TECHNOLOGY FOCUS - POLYMERS - Preferred Polypropylene: The polypropylene resin is polypropylene homopolymers, copolymers of propylene with 0.1-25% by weight of ethylene, blends of propylene homopolymers or copolymers with high density polyethylene, low density polyethylene, linear low density polyethylene or ethylene propylene rubbers or alloys of polypropylene homopolymers and copolymers with other thermoplastic polymers selected from polyamide and thermoplastic polyester resins. The level of second polymer in the blends or alloys is from 0.1-40% by weight of the polymer composition.Preferred Process: The amount of peroxide is from 0.001-10% by weight of the polypropylene resin used. The amount of crosslinking agent is from 0.01-10% by weight of the polypropylene resin. The residence time of the resin inside the extruder is from 2-50 times the half-life of the peroxide at the actual melt temperature.    TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Components: The foaming agent is carbon dioxide, 1-5C alkanes, nitrogen, water and/or chloro-fluorocarbons. The crosslinking agent is triallyl cyanurate or triallyl isocyanurate. The bubble initiator is inorganic carbonate, citric acid and/or azodicarbon amides. SPECIFIC COMPOUNDS - Tert-butyl-peroxy-n-butylfumarate, dicetyl peroxydicarbonate and dimyristyl peroxydicarbonate are specifically claimed as peroxides. Carbon dioxide and isobutane are specifically claimed as foaming agents.    EXAMPLE - Propylene homopolymer (polymer grade HO-2) having a melt index of MFI = 2 was premixed with dicetyl peroxydicarbonate (DCPDC) (0.50 wt. %) having a half-life of 1 second at 140 degrees C. This premix was extruded as strands in a twin screw extruder at 180 degrees C with the screw speed of 150 rpm to obtain a polymer resin dwell time of 8 times the half-life of DCPDC. Carbon dioxide (5.0 wt. %) was then added to the extruder at 18D downstream of the first inlet point of the extruder, followed by azodicarbonamide (1.0 wt. %). The strands of the resin were then granulated. The foam density was found to be 175Kg/m3 and the foam average cell size was 125 mum for these resins.