Abstract: An extruded composite structural artificial lumber product is manufactured from wood fiber and polyethylene, including recycled polyethylene, by reducing the wood fiber and polyethylene each to a finely divided particles, mechanically mixing the wood fiber and polyethylene particles together with a measured amount of a powdered endothermic foaming agent, mechanically mixing the mixture, without pre-pelletization, in a twin-screw extruder where it is compressed and heated until it becomes plastic and homogenous, and then extruding the plastic mixture, using vacuum to remove vaporized moisture and other excess volatiles, through a molding die which forms structural profile of the desired product.