a method for extruding a thermoplastic polymer / wood fiber composite utilizes styrene acrylonitrile as the principal thermoplastic component . acrylonitrile butadiene styrene is used as a stiffener and modifier to prevent degradation of a foaming agent &# 39 ; s efficacy . the method of the invention and the extrusion produced by the inventive method are particularly applicable to extrusions having high aspect ratio cross - sectional shapes and extrusions in which the ratio of wall thickness to interior volume is large .