Previous work has shown that numerical simulation does not provide a correct description of compression experiments if the theology of the polymer in the solid state is deduced fr om tensile tests. Therefore, a new method is proposed for the determination of the coefficients of G＇sell＇s constitutive equation, in order to simulate the forging process. It also gives access to a parameter characterizing the friction conditions between the polymer and the tool. It is based on isothermal compression of polymer cylinders with an appropriate geometry and on its complete interpretation by means of computer simulation using the finite element code FORGE2. This original procedure is applied to polyoxymethylene.