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The Main Manufacturing Techniques of Optical Glass Materials

1. Melting Process:
The primary method for manufacturing optical glass is the melting process. This involves uniformly mixing various raw materials (such as quartz sand, soda ash, limestone, etc.) and heating them to a molten state in a high-temperature furnace. The molten glass is then formed into various shapes and sizes through cooling, shaping, and annealing processes.

Specifically, the melting process is divided into the Single Crucible Melting Method and the Continuous Melting Method. The Single Crucible Melting Method can be further categorized into Clay Crucible Melting and Platinum Crucible Melting.

• Clay Crucible Melting: Can melt most crown and flint glasses. It is low-cost and used when the melting temperature exceeds the usable temperature of platinum.

• Platinum Crucible Melting: Used for melting higher-quality glasses that severely erode clay crucibles, such as dense crown, heavy barium flint, rare-earth glasses, and fluorophosphate glasses. Platinum crucibles are heated electrically, typically using silicon carbide or molybdenum disilicide rod furnaces. For glasses prone to crystallization, requiring rapid cooling, or having specific atmosphere requirements, high-frequency heating can be employed.

In recent years, the use of continuous tank furnaces lined with platinum has become widespread, significantly increasing the output and quality of optical glass. This represents the main trend in current optical glass production technology.

2. Molding (Pressing) Process:

• Mold Preparation: A mold suitable for the desired optical glass size and shape is prepared first. Molds are typically made of stainless steel or cemented carbide for high precision and durability.

• Glass Melting: The required optical glass raw materials are melted in a high-temperature furnace, typically above 1000 °C (1832 °F).

• Glass Heat Treatment: The molten optical glass is poured into the mold and undergoes heat treatment to allow it to flow fully and fill the entire mold cavity.

• Isothermal Pressing: While maintaining pressure, the mold is cooled to a temperature below the glass transition point (Tg). (Glass viscosity at the softening point is about 10^7.6 poise, and viscosity at Tg is about 10^13.4 poise). This method of isothermal pressing with the mold is called the Isothermal Pressing Method. It facilitates high precision by allowing the mold surface shape to be accurately replicated onto the glass.

3. Rolling Process:
This method is performed at room temperature and involves a series of steps to press glass raw materials into glass plates or sheets of specific shapes and thicknesses.

• Raw Material Preparation: High-quality glass raw materials (e.g., silica sand, soda ash, limestone, dolomite) are crushed, mixed, and screened to meet manufacturing requirements.

• Thermal Processing: Prepared raw materials are heated to a specific temperature to achieve a viscoelastic state. In this state, the glass material has optimal plasticity for subsequent pressing.

• Pressing: The viscoelastic glass material is placed into a metal mold. Heating is applied to reach the required temperature and fluidity for pressing. Temperature and pressure must be strictly controlled during pressing to prevent cracks or deformation.

• Surface Treatment: After pressing, optical glass undergoes surface treatment (grinding, polishing, coating) to eliminate defects and improve quality and aesthetics, ensuring a smooth, flat, flawless surface suitable for optical instruments.

• Annealing and Cutting: After surface treatment, the glass is annealed to relieve internal stresses and improve stability, then cut to the required dimensions for various optical instruments.

4. Centrifugal Casting Process:
This method uses a centrifuge to spin molten glass, solidifying it into the desired shape. It's primarily used for manufacturing complex-shaped, precisely dimensioned glass products like optical lenses and glass fibers.

In centrifugal casting, molten glass is poured into a spinning centrifuge. Centrifugal force causes the glass to uniformly coat the mold surface and solidify gradually. During solidification, the glass density increases and its internal structure becomes denser, enhancing its physical properties and chemical stability.

Advantages: Can produce complex shapes with precise dimensions; improves physical properties and chemical stability.
Limitations: Requires high-temperature equipment and molds, increasing production costs; may not be suitable for certain highly specialized shapes.

5. Float Glass Process:
Molten glass is floated onto a bath of molten metal (typically tin) to form flat glass sheets.

• Forming Process: The forming occurs in a tin bath filled with a protective atmosphere (N₂ and H₂). Molten glass continuously flows from the furnace tank and floats on the denser molten tin surface. Under gravity and surface tension, the glass spreads out, levels, and forms sheets with smooth, flat, parallel top and bottom surfaces. It then hardens, cools, and is lifted onto transition rollers.

Characteristics and Advantages: Compared to other forming methods, float glass excels in high-efficiency production of high-quality flat glass – free of waviness ("wave distortion"), with uniform thickness, perfectly flat and parallel surfaces. Production line scale isn't limited by the forming method, energy consumption per unit product is low, and finished product utilization is high. Float technology is easily managed scientifically and fully mechanized/automated, leading to high labor productivity. Continuous operation can last for years, enabling stable production.