A model for non-isothermal Newtonian flow startup in an initially empty modular self-wiping co-rotating twin screw extruder is developed. A viscous Newtonian fluid is considered to gradually fill a modular co-rotating twin screw extruder with viscous heating and heat transfer to the barrel (and screw). Thee build up of the axial cup mixing temperature profiles with time calculating for various modular screw configurations.