US 6,270,698 B1Granted

Stress-relieved acrylic optical lenses and methods for manufacture by injection coining molding

AssigneeAmerican Greetings Corporation

InventorDarrell Pope

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Abstract

Stress-relieved molded acrylic opthalmic lenses, and injection-coining mold processes for molding stress-relieved plus and minus to high-minus diopter ophthalmic lenses of optically superior acrylic resin, able to pass standardized impact drop tests for use in eyeglass lenses, are described. The injection-coining processes use two-plate and three-plate mold assemblies. The cavity of a two plate runnerless mold is partially filled under partial tonnage, and the movable half of the mold controlled to coin and densify the acrylic resin under secondary or full tonnage, when a velocity-pressure changeover point is reached. In a process which employs a three-plate mold assembly, the lens mold cavities are partially filled under less than total clamp tonnage, with a cavity plate held by hydraulic pressure against movable half of the mold. The mold assembly is then closed under full tonnage when a velocity-pressure changeover point is reached, collapsing the cavity plate against a stationary platen, bringing the mold assembly to a total stack height. The processes produce impact resistant acrylic ophthalmic lenses which are stress-relieved and without weld lines, even in the high minus diopter range, with center thickness' as low as one millimeter, the edge thickness' up to ten millimeters.

Claims19

Dependent claims

A method of molding a negative diopter stress-relieved ophthalmic lens out of acrylic resin material by an injection-coining mold process, the method comprising the steps of:

providing an injection molding machine having a mold clamp and a mold material injection screw, with a two plate fixed cavity mold having a movable half and a stationary half with at least one mold cavity defined between the movable half and stationary half, the mold cavity having varying thickness dimensions, with a center thickness less than a peripheral thickness, and an overflow area extending from the mold cavity, the movable mold half operatively controlled to exert variable clamping pressure against the stationary mold half,
preparing the injection molding machine for operation with acrylic resin as a molding material by inputting predetermined molding process control parameters into a control system, the molding process control parameters being selected to avoid formation of stresses in the lens, the molding process control parameters including plastic temperature, plastic flow rate, plastic pressure, plastic cooling rate, clamp closing velocity, velocity-pressure changeover position and coining stroke,
initiating the mold process by closing the mold halves together under a primary clamp pressure at a specified partial tonnage,
advancing the injection screw to partially fill the mold and to reach a velocity-pressure change over position, at which
a secondary clamp pressure is applied to the mold at a specified tonnage greater than the partial tonnage to perform a coining operation on the acrylic resin material in the mold,
holding the injection screw under time and pressure,
advancing the injection screw under the packing time and pressure,
retracting the injection screw back to a metering ready position,
allowing the acrylic resin material to solidify within the mold during a specified cooling time,
opening a movable half of the mold to a stop point, and
ejecting a stress-relieved acrylic lens out of the mold.

8 more elements

A method of making stress-relieved negative diopter ophthalmic lenses out of acrylic resin by injection-coining with a three-plate mold assembly in an injection molding machine having a mold clamp operative to apply variable pressures to the mold assembly, the three-plate mold assembly having a stationary plate/a movable plate/and a cavity plate and at least one mold cavity configured to form a negative diopter opthalmic lens having a center thickness dimension less than a peripheral thickness dimension, and a mold material injection screw operative to heat the acrylic resin and fill and pack a mold cavity in the mold assembly, the mold assembly having a mechanism operative to control a position of a cavity plate relative to a stationary plate of the mold assembly, the method comprising the steps of:

preparing the injection molding machine for operation with the three-plate mold assembly and acrylic resin by inputting control parameters into a control system for the injection molding machine, the control parameters including plastic temperature, plastic flow rate, plastic pressure, plastic cooling rate, clamp closing velocity, velocity-pressure changeover position, and degree of movement of the cavity plate relative to the stationary plate of the mold,
opening a moveable plate of the mold assembly to an opening set point,
extending the cavity plate to a position spaced from the stationary plate of the mold assembly;
applying a primary clamp pressure to the mold assembly;
partially filling the cavity within the mold assembly by operation of the injection screw to reach a velocity-pressure changeover position,
at which a secondary clamp pressure to the movable plate of the mold assembly which is greater than the primary clamp pressure, the secondary clamp pressure, the secondary clamp pressure causing the cavity plate to collapse against the stationary plate of the mold assembly,
holding the injection screw according to time and pressure control parameters,
advancing the injection screw according to packing time and pressure control parameters, allowing the material in the mold cavity to solidify and cool, and
moving the movable plate to open the mold assembly to allow ejection of a molded lens from the mold cavity.

6 more elements

Dependencies are inferred from the claim text.

Facts

Application: 09/318,396Filed 25 May 1999
Granted: 7 Aug 2001
Term ends: Not estimatedThe source records don’t support an estimate.

Classification10 codes

B29D 11/00192
Auxiliary operations, e.g. removing oxygen from the mould, conveying moulds from a storage to the production line in an inert atmosphere:…
B29C 45/561
Using mould parts movable during or after injection, e.g. injection-compression moulding: injection-compression moulding
B29D 11/00413
Production of simple or compound lenses: made by moulding between two mould parts which are not in direct contact with one another, e.g.…
B29D 11/00432
Production of simple or compound lenses: auxiliary operations, e.g. machines for filling the moulds