The temperature of the glass, which is not uniform when being conformed, tends to be lower close to mould’s surface. It also tends to decrease during conformation. The mould temperature tends to rise, as heat is transferred from glass to mould. By the end of the conformation, just before annealing, one tries to avoid that the temperature of the glass located close to the mould surface drops below the glass transition temperature (Tg). This prevents cracking. The most convenient temperature to pre-heat the mould depends greatly on Tg and the actual melt temperature. It also depends, presumably to a lesser extent, on other factors: weight and shape of glass piece and mould, mould material, glass composition, room temperature (Tr), annealing temperature, and time allowed for conformation. Because it would be quite difficult to quantitatively account for these effects, while no appreciable benefit could be earned from such knowledge, the mould pre-heating temperature is typically ‘ad hoc’ selected somewhere between Tr and Tg, being accepted if found suitable to avoid cracks. Graphite moulds, also often used, are typically not pre-heated, being refractory.

Hi,

well probably a bit above the glass transition temperature of your glass to allow relaxation but avoid crystalization? But thats not a trivial question at all... depends on the glass ant what you want to do with it.

Perhaps contact Prof. C. Rüssel at the Otto-Schott-Institute in Germay...

The temperature of the glass, which is not uniform when being conformed, tends to be lower close to mould’s surface. It also tends to decrease during conformation. The mould temperature tends to rise, as heat is transferred from glass to mould. By the end of the conformation, just before annealing, one tries to avoid that the temperature of the glass located close to the mould surface drops below the glass transition temperature (Tg). This prevents cracking. The most convenient temperature to pre-heat the mould depends greatly on Tg and the actual melt temperature. It also depends, presumably to a lesser extent, on other factors: weight and shape of glass piece and mould, mould material, glass composition, room temperature (Tr), annealing temperature, and time allowed for conformation. Because it would be quite difficult to quantitatively account for these effects, while no appreciable benefit could be earned from such knowledge, the mould pre-heating temperature is typically ‘ad hoc’ selected somewhere between Tr and Tg, being accepted if found suitable to avoid cracks. Graphite moulds, also often used, are typically not pre-heated, being refractory.

Thank you Dr Queiroz and Dr Wisniewski , My glass is a short glass, I mean the viscosity increases fast via temperature decreasing. I try to shape it by centrifuge casting but it broken when I want to get it out from the mold. Tg of our glass is 750 Degree Celsius. The glass melt has 1700 Degree Celsius when we pour it into the mold. The mold is made of cast Iron and has 450 degree Celsius. So you suggest I should increase the mold temperature? Indeed can I anneal the glass in the mold?

Since the conformed glass show tendency to break before annealing, that can be indicative of either a too long conforming time or a too low mould temperature. By centrifugal casting it may indeed be possible to speed-up the conformation of a bulk rod of glass. However, for a rod-like shape, the contact area between glass and mould is often higher than for a bar-like shape. That can promote faster cooling for a rod-like piece, unless such effect is compensated by higher mould temperature. A graphite mould can be quite appealing for centrifugal casting. Graphite moulds can be opened or unmoulded after annealing, what can be convenient. However, graphite oxidizes (burns) slowly in hot oxidizing atmosphere. For best mould conservation, the atmosphere during annealing should be inert or reducing. Pouring of hot glass into a graphite mould can occur in air with little or no mould damage.

Dear Farshad

Aditional tips for choosing the mould temperature.

The mold temperature Tmold should be a little above Tg and of course below the crystallization temperature on cooling Txc.

It is possible that you dont have the value of Txc. Then you can use the crystallization temperature on heating Txh. You can measure it with a DTA or DSC run, using the glass you already obtained.

Thus a choose for Tm could be: Tg < Tmold

Thank you Dr Prado