... trailer (Road milk tanker )3-60tons:All the heads are one time moulding . And all the inner ... 5-3mm / outer shell 2mm, one time molding heads.2 .Special welding process :TIG welding process, ... time molding heads and injecting PUF or use fiber glass.And each compartment use double heads back-to ... -back which make sure every compartment heads smooth and no right ...