Abstract: A method of making a foamed polymer article by injection molding, using a foaming agent that does not form byproducts which could pit or chalk the polymer surface. The foaming agent contains sodium borohydride in an amount from 0.01 to 5.0 weight percent of the polymer, and sodium dihydrogen phosphate as an activator to react with the sodium borohydride and produce hydrogen gas. The sodium borohydride and the sodium dihydrogen phosphate are kept separated from direct contact with each other, such as by encapsulating the components, until gas production is desired when the polymer is heated.