Fraunhofer Institute for Microstructure of Materials and Systems IMWS
For the first time, TS-Moulding enables the fully automated production of thermoplastic continuous fiber-reinforced sandwich components with honeycomb cores in cycle times suitable for large-scale production. The abbreviation TS stands for both "Thermoplastic Sandwich" and "Three Steps" in the thermoforming process. The process enables the production of complex-shaped sandwich components at high process speeds and comparatively low machine forces. The closing speed can be up to 80 mm/s for hot forming and up to 30 mm/s for pressing. Even taking into account the intermediate stabilization phase (1 second), the entire forming process takes no longer than three seconds. The low density of the sandwich composite guarantees short cooling phases (10-15 seconds) to derive sufficient structural strength for the subsequent injection moulding process or for direct component removal without injection moulding.
The thermoforming process
TS-Moulding technology is based on a thermoforming process that is divided into two main process steps:
- Heating the sandwich composite using infrared radiation (IR)
- Three-stage shaping of the sandwich composite in the shaping tool
During thermoforming, there are also two automated transfer phases of the sandwich blank and a final cooling phase of the formed sandwich shell in the temperature-controlled tool.
In the sandwich blank, a thermal state is set during heating by means of infrared radiation, which has surface areas with different temperature levels for subsequent hot forming and pressing. For this purpose, areas to be hot-formed in the IR oven are briefly shaded with screens, while the other areas are reached evenly and on both sides by the IR radiation field. The heated semi-finished product is then reproducibly transferred to the shaping tool within a few seconds so that the thermal condition essential for shaping is maintained. When the shaping tool, which is double-sided and at least single-segmented, is closed, shaping takes place in three process steps:
- Hot forming of the sandwich composite
- Stabilizing the hot-formed sandwich areas
- Pressing the remaining sandwich areas
Additional design options for components are possible if the thermoforming process is combined with thermoplastic injection moulding. This hybrid production approach is also possible with TS-Moulding. This results in sandwich components that utilize known functional elements of sandwich construction (inserts, edge reinforcements) as well as injection moulding applications (housing on and in the sandwich shell, screw domes). If IR heating and combined shaping are synchronized using an injection moulding machine, the hybrid process variant can achieve the production of ready-to-use sandwich components in cycle times of less than one minute in series production.
The tool concept
The TS-Moulding process requires a special design of the shaping tools used. As with many compression and injection moulds, a two-sided, metallic and temperature-controlled mould structure is required. The shaping cavity is located within the parting plane of both mould halves. The mould design in TS-Moulding has at least one mould half with a segmented cavity, which enables selective hot forming of the sandwich structure for pressing the edge finish when closing the mould. The separation of the two cavity areas must be designed in such a way that the hot forming segment is mounted within the press mould so that it can move in translation with respect to the mould closing direction. A sufficient lead between the two cavity areas must also be adjustable so that the three-stage forming process works. In the first part of the mould-closing movement, this preforming enables the characteristic hot forming of the sandwich composite selectively for subsequent stabilization and pressing. Porous tool inserts in the hot forming segment enable the stabilization and consolidation of the cover layers after hot forming or in the final cooling phase for thermoforming.