Glasses- Heat Transfer and Container Manufacture Flashcards
Temperature curves either side of glass-metal mould boundary
Glass gob is a over 1100C. This is almost constant until very close to the boundary where it decreases rapidly (this shape as low k). If there is perfect thermal contact, the temperature of the glass is the same as the metal mould at the boundary. Going into the mould the temperature doesn’t decrease much
Ideal mould-glass interface temperature
About the Tg. If it is higher the melt will tend to stick. If it is lower the glass may crack through thermal shock
What happens to temperature curves either side of mould-glass interface over time?
Over time heat flows across boundary. The local temperature gradients on each side decrease in magnitude.
Effect of glass shrinking away from mould
Creates a thermally insulating air gap and there is no longer perfect thermal contact. Temperature of glass touching air gap is much higher than mould wall. Over time heat removal slows significantly
How to solve problem from shrinkage away from mould
Glass parison is taken from mould, surface reheats by conduction from inside before 2nd stage of heat removal in the blow mould (final shaping)
Stages of container making (blow and blow)
Gob falls into blank mould. Finish (neck) is made first. Parison (shape) is blown from bottom. This requires glass distribution. Some heat is removed. Parison then inverted to upright and blown to shape. More heat removed. Uses a cast iron mould and compressed air cooling
Bottle making speed
Machines make 500 bottles per minute.
How are bottle making speeds so high?
Have 12 section machines with moulds operating in pairs so 24 blank moulds. Max glass residence time in mould 3sec. For each section, have set of parisons in transit to blow mould and further pair of gobs in blow mould adding further 2x3sec to processing. Gobs delivered to individual sections of machine which operate in synchronisation so bottles placed on conveyor belt in sequence that gives line of bottles without gaps
Other forming methods: tubes
Tubes drawn directly from melt or melt is wrapped around a hollow cone of refractory material and drawn off as air is blown along centre to prevent collapse
Other forming methods: Fibres
Optical fibres pulled from preforms. Continuous strand reinforcing fibres are pulled through a multi hole Pt brushing. Fibres for insulation blankets are aged when molten glass stream runs onto a spinning disk
Other forming methods: ribbon machine for bulbs
Like a fast bottle machine. Glass is blown against a steam cushion in a rotating mould to avoid seam line
Other forming methods: Further processing
Reheat for further shaping such as bending flat glass for windscreens, forming small bulbs from tubing, making paperweights.
Shaping by cutting, grinding, drilling
