Cold Forming

Cold forming was first applied to the manufacturing of fasteners in the mid-19th century, when it was used to produce nails. The process itself involves shaping the desired form by applying sufficient force (using appropriate tools: headers, punches, dies) to a metal semi-product – typically a section of wire or a metal strip. The applied force must exceed the material’s yield strength but remain below its ultimate tensile strength – exceeding this limit would result in the destruction of the workpiece’s internal structure.

 

The term “cold forming” refers to the fact that the material is not preheated before processing. Other terms used to describe this method include: forging, upsetting, heading, extrusion, and swaging.

For many years, cold forming was used only for simple parts – e.g. hex-head bolts. Today, it is a highly advanced manufacturing process used for producing complex components (stepped rivets, gear wheels, hollow parts, bushings, joint components, balls), ranging from very small (Ø1 mm) to large-sized parts (Ø40 mm).

 

Key advantages of this technology include: high production speed, excellent repeatability, superior surface finish, significant economies of scale, minimal or no waste, enhanced mechanical properties, and wide versatility.
Modern cold forming machines range from the simplest single- or double-blow presses (headers, punches, upsetters), to state-of-the-art multi-die, multi-blow presses, capable of producing the most complex parts.

 

This rapid development of cold forming technology has been possible mainly due to: greater availability of tooling materials (high-speed steels, powder steels, carbide), improved metallurgical properties of raw materials, increasingly precise control systems, and the technological advancement of the machines themselves.

 

In the cold forming process, wire or rod is fed into the machine, where it is cut into segments of appropriate length. The desired shape is then formed at successive stations. At each station (i.e., with each blow), the material is upset or extruded.
Upsetting (heading) involves increasing the diameter of the material using a punch or header – e.g., forming the head of a round-head rivet.

Extrusion includes two main approaches:

  • Reducing the diameter by pushing the material through a die aperture
  • Forming a hole by pressing material around a mandrel (the material flows along the shape of the mandrel)

 
During upsetting and extrusion, the amount of material remains unchanged – only the shape is modified. Other operations, which also change the volume (not just the shape), include trimming, piercing, and pointing – these too are performed using cold forming machines. Most cold forming machines (presses) are horizontal, though vertical headers also exist.
 

The final shape of the product is determined by headers and punches on one side, and dies on the other. In general, headers and punches function like custom-shaped hammers that deform the part, which is held in place by the die.

The amount of deformation achieved in a single blow (i.e., at one station in a multi-die press) depends on the material’s mechanical properties and the complexity of the part. As a result, difficult-to-form materials or complex parts require multiple forging operations (up to 8 blows/stations per machine) and larger-diameter input materials (up to Ø40 mm). Therefore, the main specifications of a cold forming machine include:

  • number of stations (i.e., number of dies and blows),
  • maximum input wire diameter,
  • maximum forming force,
  • maximum part length.