AbstractThis work concerns some selected 3‐D flow effects on residence time distribution (RTD) in a screw extruder. The Newtonian problem is emphasized in which the cross‐ and down‐channel flow components can be formally discoupled and treated as two parallel 2‐D problems: our stream‐function‐based analysis of the Newtonian cross‐channel flow was combined with the existing 2‐D solution for the down‐channel flow. The RTD was determined by tracing the streamlines generated from both velocity components for the industrially important cases of fully‐ and partially‐filled channels. The effect of the cross‐channel flow on the RTD is discussed in connection with the stagnation flows, the down‐channel pressure gradient, and the presence of a free surface in a partially‐filled channel. The cross‐channel pressure distribution is compared to the conventional 1‐D theory. The non‐Newtonian effects were assessed using the commercial software POLYCAD.