an injection molding process for forming cross - linked , foamed moldings of an ethylenic polymer by passing a molding composition comprising an ethylenic polymer / cross - linking agent / foaming agent blend through the cylinder of an injection molding machine without substantially decomposing the cross - linking agent and the foaming agent , introducing the molding composition into a molding composition holding chamber provided at the exit end of the cylinder of the injection molding machine to decompose the cross - linking agent and the foaming agent therein , and injecting the molding composition into a mold . the invention also provides apparatus for carrying out the process comprising an injection molding cylinder for conveying and plasticizing the molding composition and a molding composition holding chamber provided at the exit end of the cylinder .