Homogeneous core materials, e.g. polyurethane (PU) foams, are widely used in low cost applications (e.g. in building and automotive industry). The out-of-plane compression and shear performance is for equal density however lower compared to honeycomb cores. Recently, polypropylene foams (EPP) have become a better recyclable alternative to PU foams. However, they are more expensive and have even lower mechanical properties than PU foams.
Textile and truss cores
Production cost reductions had been one driving force for the development of truss of textile type of structured cores which provide a punctual support for the skins. K.U.Leuven had been on the forefront in the research and development of the use of textile processes for the production of sandwich panel preforms. In combination with a foam filling these textile cores can provide good mechanical properties. However, this leads again to higher cost.
Corrugated cores and cup-shaped cores
The corrugated core in cardboard is well known for its low cost packaging applications, but due to its low cost it has also been used in automotive headliners and, made from metal sheets, in building applications. The efficient in-line production technology of corrugation leads to very low costs. However, the corrugated core has rather low mechanical properties especially transvers to the corrugations. The automotive industry and the packaging industry employs thermoplastic sandwich materials with unidirectional support, e.g. extruded twin sheet panels and sandwich panels with regional support, e.g. a thermoformed cup-shaped core layer, but also they have generally considerably lower mechanical properties compared to a sandwich material having honeycomb core.
Honeycomb core materials can offer weight and cost savings thanks to their excellent performance per weight. Unlike corrugated core types, which cell openings in the in-plane direction, honeycomb core types have have vertical cell walls and only openings in the out-of-plane direction and provide a bi-directional support for the skins. The desire for lightweight design to enable savings in raw material and energy resources will continue to increase and spread to many traditionally less weight sensitive industrial sectors. However, the demands for low cost and high production capacities require automated and continuous processes.