Method of injection molding plastic lens

A cavity 22 in an injection molding assembly includes a lower mold insert 21 for shaping a lens convex surface and an upper mold insert 20 for shaping a lens concave surface. When the heated injection molding assembly is cooled and a lens is ejected after a molten resin is pressurized by the upper mold insert 20, the temperature of the lower mold insert 21 is lowered below the temperature of the upper mold insert 20, which prevents the molded lens from bending at a central portion and enable high transfer precision of insert shapes.

TECHNICAL FIELD 
The present invention relates to a method of injection molding a plastic 
lens and, more particularly, to temperature control of an injection 
molding assembly for making a highly precise lens molded in a cavity. 
BACKGROUND ART 
An injection molding technology to mold a meniscus-shaped plastic spectacle 
lens is shown in Japanese Patent Publication No. Hei 5-30608. In this 
technology, a cavity for molding the lens is formed inside an injection 
molding assembly, the cavity containing a pair of cavity forming members 
for shaping a convex surface and a concave surface of the lens disposed 
vertically opposite with each other. The injection molding assembly is 
heated before filling a molten resin in the cavity and one cavity forming 
member is moved toward the other cavity forming member to pressurize the 
molten resin filled in the cavity. Subsequently, the injection molding 
assembly is cooled to cool and solidify the molten resin, and the molded 
lens is taken out (=eject). 
It is shown in Japanese Patent Laid-open No. Hei 6-31785 that an injection 
molding assembly is heated by means of a heating fluid such as steam and 
cooled by means of a cooling fluid such as air, water. In addition, after 
a molten resin is filled in a cavity in the injection molding assembly of 
which the temperature is raised beyond flow halting temperature of the 
molten resin, the temperature of the injection molding assembly is lowered 
below a glass transition point for molding a lens by cooling and 
solidifying the molten resin. 
A lens is a precise molded product which requires high molding precision. 
Especially in a meniscus lens used for a spectacle lens, it is important 
that a convex shape and a concave shape of a pair of cavity forming 
members for shaping a convex surface and a concave surface of the lens are 
precisely transferred to the lens. However, when a lens to be molded has a 
difference in thickness between a central portion and a peripheral portion 
thereof, a thickness of the central portion being larger than that of the 
peripheral portion, the lens is easy to bend at the thin central potion. 
When such a disadvantage occurs, a high-precision lens to which a convex 
shape and a concave shape of cavity forming members are accurately 
transferred is not obtained. 
For manufacturing a high-precision lens, it is important to prevent heat 
distortion or shrinkage deformation from occurring, which requires that 
the entire molten resin filled in the cavity is uniformly cooled. However, 
since the amount of the molten resin filled in the cavity corresponds to 
the volume of the lens and differs depending on a type of the lens, 
especially lens power, uniform cooling is difficult by controlling the 
temperature uniformly. Thus, temperature control of an injection molding 
assembly is desired for molding each lens highly precisely irrespective of 
the above difference. 
An object of the present invention is to provide a plastic lens injection 
molding method to mold a high-precision lens by means of proper 
temperature control of an injection molding assembly. 
DISCLOSURE OF THE INVENTION 
A method of injection molding a plastic lens according to the present 
invention provides a cavity for molding the lens formed by a pair of 
cavity forming members disposed opposite with each other inside an 
injection molding assembly for shaping a convex surface and a 
corresponding concave surface of the lens. The injection molding assembly 
is heated before filling a molten resin in the cavity and pressurizing the 
molten resin. Thereafter, the injection molding assembly is cooled to cool 
and solidify the molten resin for molding the lens in the cavity, before 
ejecting from the cavity. In the aforementioned method of injection 
molding the plastic lens, the temperature of the cavity forming member for 
shaping the lens convex surface is lowered below the temperature of the 
cavity forming member for shaping the lens concave surface in ejecting the 
lens. 
According to the above injection molding method, when the lens is ejected, 
the temperature of the lens convex surface is lower than that of the lens 
concave surface, that is, the lens convex surface is cooled and solidified 
earlier than the lens concave surface, which prevents the lens from 
bending at a central portion thereof. Consequently, a high-precision lens 
can be obtained, where shapes of the convex surface and the corresponding 
concave surface of a pair of the cavity forming members are precisely 
transferred. 
The aforementioned injection molding method is used for molding a meniscus 
lens, especially more effective in molding a lens having larger thickness 
of a peripheral portion than the thickness of a central portion (a minus 
lens). 
When the minus lens is molded, it is preferable that the difference in 
temperature between the cavity forming member for shaping the lens convex 
surface and the cavity forming member for shaping the lens concave surface 
is enlarged in proportion to increase in the power (meaning spherical 
vertex refractive power and/or cylindrical refractive power in the present 
invention) of the lens molded in the cavity. Usually, as the lens power 
increases, the thickness of a peripheral portion becomes larger than that 
of a central portion, that is, a difference in thickness is enlarged, 
which causes the lens to bend easily at the central portion. However, the 
central portion of the lens is prevented from being bent even in a minus 
lens having large thickness difference by enlarging the temperature 
difference between the cavity forming member for shaping the lens convex 
portion and the cavity forming member for shaping the lens concave portion 
in proportion to the increase in the lens power. 
Time to cool the injection molding assembly after pressurization of the 
molten resin is preferably lengthened in proportion to the increase in the 
lens power in order to mold each of highly precise lenses having different 
power. As the lens power increases, the volume of the lens, that is, the 
amount of the molten resin filled in the cavity increases. Therefore, the 
whole molten resin in the cavity can be gradually cooled uniformly to a 
predetermined temperature by lengthening a cooling time in proportion to 
the increase of the lens power. Consequently, each of lenses with 
different powers can be molded highly precisely with little heat 
distortion, little shrinkage deformation and the like. 
In order to lower the temperature of the cavity forming member for shaping 
the lens convex surface below the temperature of the cavity forming member 
for shaping the lens concave surface, but the temperatures of the two 
cavity forming members may be the same or almost the same over the 
majority of the cooling time after pressurization of the molten resin. 
However, in order to securely lower the temperature of the lens convex 
surface below the temperature of the lens concave surface in ejecting, it 
is preferable that the cooling time of the injection molding assembly is 
controlled while differentiating the temperature of the cavity forming 
member for shaping the lens concave portion and the cavity forming member 
for shaping the lens convex portion by controlling flow rate of the 
temperature controlling fluid circulating in the injection molding 
assembly for raising and lowering the temperature of the injection molding 
assembly, thereby lowering the temperature of the cavity forming member 
for shaping the lens convex surface below the temperature of the cavity 
forming member for shaping the lens concave surface. 
In the above, a pair of the cavity forming members for shaping the convex 
surface and the concave surface of the lens may be opposed with each other 
vertically or horizontally. In other words, an injection molding machine 
in which the injection molding assembly is mounted can be vertically or 
horizontally structured. 
The number of cavities provided in the injection molding assembly is 
optional. One or more than one cavity is available. 
In the method of injection molding the plastic lens according to the 
present invention, the molten resin is filled in the cavity for molding 
the lens inside the heated injection molding assembly and pressurized. 
Subsequently, the injection molding assembly is cooled to cool and 
solidify the molten resin so as to mold the lens in the cavity before 
ejecting the lens from the cavity. The injection molding method is 
characterized in that the time to cool the injection molding assembly is 
lengthened in proportion to increase in the power of the lens molded in 
the cavity. 
According to the aforementioned injection molding method, since the cooling 
time of the injection molding assembly is controlled in proportion to 
change in power, the entire molten resin can be uniformly cooled to the 
predetermined temperature, thereby manufacturing a high-precision lens 
with little heat distortion, little shrinkage deformation and the like. 
The injection molding method is available for molding, a lens of which a 
thickness of a peripheral portion is smaller than that of a central 
portion (a plus lens), and a semi-finished lens as well as the 
aforementioned minus lens. 
Moreover, the injection molding method is applicable not only for molding a 
meniscus lens but for molding other types of lenses. 
When the lens is a spectacle lens, some spectacle lenses have the same lens 
power, and different astigmatic powers. In such spectacle lenses with the 
same power and different astigmatic powers, it is preferable that the 
cooling time of the injection molding assembly is lengthened in proportion 
to increase in the astigmatic power, since the amount and/or the shape of 
molten resin filled in the cavity change in accordance with the astigmatic 
power even when the surface area of cavities in the injection molding 
assembly have no substantial difference. 
The injection molding assembly can be heated and cooled by means of an 
electric heater, air cooling and the like. However, if the injection 
molding assembly is heated and cooled by means of a temperature 
controlling fluid of which the temperature is controlled, more 
specifically a heating fluid and a cooling fluid, temperature control can 
be performed highly precisely and easily. 
When the lens is a spectacle lens, it is preferable that the temperature of 
the injection molding assembly is controlled in accordance with 
temperature curve, at least two temperature curves being prepared for weak 
power and strong power for the minus lens and at least one for the plus 
lens. In molding a semi-finished lens of which one surface is later 
processed, it is preferable that another temperature curve is further 
prepared and the temperature of the injection molding assembly is 
controlled in accordance with the temperature curve. Thus, at least three 
temperature curves of the injection molding assembly, that is, for a weak 
minus lens, a strong minus lens and a plus lens, or four temperature 
curves can be prepared when a temperature curve for a semi-finished lens 
is included, so that temperature control can be easily conducted. 
When the minus lens and the plus lens are molded, it is preferable that 
time to cool the injection molding assembly after pressurization of the 
molten resin is set for respective groups divided by lens spherical power 
and lens astigmatic power. Especially in molding the minus lens, the 
cooling time is preferably set for respective groups divided on the basis 
of the sum of lens spherical power and lens astigmatic power.

BEST MODE FOR CARRYING OUT THE INVENTION 
An embodiment of the present invention will be described below with 
reference to the drawings. An injection molding method according to the 
present embodiment is for molding a meniscus lens for glasses. An 
injection molding assembly used for the injection molding method is shown 
in FIG. 1 to FIG. 3. FIG. 2 and FIG. 3 are sectional views taken along the 
II--II line and the III--III line in FIG. 1. The injection molding 
assembly can be formed of optional material such as glass and ceramic 
besides metal. Material for spectacle lenses as molded products is a 
thermoplastic resin such as PMMA (polymethyl methacrylate), and PC 
(polycarbonate). 
A structure of the injection molding assembly is hereunder described with 
reference to FIG. 1 to FIG. 3. The injection molding assembly is composed 
of an upper mold 1 and a lower mold 2. The upper mold 1 is a movable mold 
which opens and closes vertically in relation to the lower mold 2 as s 
fixed mold, and a parting line PL extends horizontally. The upper mold 1 
is composed of a mold body 3 on a lower side and a die fitting member 4 in 
an upper side. The mold body 3 is provided with insert guides 5, mold 
plates 6 and 7 and the like. The die fitting member 4 is provided with an 
upper member 8 and a lower member 9 and the like. The lower mold 2 is 
composed of insert guides 10, mold plates 11 and 12, a sprue bush 13 and 
the like. 
As clearly shown in FIG. 2, the mold body 3 of the upper mold 1 is mounted 
on the die fitting member 4 with a bolt 14. In this mounting, the mold 
body 3 is mounted being guided to the lower mold 2 by means of a guide rod 
15 to be freely movable within a margin S. The margin S is opened between 
the mold body 3 and the die fitting member 4. The mold body 3 is always 
resiliently biased downward by means of a plate spring 16 attached on an 
outer periphery of the bolt 14. 
A clamping cylinder (not shown) is provided above the die fitting member 4 
which is mounted on the clamping cylinder. By the clamping cylinder the 
die fitting member 4 and the mold body 3 vertically move and the upper 
mold 1 composed of the mold body 3 and the die fitting member 4 vertically 
moves to open and close in relation to the lower mold 2. This vertical 
movement is conducted while an end portion 15A of the guide rod 15 in the 
upper mold 1 is inserted into and pulled out from a guide pipe 17 in the 
lower mold 2. The upper mold 1 and the lower mold 2 are aligned in closing 
the mold by a positioning pin 18 in the upper mold 1 being inserted in a 
positioning sleeve 19 in the lower mold 2. 
A margin setting cylinder (not shown) is provided below the lower mold 2. 
When the mold body 3 in the upper mold 1 abuts on the lower mold 2 by the 
clamping cylinder and the die fitting member 4 is in close contact with 
the mold body 3, the die fitting member 4 is raised against clamping force 
of the clamping cylinder with the margin setting cylinder, thus opening 
the margin S between the mold body 3 and the die fitting member 4. 
An upper mold insert 20 is put into the insert guide 5 mounted on the mold 
body 3 in the upper mold 1 movably in vertical direction. A lower mold 
insert 21 is put into the insert guide 10 provided in the lower mold 2 so 
as to be movably in vertical direction. By the aforementioned arrangement, 
a cavity 22 for molding a spectacle lens is formed. As shown in FIG. 1, 
two of the cavities 22 are provided on right and left sides in the present 
embodiment. Therefore, the injection molding assembly is used for molding 
two spectacle lenses simultaneously. 
The upper mold insert 20 and the lower mold insert 21 form the cavity 22 
with the insert guides 5 and 10, that is, the inserts 20 and 21 are cavity 
forming members. In the present embodiment, the upper mold insert 20 is a 
cavity forming member for shaping a concave surface of the lens and the 
lower mold insert 21 is a cavity forming member for shaping a convex 
surface of the lens. 
Each of the upper mold inserts 20 is attached to a piston rod 24 of a 
hydraulic cylinder 23 disposed downward through a T-shaped clamping member 
25, the hydraulic cylinder 23 being built in the die fitting member 4 in 
the upper mold 1 so as to be slideable vertically. Each of the lower mold 
inserts 21 is attached to a piston rod 27 of a hydraulic cylinder 26 
disposed upward through a T-shaped clamping member 28, the hydraulic 
cylinder being fixed on the lower mold 2. A back insert 29, in which the 
piston rod 24 is inserted to be slideable vertically, is fixed on a lower 
surface of the hydraulic cylinder 23. 
When the upper mold 1 is raised by means of the clamping cylinder and hence 
the upper mold 1 and the lower mold 2 are parted from the parting line PL, 
the upper mold insert 20 and the lower mold insert 21 are exposed between 
the upper mold 1 and the lower mold 2 by advancing the piston rods 24 and 
27. T-shaped slots of the inserts 20 and 21, with which the T-shaped 
clamping members 25 and 28 are engaged, extend to an outer region of the 
inserts 20 and 21 for opening so that the inserts 20 and 21 are, 
respectively, inserted in and released from the piston rods 24 and 27, on 
which the T-shaped clamping members 25 and 28 are mounted, by engagement 
and disengagement of the T-shaped clamping members 25 and 28 with/from the 
T-shaped slots. Consequently, various inserts corresponding to spectacle 
lenses to be molded are exchangeably attached on the upper mold 1 and the 
lower mold 2. Meanwhile, when the piston rods 24 and 27 retract, the upper 
mold insert 20 and the lower mold insert 21 abut respectively on the back 
insert 29 and the mold plate 12 in the lower mold 2 to be seated, which 
makes the inserts 20 and 21 clamped. 
A pressure receiving member 30 mounted on an upper surface of the hydraulic 
cylinder 23 is accommodated inside a recessed portion 8A of the upper 
member 8 composing the die fitting member 4 in the upper mold 1. As shown 
in FIG. 2, a pair of guide bars 31 slidably inserted in the lower member 9 
of the die fitting member 4 are hung from the pressure receiving member 
30. By means of springs 32 attached on outer peripheries of the guide bars 
31, the pressure receiving member 30, the hydraulic cylinder 23, and the 
back insert 29 are always resiliently biased upward oppositely to the 
lower mold 2 and the pressure receiving member 30 abuts on an upper 
surface of the recessed portion 8A formed downward in relation to the 
upper member 8 of the die fitting member 4. 
A through-hole 33 leading to the recessed portion 8A is formed in the upper 
member 8 of the mold attaching member 4. An eject pin 34 is inserted in 
the through-hole 33 to move vertically by an eject cylinder (not shown). 
The eject pin 34 abuts on the pressure receiving member 30, and with 
descent of the eject pin 34 by means of the eject cylinder, the pressure 
receiving member 30, the hydraulic cylinder 23, the back insert 29, and 
the upper mold insert 20 are pressurized to move downward in relation to 
the upper mold 1. 
As shown in FIG. 1, an eject bar 35 is inserted to be movable vertically in 
central parts of the mold body 3 of the upper mold 1 and the lower member 
9 of the die fitting member 4. A pair of guide bars 37, vertically 
slidably inserted in the lower member 9, are fixedly hung from an pressure 
receiving member 36 mounted on an upper end of the eject bar 35 as shown 
in FIG. 3. By means of springs 38 attached on outer peripheries of the 
guide bars 37, a pressure receiving member 36 and the eject bar 35 are 
always resiliently biased upward. An eject pin 40, which is moved 
vertically with an eject cylinder (not shown), is inserted in a 
through-hole 39 formed in the upper member 8 of the die fitting member 4. 
With the eject pin 40, the pressure receiving member 36 and the eject bar 
35 are pressurized to move downward. 
As shown in FIG. 1, an injection nozzle 41 in an injection molding machine 
is connected to the sprue bush 13. A runner 43 is connected to an upper 
end of a sprue 42 in the sprue bush 13, the runner 43 extending to the 
cavities 22 provided on right and left side. 
The whole apparatus for adjusting and controlling the temperature of the 
injection molding assembly is shown in FIG. 4. Main lines 52 to 55 extend 
from a temperature controlling fluid feeder 51 controlled by a controller 
50. End portions of the main lines 52 to 55 lead to branch lines 52A, 52B, 
53A, 53B, 54A, 54B, 55A, and 55B. 
The branch lines in pair for every main line 52 to 55 are disposed 
correspondingly to two cavities 22 provided on both sides shown in FIG. 1. 
In other words, the branch lines 52A and 52B are connected to ring slots 
57 formed on upper surfaces of two right and left upper mold inserts 20 
through passages 56, the branch lines 53A and 53B are connected to ring 
slots 59 formed on lower surfaces of two right and left lower mold inserts 
21 through passages 58, the branch lines 54A and 54B are connected to 
peripheral slots 61 formed on side surfaces of two right and left upper 
mold insert guides 5 through passages 60, and branch lines 55A and 55B are 
connected to peripheral slots 63 formed on side surfaces of two right and 
left lower mold insert guides 10 through passages 62. 
The temperature controlling fluid feeder 51 shown in FIG. 4 circulates a 
heating fluid and a cooling fluid through the aforementioned main lines, 
branch lines, passages, ring slots and peripheral slots so as to raise and 
lower the temperature of the injection molding assembly, more 
specifically, temperature around the cavities 22 inside the injection 
molding assembly. The heating fluid is steam and the cooling fluid is air 
and water, for example. In the temperature controlling fluid feeder 51 has 
a switching valve to switch over the heating fluid and the cooling fluid, 
a closing valve to supply and suspend the fluids and the like. The 
switching valves, the closing valves and the like are controlled by means 
of signal from the controller 50 which controls the heating and cooling 
time of the injection molding assembly by the heating fluid and the 
cooling fluid from the temperature controlling fluid feeder 51. 
Incidentally, a discharge line, for discharging the heating fluid and the 
cooling fluid supplied to the injection molding assembly is provided in 
the injection molding assembly, though not shown in the drawing. 
A molding process of a plastic lens for glasses using the injection molding 
assembly is conducted as follows. The upper mold 1 and the lower mold 2 
are closed by the clamping cylinder. When the margin S is opened between 
the mold body 3 of the upper mold 1 and the die fitting member 4 upon 
operation of the margin setting cylinder and a molten resin is ready to be 
filled in the cavity 22, the injection molding assembly is heated by 
supplying the heating fluid from the temperature controlling fluid feeder 
51 to raise the temperature of the cavity 22 beyond flow halting 
temperature of the molten resin. Subsequently, the molten resin is 
injected from the injection nozzle 41 to be filled in the cavity 22 
through the sprue 42 and the runner 43, and the nozzle is shut thereafter. 
After the filling or during the filling, the die fitting member 4 is 
lowered by the clamping cylinder and the molten resin in the cavity 22 is 
pressurized by the amount corresponding to the margin S by the upper mold 
insert 20. After the pressurization, the cooling fluid is supplied to the 
injection molding assembly from the temperature controlling fluid feeder 
51. Thus, the temperature of the molten resin in the cavity 22 is lowered 
to, for example, around 100 degrees centigrade below the glass transition 
point for molding spectacle lens by cooling and solidification of the 
molten resin. 
After the spectacle lens is molded by cooling and solidification of the 
molten resin in the cavity 22, the upper mold 1 is opened from the lower 
mold 2 by the clamping cylinder. With the descent of the eject pins 34 and 
40, a molded product is pushed out by the upper mold insert 20 and the 
eject bar 35. Subsequently, the molded product provided with two spectacle 
lenses ejected from the injection molding assembly as described above is 
cut to obtain the spectacle lenses molded in the cavities 22. 
The upper mold insert 20 and the lower mold insert 21 are exchangeable as 
stated above. When molding process is conducted using the upper mold 
insert 20A and the lower mold insert 21A shown in FIG. 5, a minus lens 71 
having larger a thickness T2 of a peripheral portion than a thickness T1 
of a central portion is molded as shown in FIGS. 6 and 7. FIG. 8 is a 
sectional view taken in a direction perpendicular to FIG. 7. In FIG. 8, a 
thickness T3 of a peripheral portion is larger than the thickness T2 thus 
making the minus lens 71 as astigmatic spectacle lens by the difference 
between T2 and T3. On the other hand, when molding process is conducted 
using an upper mold insert 20B and a lower mold insert 21B shown in FIG. 
9, a plus lens 72 having smaller thickness T5 of peripheral portion than a 
thickness T4 of a central portion can be molded as shown in FIGS. 10 and 
11. FIG. 12 is a sectional view in a right-angled direction perpendicular 
to FIG. 11. In FIG. 12, a thickness T6 of peripheral portion is larger 
than T5, thus making the plus lens 72 as astigmatic spectacle lens by the 
difference between T5 and T6. 
Upper mold inserts and lower mold inserts are respectively prepared 
correspondingly to every lens power (diopter) of minus lenses and plus 
lenses. Besides, another upper mold insert and a lower mold insert are 
provided for molding a semi-finished lens, having one surface already 
finished and the other surface to be later finished. 
(A) to (D) in FIG. 13 show temperature curve of upper mold inserts and 
lower mold inserts in molding various kinds of lenses with a diameter of 
76 millimeter from pressurization of the molten resin to ejection of the 
molded product. FIG. 13(A) to (D) correspond to each lens shape and /or 
power. FIG. 13(A) shows a case of molding a minus lens (weak minus lens) 
with a power of -2.00, T1 of 1.4 mm and T2 of 4.8 mm, FIG. 13(B) shows a 
case of molding a minus lens (strong minus lens) with a power of -4.00, T1 
of 1.4 mm and T2 of 7.9 mm, FIG. 13(C) shows a case of molding a plus lens 
with a power of +2.00, T4 of 4.2 mm and T5 of 1.0 mm, and FIG. 13(D) shows 
a case of molding a semi-finished lens with a base curve of convex surface 
of 3.00 D, a thickness of a central portion of 5.4 mm and a thickness of a 
peripheral portion of 5.8 mm. These figures are for explaining basic lens 
molding patterns (basic lens molding patterns of a weak minus lens, a 
strong minus lens, a semi-finished lens and a plus lens) corresponding to 
each lens shape and/or power. 
When a minus lens is molded as shown in FIG. 13(A) and (B), the temperature 
of a lower mold insert (namely, a cavity forming member for shaping a 
convex surface of the lens) in ejecting a molded product from the 
injection molding assembly is lowered below the temperature of an upper 
mold insert (namely, a cavity forming member for shaping a concave surface 
of the lens). When there is such a difference in temperature as above 
between the lower mold insert and the upper mold insert according to the 
difference in lens shape and /or power, since the temperature of the 
convex surface of the ejected lens is low even in a minus lens of which 
thin central portion is likely to be bent and the temperature is high when 
being ejected, the convex side of the lens solidifies earlier than the 
concave side thereof, thereby efficiently preventing the central portion 
from being bent. In other words, a concave shape of the lower mold insert 
and a convex shape of the upper mold insert are precisely transferred to a 
molten resin so that a high-precision lens can be manufactured. 
It is also effective in molding a lens of which a thickness of a central 
portion is smaller than that of a peripheral portion that the temperature 
of the lower mold insert is lowered below the temperature of the upper 
mold insert when the molded product is ejected as described above. 
Therefore, when a semi-finished lens with a small difference in thickness 
between a central portion thereof and a peripheral portion thereof shown 
in (D) in FIG. 13 is molded, the temperature of the lower mold insert is 
also preferably lowered below that of the upper mold insert as described 
in molding the minus lens. 
As shown in FIG. 13(A), (B), and (D), the larger the difference in 
thickness between a central portion and a peripheral portion becomes, the 
more a difference in temperature between the lower mold insert and the 
upper mold insert is enlarged, which securely prevents various kinds of 
lenses with various differences in thickness between central portions and 
peripheral portions from bending at central portions thereof. 
As shown in FIG. 13(A), (B), and (D), particularly in FIG. 13(A) and (B), 
when a lens of which a thickness of a peripheral portion is larger than 
the thickness of a central portion is molded, the temperature of the lower 
mold insert is lowered below that of the upper mold insert from 
pressurization of the molten resin to ejection of a molded product. As a 
result, the temperature of a convex surface of the lens can be securely 
lowered below the temperature of a concave surface when the molded product 
is ejected, thereby improving transfer precision of shapes of the lower 
mold insert and the upper mold insert. 
Following Table 1 shows the cooling time of the injection molding assembly 
from the start of supplying the aforementioned cooling liquid after 
pressurizing the molten resin to ejection of a molded product when a minus 
lens and a plus lens are molded. The cooling time is divided into groups 
according to lens power (diopter) (spherical power) and astigmatic power. 
Especially in the minus lens, the grouping is based on the value of the 
sum of the spherical power and the astigmatic power. The lens spherical 
power is indicated in a vertical axis and the astigmatic power is 
indicated in a horizontal axis. A three-digit number in the Table 1 
indicates a cooling time (second). 
TABLE 1 
______________________________________ 
##STR1## 
##STR2## 
______________________________________ 
As can be seen from FIG. 13(A) and (B), the increase of lens power in a 
minus lens means that a thickness of the peripheral portion T2 becomes 
much larger than the thickness of the central portion T1. Also, the 
increase of lens power in a plus lens means that a thickness of the 
central portion T4 becomes much larger than that of the peripheral portion 
T5. As shown in Table 1, the more a lens power increases, the longer a 
cooling time is made in both the minus lens and the plus lens. When a lens 
power increases, though surface area of the cavity in the injection 
molding assembly does not substantially changes, the volume of a lens, 
that is, the amount of the molten resin filled in the cavity 22 increases. 
Accordingly, the cooling time is lengthened in proportion to the increase 
of the lens power, thereby gradually cooling the whole molten resin in the 
cavity 22 uniformly to a predetermined temperature (the temperature for 
ejecting the molded product). Consequently, a high-precision and 
high-quality plastic spectacle lens with little heat distortion, little 
shrinkage deformation and the like can be obtained. 
The cooling time of the injection molding assembly is lengthened in 
proportion to the increase of the lens power also in molding a 
semi-finished lens. 
As can be seen from a comparison of graphs of FIG. 13(A) and (B), a minus 
lens with the time from pressurization of molten resin to ejection of the 
molded product is made longer for minus lens having greater lens power in 
a minus lens with a smaller power even when the minus lenses have 
substantially the same surface area in the cavity, thereby lengthening a 
cooling time of the injection molding assembly. When a lens having larger 
thickness of a peripheral portion than the thickness of a central portion 
is molded, the temperature of the lower mold insert is lowered below the 
temperature of the upper mold insert and a cooling time of the injection 
molding assembly is lengthened as shown in FIG. 13(A) and (B), which 
prevents the lens from bending at the central portion so as to enable high 
insert transfer precision and also prevents heat distortion, thus making a 
high-quality lens. 
As described above, the thickness of peripheral portions T2 and T3 of a 
minus lens and the thickness of peripheral portions T5 and T6 of a plus 
lens respectively, show the thickness of two points which are 90 degrees 
apart in a circumferential direction. The increase of difference between 
T2 and T3, and between T5 and T6 leads to the increase of astigmatic 
power. Even in spectacle lenses with the same lens power, the volume of 
lens becomes larger and more molten resin is filled in the cavity 22 when 
an astigmatic power is made larger. 
In molding lenses with the same lens power, the larger the astigmatic power 
becomes, the longer the cooling time is made as shown in Table 1. 
Accordingly, the whole molten resin filled in the cavity 22 can be 
uniformly cooled to a predetermined temperature in the same manner as 
molding glass-lenses with different powers, thus making a high-precision 
and high-quality spectacle lens. 
In the injection molding assembly according to the embodiment described 
above, the cavity forming member for shaping the lens convex surface is 
disposed on the lower mold and the cavity forming member for shaping the 
lens concave surface is disposed on the upper mold, but the reverse 
disposition is also available. The present invention can be also 
implemented by means of an injection molding assembly having 
horizontally-opposed cavity forming members. 
According to the aforementioned embodiment, the heating fluid and the 
cooling fluid are used respectively for heating and cooling the injection 
molding assembly. However, the present invention is not limited to the 
above arrangement and preadjusted temperature controlling fluid may be 
used. Alternatively, a heating method by means of an electric heater and 
the like and a cooling method by means of forced air-cooling and the like 
can also be adopted. The cooling time required for cooling to the 
predetermined temperature is set in accordance with cooling methods. 
According to the present invention, the temperature of the cavity forming 
member for shaping the lens convex surface is lowered below the 
temperature of the cavity forming member for shaping the lens concave 
surface, which prevents the molded lens from bending at a central portion. 
As a result, the shapes of the cavity forming members are highly-precisely 
transferred to the lens so that a high-precision and high-quality lens as 
desired can be obtained. 
According to the present invention, the cooling time of the injection 
molding assembly after pressurization of the molten resin is lengthened 
for a lens with a large power requiring a larger amount of molten resin to 
be filled in the cavity, so that the entire molten resin can be uniformly 
cooled to the predetermined temperature. Consequently, a high-precision 
spectacle lens with little heat distortion and little shrinkage 
deformation and the like can be manufactured. 
INDUSTRIAL AVAILABILITY 
A method of injection molding a plastic lens according to the present 
invention is applicable for molding a plastic lens for glasses, an optical 
plastic lens etc. made of a thermoplastic resin, and especially useful for 
molding a meniscus-shaped plastic lens for glasses which requires 
high-precision molding.