In thermoforming process, the mechanical and thermal properties induced in the moulded product depend, on one hand, on the type of polymer material, and, on the other, on the two following processing conditions: (a) the optical and the thermal properties of the infrared ovens, and (b) the temperature and mechanical properties of the mould. In addition, for semi-crystalline polymers, the quality of product also depends on the crystallization processes. In this paper, the effect of the cooling temperature on a semi-crystalline polymers sheet, heated beforehand by infrared radiation, on crystallinity is numerically analyzed. For this, the energy equation is coupled to the kinetics of the polymer. For this purpose, the heat equation is expressed in terms of enthalpy, and its numerical resolution is carried out using the 3D finite element method. An example of a polyethylene terephthalate (PET) sheet is presented.