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.