The glass transition temperature Tg of a polymer (amorphous polymers) is defined as the temperature above which the polymer transitions from a hard solid state to a soft, rubbery state. The lower the glass transition temperature, the easier it is to deform and process the material. Which is why PLA, having the lowest Tg, among the available polymers for 3D printing, is the easiest material to process for 3D printing. The downside is that it is also the easiest to deform under very hot conditions and near its Tg @ 60 deg C, it will start to deform.
Polymers do not melt above the Tg but they simply start losing their structural integrity and become soft and pliable and very easy to process. For high temperature applications, it is therefore essential to use a material which has a relatively high glass transition temperature such that the structural integrity of the part remains intact at elevated temperatures. Most of the photopolymer resins have a Tg below 75 deg C and are avoided for high temperature applications. Among the polymer materials available with us, PETG-Glass polymer composite and different blends of Nylon 12 have a relatively high Tg, whereas PLA and its blends, HIPS have a relatively lower Tg. Castable materials, understandably, have a very low Tg.
For 3D printing materials available at Fabforma, generally speaking, the glass transition temperature values can be stated in the following order,
Wax / castable resins < silicone rubber < standard resins < thermoplastics < metals