Experimental methods of measurement of the flow velocity distribution of a rigid PVC through a slit and a cylindrical die were developed. In particular, the slip of the polymer at the wall of a slit die has been shown by the optical observation of the displacement of tracers mixed with the melt. The effect of lubricants on the flow velocities has been shown and a law of variation of the fluid velocity at the wall has been proposed. These data, brought together with the kinetics of the PVC thermal degradation, can be used to compute the transit time of the polymer in a die and a thermal residual stability after extrusion. On this basis, it is possible to adjust the output rates, the temperatures, or the geometrical parameters to obtain a product of constant quality during a long extrusion time.