The development of PTFE-seals in lay-down-technology is changing rapidly from an empirical to a more sophisticated engineering approach, due to the use of e.g. Finite Element Analysis (FEA) for optimization of design parameters. FEA is a powerful engineering tool, but its results are only as reliable as the input data for the model and the material.Therefore material tests with a specific PTFE-compound are performed including creep and relaxation. Then an explaination for the ‘bell mouthing’ of the seal lip is given. Further on it is analyzed how some design parameters like seal thickness, length, and grooves influence the contact pressure distribution.