Bonding of Metal and Thermoplastics
Joining Gun Generates Robust Connections within Seconds
Modern lightweight construction often requires the combination of metal with polymers. In addition, efficient process chains are required for use in industrial production bringing pretreatment and joining technology in agreement with the specific load case. Tools for process simulation and property characterization also play an important role.
The Fraunhofer Institute for Material and Beam Technology IWS Dresden, Germany, has developed a new process for joining materials with different properties. The "HeatPressCool-Integrative" (HPCI) process is highly suitable for replacing complex adhesive processes. The HPCI process combines many years of experience in adhesive bonding with modern technological developments in the laser remote technology field. The researchers thus achieved their self-defined goal of developing productive solutions for direct and form-fit joining.
Since thermoplastics and metal have very different physical properties – such as melting temperature or thermal expansion coefficient – optimizing the adhesive force between the two joining partners is of outstanding importance. For this reason, the IWS researchers developed a laser ablation process that generates structure depths of 100 µm and more at surface rates of up to 30 cm2 /s. Remote or scanner optics focus the continuously radiating laser on the metal and quickly deflect it. This process cleans the surface from adhering oils or dirt on the boundary layer. At the same time, the later penetrating polymer can fill the generated structures so that there is a positive fit between polymer and metal. This eliminates the need to clean the surface with solvents or pickling baths.
Fast Heat with a Joining Gun
The actual joining process is quite simple: the pre-structured metallic joining partner is pressed with the polymer. At the same time, the metal is heated at the joint and the thermoplastic is partially melted. In order to adapt this process for industrial use, IWS scientists developed a modularly designed joining gun that can be mounted on a robot arm instead of a spot welding gun. This allows proven system technology to be used for multi-material applications as well. A particular challenge consists in the uniform heating of the metallic joining partners. In addition to inductive heating, laser heating offers a similarly adequate solution. The use of a two-dimensional laser beam oscillation enables extremely fast beam movement and control. This procedure allows the temperature field to be adjusted dynamically to compensate for the specific heat dissipation conditions of the joined parts.
Lightweight Construction Potential
Together with industrial and research partners, Fraunhofer IWS evaluated the developed method using a complex technology demonstrator. The researchers replaced a pure welded construction steel assembly with a multi-material component made of organo sheet and metallic cover plate in order to demonstrate the lightweight construction potential. In addition to thermal direct joining, they also generated form-fit connections in web-slit design between the metal and organo sheet. The basic study showed that thermal direct joining suits multi-material and component designs, in particular due to short process times, robust process control and good automation capability.
Fraunhofer Institut für Werkstoff- und Strahltechnik IWS Dresden
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