Tube forming processes are often difficult to separate from tube fabricating processes, since fabrication processes are essentially secondary operations that shape, bend, enlarge and cut tubes.
The different tube forming processes are often intermixed and both can be essential to creating the desired tube. For example, tube rolling is technically a tube forming process because it produces tubular parts through heat curing materials that have been wrapped around a circular mold.
Tube swaging, on the other hand, is technically a form of tube fabricating because it is used to reduce or increase the diameter of tubes rather than change the actual tube formation. However, both of these processes can and often do fall under the heading of tube fabrication. Tube forming processes are essential to many industries, as tubes are used in a wide range of diverse applications.
Some examples include: industrial manufacturing, for parts such as heating elements, hydraulic cylinders and heat exchangers; marine and naval, for masts, telescopes and launcher tubes; architecture and construction, for steel handrails, grab bars and floor flanges; and automotive for fuel lines, exhaust pipes and other power transmission applications.
Although there are numerous tube forming processes, there are two that are considered to be the most common: tube rolling and tube extrusion. As one of the two main processes in which raw materials are used to form a tube, tube rolling involves three main steps: cutting pre-impregnated materials and rolling them around a mold or mandrel; wrapping the mandrel in a sleeve or film to eliminate retained air; and heat-curing, after which the mandrel can be removed from the formed hollow tube. The other main process is through extrusion, which is more complex.
In order to begin extrusion, a round metal billet is pressed by a ram through a die, which is a hollow profile that shapes the metal into a specific extruded shape as the billet is squeezed through. Tube forming often requires high temperatures. Hot extrusion is the process in which the metal is fully plasticized by heat; this is often performed in a vacuum in order to avoid oxidation. The metal can be extruded through the die using two different methods of extrusion: indirect extrusion and direct extrusion.
In direct extrusion, the die is held stationary while the ram pushes the metal billet through the die opening. In indirect extrusion, the die is held stationary as the hollow ram moves into the stationary billet from one end, forcing the metal to flow through the die. After the metal tubing has been extruded, it is straightened by a stretcher into the desired length of tube.