Polytetrafluoroethylene (PTFE) has some very interesting properties such as low friction coefficient or high thermal stability. However, its processing is rather difficult due to its high viscosity at molten state. One of the manufacturing process consists in sintering large parts of compacted PTFE powder. During such a thermal cycle, PTFE is melted and then re-crystallized in order to make PTFE particles coalesce and close the porosity. The difficulties of this process come from the coupling of thermal gradients and large strains (due to melting and crystallization) that can lead to mechanical incompatibilities inside the part. This can result in damaging the part. This work aims to model the stress-free strain generated by a given thermal treatment. The strain will be decomposed into different terms referring to physical mechanisms observed experimentally: (a) a thermal expansion depending on the composition of the material (crystallinity content) and possibly the temperature, (b) a strain due to volume change caused by phase changes (melting/crystallization), (c) a strain representing the porosity closure. A user material subroutine based on the constitutive equations developed is implemented in a finite element code. Results of first simulations are shown and compared with the experimental results. This model is first step for the simulation of the sintering process of PTFE parts. The characterization and modeling of PTFE mechanical behavior along the thermal cycle will enable to simulate the stress and strain inside the part and consequently to optimize the manufacturing process parameters to avoid any degradation of the parts.