Clinching
Clinching is a mechanical cold-forming process for permanently joining thin sheet metals by deforming the material to create a localized interlock, eliminating the need for welding, adhesives, or separate fasteners. The process was patented in 1897 by Dr. Louis Thies.[1][2][3] This technique, also known as press joining or sealed clinch joining, involves clamping two or more overlapping sheets between a punch and a die, where the punch forces the top sheet into the bottom one, causing the material to flow radially and form a button-like protrusion with a mechanical bond.[1][4] The process is high-speed and operates at room temperature, making it suitable for a wide range of metals, including steel, aluminum, and even dissimilar materials like aluminum to steel, without generating heat-affected zones that could compromise material properties.[1][5] Clinching offers significant advantages in manufacturing, including reduced production costs due to minimal tooling requirements and no consumables, improved environmental sustainability by avoiding fumes or waste from welding, and enhanced joint strength that can withstand shear, tension, and fatigue loads comparable to or exceeding those of riveted joints.[6][7] It is widely applied in industries such as automotive, aerospace, and appliance production for assembling components like car bodies, aircraft panels, and HVAC systems, where lightweight construction and rapid assembly are critical.[5][8] Variations of clinching include round clinching for general-purpose joints and rectangular clinching for higher load-bearing applications, with tooling designs tailored to sheet thickness—typically ranging from 0.4 to 6 mm—and material hardness to ensure optimal interlock formation.[1][4] While highly efficient for high-volume production, the process requires precise control to avoid issues like cracking in brittle materials, and post-joining inspections often use ultrasonic or visual methods to verify joint integrity.[2][8]Introduction
Definition and Principles
Clinching is a cold sheet metal forming process that joins thin sheets, typically ranging from 0.5 to 6 mm in thickness, through localized plastic deformation to create a mechanical interlock without the use of additional fasteners, heat, or adhesives.[9] This method relies on the compressive action of a punch and die to deform overlapping sheets, forming a protruding "button" or geometric feature that locks the materials together.[9] The process is particularly suited for ductile materials, as it requires sufficient material flow to achieve deformation without cracking, enabling joins of single or multiple layers, including dissimilar metals.[9] The core principles of clinching involve cold forming at room temperature, where compressive forces cause the sheets to draw and upset, resulting in an interlocked structure that gains strength through work-hardening during plastic deformation.[9] Key to joint integrity is the interlock geometry, defined by dimensions such as neck thickness (the thinnest section preventing pull-out), undercut (the groove securing the upper sheet), and button height (the protrusion on the lower sheet), which collectively determine shear and peel resistance.[9] These parameters are influenced by tool design and material properties, ensuring a permanent, non-detachable connection without filler materials.[10] Clinching offers an eco-friendly alternative to thermal joining methods, producing no fumes, sparks, or waste, and completing the join in under one second for many applications, which supports high-volume manufacturing.[9][11] The absence of heat preserves material properties, making it ideal for lightweight alloys like aluminum and magnesium, while the reliance on mechanical interlocking provides reliable strength comparable to the base material in suitable configurations.[10]History
The clinching process originated with German inventor Dr. Louis Thies, who patented a press-joining system for connecting metal sheets without additional fasteners (DRP No. 97517) on October 18, 1897.[3] This early innovation emphasized cold forming techniques to prevent heat-related distortion in the materials, distinguishing it from thermal joining methods.[12] Initially developed as a mechanical alternative to riveting for sheet metal assembly, clinching saw limited adoption through the early 20th century, constrained by rudimentary tooling and the dominance of established fastening practices.[13] Widespread use did not emerge until the late 20th century, particularly in the 1970s and 1980s, when improvements in press technology and material handling enabled more reliable implementation.[13] Commercialization accelerated in the 1970s and 1980s, with companies like TOX Pressotechnik, founded in 1978, pioneering practical applications of the technology.[14] By the mid-1980s, clinching was introduced to automotive manufacturing, marking its transition from niche use to industrial scalability.[15] The 1990s brought broader integration into assembly lines, highlighted by the first large-scale adoption at Audi in 1985 for vehicle body production, which demonstrated its viability for high-volume operations.[16] Entering the early 2010s, clinching had benefited from over 25 years of refinement, particularly in the evolution of portable tools that enhanced flexibility in field and small-scale applications.[16] Key milestones in the 2000s included a pronounced shift toward automated clinching systems, which improved precision and throughput in manufacturing environments compared to manual setups.[17] Post-2020 developments have centered on advancements for joining hybrid materials in lightweight structures, such as effective clinching of carbon fiber-reinforced composites to aluminum alloys for enhanced structural efficiency.[18] This growth has been propelled by environmental regulations favoring low-emission processes, as clinching avoids the fumes, energy demands, and waste associated with welding.[19]Clinching Process
Stages of Clinching
The clinching process involves a sequence of mechanical stages that deform two or more overlapping sheet materials to form an interlocking joint without additional fasteners or heat. These stages—initial indentation, drawing, and bottom forming or upsetting—occur under controlled force from a punch and die, resulting in plastic material flow that creates a characteristic button protrusion. These stages describe the standard deformation process without cutting, as used in round clinching; variants like lance clinching incorporate a cutting action. The entire process typically completes in less than 1 second, enabling high-speed production applications.[20] In the initial indentation stage, the punch indents the upper sheet, initiating localized plastic deformation by displacing the material into the die cavity without cutting or separating the sheets. This step initiates localized plastic deformation, with the punch force rising sharply as it overcomes the material's yield strength and begins to displace the sheets into the die cavity. Material flow at this point is primarily axial, pushing the upper sheet downward while the lower sheet resists and begins to bulge slightly, forming the foundation for interlocking. Insufficient material ductility during this stage can lead to cracking, particularly in brittle alloys.[21][22] The drawing stage follows, where continued punch advancement draws the deformed material radially into the die groove, elongating the upper sheet to form a frustum-shaped neck between the sheets. Force progression here stabilizes at a steady level after the initial peak, allowing controlled plastic flow that thins the neck while expanding the joint perimeter. This phase shapes the interlocking geometry through combined shear and compression, with material from both sheets flowing together to prevent separation. Typical joint depths emerge here, setting the stage for a final button height of 1-2 mm.[21][23] During the final bottom forming or upsetting stage, the punch compresses the protruding material against the die bottom, flaring the edges to create a locked undercut and rounded button. Force reaches its maximum as the material upsets outward, solidifying the interlock via enhanced plastic flow that hooks the sheets together. This completes the joint's mechanical integrity, with the button serving as a visible indicator of formation quality. Variants include single-stroke clinching, where all stages occur in one continuous motion for simple joins, and double-stroke processes, which separate indentation from forming for stronger interlocks in thicker or dissimilar materials. Failure modes such as neck cracking can arise if ductility is inadequate, compromising the joint's hold. Visually, the material flow transforms flat sheets into a protruding, button-like structure, as illustrated in cross-sectional diagrams showing radial expansion and axial compression from initial contact to final undercut.[21][24][22]Types of Clinch Joints
Clinch joints are primarily categorized by their geometric configurations, which determine their mechanical performance and suitability for specific applications. The most prevalent type is the round clinch joint, formed by deforming overlapping sheets into a circular button that creates a mechanical interlock without material cutting.[17] This configuration originated from the basic press-joining concept patented in 1897 by Dr. Louis Thies, which laid the foundation for modern clinching techniques.[3] Round clinch joints feature several subtypes based on die design, each tailored to material ductility and interlock depth. Fixed grooved dies produce a simple, symmetrical joint ideal for ductile metals like aluminum, where the sheets flow evenly into the die groove to form a protruding button typically 3 to 5 mm in diameter.[20] Split dies, which part during deformation, enable deeper interlocks for harder materials by allowing greater material flow, resulting in enhanced shear strength up to 3-5 kN.[17] Flat dies minimize protrusion on the bottom sheet, creating a low-profile joint suitable for aesthetic or space-constrained assemblies, though with reduced interlock compared to grooved variants.[17] Key geometric parameters for round joints include neck thickness (typically 0.1-0.3 mm) and interlock depth, which govern resistance to separation under load.[25] Rectangular or lance joints adopt an elongated shape, often incorporating a cutting action to penetrate tougher sheets, providing higher shear strength due to the extended contact area. These are particularly suited for stainless steel or high-strength steels in corrosion-resistant applications, where the lance design—featuring a slotted interlock—improves load distribution and joint integrity.[4] Peel strength in lance joints varies by material thickness, often achieving 0.4-0.8 kN for 18-24 gauge stainless steel.[4] Specialized variants have emerged since the 2000s to address limitations in standard clinching, including modified round joints for hybrid configurations and clinch-adhesive bonding for superior pull-out resistance. In hybrid clinching, adhesives are integrated during formation to reinforce the mechanical interlock, increasing overall joint durability in dynamic loading scenarios.[26] These evolutions build on the 1897 patent's principles, adapting round joints for multi-layer or dissimilar material assemblies prevalent in automotive and aerospace sectors.[11]Tools and Equipment
Punches and Dies
In clinching, punches and dies form the core tooling components that shape and interlock sheet materials through localized cold forming. The punch applies force to deform the sheets into the die cavity, creating a mechanical interlock without additional fasteners. These tools must withstand high pressures, typically up to several tons, while ensuring consistent joint quality across varying material thicknesses.[12] Punch designs are primarily cylindrical for producing round clinch joints, which provide uniform material flow and are suitable for most sheet metals, or rectangular for lance-style joints that enable elongated or modified interlocks. Cylindrical punches often feature a flat or slightly rounded tip to initiate drawing, while rectangular variants include notched edges for preliminary bonding. Punches are commonly made from high-strength tool steel to endure repeated impacts, with features like spring-loaded mechanisms for automatic retraction and to prevent sheet sticking post-forming.[27][28][29] Die types include round grooved designs, which feature an annular groove to guide material extrusion and form the characteristic button on the die side; these can be fixed for standard applications or split (e.g., two- or four-segment) to control material flow and reduce indentation on the upper sheet. Flat dies produce flush joints with minimal protrusion, ideal for aesthetic or space-constrained assemblies, while rectangular dies accommodate lance configurations. Die clearance, typically 0.1-0.5 mm, is critical for optimal interlock formation, as it influences the neck thickness and bottom thickness of the joint. Like punches, dies are constructed from tool steel for durability.[12][13] Tool wear is minimized through advanced coatings such as diamond-like carbon (DLC), which reduce friction and extend service life beyond 300,000 cycles in demanding environments. Customization is essential, with smaller punch and die geometries (e.g., reduced radii) used for thinner sheets to avoid cracking, while larger tools suit thicker materials up to 6 mm total thickness. Standard punch diameters range from 3 to 6 mm, allowing versatility in joint size and strength. Interchangeable tooling systems enable quick swaps in multi-material production lines, supporting automation. Maintenance involves periodic reshaping of worn surfaces, particularly when die depth decreases by more than 0.1 mm, to restore precision and prevent joint failure.[30][31]60179-4)| Punch Diameter (mm) | Typical Application | Source |
|---|---|---|
| 3.0 | Thin sheets (e.g., 0.5-1.5 mm total) | [31] |
| 4.0-5.0 | Standard automotive panels | [29] |
| 6.0 | Thicker hybrids (up to 6 mm) | [28] |