An extruder 10 comprises a barrel 12 and a screw 24 mounted in the barrel for rotation. The screw comprises screw threads 26a, 26b, 26c. The barrel has an inlet funnel 22 toward one end for material to be extruded and a die plate 18 with openings 20 at the other end and an air inlet port 28 toward one end of the barrel and an air outlet port 30 toward the other end. The screw threads are such that material 34 in the barrel is moved more quickly along the portion of the barrel between the air inlet port and the air outlet port than upstream of that portion. The air outlet is connected to a pump 32 so that there is a pressure difference between the air inlet and the air outlet.
