BACKGROUND OF THE INVENTION 
     The invention concerns a method for anchoring a joining element in an object and a joining element to be used in the method. The joining element comprises a thermoplastic material (or a material having a thermoplastic component) and anchoring is effected with the aid of the thermoplastic material and mechanical vibration (e.g. ultrasonic vibration) through which the thermoplastic material is at least locally liquefied. The joining element serves for connecting a further object to the named object in which it is anchored or the joining element is part of such further object. 
     It is known from the publications WO 98/42988 and WO 00/79137, to anchor joining elements in objects with the aid of a thermoplastic material which is liquefied by mechanical vibration, and which, in the liquefied state, is pressed into suitable surface structures in the object, where on re-solidification it forms a positive fit connection with the named surface structures and therewith anchors the joining element in the object. The material of the object in which the joining element is to be anchored possesses e.g. an open porosity or the surface of the object in a region where anchorage is to be affected comprises a plurality of corresponding cavities or protrusions. The thermoplastic material of the joining element usually has an elasticity module which is greater than 0.5 GPa and therefore this material can be liquefied by mechanical vibration only if contact with the object is limited to a plurality of small locations (energy directors). This is achieved e.g. by providing energy directors in the form of elements protruding from a main surface of the joining element which protruding elements taper to an edge or tip. 
     In many cases in which a joining element is anchored in an object with the aid of a thermoplastic material and mechanical vibration, an opening is provided in the object and it is desired that the joining element is anchored either in the walls of the opening or in the walls and in the bottom of the opening. Anchorage in the walls of the opening is achieved usually by dimensioning the joining element and the opening such that the circumference of a cross section of the joining element including energy directors is somewhat larger than the corresponding circumference of the cross section of the opening, such that on introducing the joining element into the opening, the circumference of the joining element is at least locally (energy directors) pressed against the wall of the opening. 
     In particular, if anchorage in the walls of the opening is desired in the immediate vicinity of the mouth of the opening, it often happens that the liquefied material interpenetrates the surface of the object in the vicinity of the opening and small amounts of the material protrude onto this surface. This effect blurs the contours of the opening and/or of the joining element in the mouth region such that these contours do not appear as clear cut as this may be desired for various reasons. In particular, if a proximal face of the joining element, which is anchored in an opening provided in the object, is to be flush with the object surface it is desirable mainly for esthetic reasons that the circumference of the joining element and of the opening are clearly defined, i.e. not blurred at all. 
     Anchorage in the immediate vicinity of the mouth of the opening is in many cases desired and it is necessary, if the object is very thin or if the opening is very shallow. This is the case if the object is a sandwich board with thin outer layers and an inner layer, wherein anchorage is possible only in the outer layers. An example of such a board is a so called hollow core board comprising outer layers of wood or a wood-like material and an inner layer being constituted by a honeycomb structure of cardboard, in which the named anchorage is not possible. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the invention to create a method for anchoring a joining element in an object and a joining element to be used in the method, wherein for anchoring the joining element, an opening is provided in the object, wherein the joining element comprises a thermoplastic material (or a material having a thermoplastic component) and anchorage is achieved by liquefaction of the thermoplastic material through mechanical vibration being applied to the joining element when it is forced into the opening. The anchoring method and the joining element according to the invention are to enable anchorage of the joining element in the wall of the opening, in particular in the immediate vicinity of a mouth of the opening wherein the contours of the mouth and the joining element at the level of the mouth are to remain as clear cut when the joining element is anchored as before anchoring. 
     This object is achieved by the anchoring method and the joining element as defined in the corresponding independent claims. Preferred embodiments are defined in the dependent claims. 
     For anchoring the joining element in a substantially cylindrical or slightly conical wall (having a round or non-round cross section) of an opening in the object, which wall extends away from the mouth of the opening, the joining element comprises a substantially cylindrical or slightly conical wall anchorage portion which is adapted regarding form and size to the named wall and whose circumferential surface comprises the thermoplastic material and is equipped with energy directors, i.e. with ribs and/or bumps which taper towards the outside in more or less sharp edges or tips. For anchorage, this wall anchorage region, whose cross section is slightly larger than the cross section of the mouth and opening wall is forced through the mouth by applying mechanical vibration (e.g. ultrasonic vibration) to a proximal end of the joining element and at the same time forcing the wall anchoring portion of the joining element through the mouth of the opening. Thereby the thermoplastic material of the circumferential surface of the wall anchoring portion is at least partly liquefied and is pressed into pores, unevenesses or specifically provided cavities of the walls and on re-solidification anchors the joining element therein. 
     For preventing the liquefied material from protruding through the mouth of the opening on to the surface of the object in which the opening is provided, a sealing means is forced through the mouth immediately following the wall anchoring portion of the joining element, the sealing means being again substantially cylindrical or slightly conical and having a cross section which corresponds in form to the cross section of the mouth of the opening but is slightly larger in cross section than the latter such that it needs to be forced into the mouth and effects sealing by a press-fit. The circumferential surface of the sealing means is smooth for effective sealing and, if the sealing means consists of the thermoplastic material, for preventing liquefaction thereof (no energy directors). 
     As mentioned above, the cross section of the sealing means has the same form as the cross section of the mouth and is on all sides slightly larger than the latter by about 0.1 to 1 mm. Furthermore, the cross section of the sealing means envelopes the cross section of the wall anchoring portion of the joining element, i.e. the edges and tips of the energy directors provided on the circumferential surface of the wall anchoring portion have the same distances from a joining element axis as the corresponding part of the circumferential surface of the sealing means. 
     In a preferred embodiment of the joining element according to the invention, the sealing means is part of the joining element, i.e. it is arranged adjoining the wall anchoring portion on the proximal side thereof, either directly or with a narrow groove therebetween, the groove region having a smaller cross section than the sealing means. However, the sealing means may also be constituted by the distal end of a tool which is applied to the proximal end of the joining element for coupling vibration and force into the joining element. 
     A joining element to be used in the method according to the invention comprises a thermoplastic material, preferably with a modulus of elasticity of more than 0.5 GPa. The joining element further comprises a substantially cylindrical wall anchoring portion between a distal end and a proximal end distanced from each other along a joining element axis. The joining element is dimensioned such that, on the anchored joining element, the wall anchoring portion is situated inside of the moth of the opening. The region of the proximal end comprises a contact face suitable for being contacted with a vibrating tool (e.g. a sonotrode of an ultrasonic device), which contact face advantageously extends perpendicular to the joining element axis. The thermoplastic material forms at least part of the circumferential surface of the joining element in the wall anchoring portion. The distance between the distal end of the joining element and the proximal end of its wall anchoring portion substantially defines a depth to which the joining element is to be introduced and anchored in the opening. 
     A preferred embodiment of the joining element to be used in the method according to the invention also comprises the sealing means which, as a sealing portion, is arranged coaxially adjoining the wall anchoring portion at its proximal side, the cross section of the sealing portion enveloping the cross section of the wall anchoring portion, wherein the circumferential surface of the sealing portion is smooth and the circumferential surface of the wall anchoring portion comprises a plurality of recesses forming between them a plurality of energy concentrating elements, i.e. a plurality of rib- or hump-shaped elements tapering off to edges or tips. The smooth circumferential surface of the sealing portion and the recess containing surface of the wall anchoring portion meet along a border line or narrow border groove running around the circumference of the joining element, which border line or groove is adapted to the shape of the surface of the object in which the opening is provided. For instance, if the named object surface is even and the opening extends substantially perpendicular to this surface, the border line extends in a plane which is perpendicular to the joining element axis. 
     The joining element is forced into the mouth of the opening provided in the object until the sealing portion has passed the mouth and is situated at a distance from the mouth of at least 0.5 mm preferably of at least 1 mm. 
     The sealing portion of the joining element or even the whole joining element may consists of the thermoplastic material. The smooth circumferential surface of the sealing portion which is in contact with the wall of the opening in the anchoring process does not enable liquefaction because there are no places where the vibrating energy is concentrated. However, it is possible also to use non-thermoplastic materials or thermoplastic materials melting at much higher temperatures or metals for the sealing portion and also for a core region of the joining element. 
     The wall anchoring portion of the joining element may continue to the distal end thereof. Alternatively, the joining element may further comprise a distal end region adjoining the wall anchoring portion on its distal side and having another shape and/or surface than the latter, wherein this distal end region, for being able to pass through the mouth of the opening, has a smaller cross section than the wall anchoring portion. The sealing portion of the joining element may form the proximal end of the joining element. Alternatively, the joining element may further comprise a proximal end region adjoining the sealing portion and having another shape and/or surface than the latter, wherein this proximal end region may have a larger or smaller cross section than the sealing portion. 
     If the joining element is to be anchored in the opening with its proximal face flush with the object surface in which the mouth of the opening is situated, then the sealing portion has a small axial length and carries the proximal contact face. In such cases, the axial extent of the sealing portion, i.e. the distance between the above defined border line and the proximal face is preferably at least 0.5 mm, preferably at least 1 mm. 
     It is possible also that in the method according to the invention the sealing function is taken over by a distal end region of a tool used for pressing the joining element into the opening provided in the object and for coupling the mechanical vibration into the joining element. In such cases, the wall anchoring portion of the joining element forms its proximal end and is forced with the tool to just below the mouth of the opening in the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The method for anchoring the joining element in the object and a plurality of exemplary embodiments of the joining element according to the invention are described in detail in connection with the following figures. 
         FIG. 1A  is a perspective view of the joining element; 
         FIG. 1B  is a cross sectional plan view through a wall anchoring region; 
         FIG. 1C  is an elevation view illustrating the joining element being anchored in an opening provided in the object; 
         FIG. 1D  is an elevation view illustrating detail of a proximal end of the wall anchoring region; 
         FIG. 1E  is an elevation view illustrating detail of the proximal end of the wall anchoring region; 
         FIG. 2A  is an elevation view of the joining element; 
         FIG. 2B  is cross sectional plan view of the wall anchoring portion; 
         FIG. 2C  is a elevation view of the joining element being anchored in an opening provided in the object; 
         FIG. 3A  is a plan view illustrating a hollow core board before the anchoring process; 
         FIG. 3B  is a plan view illustrating the hollow core board after the anchoring process; 
         FIG. 4  is a perspective view illustrating a further exemplary embodiment of the joining element according to the invention; 
         FIG. 5  is an elevation view illustrating a joining element and tool for carrying out the method according to the invention; 
         FIG. 6  is an elevation view illustrating the joining element and tool for carrying out the method according to the invention; and 
         FIG. 7  is an elevation view illustrating the joining element and tool for carrying out the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A to 1C  illustrate a first exemplary embodiment of a joining element according to the invention. Furthermore,  FIGS. 1D and 1E  show two different embodiments of a proximal end of a wall anchoring portion of the joining element. 
     The joining element consists fully of the thermoplastic material and comprises a proximal end  1  and a distal end  2 , wherein the proximal end  1  and the distal end  2  are distanced from each other along a joining element axis A. Between the proximal end  1  and distal end  2 , the joining element comprises a mouth region  3 , which is the region where a sealing portion  3 . 1  and a wall anchoring portion  3 . 2  meet. The mouth region  3  is substantially cylindrical or slightly conical and has a cross section, as shown in  FIG. 1B  as substantially round. The proximal end  1  comprises a contact face  1 . 1  suitable for applying a tool for coupling mechanical vibration and a force in the direction of the joining element axis A into the joining element. The proximal end  1  may also comprise means for attaching the joining element to the tool or for guiding the tool, e.g. an axial recess  1 . 2 . The distal end  2  is adapted to the opening in which it is to be anchored, the opening  5  provided in the object  10  being e.g. a blind bore. 
     The circumferential surface of the sealing portion  3 . 1  is smooth. The circumferential surface of the wall anchoring portion  3 . 2  comprises a plurality of recesses  4 . 1  (e.g. in a regular pattern) and between recesses  4 . 1 , a plurality of energy concentrating elements  4 . 2  in the form of axial ribs  4  tapering off to more or less sharp edges, which have similar distances from the joining element axis A as the smooth circumferential surface of the sealing portion  3 . 1 . This means that the cross section of the sealing portion  3 . 1  envelopes the cross section of the wall anchoring portion  3 . 2  (including energy concentrating elements) as illustrated in  FIG. 1B , which is a cross section through the wall anchoring portion  3 . 2  viewed towards the sealing portion  3 . 2 . The sealing portion  3 . 1  and the wall anchoring portion  3 . 2  meet in a mouth region  3  of the joining element along a border line  6 , which lies in a plane perpendicular to the joining element axis A. 
     For anchoring the joining element according to  FIGS. 1A and 1B , the blind bore (opening  5 ) is provided in the board  10 , wherein a bore axis A′ is oriented substantially perpendicular to the surface of the board  10  and wherein, of the bore  5 , at least the mouth  5 . 1  and a piece of the wall extending away from the mouth  5 . 1  have a cross section which regarding form is adapted to the cross section of the sealing portion  3 . 1  of the joining element, but is slightly smaller than the latter (difference between diameters preferably between 0.2 and 2 mm). If the joining element is to be anchored not only in the bore walls but also in the bottom of the bore  5 , the depth of the bore  5  is smaller than the distance between the distal end of the joining element and the border line  6 . 
     For being anchored in the opening  5 , the joining element is positioned in or on the mouth  5 . 1  of the opening  5  and using a suitable tool (e.g. the sonotrode of an ultrasonic device) which is positioned on and pressed against the contact face  1 . 1  of the joining element, the joining element is forced into the opening  5  while being vibrated. The ribs (energy concentrating elements  4 . 2 ) are pressed against the wall of the opening  5  and the thermoplastic material is liquefied there and is pressed into the porous structure of these walls. When the wall anchoring portion  3 . 2  and the border line  6  have passed the mouth  5 . 1  the distal end of the sealing portion  3 . 1 , which is not equipped with energy concentrating elements (smooth circumferential surface) is forced into the mouth  5 . 1  wherein, because of the lack of energy directors, hardly any material is liquefied such that this sealing portion  3 . 1  is able to seal the mouth  5 . 1  in a clean manner as illustrated in  FIG. 1C . The border line  6  which is shown in  FIG. 1C  as a dash-dotted line is in reality hardly distinguishable after the anchoring process. 
     As shown in  FIGS. 1D and 1E , the border line  6  may be a sharp line as illustrated in  FIG. 1D , or it may be a narrow grove (border groove  6 . 1 ) extending between the sealing portion  3 . 1  and the wall anchoring portion  3 . 2  and serving for accommodating a surplus of liquefied material. 
     Possible alternative features of the joining element and the anchoring method as illustrated in  FIGS. 1A to 1E  are listed hereinbelow. 
     The cross section of the mouth  5 . 1  and the sealing portion  3 . 1  are not round (see also  FIGS. 2A to 2C ). 
     The sealing portion  3 . 1  has a considerably smaller axial length and the joining element is to be forced into the opening  5  until the contact face  1 . 1  or a proximal face is flush with the surface of the board  10 . 
     The sealing portion  3 . 1  does not extend to the proximal end of the joining element and the region of the proximal end  1  is designed as a fitting to be attached to the board  10  or a means for fixing such a fitting. 
     The energy concentrating elements  4 . 2  of the wall anchoring portion  3 . 2  are not axially extending ribs, but differently oriented ribs or hump-shaped elements (see also  FIGS. 2A and 2B ). 
     The joining element does not consist fully of the thermoplastic material, but comprises a core made of e.g. a metal. 
     The joining element is hollow (see also  FIGS. 3A ,  3 B and  4 ). 
     The object in which the joining element is anchored is not a wooden board  10  or a chip board but consists of another porous material (e.g. concrete, sand stone, metal foam, sintered ceramic or sintered metal) or it consists of a non-porous material and comprises, at least on the walls of the opening  5 , surface structures suitable for being interpenetrated by the liquefied thermoplastic material of the joining element. 
     The opening  5  reaches right through the board. 
     The axis A′ of the opening  5  is oriented at an angle other than a right angle relative to the surface of the board  10  and the border line  6  of the joining element lies in a plane at a same angle relative to the joining element axis A. 
     Finally, the surface of the object in which the opening is provided is not even but, curved and the border line  6  is similarly curved. 
       FIGS. 2A to 2C  illustrate in the same manner as  FIGS. 1A to 1C , a further exemplary embodiment of the joining element according to the invention and the anchorage thereof in an opening  5  provided in an object, e.g. in a wooden board  10 . As mentioned already further above, the joining element according to  FIGS. 2A to 2C  comprises a cylindrical mouth region  3  with a substantially square cross section (without sharp corners), the energy concentrating elements  4 . 2  of the circumferential surface of the wall anchoring portion  3 . 2  of the mouth region  3  are on the one hand humps tapering off to a tip and on the other hand, sharp edges of the square cross section forming axially extending ribs. The region of the proximal end  1  of the joining element consists substantially of a contact face  1 . 1  only, which contact face  1 . 1  terminates the sealing portion  3 . 1 , which has an axial length of at least 0.5, preferably at least 1 mm. The joining element is forced into the opening  5  to a depth such that the contact face  1 . 1  is flush with the surface of the object in which the opening  5  is provided. 
     The region of the distal end  2  of the joining element according to  FIGS. 2A to 2C  comprises a step-like reduction  2 . 1  in cross section which is adapted to a similar step  5 . 2  in the opening  5 . This results in anchorage not only in the wall of the opening  5  in the vicinity of the mouth  5 . 1  but also on the step  5 . 2  and possibly also on the bottom of the opening  5 . 
       FIGS. 3A and 3B  illustrate a further embodiment of the joining element according to the invention and anchorage thereof in a hollow core board  20 . Both  FIGS. 3A and 3B  are axial sections wherein  FIG. 3A  shows the joining element before being forced into the opening  5  while being vibrated and  FIG. 3B  thereafter. 
     The hollow core board  20  comprises a first outer layer  20 . 1  of e.g. wood, a second outer layer  20 . 2  of e.g. wood and a middle layer  20 . 3  consisting e.g. of a cardboard honeycomb. The first and second outer layers  20 . 1 ,  20 . 2  have a thickness of 3 mm, the middle layer  20 . 3  of 32 mm. The bore  5  penetrates the first outer layer  20 . 1  and the middle layer  20 . 3 . The joining element is to be anchored in the walls of the opening in the first outer layer  20 . 1  and preferably on the inner surface of the second outer layer  20 . 2 . The joining element comprises a central tube  2 . 1  reaching to the distal end  2  of the joining element. Within the central tube  2 . 1  an inner metal core piece (not shown) may be arranged having an inner thread for fixing a further object on the hollow core board  20 . 
     The joining element again comprises a cylindrical sealing portion  3 . 1  and a corresponding wall anchoring portion  3 . 2  which together form a collar on the central tube  2 . 1 . The sealing portion  3 . 1  has an axial extension of 1 mm and the wall anchoring portion  3 . 2  has an axial extension of 2.5 mm and its circumferential surface is equipped with recesses and axially extending ribs between the recesses, wherein the depth of the recesses is at least 0.2, preferably 0.3 mm. 
     The central tube  2 . 1  has a smaller diameter than the mouth region  3  and tapers off to an edge  21  constituting the distal end  2  of the joining element. 
     The joining element according to  FIGS. 3A and 3B  is positioned in the mouth of the bore  5 , wherein the dimensions of the joining element and the bore  5  are matched such that the wall anchoring portion  3 . 2  sits on the mouth  5 . 1  when the joining element is positioned but can be forced into the mouth  5 . 1  on application of vibration and force parallel to the axis A, and such that the edge  21  touches the inner surface of the second outer layer  20 . 2 , when the joining element is positioned. The joining element is then forced into the opening  5 , wherein the thermoplastic material is liquefied in the area of the edge  21  anchoring the joining element in the second outer layer  20 . 2  of the hollow core board  20  and in the area of the wall anchoring portion  3 . 2  anchoring the joining element in the walls of the opening through the first outer layer  20 . 1  of the hollow core board  20 . 
     For guiding the joining element and for enhancing anchorage in the second outer layer  20 . 2 , a blind or through bore may be provided therein, the axis of such bore being aligned with the axis of the bore in the first outer layer  20 . 1 . 
     The joining element is forced into the opening  5  until the contact face  1 . 1  thereof is flush with the outer surface of the first outer layer  20 . 1  of the hollow core board  20 , i.e. until the border line  6  is situated at 1 mm inside the mouth. Experiments show that on removal of the vibrating tool from the contact face  1 . 1 , the contours of the joining element and of the mouth as seen from the outer surface of the first outer layer  20 . 1  of the hollow core board  20  are not blurred at all, but are as clear cut as before the anchoring process. 
     The hollow core board  20  according to  FIGS. 3A and 3B  comprises a middle layer  20 . 3  comprising e.g. a honeycomb structure of card board. Other known middle layers of similar hollow core boards are foamed plastics and other similar materials. However, it is advantageous also to use the joining elements as sole spacers between the first and second outer layers  20 . 1  and  20 . 2  such that the middle layer  20 . 3  is constituted by nothing more than a plurality of such joining elements. If the board is relatively small all the joining elements may be positioned in locations where further objects (e.g. fittings, hinges, other hollow core boards) are to be fixed on the board. If the board is larger, it is advantageous to provide more joining elements, wherein some of the joining elements serve solely as spacers. 
     For manufacturing a hollow core board  20  whose middle layer  20 . 3  is constituted solely by joining elements as described above, prior to introduction and anchoring of the joining elements corresponding openings through the first outer layer  20 . 1  are to be provided and the two outer layers  20 . 1  and  20 . 2  are to be positioned at the desired distance from each other. 
     A hollow core board  20  whose middle layer  20 . 3  is constituted solely by joining elements as described further above may be used as a table top, wherein at locations at which the table legs are to be attached to the underside of the table top, the hollow core board  20  comprises joining elements being equipped with means for attaching the legs, e.g. with inner threads into which a treaded bolt can be screwed or with a suitable inner opening into which a corresponding protrusion of the table leg can be glued. Depending on the stiffness of the outer layers  20 . 1  and  20 . 2  of the hollow core board  20  forming the table top, on the locations of the leg attachment and on the overall size of the table top, additional inserts may be provided at other locations than the leg attachment locations or not. 
       FIG. 4  shows a last exemplified embodiment of the joining element according to the invention. The same as the joining element according to  FIGS. 3A and 3B , the joining element according to  FIG. 4  is particularly suitable for being anchored in the two outer layers  20 . 2 ,  20 . 3  of a hollow core board  20  such that the contact face  1 . 1  constituting the proximal end of the joining element is flush with the outer surface of the first outer layer of the hollow core board  20 . The region between the wall anchoring portion  3 . 2  and the distal end  2  comprises a central tube  2 . 1  and radial wings  2 . 2  extending radially into the recesses of the wall anchoring portion  3 . 2  and reaching axially beyond the distal end of the central tube  2 . 1  and having a thickness which gets smaller towards the distal end. The sealing portion  3 . 1  together with the wall anchoring portion  3 . 2 , whose circumferential surfaces are equipped in the same manner as described above for the joining element according to  FIGS. 3A and 3B  form together a collar on the central tube  2 . 1  and cover the wings  2 . 2  leaving the proximal end of the central tube  2 . 2  open. 
     When the joining element according to  FIG. 4  are anchored in a hollow core board  20  as substantially described in connection with  FIGS. 3A and 3B , a further object can be fixed to the hollow core board  20  by suitable means fitting into the central tube  2 . 1  of the joining element. It is also possible to use the proximal opening of the central tube for introducing a corresponding protrusion on a distal face of a vibrating tool and therewith enabling guidance of the joining element into the opening by the tool. 
     The joining element according to  FIG. 4  proves to be able to be anchored in a hollow core board  20  with a pull-out resistance which is in relation to the amount of material in the joining element superior to other joining element forms. 
     As mentioned already at the beginning, in addition to the embodiments in which the sealing portion  3 . 1  is part of the joining element as shown in  FIGS. 1 to 4 , it is also possible to carry out the method according to the invention with a joining element whose wall anchoring portion forms the proximal end and with a tool whose distal end is formed for the function of the sealing portion. Such embodiments of the method according to the invention are illustrated in  FIGS. 6 and 7 . For a comparison,  FIG. 5  illustrates in the same manner the method in which a joining element with sealing portion is used. 
     A tool  30  to be used in the method according to the invention is e.g. a sonotrode of an ultrasonic device, and comprises a distal face  31  adapted to the contact face of the joining element in a per se known manner and suitable for a smooth transfer of the vibration from the tool  30  into the joining element and for coupling a force from the tool  30  into the joining element. 
     According to  FIG. 5 , the joining element comprises in the above described manner a sealing portion  3 . 2  and a wall anchoring portion  3 . 1  which meet along a border line  6  in a mouth region  3  of the joining element. As the above described sealing function is taken over by the sealing portion  3 . 2  being part of the joining element, the distal face of the tool  30  does not need to have the same form and size as the proximal face of the joining element. It may be smaller as shown in  FIG. 5 . 
     According to  FIGS. 6 and 7 , the sealing function is taken over by a distal (or sealing) portion  30 . 1  of the tool  30 , which for being suitable for the sealing function needs to be formed as discussed further above for the sealing portion  3 . 1  of the joining element. According to  FIG. 6 , the distal face  31  of the tool  30  is positioned against the proximal end of the wall anchoring portion  3 . 2  of the joining element. The joining element and the tool  30  are to be forced into the opening until the border line  6  between the joining element or its wall anchoring portion  3 . 2  respectively and the tool  30  has reached a position beyond the mouth. This means that on removing the tool  30 , the proximal face of the joining element is positioned not flush with the object surface  33  but slightly deeper in the opening (at least about 0.5 mm). The same applies to the method as illustrated in  FIG. 7  with the difference that, in analogy to the embodiment shown in  FIG. 1E , a narrow border groove  6 . 1  is provided between the wall anchoring portion  3 . 2  and the distal or sealing portion  30 . 1  of the tool  30 , the groove portion being part of the joining element and carrying the proximal face of the joining element. For being suitable for carrying out the sealing function, the distal face  31  of the tool  30  needs to be larger than the portion of the groove by the groove depth. 
     Suitable thermoplastic materials for the joining element are e.g. ABS (acrylonitrile butadiene styrene), PA (polyamide), ASA (acrylonitrile styrene acrylate) or PS (polystyrene).
A joining element to be anchored in an object ( 10) with the aid of a thermoplastic material and mechanical vibration comprises a sealing portion ( 3.1) and a wall anchoring portion ( 3.2). The sealing portion ( 3.1) is substantially cylindrical and has a smooth circumferential surface. The wall anchoring portion ( 3.2) has a circumferential surface which comprises the thermoplastic material and energy directors. A cross section of the sealing portion ( 3.1) envelops the cross section (including energy directors) of the wall anchoring portion and is arranged adjoining to the latter on its proximal side. The joining element is anchored in an opening ( 5) provided in the object ( 10) wherein a cross section of the opening corresponds to the cross section of the sealing portion ( 3.1) of the joining element such that the latter can be forced into a mouth ( 5.1) of the opening ( 5) resulting in a press fit. For anchoring the joining element in the opening ( 5), mechanical vibrations e.g. Ultrasonic vibrations are coupled into the joining element and the latter is forced into the opening ( 5) until at least a distal part of the sealing portion ( 3.1) is positioned within the opening. Thereby, the thermoplastic material of the circumferential surface of the wall anchoring portion ( 3.2) is liquefied and pressed into the opening wall, wherein the sealing portion ( 3.1) prevents the liquefied material to penetrate through the mouth and therefore the mouth ( 5.1) remains clear cut, i.e. Does not get blurred by the liquefied material.