A great effort has been made to develop technologies for cast molding of hydrogel contact lenses with high precision, fidelity and reproducibility and at low cost. One of such manufacturing technologies is the so-called Lightstream Technology™ (CIBA Vision) involving a lens-forming composition being substantially free of monomers and comprising a substantially purified prepolymer with ethylenically-unsaturated groups, reusable molds, and curing under a spatial limitation of actinic radiation (e.g., UV), as described in U.S. Pat. Nos. 5,508,317, 5,583,463, 5,789,464, and 5,849,810. The Lightstream Technology™ for making contact lenses have several advantages. First, the curing process is fast, at a time scale of seconds. Fast curing can ensure design and adaptation of a high speed, continuous and automatic lens production involving on-line lens curing. Second, by using a composition comprising a prepolymer and being substantially free of monomers, subsequent extraction steps (removing unpolymerized monomers from the lenses) required in a traditional cast-molding manufacturing process are eliminated. Without lens extraction, the production cost can be reduced and the production efficiency can be further enhanced. Third, reusable quartz/glass molds or reusable plastic molds, not disposable plastic molds, can be used, because, following the production of a lens, these molds can be cleaned rapidly and effectively of the uncrosslinked prepolymer and other residues, using a suitable solvent and can be blown dried with air. Disposable plastic molds inherently have variations in the dimensions, because, during injection-molding of plastic molds, fluctuations in the dimensions of molds can occur as a result of fluctuations in the production process (temperatures, pressures, material properties), and also because the resultant molds may undergo non-uniformly shrinking after the injection molding. These dimensional changes in the mold may lead to fluctuations in the parameters of contact lenses to be produced (peak refractive index, diameter, basic curve, central thickness etc.) and to a low fidelity in duplicating complex lens design. By using reusable molds which are produced in high precision, one can eliminate dimensional variations inherently presented in disposable molds and thereby variation in contact lenses produced therefrom. Lenses produced according to the Lightstream Technology™ can have high consistency and high fidelity to the original lens design.
However, there are some practical limitations which hinder realization of all of the great potentials of such technology in the production of silicone hydrogel contact lenses. For example, when a silicone-containing prepolymer disclosed in commonly-owned U.S. Pat. Nos. 7,091,283, 7,268,189 and 7,238,750 is used to prepare a silicone hydrogel lens formulation, an organic solvent is generally required to solubilize the prepolymer. When such lens formulation is used to produce silicone hydrogel according to the Lightstream Technology™, the cured lens in the mold after UV crosslinking is still swollen in the organic solvent before the solvent exchange to water. Such lens may not be able to survive the mold opening and de-molding process, because the cured lens is in the swellon state in the organic solvent and has an inadequate stiffness and toughness (e.g., too low). As such, the production yield may be low and the production cost could be higher due to low production yield derived from the lens defects created during mold opening and de-molding process.
Accordingly, there is still a need for a lens manufacturing process in which lens defects generated during mold opening and de-molding process can be minimized.