Glasses- Viscosity and Glass Fabrication Flashcards
What is needed to make glass shaping relatively easy?
High viscosity of the glass forming melt
What normally occurs simultaneously with shaping?
Cooling. Reheating allows further shaping
Formula involving shear stress and viscosity
Shear stress=ηxvelocity gradient
Where η is viscosity
Are most glasses Newtonian or non-Newtonian fluids?
Newtonian. So η independent of stress
Forced needed for shaping at high temperatures
Don’t need large forces to shale glass melt. Surface tension and gravity control forming
Forced needed for shaping at lower temperatures
Need greater forces to press or blow glass. Gravity and surface tension play a lesser role
Describe glass viscosity curves
Log(η) on y-axis (η in dPa.s) and temperature (in °C) on x-axis (500-2000). Curves start high (10-15) at 500C and curve down like decay curve past 1500C between 0 and 5 on y-axis.
Where is Tg on a glass viscosity curve?
Log(η)=13. Is effectively the glass setting point
Fulcher equation
Log(η)=A+B/(T-T0)
A, B and T0 are constants which can be calculated as a function of composition
Key viscosities for key operations in commercial SLS glasses
Melting: Log(η) 1.5-2.5, 1500C Forming gobs/drawing sheet: 3.6-4.4, 1200C Deformation limit by gravity: 11, 620C Belt marks on containers: 11.7, 590C Annealing: 13, 550C
Effect of liquidus temperature
Forming usually starts above it. Much of operation is below it. Must optimise glass stability (low nucleation, crystal growth rates) and minimise time for forming to avoid crystallisation
What is true about critical cooling rates for most commercial glasses?
They are much lower than actual cooling rates
What problems do crystals cause?
Can spoil optical characteristics. Can significantly weaken the material produced
What is devitrification?
Uncontrolled crystal growth. Nucleation and growth of stable/metastable crystal phases below the liquidus temperature of the melt. Means melting must be carried out above liquidus temperature
How fast must forming be carried out?
Quickly. Faster than the critical cooling rate
