Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Changing mold size or shape during molding or with shrinkage...
Reissue Patent
2001-11-06
2004-10-12
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Changing mold size or shape during molding or with shrinkage...
C264S002500, C264S040600
Reissue Patent
active
RE038617
ABSTRACT:
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
less
than that of the peripheral portion, the lens is easy to bend at the thin central
potion
portion
. 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 from 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 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 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 cavi
Asami Hiroshi
Saito Kiyohiro
Hoya Corporation
Vargot Mathieu D.
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