Patent ID: 7431872

Claim:
A process for simultaneously forming a stable cellular wood plastic composite, the composite having an outer layer and an inner layer wherein the inner layer density is lower than the outer layer density and wherein the composite is formed in an extruder, the extruder having a feeding zone 1 , a first metering zone 2 , a venting zone 3 , and a second metering zone 4 ; the process comprising: a. mixing finely ground wood flour with a thermoplastic material at a temperature between about 325 and 350° F. (163 and 177° C.) in the feeding zone 1 to form a plasticated mixture; b. metering the plasticated mixture in the first metering zone 2 at a temperature of about 340° F. (171° C.); c. introducing a physical blowing agent into the plasticated mixture in the venting zone 3 at a pressure of between 40 psi and 500 psi and a temperature of about 325° F. (163° C.); d. metering the plasticated mixture of venting zone 3 in the second metering zone 4 at a pressure between about 600 and 6000 psi and a temperature of about 270° F. (132° C.) to drive the physical blowing agent into the plasticated mixture to form cells within the plasticated mixture, wherein the physical blowing agent is incorporated under a pressure gradient in the extruder sufficient to increase the solubility of the physical blowing agent in the plasticated mixture and under a temperature sufficient to prevent collapse of the cells; and e. reducing the pressure prior to removing the plasticated mixture from the extruder.