When drawing special-shaped pipes, the movement of metal particles is mainly transverse movement, but the longitudinal movement is very small. The radial movement of the metal is achieved by the radial pressure given by the mold wall. Radial pressure is generated by the drawing force. The metal moves laterally under the action of radial pressure and is also affected by the friction resistance of the mold wall.

Therefore, the stress state of drawing special-shaped pipes with constant wall thickness is also a stress state of one pulling and two pressing. When drawing special-shaped tubes from round tubes, the contact area where the round tube begins to enter the special-shaped die and the starting contact position along the circumference are various. In order to reduce the non-synchronous deformation along the circumference, the drawing die is designed When grooving at the entrance, the simultaneous contact area should be increased as much as possible to prevent the pipe from becoming unstable during deformation.

The metal deformation of special-shaped pipes when stretched is very uneven. This is due to the different deformation amounts of each part and the different metal deformation of the inner and outer layers. For example, when drawing a rectangular tube, due to the different metal extensions at the corners and the flat part, generally the metal extension at the corners is large and the metal extension at the flat part is small, so the angle is subject to axial compressive stress, while the flat part is subject to axial tensile stress, and the deformation is very insignificant. If it is uniform, cracks will occur in the flat part. The reason for this uneven extension of various parts is that the arc-shaped surface of the round pipe material is bent into a flat surface. The metal on the inner surface of the flat part flows to the corner, while the metal on the outer surface of the corner part flows to the flat part, because the former is much larger. There are many, so the corners are extended greatly.