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Lightweight Design worldwide

, Volume 11, Issue 3, pp 12–17 | Cite as

Ready-to-install sandwich components at minute intervals

  • Johannes Knöchel
  • Mathias Mühlbacher
  • Joachim Starke
  • Jonas Beck
Cover Story Large-Scale Production
  • 189 Downloads

Neue Materialien Bayreuth teamed up with BMW, Werkzeugbau Siegfried Hofmann and six other partners to develop a technology for manufacturing thermoplastic structural components using a sandwich construction. It ensures a high degree of functional integration through local injection and shortens cycle times.

Structural Components

Besides their potential for lightweight design, sandwich structures with foam cores offer excellent thermal and acoustic insulation properties. Thermoplastic materials offer the potential for further savings in regard to the system costs through a high degree of functional integration. Until now, however, process technologies permitting sandwich construction methods to be implemented economically for high production rates were only used for trim parts, not structural components. As part of the MAI Sandwich joint project funded by the MAI Carbon Excellence Cluster, a technology for the efficient production of thermoplastic sandwich structural components has now been developed and demonstrated.

Advantages of Thermoplastic Sandwich Structures

Thermoplastic sandwich components offer a number of economic and technical advantages that make them a particularly suitable for large-volume applications. Similar or miscible thermoplastic materials can be firmly thermally bonded [1]. This effect can be used when manufacturing sandwich structures to connect fiber-reinforced surface layers with a thermoplastic matrix to thermoplastic core systems, preferably foam cores [2]. Furthermore, the behavior of thermoplastic materials allows complex structures to be produced through thermoforming. Functional elements like ribs, edge trims, snap-fits and inserts can be applied swiftly, efficiently and with high geometric precision as part of an injection-molding process. Combining thermal joints, thermoforming and injection enables short cycle times and highly integrated processes for ready-to-install sandwich components that do not require secondary finishing. However, the combination of these technologies has so far been neither technologically nor economically feasible for sandwich components.

In technical terms, these sandwich structures — with their ductile, thermoplastic behavior of the matrix — generally excel in terms of impact characteristics. At the same time, polymer foams in the core provide outstanding thermal insulation properties. Thermoplastic sandwich structures can be implemented homogeneously based on a single polymer, using glass or carbon fiber reinforcement in the surface layers. This makes recycling easier at the end of the component’s life cycle. The easiest way to implement this is to regranulate and feed the polymer-fiber granulate into the injection molding process.

Manufacture

Thermoplastic plastics offer the potential to produce sandwich components cost-efficiently. However, no automated process has yet been established to realize the material potential in processes for highly integrated thermoplastic sandwich structures with structural functions in high volumes. The main challenges when planning manufacturing processes include limited bending radii and the low compressive rigidity for foam cores of typically between 50 and 100 MPa. The low compressive rigidity means the core collapses when higher pressures are applied [3]. This makes injection-molding of functional elements and achieving low- porous consolidation impossible, since the foam core cannot withstand the resulting forces. Preliminary investigations by automotive and aviation partners showed that manufacturing structural components requiring a fully consolidated, multi-ply surface layer is not possible in a single-step process. [4, 5]

The aim was the cost-efficient manufacture of weight-optimized sandwich structures with a high level of functional integration.

One possible solution is to split the processes over several work stations. The surface layers are first thermoformed and injection- molded. In a subsequent work step, the foam core is joined to the surface layers to form the sandwich component. This process allows complex structures and paves the way to functionalizing the sandwich component through injection molding. Conversely, this requires high capital expenditures on a number of tools and process stations such as presses and injection-molding equipment.

From One Mold

A new type of manufacturing process was developed within the project to optimally realize the thermoplastic structures. The aim was to demonstrate the cost-efficient production of weight-optimized sandwich structures with a high degree of functional integration for use in automotive design and in aviation. The project consortium had the goal of successfully producing sandwich structures in a single machine and using only one mold. To this end, the process outlined above and the necessary tool technology had to be developed.

To validate the project objectives, the consortium developed a demonstrator for the greatest possible range of applications, Figure 1, in which surface layers comprising textile organic sheets, unidirectional fiber-reinforced tapes and fleeces were processed. Polymer foams, foldcombs and an injection-molded rib core were also used as core materials. The demonstrator exhibits a slight curvature in order to evaluate the formation of free-form surfaces, an overmolded lip and a rib bay on the top of the component. The injection-molded edge shows the potential for producing parts requiring no secondary finishing. The rib bay demonstrates the possibility of applying local stiffening structures in the form of clips, cable guides or component mounts through injection molding. A fully automated, research-scale process had to be developed to prove its viability for large-scale production. A demonstrator measuring 360 × 460 mm was used for validation within the limited project scope. These dimensions correspond to around a quarter of the size of an engine hood or roof module.
Figure 1

Demonstrator component developed in the course of the project (© Neue Materialien Bayreuth)

Fully Automated, Flexible Process Chain

The basis for the development of the new production process is the Force-Molding production cell at Neue Materialien Bayreuth (NMB), which consists of an Engel-Austria injection press with two injection molding units and an infrared heating station. The process was designed for complete automation from the start to systematically pursue the objective of a cost-efficient series production. For this purpose, the equipment was adapted to meet project-specific requirements. A new handling concept was developed that allows different semi-finished forms as well as different materials to be transported, from polypropylene or polyamide (automotive applications) to polyethersulfone (aviation application). In order to minimize the cooling of the carbon-reinforced surface layer materials during transport, the focus of process development was on a fast transport of the semi-finished products and a flexible heating solution. These requirements were met by using an infrared heating system with precisely configurable surface energy distribution in a handling system adapted to the process, Figure 2, and a freely usable needle gripper solution, Figure 3. For some materials the use of a vacuum gripper system was not possible at system temperatures up to or exceeding 400 °C. The system was developed and set up by NMB in collaboration with Aumo.
Figure 2

CAD view of the parts of the handling system comprising an cart with infrared heating fields (left), and a heating and transport unit (right) (© Aumo)

Figure 3

Needle gripper and mold (© Neue Materialien Bayreuth)

A fully automated, research-scale process was necessary to prove feasibility for large-scale production.

Tool Engineering

In order to mechanically separate the foam core from consolidation and injection-molding pressure, a new process concept was developed, to which the project partner “Hofmann Ihr Impulsgeber” contributed a new type of mould technology.

The tool is an innovative hybrid concept, combining a sliding table with a three-plate tool and a core dummy. The core dummy, a mold plate made from steel weighing around 1 t, concentrates the high process pressures during consolidation and injection molding into the mold. This allowed three different cavities to be created in a single mold: two cavities each for the thermoforming and functionalization processes of the top layer, and one for the thermal joining of the surface layers with the foam core.

Preferred areas of application for the technology are large-area components with high bending stiffness.

The mold concept allows the simultaneous molding of both surface layers in the first two cavities, i.e. the thermoplastic surface layers are pressed and molded against the core dummy, as in Figure 4. The functional elements, e.g. ribs or screw bosses, are injected via a hot runner. The steel core dummy withstands the high pressure and needs only be extended from the tool horizontally to allow the actual foam core to be inserted into the tool and joined with the surface layers. The surface layers are then heated up to the required joining temperature by an infrared system only in the near-surface portion. After automated transport of the externally preheated form core into the mold, it is thermally joined to the surface layers. It is crucial for both materials in the joining zone to briefly exceed the melting temperature. Finally, a second hot runner is used to form the lip of the component through overmolding. The injection-molding cavity for the lip is designed in such a way that the foam core and casting compound do not come into direct contact, while further secondary finishing steps, such as trimming or stamping of the final contour, are eliminated entirely.
Figure 4

Drawing of the process and mold concept (© Neue Materialien Bayreuth)

Economic View

Today, sandwich components are hardly ever used in structural applications due to the costly components. Cost drivers in production are primarily the complex manufacturing and secondary finishing steps, and the high costs of semi-finished parts for surface layers and foam cores. Owing to the secondary finishing steps such as edge milling or stamping that are customary nowadays, part of the material deployed is not used in the end product. The newly developed process, Figure 5, demonstrates a high potential for cost savings across the board. A single piece of equipment is all the process needs, thanks to compact and highly integrated mold and handling technology. This makes investment costs significantly lower than for multi-step sandwich production on several tools and pieces of equipment.
Figure 5

Schematic diagram of the manufacturing process (© Neue Materialien Bayreuth | Hofmann Werkzeugbau)

The potential to remold and join semi- finished products paves the way for using plate-shaped, semi-finished materials. They can be cut in fully automated processes using intelligent pattern distribution and generating minimal material waste. Alternatively, individual structures for surface layers can be produced on a tape-laying machine, to further minimize scrap. Eliminating the need for secondary finishing of components can help slash process waste.

The main advantage of the new process is its potential of manufacturing sandwich components for automotive applications in under 2.5 min, Figure 6. The potential for greater functional integration thus enables whole assemblies to be substituted.
Figure 6

Cycle time for a fully automated process of manufacturing of a sandwich component for the automotive industry (© Neue Materialien Bayreuth)

Cost-efficient Large-scale Production

With its very high lightweight design potential, the sandwich technology can be used for automotive and aircraft applications in the realized tool and process concept for large series and is economically competitive. Preferred areas of application of the technology are large-area components with high bending stiffness. Such applications can be found in partitions and rear shelves in many vehicles, or interior trims in aircraft. The systematic use of lightweight materials, such as fiber composites made from carbon fiber, further increases the level of lightweight design, allowing sandwich components to be used for structural applications. Thermoplastics help shorten cycle times, while allowing a high degree of functional integration thanks to local injection. Moreover, using secondary materials such as secondary carbon fibers can help reduce the CO2 footprint.

The process developed as a result of the MAI Sandwich project shows that these expectations can be met. The developed tool, plant and process technology allows the processing of surface layer and core materials based on wide-ranging polymers, thereby enabling implementation in automotive and aviation applications. The manufacturing concept realized in the demonstrator and the cycle time derived for a series production process constitute a commercially interesting approach to series production.

The high practical relevance and the above-average innovation of the project were honored with the JEC Innovation Award at the world’s leading composite trade show, the JEC World, in 2018 in Paris.

Notes

Thanks

The authors would like to thank the other co-authors of this article, Dr. Thomas Neumeyer, Head of the Plastics Business Unit at Neue Materialien Bayreuth, and Prof. Volker Altstädt, Managing Director of Neue Materialien Bayreuth GmbH and holder of the Chair in Polymer Materials at the University of Bayreuth.

The authors would also like to thank the participating project partners: Airbus, BASF, BMW, Nennah Gessner, Neue Materialien Bayreuth, Werkzeugbau Siegfried Hofmann, Foldcore, TU Munich and SGL for the successful collaboration.

This article deals primarily with the process that was developed. The project also tested various core and surface layer systems of the partners, as well as foldcores and an injection-molded honeycomb core.

The authors also thank the Federal Ministry of Education and Research (BMBF) for funding the MAI Sandwich research initiative as part of the MAI Carbon Excellence Cluster (FKZ: 03MAI32).

References

  1. [1]
    Mühlbacher, M.; Hornfeck, C.; Neumeyer, T.; Altstädt, V.: Thermoplastische Sandwichstrukturen aus Luftfahrtmaterialien. In: lightweight.design 9 (2016), No. 4, pp. 44–49CrossRefGoogle Scholar
  2. [2]
    Grünewald, J.; Parlevliet, P.; Altstädt, V.: Manufacturing of Thermoplastic Composite Sandwich Structures — a Review of Literature. In: Journal of Thermoplastic Composite Materials 30 (2015), No. 4, pp. 737–464Google Scholar
  3. [3]
    Knöchel, J.; Neumeyer, T.; Mühlbacher, M.; Altstädt, V.: Funktionsintegration in thermoplastischen Sandwichstrukturen durch Spritzgießen. In: Tagungsband LLC 2017, No. 3, pp. 106–115Google Scholar

Copyright information

© Springer Fachmedien Wiesbaden 2018

Authors and Affiliations

  • Johannes Knöchel
    • 1
  • Mathias Mühlbacher
    • 1
  • Joachim Starke
    • 2
  • Jonas Beck
    • 3
  1. 1.Neue Materialien Bayreuth GmbHGermany
  2. 2.BMW GroupMunichGermany
  3. 3.Werkzeugbau Siegfried Hofmann GmbHLichtenfelsGermany

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