Manufacturing Process

SOTEP transforms standard tubes (known as basic or draft tubes) into precision tubes by means of a cold drawing process.

Drawing involves cold pulling a bar or a tube through a die to reduce its cross-section and increase its length. This is a process which enables the outer and inner diameters to be calibrated according to criteria set by the customer, with good control of dimensional tolerances, a good surface finish and mechanical properties as required by the stipulated standards or specifications.

The drawing process uses a die which calibrates the outer diameter of the tube and internal tools to calibrate either the thickness or the internal diameter.

Cold drawing

The tube is prepared so that it can be gripped by the jaws of the chain bench, which pull the tube. For this, one end of the tube is prepared in a specific way (by hammering or by seaming) to pass through the die and therefore be gripped by the drawing system. This part of the tube is called a tang.

In order to change the thickness or the inner diameter at the same time as the outer diameter, different tools may be used:

  • Long mandrel
  • Fixed mandrel or olive or short mandrel
  • Floating plu

The long mandrel is only used for the first stages of drawing, as the mandrel then has to be removed from the tube through a mandrel removal stage which affects the dimensional precision of the tube.

Finishing methods


Drawing is carried out in the presence of a film of grease in order to prevent any material wrenching. This film of grease can then affect the structure of the metal owing to some of its components during heat treatment. In order to avoid this, the tubes must be perfectly degreased.

Heat treatment

Between each drawing stage, the hammer-hardened tubes undergo a heat treatment stage at a very specific temperature and with a previously calculated passage speed in order to give them a sufficient ductility level to enable a new drawing operation and thus continue to reduce the cross-section to the desired end dimensions. Heat treatment is followed by rapid cooling to prevent harmful phases from forming in the metal. Heat treatment is performed in a protective atmosphere. SOTEP has latest-generation probe ovens and recording devices which ensure total traceability of this special process which is vital for the quality of the end product.