Ophthalmic Lens Blank Manufacture
Ophthalmic lenses are manufactured from either glass or plastic materials. The process involves multiple steps, including raw material selection, melting, molding, grinding, polishing, and coating. Below is a detailed explanation of the manufacturing process of ophthalmic lens blanks for both glass and plastic lenses.
1. Glass Lens Blank Manufacturing
Raw Materials:
- Silica (SiO₂) – Main component
- Boric oxide (B₂O₃) – Improves chemical durability
- Sodium oxide (Na₂O) & Potassium oxide (K₂O) – Lower melting point
- Lead oxide (PbO) – Increases refractive index (used in high-index lenses)
- Barium oxide (BaO) – Enhances light transmission
Manufacturing Process:
Step 1: Melting and Mixing
- Raw materials are mixed and heated in a furnace at 1400–1600°C to form a homogeneous glass melt.
- The molten glass is refined to remove bubbles and impurities.
Step 2: Molding into Lens Blanks
- The molten glass is poured into molds to form lens blanks.
- The blanks are then annealed (slowly cooled) in an annealing oven to relieve internal stresses.
Step 3: Grinding and Polishing
- Lens blanks undergo surface grinding using abrasive wheels to shape the optical surface.
- They are polished using fine abrasives to achieve optical clarity.
Step 4: Coating
- Anti-reflective (AR), UV, and scratch-resistant coatings are applied to improve optical performance.
2. Plastic Lens Blank Manufacturing
Types of Plastic Materials Used:
- CR-39 (Columbia Resin 39) – Most commonly used, lightweight, and impact-resistant.
- Polycarbonate – High-impact resistance, ideal for sports and children’s eyewear.
- Trivex – Stronger and lighter than polycarbonate, good for rimless frames.
- High-Index Plastics (1.60, 1.67, 1.74) – Thinner and lighter lenses for high prescriptions.
Manufacturing Process:
Step 1: Monomer Preparation
- The liquid monomer (e.g., CR-39) is mixed with catalysts and dyes (if tinted lenses are needed).
Step 2: Molding and Polymerization
- The monomer mixture is poured into a mold (glass or metal) with the desired curvature.
- The mold is sealed and heated in an oven at 90–100°C for several hours to initiate polymerization.
Step 3: Cooling and Removal
- The lens blanks are slowly cooled and removed from the mold.
Step 4: Surface Treatment
- The lens surface is ground and polished to achieve optical clarity.
- Some plastic lenses undergo hard coating to improve scratch resistance.
Step 5: Coating Applications
- Anti-Reflective (AR) Coating – Reduces reflections and glare.
- UV Coating – Blocks harmful UV rays.
- Tinting – For sunglasses or fashion lenses.
- Photochromic Treatment – Allows lenses to darken in sunlight (e.g., Transitions lenses).
Comparison: Glass vs. Plastic Lens Manufacturing
| Feature | Glass Lenses | Plastic Lenses |
|---|---|---|
| Weight | Heavy | Lightweight |
| Impact Resistance | Brittle, can shatter | Highly impact-resistant (polycarbonate, Trivex) |
| Optical Clarity | Excellent | Good, but can be enhanced with coatings |
| Refractive Index | Higher (1.50 - 1.90) | Lower, but high-index plastics available (1.50 - 1.74) |
| Scratch Resistance | Naturally scratch-resistant | Needs additional scratch- resistant coating |
| UV Protection | Limited, requires coating | Most plastic lenses naturally block UV rays |
| Manufacturing Cost | More expensive | Cost-effective |
Conclusion
The manufacturing of ophthalmic lens blanks involves precise engineering to ensure optimal optical performance. Glass lenses offer superior clarity and scratch resistance but are heavier and prone to shattering. Plastic lenses, especially CR-39, polycarbonate, and Trivex, are more commonly used today due to their lightweight, impact resistance, and ability to incorporate advanced coatings.
Tags:
Optometry