The molding pressure is in a range of 0.05 to 20 kg/cm2 G preferably 0.1 to 10 kg/cm2 G especially 1 to 5 kg/cm2 G when the molded product having three layer structure is prepared, and it is in a range of 1 to 75 kg/cm2 G preferably 5 to 40 kg/cm2 G when the molded product having two layer structure is prepared.