A plastic working method in which a metal blank is subjected to a pulling force to pass through a die hole to obtain an article having the same cross-sectional size and shape as a die hole is called drawing. Drawing is one of the main production methods for pipes, bars, profiles and wires.
According to the cross-sectional shape of the product, the drawing can be divided into solid material drawing and hollow material drawing.
The drawing of hollow materials mainly includes the drawing of round pipes and special-shaped pipes. The following basic methods are used for drawing hollow materials.
(1) empty pull
When drawing, the core of the tube blank is not placed, that is, the core is not pulled, mainly for the purpose of reducing the outer diameter of the tube blank. The wall thickness of the drawn pipe will generally vary slightly, and the wall thickness may increase or decrease. After many times of empty drawing of the pipe, the inner surface is rough and cracks may occur in severe cases. The air pulling method is suitable for small diameter pipes, special-shaped pipes, coil drawing, and reduction and shaping of small diameter reduction.
(2) long core rod drawing
The tube blank is freely sleeved on the surface-polished core rod, and the core rod and the tube blank are pulled together through the die hole to reduce the diameter and reduce the wall. This method becomes a long core rod drawing. The length of the mandrel should be slightly larger than the length of the pipe after drawing. After drawing one time, the core rod needs to be taken out by the pipe removal method or the rolling method.
Long core rod drawing is characterized by a large processing rate of 63%, but due to the need to prepare many long core rods of different diameters and increase the tube removal process, it is rarely used in production, it is suitable for thin wall. Production of pipes and tungsten and molybdenum pipes with poor plasticity.
(3) Fixed core drawing
The core rod with the core is fixed during drawing, and the tube blank is reduced or reduced in wall through the die hole. The inner surface quality of the pipe drawn by the fixed core is better than that of the air pull. This method is the most widely used in pipe production, but it is difficult to draw the thin pipe and it cannot produce long pipes.
(4) swimming core pull
During the drawing process, the core is not fixed to the core rod, but the force balance established by its own shape is stabilized in the mold hole. A more advanced method of pipe drawing when the core is drawn, it is very suitable for long pipe and coil production, and is extremely beneficial for improving drawing productivity, yield and surface quality of the pipe. However, compared with the fixed core drawing, the swimming core is difficult to pull, the process conditions and technical requirements are high, and the matching mode has certain limitations, so it is impossible to completely replace the fixed core drawing.