an apparatus and methods for mixing and injecting a foam core within an extruded shell preferably of plastic downstream from an extrusion die to produce a uniform building product . the preferred apparatus includes a mixing head injector for mixing a binary system foam . the mixing head injector incorporates pressurized gas injection for homogenization of the foam core . a preferred method employs the mixing head injector and a novel calibrator thereby allowing injection of a foam core within the shell as it passes through a calibrator to reliably produce a uniform building product . an alternative method employs a second extruder . alternative methods employ a mounting fixture downstream of a first calibrator .