There is a hot-melt fastening technology in the fastener industry, which does not require pre-opening holes, and can be directly tapped under the closed profile to realize the connection. In the automotive industry such as BMW, Mercedes-Benz and Audi are using this technology, and more and more applications. So what kind of technology is this? What are the advantages and features?
First of all, I would like to introduce a concept to you, the "flow drilling screw" process (another name: hot-melt self-tapping/hot-melt fastening system). In English, some are translated as Flow drill Screws (FDS), and some are translated as Flow Form Screws (FFS), while "flow drill screw" refers to a connection process of high-speed rotation, self-tapping and final tightening. This process can realize unilateral connection with less deformation, and it is a detachable fastening method.
The hot-melt fastening technology is a cold forming process that conducts the high-speed rotation of the motor through the tightening shaft in the center of the equipment to the plastic deformation of the plates to be connected due to friction and heat, and then self-tapping and screwing.
At the beginning of the tightening process, the hot-melt screw is placed on the surface of aluminum or sheet steel without pre-drilled holes, and the high-speed rotating screw exerts high downforce on the workpiece surface while generating extremely high frictional heat, melting the metal. The tip of the screw then penetrates the material, forming a perforation, and gradually taps the thread. Once the thread is formed, the screw is tightened to a preset torque.
Hot melt fastening process steps and process description, including six stages: rotation (heating) → penetration → through hole → tapping → threading → fastening.
In the initial stage, the high-speed rotating motor drives the screw to contact the surface of the workpiece, and exerts downward axial pressure (the axial force can reach up to 1.5kN, and the rotation speed can reach up to 8000r/min), the screw head and the sheet metal part The surface rubs and generates high temperature, the temperature is basically 600 ℃ ~ 900 ℃, the metal in the area near the screw softens rapidly, and the heated material extends upward along the taper of the drill bit.
When the FDS screw penetrates the material, most of the hot-melt sheet metal material will flow to the lower part of the drilled hole to form a metal bushing with a thickness of 1~3 times. Tighten. The entire processing process only takes 1~6s to complete the fastening connection effect, and the torque can reach 15N·m.
At the same time, after the FDS nail is fastened in place, it will remain for a period of time. Under the action of the rotating torque, a pre-tightening force will be generated, which can improve the reliability of the threaded connection, the anti-loosening ability and the fatigue strength of the thread, and enhance the tightness of the connection. and rigid.
The question is, now that there are many connection technologies in the automotive industry, why use this connection method?
Now more and more automobile companies are beginning to apply the steel-aluminum hybrid body frame structure, using engineering plastics and composite materials such as aluminum profiles, aluminum stampings, aluminum castings, and carbon fibers, combined with various ultra-high-strength steel plates to replace traditional steel plate components or components to achieve lightweight and high strength body frame structures.
In the automotive industry, the main ways of joining aluminum are traditional mechanical fastening SPR (self-piercing riveting technology) and resistance spot welding RSW. SPR is a robust and replaceable process, but due to the need for a variety of rivet-rivet combinations, the equipment is expensive; although resistance spot welding is a mature technology and high economy, it is limited by the inability to connect dissimilar metals. Both processes require bilateral access, which severely limits the design.
Therefore, the technology of hot-melt fasteners came into being. The picture below is a description of the advantages of hot melt fastening technology.
There is no need for pre-punching holes, single-sided connection, and the mutual fastening connection between aluminum parts and aluminum parts, aluminum parts and steel parts can be completed.
Many European car manufacturers have already started to use hot-melt fastening technology, such as Jaguar XK and X150, Audi R8, A8, TT Coupe, A6, etc.; European car brands such as Nissan, Land Rover, Porsche, BMW, etc. have also introduced this connection Craftsmanship: The Cadillac CT6 model just launched by SAIC-GM has achieved the first localization of steel-aluminum body-in-white, combining 11 different materials and extensively using this connection technology.
FDS nails are made of hard alloy materials with special formula. The procurement of this kind of screws is mainly imported from abroad. Although many foreign car companies have used this fastening technology for many years, domestic self-owned brand car companies rarely use it. .
