Abstract: A polyethylene or polypropylene foam and a method for its manufacture in which polyethylene or polypropylene powder, is mixed with a liquid component, e.g. mineral oil, to a substantially uniform blend and placed in a container such as a closed mold. The mixture is heated to a temperature at which the polymer flows and is in a substantially liquid state such that the intimately mixed polymer and mineral oil enter into a solution, the solution forming in situ in the container. Thereafter, upon further heating, the mineral oil is caused to undergo a phase change from a liquid to a vapor at its boiling temperature, whereupon the mineral oil acts to expand the polymer into a foam. The temperature to which the solution is heated is maintained to less than the temperature at which the polymer foam collapses due to lowering of its viscosity, thereby ensuring that the foam structure is maintained. The foam thus produced is cooled and removed from the mold.