Mold for molding optical elements

A mold arranged between a pair of heater blocks for molding an optical element comprises a pair of first and second elements each having a molding surface for defining a cavity and an outer surface opposite to the molding surface and in contact with a corresponding heater block, and a ring member for guiding the first and second elements. The rear surface of at least one of the first and second elements has a part which is not in contact with the corresponding heater block. Also, another mold comprises a pair of first and second elements each having a molding surface for defining a cavity and an outer surface opposite to the molding surface, a first ring member for guiding the first and second elements, and a second outer ring member made of a material with a lower thermal conductivity than that of the first ring member. A molding method for optical elements is such that a glass with a viscosity of 10.sup.12 poise or more is fed to a mold with a temperature at or below the glass transition point, heated to a temperature corresponding to the glass viscosity of 10.sup.8 to 10.sup.10 poise and pressed for 3 to 90 seconds. The glass is then cooled at a rate of 1.5.degree. to 2.5.degree. C./sec to a viscosity of 10.sup.10 to 10.sup.11 poise and thereafter at a rate of 0.2.degree. to 1.5.degree. C./sec with the glass pressurized, and removed as an optical element at or below the glass transition point.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a mold used mainly for press molding of a 
highly precise optical element and a method for molding optical elements. 
2. Description of the Prior Art 
Recently, optical elements have tended to have a nonspherical surface with 
which both simplification of lens configuration and weight reduction of 
the lens part in an optical apparatus can be simultaneously achieved. U.S. 
Pat. No. 4,481,023 proposes a method of manufacturing a highly precise 
glass product with a low cost, by which a highly precise optical glass 
element can be obtained by shortening the time required for a 
pressurization cycle. The method proposed is outlined as follows. Using a 
glass preform having a shape similar to its final product, a mold and the 
glass preform are heated separately from each other, a pressing operation 
is initiated at a temperature corresponding to a glass viscosity of 
10.sup.8 poise or more and 10.sup.12 poise or less, respectively, for 
glass and metal molds, and the pressing operation is finished at a 
temperature corresponding to a glass viscosity less than 10.sup.13 poise. 
Then the glass is taken out. 
Japanese Patent Publication No. 56-378/1981 discloses a molding method for 
molding a raw lens material in which the temperature of a mold is kept at 
a value equal to or greater than the transition point and the softening 
point of a molded glass, a glass having flow properties is poured into the 
mold to be press molded. This condition is held for 20 seconds or more 
until the temperature distribution of the glass becomes uniform. 
However, in the prior art examples mentioned above, a glass and a mold have 
been separately heated and then the glass has been supplied to inside the 
mold to be press molded, so that in the course of press molding the 
temperature is very liable to be non-uniform, requiring the press to be 
held for a certain time. Since a heating section is provided on the entire 
peripheral portion of the mold, rapid cooling is impossible. Accordingly, 
this has negatively affected cycle time, with the result that a low-cost 
lens cannot be manufactured. 
As disclosed in Japanese Patent laid open application No. 61-26528/1986, a 
method has been known by which, after press molding, cooling is performed 
by using many molds and sequentially transferring them to a 
temperature-decline type slow cooling chamber. Also, the cooling rate 
after press molding is a very important parameter which affects lens 
performance. Japanese Patent laid open application No. 61-53126/1986 
discloses a cooling at a constant rate of 0.9.degree. C./sec. or less 
without pressurization. 
Thus, the temperature control from press molding to cooling has been known 
to be important in order to obtain a highly precise transferred surface. 
Slowing the cooling rate causes cycle time to become long, making 
difficult the supply of a low-cost optical element. Also, an optical 
element with a substantially different wall thickness between its center 
portion and peripheral portion is liable to develop a temperature 
difference, making it difficult to obtain a desired lens performance. 
Further, U.S. Pat. No. 2,292,917 discloses a mold through which a 
thick-walled lens part is liable to cool and which uses two different 
materials in thermal conduction in order to make it difficult to develop a 
temperature difference between the center portion and the peripheral 
portion of an optical element. However, the mold is complex in structure, 
making it difficult for the mold to be located between a pair of heater 
blocks and transferred. Also, U.S. patent application Ser. No. 07/198,929 
(filed on May 24, 1988) discloses a method by which a thin-walled lens 
portion is heated so as not to develop a temperature difference in the 
plastic inside the cavity. However, a heating section is provided on the 
peripheral portion of a mold, causing the cycle time to become long. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a mold and a molding 
method for optical elements for making it easy to reduce a temperature 
difference in a glass to be molded and for quickly manufacturing a highly 
precise and low-cost optical element. 
In order to achieve the object mentioned above, a mold for molding an 
optical element according to the present invention, which is to be 
arranged between a pair of heater blocks to mold an optical element, 
comprises a pair of first and second elements each having a molding 
surface for defining a cavity and a rear or outer surface opposite to the 
molding surface and in contact with a corresponding heater block, and a 
ring member for guiding said first and said second elements, the rear 
surface of at least one of said first and said second elements having a 
non-contact part which will not be in contact with the corresponding 
heater block at an area near a thin portion of the optical element. 
With the configuration mentioned above, the rear or outer surface of the 
mold near a thin portion of the optical element to be molded has a 
non-contact part which is not in contact with the corresponding heater 
block in each stage, so that thermal conduction is inhibited at the 
non-contact part. This arrangement inhibits the development of a 
temperature difference in the glass. When there are different curvatures, 
the internal portion of the glass on the mold side which has a smaller 
curvature is liable to develop a temperature different than the outer 
portions of the glass. Accordingly, a positive effect can be more easily 
obtained by providing the mold with a non-contact part which is not in 
contact with the heater block. 
Also, another mold for molding an optical element comprises a pair of first 
and second elements each having a molding surface for defining a cavity, a 
first ring member for guiding the first and the second elements, and a 
second ring member, made of a material with a lower thermal conductivity 
than that of the first ring member, disposed to the outside of the first 
ring member. 
With this configuration, the mold has the second ring member made of a 
material with a lower thermal conductivity than that of the first ring 
member and disposed on the outside of the first ring member. The second 
ring member prevents heat from escaping from the peripheral portion of an 
optical element and temperature difference from developing in the glass. 
Preferably, a more positive effect can be obtained by providing a 
clearance of 3 mm or less between said first and second ring members. 
Further, with said second ring member made longer than said first ring 
member, the heater blocks come in contact with said second ring member, 
permitting the thickness of the optical element to be predetermined. At 
this time, finishing the second ring member with a high precision allows 
the parallelism of a lens to be predetermined. The properties to be 
considered in selecting the second ring member materials include superior 
oxidation-resistance, large compressive strength and low cost. A suitable 
material in which the above properties can be found is stainless steel, 
preferably that of austenitic base or of martensitic bas. The first ring 
member guides said first element and said second element. When a mold 
having a molding surface is inserted into the first ring member, the first 
molding surface and the second molding surface become perpendicular to the 
internal surface of the first ring member, thereby allowing the optical 
axes to be aligned. 
A molding method for molding an optical element according to the present 
invention to achieve the object mentioned above is a method in which a 
glass with a viscosity of 10.sup.12 poise or more is supplied to a mold 
with a temperature at or below the transition point, is heated up to a 
temperature corresponding to the glass viscosity of 10.sup.8 to 10.sup.10 
poise, is press molded for 3 to 90 seconds, and is then cooled at a 
cooling rate of 1.5.degree. to 2.5.degree. C./sec. to 10.sup.10 to 
10.sup.11 poise or less with the glass pressurized. Thereafter, the glass 
is cooled at a cooling rate of 0.2.degree. to 1.5.degree. C./sec., and is 
then taken out as an optical element at a temperature at or below the 
glass transition point. 
The method mentioned above allows a highly precise and low-cost optical 
element to be quickly manufactured. The molding method for optical 
elements according to the present invention is performed within a range of 
viscosities of 10.sup.8 to 10.sup.12.75 poise, so that both the glass and 
the mold are cooled to prevent deformations in the shape of a molded 
product and to prevent the development of distortions due to cooling. 
Also, when under a pressurized condition the cooling rate is controlled 
such that the temperature of the mold is the same as that of the glass in 
the cooling process before annealing, no change in surface accuracy is 
found and there is little residual distortion even though annealing is 
performed. On the contrary, an optical element having been molded without 
being processed with such cooling process cannot maintain a shape obtained 
before the cooling process when annealing is performed. 
A press molding machine used for the molding method for optical elements is 
a simplified press molding machine which has no special space for 
replacing air with inert gas and performs no vacuum exhaust.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings, molds for molding optical elements according to 
the present invention will be explained hereinafter. 
FIGS. 1(a), (b) and (c) show sectional views of molds for molding a 
bi-convex lens. In each drawing, an upper element 1 and a lower element 2 
are arranged such that they are guided by a ring member 3. The upper 
element 1 has a molding surface 1a and a rear or outer surface 1b opposite 
to the molding surface 1a, and the lower element 2 has a molding surface 
2a and a rear or outer surface 2b opposite to the molding surface 2a. The 
molding surfaces 1a and 2a define a cavity. The upper element 1 is in 
contact with an upper heater block 16 at its rear surface 1b, and the 
lower element 2 is in contact with a lower heater block 17 at its rear 
surface 2b. The upper and lower heater blocks 16 and 17 are heated by 
cartridge heaters 16a and 17a as heating sources, respectively. Between 
the upper element 1 and the lower element 2, a raw glass material 4 for a 
lens to be molded is held. The raw glass material 4 is heated with the 
heat produced by the heater blocks, and press molded with a pressure 
applied from either or both of the heater blocks 16 and 17. 
In a cooling process after having been heated and press molded, the whole 
of the lens to be molded must be uniformly cooled. Accordingly, a part of 
the rear surface of either or both of the upper element 1 and the lower 
element 2 is arranged such that it is not in contact with the 
corresponding heater block. The part which is not in contact with the 
heater block corresponds to a thin portion of the lens. 
In the structure shown in FIG. 1(a), the shape of the raw glass material 4 
is nearly spherical. A groove 5a whose cross section is triangular is 
provided on a part near the outer periphery of the rear surface 2b of the 
lower element 2. Accordingly, the rear surface 2b of the lower element 2 
is not in contact with the lower heater block 17 at the groove 5a. Since, 
near the non-contact part (or groove) 5a, heat is difficult to conduct to 
the heater block, the cooling rate at the periphery (thin portion) of the 
lens to be molded becomes slower than that of the center portion (thick 
portion). Accordingly, the lens is uniformly cooled, allowing a uniform 
molded lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 45 mm, the radius of the curvature on the lower side 
is 32 mm, the diameter is 30 mm, the thickness at the center portion is 
7.5 mm, and the thickness at the periphery is 1.2 mm. In this case, the 
cross section is 3 mm deep with a triangular section located at a diameter 
of 24 mm corresponding to 80% of the diameter of the lower element 2 and 
defining the part 5a which is not in contact with the heater block 17. 
In FIG. 1(b), the raw lens material 4 is cylindrical. The upper element 1 
is provided with a groove 5b of semi-circular cross section at a part near 
the periphery of its rear surface 1b such that part of the upper element 1 
is not in contact with the upper heater block 16. The lower element 2 is 
provided with a groove 5b' of quadrangular cross section at a part near 
the periphery of its rear surface, such that part of the lower element 2 
is not in contact with the lower heater block 17. With the grooves 5b and 
5b', the cooling at the lens periphery is inhibited, so that the lens is 
uniformly cooled, allowing a uniform molded lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 50 mm, the radius of the curvature on the lower side 
is 33 mm, the diameter is 30 mm, the thickness at the center portion is 7 
mm, and the thickness at the periphery is 1.1 mm. In this case, the cross 
section is 8 mm deep with a quadrangular section located at a diameter of 
24 mm corresponding to 80% of the diameter of the lower element 2 with the 
small radius of curvature to its inside. The groove 5b' near the periphery 
of the rear surface of the lower element defines the part 5b' which is not 
in contact with the heater block 17, and the cross section is 1 mm deep 
with a semicircular section located at a diameter of 21 mm corresponding 
to 70% of the diameter of the upper element 1. 
In FIG. 1(c), the raw lens material is cubic. Near the periphery of the 
rear surface 2b of the lower element 2, a groove 5c whose cross section is 
triangular is provided and forms a non-contact part not in contact with 
the lower heater block 17. With the non-contact part (or groove) 5c which 
is not in contact with the heater block 17, the cooling of the lens 
periphery is inhibited, so that the lens is uniformly cooled, allowing a 
uniform molded lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 50 mm, the radius of curvature on the lower side is 
33 mm, the diameter is 30 mm, the thickness at the center portion is 7 mm, 
and the thickness at the periphery is 1.1 mm, which is the same as for the 
example of FIG. 1(b) and gives a larger deformation because the raw glass 
material 4 to be molded is cubic. Accordingly, the lower element 2 is made 
short, so that the part 5c not in contact with the heater block 17 is 
configured to form a triangular cross section. 
FIGS. 2(a), (b) and (c) show sectional views of molds for molding a 
bi-concave lens. 
In the structure shown in FIG. 2(a), the shape of the raw glass material 4 
is cylindrical. A groove 5d whose cross section is quadrangular is 
provided on the part of the lower element 2 near the center portion of the 
rear surface 2b thereof. Accordingly, the rear surface 2b of the lower 
element 2 is not in contact with the lower heater block 17 at the groove 
5d. Since, near the non-contact part 5d, heat is difficult to conduct to 
the heater block, the cooling rate at the center portion (thin portion) of 
the lens to be molded becomes slower than that at the periphery (thick 
portion). Accordingly, the lens is uniformly cooled, allowing a uniform 
molded lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 90 mm, the radius of curvature on the lower side is 
80 mm, the diameter is 25 mm, the thickness at the center portion is 3.2 
mm, and the thickness at the perimeter is 5 mm. In this case, a groove 5d 
which is 2 mm deep and forms a part which is not in contact with the 
heater block 17 is defined at a diameter of 8 mm and encompasses 40% or 
less of the diameter of the lower element 2. 
In FIG. 2(b), the raw lens material 4 is cylindrical. Near the center of 
the rear surface 1b of the upper element 1, a groove 5e whose cross 
section is quadrangular is provided, and forms a non-contact part not in 
contact with the upper heater block 16. Near the center of the rear 
surface 2b of the lower element 2, a groove 5e' whose cross section is 
quadrangular is also provided, and forms a non-contact part not in contact 
with the lower heater block 17. With the parts 5e and 5e' which are not in 
contact with the heater blocks, the cooling of the lens center portion is 
inhibited, so that the lens is uniformly cooled, allowing a uniform molded 
lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 88 mm, the radius of curvature on the lower side is 
50 mm, the diameter is 15 mm, the thickness at the center portion is 1.1 
mm, and the thickness at the perimeter is 2 mm. In this case, the part 5e' 
which is 6 mm deep is defined at a diameter of 4.5 mm corresponding to 30% 
of the diameter of the lower element 2, and the part 5e which is 2 mm deep 
is defined at a diameter of 4.5 mm corresponding to 30% of the diameter of 
the upper element 1. 
In FIG. 2(c), the raw lens material 4 is of disc shape with a rounded 
periphery. Near the center of the rear surface 2b of the lower element 2, 
a groove 5f whose cross section is semicircular is provided, and forms a 
non-contact part not in contact with the heater block 17. With the part 5f 
which is not in contact with the heater block 17, the cooling of the lens 
center portion is inhibited, so that the lens is uniformly cooled, 
allowing a uniform molded lens to be obtained. 
As an example, a lens has a configuration in which the radius of curvature 
on the upper side is 90 mm, the radius of curvature on the lower side is 
46 mm, the diameter is 15 mm, the thickness at the center portion is 1 mm, 
and the thickness at the perimeter is 2 mm. The semicircular non-contact 
part 5f is defined at a diameter of 6 mm corresponding to 40% or less of 
the diameter of the lower element 2. 
Referring to the drawings, another mold for optical elements according to 
the present invention will be explained hereinafter. 
FIGS. 3(a) and (b) show sectional views of molds for molding a bi-convex 
lens. In each drawing, the upper element 1 and the lower element 2 are 
arranged such that they are guided by a first ring member 6. Further, a 
second ring member 7 is provided on the outside of the first ring member. 
The upper and lower heater blocks 16 and 17 are heated by cartridge 
heaters 16a and 17a as heating sources, respectively. Between the upper 
element 1 and the lower element 2, a raw glass material 4 for a lens to be 
molded is held. The raw glass material 4 is heated with the heat produced 
by the heater blocks, and press molded with the pressure applied from 
either or both of the heater blocks 16 and 17. 
In a cooling process after having been heated and press molded, the whole 
of the lens to be molded must be uniformly cooled. Accordingly, the second 
ring member made of a material with a lower thermal conductivity than that 
of the first ring member is provided on the outside of the first ring 
member. The first ring member material is more easily kept at a uniform 
temperature by using the same material as that of the mold. A material 
which is suitable and has a superior thermal penetrability is used for the 
mold and the first ring member according to the present invention is a 
super hard alloy containing, as a main component, a tungsten carbide (WC) 
with a thermal conductivity of 43.2 kcal/mh.degree. C. Also, a cermet 
consisting mainly of TiN, TiC or Cr.sub.3 C.sub.2, may be used. Further, 
any other material may be used if its thermal conductivity is 30 
kcal/mh.degree. C. or more. At this time, with the clearance between the 
outer periphery of the upper and lower elements and the inner periphery of 
the first ring member made very small to maintain perpendicularity, the 
optical axis of the optical element to be molded, the center axis of each 
of the upper and lower elements and the center axis of the first ring 
member can be made coincident with each other. In order to insulate 
against heat loss during cooling while using the second ring member, 
austenitic base stainless steel with a superior heat resistance, such as 
SUS316 with a thermal conductivity of 13.4 kcal/mh.degree. C. or SUS304 
with a thermal conductivity of 14.0 kcal/mh.degree. C., and martensitic 
base stainless steel such as SUS431 with a thermal conductivity of 17.5 
kcal/mh.degree. C. can be used. Further, with the second ring member made 
longer than the first ring member and with the end faces of the second 
ring member made mutually parallel, both the resultant lens thickness and 
the parallelism of the lens surfaces can be predetermined. That is, the 
second ring member is adapted to be in contact at the end faces thereof 
with the upper and lower heater blocks in the mold shown in FIG. 3(a), and 
with flange parts of the upper and lower elements in the mold shown in 
FIG. 3(b), to thereby predetermine the thickness of the resulting lens. 
Also, when the clearance between the outer diameter of the first ring 
member and the inner diameter of the second ring member is excessively 
wide, the insulation against loss of heat is less effective, so that the 
clearance should be 5 mm or less, preferably 3 mm or less. The difference 
between the molds shown in FIGS. 3(a) and (b) is that the mold of FIG. 
3(b) has flange parts extending outwardly from the rear surfaces of the 
upper and lower elements and the mold of FIG. 3(a) does not. Although the 
mold shown in FIG. 3(b) is usually preferable for purposes of being 
transferred, the mold shown in FIG. 3(a) is preferable to provide an 
improved time cycle. 
Referring to the drawings, the method and apparatus for molding optical 
elements using the mold for optical elements according to the present 
invention will be explained hereinafter. 
FIG. 4 is a perspective view of a press molding machine used in the present 
invention, and FIG. 5 is a plan view of FIG. 4. A mold holding a raw glass 
material with a viscosity of 10.sup.12 poise or more is fed into a mold 
feeding port, allowed to slide on a feeding rail 22 by a suitable transfer 
means (not shown), and fed to a heating stage 25. The raw glass material 
is preferably a glass for molding whose shape is configured by a 
relational expression 0.02X&lt;Y&lt;0.8X where Y is a difference between the 
diameter of an optical element to be molded and that of the glass for 
molding, and X is a movement of the mold. The heating stage 25 and a press 
stage 26 have been heated to a temperature corresponding to a viscosity of 
the glass of 10.sup.8 to 10.sup.10 poise. After the mold has been 
sufficiently heated, the mold is allowed to slide on a transfer rail 31 by 
a transfer member 24, and sent to the press stage 26. Then, with a press 
cylinder 21 lowered by a pressurizing device 30, the mold is pressed by 
the upper heater block 16 (FIG. 6). Press pressure is sufficient when it 
can deform the raw glass material for a short time, and is preferably 500 
kg/cm.sup.2 or less. After having been pressed for 30 to 90 seconds, with 
the press cylinder 21 raised, the mold is transferred to a first cooling 
stage 27 by the transfer member 24. The first cooling stage 27 is set to a 
temperature lower than that of the press stage. Although a second cooling 
stage 28 is provided in this embodiment, in some cases only one stage will 
be necessary. Accordingly, in a pressed condition after having been 
transferred to these cooling stages, the mold is cooled at a cooling rate 
of 1.5.degree. to 2.5.degree. C./sec. for viscosities of 10.sup.10 to 
10.sup.11 poise or less and at a cooling rate of 0.2.degree. to 
1.5.degree. C./sec. for viscosities of 10.sup.10 to 10.sup.11 poise or 
more. Finally, the mold whose temperature is at or below the transition 
point of the glass is transferred to the position between rapid cooling 
stages 18 and 19 to be cooled (FIG. 7), and is removed via an exit port by 
a removal rail 23. At this time, any distortions can be removed by feeding 
the mold to an annealing furnace (not shown) without transferring it to 
the rapid cooling stages. In a molding chamber 33, nitrogen gas is being 
blown off through nitrogen gas blow off ports 32, making the inside of the 
chamber inactive. The transfer member 24 moves such that it moves in the y 
(-) direction to hold the mold, and moves in the x (-) direction to 
transfer the mold to the next stage. Thereafter, it moves in the y (+) 
direction followed by the x (+) direction. With the movement repeated, the 
whole transferring process can be carried out. 
EXAMPLE 1 
The molding method for optical elements in the Example was such that 
molding was performed using the mold shown in FIG. 1(b). The glass 
material to be pressed was SF3. First, the glass was preheated to 
200.degree. C. at a location not shown, fed into a mold at room 
temperature, and heated to 480.degree. C. (corresponding to a glass 
viscosity of 10.sup.8.5 poise) in the heating stage. Then, the glass was 
transferred to the press stage 26, and, with the press cylinder 21 
lowered, pressed for 10 seconds. The press time is not particularly 
important where it does not affect cycle time. The press pressure was 500 
kg/cm.sup.2. Although the press pressure raises no problem even when it is 
500 kg/cm.sup.2 or more, a press pressure of 1000 kg/cm.sup.2 or more is 
not preferable because it may effect the highly precise mold shape. Then, 
after the press pressure had been released, the glass was transferred to 
the first cooling stage 27. After having been transferred to the first 
cooling stage 27 which was constantly kept at 435.degree. C., a press 
pressure was applied to the glass for 22 seconds. At a cooling rate of 
1.8.degree. C./sec., the glass was cooled to 440.degree. C. (corresponding 
to a glass viscosity of 10.sup.11 poise) Then, the glass was transferred 
to the second cooling stage 28 kept at 400.degree. C., and held for 50 
seconds in a pressed condition. At this time, the glass temperature became 
405.degree. C. (corresponding to a glass viscosity of 10.sup.13 poise) at 
a cooling rate of 0.7.degree. C./sec. Then, the glass was rapidly cooled 
and taken out as a lens. The optical performance of the lens was superior. 
EXAMPLE 2 
Press molding was performed using the mold shown in FIG. 2(a). The glass 
material to be molded was SF8. First, the raw glass material was fed into 
a mold at room temperature, and transferred to the heating stage 25. The 
glass was heated to 540.degree. C. (corresponding to a glass viscosity of 
10.sup.9 poise) in the heating stage 25. Although a heating temperature 
corresponding to a glass viscosity of 10.sup.10 poise or less allows 
formation to be completed in a short time, a temperature corresponding to 
a glass viscosity of 10.sup.10 poise or more is not preferable because it 
makes deformation difficult. Then, the glass was transferred to the press 
stage 26, and, with the press cylinder 21 lowered, pressed for 10 seconds. 
The press pressure was 100 kg/cm.sup.2. Then, after the press pressure had 
been released, the glass was transferred to the first cooling stage 27. 
After having been transferred to the first cooling stage 27 which was 
constantly kept at 415.degree. C., press pressure was applied to the glass 
for 70 seconds. The cooling rate was 2.5.degree. C./sec. for glass 
viscosities of 10.sup.11 poise or less and 1.5.degree. C./sec. for glass 
viscosities of 10.sup.11 to 10.sup.13 poise. Thus, after the glass had 
been pressed to at or below its transition point, the press cylinder 21 
was raised. Thereafter, even when the lens, cooled to at or below the 
glass transition point, is cooled rapidly, the shape accuracy is not 
affected. However, since the residual distortion is as large as 200 nm/cm 
due to cooling in a short time, without being transferred to the second 
cooling stage and the quenching stage, the glass was fed to an annealing 
furnace by means (not shown) to remove the residual distortion to 4 nm/cm 
or less. The wavefront aberration of the pressed lens thus obtained was 
highly precise at RMS=0.028.lambda.. On the other hand, in this case, the 
first cooling stage was constantly kept at 415.degree. C., so that the 
glass was not released from the mold, allowing a highly precise pressed 
lens to be molded. However, when the first cooling stage was constantly 
kept at 380.degree. C., the cooling rate for glass viscosities of 
10.sup.11 poise or less became 2.8.degree. C./sec., and that for glass 
viscosities of 10.sup.11 to 10.sup.13 poise became 2.0.degree. C./sec. 
Accordingly, the glass was released from the mold at a viscosity near 
10.sup.11 poise, causing no lens performance to be obtained. 
EXAMPLE 3 
Press molding was performed using the mold shown in FIG. 3(a). The glass 
material to be molded was SF6. First, the raw glass material was fed into 
a mold at room temperature, and transferred to the heating stage 25. The 
glass was heated to 520.degree. C. (corresponding to a glass viscosity of 
10.sup.8 poise) in the heating stage. At this time, although the glass may 
be heated to a viscosity of 10.sup.8 poise or more, the heating time 
becomes long which does not improve lens performance, so that the heating 
up to a viscosity of 10.sup.8 poise is sufficient with respect to cycle 
time. Then, the glass was transferred to the press stage 25, and, with the 
press cylinder 21 lowered, pressed for 10 seconds. The press pressure was 
10 kg/cm.sup.2. Then, after the press pressure has been released, the 
glass was transferred to the first cooling stage 27. After having been 
transferred to the first cooling stage 27 which was constantly kept at 
465.degree. C., a press pressure was applied to the glass for 30 seconds. 
At this time, at the cooling rate of 1.5.degree. C./sec., the glass was 
cooled to 470.degree. C. (corresponding to a glass viscosity of 10.sup.10 
poise). Then, the glass was transferred to the second cooling stage 28 
kept at 430.degree. C., and held for 80 seconds in a pressed condition. At 
this time, the glass became 431.degree. C. (corresponding to a glass 
viscosity of 10.sup.13 poise), and the cooling rate was 0.5.degree. 
C./sec. However, an attempt was then made to cool the glass at the cooling 
rate of 1.0.degree. C./sec. for viscosities of up to 10.sup.10 poise and 
0.1.degree. C./sec. for viscosities up to 10.sup.13 poise in order to 
improve lens performance, with the result that the lens performance was 
nearly the same and the cooling time for viscosities up to 10.sup.13 poise 
increased by 300 seconds, causing cycle time to be long and lens cost to 
be high. The pressed lens thus cooled was transferred to the rapid cooling 
stage 29, cooled rapidly and taken out. The wavefront aberration of the 
pressed lens thus obtained was highly precise at RMS=0.025.lambda.. Also, 
in this case, although the mold had no non-contact part not in contact 
with the heater block, a pressed lens having a very good lens performance 
was molded by using the second ring element. 
EXAMPLE 4 
Using the mold shown in FIG. 1(b), the molding method was also the same as 
for the Example 1. The shape of the raw glass material used was 
cylindrical with a diameter of 27 mm and a height of 11 mm. A convex lens 
was molded with a configuration in which the lens diameter was 30 mm and 
the thickness at the center portion was 7 mm after having been pressed. 
Accordingly, the difference between the diameter of the resulting optical 
element and the diameter of the raw glass material was 3 mm (Y), and the 
movement of the mold was 4 mm (X), resulting in Y=0.75 X. The wavefront 
aberration of the pressed lens obtained by this molding method was highly 
precise at RMS=0.024.lambda.. Using another raw glass material with a 
diameter of 27 mm and a height of 17 mm, a convex lens was molded with a 
configuration in which the lens diameter was 30 mm and the thickness at 
the center portion was 7 mm after having been pressed. Accordingly, the 
difference between the diameter of the resulting optical element and that 
of the raw glass material was 3 mm (Y) and the movement of the mold 10 mm 
(X), resulting in Y=0.3 X. The wavefront aberration of the pressed lens 
obtained by this molding method was highly precise at RMS=0.030.lambda.. 
However, using a glass preform as a raw glass material with a diameter of 
29.95 mm and a height of 10 mm, a convex lens was molded with a 
configuration in which the lens diameter was 30 mm and the thickness at 
the center portions was 7 mm after having been pressed. Accordingly, the 
difference between the diameter of the resulting optical element and the 
diameter of the raw glass material was 0.05 mm (Y), and the movement of 
the mold was 3 mm (X), resulting in Y=0.016X. The wave front aberration of 
the pressed lens obtained by this molding method was RMS=0.059.lambda.. 
Thus, the result was that a highly precise pressed lens was not 
manufactured. Further, using a ball-shaped raw glass material with a 
diameter of 19 mm, a convex lens was molded with a configuration in which 
the lens diameter was 30 mm and the thickness at the center portion was 7 
mm after having been pressed. Accordingly, the difference between the 
diameter of the resulting optical element and the diameter of the raw 
glass material was 11 mm (Y) and the movement of the mold was 12 mm (X), 
resulting in Y=0.916X. The wavefront aberration of the pressed lens 
obtained by this molding method was RMS=0.078.lambda.. Thus, the result 
was again that a highly precise pressed lens was not manufactured. Thus, 
where the value of the Y of the raw glass material used exceeds 0.8X, the 
lens is liable to develop an eccentricity, and where the value is less 
than 0.02X, it gives poor transferring properties, so that it is 
preferable to use the raw glass material within that range.