Viscosity plays a crucial role in the behavior and processing of glass. It influences every stage of the production process, from melting and clarification to homogenization, forming, annealing, and heat treatment. Understanding viscosity is essential for controlling the quality and performance of the final product.
The viscosity of glass increases as temperature decreases. Unlike crystalline materials, which undergo a sudden change in viscosity at their melting point, glass exhibits a smooth and continuous transition from a molten to a solid state. This gradual change is one of the defining characteristics of glass and is key to its unique properties. The rate at which viscosity changes with temperature is an important factor in glass manufacturing, often referred to as the material’s thermal response. Glass that shows a rapid change in viscosity with temperature is classified as "short glass," while glass with a slower change is called "long glass."
Several reference points are commonly used in the industry to describe different stages of glass behavior based on viscosity:
(1) Strain Point: This is the temperature at which internal stresses can be relieved within a few hours. It corresponds to a viscosity of approximately 1013.6 Pa·s.
(2) Transition Point (Tg): This is the temperature where the glass transitions from a rigid to a more viscous state, corresponding to a viscosity of about 1012.4 Pa·s.
(3) Annealing Point: At this temperature, stress can be removed within minutes. It is typically associated with a viscosity of around 1012 Pa·s.
(4) Deformation Point: This occurs when the viscosity is between 1010 and 1011 Pa·s, marking the temperature range where the glass begins to deform under its own weight.
(5) Softening Temperature (Tf): This is the temperature at which the glass becomes soft enough to shape, with a viscosity of approximately 106.6 Pa·s. It is influenced by factors such as density and surface tension.
(6) Working Range: This refers to the temperature range in which glass can be formed. The upper limit corresponds to a viscosity of 102–103 Pa·s, while the lower limit is around 105 Pa·s, where the shape remains stable during molding. The working range generally spans from 103 to 106.6 Pa·s.
(7) Melting Temperature (Tm): This is the temperature at which the glass reaches a viscosity of around 10 Pa·s, allowing it to melt efficiently for further processing.
(8) Feeder Viscosity: Used in automated feeding systems, this corresponds to a viscosity of 102–103 Pa·s.
(9) Hand Picking Viscosity: This is the viscosity at which manual handling of the glass is feasible, typically around 102.2 Pa·s.
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