Injection compression molding method and injection...

Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Changing mold size or shape during molding or with shrinkage...

Reexamination Certificate

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Details

C264S040100, C264S328700, C425S145000, C425S562000, C425S808000

Reexamination Certificate

active

06767482

ABSTRACT:

1. Field of the Invention
The present invention relates to an injection compression molding method and an injection compression molding machine for molding products by molding thermoplastic resin being injected and compressed.
2. Description of Related Art
Injection compression molding method is known as a molding method for obtaining lenses from thermoplastic resin as disclosed in Japanese Patent Publication Laid-Open No. Hei 2-26723 and Japanese Utility Model No. Hei 6-9826. Incidentally, spectacle lenses having lens power ranging from +4.00 to −8.00 D(D: diopter) are preferably used for the molding method.
The injection compression molding method is a molding method for correcting shrinkage of the molten resin to obtain uniform and highly accurate configuration, in which molds are closed while retaining a predetermined compression margin in a lens forming cavity and, subsequently to injecting the molten resin to fill the lens forming cavity, the compression margin is compressed to obtain lenses.
Incidentally, when curvature of front side and rear side of the lens differ, thickness difference is caused to the thickness of the lens. As lenses having the thickness difference, there are two kinds of lenses, i.e. a plus lens having thicker central portion than peripheral portion thereof and a minus lens having thinner central portion than peripheral portion thereof.
The lens thickness differs according to a design of an optical lens. For instance, an aspheric lens having refractivity of approximate 1.50 and lens power of +2.00 D has approximate central thickness of 4.2 mm and approximate peripheral thickness of 1.0 mm. When the lens power is −2.00 D, the central thickness is designed to be approximately 1.4 mm and the peripheral thickness is designed to be approximately 4.8 mm. When the lens power is −4.00 D, the central thickness is designed to be approximately 1.44 mm and the peripheral thickness is designed to be approximately 7.9 mm.
In injection-compression-molding such lenses having the thickness difference, following disadvantage occurs according to difference in configuration characteristics.
Since the peripheral portion of the plus lens is thinner than the central portion thereof, a deformation containing molding shrinkage called “hi-ke” (surface sink) is likely to be caused around the gate in molding plus lenses, thereby making it difficult to keep configuration accuracy of the peripheral portion.
Since the central portion of the minus lens is thinner than the peripheral portion thereof, central portion of the cavity has greater flow resistance. Therefore, the molten resin injected into the cavity flows to branch into the peripheral portion and around into the central portion on account of difficulty of flowing the central portion of the cavity, thereby often causing weld mark in the central portion.
Accordingly, conventional molding die for the plus lens has a small gate opening configuration in order to restrain the development of the “hi-ke” (surface sink).
On the other hand, since the weld mark tends to be generated when the molten resin is not smoothly flown into the cavity, conventional molding die for the minus lens has a relatively large gate opening configuration in order to ensure the fluidity of the molten resin.
However, since there is delicate difference in the gate opening configuration according to lens thickness difference, numerous molding dies having different gate opening configuration have to be prepared, thereby increasing financial burden.
In view of the above, the Applicant of the present application has proposed an injection molding die for molding lens, in which the gate portion is arranged to be exchangeable (Japanese Patent Application No. Hei 8-6407).
The gate portion of the molding die is arranged to be exchangeable. Furthermore, a plurality type of gate top member for defining the gate opening configuration is prepared in advance. A gate top member having gate opening configuration corresponding to the lens configuration to be molded is selected from the gate top members and is exchangeably attached to the gate portion of the molding die.
However, even when the gate top member is selected in accordance with configuration property of the lens to be molded, the gate top member has to be exchanged in accordance with the lens configuration to cause much trouble, and the productivity can not be greatly improved.
Further, it is usually preferable that the gate is arranged to have small resistance in filling the cavity with the molten resin and the molten resin is preferably shut in the cavity after the cavity is packed with the molten resin. Accordingly, the configuration and the size of the gate has to be determined by a compromise between the conflicting demands.
Accordingly, as mentioned above, even when the gate top members having the gate opening configuration in accordance with the lens configuration are prepared in advance, since the gate opening configuration has to be determined by an appropriate compromise of the contradictory demands, the contradictory demands can not be satisfied simultaneously. Further, in order to satisfy the demands as best as possible, the gate opening configuration gets too complicated, thereby increasing production cost.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an injection compression molding method and injection compression molding machine for solving the above-mentioned conventional problem to improve the productivity and for obtaining molding products having high configuration accuracy and high quality.
The present invention is made by taking notice of the fact that both of the above two demands for the gate (fewer or lower resistance in filling the molten resin into the cavity and shutting in the molten resin after the molten resin is packed in the cavity) can be met utilizing time lag between the respective demands, in which the opening of the gate is closed after injecting and filling the molten resin into the cavity.
A injection compression molding method according to the present invention includes the following steps: providing an openable/closable molding die having at least one cavity including a pair of relatively movable cavity forming member and a resin flow channel in communication with the cavity through a gate; closing the molding die; setting a size of the cavity by relatively moving the pair of cavity forming members to form a predetermined compression margin inside the cavity; injecting molten resin to fill the cavity; and at least after a time point prior to completion of injection of the molten resin, reducing the size of the cavity by relatively moving the pair of cavity forming members to compress the molten resin. The present invention is characterized in that an opening of the gate is shut after completion of injecting the molten resin; and the gate shut operation is conducted in synchronization with the relative movement of the pair of cavity forming member in reducing the size of the cavity.
In the above, the shutting timing of the opening of the gate is set after completion of injection of the molten resin, more preferably, immediately after completion of injection of the molten resin, and the shutting action of the opening of the gate may be initiated before completion of injecting the molten resin.
When the opening of the gate is closed, a clearance enough to prevent the molten resin injected and filled in the cavity from flowing back to the resin channel from the gate during cooling process, preferably a clearance of less than or the same as 2 mm may be secured. Or alternatively, the opening of the gate may be completely closed.
Further, the number of the cavity may either be single or plural. The type of the cavity is not limited, but a lens-forming cavity having meniscus shape, which requires high accuracy and high quality, is preferably employed.
According to the injection compression molding method, since the size of the cavity is initially set; the molten resin is injected and filled into the cavity; the opening

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