Patent Description:
Panels are known that are configured to be assembled by a vertical displacement and to be locked together in a vertical direction and in a horizontal direction. Such panels are disclosed in, e.g., <CIT>. A tongue and groove connection locks a first edge of a first panel to a second edge of the second panel. The first and the second edge furthermore comprise a locking element configured to cooperate with a locking groove for locking in the vertical and the horizontal direction.

Furthermore, <CIT> discloses a floor panel for forming a floor covering,
wherein the floor covering consists of floor panels, which, on at least one pair of edges, are provided with coupling parts. The coupling parts substantially are manufactured from the material of the floor panel, and the coupling parts are configured such that two such floor panels, at the pair of edges, can be installed and locked to each other by means of a downward movement and/or by means of the fold-down principle.

Embodiments of the present invention address a need to provide an easier assembling and/or an increased locking strength of the panels.

It is an object of at least certain embodiments of the present invention to provide an improvement over the above described techniques and known art.

A further object of at least certain embodiments of the present invention is to facilitate assembling of panels configured to be assembled by a vertical displacement and locked together in the vertical direction and the horizontal direction.

Another object of at least certain embodiment of the present invention is to increase the locking strength by preventing or at least decreasing damage of edges of the panels, particularly parts of the edges that have a locking function. The locking strength may also be increased by an improved configuration of locking surfaces at the edges of the panels.

At least some of these and other objects and advantages that may be apparent from the description have been achieved by a first aspect of the invention comprising a set of panels comprising a first panel and a second panel. The first edge of the first panel and a second edge of the second are configured to be locked together and assembled by a vertical displacement of the second edge relative the first edge. The first edge comprises a locking element configured to cooperate with a locking groove at the second edge for locking in a horizontal and in the vertical direction. The first edge comprises a tongue configured to cooperate with a tongue groove at the second edge for locking in a vertical direction. An upper part of the first edge comprises a first guiding surface and a lower edge of a lower lip of the tongue groove comprises a second guiding surface, which are configured to cooperate during the vertical displacement. An upper part of the locking element comprises a third guiding surface and a lower edge of the locking groove comprises a fourth guiding surface, which are configured to cooperate during the vertical displacement. A tip of the tongue comprises a fifth guiding surface and a tip of the lower lip of the tongue groove comprises a sixth guiding surface, which are configured to cooperate during the vertical displacement.

The three pairs of guiding surface may have the advantage that damages of particularly the tongue may be prevented during the vertical displacement.

The tongue at the first edge, at the same edge as the locking element, may provide an improved guiding and at the same time an improved configuration for the cooperation with the tongue groove.

The third and the fourth guiding surfaces and fifth and the sixth guiding surfaces, respectively, may be configure cooperate at the same time during the vertical displacement.

The first and the second guiding surfaces may be configured to cooperate at a first guiding position, during the vertical displacement, before a second guiding position comprising the guiding by the third and the fourth guiding surfaces. This may have the advantage that the second edge is in a correct position relative the first edge when edges are guided by the third and the fourth guiding surfaces and fifth and the sixth guiding surfaces. This second guiding position may occur at the same time as a bending of the locking strip and/or a compressing of parts of the first and the second edge in order to displace the first and the second edge to a locked position. The bending and the compression involve a stress of the first and the second edges. A correct position may decrease the stress.

The fifth and the sixth guiding surfaces may be essentially parallel, preferably parallel and/or essentially vertical, preferably vertical.

The first guiding surface may be positioned above the fifth guiding surface.

The second guiding surface may be positioned below the fifth guiding surface.

An upper lip of the tongue groove may be configured to overlap the first guiding surface in a locked position of the first and the second edge.

A lower surface of the upper lip may be configured cooperate with the first guiding surface for restraining penetration of moisture and/or dirt between the first and the second edge in the locked position.

The first guiding surface may be a bevel or a rounding of the uppermost part of the first edge.

The first guiding surface may be adjacent, preferably transition into, the fifth guiding surface. A close positioning of the first guiding surface and the fifth guiding surface may provide room for a larger guiding surface that may improve the guiding.

A locking strip may protrude from the first edge under the tongue, wherein the locking strip comprises the locking element.

A first locking surface of the locking element may be configured to cooperate with a second locking surface of the locking groove for locking in the vertical and the horizontal direction, wherein a first locking angle between the first locking surface and an upper surface of the first panel is preferably within the range of about <NUM>° to about <NUM>°, preferably within the range of about <NUM>° to about <NUM>° or preferably about <NUM>°.

The tip of the tongue may have a blunt shape which may improve the strength of the tip.

The tip of the lower lip of the tongue groove may have a blunt shape which may improve the strength of the tip.

The first edge and the second edge are preferably produced by mechanically cutting, such as milling.

The locking surfaces and the guiding surfaces may comprise a material of the core of the first panel and/or the second panel.

The first panel and the second panel may be resilient panels. The resilient panels may comprise a core comprising thermoplastic material. The thermoplastic material may be foamed.

The thermoplastic material may comprise polyvinyl chloride (PVC), polyester, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyethylene terephthalate (PET), polyacrylate, methacrylate, polycarbonate, polyvinyl butyral, polybutylene terephthalate, or a combination thereof. The core may be formed of several layers.

The first panel and the second panel may comprise a decorative layer, such as a decorative foil comprising a thermoplastic material. The thermoplastic material of the decorative layer may be or comprise polyvinyl chloride (PVC), polyester, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyethylene terephthalate (PET), polyacrylate, methacrylate, polycarbonate, polyvinyl butyral, polybutylene terephthalate, or a combination thereof. The decorative foil is preferably printed, for example by direct printing, rotogravure, or digital printing.

The firs panel and the second panel may comprise a wear layer such as a film or foil. The wear layer may comprise thermoplastic material. The thermoplastic material may be polyvinyl chloride (PVC), polyester, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyethylene terephthalate (PET), polyacrylate, methacrylate, polycarbonate, polyvinyl butyral, polybutylene terephthalate, or a combination thereof.

Embodiments of the invention may be particularly advantageous for panels comprising guiding surfaces with higher friction and a tongue comprising a less elastic thermoplastic material.

The first and the second panel may comprise a wood-based core, such as HDF, MDF or plywood.

The first guiding surface preferably extends through the decorative layer and/or the wear layer and a part of the first guiding surface comprises material of the core.

The first panel and the second panel may be configured to be disassembled by downwardly rotating the first and/or the second panel.

A second aspect of the invention comprising a set of panels comprising a first panel and a second panel. The first edge of the first panel and a second edge of the second are configured to be locked together and assembled by a vertical displacement of the second edge relative the first edge. The first edge comprises a locking element configured to cooperate with a locking groove at the second edge for locking in a horizontal and in the vertical direction. The first edge comprises a tongue configured to cooperate with a tongue groove at the second edge for locking in a vertical direction. An upper part of the locking element comprises a third guiding surface and a lower edge of the locking groove comprises a fourth guiding surface, which are configured to cooperate during the vertical displacement. A tip of the tongue comprises a fifth guiding surface and a tip of the lower lip of the tongue groove comprises a sixth guiding surface, which are configured to cooperate during the vertical displacement. A locking surface of the tongue is configured to cooperate with a locking surface of the tongue groove for locking in the vertical direction. The locking surfaces are positioned at a distinguishing angle to each other in a locked position of the first and the second edge.

An advantage may be that the locking surface of the tongue may cooperate with the locking surface of the tongue groove even if the locking surfaces are positioned incorrect due to production tolerances and/or dust or other particles between surfaces of the first edge and the second edge.

A locking strip may protrude from the first edge under the tongue, wherein the locking strip comprises the locking element. The locking strip is preferably configured to be bent downwards during the vertical displacement and to spring back to the locked position.

In a locked position with an incorrect positioned locking surface, of the tongue and/or of the tongue groove, the locking strip may stay slightly downwardly bent.

The distinguishing angle between the locking surface of the tongue and the locking surface of the tongue groove may be about <NUM>°, or in the range of about <NUM>° to about <NUM>°.

The second aspect of the invention may be combined with the embodiments of the first aspect of the invention described above or with the dependent claim or with the embodiments described below.

These and other aspects, features and advantages of which embodiments of the invention are capable of, will be apparent and elucidated from the following description of embodiments of the present invention, reference being made to the accompanying drawings, in which.

A known set of panels, such as floorboards, are shown in <FIG>. The set comprising a first panel <NUM> and a second panel <NUM>. A first edge of the first panel is configured to be locked to a second edge of the second panel. The first edge and the second edge is assembled by a relative vertical displacement <NUM>. The first edge comprising a tongue groove <NUM> and the second edge comprises a tongue <NUM> which are configured for locking the first edge to the second edge in a vertical direction. The first edge comprises a protruding locking strip with locking element <NUM> which is configured to cooperate with a locking groove <NUM> at the second edge for locking in a horizontal direction and the vertical direction.

An embodiment of the invention is shown during assembling in <FIG>. The embodiment comprises a set of panels comprising a first panel <NUM> and a second panel <NUM>, wherein a first edge of the first panel and a second edge of the second panel are configured to be locked together and assembled by a vertical displacement <NUM> of the second edge relative the first edge. An enlargement of the first edge is shown in <FIG> and an enlargement of the second edge is shown in <FIG>.

The first panel and the second panel may have a thickness in the range of about <NUM> to about <NUM>.

The first edge comprises a locking element <NUM> configured to cooperate with a locking groove <NUM> at the second edge for locking in a horizontal direction and in the vertical direction. The first edge comprises a tongue <NUM> configured to cooperate with a tongue groove <NUM> at the second edge for locking in a vertical direction. The tongue groove <NUM> has an upper lip <NUM> and a lower lip <NUM>. An upper part of the first edge comprises a first guiding surface <NUM> and a lower edge of the lower lip <NUM> comprises a second guiding surface <NUM>, which are configured to cooperate during the vertical displacement. An upper part of the locking element comprises a third guiding surface <NUM> and a lower edge of the locking groove comprises a fourth guiding surface <NUM>, which are configured to cooperate during the vertical displacement. A tip of the tongue comprises a fifth guiding surface <NUM> and a tip of a lower lip <NUM> of the tongue groove <NUM> comprises a sixth guiding surface <NUM>, which are configured to cooperate during the vertical displacement. This embodiment of the invention may be easier to install compared to the known set of panels due to the guiding surfaces. An improved guiding may be critical for assembling panels having surfaces with a high friction and particularly if the panel edges comprise a less elastic material. Without the improved guiding such panels may be difficult to assemble or the panels or part of the panel, e.g., the tongue may break during the assembling.

A first guiding position is shown in <FIG> comprising the cooperation between the first guiding surface <NUM> and the second guiding surface <NUM>.

A second guiding position is shown in <FIG> comprising the cooperation between the third guiding surface <NUM> and the fourth guiding surface <NUM>, and the cooperation between the fifth guiding surface <NUM> the sixth guiding surface <NUM>.

As is shown in <FIG> the embodiment comprises three pairs of guiding surfaces, a first pair comprising the first guiding surface <NUM> and the second guiding surface <NUM>, a second pair comprising the third guiding surface <NUM> and the fourth guiding surface <NUM>, and a third pair comprising the fifth guiding surface <NUM> the sixth guiding surface <NUM>.

The third guiding surface <NUM> and the fourth guiding surface <NUM> may be roundings. This embodiment may be advantageous for thinner panels with a thickness of about <NUM> to about <NUM>. The roundings may have a radius which is in the range of about <NUM>,<NUM> to about <NUM>,<NUM>, or about <NUM>,<NUM>. The radius may be in the range of about <NUM>% to about <NUM>% of the thickness of the panels.

The second edge may be provided with a calibrating groove <NUM> adjacent said locking groove <NUM>. The calibrating groove may compensate for floorboards having different thickness, especially any difference in thickness at the edges of the floorboards. The calibrating groove allows that the second edge may be pushed towards a sub-floor on which the floorboards are arranged. The shown calibrating groove is of a rectangular shape, however the calibrating groove may have other shapes such as a bevel.

A locked position of the first edge and the second edge is shown in <FIG>.

A vertical locking both at the tongue and the tongue groove and at the locking element and the locking groove may be an advantage, especially for panels with a locking in a resilient material. The double vertical locking may decrease the risk of unlocking and a separation of the first and the second edges.

The tongue <NUM> may comprise a locking surface <NUM> at a lower side and the tongue groove may comprise a locking surface <NUM> at an upper side of the lower lip <NUM>.

The locking surface <NUM> of the tongue <NUM> may be parallel or essentially parallel to the locking surface <NUM> of the tongue groove.

The tongue <NUM> at the first edge, at the same edge as the locking element, provides an improved guiding. Furthermore, the locking surfaces <NUM>,<NUM> at the tongue <NUM> and the lower lip <NUM> may be larger and at a more advantageous locking angle of the locking surfaces which may increase the locking strength as compared to known locking systems.

The third and the fourth guiding surfaces <NUM>, <NUM> and fifth and the sixth guiding surfaces <NUM>,<NUM>, respectively, are configure cooperate at the same time during the vertical displacement.

The first and the second guiding surfaces <NUM>,<NUM> are configured to cooperate before the third and the fourth guiding surfaces <NUM>,<NUM> during the vertical displacement.

The fifth and the sixth guiding surfaces <NUM>,<NUM> may be essentially parallel to each other, and preferably extend essentially vertical or may be within a range of about <NUM>° to about <NUM>°, or about <NUM>° to the vertical direction.

The first guiding surface <NUM> is positioned above the fifth guiding surface <NUM>.

The second guiding surface <NUM> is positioned below the sixth guiding surface <NUM>.

An upper lip <NUM> of the tongue groove is configured to overlap the first guiding surface <NUM> in a locked position of the first and the second edge. A lower surface <NUM> of the upper lip <NUM> may be configured cooperate with the first guiding surface <NUM> for restraining penetration of moisture and/or dirt between the first and the second edge in the locked position.

The first guiding surface <NUM> may be a bevel or a rounding of the uppermost part of the first edge. The first guiding surface <NUM> may be adjacent, preferably transitions into, the fifth guiding surface <NUM>.

A locking strip <NUM> protrudes from the first edge under the tongue <NUM> and the locking strip comprises the locking element <NUM>. The locking strip is preferably configured to be downwardly bent during the vertical displacement and spring back to the locked position.

A first locking surface <NUM> of the locking element <NUM> is configured to cooperate with a second locking surface <NUM> of the locking groove <NUM> for locking in the vertical and the horizontal direction.

A curved surface <NUM> which is between the first locking surface and an upper surface of the locking strip may be configured to cooperate in the locked position with the fourth guiding surface <NUM>. This may further increase the locking strength in the horizontal direction.

A first locking angle <NUM> which is between the first locking surface <NUM> and an upper surface of the first panel may be within the range of about <NUM>° to about <NUM>°, preferably within the range of about <NUM>° to about <NUM>°, or preferably about <NUM>°.

A second locking angle <NUM> which is between the second locking surface <NUM> and an upper surface of the second panel may be within the range of about <NUM>° to about <NUM>°, preferably within the range of about <NUM>° to about <NUM>°, or preferably about <NUM>°, and may be essentially equal to the first locking angle <NUM>.

An embodiment of the invention comprises a first locking angle <NUM> which is larger than the second locking angle, such as about <NUM>-<NUM>° larger. This may have the effect that a greater part of a pulling load is applied on curved surface cooperating with fourth guiding surface <NUM>, and that a greater locking strength is obtained.

The locking element may have a first side, an opposite second side and an upper side. The first side is closer to the tongue <NUM> than the second side. The first locking surface is preferably at the first side. There may be a space between the second side and the locking groove <NUM>. There may be a space between the upper side and the locking groove <NUM>.

The tip of the tongue <NUM> may have a blunt shape which may improve the strength of the tip and improve the guiding. Each angle between adjacent surfaces, at the tip of the tongue, is preferably more than <NUM>°.

<FIG> shows an embodiment of the tongue comprising a first tongue angle <NUM> between the first guiding surface <NUM> and the fifth guiding surface <NUM> and a second tongue angle <NUM> between the fifth guiding surface <NUM> and the locking surface <NUM> of the tongue. The first tongue angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°. The second tongue angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°, for example in the range of about <NUM>° to about <NUM>°.

A guiding angle between the first guiding surface <NUM> and the vertical direction may be about <NUM>° or in the range of about <NUM>° to about <NUM>°.

The tip of the lower lip <NUM> of the tongue groove <NUM> may have a blunt shape which may improve the strength of the tip and improve the guiding. Each angle between adjacent surfaces, at the tip of the lower lip <NUM>, is preferably more than <NUM>°.

<FIG> shows an embodiment of the lower lip <NUM> comprising a first lip angle <NUM> between the locking surface <NUM> of the lower lip <NUM> and the sixth guiding surface <NUM> and a second lip angle <NUM> between the sixth guiding surface <NUM> and the second guiding surface <NUM>. The first lip angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°, for example in the range of about <NUM>° to about <NUM>°. The second lip angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°.

<FIG> shows an embodiment of the locking surface <NUM> of the tongue <NUM> and the locking surface <NUM> of the tongue groove at the lower lip <NUM>. Said locking surfaces <NUM>,<NUM> are positioned at a distinguishing angle <NUM> to each other in the locked position. An advantage may be that locking surface <NUM> of the tongue may cooperate with the locking surface <NUM> of the tongue groove even if the locking surfaces are positioned incorrect due to production tolerances and/or dust or other particles between surfaces of the first edge and the second edge. In a locked position with an incorrect positioned locking surface, of the tongue and/or of the tongue groove, the locking strip may stay slightly downwardly bent.

The distinguishing angle <NUM> between the locking surface <NUM> of the tongue <NUM> and the locking surface <NUM> of the tongue groove may be about <NUM>°, or in the range of about <NUM>° to about <NUM>°.

A first locking surface angle <NUM> is between the locking surface of the tongue and an upper surface of the first panel. A second locking surface angle <NUM> is between the locking surface <NUM> of the tongue groove and an upper surface of the second panel <NUM>. The first locking surface angle <NUM> is preferably larger than the second surface angle <NUM>.

The first locking surface angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°, or preferably in the range of about <NUM>° to about <NUM>°.

The second locking surface angle <NUM> may be about <NUM>° or in the range of about <NUM>° to about <NUM>°, or preferably in the range of about <NUM>° to about <NUM>°.

An upper surface <NUM> of the locking strip may be configured to cooperate in a locked position with a lower surface <NUM> of the second edge.

The upper surface <NUM> of the locking strip may be essentially parallel to an upper surface of the first panel.

The upper surface <NUM> of the locking strip is preferably parallel to lower surface <NUM> of the second edge.

The first panel <NUM> and the second panel <NUM> may be configured to be disassembled by downwardly rotating <NUM> the first and/or the second panel, see <FIG>.

A lower side of the locking strip may comprise a space <NUM>, such as a recess or bevel, under the locking element to facilitate bending of the locking strip during the assembling.

The first edge may comprise a lower groove <NUM> below the tongue <NUM>. The lower lip <NUM> may be configured to be inserted into the lower groove <NUM> during the vertical displacement.

The assembling may comprise a displacement in the horizontal direction.

The firs edge and the second edge may be short edges of the first and the second panel.

The assembling may also comprise an angling motion along a long side of the first and or the second panel.

An embodiment of the set of panels is shown during assembling in <FIG>. The third and the fourth guiding surfaces <NUM>, <NUM> are in this embodiment bevels comprising an essentially flat surface. This embodiment may be advantageous for thicker panels and/or panels comprising a wood based core, such as HDF, MDF or plywood. The thicker panels may have a thickness in the range of about <NUM> to about <NUM>.

<FIG> show embodiments of the first edge comprising a locking strip with a larger space <NUM>. The first edge has a first distance <NUM> between a bottom surface <NUM> of the lower groove <NUM> and an outer surface <NUM> of the locking strip <NUM>. The space <NUM> extends from outer surface <NUM> towards the bottom surface <NUM> by a second distance <NUM>. The second distance may be within the range of about <NUM>/<NUM> to about equal the first distance, preferably about <NUM>/<NUM> to about ½ of the first distance, or more preferably <NUM>/<NUM> to about <NUM>/<NUM> of the first distance. The space <NUM> may have a height of <NUM>,<NUM> to <NUM>, preferably about <NUM>.

The space in the embodiment in <FIG> is a recess under the locking strip and the space in <FIG> is a groove in the locking strip <NUM>. These embodiments may be advantageous for panels with a larger thickness.

The embodiments described above may be resilient panels. The resilient panels may comprise a core comprising thermoplastic material. The thermoplastic material may be foamed.

The embodiments described above may comprise a decorative layer, such as a decorative foil comprising a thermoplastic material. The thermoplastic material of the decorative layer may be or comprise polyvinyl chloride (PVC), polyester, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyethylene terephthalate (PET), polyacrylate, methacrylate, polycarbonate, polyvinyl butyral, polybutylene terephthalate, or a combination thereof. The decorative foil is preferably printed, for example by direct printing, rotogravure, or digital printing.

The embodiments described above may comprise a wear layer such as a film or foil. The wear layer may comprise thermoplastic material. The thermoplastic material may be polyvinyl chloride (PVC), polyester, polypropylene (PP), polyethylene (PE), polystyrene (PS), polyurethane (PU), polyethylene terephthalate (PET), polyacrylate, methacrylate, polycarbonate, polyvinyl butyral, polybutylene terephthalate, or a combination thereof.

The embodiments described above may comprise a wood base core, such as HDF, MDF or plywood.

<FIG> shows that the first panel and the second panel may each comprise an upper layer <NUM>, such as a decorative layer and/or wear layer. The first guiding surface <NUM> preferably extends through the upper layer <NUM> and a part of the first guiding surface comprises material of the core.

<FIG> shows that the first panel and the second panel may each comprise one or more enforcement layers <NUM> comprising, e.g., glass fibres. The enforcement layers <NUM> are preferably positioned outside the third and the fourth guiding surfaces <NUM>, <NUM> and fifth and the sixth guiding surfaces <NUM>,<NUM>. Enforcement layers at these guiding surfaces may increase the friction forces during assembling of the first panel and the second panel.

<FIG> shows that the second edge of the second panel <NUM> may comprise a second groove <NUM> adjacent the second locking surface <NUM>. This groove <NUM> may facilitate disassembling by downwardly rotating <NUM> the first and/or the second panel.

<FIG> shows that the first edge of the first panel <NUM> may comprise a first groove <NUM> adjacent the first locking surface <NUM>. This groove <NUM> may facilitate disassembling by downwardly rotating <NUM> the first and/or the second panel.

The first edge with the first groove <NUM> may be combined with the second edge with the second groove <NUM>.

An embodiment of the first edge and the second edge is shown in <FIG>. The embodiment includes a locking strip <NUM> which comprises a notch <NUM>, adjacent the lower groove <NUM>, which is configured to cooperate with a protrusion <NUM> at lower surface of the second edge and adjacent the lower lip. An advantage with the notch <NUM> and the protrusion <NUM> may be that the horizontal strength is further increased.

The notch <NUM> may comprise a notch surface <NUM> which may be configured to cooperate for locking in the horizontal direction with a protrusion surface <NUM> of the protrusion <NUM>. The notch surface <NUM> and the protrusion surface <NUM> are preferably parallel and arranged at angle <NUM> which is within the range of about <NUM>° to about <NUM>° to the upper surface of the first panel <NUM>.

The notch surface and the protrusion surface may be configured to be positioned at a distance <NUM> from each other, such as about <NUM>,<NUM> to about <NUM>,<NUM>, or about <NUM>,<NUM>, in the locked position. An advantage with the distance may be that assembling of the first panel and the second panel is facilitated.

Claim 1:
A set of panels comprising a first panel (<NUM>) and a second panel (<NUM>), wherein a first edge of the first panel and a second edge of the second are configured to be locked together and assembled by a vertical displacement (<NUM>) of the second edge relative the first edge,
wherein the first edge comprises a locking element (<NUM>) configured to cooperate with a locking groove (<NUM>) at the second edge for locking in a horizontal and in the vertical direction, wherein
the first edge comprises a tongue (<NUM>) configured to cooperate with a tongue groove (<NUM>) at the second edge for locking in a vertical direction,
wherein the tongue groove (<NUM>) has an upper lip (<NUM>) and a lower lip (<NUM>),
wherein an upper part of the locking element comprises a third guiding surface (<NUM>) and a lower edge of the locking groove comprises a fourth guiding surface (<NUM>), which are configured to cooperate during the vertical displacement, characterised in that an upper part of the first edge comprises a first guiding surface (<NUM>) and a lower edge of the lower lip (<NUM>) comprises a second guiding surface (<NUM>), which are configured to cooperate during the vertical displacement,
that a tip of the tongue comprises a fifth guiding surface (<NUM>) and a tip of a lower lip (<NUM>) of the tongue groove (<NUM>) comprises a sixth guiding surface (<NUM>), which are configured to cooperate during the vertical displacement, and
that the first guiding surface (<NUM>) is a bevel or a rounding of an uppermost part of the first edge.