A theoretical study of pressure-controlled sequential sandwich transfer molding of rubber compounds under isothermal conditions has been carried out to obtain a two-layered sandwich structure. A physical model, numerical simulation procedure, and a numerical algorithm have been formulated based on the Hele-Shaw approximation along with the kinematics and dynamics of interface evolution. Based on the developed numerical simulation code, the effect of various processing conditions on the material distribution and interface shape can be evaluated and predicted. Comparison with experiments is carried out in Part II of this study. (C) 2004 Society of Plastics Engineers.