The elbow is a pipe for the pipe turning. The largest proportion of pipe fittings used in piping system is about 80%. In general, different forming processes are chosen for elbow with different material or wall thickness. At present, the elbow forming processes commonly used in manufactory include thermal push, stamping and extrusion.
Hot push forming
The forming process of the hot push elbow is a special bending machine, a core mold and a heating device, so that the blanks on the die are moved forward under the push of the push machine and are heated, expanded and formed in motion. The deformation characteristic of the hot push elbow is to determine the diameter of the tube blank according to the constant volume of the plastic deformation before and after the metal material. The diameter of the tube is less than the diameter of the elbow, and the deformation process of the blank is controlled by the core die, and the flow of the compressed metal in the inner arc is compensated to the other parts which have been thinned because of the expansion diameter, thus the wall thickness is obtained. A uniform elbow.
The hot push bending process has the characteristics of beautiful appearance, uniform wall thickness and continuous operation, suitable for mass production, thus becoming the main forming method for the bending head of carbon steel and alloy steel, and also used in the forming of some specifications of the stainless steel elbow. The heating mode of the forming process includes medium frequency or high frequency induction heating (the heating ring can be multi ring or single ring), flame heating and reflection furnace heating. Which heating mode depends on the requirements and energy conditions of forming products.
Stamping bending is the first forming process for mass production of seamless elbow. At present, it has been replaced by hot push or other forming processes in the production of common specifications, but the production quantity is less, the thickness of the wall is too thick or too thin in some specifications. The product is still in use when it has special requirements. The stamping process of elbow is made of tube blank equal to the elbow diameter, and the press is directly pressed in the mould by pressing machine. Before stamping, the tube blank is placed on the lower die, the inner core and the end die are loaded into the tube billet, and the upper die starts to press down, and the elbow is formed by the constraint of the outer die and the support of the internal mold.
Compared with the hot pushing process, the appearance quality of the stamping is not as good as the former, and the outer arc in the forming is stretched when the stamping elbow is formed, and there is no excess metal in other parts to compensate, so the wall thickness of the outer arc is about 10% thinner. However, due to the characteristics of single piece production and low cost, the stamping elbow technology is mostly used for the manufacture of small batch and thick wall elbow.
Stamping elbow is divided into two parts: cold stamping and hot stamping. Cold stamping or hot stamping is usually selected according to material properties and equipment capabilities. The forming process of the cold extrusion elbow is to use a special elbow forming machine and put the tube into the outer die. After the upper and lower die closing, the forming process is completed by the clearance movement of the inner and outer dies under the push of the push rod. The bending head made with internal and external die cold extrusion process has beautiful shape, uniform wall thickness and small size deviation, so the stainless steel elbow, especially the thin-walled stainless steel elbow, is made by this process. The accuracy of the internal and external die used in this process is high, and the wall thickness deviation of the tube blank is also very strict.
Other forming methods
In addition to the three common forming processes mentioned above, the seamless bending is also used to squeeze the tube into the outer die, and then through the shaping process of the open ball in the tube. However, this process is relatively complex, difficult to operate, and the forming quality is not as good as the process mentioned above.