During the solidification process, some polymers are able to form an internal structure consisting of regular shapes with surfaces in an even arrangement. This internal arrangement of macromolecules is called the crystalline structure. Whether polymers are able to crystallize or not depends on their molecular structure. In general, the unbranched macromolecules with no or only a very few regularly spaced constituents crystallize more easily. In all cases, the crystalline structure covers only a part of the entire volume, while the rest of it is formed by the polymer in the amorphous state. Polymers that contain both crystalline and amorphous regions are called semi-crystalline polymers. The initial arrangement of macromolecules in the melt is quite accidental because the valence forces between the molecules are weak and the molecules are not fixed in place. Upon cooling of the melt, the valence forces increase and every molecule tries to find the most advantageous position. This process depends on the properties of molecules and on the thermodynamic conditions. From a thermodynamic point of view, the system of semi-crystalline thermoplastics can be considered as an open system, i.e. Energy exchange with the surroundings is permitted The state of the system can be characterized by the enthalpy: The lower its value, the more stable the system. A change of state, for example a transition from the molten amorphous state into the crystalline state or a post-crystaIlization which takes place after processing the material, can be described by the change in the free enthalpy