A model for flow start-up of a Newtonian liquid in an initially empty modular self-wiping co-rotating twin screw extruder is developed. The changes of length of fill in front of the die and kneading disc block elements and output now rate with time were predicted for various modular screw configurations. Experiments were also carried out on a laboratory modular machine with windows to verify predictions. Generally, good agreement with the flow analysis was found.