In metalworking, clinching or press-joining is a bulk sheet metal forming process aimed at joining thin metal sheets without additional components, using special tools to plastically form an interlock between two or more sheets. The process is generally performed at room temperature, but in some special cases the sheets can be pre-heated to improve the material ductility and thereby avoid the formation of cracks during the process. Clinching is characterized by a series of advantages over competitive technologies: Reduced joining time (the joining time is less than a second) Reduced cost and weight: the process does not involve additional elements such as screws, rivets or adhesives Reduced cost of the machine No pre-holes are required Can be adopted to join different materials including metals, polymers, wood, and composite materials Can be easily automated and does not require qualified workers Eco-friendly: it does not require pretreatments with solvents, acids, and other harmful liquids The mechanical strength of the metal material near the joint is generally increased due to work-hardening Cleanness: the process does not produce flashes or fumes Repeatability Flexibility: the same tools can be employed for a wide series of materials Reduced joining forces Tools Because the process involves relatively low forces (ranging from 5 to 50 kN depending on the material to join, type of tools and sheet thicknesses), clinching generally involves reduced size (often portable) machines. The tools typically consist of a punch and a die. Different tools have been developed so far, which can be classified in round and rectangular tools. Round clinching tools include: fixed grooved dies, split dies (with 2–4 movable sectors) and flat dies. Such tools produce round joints which show almost identical mechanical behaviors in all plane directions. When round tools are adopted, the integrity of the sheet in the joint must be guaranteed in order to preserve a good mechanical behavior of the joints. On the other hand, rectangular clinched joints exhibit behaviors which depend on the loading direction and both sheets are intentionally sheared along the "long direction" in order to produce the interlock. The choice of the tools is highly influenced by: Material ductility Loading direction Thickness of the sheets In addition, the choice of the clinching tools highly affects the joining strength and the absorbed energy of a clinched connection other than the joining force. Rectangular tools, for example, require lower joining forces than round tools since the material