JPS5939526A - Production of plastic lens - Google Patents

Abstract

PURPOSE:To obtain a large-type plastic lens for video cameras or the like at a high productivity, by a method wherein a thin film of the same material as that for a lens blank is polymerized on the surface of the blank, whereby the accuracy of the lens is enhanced through the minute contraction in polymerization of the film and the period of time required for polymerization is shortened. CONSTITUTION:The lens blank 3 freed from optical strain and having a substantially desired shape which blank is obtained, for example, by injection molding an ultraviolet ray-polymerizable plastic material such as polymethyl methacrylate followed by annealing is inserted into a cavity 8 having a desired shape. Then, a fluid film 4 of the same material as that constituting the lens blank 3 is formed in the gap between the lens blank 3 and force surfaces 5, 5' of the cavity 8 formed by forces 6, 6', and is irradiated with ultraviolet rays, whereby the film 4 is polymerized to produce the plastic lens 7.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method of manufacturing a plastic lens suitable for making relatively large lenses such as video camera lenses into plastic.

[Prior art]

In recent years, with the spread of video cameras, there has been a great desire for optical devices such as zoom lenses to be lighter and less expensive, and plastic lenses have come to be used as one solution to this.

Traditionally, plastic lenses have been manufactured using the so-called cast molding method, in which a fluid plastic material such as a monomer or oligomer is injected into a mold and polymerized using heat, radiation, etc., or by heat-melting a thermoplastic polymer. They were molded using injection molding methods, compression molding methods, etc. However, these molding methods have the following drawbacks:
In particular, it is unsuitable for molding large plastic lenses.

In other words, the cast molding method can produce lenses with fewer optical defects such as internal distortion when molding lenses with relatively little change in wall thickness, such as eyeglass lenses, but on the other hand, it takes time to polymerize, making production difficult. When molding a lens of high power with poor properties and extreme differences in wall thickness, shrinkage due to polymerization cannot be fully compensated for, making it impossible to obtain a lens with high precision. The injection molding method has high productivity, but
Optical distortion occurred due to material flow and cooling and solidification, and sink marks occurred due to molding shrinkage, making it impossible to obtain high-quality, high-precision lenses. Even in compression molding, which is said to be able to form lenses with a relatively high angle of 17 degrees, temperature non-uniformity occurs during cooling because the material is heated to a melting or flowing temperature and then cooled and solidified during shaping. Warpage or non-uniform thermal contraction occurred in the lens itself, and as a result, it was not possible to form a lens with high precision.

In order to eliminate these drawbacks, for example, methods have been proposed that incorporate radiation polymerization in order to increase the polymerization reaction rate in the cast molding method, but since radiation is used, work safety is a concern. There is an interval in.

[Object of the present invention]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for manufacturing a plastic lens that eliminates the drawbacks of the above-mentioned prior art, improves lens precision, prevents optical distortion, and has excellent productivity.

[Overview of the present invention]

In order to achieve this objective, the present invention forms a thin layer of the same material that forms the lens blank on the surface of a lens blank having an approximately desired shape without optical distortion, and overlaps the layers to obtain the desired shape. The present invention is characterized in that it is possible to mold a lens of the same shape, improve the lens precision by the minute amount of polymerization shrinkage of the layer, and shorten the polymerization reaction time.

[Example of the present invention]

Embodiments of the present invention will be described below with reference to the drawings.

First, a generally shaped article is injection molded from a lens material such as polymethyl methacrylate. As mentioned above, injection-molded plastic lenses can be manufactured with a high die strength, but on the other hand, optical distortion (birefringence, striae) due to the flow of the polymer is likely to occur. These defects are removed by so-called annealing. In the case of polymethyl methacrylate, annealing is performed at 80° C. for 4 hours. However, the shape imposed by injection molding is deformed by annealing.

FIG. 1(a) shows the lens after injection molding, and has optical distortion 2 near the gate 1 due to internal distortion. FIG. 1(b) shows the case where such a lens is annealed, and the optical distortion is removed, but the lens is deformed. The amounts of deformation A, B, and C vary depending on the shape of the lens, molding conditions, etc., but each is usually about several tens of μm.

2(a), (b) j (C) are schematic process diagrams for explaining the outline of each step of the method for manufacturing a plastic lens according to the present invention, 3 is a lens blank, 4 is a
is syrup, 5 and 5' are mold surfaces, 6 and 6' are molds, 7 is a plastic lens, and 8 is a cavity.

As described above, the lens blank 3 is made of polymethyl methacrylate and is obtained through injection molding and annealing processes, and although the imposed shape is deformed, optical distortion is eliminated. A syrup 4 of methyl methacrylate containing a polymerization initiator is applied to the outer surface of the lens blank 3, and the lens blank 3 is placed in a mold 6°6' having a mold surface 5, 5/ of the finished curvature of the lens (FIG. 2(a)). . Next, the molds 6, 6', which have been divided into two, are closed to form a lens-shaped cavity 8, and the molds 6, 6' are heated at 60°C to 80°C, thereby polymerizing the syrup 4. Then, it is integrated with the lens blank 3 (FIG. 2(b)). 4~ for polymerization
Keep warm for 10 hours. When the polymerization is completed, the molds 6 and 6' are cooled to room temperature and separated into plastic lenses 7.
(Figure 2 (C)).

As the syrup 4, a methyl methacrylate monomer to which a polymerization initiator such as benzoyl peroxide is added, a methyl methacrylate oligomer, or a solution of polymethyl methacrylate C polymer) in a methyl methacrylate monomer is used. The polymerization shrinkage rate of these syrups is 5 to 10 by volume, but if the thickness of the syrup 4 is about 10 to 100 μm, the amount of shrinkage (thickness) is 0.
It is about 2 to 3 μm, which is negligible in terms of optical properties.

By the way, when polymerization is carried out by heating, not only does it take a long time to polymerize, but also bubbles may form on the lens surface, and as the polymerization progresses, there may be a shortage of syrup near the outer periphery of the lens (possibly due to polymerization shrinkage). ) defects are likely to occur. In addition, when using the thermal polymerization method, the temperature of the lens blank itself is raised to the polymerization temperature, so when the temperature is lowered to room temperature after the polymerization of the syrup is completed, the amount of thermal shrinkage is not small.
There is a tendency that the curvature of the mold surface 5.5' cannot be completely transferred.

Most of these trends are overcome by UV polymerizing the syrup rather than thermally polymerizing it. That is, Fig. 2 (a
), (b), (C), the mold 6,6/ is made of silicate glass or Fe2O, with a content of 0.01
4% by weight of benzophenone as a syrup formed with a UV-transparent glass such as soda-lime glass.

By using a material that has improved UV polymerizability by adding a sensitizer such as benzaldehyde, and irradiating UV rays from outside the mold 6°6' during polymerization, the polymerization time can be increased from several tens of seconds to a few seconds.
It only takes a few minutes, and the temperature rise can be suppressed slightly. However, even if the ultraviolet polymerization method is used, it is not possible to completely suppress the generation of bubbles on the lens surface, especially on the outer periphery.

FIG. 3 is a cross-sectional view of a polymerization mold showing an embodiment of the method for manufacturing a plastic lens according to the present invention, 6a, 6
a' is the outer surface, 9, 9' are the frame, 10, 11 are the communicating pipes, 1σ, 11 is the flow path, 12, 13 are the valves, 14
1 is a parting surface, 15 is an O-ring, and parts corresponding to those in FIG. 2 are given the same reference numerals.

The lens blank 3 produced by the method described above is placed on a frame 9.
．． The final lens-shaped cavity 8 formed by the mold 6.6', which has a lens surface shape made of UV-transparent glass, is held in the mold and communicates with a tank (not shown) containing defoamed syrup. A valve 12 provided in the middle of the flow path 1σ is closed, and a vacuum pump (not shown) is connected to the cavity 8 through a communicating pipe 11.
The valve 13 provided in the middle of the flow path IY for exhausting the air inside is opened to bring the inside of the cavity 8 into a reduced pressure state. Thereafter, the syrup 4 is sucked into the cavity 8 by closing the valve 13 and opening the valve 12. At this time, if necessary, the tank containing the syrup may be pressurized and the syrup may be injected under pressure. When the gap between the lens blank 3 and the mold surface formed in the cavity 8 is completely filled with the syrup 4, the valve 12 is closed and the ultraviolet transparent mold 6, 6/
Ultraviolet rays are irradiated from the outer surfaces 6a and 6a' using a mercury lamp (not shown) or the like. After the polymerization of the syrup 40 filled between the lens blank 3 and the mold surface is completed, the molds 6, 6' are placed on the frame 9. The lens is divided along with γ from the parting surface 14 to obtain the final lens.

In this embodiment, it is necessary to provide an airtight fit between the molds 6, 6' and the frames 9, 9', and for this purpose, O-rings 15 are inserted at key points. Further, since the mercury lamp used for ultraviolet polymerization generates heat as well as ultraviolet rays, which tends to raise the temperature of the molds 6 and 6', cooling water passages are provided in the frames 9 and 9' as necessary.

Next, materials will be explained. This article will focus on polymethyl methacrylate (including copolymers), which is used in the largest quantities in plastic lenses. Polymethyl methacrylate for injection molding usually has an average molecular weight of 100,000 to 150,000 and a refractive index of 20'C.

The measured value at line D is 1.4913. On the other hand, polymers obtained by polymerizing methyl methacrylate to which 0.1, 0.2, and 0.3 weight percent of benzoin as a sensitizer has been added are irradiated with a medium-high pressure mercury lamp (3 kW) from a distance of 10 steel, and the molecular weight is 700,000 to 1,000,000, but the difference in refractive index from that for injection molding is within 0.0004, and no optical defects occur in the plastic lens according to the present invention. The syrup may be a monomer alone, an oligomer, or a monomer with a polymer dissolved therein, and there are no particular restrictions as long as it maintains fluidity and has the same refractive index as the lens blank upon completion of polymerization. As the sensitizer, benzophenone, penzanthrone, benzaldehyde, fenathraquinone, etc. can be used. The same applies to polystyrene used as a flint glass as a plastic lens. However, the commercially available reagent styrene takes a long time to polymerize.

This is thought to be due to the addition of a polymerization inhibitor. If you use styrene that has been distilled under reduced pressure, this will not happen. Furthermore, since the polymerization is carried out using ultraviolet rays, the polymerization reaction rate is higher than that of thermal polymerization.

In this embodiment, a biconvex lens is shown, but it goes without saying that the shape of the lens, such as biconcave or uneven, is not subject to IvFK regulations. The present invention can eliminate weld lines even in concave lenses with extreme thickness changes that are unavoidable in injection molding.

〔Effect of the invention〕

As explained above, according to the present invention, a plastic lens can be molded with high precision and without optical distortion, productivity is improved, and a method for manufacturing a plastic lens with excellent functions not available in the prior art is provided. be able to.

[Brief explanation of drawings]

FIG. 1(a) is an explanatory diagram showing the optical distortion of the lens after injection molding, FIG. 1(b) is an explanatory diagram showing the deformation of the injection molded lens after annealing, and FIG. 2 is an explanatory diagram showing the deformation of the injection molded lens after annealing. FIG. 3 is a schematic process diagram for explaining the outline of each step of the method for manufacturing a lens, and is a cross section of a polymerization mold showing an embodiment of the method for manufacturing a plastic lens according to the present invention. 3...Lens blank, 4.Old...Syrup,
5゜5'... prayer-shaped surface, 6, 6'...
・Mold, 18...Cavity. Sai 1 figure ((1) '
(b) Figure 2

Claims (1)

[Claims] In a method for manufacturing a plastic lens using a plastic material that is polymerized by ultraviolet rays, a lens blank with optical distortion removed and having an approximately desired shape is inserted into a gap of the desired shape, and a mold surface of the gap formed by the lens blank and a mold is inserted. A method for manufacturing a plastic lens, comprising forming a fluid layer of the plastic material forming the lens blank in the gap between the lenses, irradiating the lens blank with ultraviolet rays, and superposing the fluid layer.