A method for the assembly of doors having a pair of skins and a core of synthetic resin forms a first door element including forming a first skin and a peripheral wall to provide an upwardly opening cavity into which a foamable synthetic resin formulation is introduced. A second skin is then assembled over the cavity, and the resultant door assembly is passed through an elongated heating zone while controlling the thickness dimension of the door assembly to expand and cure the resin formulation in the cavity into a cellular resin insulating core and to bond the skins and core. The several steps are conducted seriatim at stations along a conveyor assembly.