Patent Publication Number: US-11391050-B2

Title: Mechanical locking system for floor panels

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. application Ser. No. 15/901,614, filed on 21 Feb. 2018, which is a continuation of U.S. application Ser. No. 15/028,831, filed on Apr. 12, 2016, now U.S. Pat. No. 10,041,258, which is a U.S. National Stage of International Application No. PCT/SE2014/051251, filed on Oct. 24, 2014, which claims the benefit of Swedish Application No. 1351273-6, filed on Oct. 25, 2013. The entire contents of each of U.S. application Ser. No. 15/901,614, U.S. application Ser. No. 15/028,831, International Application No. PCT/SE2014/051251, and Swedish Application No. 1351273-6 are hereby incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The disclosure generally relates to the field of mechanical locking systems for floor panels and building panels. The disclosure shows floorboards, locking systems and production methods. 
     FIELD OF APPLICATION 
     The present invention is particularly suitable for use in thin floating floors, which are formed of floor panels which are joined mechanically with a locking system preferably integrated with the floor panel, i.e. mounted at the factory, are made up of one or more upper layers of thermoplastic or thermosetting material or wood veneer, an intermediate core of wood-fibre-based material or plastic material and preferably a lower balancing layer on the rear side of the core. The invention can also be used for joining building panels which preferably contain a board material for instance wall panels, ceilings, furniture components and similar. Parts of the locking system may also be supplied as separate components, which may be connected to a panel during installation. 
     The following description of prior-art technique, problems of known systems and objects and features of the invention will therefore, as a non-restrictive example, be aimed above all at this field of application and in particular at thin panels formed as rectangular floor panels with long and shorts edges intended to be mechanically joined to each other on both long and short edges. 
     The long and short edges are mainly used to simplify the description of the invention. The panels may be square. It should be emphasized that the invention can be used in any floor panel on long and/or short edges and it may be combined with all types of known locking system that lock the panels in the horizontal and/or vertical direction. 
     The following description of prior-art technique, problems of known systems and objects and features of the invention will, as a non-restrictive example, be aimed above all at floor panels and especially at thin resilient thermoplastic floor panels such as so called luxury vinyl tiles, generally referred to as LVT, 
     BACKGROUND OF THE INVENTION 
     LVT flooring usually comprises a transparent wear layer which may be coated by a UV cured PU lacquer, a decorative plastic foil and one or several core layers which generally are of different density and hardness. Relevant parts of this prior art description are also a part of the invention. 
     Thin LVT floors with a thickness of 2-3 mm have traditionally been installed by gluing to the sub floor. Recently LVT floors have been introduced on the market that comprises a mechanical locking system, which allows a floating installation without glue. This facilitates installation and eliminates a lot of work to prepare the sub floor for gluing. 
     Such LVT floors have generally a thickness of about 5 mm. This thickness is mainly required in order to form the locking system. The panel itself is strong and flexible and a thickness of about 3 mm would in many application be sufficient but cannot be used since it is not possible to form a strong and cost efficient locking system in such thin floors. 
     Such problems related to minimum thickness requirements due to the forming of locking systems are also applicable in other thin floor panels such as laminate floors and wood powder based floors where material and weight savings may be accomplished with lower thicknesses, preferably below 6 mm. 
     Laminate flooring usually comprise a core of a 6-12 mm fibre board, a 0.2-0.8 mm thick upper decorative surface layer of laminate and a 0.1-0.6 mm thick lower balancing layer of laminate, plastic, paper or like material. A laminate surface comprises melamine-impregnated paper. The most common core material is fibreboard with high density and good stability usually called HDF—High Density Fibreboard. Sometimes also MDF—Medium Density Fibreboard—is used as core. 
     Laminate floor panels of this type have been joined mechanically by means of so-called mechanical locking systems. These systems comprise locking means, which lock the panels horizontally and vertically. The mechanical locking systems are usually formed by machining of the core of the panel. Alternatively, parts of the locking system can be formed of a separate material, for instance aluminium or HDF, which are integrated with the floor panel, i.e. joined with the floor panel in connection with the manufacture thereof. 
     The main advantages of floating floors with mechanical locking systems are that they are easy to install. They can also easily be taken up again and used once more at a different location. 
     DEFINITION OF SOME TERMS 
     In the following text, the visible surface of the installed floor panel is called “front side”, while the opposite side of the floor panel, facing the sub floor, is called “rear side”. The edge between the front and rear side is called “joint edge”. By “horizontal plane” is meant a plane, which extends parallel to the front side. Immediately juxtaposed upper parts of two adjacent joint edges of two joined floor panels together define a “vertical plane” perpendicular to the horizontal plane. By “vertical locking” is meant locking parallel to the vertical plane. By “horizontal locking” is meant locking parallel to the horizontal plane. By “up” is meant towards the front side, by “down” towards the rear side, by “inwardly” mainly horizontally towards an inner and centre part of the panel and by “outwardly” mainly horizontally away from the centre part of the panel. 
     RELATED ART AND PROBLEMS THEREOF 
     For mechanical joining of long edges as well as short edges in the vertical and horizontal direction perpendicular to the edges several methods may be used. One of the most used methods is the angle-snap method. The long edges are installed by angling. The short edges are locked by horizontal snapping. The vertical connection is generally a tongue and a groove and the horizontal connection is a strip with a locking element that cooperates with a locking groove in the adjacent edge. 
     Similar locking systems may also be produced with a rigid strip and they are connected with an angling-angling method where both short and long edges are angled into a locked position. 
     Advanced so-called fold down locking systems with a separate and flexible tongue on the short edges have been introduced where both the long and short edges are locked with an angling action. 
     It is known that a locking strip may be formed of a separate material such as aluminium and that such strip may be clamped in undercut grooves. Such systems are described in WO94/26999. The separate metal strip may be used to lock very thin panes with a thickness of about 3 mm provided that the core is made of a strong material for example compact laminate or a high quality HDF and that the strip extends along essentially the whole edge. The strip is used to accomplish vertical and horizontal locking. 
     WO 99/66152 describes a locking system with a tongue and a tongue groove and a separate metal strip that is attached to the lower lip of the tongue groove and that in locked position is located vertically under the tongue. Such locking system is not suitable for thin flooring since the thickness must be sufficient to form the tongue groove and a connecting part for the strip under the groove. Generally ⅓ of the panel thickens is used to form the upper lip, ⅓ is used to form the tongue and ⅓ remains to form the lower lip, The available material thickness that may be used to form the strip under the tongue is generally less than ⅓ of the panel thickness. A connection to the outer part of the lower lip is also disadvantage in panels with a soft and flexible core such as LVT. A lower lip formed in soft and flexible material bends downwards when the strip is exposed to rather low separation forces and a strong strip will not improve the locking strength due to inferior connection to the panel edge. 
     It is known from CN 201588375 that clips may be used to accomplish horizontal and vertical locking. Such clips may provide cost advantages over a locking strip that extends along the whole edge. A disadvantage is that a considerable part of the edge between the clips is not locked vertically and the edges will move vertically when exposed to high load especially if the floor panels are thin and flexible. 
     US 2001/0010139 A1 shows a locking system similar to embodiments shown in WO 94/26999. A separate clip is connected to an outer part of a lower lip that is positioned beyond an upper lip. The geometry of the lower lip, the tongue and the tongue groove is not suitable to form a strong locking in soft and flexible core materials. 
     It is also known from WO 2013/025165 that a tongue and a groove formed in one piece with the core may be used for vertical locking and several strip parts spaced form each other may be attached to an edge in order to obtain horizontal locking. A disadvantage is that such locking system are not suitable for thin floors since the strip part is connected in a separate groove that extend along the whole edge and that is located under the lower part of the tongue. The connection of the strip part is not sufficient to prevent backwards bending of the strip body and edge separation when the edges are exposed to pulling forces. This is a disadvantage in thin laminate floors and floors with a rather soft core such as LVT floors. 
     It would be an advantage if separate clips that comprise a stronger material than the core may be used to accomplish a horizontal locking in thin floors and if such horizontal locking may be combined with a vertical locking comprising a tongue and a grove that extends along the whole edge and is made in one piece with the core. 
     SUMMARY OF THE INVENTION AND OBJECTS THEREOF 
     An overall objective of the present invention is to provide an improved and more cost efficient locking system for primarily adjacent long edges of thin and flexible floor panels that may be locked to each with angling. 
     A first specific objective is to provide a locking system for thin flooring comprising a tongue and groove for vertical connection and a separate clip that may be attached to the panel edge and provide a strong locking in panels with a thin and flexible core. 
     A second specific objective is to provide a flooring system comprising two types of panels that may be locked in a more flexible way in order to allow installation of advanced floor patterns. 
     The above objects of the invention may be achieved by embodiments of the invention. 
     According to a first aspect of the invention building panels are provided with a locking system comprising a tongue at a second edge of a second panel. The tongue is configured to cooperate with a tongue groove at a first edge of a first panel for locking in a vertical direction. The tongue groove comprises an upper lip and a lower lip. The locking system further comprises one or more clips attached to the first edge and a downwardly open locking groove formed at the second edge. Each clip comprises an upwardly extending locking element, which is configured to cooperate with the locking groove for locking the first edge and the second edge in a horizontal direction. The clip comprises a clip body at a rear side of the first panel. Said clip body is provided with an inner part, which extends inwardly from the first edge and an outer part, which extends outwardly from said first edge. The inner part comprises a fixing element that cooperates with a downwardly open fixing groove, formed on the rear side of the first panel, for locking the clip to the first edge in a horizontal direction. The clip comprises a locking protrusion that protrudes upwardly from the clip body. The locking protrusion is configured to lock the clip to the first edge in a vertical direction. The lower lip of the tongue groove comprises a recess and the locking protrusion is in a locked position positioned in the recess. 
     The locking protrusion may have a part that is located in the tongue groove. 
     A part of the locking protrusion may be located below the tongue. 
     The locking protrusion may be spaced horizontally inwardly in the tongue groove beyond the outer tip of the tongue. 
     The locking protrusion may comprise a first part that extends upwardly from the clip body and a second part that extends inwardly into the tongue groove. 
     The locking protrusion may be located inwardly and spaced horizontally from the vertical plane 
     The panel may comprise a core of plastic material. 
     The panel may comprise a surface of thermoplastic material. 
     The panel may comprise a core with an upper core layer and a lower core layer and the locking protrusion may protrude vertically beyond the lower core layer. 
     According to a second aspect of the invention a flooring system is provided comprising a first panel and a second panel provided with a locking system comprising clips. Said clips being arranged at a first edge and at an opposite second edge of the first and the second panel. The locking system is configured to lock the first edge of the first panel to the second edge of the second panel in a horizontal and a vertical direction. 
     The first edge and the second edge may each comprises a horizontal groove comprising a lower lip. 
     Each clip may comprise a vertically extending locking protrusion with an upper part that is located essentially above the lower lip of the first and the second panel, respectively. 
     Each lower lip may be spaced horizontally and inwardly from an upper part of the edge. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure will in the following be described in connection to exemplary embodiments and in greater detail with reference to the appended exemplary drawings, wherein: 
         FIGS. 1 a - f    illustrates locking systems according to known technology. 
         FIGS. 2 a - f    illustrate a clip that may be used to lock thin floor panels according to an embodiment of the invention. 
         FIGS. 3 a - h    illustrate clips and a production methods to connect a clip to an edge according to embodiments of the invention. 
         FIGS. 4 a - c    illustrate a locking system according to an embodiment of the invention. 
         FIGS. 5 a - d    illustrate a locking system according to an embodiment of the invention. 
         FIGS. 6 a - c    illustrate a locking system according to an embodiment of the invention. 
         FIGS. 7 a - d    illustrate a locking system according to an embodiment of the invention. 
         FIGS. 8 a - c    illustrate a locking system and a LVT floor panel with a core comprising several layers according to an embodiment of the invention. 
         FIGS. 9 a - b    illustrate a panels with clips on long and short edges according to an embodiment of the invention. 
         FIGS. 10 a -10 e    illustrate A and B panels comprising clips on both adjacent edges. 
     
    
    
     DESCRIPTION OF EMBODIMENTS OF THE INVENTION 
       FIGS. 1 a -1 f    show known locking systems.  FIG. 1 a    shows a conventional locking system formed in one piece with the core  5  and configured to lock with angling. The floor panel  1 ,  1 ′ comprises a locking system that has a tongue  10  and a tongue groove  9  that lock vertically and a strip  5  with a locking element  8  that cooperates with a locking groove  14  and locks the edges horizontally. 
       FIG. 1 b  and 1 c    shows a locking system with a separate strip  5  that comprises a locking protrusion  17  connected to a lower lip  12  of the tongue groove  9  that protrudes beyond a vertical plane VP. The locking protrusion  17  is located under a horizontal plane HP that intersects the lower part of the tongue  10 . Such locking system may not provide sufficient locking strength in thin and flexible core material since the lower lip  12  and the outer part of the strip  5  will bend downwards when the edges are exposed to pulling forces and the locking element  8  will slide out from the locking groove  14 . 
       FIGS. 1 d -1 f    show similar locking systems comprising a plastic or metal clip  6  with a locking protrusion  17  connected to an upper part of the lower lip  12  which is located under the tongue  10  and under the cooperating locking surfaces between the tongue and the lower lip  12 . The clip is connected to an outer part of a lower lip  12  that is positioned beyond the upper lip and beyond the vertical plane VP. 
     To facilitate understanding of the described invention, several locking systems in the figures are shown schematically. It should be emphasized that improved or different functions can be achieved using combinations of the preferred embodiments. 
     All embodiments may be used separately or in combinations. Angles, dimensions, rounded parts, spaces between surfaces etc. are only examples and may be adjusted within the basic principles of the invention. 
       FIGS. 2 a -2 f    show a first embodiment of the invention. 
       FIG. 2 a    show a cross section of a first and second panel  1 ,  1 ′ each provided with a surface layer  2  comprising a transparent wear layer  20  which may be coated by a UV cured PU lacquer. The first and the second panels  1 ,  1 ′ are preferably LVT panels. A decorative plastic foil  21  is attached to a core  3  and under the transparent layer  20 . The core  3  that preferably comprises a thermosetting plastic material with a filler may have several core layers, which may have different density and hardness. The locking system comprises a tongue  10  at the second edge of the second panel  1 ′, a tongue groove  9  at a first edge of the first panel and a clip  6 , that preferably is formed by punching a metal sheet, for example a 0.3-0.6 mm aluminium or steel sheet. The clip  6  comprises a clip body  7  at a rear side of a first panel  1 . The clip body comprises an inner part IP that extends inwardly from a first edge of the first panel and an outer part OP that extends outwardly from the first edge of the first panel  1 . 
     The clip  6  comprises a fixing element  16  located in a fixing groove  15  in the first panel  1  and a locking element  8  located in a locking groove  14  formed in an adjacent second panel  1 ′ that lock the panel edges horizontally and prevents horizontal separation. The clip  6  comprises a locking protrusion  17  formed on the strip body  7  between the locking element  8  and the fixing element  16 . The locking protrusion  17  projects vertically upwardly from the strip body and is located in a recess  18  formed in the lower lip  12  of the tongue groove  9 . The recess  18  extends vertically from an upper to a lower part of the lower lip  12 . The locking protrusion  17  is in this embodiment located such that it is displaced inwardly from the vertical plane VP. A part of the locking protrusion  17  extends inwardly into the tongue groove  9  and beyond the outer part of the tongue  10 . An upper part of the locking protrusion  17  is preferably located above a horizontal plane HP that intersects the lower part of the tongue  10  and the upper part of the lower lip  12 . The locking protrusion  17  connects the clip  6  vertically to the first panel  1  edge and prevents downward bending of the clip  6  when the edges of the first  1  and the second  1 ′ panels are exposed to separation forces. The locking protrusion  17  prevents a displacement of the clip  6  inwardly such that the clip  6  is accurately fixed and positioned in a pre-determined position by the locking protrusion  17  and the fixing element  16 . 
     An advantage is that the clip  6  may be connected to the core  3  in a horizontal plane HP that is located above the lower lip  12  and to an edge part that is more rigid than an outer part of the lower lip. The whole vertical extension of the lower lip  12  and tongue groove  9  may be used to accomplish a strong connection without any essential negative effect on the vertical tongue  10  and tongue groove  9  connection since only a small part of the lower lip  12  will be partially removed when the recess  18  is formed. The upper contact surfaces between the tongue  10  and the upper lip  11  are unchanged and may provide an unchanged sealing against moisture penetration into the joint. The locking protrusion may be connected to an edge part that comprises sufficient material to allow a strong connection even when the panels are thin for example 3-4 mm and comprise a core  3  of flexible material, such as thermoplastic material mixed with a filler, which is a material composition generally used in LVT floors. 
       FIG. 2 b    is a top view of the clip  6 .  FIG. 2 c    shows a clip  6  that has a length direction L along the edge and a width direction W perpendicular to the length. A clip with a length of about 3 cm and a width of about 2 cm may provide a locking strength that corresponds to a pulling force of about 200 N.  10  clips/m are sufficient to provide a locking strength on a long edge of about 2000 N. 
       FIG. 2 d    shows an edge section  1  that comprises a recess  18  formed in the lower lip  12 . 
       FIG. 2 e    shows the same edge section  1  with the surface layer  2  pointing downwards and the recess  18  formed in the lower lip  12 . 
       FIG. 2 f    shows a clip  6  connected to an edge section  1 . The locking protrusion is located in a recess  18  formed in the lower lip  12 . 
       FIG. 3 a    shows that the locking system may be locked with angling. The lower lip  12  comprises preferably a sliding surface  19  that guides the tongue  10  into the tongue groove  9  during angling but also during horizontal snapping. The sliding surface  19  and a part of the lower lip  12  are located above the outer part OP of the clip body  7   
       FIG. 3 b    shows that the clip  6  may be connected with angling and pressing of the fixing element  16  with a pressing tool P into the fixing groove  15 . The recess  19  is preferably formed by a vertically rotating tool T that cuts the edge as a saw blade. 
       FIGS. 3 c , 3 d  and 3 e    show that the clip  6  may be connected by a horizontal displacement and pressing against the fixing element  16  such that a bending of the fixing element  16  takes place. 
       FIG. 3 f    shows that the fixing element  16  may be pressed into the core  3  and the fixing groove  15  is formed by the fixing element  16 . The fixing groove may be precut with a knife. Glue may also be used to connect the clip  6  to a panel edge. Glue may in some applications replace the fixing grove  15  and the fixing element  16 . 
       FIG. 3 g    shows that several clips  6   a ,  6   b  may be formed by punching a metal sheet and may be inserted after separation from a clip blank comprising several clips.  FIG. 3 h    shows that the clip  6  may have several locking protrusions  17   a ,  17   b.    
       FIGS. 4 a -4 c    show that the clip  6  may comprise guiding parts  22  having an upwardly extending sliding surface  19  that may facilitate the guiding of the tongue  10  into the tongue groove  9  during angling and/or horizontal snapping. The guiding part  22  may also be used to position the clip  6  horizontally against the lower lip  12 . 
       FIGS. 5 a -5 d    show that the recess  18  may be formed in an upper surface of the lower lip  12  and extend along a part of the lower lip. 
       FIGS. 6 a -6 c    show that the recess  18  may be formed in a lower part of the tongue  10  as shown in  FIG. 6 c    where the panel  1 ′ is shown with the rear side pointing upwards. The locking protrusion  17  is in locked position connected into the tongue groove  9  and located in the recess  18  formed in the lower part of the tongue  10 . 
       FIGS. 7 a  and 7 b    show that the recess  18 ,  18 ′ may extend from the tongue  10  and to the locking groove  14  in order to accommodate the outer part OP of the clip  6  that extends beyond the upper edge of the panel  1 .  FIGS. 7 a  and 7 b    show that the clip  6  may be an extruded section, for example a plastic or aluminium section. 
       FIGS. 8 a  and 8 b    show panels  1 ,  1 ′ comprising a core  3  with an upper core layer  4   a  and a lower core layer  4   b  layer and wherein the locking protrusion  17  protrudes vertically beyond the lower layer  4   b .  FIG. 8 c    shows that the core  3  may comprise a glass fibre layer  4   c  and the upper part of the locking protrusion may be located above such glass fibre layer  4   c.    
       FIG. 9 a    shows a floor panel  1  comprising several clips  6  and recesses  18   a  on one of the long edges and several recesses  18   b  on the opposite long edge. The panel comprises a locking system on the short edges that is formed in one piece with the core.  FIG. 9 b    shows a locking system comprising clips  6  on long and short edges. 
       FIGS. 10 a -10 e    show that all embodiment of this disclosure may be adapted such that a flooring system may comprise a first A panel and a second B panel comprising clips  6  on at least two opposite edges, a first edge  23   a  and a second edge  23   b . The locking system is configured such that a first edge  23   a  of a first A panel may be locked to a second edge  23   b  and a first edge  23   a  of a second panel B. 
       FIG. 1 a    shows a cross section C 1 -C 1  of two adjacent edges  23   a  and  23   b  according to  FIG. 10 e   . Both edges comprise a horizontal groove  9   a  and  9   b  and a lower lip  9   a ,  9   b . The locking protrusion  17  is preferably located essentially above the lower lip  12   a  and the lower lip is preferably spaced horizontally from the vertical plane VP. 
       FIG. 10 b    show the cross section C 2 -C 2  in  FIG. 10 d    and  FIG. 10 c    shows the cross section C 1 -C 1  in locked position. 
     The clips are offset along the adjacent edges such that they may be inserted between each other. 
       FIG. 10 d    shows that a first edge  23   a  of a first panel A may be locked to a second edge  23   b  of a second panel B.  FIG. 10 e    shows that a first edge  23   a  of the first panel A may also be connected to a first edge  23   a  of the second panel B. 
     The above-described locking system may be used to lock all types of floor panels. Ceramic tiles may be installed with a space between the upper edges. This allows that the outer part of the lower lip  12  may be located at the vertical plane VP or may even protrude horizontally beyond the vertical plane VP and the upper part of the edge. 
     Embodiments 
     1. Building panels provided with a locking system comprising a tongue ( 10 ) at a second edge of a second panel ( 1 ′), the tongue being configured to cooperate with a tongue groove ( 9 ) at a first edge of a first panel ( 1 ) for locking in a vertical direction, the tongue groove ( 9 ) comprising an upper lip ( 11 ) and a lower lip ( 12 ), the locking system further comprising one or more clips ( 6 ) attached to the first edge and a downwardly open locking groove ( 14 ) formed at the second edge, each clip ( 6 ) comprises an upwardly extending locking element ( 8 ), which is configured to cooperate with the locking groove ( 14 ) for locking the first edge and the second edge in a horizontal direction characterized in: 
     that the clip ( 6 ) comprises a clip body ( 7 ) at a rear side of the first panel ( 1 ), said clip body ( 7 ) is provided with an inner part (IP), which extends inwardly from the first edge, and an outer part (OP), which extends outwardly from said first edge, 
     that the inner strip part (IP) comprises a fixing element ( 16 ) that cooperates with a downwardly open fixing groove ( 15 ), formed on the rear side of the first panel ( 1 ), for locking the clip ( 6 ) to the first edge in a horizontal direction, 
     that the clip ( 6 ) comprises a locking protrusion ( 17 ), which protrudes upwardly from the clip body ( 7 ), said locking protrusion ( 17 ) is configured to lock the clip ( 6 ) to the first edge in a vertical direction, 
     the lower lip ( 12 ) or the tongue ( 10 ) comprises a recess ( 18 ), and 
     that the locking protrusion ( 17 ) is in a locked position positioned in the recess ( 18 ). 
     2. The building panels as in embodiment 1, wherein a part of the locking protrusion ( 17 ) is located in the tongue groove ( 9 ). 
     3. The building panels as in embodiment 1 or 2, wherein a part of the locking protrusion ( 17 ) is located below the tongue ( 10 ). 
     4. The building panels as in any one of the preceding embodiments 1-3, wherein said locking protrusion ( 17 ) is spaced horizontally inwardly in the tongue groove ( 9 ) beyond the outer tip of the tongue ( 10 ) 
     5. The building panels as in any one of the preceding embodiments 1-5, wherein the locking protrusion ( 17 ) comprises a first part ( 17   a ) that extends upwardly from the clip body ( 7 ) and a second part ( 17   b ) that extends inwardly into the tongue groove ( 9 ). 
     6. The building panel as in any one of the preceding embodiments 1-5 wherein the locking protrusion is located inwardly and spaced horizontally from the vertical plane 
     7. The building panels as in any one of the embodiments 1-6, wherein the panel comprises a core of plastic material 
     8. The building panels as in any one of the preceding embodiments 1-7, wherein the panel comprises a surface of thermoplastic material 
     9. The building panels as in any one of the preceding embodiments 1-8, wherein the panel comprises a core ( 3 ) with an upper core layer ( 4   a ) and a lower core layer ( 4   b ) and wherein the locking protrusion ( 17 ) protrudes vertically beyond the lower core layer ( 4   b ). 
     10. A flooring system comprising a first panel (A) and a second panel (B) being provided with a locking system comprising clips, said clips ( 6 ) being arranged at a first edge ( 23   a ) and at an opposite second edge ( 23   b ) of the first and the second panel (A,B), characterized in that the locking system is configured to lock the first edge ( 23   a ) of the first panel (A) to the second edge ( 23   b ) of the second panel (B) in a horizontal and a vertical direction. 
     11. The flooring system as in embodiment 10, wherein the first edge ( 23   a ) and the second edge ( 23   b ) each comprises a horizontal groove ( 9   a ,  9   b ) comprising a lower lip ( 12   a ,  12   b ). 
     12. The flooring system as in embodiment 11, wherein each clip ( 6 ) comprises a vertically extending locking protrusion ( 17 ) with an upper part that is located essentially above the lower lip ( 12   a ,  12   b ) of the first and the second panel, respectively. 
     13. The flooring system as in embodiment 12, wherein each lower lip is spaced horizontally and inwardly from an upper part of the edge.