he effect of the melt viscosity on the residence time distribution (RTD), pressures and temperatures in a modular co-kneader was investigated during the extrusion polypropylene samples, by varying the regulation temperature of the kneader or the molecular weight of the polymer. The co-kneader is a special modular single-screw extruder with simultaneous rotating and axial movements of the screw, with pins on the barrel fitting threads of the screw, commonly named reciprocating single-screw extruder. The RTD was obtained using a colored tracer. Material temperature was measured by sensors integrated in the pins of the barrel. Pressure was measured at the die. Experiments with four PP samples were carried out at various feed rates. One PP was selected for experiments varying feed rates and screw speeds. Both series of experimental results consistently show an increase of the temperature of the melt and of the pressure at the die when the viscosity of the melt increases. Increasing the feed rate increases the die pressure but reduces the temperature, while increasing screw speed produces self-heating. The experimental RTD remains the same at a given feed rate and screw speed, regardless of the viscosity of the material induced either by changing the barrel temperature or the molecular weight distribution of the polymer.