JPH033570B2 - - Google Patents

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 cheaper, and plastic lenses have come to be used as one solution to this demand.

Conventionally, plastic lenses have been produced 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 hot melting a thermoplastic polymer. It was molded using injection molding, compression molding, etc. However, these molding methods have the following drawbacks and are particularly unsuitable for molding large plastic lenses.

In other words, with the cast molding method, lenses with fewer optical defects such as internal distortion can be obtained when molding lenses with relatively little change in wall thickness, such as eyeglass lenses, but on the other hand, polymerization takes time and production is slow. When molding a high-power lens with poor properties and extreme differences in wall thickness, shrinkage due to polymerization cannot be fully compensated for, making it impossible to obtain a high-precision lens. The injection molding method has high productivity, but
Optical distortion may occur due to material flow, cooling and solidification, and sink marks may occur due to molding shrinkage.
A high-precision lens could not be obtained. Even in compression molding, which is said to be able to form relatively high-precision lenses, the material is heated to a melting or flowing temperature and then cooled and solidified during shaping, resulting in uneven temperature during cooling, causing damage to the lens itself. Warpage or non-uniform thermal contraction occurred, 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 a problem.

[Object of the present invention]

The purpose of the present invention is to eliminate the drawbacks of the above-mentioned prior art,
To provide a method for manufacturing a plastic lens that can improve lens precision, prevent optical distortion, and has excellent productivity.

[Summary of the invention]

In order to achieve this objective, the present invention forms a thin layer of the same material forming the lens blank on the surface of a lens blank having an approximately desired shape without optical distortion, and overlaps the layers to form a desired shape. The lens is molded, and the lens precision is improved by the minute amount of polymerization shrinkage of the layer, and the polymerization reaction time can be shortened.

[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 previously mentioned, injection-molded plastic lenses can be manufactured with high efficiency, but on the other hand, optical distortions (birefringence, striae, etc.) are likely to occur due to the flow of the polymer. 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. 1a shows the lens after injection molding, and has optical distortion 2 near the gate 1 due to internal distortion. 1st
Figure b shows the case where such a lens is annealed,
Optical distortion is removed but 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.

2a, b, and 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, in which 3 is a lens blank; 4 is a lens blank;
is the syrup, 5, 5' is the mold surface, 6, 6' is the mold,
7 is a plastic lens, and 8 is a cavity.

As mentioned 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 mold surfaces 5, 5' having the finished curvature of the lens (FIG. 2a). Next, the molds 6 and 6' that have been divided into two are closed to form a lens-shaped cavity 8, and by heating the molds 6 and 6' at 60°C to 80°C, the syrup 4 is polymerized and the lens is formed. It is integrated with the blank 3 (Fig. 2b). Incubate for 4 to 10 hours for polymerization. When the polymerization is completed, the molds 6 and 6' are cooled to room temperature and separated to obtain a plastic lens 7 (FIG. 2c).

As the syrup 4, a methyl methacrylate monomer added with a polymerization initiator such as benzoyl peroxide, an oligomer of methyl methacrylate, or a solution of polymethyl methacrylate (polymer) in a methyl methacrylate monomer is used. The polymerization shrinkage rate of these syrups is 5
~10% by volume, but the thickness of syrup 4 is ~10~
If it is about 100μm, the amount of shrinkage (thickness) is 0.2~3μ
m, which is an amount that can be ignored in terms of optical characteristics.

By the way, when polymerizing by heating, not only does the polymerization take a long time, but also bubbles may form on the lens surface. Especially as the polymerization progresses, there may be a shortage of syrup near the outer periphery of the lens (due to polymerization shrinkage). (possible) 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 contraction is not small, and the curvature of the mold surfaces 5 and 5' is transferred. I tend not to be able to do it.

Most of these trends are overcome by UV polymerizing the syrup rather than thermally polymerizing it. That is, in FIGS. 2a, b, and c, the molds 6 and 6' are
It is made of UV-transparent glass such as silicate glass or soda-lime glass with a Fe ₂ O ₃ content of 0.01% by weight or less, and a sensitizer such as benzophenone or benzaldehyde is added as syrup 4 to make it UV polymerizable. By using improved materials and irradiating ultraviolet rays from the outside of the molds 6 and 6' during polymerization,
The polymerization time can be reduced to several tens of seconds to several minutes, and the temperature rise can be suppressed slightly. However, even if ultraviolet polymerization 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 sectional view of a polymerization mold showing an embodiment of the method for manufacturing a plastic lens according to the present invention, in which 6a and 6a' are outer surfaces, 9 and 9' are frame bodies, 10,
11 is a communication pipe, 10', 11' are flow channels, 12, 13
14 is a parting surface, 15 is an O-ring, and parts corresponding to those in FIG. 2 are given the same reference numerals.

Lens blank 3 produced by the method described above
is held by frames 9 and 9', and placed in a cavity 8 in the final lens shape formed by molds 6 and 6' having lens surface shapes made of ultraviolet-transparent glass, and then placed in a tank containing defoamed syrup ( (not shown) through a communication pipe 10.
The valve 12 provided in the middle of the cavity 8 is closed, and the valve 13 provided in the middle of the passage 11' for exhausting the air in the cavity 8 through a communication pipe 11 to a vacuum pump (not shown) is opened. Bring to a reduced pressure state. After that, valve 13 is closed and valve 1 is closed.
By opening 2, the syrup 4 enters the cavity 8.
inhaled into the body. At this time, if necessary, the tank containing the syrup may be pressurized and the syrup may be injected under pressure.
When the syrup 4 is completely filled into the gap formed in the cavity 8 between the lens blank 3 and the mold surface, the valve 12 is closed and a mercury lamp is applied from the outer surfaces 6a, 6a' of the ultraviolet-transparent molds 6, 6'. (not shown) or the like. Shirob 4 filled between the lens blank 3 and the mold surface
After the polymerization of the molds 6, 6' is completed, the molds 6, 6'
9' and the parting surface 14 to obtain the final lens.

In this embodiment, the molds 6, 6' and the frame 9,
9' must be airtightly fitted, and for this purpose O-rings 15 are inserted at key points. In addition, the mercury lamp used for ultraviolet polymerization generates heat as well as ultraviolet light, which tends to raise the temperature of the molds 6 and 6'.
Cooling water passages are provided in the frames 9, 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℃,
The measured value at line D is 1.4913. On the other hand, polymers made by polymerizing methyl methacrylate to which 0.1, 0.2, and 0.3 weight percent benzoin is added as a sensitizer by irradiating it with a medium-high pressure mercury lamp (3 kW) from a distance of 10 cm have a molecular weight of 700,000 to 1,000,000. 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, benzanthrone, benzaldehyde, fuenathraquinone, 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 polymerization is carried out using ultraviolet rays, the polymerization reaction rate is higher than that in thermal polymerization.

In this embodiment, a biconvex lens is shown, but it goes without saying that the shape, such as biconcave or uneven, is not particularly limited. The present invention can eliminate weld lines even in the case of 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 without optical distortion, productivity is improved, and a method for manufacturing a plastic lens with excellent functions not found in the prior art is provided. be able to.

[Brief explanation of the drawing]

FIG. 1a is an explanatory diagram showing optical distortion of a lens after injection molding, FIG. 1b is an explanatory diagram showing deformation of an injection molded lens after annealing, and FIG. 2 is an explanatory diagram showing a method for manufacturing a plastic lens according to the present invention. FIG. 3 is a cross-sectional view of a polymerization mold showing an embodiment of the method for producing a plastic lens according to the present invention. 3...Lens blank, 4...Syrup, 5,
5'... Mold surface, 6,6'... Mold, 8... Cavity.

Claims (1)

[Scope of Claims] 1. A method for manufacturing a plastic lens made of a plastic material such as polymethyl methacrylate (including copolymers) or polystyrene that is polymerized by ultraviolet rays, comprising the steps of: producing a lens blank having an approximately desired shape from which optical distortion has been removed; The plastic material of polymethyl methacrylate (including copolymer) or polystyrene forming the lens blank is inserted into the cavity and placed in the space between the lens blank and the mold surface of the cavity formed by the mold. 1. A method for producing a plastic lens, comprising forming a fluid layer, irradiating the fluid layer with ultraviolet rays, and polymerizing the fluid layer.