Glasses- Glass Forming

Question 1

What takes place at the same time as shaping?

Answer 1

Cooling. Reheating allows further shaping

Question 2

Working/forming temperature

Answer 2

Temperature below which the melt is sufficiently viscous to start forming

Question 3

Forming range

Answer 3

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

Question 4

Softening point

Answer 4

Temperature where a glass fibre of specified dimensions heated at 5°C/min extends at 1mm/min under its own weight

Question 5

Annealing temperature

Answer 5

Where 95% of the strain is released within 15mins

Question 6

Strain point

Answer 6

Where 95% of the strain is released over 6 hours

Question 7

Oder the standard viscosity points from highest to lowest temperature

Answer 7

Working/forming T
Forming range
Softening point
Annealing T
Strain point

Question 8

Above what temperature does forming start?

Answer 8

Liquidus temperature (above 1000C for a SLS glass)

Question 9

How to avoid crystallisation during forming

Answer 9

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

Question 10

How does pressing work?

Answer 10

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

Question 11

What happens if mould is too hot or cold for pressing?

Answer 11

Too hot leads to sticking of final part to mould. Too cold means raid cooling of outer layer of melt which causes cracking

Question 12

Uses of pressing

Answer 12

Tableware, TV tubes, car headlamps. Need accurate temperature control

Question 13

Surface quality from pressing

Answer 13

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

Question 14

Good things about pressing

Answer 14

Rapid cooling rates and high production rates. Reproducible simple shapes. Split moulds can give more flexibility

Question 15

Which glass surfaces have optical quality?

Answer 15

Only fire polished, untouched surfaces

Question 16

Early method of making flat glass for windows and mirrors

Answer 16

Use casting which gives one high quality surface but the other one has to be polished

Question 17

Blowing for making optical quality glass

Answer 17

Long, large cylinders or large bubble opened and spun into disk to avoid contact. Doesn’t give flat surfaces

Question 18

Float process

Answer 18

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

Question 19

What can’t be made by float process?

Answer 19

Thin, flexible sheets for display panels (<0.5mm) use special low-alkali glasses. Have to be made by other methods

Question 20

Stages of producing gobs for containers

Answer 20

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.

Question 21

Forehearth length

Answer 21

Distance between working end (furnace) and feeder bowl over which the melt is cooled to suitable forming temperature

Question 22

Why does the forehearth need to be long?

Answer 22

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

Question 23

How to minimise process time for making gobs

Answer 23

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

Question 24

How are gobs made at the end?

Answer 24

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