a process for forming hollow articles from a thermoplastic polymer sheet material . polymer sheet material is heated to a forming temperature and clamp between a ported mold plug and a jacketed mold cavity . the mold plug is moved into the mold cavity to stretch the material into the cavity . the clamping is released during the moving of the plug and a substantial portion of the air trapped between the material and mold cavity is evacuated . pressurized forming gas is injected through flow means around the mold plug to further expand the material into the mold cavity . thereafter , the injecting of forming gas is discontinued and coolant is flowed through the jacketed mold cavity and a cooling fluid is injected through the ported mold plug against the material while the pressurized forming gas is simultaneously purged from the area between the mold plug and expanded material through the flow means .