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Most consumers have probably never considered the relationship between carbon graphite and the glass industry. Actually graphite is the ideal material for glass manufacturing due to its physical and chemical composition properties.
Home Products Glass Industry / Graphite Barriers for Manufacturing Float Glass
Graphite barriers (edge boards/dams) are critical components in the float glass manufacturing process, specifically within the float bath. Their unique properties – high temperature resistance, non-wetting characteristics, lubricity, thermal conductivity, and chemical inertness – make them ideal for controlling the flow, width, and shape of the molten glass as it floats on the molten tin, resulting in the smooth, flat, and high-quality float glass we use every day. The specific type and grade of graphite are carefully selected based on the specific requirements of the float glass production line.
Graphite barriers can play a crucial role in the manufacturing of float glass, primarily during the float bath stage.
Melting: Raw materials (silica sand, soda ash, limestone, etc.) are melted in a furnace at extremely high temperatures.
Float Bath: The molten glass flows from the furnace onto a bath of molten tin. This is where the key shaping and polishing occur. The glass floats on the tin, creating a perfectly flat and smooth surface.
Annealing: The glass ribbon is slowly cooled in an annealing lehr to relieve internal stresses.
Cutting & Finishing: The glass is cut to size and may undergo further processing.
Role of Graphite in the Float Bath:
Edge Restraint & Control (Edge Boards/Edge Dams):
Function: Graphite barriers, typically in the form of edge boards or edge dams, are positioned on the edges of the molten tin bath. They confine the molten glass as it flows onto the tin, preventing it from spreading uncontrollably.
High Temperature Resistance: The float bath operates at temperatures around 1000-1100°C (1832-2012°F). Graphite can withstand these temperatures without melting or degrading.
Non-Wetting by Molten Glass: Molten glass does not easily wet or adhere to graphite. This prevents the glass from sticking to the barriers, ensuring a smooth, continuous flow. If the glass stuck, it would create imperfections in the final product.
Lubricity: Graphite is inherently lubricating, which aids in the smooth movement of the glass across the tin bath and along the barriers.
Thermal Conductivity: Graphite's thermal conductivity helps to regulate the temperature of the glass at the edges, promoting uniform cooling and preventing edge defects.
Chemical Inertness: Graphite is generally inert to the molten tin and the atmospheric conditions within the float bath. This prevents contamination of the glass.
Width Control: The distance between the graphite edge boards determines the width of the glass ribbon produced. By adjusting the position of these barriers, the glass manufacturers can control the width of the final product.
In some more specialized applications, graphite can be used to create more complex shapes in the glass ribbon before it cools and solidifies. This is less common in standard float glass production but might be used in creating patterned or textured glass.
High Purity: Impurities in the graphite can contaminate the glass or the tin bath. Therefore, high-purity graphite is essential.
Fine Grain Size: A fine grain structure provides a smoother surface and reduces the risk of particulate contamination.
Controlled Porosity: Porosity can affect the wetting characteristics of the graphite. Manufacturers often control the porosity to optimize performance.
Mechanical Strength: The graphite barriers need sufficient mechanical strength to withstand the pressure of the molten glass and the stresses of the operating environment.
Thermal Shock Resistance: The graphite must be able to withstand rapid temperature changes without cracking or spalling.
Isotropic Graphite: Often preferred because its properties are uniform in all directions. This helps ensure consistent performance and reduces the risk of warping or cracking.
Extruded Graphite: Can be used, but may be less desirable due to potential variations in properties along the extrusion direction.
Carbon Fiber Reinforced Carbon (C/C): For even higher strength and thermal shock resistance, especially in demanding applications or larger components. However, this is more expensive.
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