Extruded polymer foam process with staged forming

A process for preparing extruded polymeric foam comprising the following steps:

• a. providing a foamable polymer composition in an extruder at an initial temperature and an initial pressure, the foamable polymer composition comprising a blowing agent and a polymer matrix that comprises a polymer component with a softening temperature, the initial temperature exceeding the softening temperature of the polymer component and the initial pressure high enough to preclude foaming of the foamable polymer composition;
• and
• b. extruding the foamable polymer composition in an extrusion direction into an atmosphere at a pressure below the initial pressure and allowing the foamable polymer composition to expand into a polymeric foam prior to cooling to a temperature below the softening point of the foamable polymer composition;
• wherein prior to cooling to a temperature below the softening point in step (b), the foamable polymer composition expands with a thickness expansion ratio of at least 30:1 while traveling through and contacting opposing forming plates that provide at least two sequential constraining sections along the extrusion direction, the forming plates in each constraining section comprising essentially parallel constraining walls that restrict expansion of the foamable polymer composition in a thickness dimension and wherein the constraining section closest to the extruder has essentially parallel walls defining a spacing in the thickness dimension that is smaller than the spacing of parallel walls defining the second constraining section and wherein the polymer composition expands into a polymeric foam having a thickness of 10 centimeters or more and essentially uniform density and compressive strength profiles along the thickness dimension of the polymeric foam.