An experimental study of the dynamics of thermoforming a high-impact polystyrene sheet was undertaken to evaluate the effect of evacuation rate and temperature on the rate of sheet deformation and the wall thickness distribution of the molded part. The studies were conducted using an instrumented cylindrical mold having an adjustable bottom insert to vary the depth of draw. The evacuation rate was varied by introducing a flow restriction in the form of an orifice plate in the base of the mold. The deformation rate of the sheet was determined by means of fiber-optic infrared detectors located at various depths within the mold. Mold contact sensors also provided information with regard to the deformation process after contact with the mold surface had been achieved. The evacuation characteristics were monitored by a pressure transducer. A three-level, two-variable factorial design was conducted to provide information of the influence of the principal operating conditions and their interactions on the measured response variables. Reliable predictive expressions were derived for the minimum pressure, time to reach minimum pressure, overall forming time, and wall thickness near the lip of the mold. The wall thickness at other locations around the periphery of the mold contour was found to be relatively insensitive to the rate of evacuation and material temperature.