elongating and then starts contracting due to deformation 
caused by softening of the glass with elevating of the temperature. This 
inflexion point where glass is apparently changed from the expansion to 
the contraction, is called the yield point. 
Mold member s shown in FIGS. 1A to 1E are contained in a casing (not 
shown), into which nitrogen is introduced after evacuation for example to 
a pressure of 1.times.10.sup.-2 Torr. An upper mold member 2 and a lower 
mold member 3 are heated close to 620.degree. C. (corresponding to a glass 
viscosity of 10.sup.9.7 (poise) for example by a heater (not shown) 
provided in a cylindrical mold 4 surrounding the molds. When the mold 
members 2, 3 are heated to the above-mentioned temperature, the glass 
material 1 which is preliminarily heated (for example to 620.degree. C.) 
in advance in the same casing, is taken up for example by a suction hand 9 
and is placed on a molding face 3a of the lower mold member 3 through an 
aperture provided in the cylindrical mold 4 (FIG. 1A). The cylindrical 
mold 4 is mounted on a base member 5. 
Then, the upper mold member 2 is lowered by operation means 6 such as a 
ram, thereby effecting press molding. The glass material 1 may be heated 
to 620.degree. C. (corresponding to a glass viscosity of 10.sup.9.7 poise) 
in advance as explained above, and then loaded between the mold members 2, 
3, but it may be heated to the temperature after loading. The descent of 
the upper mold member 2 is continued until a stopper portion provided at 
the upper end of the mold 2 member comes into contact with the upper face 
4a of the cylindrical mold 4, and, in the course of the descent, the 
shapes of molding faces 2a, 3a of the mold members 2, 3 are transferred to 
the surface of the glass material 1, whereby it is molded into a 
predetermined product (FIG. 1B). The employed pressing load is 320 kgf, 
and the thickness of the molded article 7 is determined by the contacting 
level between the stopper portion and the upper face 4a of the cylindrical 
mold 4. 
Then, the heater is deactivated, and the cooling is executed by 
introducing, for example, nitrogen into cooling paths (not shown) provided 
in the mold members 2, 3. When the molded article (optical element) 7 
reaches 580.degree. C. (corresponding to a glass viscosity of 10.sup.11 
poise; molding pressure being zero), the molded article 7 is interposed 
from both sides thereof by fixing means 8 provided with spring mechanisms, 
and the upper mold member 2 is elevated to separate the article 7 from the 
molding face 2a. Then, the fixing means 8 is slightly elevated to separate 
the molded article 7 from the molding face 3a of the lower mold member 3 
(FIG. 1C). 
Subsequently, the fixing means 8 is returned to the original level, thereby 
placing the molded article 7 on the molding face 3a, then the upper mold 
member 2 is lowered to the original state (FIG. 1B), and the fixing means 
8 is retracted from the mold to the outside (FIG. 1D). The above-mentioned 
operations are executed promptly (with a temperature descent of about 
5.degree. C.). After the mold members are closed (with zero pressure), the 
upper and lower mold members 2, 3 and the molded article 7 are cooled to a 
temperature below the transition point of glass, for example, to 
480.degree. C. During this cooling operation, the molded article 7 is 
prevented from deformation caused by the dead weight thereof, by the 
function of the molding faces 2a, 3a of the mold members 2, 3, thereby 
maintaining the desired shape. The thermal stress and strain generated in 
the glass material before the mold members are opened, due to the 
difference in thermal expansion coefficient from the mold members, are 
eliminated by the viscoelasticity of the glass material. 
The variations in temperature, molding pressure and stress generated in the 
glass material in the molding method of the present invention can be 
represented by a chart shown in FIG. 5. In FIG. 5, a broken line indicates 
the variation in stress in the conventional method, in which the opening 
of the mold members in the course of cooling step as the present 
invention, is not conducted. A point t.sub.1 indicates the start of 
cooling, while a point t.sub.2 indicates the timing of temporary mold 
releasing, and a point t.sub.3 indicates the timing of removing the molded 
article from the mold members. 
FIGS. 6A to 6D schematically show the state of thermal stress generated in 
the glass material. Since the thermal contraction B of the glass material 
is larger than the thermal contraction A of the mold members 2, 3 (FIG. 
6A), a stress corresponding to the difference of the contractions is 
generated in a direction opposite to the illustrated arrows (FIG. 6B). 
This is presumably due to generation, on the molding faces of the mold 
members at the press molding, of an adhesion state such as molecular 
bonding between the mold surface and the glass surface in physical sense. 
If the cooling is continued in this state and the mold members 2, 3 are 
opened at the end of cooling as in the conventional molding method, the 
internal stress is released at once as shown in FIG. 6C upon releasing of 
the glass from the mold, whereby a deformation is induced in the glass 
material (arrows indicating forces generated by the release of stress). 
In the molding method of the present invention, however, in the course of 
cooling after press molding, the mold members 2, 3 are opened to separate 
the glass material from the molding faces of the mold members and to 
eliminate the adhesion state between the molding faces and the glass 
material in the press molding at a temperature range of the glass material 
corresponding to a glass viscosity of 10.sup.10 to 10.sup.12 poise, 
preferably at a glass temperature corresponding to a glass viscosity of 
10.sup.11 poise, thereby relaxing the internal stress of glass generated 
in the cooling process up to this point and releasing the thermal stress 
accumulated thus far. Consequently, when the mold members are closed again 
in order to maintain the molded state of glass and the molded glass is 
cooled in this state to the taking-out temperature, there is no longer 
generated the thermal stress in this process. This is presumably due to a 
fact that, once the molded glass surface is separated from the molding 
faces, the aforementioned molecular bond state is not established even 
when the mold members 2, 3 are closed again and the molecules of the 
atmosphere enter between the surfaces, thereby preventing re-establishment 
of such molecular bond state and enabling mutual sliding movement of the 
surfaces. 
Then, the mold members are again opened at a predetermined taking-out 
temperature (where the molded glass is not deformed by the dead weight 
thereof), for example, 480.degree. C. (corresponding to a glass viscosity 
of 10.sup.13 poise or less), and the completed molded article 7 without 
thermal stress or strain can be removed by the suction hand 9 (FIG. 1E). 
In this operation, the suction hand is maintained at about 400.degree. C., 
in order not to give thermal shock to the molded article 7 and in 
consideration of the subsequent temperature loss of the molded article to 
the taking-out (removing) operation. 
The optical element actually molded in the above-explained molding 
apparatus showed a satisfactory surfacial precision of 1/4 Newton's rings 
or less, both in the astigmatism and in the contour map of the surface. 
Another embodiment shown in FIG. 2 employs flint glass F8 for molding a 
concave optical element. In this case, after the glass blank is placed on 
the lower mold member 3' for molding concave lens, it is positioned at the 
center of the mold member by means of the aforementioned fixing means 8 
between lower member 3' and upper member 2', for molding concave lens 7'. 
Other operations and the obtained results are similar to those in the 
foregoing embodiment both the formation of the concave lens 7'. The 
concave element has a shape and dimensions as shown in FIG. 8. 
In the foregoing embodiments, the optical element may have convex or 
concave optically functional faces. The operation of separating the upper 
and lower mold members from the glass material is preferably conducted in 
prompt manner (for example within about 4 seconds). 
FIGS. 3 and 4 show comparison of the radii of curvature of the desired 
optical element (having a convex face with R=51 mm and a concave face with 
R=9 mm) and those of the molding faces of the mold members, when the 
molding method of the present invention is executed. The radii of 
curvature of the mold members are determined in consideration of the 
difference in thermal expansion coefficient, between the mold members and 
the molded product. 
In the following there will be explained a simulation of the thermal stress 
generated in the glass material in the course of cooling, utilizing a 
general-purpose structure analyzing software "MARC", supplied by Japan 
Marc Co., Ltd. The physical properties of glass and mold members, shown in 
Tables 1 and 2, are entered as the reference data. The temperature 
dependences of the thermal expansion coefficients of glass and mold 
members are as shown in FIG. 9. 
TABLE 2 
______________________________________ 
Physical Mold 
property Unit Glass members 
______________________________________ 
Thermal W/m .multidot. K 
1.34 71.2 
conductivity 
Specific heat 
kj/kg .multidot. K 
0.967 0.265 
Density kg/m.sup.3 3.19 .times. 10.sup.3 
1.47 .times. 10.sup.3 
Poisson's ratio 0.25 0.22 
Expansion /.degree.C. 
.sup. 9.0 .times. 10.sup.-6 
.sup. 3.78 .times. 10.sup.-6 
coefficient 
values being at room temperature (temperature 
dependence shown in FIG. 9 being taken 
into consideration) 
Elastic modulus 
kg/m.sup.2 6.22 .times. 10.sup.10 
(mold) 
Viscoelasticity 
Data on viscoelastic properties of glass 
______________________________________ 
The viscoelastic properties of glass shown in Table 2 can be determined in 
the following manner. At first a glass specimen in a viscoelastic 
temperature range is subjected to a bending test in which the specimen 
maintained at a constant temperature is subjected to a constant load in 
three-point support, and the deflection of the specimen is measured, and 
the creep compliance is calculated from the following equation. This 
calculation is repeated at slightly different temperatures, thereby 
determining a creep curve: 
EQU D.sub.c (t, TO)=4bd.sup.3 /1.sup.3 .times.v(t)/WO 
wherein 
D.sub.c (t, TO): creep compliance 
b: width of specimen 
d: length of specimen 
l: length of span 
v(t): deflection at load point 
WO: load. 
Since the glass in the viscoelastic temperature region has simple 
thermo-rheological properties, the creep compliance curves at different 
temperatures can be normalized to a master curve by a displacement in 
lateral direction (in time axis). The relationship between temperature and 
time in this case can be represented by a time-temperature shift factor. 
The time-temperature shift factor for the glass in this case can be 
approximated by two straight lines (Arrhenius' equation), and the 
temperature of the crossing point thereof is somewhat lower than the glass 
transition point. 
The relaxing elastic coefficient (corresponding to the elastic coefficient 
in an elastomer) can be represented as a function of temperature and time, 
because of the influence of decrease of stress relaxation. However, since 
the glass has a simple behavior in thermo-rheological properties, there 
can be obtained a master curve, as in the case of the creep compliance. 
(In general, the master curve of the relaxing elastic coefficient can be 
approximated by the reciprocal of the creep compliance.) 
The behavior of a viscoelastic substance, which is simple in the 
thermo-rheological properties, such as glass, can be represented by the 
following hysteresis integration in the linear viscoelastic theory, by 
determining the relaxing elastic coefficient E.sub.r (t, TO) at a 
temperature and a time from the master curve of the relaxing elastic 
coefficient and the time-temperature shift factor: 
##EQU1## 
wherein .tau. is time of analysis, .sigma.(t) is stress, and .epsilon.(t) 
is strain. 
Therefore, in order to add the viscoelastic properties to the numerical 
analysis, it becomes necessary to represent the master curve of the 
relaxing elastic coefficient and the time-temperature shift factor in 
numerical equations. Since the time-temperature shift factor can be 
approximated by the Arrhenius' equation as explained above, it can be 
analyzed by the analyzing software MARC of Japan MARC Co., Ltd., with the 
data inputs of the equations of straight lines and the crossing point 
thereof. Also, the master curve of the relaxing elastic coefficient can be 
approximated by the following Prony development: 
##EQU2## 
wherein t'.sup.n is conversion time of n-th order, and E.sub.r.sup.n is 
relaxing elastic coefficient of n-th order. 
As a result of such simulation, in the method of the present invention in 
which a temporary mold separation is conducted in the course of cooling, 
the thermal stress generated in the glass is almost zero after said mold 
separation as indicated by a monitor display shown in FIG. 10, whereas the 
internal thermal stress in the conventional method is gradually 
accumulated until the molded article is taken out from the molds as shown 
in FIG. 11, thus inducing a significant strain. 
As detailedly explained in the foregoing, the present invention is featured 
by opening the mold members thereby releasing the glass material from the 
molding faces of the mold members in the course of cooling after the press 
molding, then closing the mold members again in order to maintain the 
molded state of the glass material, and cooling said glass material in 
this state until the taking-out temperature. It is thus rendered possible 
to obtain an excellent effect of maintaining the optically functional 
faces of the final molded article in a highly precise desired state of 
molding, without leaving unnecessary thermal stress or strain in the 
course of cooling.