Glasses- Glass Forming Flashcards
What takes place at the same time as shaping?
Cooling. Reheating allows further shaping
Working/forming temperature
Temperature below which the melt is sufficiently viscous to start forming
Forming range
Temperature range over which melt goes from being viscous enough to start forming to a temperature where it can just stand under its own weight
Softening point
Temperature where a glass fibre of specified dimensions heated at 5°C/min extends at 1mm/min under its own weight
Annealing temperature
Where 95% of the strain is released within 15mins
Strain point
Where 95% of the strain is released over 6 hours
Oder the standard viscosity points from highest to lowest temperature
Working/forming T Forming range Softening point Annealing T Strain point
Above what temperature does forming start?
Liquidus temperature (above 1000C for a SLS glass)
How to avoid crystallisation during forming
Much of forming occurs below liquidus T. Must optimise glass stability by composition control (low nucleation and crystal growth rates). Minimise forming time.
Most commercial glasses have critical cooling rates much lower than actual cooling rates
How does pressing work?
Simple mould, cover, plunger. Pour glass melt into mould and insert plunger to the desired depth. Use cast iron mould heated to near Tg. Air/water extracts heat transferred from melt into mould
What happens if mould is too hot or cold for pressing?
Too hot leads to sticking of final part to mould. Too cold means raid cooling of outer layer of melt which causes cracking
Uses of pressing
Tableware, TV tubes, car headlamps. Need accurate temperature control
Surface quality from pressing
Involves contact between melt and mould surface. Damage occurs which weakens glass. Difficult to form optically perfect surface by contact. So unsuitable for windows, mirrors
Good things about pressing
Rapid cooling rates and high production rates. Reproducible simple shapes. Split moulds can give more flexibility
Which glass surfaces have optical quality?
Only fire polished, untouched surfaces
Early method of making flat glass for windows and mirrors
Use casting which gives one high quality surface but the other one has to be polished
Blowing for making optical quality glass
Long, large cylinders or large bubble opened and spun into disk to avoid contact. Doesn’t give flat surfaces
Float process
Process for making flat glass with optical quality. Glas smelt floated on molten tin under H2/N2 atmosphere. Equilibrium thickness of sheet defined by densities and surface tensions of tin and glass is 7mm (close to main commercial requirement 6mm). Vary thickness by stretching or using side arms from 1-24mm. Both surfaces fire polished and optically of high quality. Glass can be almost flat on atomic scale
What can’t be made by float process?
Thin, flexible sheets for display panels (<0.5mm) use special low-alkali glasses. Have to be made by other methods
Stages of producing gobs for containers
Melt comes out of furnace and flows along the cooling system. Goes through the cooling zone and then the equalising zone until it reaches the feeder bowl. Needs temperature control and uniformity of under +/-1°C from conditioning zone.
Forehearth length
Distance between working end (furnace) and feeder bowl over which the melt is cooled to suitable forming temperature
Why does the forehearth need to be long?
May need to cool melt by over 200°C before it can be formed. The temperature gradient created can be removed only slowly (due to chemical homogenisation). Length often 10m with a slope to promote flow
How to minimise process time for making gobs
Use shallow forehearth (15cm) as time is proportional to depth squared. Cool rapidly from top then reheat so give an S shaped temperature profile rather than a U. This means shorter distances for heat to diffuse and faster time to equilibrium. Accurate mathematical models for melt flow and temperature exist allowing optimisation of process
How are gobs made at the end?
Blowing a glass freeform produces a sphere. So blowing directly into mould gives uneven glass distribution. Modern feeders produce gobs of controlled shape to match vertical glass distribution in final article. Glass flow accelerated by plunger. Shear the melt as plunger starts to rise. Plunger pulls glass up from shears which have cut away a gob underneath
