ACB

Stretch forming

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STRETCH FORMING PROCESS

cold_stretching The basic principle of this technology is to stretch the material above the elastic limit in order to reach the plastic mode so that permanent elongation with less spring back is obtained.

This principle can be applied to metallic sheets and profiles and in various directions depending on the final part shape.

Calculation of the stretching force :
F = 0,1 S * (Rm+Re) / 2

with :
F = force in daN
S = profile section in mm2 <<Click to enlarge
Rm and Re in MPa

Usual materials :
Aluminum, Cold formable Titanium, Stainless steel, etc.


Advantages

Stretch forming is widespread in the aerospace industry. It allows the manufacture of large parts, most often made of aluminium, with lower tooling costs than those of the drawing tools, as a single run is required to form the part, on a single die.

> on the workpiece:
  • gives very low and homogeneous residual stresses,
  • greatly reduces spring back,
  • allows without deformation, machining or assembling operations,
  • increases hardness by about 2 %,
  • allows the production of sheets, bars and rolled or extruded sections,
  • allows a final normalised metallurgical state of the product after completion of the various forming and heat treatments phases.

> on the press and the tool:
  • allows simple and fast tools changeover,
  • finished product matching the shape of the tool,
  • cheap form tools (1 form instead of 1 die + 1 punch) : about one-third of conventional tools,


Sheet Stretch Forming

The sheet to be formed is stretched around a tool through a traction force applied on the sheet. This force induces a traction stress in excess of the yield point of the material.

To apply this traction force, the sheet is held at both ends in gripper jaws. The latter are mounted on stretching heads which ensure positioning and stretching of the sheet.

The kinematics generated by the machine to the set of jaws allows bringing the part in a direction tangential to the tool throughout the forming operation.


Profile Stretch Forming

All throughout forming operation, outer and inner fibres remain extended.

With an elastoplastic material, spring-back effect does not appear and the part is satisfactorily formed in one press run with little and sometimes no hand finishing.

Extending the inner fibres will prevent the presence of wrinklings.

The application of stretch-bending process to sections (for materials as steel, stainless steel, aluminium alloy and Titanium) is shown on diagram below :

This process is particularly suitable for those sections with a low resistance to compression buckling such as open sections (for example handling rails).