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.
1. A method to produce essentially flat floor panels, each floor panel having a surface layer on a front side, a backing layer on a rear side, and an intermediate core, wherein the surface layer and the backing layer comprise thermosetting resins, and the method comprises:
forming a board with convex pretension so that the board is curved downward at opposite lateral edges of the board, by connecting the core, the surface layer and the backing layer with heat and pressure;
dividing the board into several floor panels; and
forming core grooves at the rear side of the floor panels such that the convex pretension is at least partly released.
2. The method as claimed in claim 1 , wherein the core grooves are formed after dividing the board into the several floor panels.
3. The method as claimed in claim 1 , further comprising covering the core grooves with a separate covering layer.
4. The method as claimed in claim 3 , wherein the separate covering layer is one of a paper, a plastic foil, foam, cork and a wood veneer.
5. The method as claimed in claim 1 , further comprising forming components of a mechanical locking system at two opposite edges of the floor panels.
6. The method as claimed in claim 5 , wherein a groove length of the core grooves is smaller than a distance between the components of the mechanical locking system at the opposite edges.
7. The method as claimed in claim 5 , wherein the core grooves are curved at one short edge of the floor panels and parallel with the surface layer at the opposite short edge of the floor panels such that the core grooves at the opposite short edge intersect a part of a locking groove provided in the mechanical locking system.
8. The method as claimed in claim 1 , wherein the core grooves are curved at the short edges.
9. The method as claimed in claim 1 , wherein at least two of the core grooves have different shapes.
10. The method as claimed in claim 1 , wherein the core grooves comprise inner core grooves and outer core grooves, and the inner core grooves are formed with a smaller groove depth than the outer grooves.
11. The method as claimed in claim 1 , wherein the core grooves are formed by undercutting.
12. The method as claimed in claim 1 , wherein the convex pretension is at least partly accomplished by providing more resins in the backing layer than in the surface layer.
13. The method as claimed in claim 1 , wherein the convex pretension is at least partly accomplished by curing the backing layer at a higher temperature than the surface layer.
14. The method as claimed in claim 1 , wherein the convex pretension is at least partly accomplished by mechanical bending of the board while the board is hot.
15. The method as claimed in claim 1 , wherein the floor panels are essentially flat after forming the core grooves at the rear side of the floor panels.
16. The method as claimed in claim 1 , wherein a vertically extending groove depth of said grooves is between 0.1 and 0.5 times a floor thickness of the floor panels.
17. The method as claimed in claim 1 , wherein there is a horizontally extending space between the core grooves.
18. The method as claimed in claim 17 , wherein the horizontally extending space is between 0.2 and 1 times a floor thickness of the floor panels.
19. The method as claimed in claim 1 , further comprising filling the core grooves with material.
20. The method as claimed in claim 1 , wherein the area of the backing layer is less than about 90% of the area of the surface layer.