7.3 Mechanism of Thermosetting Heat setting stresses in the amorphous regions of thermoplastic fibers. When the fiber is heated above its glass transition temperature, the molecules in the amorphous regions can move, and the material can be formed into a new shapes Thus, crease that have developed in the fabric can be pulled out. and the width of the fabric can be changed somewhat in the heat setting proces7.3 Mechanism of Thermosetting • Heat setting stresses in the amorphous regions of thermoplastic fibers. When the fiber is heated above its glass transition temperature, the molecules in the amorphous regions can move, and the material can be formed into a new shapes. Thus, crease that have developed in the fabric can be pulled out, and the width of the fabric can be changed somewhat in the heat setting process