a process for producing plastic / wood fiber composite foamed structures includes the steps of pre - drying wood fiber filler ; mixing it with plastic to form a mixture ; feeding the mixture into an extruder ; introducing and mixing a blowing agent ; subject the mixture to high shear forces and extruding the mixture to produce a plastic / wood fiber composite foamed structure . the filler has a degradation temperature and an active volatization temperature . during the pre - drying step the temperature is maintained below the degradation temperature . during the mixing step the mixing temperature is maintained below the active volatilizing temperature . during the introducing and mixing step a blowing agent is introduced into the plastic / wood fiber mixture and is mixed therewith to produce a plastic / wood fiber / gas mixture . during the subjecting step the plastic / wood fiber / gas mixture is subjected to high shear forces in the presence of high pressures and the temperature is maintained below an active volatilizing temperature .