JPH0139336B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing plastic lenses.

Plastic lenses are becoming more widely used in optical products because they are lighter than traditional glass lenses, have the potential for mass production, and are expected to be inexpensive. . The resin currently mainly used as a plastic lens material is diethylene glycol bisallyl carbonate (hereinafter referred to as CR-39).
Or polymethyl methacrylate (hereinafter
(referred to as PMMA).

CR-39 is mainly used for eyeglass lenses, and PMMA is often used for sunglasses and loupes.
These lenses have a relatively small diameter and are manufactured primarily by injection molding in the case of PMMA and by CR-
39 manufactures lenses using the casting method.

Incidentally, attempts have recently been made to make lenses that have a relatively large diameter and a large deviation in thickness from plastic. When manufacturing such a lens by injection molding, in order to obtain a lens with good shape accuracy, the cylinder → nozzle → sprue →
It is necessary to apply pressure along the flow path of the resin from the gate to the product to prevent the product from becoming insufficiently filled (short shot), but the part of the lens product section that is particularly close to the gate must be exposed to strong pressure. It is added to the cavity and tends to generate residual stress, resulting in distortion. Also, due to differences in the thickness of lens product parts called uneven thickness, sink marks are likely to occur in the thick parts of the lens. As described above, it is difficult to obtain a plastic lens with high shape accuracy and little distortion by injection molding because of the application of high injection pressure and uneven thickness.

On the other hand, attempts have also been made to manufacture such lenses by a casting method. However, this casting method has the problem of shape distortion due to polymerization shrinkage of the monomer. Therefore, the mold for cast molding is
Despite having a predetermined lens shape,
The lens obtained after molding is quite different from the predetermined one, and it is difficult to obtain a plastic lens with good shape precision by the casting method.

In view of the above points, the present invention has a relatively large diameter,
Moreover, the present invention provides a method for obtaining a plastic lens with a large thickness deviation. In other words, the present invention is based on the fact that plastic lenses made by casting process have significantly less optical distortion than lenses made by injection molding process, and that compression molding can provide products with good shape accuracy. A monomer or semi-polymer is poured into a casting mold having a shape approximating the shape of a lens, and polymerized in bulk to produce a plastic lens material. The lens material is placed in a compression molding mold having a predetermined lens shape that has been heated to a temperature that minimizes the stress strain of the lens material. In addition, since the temperature near the surface of the lens material is almost equal to the compression molding mold temperature, the temperature near the surface of the lens material (surface layer) is lowered when the internal temperature of the lens material is lower than the temperature near the surface. By compression molding the plastic lens once it has melted and become flowable, it is possible to manufacture a plastic lens with a large diameter and uneven thickness.

The manufacturing method of the present invention will be explained in more detail below.

Suitable materials for making plastic lens materials obtained by bulk polymerization include transparent monomers such as methyl methacrylate, styrene, or semipolymers thereof. The average molecular weight of the plastic lens material obtained by bulk polymerization is considerably greater than the average molecular weight of the injection molding material. Since almost no undue pressure is applied during bulk polymerization, no residual stress is generated, and the occurrence of distortion is extremely low compared to injection molding. Furthermore, since there is a compression molding step after bulk polymerization, the accuracy of the casting mold does not need to be very high.

Next, the lens material made by bulk polymerization is placed into a compression molding mold having a predetermined lens shape, which has been heated in advance to a temperature that minimizes the stress strain of this lens material, and compression molding is performed. . At this time, the resin must be heated to change from a rubber state to a fluid state. In the compression molding process, the entire plastic lens material is not melted, but the internal temperature of the lens material is 10°C higher than the glass transition point of the lens material or less, and the temperature near the surface of the lens material is not melted. By making the temperature approximately equal to the mold temperature, only the area near the surface of the lens material is melted and brought into a fluid state while the internal temperature of the lens material is lower than the temperature near the surface. The reason for this is that melting the entire lens material has a large heat capacity and takes a long time, and the thinner parts of the lens solidify first during the cooling process, resulting in sink marks as the thicker parts of the lens solidify. It is from.

In the compression molding of the present invention, even though the lens shape is a product with uneven thickness, the thickness of the part that can be melted and fluidized is almost constant throughout, making it as if a product of equal thickness was molded. The molding is carried out as if. At this time, the temperature near the surface where the lens material melts and fluidizes is the temperature of the compression molding mold,
In other words, the temperature is close to the temperature at which the stress strain of the lens material is minimized. If the temperature near the surface of the lens material is higher than the temperature at which the stress strain of the lens material is minimized, the resin will shrink during cooling after compression molding, so when the entire lens material is melted, Distortion occurs because the amount of contraction differs between thick and thin parts of the lens. On the other hand, in the case of products of equal thickness, the amount of shrinkage is constant, so no distortion occurs due to shrinkage during cooling. On the other hand, if the temperature near the surface of the lens material is lower than the temperature at which the stress strain of this lens material is the smallest,
Since the rubbery resin is forced to move by applying pressure, the molecular order is disturbed and distortion occurs. In addition, the heating for melting the lens material can be carried out in advance outside the molding machine. In this case, only the vicinity of the surface of the lens material is melted, and when it becomes flowable, it is heated inside the molding machine. Transferring the mold and performing compression molding, etc.
It is useful to perform preheating, etc., which is used in general compression molding. Also, at the same time as compression pressure starts to be applied, cooling of the mold begins.

Hereinafter, the above-described compression molding process will be explained in more detail using the drawings. FIGS. 1 to 3 are diagrams schematically showing the compression molding process divided into three stages. That is, FIG. 1 is a diagram for explaining the stage of setting the lens material, FIG. 2 is the stage of pressurization and cooling, and FIG. 3 is a diagram for explaining the stage of product removal.

First, in FIG. 1, the lens material 2 manufactured by the casting method in the previous step is set between the cores 6 and 7 incorporated in the templates 1 and 3 with the mold open. In this case, the mold is heated to a constant temperature by flowing a temperature regulating medium (usually oil) through the mold temperature regulating medium passage hole 11 for adjusting the temperature.

Next, the mold is closed as shown in FIG. 2 using a device not shown. However, the mold should only be closed so that almost no force is applied to the mold. This state is left until only the vicinity of the surface of the lens material 2 melts and becomes fluid. Thereafter, compression molding is performed by applying a large pressure to the mold using a device (not shown). At that time, at the same time as starting to apply compression pressure, the circulation of the temperature regulating medium that was flowing through the mold temperature regulating medium passage hole 11 is stopped, and the cooling medium (oil, water, etc.) is allowed to flow through the mold cooling medium passage hole 12. This starts cooling the mold.

After reaching a temperature at which the molded product can be taken out, the mold is opened as shown in FIG. Take out lens 2'.

Examples of the present invention will be described below.

To obtain a plastic lens that is a biconvex lens with a diameter of 100 mm, one spherical surface has a radius of 200 mm, the other spherical surface has a radius of 100 mm, the edge thickness is 5 mm, and the center thickness is 24.75 mm. 1 concave glass mold, diameter 105mm, spherical radius 205
A glass mold with a concave size of mm was prepared for casting. A gasket was sandwiched between these two molds so that the thickness was 5 mm, and a half-polymer of methyl methacrylate prepolymerized was poured into it.
℃ for 20 hours, further bulk polymerized at 110℃ for 5 hours,
Thereafter, the mold was cooled and released to obtain a plastic lens material. On the other hand, as shown in the drawing, a concave core with a diameter of 100 mm and a spherical radius of 100 mm and a concave core with a diameter of 100 mm and a spherical radius of 2000 mm were prepared and assembled into the mold. This mold is heated to 150℃
It was heated to. After that, the compression molding mold is opened and the plastic lens material made in the previous process is set on the core surface, and then the mold is closed until only the lens material near the surface melts and can flow.
I waited 20 seconds. Thereafter, a compression pressure of 50 kg/cm ² was applied for 10 minutes. At the same time as applying compression pressure, cooling of the mold began. After tightening the compression pressure, the mold was opened to obtain the desired plastic lens.

The shape of the obtained plastic lens was measured using a coordinate measuring machine. As a result, the variation from the theoretical value for both sides of the spherical radius of 100 mm and 200 mm was within ±3 μm. Further, when optical distortion was observed using a polarizing plate, almost no distortion was observed.

As described above, the present invention produces a plastic lens material by bulk polymerization in a cast molding mold having a shape approximate to the final lens shape, and prepares the plastic lens material in advance so that the stress strain of this lens material is minimized. It was heated to temperature. Place the lens material into a compression molding mold having a predetermined lens shape, and make sure that the internal temperature of the lens material is 10°C higher than the glass transition point of the lens material or less, and the temperature near the surface of the lens material is for compression molding. The internal temperature of the lens material is lower than the temperature near the surface by almost equalizing the mold temperature, and compression molding is performed when only the surface of the lens material melts and becomes flowable. This method makes it possible to provide a plastic lens with a large diameter and uneven thickness, which could not be obtained using the method described above. Furthermore, when performing compression molding, the internal temperature of the plastic lens material is lower than the temperature near the surface, and compression molding is performed when only the surface of the lens material melts and can flow. This has the effect of reducing optical distortion caused by non-uniform cooling. In addition, the temperature distribution of the lens material during cooling in compression molding is considerably smaller than that of a method in which the entire lens material is heated to a uniform temperature and then cooled, since cooling is done from the surface. Since cooling is performed uniformly, cooling distortion is less likely to occur.

Furthermore, during compression molding, the entire lens material is not heated to a uniform temperature, so the heating time can be considerably shortened, and the cooling time is also reduced so that the internal temperature of the lens material does not rise that much. It also has the advantage of being short.

Furthermore, during compression molding, pressure is applied in a mold that has been preheated to a temperature where the stress strain of the lens material is minimized, and when only the surface of the lens material becomes fluidized, this pressure is applied. The amount of deformation of the compression molding mold is small, and the resulting plastic lens has good shape accuracy, so its industrial value is great. There is. It goes without saying that in the present invention, well-known treatments such as surface hardening treatment and antireflection coating may be performed after compression molding.

[Brief explanation of drawings]

FIG. 1, FIG. 2, and FIG. 3 are diagrams for explaining each step of compression molding in the present invention. 1, 3...template, 2...lens material, 2'...
Final plastic cleanser, 4... Ejector sleeve (ejector pin), 5... Ejector plate, 6, 7... Core, 11... Mold temperature control medium passage hole, 12... Mold cooling medium passage hole .

Claims (1)

[Claims] 1 Pour the monomer or semi-polymer into a cast mold having a shape similar to the final lens shape,
Bulk polymerization is used to create plastic cleansing materials.
Thereafter, this plastic lens material is placed in a compression molding mold having a predetermined lens shape that has been heated in advance to a temperature that minimizes stress strain on the plastic lens material, and the internal temperature of the lens material is The temperature near the surface of the lens material is approximately equal to the compression molding mold temperature, and the internal temperature of the lens material is lower than the temperature near the surface. A method for producing a plastic lens, characterized in that compression molding is carried out at a point when only the surface vicinity of the lens material melts in a low temperature state and becomes flowable.