Ready-to-install sandwich components at minute intervals
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.
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 . 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 . 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.
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 . 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.
Fully Automated, Flexible Process Chain
A fully automated, research-scale process was necessary to prove feasibility for large-scale production.
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 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.
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.
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).
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