Disclosed are low density foam planks of ethylenic polymers, which planks have a density of from about 0.8 to about 1.6 pounds per cubic foot, dimensional stability, substantially closed-cell structure, a cross-sectional area of at least six square inches and a minimum cross-sectional dimension of at least 0.5 inch. Such foam planks are made from polyethylene resins (e.g., low density polyethylene) by release to lower pressure of a flowable, foamable gel under pressure, e.g., by extrusion foaming, wherein the gel comprises at least one polyethylene resin and at least one volatile blowing agent. The process of making such foam planks is particularly characterized by the improvement wherein the flowable gel is mixed sufficiently to disperse the blowing agent in the gel to an amount from about 13.5 to 35.times.10.sup.4 moles per gram of the ethylenic polymer and such process is especially beneficial in that it enables the preparation of substantially closed-cell polyethylene foams having densities of from 0.8 to about 1.9 via extrusion foaming in conjunction with certain C.sub.2 -C.sub.4 fluorocarbon blowing agents.