Extruders for processing material have a selectively variable extruder volume. The extruders include an extruder barrel containing a bore sized to process 5 to 50 grams of material, wherein the bore has an upstream inlet portion for feeding the material to be processed and a downstream outlet portion connectable to an extruder exit for material processed through the extruder, and at least two recycle channels which differ from one another in at least one of volume, area of cross-section and length. A diverter valve of a flow control system is positioned at the outlet of the extruder for controllably directing material flow from the bore to either the extruder exit or at least one of at least two recycle channels.