Abstract: A low density foam having a density less than 15 pounds per cubic foot (240 kg/m.sup.3) is formed from an amorphous polyethylene terephthalate polymer. An expandable resin composition is also disclosed, comprising an amorphous polyethylene terephthalate polymer and a blowing agent mixture comprising a low permeability blowing agent and a high solubility blowing agent. Unexpanded and partially expanded foam particles, impregnated with the blowing agent mixture are disclosed. Numerous methods are disclosed for producing lightweight foams from amorphous polyethylene terephthalate polymers. One such method utilizes an Accumulating Extrusion Expansion System which avoids problems experienced with the prior art when attempting to extrusion expand polyesters.