The plasticating sequence in barrier screws is modeled from hopper to die. The aim is to couple this routine to an optimization algorithm and use the software to set optimal operating conditions and screw design. The procedure developed to describe the geometry of the barrier section enables tackling most of the existing solutions. Calculations encompass gravity solids conveying in hopper, drag solids conveying, delay in melting, melting, and melt conveying. The onset and length of melting are decoupled from the beginning and profile of the barrier, providing a more realistic description of the effect of operating conditions and screw geometry. Predictions of melting and pressure axial profiles, output, distributive mixing, melt exit temperature, viscous dissipation, and mechanical power consumption are presented, and the effect of screw speed is discussed for a variety of barrier screw designs. (C) 2013 Society of Plastics Engineers