In today's extrusion plants, production flexibility is an increasingly important requirement. Moreover, following strategies of minimized stock keeping and just-in-production, plastics converters must be able to produce small batch sizes economically. Due to the resulting high number of consecutive material and color changes on a production line, residence time in the extruder plays a decisive role. In this work, the residence time distribution is determined for three different extruder concepts using ultrasound measurement technology. A smooth-bore, a grooved feed and an extruder with a grooved plasticizing zone are investigated. A 35 mm barrier screw without mixing elements is used in each case. The residence time distribution allows an evaluation of the melt homogeneity in reference to axial mixing in the extruder. Calcium carbonate is used as an indicator material, since small percentages are already sufficient to produce a detectable attenuation of the ultrasound signal. A measuring flange between the extruder and the throttle die enables the recording of the amplitude in an inline measuring set-up. For each of the respective extruder concepts, screw speeds of up to 300 rpm are applied with counter-pressures of 150 and 300 bar. For the tests, a PE-HD and a PE-LD are used in order to examine the influence of wall slipping and wall adhesive materials. The results point out that greater residence time distributions are obtained with the smooth-bore extruder than with the two force-feeding extruders which was to be expected. The results of the grooved feed bush extruder and grooved plasticizing zone extruder are very similar. A clear correlation between viscosity and wall slipping behavior cannot be determined by the measuring results. However, repeated tests demonstrate the reproducibility. There seem to be additional influential factor, not yet identified, which should be focus of future research activities.