Method for manufacturing toric contact lenses
A method of manufacturing of an astigmatic contact lens having a toric portion and a thickness differential feature to provide lens orientation on eye portion such that said thickness differential causes the toric portion of the contact lens to properly orient in the eye of the wearer. The toric lenses are manufactured by an effective process control method for cylinder power in toric lens production by determining an amount of a mold cylinder compensation which is caused by processes in a toric lens manufacturing system including tool making, injection molding, casting and curing, wherein the mold cylinder is defined as the difference in measured radius of curvature at two orthogonal directions. A control metric is established by using the amount of a mold cylinder compensation and tolerance range and reject mold out of the control limits and improve the production yield for toric lens manufacturing.
1. A method for manufacturing a toric contact lens, the method comprising the steps of:
a. determining an amount of a mold cylinder compensation which is caused by processes in a toric lens manufacturing system including tool making, injection molding, casting and curing wherein the mold cylinder is defined as the difference in measured radius of curvature at two orthogonal directions,
b. Injection molding to form an anterior mold half and a posterior mold half for the same toric lens manufacturing system as in step a), wherein the posterior mold half or the anterior mold half has a toric portion and the other mold half has a prism ballast or double slab off, wherein each mold half has a tab or other locating features for facilitating a determination of orientation of the mold,
c. measuring a mold cylinder value for the mold half having the prism ballast or double slab off,
d. controlling the mold half having the prism ballast or double slab off to have a mold cylinder value between a high limit and a low limit by rejecting the mold half having the prism ballast or double slab off falling out of the limits,
wherein the high limit is equal to or larger than the amount of the mold cylinder compensation plus a control tolerance and the low limit is equal to or less than the amount of the mold cylinder compensation minus the control tolerance, wherein the control tolerance is equal to or less than 0.050 mm,
e. introducing a lens forming material into the anterior mold half,
f. assembling the anterior mold half and the posterior mold half to form a lens shaped cavity there between,
g. curing the lens forming material in the lens shaped cavity to form the toric lens,
h. repeating the steps b) to g) for the same toric lens manufacturing system.
2. The method of claim 1 , wherein the posterior mold half has a toric portion and the anterior mold half has the prism ballast or double slab off.
3. The method of claim 1 , wherein the anterior mold half has a toric portion and the posterior mold half has the prism ballast or double slab off.
4. The method of claim 1 , wherein the lens forming material comprises at least one member selected from the group consisting of a hydrophilic vinylic monomer, a silicone containing vinylic monomer, a silicone-containing crosslinker, a silicone-containing prepolymer, a hydrophilic prepolymer free of silicone, and combinations thereof.
5. The method of claim 1 , wherein the curing step is performed by an ultraviolet curing.
6. The method of claim 1 , wherein the curing step is performed by a thermal curing.
7. The method of claim 1 , wherein the control tolerance is 0.040 mm.
8. The method of claim 1 , wherein the control tolerance is 0.030 mm.
9. The method of claim 1 , wherein the control tolerance is 0.020 mm.
10. The method of claim 1 , wherein the control tolerance is 0.010 mm.