The manufacturing process for contact lenses depends on whether the lenses are soft contacts, rigid gas permeable contacts or hybrid lenses.
How Soft Contacts Are Made
Soft contacts are made of hydrophilic ("water-loving") plastic polymers called hydrogels. These materials can absorb water and become soft and pliable without losing their optical qualities.
Soft contacts including new highly oxygen-permeable varieties called silicone hydrogel lenses can be made with either a lathe cutting process or an injection molding process.
- Lathe cutting. In this process, non-hydrated disks (or "buttons") of soft contact lens material are individually mounted on spinning shafts and are shaped with computer-controlled precision cutting tools. After the front and back surfaces are shaped with the cutting tool, the lens is then removed from the lathe and hydrated to soften it. The finished lenses then undergo quality assurance testing.
Though the lathe cutting process has more steps and is more time-consuming than an injection molding process, over the years the process has become more automated. With computers and industrial robotics, it now takes only a few minutes to create a lathe-cut soft contact lens.
- Injection molding. In this process, the soft contact lens material is heated to a molten state and is then injected into computer-designed molds under pressure. The lenses are then quickly cooled and removed from the molds. The edges of the lenses are polished smooth, and the lenses are hydrated to soften them prior to undergoing quality assurance testing.
Most disposable contact lenses are made with an injection molding process, as this method is faster and less expensive than lathe cutting processes.
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How Gas Permeable Contacts Are Made
Most rigid gas permeable lenses (RGP or GP lenses) are made of oxygen-permeable plastic polymers containing silicone and fluorine. GP lenses contain very little water and remain rigid on the eye.
Gas permeable lenses are custom-made to specifications supplied by the prescribing doctor and hence are more costly than mass-produced soft lenses.
A greater degree of customization is needed for GP contacts because they maintain their shape and don't conform to the eye like soft lenses. Minute differences in lens design can be the difference between a comfortable fit and contact lens failure with gas permeable lenses.
GP contacts are made with a computerized precision lathe cutting process similar to that used for lathe-cut soft lenses. But generally they are shipped dry to the prescribing doctor. The doctor's office then soaks the lenses in a GP contact lens care solution prior to dispensing them. This solution "conditions" the lens surfaces for greater wearing comfort.
How Hybrid Contact Lenses Are Made
Hybrid contact lenses have a central optical zone made of rigid gas permeable plastic, surrounded by a peripheral fitting zone made of a soft contact lens material.
Hybrid lenses are made with a process very similar to lathe-cut soft contact lenses, with one very significant difference: the plastic disks cut with the lathe have a GP center, surrounded by non-hydrated soft contact lens material.
The two materials are bonded together with proprietary technology to prevent separation of the materials after the lenses are cut and hydrated.
About the Author: Gary Heiting, OD, is senior editor of AllAboutVision.com. Dr. Heiting has more than 25 years of experience as an eye care provider, health educator and consultant to the eyewear industry. His special interests include contact lenses, nutrition and preventive vision care. Connect with Dr. Heiting via Google+.
[Page updated June 2011]
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