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Lightweight Design worldwide

, Volume 11, Issue 5, pp 42–47 | Cite as

Tool Technology for Lightweight Structures in 3-D Hybrid Designs

  • Ralf Drössler
  • Robert Waffler
  • Michael Krahl
  • Daniel Haider
Design
  • 158 Downloads

Novel construction methods for the series-capable production of hybrid lightweight structures demand innovative tool technologies. The builder of models and molds Siebenwurst is developing suitable tool solutions for the large series-compatible production of intrinsically bonded hybrid structures in 3-D hybrid construction.

3-D Hybrid Technology

The automotive industry is constantly looking for new solutions to reduce vehicle weight. It is especially important to find intelligent construction methods in the bodywork structure so that they are not only lighter but can also be manufactured economically in large series-compatible volumes. From the classic pure steel construction of the past, hybrid constructions have increasingly become established in automotive building over the last few years. The intelligent combination of different materials such as aluminum, magnesium and steel means that their specific properties can be used in a more targeted manner, improving component performance and increasing the proportion of lightweight materials used in the bodywork structure.

For crash structures that are exposed to high stress in particular, however, purely metallic hybrid structures are also reaching the limits of their capabilities. Especially for electric cars, new hybrid designs are needed so that the vehicle crash requirements due to the increased vehicle mass caused by the battery can continue to be met, and so that the installation space to accommodate the battery systems can be maximized. To this end, in collaboration with our partners Porsche, Mitras, the lightweight design center in Saxony (Germany) (Leichtbau-Zentrum Sachsen, LZS) and the Institute of Lightweight Engineering and Polymer Technology (ILK) at the Technical University of Dresden (Germany), new solutions have been drafted as part of the "3-D hybrid" joint project sponsored by the Sächsische Aufbaubank (SAB) aimed at investigating novel, innovative steel-fiber composite hybrid designs for use in high-stress car bodywork structures, Figure 1. The aim was to transfer the previous hybrid solutions to a new combination technology. According to the technological state of the art, there are already numerous hybrid designs around for combining metallic shells or profiles with short or Long Fiber-reinforced Thermoplastic compounds (LFTs) or endless Fiber-Reinforced semi-finished Products (FRP), Figure 2. Shaping reinforcing ridges using injection molding technology contributes to the stabilization of the profile cross section, however, the specific stabilities of this design are severely restricted in comparative terms. One advantage comes from the metallic flange areas that allow the use of conventional metallic bonding technologies such as spot welding or clinching in large series-compatible assembly processes. This is also possible with steel-FRP hybrids. In this instance, endless fiber-reinforcement provides a significant increase in the specific stability. The missing ridges on the inside of the profile, however, mean that only restricted bending stiffnesses can be achieved. Through the combination of organo sheets and LFT reinforcing ridges, component performance can be improved significantly in terms of the required stabilities. These thermoplastic hybrid structures, however, call for alternative bonding concepts in shell designs, with the integration of such lightweight designs into existing assembly lines being severely restricted. The 3-D hybrid technology developed as part of the joint project brings together the advantages of the different hybrid designs [1]. For example, it is possible to use a classic thermoformed or cold-formed steel sheet, which is reinforced locally with an organo sheet. The cross section stabilization is achieved through the long fiber-reinforced ridge structure.

Figure 1 Components of the 3-D hybrid A-pillar (© ILK)

Figure 2 Combination of advantages in 3-D hybrid designs [1] (© Porsche)

The potential of this construction was highlighted in an impressive way as part of the sponsored 3-D hybrid project using the example of a lightweight-design B-pillar. Compared to a conventional B-pillar, the number of parts was reduced significantly, the weight of the component reduced by 14 % and the energy absorption increased by 25 %. All of the required load constraints were demonstrated successfully in component and bodywork tests.

Pressing Tools for 3-D Hybrid Structures

The processing and process-integrated connection of cold-formed and thermoformed steel sheets with fiberglass- reinforced organo sheets and a long fiber-reinforced pressing compound to create 3-D hybrid structures requires an adapted process and tool technologies. To do this, in 2013, Siebenwurst developed and produced a special molding tool for impact extrusion processing and put it into operation together with its project partners at the ILK's process development center. Figure 3 shows the particular features of the tool system. There are several pneumatic clamps integrated into the upper tool which allow reliable positioning of the steel sheets during the pressing process. The lower tool is equipped with multiple tool elements that can be moved towards each other using hydraulic or gas cylinders. This ensures defined forming and pressing of the organo sheet onto the steel shell in the tool's closing movement. The mobile sealing frame prevents any leakage of the thermoplastic impact extrusion compound during the pressing process. Once the ridge structure in the cavity is completely filled and the molding compound has set, ejector pins can be used to finally de-mold the component.

Figure 3 Molding tool for pressing a lightweight-design B-pillar using 3-D hybrid technology (© Siebenwurst)

In production trials, the functionality of the tool concept was demonstrated for automated component production. Safeguarding reproducible component quality, however, requires precise coordination of the tool and handling system. The transfer and defined depositing of the individual plasticized LFT compound strands, as well as the organo sheet sections, are an extremely challenging task in terms of process technology that leads to increased process complexity.

3-D Hybrid in Thermoplastic Injection Molding

This is why, in a follow-up project, the expertise gained will be used to further improve the 3-D hybrid technology in terms of component quality, reproducibility and cost-effectiveness. To this end, in 2015, seven industrial companies (Porsche, Volkswagen, Siebenwurst, Mitras Composites, ESI, Trumpf, Hengstmann Solutions) and three German research organizations (TU Dresden's ILK, University of Stuttgart's IKT and the Bavarian Laser Center) came together as part of a research consortium on the Forel joint project "Q-Pro." The group focused on a number of different goals. For the process-integrated bonding of the organo-reinforcements to the sheet metal shells, the use of laser-structured surfaces was also investigated alongside firmly bonded solutions. To minimize future development efforts and to improve component quality, an end-to-end process chain was also established and extensively investigated for the simulative and experimental mapping of the 3-D hybrid production process.

A basic profile and, for the component tests, a hybrid lightweight-design A-pillar were developed as project demonstrators for the basic investigations, Figure 4. For Siebenwurst, the core task was the further development of large series-compatible tool technology for 3-D hybrid structures, with the original impact extrusion forming process being substituted by an injection molding forming process. Along with tool technology, Siebenwurst was also responsible for the tool and process-compatible adaptation of the component structures. In close collaboration with the component design processes at Porsche and with constant process simulations (forming, mold filling and warpage simulation) at the facilities of the project partners (ILK, VW and ESI), the tool design process was advanced in an iterative development process. It was only thanks to the excellent interdisciplinary cooperation between the project partners that parallel tool and component designs were implemented efficiently.

Figure 4 Q-pro demonstrator structures in 3-D hybrid technology; basic profile (left) and hybrid lightweight-design A-pillar (right) (© ILK)

The component structures were produced in ILK's process development center. To do this, a vertical pressing unit with a horizontally arranged bolt-on injection molding assembly, infrared heat field and robot automation was provided. For the supply of thermoplastic melted compound to the forming tool, Siebenwurst developed a modular hotrunner system that can be used for a range of different tools, Figure 5. This allowed savings to be achieved in terms of tool costs. The floor-level bolt-on assembly docks directly onto a hotrunner plate. From there, the melted compound is deflected by 90° and then, through an insertion nozzle, pumped into the actual hotrunner of the molding tool. The respective tool also features a complex hotrunner which then guides the melted compound to the component cavity and injects it into the mold via needle closure nozzles.

Figure 5 Hotrunner system for the transfer of thermoplastic melted compound to the injection molding tool (© Siebenwurst)

Both with the basic profile and on the A-pillar, the tolerances of the sheet metal shells from the upstream metal forming process were a major challenge when it came to the design of the molding tools. To address this challenge, extensive geometry measurements were carried out on the sheet metal shells. If deviations from the specified sheet metal tolerances are not adequately accommodated, this can result in costly damage to the tool and to reduced component quality. Under-sized sheet metal shells, for example, can lead to increased tool wear since the shells are widened and shaped as a result of the closing movement and injection pressure. After the de-molding of the component, there is also the risk of the steel sheet springing back, and tension in the component can lead to loosening of the injection-molded ridge structures.

The sheet metal shells are secured in the upper tool by means of three hydraulic compact clamps, Figure 6. The position of the compact clamps, i.e. the securing of the steel sheet, is monitored by sensors. A camera system on the robot handling mechanism also monitors the position of the shell after deposition in the tool. Only following approval by the optical monitoring system is approval given for the downstream pressing process.

Figure 6 CAD representation of the upper tool with tool guide and clamping elements (© Siebenwurst)

For the deposition and accurately positioned draping of the organo sheet, four retaining pins with a conical-shaped tip were installed in the lower part of the tool, Figure 7. These retaining pins are extended and retracted with the ejector package. To ensure that these retaining pins and therefore the superimposed organo sheet are retracted slowly by the tool closure movement and not suddenly, four gas compression springs were installed in the tool. These allow gentle gliding and therefore smooth draping of the organo sheet.

Figure 7 Lower tool part with positioning pins for organic sheet fixation and sleeve ejector for component de-molding (© Siebenwurst)

To de-mold the finished component, sleeve ejectors were installed at the crossover points of the ridges. One key challenge of the Q-Pro project was the implementation of end-to-end quality assurance. To achieve this, a range of different sensors were installed to capture the process parameters in the individual process stages throughout the entire process chain. To measure the internal tool pressures and temperatures during the filling process and the cooling phase, combined pressure and temperature sensors from Kistler were installed in the lower tool cavity at selected positions using small molded inserts to integrate them into the tool.

Summary

For the electric vehicles of the future especially, innovative solutions for component structures exposed to high levels of mechanical stress are needed so that the strict crash requirements can be met while maintaining the maximum installation space. The potential of 3-D hybrid technology for this was demonstrated very impressively in the development projects listed above. In addition to the wide range of material, design and simulation issues, however, it has also become clear that tool technology is an essential component in order to ensure a manufacturing process that can be used in large series production runs and reproducible component quality. With its solutions, Siebenwurst was able to make a considerable contribution to this, allowing the transfer to series applications to take place without any obstacles from a technical perspective. |

References

  1. [1]

    Kellner, P.; Steinbach, K.: Die 3D-Hybrid Leichtbautechnologie: Eine neuartige Stahl-GFK-Hybridbauweise für höchstbelastete Karosseriestrukturen. Conference: 18. Dresdner Leichtbausymposium, Dresden, 2014

     

Thanks

The authors are grateful to the project partners Porsche, Volkswagen, ESI, Mitras, Trumpf, Hengstmann, the Bavarian Laser Center, the ILK Dresden and the IKT Stuttgart for their highly successful cooperation. The authors are also grateful for the sponsorship of the Q-Pro project (sponsorship identifier 02P14Z040 - 02P14Z049) by the Federal Ministry for Education and Research (BMBF) and the project sponsor Karlsruhe (PTKA), as well as for the sponsorship of the 3-D hybrid project by the Sächsische Aufbaubank (SAB).

Copyright information

© Springer Fachmedien Wiesbaden GmbH, part of Springer Nature 2018

Authors and Affiliations

  • Ralf Drössler
    • 1
  • Robert Waffler
    • 1
  • Michael Krahl
  • Daniel Haider
  1. 1.SiebenwurstDietfurtGermany

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