Tube Manufacturing – Tube Drawing and Tube Extrusion
Tube drawing is a process to size a tube by shrinking a large diameter tube into a smaller one, by drawing the tube through a die. This process produces high-quality tubing with precise dimensions, good surface finish, and the added strength of cold working. For this reason this process is established for many materials, namely metalworking but also glass. Because it is so versatile, tube drawing is suitable for both large- and small-scale production.The large-scale production of glass typically uses a one step process where glass is directly drawn into a tube from a melting tank.
There are five types of tube drawing:
- tube sinking,
- mandrel drawing,
- stationary mandrel,
- moving mandrel, and
- floating mandrel.
A mandrel is used in many of the types to prevent buckling or wrinkling in the workpiece.
The ‘drawing’ process can also be used for tube drawing. Tube drawing does not mean manufacturing a tube from solid raw material. It means lengthening a tube reducing its diameter. Various arrangements used for tube drawing are shown in figure
- The method shown in Fig. (a) is the most common method used for tube drawing.
- A conventional tube drawing bench is used. Method shown in Fig. (b) employes a floating mandrel.
- Method shown in Fig. (c) uses a long circular rod to control the size of tube-bore.
- Method shown in Fig. (d) uses neither a mandrel nor a bar and controlling size of bore is difficult.
TUBE EXTRUSION :
Tubes and pipes are required in large quantities by industries all over the world. Tubes are basically of two types. They are either seamless (i.e., without any joint) or with joint all along the length of the tube.
Seamless tubes are made by processes such as casting, extrusion or rolling. Tubes with joint are made by welding. Usually, the weld joint is made by electric resistance welding process, such tubes are referred to as ERW tubes. The size of a tube or pipe is indicated by the size of its bore in mm. Since the requirement of tubes is so large, a special rolling process called Mannesmann rotary piercing process has been developed. In this process, a heated round billet with its leading end, in the centre of which a short guide hole has been punched or drilled, is pushed longitudinally between two large tapered rolls as shown in Fig.
The rolls revolve in the same direction and their axes are inclined at opposite angles of approx 6° from the axis of the billet. As the billet is caught by the rolls and is rotated, their inclination causes the material to be drawn forward. The small clearance between the rolls forces the material to deform into an elliptical shape. Due to compressive forces, secondary tensile stresses start acting in a direction perpendicular to the direction of the compressive stresses. The guide hole drilled/punched at centre of billet tears open. This action is assisted by a suitably placed mandrel.
As the billet mores forward and keeps rotating the tearing action is propagated throughout the length of the billet. End result is a roughly formed seamless tube of ellipitical cross-section.
This roughly formed seamless tube is further rolled in a “plug rolling mill”. The final operations of “reeling” and “sizing” are further conducted on cooled tube to improves size and finish of tubes
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