Abstract: In combination with a mixer/extruder of the type designed to mix, heat and extrude a mixture of shredded plastic material and shredded fibrous material, the heated mixture being extruded in the form of a continuous beam from an extrusion nozzle, the improvement which comprises a first matrix spaced horizontally downstream from the nozzle for receiving the beam therein and a second matrix spaced horizontally downstream from the first matrix for receiving the beam therein, the first matrix having a plurality of moveable walls which reciprocate in a horizontal direction against the outer surface of the beam, the second matrix having a plurality of moveable walls which reciprocate in a horizontal direction against the outer surface of the beam, the second matrix having means for conducting cooling water therethrough for cooling the beam as it passes through the second matrix.