An online numerical simulation is presented that is capable of predicting state variables such as flow rate, melt temperature, shear rate, and melt viscosity by using real time data from a nozzle pressure sensor. The simulation solves the non-Newtonian nonisothermal polymer flow into multicavity tools while executing rapidly enough for real time process control. Numerical accuracy and stability were first validated offline by comparing the online simulation to results obtained from a commercial mold filling simulation. Simulation-based process control was then demonstrated by transferring a molding machine from fill to pack-based on the predicted flow front position. The simulation-based controller dynamically determined the appropriate transfer position for each part and transferred the machine at the correct time, thereby eliminating flash. The simulation, however, did increase process variability slightly due to delay times associated with the controller-machine interface. A full factorial design of experiments (DOE) was performed varying injection velocity, mold temperature, and melt temperature. Results show that while the simulation dynamically adjusted the process on a part-by-part basis, it did not fully account for the process changes. Accuracy could potentially be improved by incorporating data from additional process sensors, by developing adaptive viscosity models, and by accounting for the melt compressibility. POLYM. ENG. SCI., 49:2482-2491, 2009. (C) 2009 Society of Plastics Engineers