Glass transition (Tg) only applies to amorphous and semicrystalline polymers. Simply speaking, when a polymer is heated, the glass transition point is the temperature at which the polymer begins to soften before reaching its melting point (Tm) and demonstrating melt flow characteristics. Conversely, in extrusion when cooling a polymer, Tg is reached when the polymer hardens to the point that it becomes brittle. The relationship of Tg and Tm is relevant to the extruder in that if the Tg and Tm are very close (only a few degrees apart, for instance), then the temperature range for a successful extrusion of the polymer probably will be very narrow, as well. In this case, control of the process temperatures becomes more exact.