A method for decreasing the density of a substantially closed-cell thermoplastic resin foam material by the secondary expansion thereof. The method comprises increasing the gas pressure within the cells, retaining that elevated cell pressure for prolonged periods and then heating the foam material above its heat plastifying temperature to cause secondary expansion thereof. In an example, a low density polyethylene material was foamed to produce a foam with a density of approximately 0.085 grams per cubic centimeter. This foam was then pressurized with nitrogen and afterward secondarily expanded to produce a foam with a density of approximately 0.036 grams per cubic centimeter.