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Thanks to their good mechanical properties and low structural weight, multi-material structures are a promising approach in the automotive industry to lightweight design, body construction and functionalization. Especially metal and plastic are mainly combined to achieve improved properties of the final component compared to mono-material structures.
This paper describes the development of a manufacturing cell for the joint forming and heat-assisted press joining of steels and continuous fiber-reinforced thermoplastics in the form of unidirectional carbon-fiber tapes. The structure of our so-called sandwich components is as follows: Steel, PA6-slide, carbon-fiber tapes, PA6-slide, steel. Such components are usually manufactured in variothermal tools. These tools are characterised by the fact that they have to be heated when the material has been inserted and then the material can be formed. Afterwards, the formed compound must solidify again in the tool, which is done by cooling down the entire tool. Process times of five minutes are not uncommon. In order to achieve shorter cycle times and to ensure economical production, a manufacturing cell, supplemented with automated handling by means of two robots and an isothermal, two-part tool concept was developed and tested.
By the first robot, the individual components are stacked to a semi-finished product and thermally joined at specific points so that they are secured against slipping. The second robot is responsible for transferring the semi-finished products into the heating/forming tool. Simultaneously, while the sandwiches are undergoing the heating and forming process, the first robot stacks a new batch of semi-finished products. Every 30 seconds, the press opens and a finished part is removed, a heated part is placed from the heating die into the forming die, and a new stacked sandwich composite is transferred into the heating die. This means that both the heating tool and the forming tool are in operation during each press stroke. The temperature of the forming tool is set in such a way that the forming can be completed before the plastic solidifies and the composite component can be removed by the second robot immediately after the forming process without cooling the forming tool. A process window for joining was developed by testing and analyzing different holding times in the mold, pressures and temperatures.
The composite components produced were tested with regard to their dimensional accuracy, the mechanical performance and the adhesion between the joining partners. The feasibility of the production was demonstrated. All composite components had a higher specific load capacity than a pure steel component. Cycle times of well under 60 seconds were achieved. An enormous reduction in process time compared to variothermal tool concepts could be achieved with the new manufacturing cell.