Controlling the forming of large thermoplastic parts from a simulation requires very precise predictions of the pressure and volume profile evolution. Present pressure profile based simulations adequately predict the thickness distribution of a part, but the forming pressure and volume profile development lack the precision required for process control. However new simulations based on the amount of power required to form the material can accurately predict these pressure and volume profiles. In addition online monitoring of the forming power on existing machines can be easily implemented by installing a flow rate and pressure meter at the gas entrance, and if necessary, exits of the part. An important additional benefit is that a machine thus equipped can function as an online rheometer that can characterize the viscosity of the material at the operating point by tuning the simulation to the online measurements. This method can characterize materials at the actual, very high strain rates, that are not attainable by conventional characterization methods, and also allows online detection of variations in the rheology of the resin. The rheological model of the material is then updated to fit this point, which would normally lie outside of the characterization range. A basic implementation for a cycle to cycle control system for thermoforming of plastic materials is proposed. The system controls the development of the pressure and volume profile in the part while adjusting to operating conditions, including changes in resin rheology. It ensures that the part is consistently formed cycle after cycle using the same strain rate profiles. A forming power based simulation is run in the background with the rheological model tuned to fit the operating point as shown above. A simple linear sensitivity matrix is then derived from the simulation. Since this linear sensitivity matrix is defined in the vicinity of the actual operating point and provides a much faster response time than the simulation, it can be used as the core of an online forming process control system. The system is applicable to large parts with slow deformation such as gas tanks, and small parts with very high rates of deformation such as angioplasty balloons.