Mold halves and molding assembly for making contact lenses

A mold assembly useful in the production of a contact lens, the lens defined by a front curve and a back curve, by the polymerization of a polymerizable composition in the mold assembly, the mold assembly comprising a front mold half and a back mold half in contact therewith thereby defining and enclosing a cavity therebetween, and a polymerizable composition in the cavity in contact with both said mold halves, wherein

• the front mold half comprises a first article of thermoplastic polymer transparent to ultraviolet light, the article having a central curved section with a concave surface, a convex surface and a circular circumferential edge, wherein the central section of the concave surface is in contact with the polymerizable composition and imparts a curvature to the front curve of a contact lens to be produced in the mold assembly, and is sufficiently smooth that the contact lens formed by polymerization of the polymerizable composition in contact with the central section is optically acceptable,
• the first article also having an axis of symmetry about the concave surface and further having an annular flange integral with and surrounding the circular circumferential edge and extending therefrom in a plane normal to the axis of the concave surface, and a generally triangular tab situated in a plane normal to the axis and extending from the flange;
• the back mold half comprises a second article of thermoplastic polymer transparent to ultraviolet light, the article having a central curved section with a concave surface, a convex surface and a circular circumferential edge, wherein the central section of the convex surface is in contact with the polymerizable composition and imparts a curvature to the back curve of a contact lens to be produced in the mold assembly and is sufficiently smooth that the contact lens formed by polymerization of the polymerizable composition in contact with the central section is optically acceptable,
• the second article also having an axis of symmetry about the convex surface and further having an annular flange integral with and surrounding the circular circumferential edge and extending therefrom in a plane normal to the axis of the convex surface, and a generally triangular tab situated in a plane normal to the axis and extending from the flange;
• wherein the convex surface of the back mold half contacts the circumferential edge of the front mold half;
• the front and back mold halves having a thinness and rigidity effective to transmit heat therethrough rapidly and to withstand prying forces applied to separate the mold halves from the mold assembly after polymerization of the polymerizable composition in the cavity.

A method for molding a mold half useful in subsequently molding a contact lens, comprising

• injecting molten thermoplastic under pressure at a selectable temperature and mass flow rate into a mold in a given direction at an injection point,
• immobilizing a first injected portion of the molten thermoplastic upon injection thereof,
• using the pressure to cause the molten thermoplastic to flow through and fill a planarizing zone in the mold which has a given thickness and a width which diverges from the injection point in a plane generally oblique to the given direction, wherein the planarizing zone is configured and the temperature and mass flow rate of the molten thermoplastic are selected so that the molten thermoplastic emerges from the planarizing zone as a thin, smoothly flowing layer having a given width,
• using the pressure to cause the layer emerging from the planarizing zone to flow into a flange region of the mold which defines a circular flange having the given thickness and having a diameter greater than the given width wherein the flange lies in a plane essentially parallel to the plane of the planarizing zone, and into a concavo-convex region of the mold having the given thickness and circumscribed by and intersecting said flange in a circle, while maintaining the temperature and the mass flow rate of the molten thermoplastic and a rate of heat withdrawal therefrom effective to fill said regions and form in the concavo-convex region a uniform layer of the thermoplastic at least one surface of which is of optical quality,
• withdrawing sufficient heat from the molten thermoplastic that the thermoplastic in the planarizing zone, the flange region and the concavo-convex region forms a unitary solid article, and
• withdrawing the article from the mold.