- When making soft contact lenses, manufacturers start with a polymer that has been cut into buttons. Guided by a computer, a digital lathe shapes the inside of the contact based on the lens prescription. The inside of the lens is then polished with an abrasive paste to remove any imperfections that could interfere with vision or comfort. Lens thickness is measured and, if correct, the lens is coated in wax so that the outside of the lens can be molded to the desired shape. Once the outside of the lens has been shaped, ultrasound is used to remove the wax and the outer surface and rims of the lens are polished. The lens is then left in a saline solution for 24 hours so that it can absorb enough liquid to become soft and flexible. The lens is then checked to verify that it matches the prescription and correctly refracts light. After a final cleaning, the lenses are sealed in bottles of a salt solution. These containers are then sterilized and can remain so for up to 7 years.
- Lenses can also be made by a process known as spin casting. A mold is made and filled with liquid monomer. Monomers are tiny molecules that can be combined in different ways to make polymers--a series of molecules that take on different characteristics depending on how they are put together. The mold is then rotated which creates a lens by polymerizing the monomer. Lens qualities are changed by changing the speed at which the mold rotates or the shape of the mold. Lenses then go through a finishing process of polishing, inspection, sterilization and packaging.
- Lenses can also be made using an injection molding process. This process involves a mold made up of two halves. The mold pieces are put together and the lens polymer is injected into the mold under pressure. After injection the mold is removed from the lens and cooled. As in the other methods, lenses are then polished, checked for quality control, sterilized and packaged.
