Multi-layered lightweight panels made by in-process foaming : comparison of core materials

Sandwich panels have been widely used during recent decades. Still, their use in typical wood-based panel applications is limited due to some basic disadvantages. The production has to be done by several processing steps, the density gradient between the layers of multi-material sandwiches is very steep and direct painting of the edges is not possible. When aiming to produce sandwich panels, the wood based panel industry faces two major challenges: Reducing the density of a panel shall not cause deterioration of its mechanical properties, and products have to stay competitive despite increasing raw material and energy costs. On the other hand, the customers and furniture producers demand for weight-reduced solutionsThe development of an innovative process as presented in this paper allows the production of lightweight foam core sandwich panels in a one step process. Such process includes resource efficient material and energy usage. The panels are manufactured from a three layered mat without additional gluing between the face and core layers. The surface layers comprise of resinated wood particles and the core is formed from an expandable material. Such mat is then processed in a hot press. The press cycle is divided into three phases. The resinated particles in the surface layers are compacted and cured in the first phase. When the temperature of the thermo-sensitive core materials reaches the activation point, the press opens to the predefined distance (final thickness of the panel) to allow core expansion. At this time, the pressing distance is kept constant until the expansion is finished. With the attained experience and the know-how needed for adapting the press parameters, high quality foam core sandwich panels can be made in a one step process.
Lightweight panels have been produced following the above process. Expandable microspheres (MS) and polystyrene (EPS) were used as core materials. The influence of surface thickness and core materials on mechanical and physical panel properties was investigated. In addition, FE-SEM-microscopy and gamma-ray densitometry were used to characterize the panels.