An extruder flow channel 10 has a pair of flow passages 40, 50 and a flow dam 12 interposed between the flow passages and a flow splitter 14 that locally increases or locally decreases the flow area of the first flow passage 40 relative to the second flow passage 50. The preferred flow channel 10 adjusts the mass balance of the flows 6, 7 between passages 40, 50 automatically.