Kinds of Tube Drawing
Tubes are an integral part of the modern world. Its use is diverse – apart from the transfer of liquid and gas, tubes are used for structural purposes. Thus, tubing finds use in various industries, including construction, aircraft, oil, gas, and petrochemical, automotive, chemical, water treatment, sanitary systems, medical, pharmaceutical, etc.
This variety also necessitates tubes with various inner diameters (IDs), outer diameters (ODs), wall thicknesses, and materials used (View our Tube Sizing Chart). Moreover, some industries need tubing with variable diameters from one end to another.
Which brings us to Custom tubing and tube drawing. This method is widely used for creating metal or alloy tubing with various diameters and wall thicknesses. Let’s take a deeper look.
What is Tube Drawing?
Shortly, tube drawing is a process where a tube is drawn through a die to make it smaller. With this process, you can change the outer and inner diameter of the tube, but also the wall thickness.
Furthermore, tube drawing also refines the grain structure, which increases strength and durability. Ultimately, the method improves the surface finish.
Various tube drawing methods are utilized, depending on the quality required, tubing length, and material used. These include sinking, rod drawing, fixed plug drawing, floating plug drawing, and tethered plug drawing.
1. Tube Sinking
Tube sinking (free tube drawing) is the simplest and cheapest drawing method since it doesn’t require a mandrel inside the tube. And, while it is good for reducing cost, sinking produces tubing with worse surface quality.
This process is very efficient for changing the inner and outer diameters of the tube. However, tube sinking doesn’t change the wall thickness, at least not significantly.
Specifically, the wall thickness can change according to the die angle used. To keep the wall thickness, one should use a 24-degree angle. Meanwhile, higher angles can thin out the tube wall, whereas lower angles will make it thicker.
2. Rod Drawing
Rod drawing is a process where the tube is drawn over a mandrel. In this case, the mandrel isn’t static – it moves together with the tube. This method is quicker than tube sinking and produces tubing with a better surface finish.
Nonetheless, rod drawing has disadvantages in terms of the length of the tubes, since the length of the mandrel affects the length of the tube. In practice, one can produce tubes up to 100 feet (30 meters) in length.
3. Fixed Plug Drawing
Fixed plug drawing is a method where the mandrel sits stationary and doesn’t move with the tubing. This is the oldest known method for tube drawing.
Compared to other methods, it produces the highest-quality finish. However, it is also much slower and can’t be used for large reductions in diameter or very long tubes.
4. Floating Plug Drawing
Compared to the fixed plug drawing, in this process, the mandrel isn’t fixed. Instead, the plug is held in place by friction forces.
Floating plug drawing can be used for very long tubing, longer than 1,000 feet (300 meters). It also produces tubes with a good surface finish. For those reasons, this process finds use in down-hole oil exploration.
Nonetheless, this method requires adequate lubrication and material purity in order to be successful.
5. Tethered Plug Drawing
Tethered plug drawing is a process where the mandrel floats, just like in floating plug drawing, but it’s also anchored via a tether, just like in fixed plug drawing. You won’t be mistaken if you view it as a mix of the two.
This method is used for producing long straight tubes and produces higher-quality inner surface finish than rod drawing.