The Fraunhofer IMWS is advancing next-generation joining technologies aimed at unlocking the commercial viability of thermoplastic sandwich structures, particularly for small- to medium-scale production environments.

New joining technologies must be developed to make thermoplastic sandwich components economically joinable and repairable in small to medium-sized production runs. The Fraunhofer Institute for Microstructure of Materials and Systems IMWS is working on this together with the SKZ Plastics Center and partners in a new project. The core of the project is the forming of joining surfaces directly during the manufacturing process (thermoforming), supplemented by welding and bonding as well as repair concepts. The goals are low weight, cost-efficiency, and recyclability with concrete demonstrators and technology transfer measures for SMEs in industrial sectors.

The demand for resource-efficient lightweight construction solutions is growing. Thermoplastic sandwich structures can contribute to this: They combine low weight with recyclability and are suitable for small to medium production runs of 10 to 1,000 units per year. However, there is a lack of practical joining techniques that make these materials usable in processes suitable for small and medium-sized enterprises (SMEs), which often produce in such quantities. The project Thermoplastic Sandwich Construction Elements (TheSaKo) aims to close this gap. To this end, Fraunhofer IMWS and SKZ are collaborating with partners from the fields of plastics processing, lightweight construction, transport/commercial vehicle manufacturing, mechanical engineering, injection molding, surface and adhesive technology, and the circular economy.

Shorter Setup Times and Greater Design Flexibility for SMEs

For SMEs, the new joining technologies result in shorter setup times, fewer foreign materials (no or few joining elements), better recyclability, and expanded design flexibility. The concepts address small series and modular large-scale structures that were previously difficult to realize due to high process complexity.

Current joining methods primarily address thermoset sandwich structures and use, for example, mechanical fasteners. This often leads to long cycle times in manufacturing and limited recyclability. Joining techniques for thermoplastic sandwich structures, such as welding or bonding, offer potential but require specific, load-dependent joint surfaces and process control, particularly for large-area sandwich structures. The project partners goal is the cost-effective use of thermoplastic sandwich components in small to medium-volume production through:

• Developing deformable joining surfaces directly during the manufacturing process (thermoforming)
•  Adapting welding and bonding technologies to these surfaces
• Lower material and joining element costs, moderate costs for adhesives and surface treatment
• Integration of repair concepts that withstand moisture and temperature cycles and keep recycling pathways open, to extend product lifespan
• Demonstrators for modular large-scale structures and localized functionalization.

For the first time, the goal is to achieve a combination of thermoformable edge shaping, load-path-optimized joining zones, weld- and adhesive-optimized joints, and repair-friendly concepts. The solutions developed are to be scaled up to the SME level within the project. Together, we want to make it possible to economically join and repair thermoplastic sandwich structures in small to medium-sized production runs   this makes lightweight construction a concrete reality in the SME sector, says Dr.-Ing. Ralf Schlimper, project manager at Fraunhofer IMWS.

TS Molding Enables Automatable, Reproducible Hot-Forming Processes

A key component of this is the Thermoplastic Sandwich Molding (TS-Molding) technology developed at Fraunhofer IMWS for automatable, reproducible hot-forming processes. In this process, the forming of joining interfaces takes place directly within the component. This enables the integration of joining technologies with locally preformed areas. In this two-year project, the institute will also focus intensively on the development of joint connection surfaces, i.e., the design and testing of local forming and joining zones (2D/3D deformation) for load-path-appropriate force transmission during bonding and welding.

The research team in Halle (Saale) is also contributing its expertise to the development of repair concepts and testing methods, as well as to the implementation of technology transfer initiatives. The goal is to produce industry-relevant demonstrators (modular large-scale structures and localized functionalization) for practical evaluation. By combining thermoformable joining surfaces, adapted joining technologies, and repair-friendly concepts, we are building a bridge from basic research to industrial practice   including recyclability and cost-efficiency, emphasizes Michael Heilig, project manager at SKZ.

Solutions for Large-Area Geometries, Process Integration, and Testing Methods

Among the technological challenges is the realization of large-area, joinable edge geometries in sandwich structures without damaging the cover layers or impairing the fiber orientations. The project must also develop viable solutions for the simultaneous optimization of multiple forming parameters (heating temperature, transfer time, mold temperature, forming pressure) as well as process integration (linking thermoforming, welding, and bonding in practical, small-scale production runs; developing robust process corridors and guidelines for SMEs). The consortium is also keeping an eye on the topic of testing and standardization, as industry-specific standards for the welding and bonding of thermoplastic sandwich structures are currently lacking. To this end, the research partners are developing load-appropriate testing methods.

The results can support companies in plastics processing, lightweight manufacturing, automotive engineering, and the production of furniture and building components. To ensure the transfer of project results, the work is supported by an advisory committee in which Automation Steeg und Hoffmeyer GmbH, EDAG Engineering GmbH, Joining Plastics Consulting, Sika Deutschland GmbH, Reese-Solutions GmbH, SABA Bocholt GmbH, ThermHex Waben GmbH, Weber Fibertech GmbH, and Wegener International GmbH are participating.

SMEs that would like to join the advisory committee to contribute requirements, market needs, and practical user questions to the ongoing research are cordially invited to do so. The Composite Sandwich Conference CSC26, taking place on May 4 5, 2026, in Halle (Saale), offers an excellent opportunity to do so. This industrial symposium facilitates interdisciplinary exchange on material and technology innovations as well as applications of composite sandwich structures.

The TheSaKo project of the research association F rdergemeinschaft f r das S ddeutsche Kunststoff-Zentrum e. V.  is funded by the Federal Ministry for Economic Affairs and Energy (BMWE) through the DLR Project Management Agency as part of the Industrial Collaborative Research (IGF) program, based on a resolution of the German Bundestag.