Abstract: An expanded polyolefin foam with uniform fine closed cells is produced by placing a pre-form of a heat-expandable polyolefin resin composition in a mold cavity of generally uniform depth and closed by a platen, heating the mold to a temperature above the blow activation temperature of the pre-form while constraining the pre-form against any expansion under heavy controlled pressure, and then after cooling of the thus treated pre-form, removing the same from the mold and heating to effect expansion thereof. The pre-form is in a solid form with a volume exceeding the volume of the mold by a controlled amount in the range of 0.1-1 percent. The area of the pre-form is such as to fit readily within the mold cavity and its thickness exceeds the mold depth so that as the mold is closed by the platen the excess volume of the pre-form extrudes out around the peripheral edge of the mold before activation of the blowing and cross-linking agents.