A dynamic finite element method is used to analyze the thermoformability of composites containing wood and a thermoplastic matrix for five different proportions of wood flour. Linear viscoelastic properties can be obtained by small amplitude oscillatory shear tests and the viscoelastic behavior is characterized using the Lodge model. To account for enclosed gas volume, which inflates the thermoplastic composite membrane, a thermodynamic approach is used to express the external work in terms of a closed volume. Pressure load is deduced by thermodynamic law using the Redlich-Kwong gas equation. The Lagrangian method together with the assumption of membrane theory is used in the finite element implementation. In addition, the influence of air flow on thickness and stress and the energy required to form a thin polymeric part in the thermoforming process are analyzed for five different proportions of wood flour in the HDPE material. POLYM. ENG. SCI., 49:1594-1602, 2009. (C) 2009 Society of Plastics Engineers