The substitution of sheet steel by lightweight fiber-reinforced plastics for sheet steel is making great strides in automotive and vehicle construction. In many places, this is also accelerating the process-related changing of the guard in the area of connection methods and joining technology. Modern adhesive technology is taking over where welding was previously used. The use of an innovative multi-segment heating “mirror” from hotset in robot-assisted bonding of large CFRP body parts in a new production cell of thyssenkrupp’s body construction division illustrates why customized high-end temperature control systems play a key role here.
The trend seems irreversible. Whereas metal sheets were welded together in vehicle construction in the recent past, 2‑component bonding of molded parts made of fiber-reinforced plastics now dominates the scene. The innovative leaps forward in this field are documented by a current project by thyssenkrupp Automotive Body Solutions: The implementation of a fully automated, robot-supported assembly system for continuous high-precision bonding of the inner and outer CFRP molded parts of vehicle doors. The fact that the project, despite its complexity, is now about to be transferred to series production of a well-known sports car manufacturer after a runtime of just under one year is not least the result of successful project coordination. The German thermodynamics specialist hotset played a central role in this regard. In close cooperation with the engineers from thyssenkrupp, the Lüdenscheid-based company produced one of the functional centerpieces of the new production cell: A production-ready temperature control system that heats both door halves quickly, precisely, reliably and simultaneously to up to 90° C only on those edge surfaces on which the previously applied adhesive beads are located, so that these cure as quickly as possible during pressing – and can be passed on to the next processing stages after just a few minutes.
Robot-assisted bonding of large CFRP body parts: The multi-segment heating mirror (brown) from hotset in the fixture of the new production cell of thyssenkrupp’s body construction division. (Image: thyssenkrupp)
A mirror with 17 segments
“The technological benchmark was very high for this project from the very beginning. In addition, the innovative nature of the project required a certain degree of flexibility, as new questions frequently arose in the course of it”, recalls Andreas Filler, who has been project manager at hotset for many years. However, the solution implemented under his responsibility is a process engineering highlight of modern temperature control technology and could be groundbreaking for the further establishment of CFRP bonding in other industries. It is a system consisting of two large heating frames whose geometries and dimensions follow the design of the vehicle doors (1,290 x 636mm). “However, while such frames – also known as heating mirrors in trade language – have so far usually been manufactured from one piece, thyssenkrupp’s design specifications in this case called for the new temperature control system to consist of a total of 17 firmly interconnected heating segments, whose contact surfaces are precisely contoured to match the various topologies of the door areas to be bonded”, explains Andreas Filler. In its design, each of these segments is therefore a three-dimensional unique specimen, milled with high precision from a special aluminum alloy, refined with finely finished contact surfaces and equipped with an internal architecture of heating elements. Since each segment also has several individually controllable control zones, the heating energy of the complete temperature control system can be provided precisely, efficiently and as needed via a total of 19 control zones. Patrick Hofsommer, project manager at thyssenkrupp Automotive Body Solutions, explains what this means in practice: “Due to its unique control technology and the precisely fitting segments with their perfectly contacting surfaces, the new heating mirror is a real innovation. It ensures optimum, highly uniform and at the same time also flexible temperature control to all gluing seams of the two door halves. In hotset’s high-precision design, it makes a decisive contribution to the process efficiency of our new CFRP bonding line.”
Precise heat control through segment design: The new temperature control system from hotset consists of 17 interconnected heating segments whose contact surfaces are precisely contoured to the various topologies of the areas of the carbon door to be bonded (black in the picture). (Image: thyssenkrupp)
Cured in minutes
Two such multi-segment heating mirrors act as temperature control performance components of thyssenkrupp’s new assembly cell. They are functionally integrated into the complex handling and control system of the 11 x 9‑meter system, which glues four CFRP molded parts to form two vehicle doors – one left and one right – in a clocked and synchronized interplay of five robots. One inner door shell (inner assy) is taken from each of the load carriers provided and placed in the lower fixture of a clamping device, which also holds the lower frame of the heating mirror. Shortly afterwards, adhesive beads consisting of a two-component adhesive are applied to the defined edge zones. When the outer panel and the upper frame of the heating mirror are placed and clamped, the temperature control system boots up and heats the adhesive beads uniformly from both sides, i.e. through the CFRP half-shells, to up to 90 °C. Curing of the adhesive is then completed after approx. 4.5 minutes. In total, the process cycle time is eleven minutes for each two vehicle doors. Both CFRP door halves including adhesive bead are between 6.0 and 7.0 mm thick.
Patrick Hofsommer: “Due to its unique control technology and the precisely fitting segments with their perfectly fitting contact surfaces, the new heating mirror from hotset is a real innovation.” (Image: thyssenkrupp)
Temperature-compensated design
Involved as a project partner in the development of thyssenkrupp’s new CFRP bonding line, hotset accompanied the project over several months, ultimately providing its innovative temperature control system as a complete solution. In concrete terms, this means that the system manufacturer received several customized, ready-to-use heating mirrors including insulation technology, sensor technology and decentralized control technology for integration into its handling and control technology infrastructure (Siemens S7/ CAN bus). It is obvious that all kinds of special aspects had to be taken into account here. This included, for example, having to repeatedly adapt the engineering of the temperature control system to the various iteration stages of the project and designing the heating mirror and its segments subject to the specifications for temperature compensation. “In such cases, all critical components have to be designed and manufactured a few percentage points smaller because, after all, the system expands in practical use due to the influence of temperature”, explains Andreas Filler. Other important requirements included the fact that the entire heating mirror had to comply with defined weight limits, as the robots of the assembly system can lift and position a maximum of 300kg, and that silicone was ruled out as a material for bonding right from the start.
“Our new heating mirror with its segment design and the resulting advantages for control accuracy and contour-precise heat guidance represents a new generation of high-precision heating elements for use in modern bonding technology.” (Image: Hotset GmbH)
Groundbreaking system solution
Once again, hotset was able to demonstrate its capabilities as a competent engineering and systems partner during the realization of the new heating mirror for thyssenkrupp’s CFRP bonding line. As is usual with projects of this complexity and size, the Lüdenscheid-based company has thus also considerably expanded its practically usable expertise in the field of production-integrated temperature control systems for industrial plant engineering. In this context, Andreas Filler particularly emphasizes that “the new heating mirror with its segment design and the resulting advantages for control accuracy and contour-precise heat guidance embodies a new generation of high-precision heating elements for use in modern bonding technology”. It is therefore quite conceivable that this high-end solution, initially developed for the sports car industry, could also provide forward-looking impetus for the further establishment of 2‑component bonding of CFRP sheets in many other areas of series-oriented lightweight construction. The thermodynamics specialists at hotset have their sight set on numerous applications, for example in electromobility.
