Panel forming

1. Floor 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 layer and the backing layer all comprise a thermoplastic material, said floor panels being provided with a locking system for vertical and horizontal locking of a first edge of a first floor panel to an adjacent second edge of a second floor panel, wherein upper parts of the first edge and the second edge in a locked position together define a vertical plane being perpendicular to a horizontal plane, which is parallel to the surface layer, said locking system comprising a tongue and a tongue groove configured to cooperate for vertical locking, and at the first edge a strip provided with a locking element, which is configured to cooperate for horizontal locking with a downwardly open locking groove formed in the second edge,

wherein the backing layer and the intermediate core comprise at least two vertically extending core grooves with an opening towards the rear side, and

wherein an area of the backing layer is less than about 90% of an area of the surface layer.

2. The floor panels as claimed in claim 1 , wherein the backing layer comprises at least three core grooves spaced horizontally and inwardly from the locking system at one pair of opposite edges.

3. The floor panels as claimed in claim 1 , wherein the area of the backing layer is less than 80% of the area of the surface layer.

4. The floor panels as claimed in claim 1 , wherein the entire parts of at least one core groove are arranged inside the vertical plane at all edges.

5. The floor panels as claimed in claim 1 , wherein the floor panels are rectangular with long edges and short edges and wherein the core grooves are essentially parallel with the long edges.

6. The floor panels as claimed in claim 5 , wherein the core grooves have a core groove length that is smaller than a distance between locking systems at opposite short edges.

7. The floor panels as claimed in claim 1 , wherein the core groove depth is at least 0.3 times a floor thickness of the floor panels.

8. The floor panels as claimed in claim 1 , wherein the core grooves comprise an opening with a core groove width that is larger than an inner part of said core groove.

9. The floor panels as claimed in claim 1 , wherein the thermoplastic material is PVC, PET or vinyl.

10. The floor panels as claimed in claim 1 , wherein the intermediate core, the surface layer and the backing layer each comprise a filler.

11. The floor panels as claimed in claim 1 , wherein the floor panels have a fold down locking system on the short edges with a flexible tongue that allows locking with vertical folding.

12. The floor panels as claimed in claim 1 , wherein the core grooves intersect the strip and the locking groove.

13. The floor panels as claimed in claim 1 , wherein the core grooves extend from a first edge to a second edge of the floor panels.

14. The floor panels as claimed in claim 1 , wherein the core grooves have a longitudinal length that is smaller than a longitudinal length of the backing layer.

15. The floor panels as claimed in claim 1 , wherein the core grooves are curved at the edges along a longitudinal direction.

16. The floor panels as claimed in claim 1 , wherein the core grooves are discontinuous.

17. The floor panels as claimed in claim 16 , wherein the core grooves are arranged in a plurality of rows along a longitudinal direction of the floor panels, each row comprising a plurality of core grooves.

18. The floor panels as claimed in claim 17 , wherein core grooves in adjacent rows extend side by side along the longitudinal direction.

19. The floor panels as claimed in claim 17 , wherein core grooves in adjacent rows are spaced from each other in the longitudinal direction.