Method for forming tube with high internal pressure and low external pressure, and forming machine

Disclosed is a method for forming a tube with high internal pressure and low external pressure. The method comprises: putting a tube to be formed into a cavity, such that a bulging area is formed between the tube to be formed and an inner wall of the cavity; continuously injecting a low-pressure fluid into the bulging area, and then injecting a high-pressure fluid into the interior of the tube to be formed, the high-pressure fluid creating a pressure on an outer wall of the tube to be formed such that the tube to be formed starts to expand to the bulging area; stopping the supply of the low-pressure fluid when the tube is bulged to conform to the inner wall of the cavity; and taking the formed tube out to complete a working cycle. Further disclosed is a forming machine for forming a tube according to the above method. According to the machine for forming a tube with high internal pressure and low external pressure, a mold body is provided with a first through hole and a second through hole for circulating a low-pressure fluid into and out of a bulging area, so as to create a certain pressure P on the tube and ensure uniform bulging of the entire tube, thereby achieving a better forming effect.