Building panels, especially laminated floor panels are shown, which are provided with a locking system and several core grooves at the rear side in order to save material and decrease weight. Building panels, each having a surface layer on a front side, a backing layer on a rear side and an intermediate core, wherein the intermediate core and the surface and the backing layer all comprise wood fibers and thermosetting resins, said building panels are provided with a locking system for vertical and horizontal locking of a first edge of a first building panel to an adjacent second edge of a second building panel.