CA2283889C

CA2283889C - Method for anchoring joining elements in a material having pores or cavities, as well as joining elements for anchoring

Info

Publication number: CA2283889C
Application number: CA002283889A
Authority: CA
Inventors: Marcel Aeschlimann; Elmar Mock; Laurent Torriani; Heinz Koster
Original assignee: Woodwelding AG
Current assignee: Woodwelding AG
Priority date: 1997-03-21
Filing date: 1998-03-19
Publication date: 2008-09-16
Anticipated expiration: 2018-03-19
Also published as: NO994558D0; NO322961B1; JP2001524188A; ATE259945T1; NO994558L; JP4258745B2; AU6606298A; DK0968373T3; EP0968373B1; EP0968373A1; CA2283889A1; AU744974B2; DE59810798D1; ES2216273T3; WO1998042988A1; US6913666B1

Abstract

The invention concerns a connecting pin (3.2) with which two parts (1 and 2)-for instance, made of a porous material, particularly of wood or similar wood-like material-can be connected and anchored at predetermined anchoring positions (31, 33) in porous material. For this purpose, a bore (42) with a closed end (41) is placed in parts (1 and 2). The form of this bore (42) is adjuste d to the connecting pin (3.2) in such a way that the pin can be guided into the bore without effort and positioned in a first position so that pressure builds up at at least one predetermined anchoring position (31, 33) between the connecting p in (3.2) and the wall of the bore (4.2), if the connecting pin (3.2) is forced with a pressing force (F) into a second position deeper into the bore. Energy is directly focused onto the connecting pin so that the thermoplastic synthetic material of the connecting pin (3.2) is plasticized at the predetermined anchoring positions (31, 32). The locally plasticized synthetic material is pressed into the porous material of the parts by applying the local pressure and forms local, macroscopic anchorings (10, 20). The connecting pin (3.2) consists, f or instance, entirely of a thermoplastic synthetic material, and the energy for plasticizing is gained from the input of ultrasonic vibration .

Description

Doc No. 35-40 CA/PCT Patent METHOD FOR ANCHORING JOINING ELEMENTS IN A MATERIAL HAVING PORES OR
CAVITIES, AS WELL AS JOINING ELEMENTS FOR ANCHORING

The invention relates to a method used for anchoring joining elements in a material having pores or cavities, particularly in wood or woodlike materials (e.g. chipboards). The invention also relates to joining elements for use in the method. The joining elements anchored according to the method of the invention are more particularly used for producing constructions constituted by different parts or for fixing fittings.
According to the prior art parts made from wood or woody materials are interconnected e.g. using joining elements in the form of nails or screws, in that they are driven through one of the parts to be joined into the other part to be joined. Screws and nails are generally made from metal and have in surface areas of one of the parts to be joined a head and are at least frictionally or positively anchored in another of the parts to be joined. These pin-like joining elements constitute in wooden structures metallic, often corrosive foreign bodies, which can be prejudicial to working after the joining of the parts and which represent heat transfer bridges in the finished structures.

It is also known to join parts made from the fibrous materials, including wood and wooklike materials, to parts made from thermoplastics, in that the thermoplastic is plasticized at its surface facing the fibrous material part and the two surfaces are pressed onto one another. The plastic is thereby applied in plasticized form to the surface of the fibrous material or it is plasticized in the final position, e.g. by ultrasonic excitation.
In all these methods a joint is formed at the interface between the plastic and the fibrous material in the sense of a microscopic self-closure in that the plasticized plastics material is pressed into surface unevennesses of the fibrous material. Such methods are e.g. described in FR-2455502, FR-1495999, DE-3828340, or EP-269476. According to WO-96/01377 the plastics part can also be a dowel, which joins together two wooden parts. The same principle forms the basis for known methods for joining parts made from wood or woodlike materials, in which between the parts to be joined is placed a layer of a thermoplastics material, e.g. a paint layer and the parts are then pressed together and subject to ultrasonic action Doc. No. 35-40 CA/PCT - 2_ Patent (JP-52127937, WO-96/01377).

In all the aforementioned methods synthetic material and fibrous material are joined together by surface adhesion results from microscopic self-closure and this occurs in the same way in conventional bonding or adhesion processes. The above methods have many of the same disadvantages as bonding methods, particularly their sensitivity to moisture and thermal stresses, in which the two joined together surfaces expand to varying extents and considerable shear forces arise, which weaken or even destroy the joint.

The problem of the invention is to provide a method for anchoring joining elements, e.g. joining pins, in parts made from a material having pores or cavities, particularly in parts made from wood or woodlike materials, said method being based on the aforementioned methods for joining plastic/synthetic material and wooden parts, but in which the disadvantages thereof are at least reduced, i.e. in particular provides anchorings, which are more stable under thermal and/or moisture stresses and loads. The method must be simple, but still adaptable in specific manner to joining elements with different functions and to different material types. The method must also be performable with known means and tools and at a limited cost.
According to the method of the invention for an anchoring plasticized synthetic material is not only pressed into surface unevennesses as in the known methods, but instead into pores or cavities within the part in which the joining elements is to be anchored, so that a macroscopic anchoring is formed. This macroscopic anchoring is based on the penetration of the plasticized synthetic material into microscopic pores of the material and to the formation of a type of composite comprising the original porous material and the synthetic material which has penetrated it or to macroscopic self-closure, which results from the fact that the plasticized synthetic material is pressed into macroscopic cavities.

According to the method of the invention in the part in which the joining , , , ~ ~ =~ .

element is to be anchored an opening with a closed end, e.g. a bore (blind hole) is formed and then in said bore is positioned the joining element, which is wholly or partly made from a thermoplastic material.

The shapes of the bore and the joining element are so matched to one another that the joining element, without force expenditure, can be intro-duced into the bore up to a first position and that, if it is driven with the aid of a pressing force parallel to the bore axis from said first posi-tion towards the closed end of the bore into a second, final position, pressure builds up at at least one predetermined anchoring point between the joining element and the bore wall, whereas no pressure arises at other points.

Simultaneously with the pressing in of the joining element from its first into its second position in the bore or immediately prior thereto energy is supplied to the joining element in such a way that the plastics material is locally plasticized at the aforementioned, predetermined anchoring points, where the pressure is concentrated. Such a planned, local plasti-cizing can e.g. be achieved in that:

- the joining element is entirely made from a thermoplastic material or at least in areas of the predetermined anchoring points has at least surface areas made from a thermoplastic material and in that such a joining element is subject to ultrasonic or some other appropriate vibration action for the supply of energy, so that at the pressure concentration points (predeter-mined anchoring points) the greatest friction and therefore the maximum heat is produced and consequently the thermoplastic material is locally plasticized (related to joining methods such as ultrasonic, friction, vibration or orbital welding);

- in that the joining element, in areas of the predetermined anchoring points, has at least surface areas of a thermoplastic material, which is plasticizable at a lower temperature than the remaining materials of the joining element and in that such a joining element is heated by heat supply;
- in that the joining element, in areas of the predetermined anchoring = 1 1 i ~

points, has at least surface areas of a thermoplastic material, in which are incorporated metal particles and in that such a joining element is inductively heated.

On its inside which, in the closed bore is directed towards the closed end of the bore, the joining element has at least one first, predetermined anchoring point. On its opposite outside, i.e. on the side projecting from the bore or positioned in the bore opening, the joining element has a thickening serving as a head or a means for fixing a further part or has further anchoring points spaced from the first anchoring point towards the outside. It is also conceivable to have pin-like joining elements with several predetermined anchoring points.

Whilst the joining element is pressed in the second, final position in the bore and energy is simultaneously supplied thereto, at the predetermined anchoring points where a high pressure occurs between the joining element and the bore wall, the joining element material is plasticized and as a result of the pressure at these points is pressed into the bore wall or into pores or cavities in the material, which is adjacent to the bore, whereas it remains unchanged at other points.

To ensure that the plasticized synthetic material is pressed by the pres-sure produced at the anchoring points into the bore wall, it must have a porosity or openings/cavities or the bore wall must be created in such a way that as a result of the pressure formed pores or cavities are produced in which the plasticized material can be pressed. Porous materials suit-able for anchorings according to the method of the invention are in partic-ular wood or woodlike materials, but also sandstone, ceramic materials, burnt brick or concrete, etc. Cavities suitable for producing anchorings according to the invention open substantially transversely to the bore axis and are in particular found in lightweight structural components.

The attainable depth of the anchoring of a synthetic material in a porous material is dependent on its structure (e.g. for wood on the density of the wood fibres), but also on the pressure applied and the locally available plasticized material quantity. As will be shown hereinafter, in solid wood = '. ~ = , ~ . ' .

e.g. anchoring depths of 1 to 4 cm are readily attainable.

The desirable depth of the anchoring of the synthetic material in a porous material is dependent on the loadability of the material and can be con-trolled via the quantity of material to be pressed in and/or via the applied pressing force level. The shape of the anchoring can be substan-tially controlled by a corresponding matching of the bore shape and the joining element shape. Thus, using the method according to the invention it is possible to produce anchorings, which are specifically adapted to the character of the material (e.g. wood type, orientation of the graining or density gradient relative to the orientation of the bore, etc.) in which the anchoring is to be produced and to the function and loading to be absorbed by the joining element.

The most advantageous bore and joining element shapes for a specific appli-cation, as well as the pressing force level and the quantity of energy to be supplied to the joining element are to be established experimentally in each specific case.

The inventive method for anchoring joining elements in a part made from a porous material, particularly wood or a woodlike material, or a material having suitable cavities and different types of joining elements are des-cribed in greater detail hereinafter relative to the attached drawings, wherein show:

Fig. 1 A specific method variant for anchoring a joining pin with a head in one wooden part for joining two wooden parts.
Fig. 2 Another exemplified method variant for anchoring a joining pin in two wooden parts to be joined.

Figs. 3 to 5 Three exemplified embodiments of the anchoring point in areas of the closed bore end.

'= , , . ~ ' , ' .

Fig. 6 An example of a fitting, which is fixed with the aid of a plurality of joining pins anchored in a wooden part to said part.

Fig. 7 Another exemplified method variant for anchoring a joining element, which has an internal thread for fixing further parts.

Fig. 8 An example of an anchoring according to the invention in a lightweight component with cavities.

In all the drawings the joining elements, bores and anchorings are shown in section along the bore axis.

Fig. 1 shows as the first, exemplified variant of the inventive method an anchoring of a pin-like joining element with head (joining pin 3.1) in a first, wooden part 1 for joining said first part 1 to a second part 2.1, which is e.g. also made from wood.

In the vicinity of its inner end the joining pin 3.1 has a predetermined, first anchoring point 31 and at its outer end a head 32. The bore 4.1 which passes entirely through the part 2.1 and has in part 1 a closed end 41 is less deep than the joining pin 3.1 is long and has at its open end e.g. a widened depression for countersinking the head 32. The cross-section of the bore 4.1 is so matched to the cross-section of the joining pin 3.1, that without force expenditure the pin can be introduced into the bore up to the closed end 41 thereof. This is the first position of the joining pin 3.1 in the bore 4.1.

From the first position the joining pin 3.1 is pressed further into the bore 4.1 with a pressing force F oriented substantially parallel to the bore axis. The only point at which the pressing force F gives rise to a pressure between the joining pin 3.1 and the wall of the bore 4.1 is the area of the closed bore end 41. If in the above-described manner, by supplying energy to the joining pin it is ensured that during pressing in the material of the joining pin is only plasticized at this point, only at '. ~ . ' . = ' ' this point is there an anchoring 10 of the joining pin in the part 1 to be joined.

In the drawing this anchoring 10 is shown as a synthetic material area, but in fact is constituted by an intimate mixture of wood fibres and synthetic material, which can be likened to a composite material and which can e.g. be of pinewood, whose graining is oriented parallel to the bore axis, where it has a depth of up to 2 cm.

The length of the joining pin 3.1, the depth of the bore 4.1, the force F
and the quantity of energy to be supplied are so matched to one another that the anchoring fulfils the desired strength conditions and that the two parts are firmly fixed together between the joining pin head 32 and the anchoring 10.

The joining pin 3.1 of fig. 1 is anchored in the part 1 by the anchoring point 10, which is only possible in a part made from a porous material, particularly wood or a woodlike material, or, if the bore wall material has suitable cavities in the vicinity of the anchoring point or if the pressure exerted on the joining pin produces such openings in the bore wall.

Part 2.1 can also be made from wood or some other, non-porous material (metal, plastic). As shown in fig. 1, the head 32 can be a component of the joining pin. =However, the head can also be placed on the joining pin after producing the anchoring, e.g. can be screwed into a thread provided in the joining pin. The head 32 can be in any random form and can e.g. also represent a fitting with a specific function.

An advantage of joining two parts in the manner shown in fig. 1 compared with other joining methods, in which plasticized materials are used as joining materials, is that in all cases it is possible to prevent plasti-cized material being pressed into the gap between the parts 1 and 2.1 to be joined resulting in the pressing apart thereof. This is prevented in that in the areas of such a gap no pressure is built up and the material of the joining pin is not plasticized in said area.

. , .

-s-If the energy to be supplied to the joining pin is supplied in the form of ultrasonic waves, in the manner shown the joining pin 3.1 must be made from a thermoplastic material in the area of its inner end to be positioned, as shown, on the closed bore end 41. The remainder of the pin can be made from the same material or a different material.

If the energy to be supplied to the joining pin 3.1 is supplied in the form of heat, in the vicinity of the anchoring point it is constituted by a plastics material, which is plasticizable at a lower temperature than the material from which the joining pin is made in other areas. It is also conceivable in such a case for the joining pin to have a "core" of a heat conducting material, e.g. metal, by means of which core the heat to be supplied to the joining pin can be conducted against the anchoring point.
If the energy to be supplied to the joining pin is supplied inductively, the thermoplastic material of the predetermined anchoring point 31 contains incorporated metal particles.

Fig. 2 shows as a further exemplified variant of the inventive method an anchoring of a pin-like joining element (joining pin 3.2) in two parts 1 and 2.2 to be joined together and which are e.g. of wood, the joining pin 3.2 being anchored in both parts 1 and 2.2 (anchorings 10 and 20).

Like the joining pin 3.1 of fig. 1, the joining pin 3.2 has a predetermined, first anchoring point 31 at its inner end to be inserted in the bore. It also has a predetermined, second anchoring point 33, which is in the form of a step-like cross-sectional reduction and on the pin is positioned where it is located in the second part 2.2 to be joined.

The bore 4.2 has a cross-sectional reduction 42 corresponding to the cross-sectional reduction on the joining pin 3.2 and on it rests the joining pin in its first position. If the joining pin 3.2 is pressed by the pressing force F more deeply into the bore 4.2, pressure builds up not only in the vicinity of the closed end 41 of the bore 4.2, but also in the area of the cross-sectional reduction 42, is pressed through the plastics material plasticized at this point into the wall of the bore 4.2 and consequently '. , = ' . = =

forms a second anchoring 20.

The bores 4.1 and 4.2 of figs. 1 and 2 advantageously have a circular cross-section. The joining pins 3.1 and 3.2 can also be circular. However, they can also have some other cross-section fitting into the corresponding bore. For example, in the vicinity of its smaller cross-section, the join-ing pin 3.2 can be circular and in the area of its larger cross-section can have an angular cross-section (e.g. square), only the areas of the edges resting on the step 42.

In both figs. 1 and 2 the closed end 41 of the bore is shown flat and the joining pin in its first position rests with a flat face in the bore. With such a shaping of the bore and joining pin, on pressing in the pin, a substantially uniform pressure builds up over the entire face. The plast-icized material is mainly driven into the wood parallel to the longitud-inal axis of the joining pin, so that the cross-section of the anchoring 10 is only slightly larger than the cross-section of the joining pin.

Such a construction of the predetermined, first anchoring point is advan-tageous for applications in which, in the vicinity of the first anchoring 10, the wood graining is oriented parallel to the bore axis and the wood of part 1 splits in the case of a limited displacement. Roughly the same effect is obtainable with a pin end tapered to a point, which in its first position rests on=a roughly identically tapering bore end.

Figs. 3 to 5 show further embodiments advantageous for specific applica-tions of predetermined, first anchoring points 31 on e.g. pin-like joining elements 3 and cooperating, closed ends 41 of bores 4, which particularly in the case of ultrasonic application leads to different anchorings 10.
Fig. 3 shows in two variants one end of a joining pin 3, which is placed in a closed end of a bore 4. In both cases the pin end is pointed and specifically more sharply than the bore end. As a result the pressure arising on pressing the joining pin 3 into the bore 4 is centrally concen-trated, so that the material is to an even greater extent pressed parallel to the pin axis into the part 1, so that also here the resulting anchoring extends more in the axial direction than at right angles thereto. The strength of such an anchoring is more particularly due to an enlargement of the shear-loaded surfaces in the wood.

Fig. 4 shows one end of a pin 3 having a concave shape. On pressing this pin into a bore with a flat or pointed, closed end, the pressure mainly builds up at radial positions, which gives rise to an anchoring 10, which extends to a greater extent transversely to the pin axis. Such an anchor-ing is more particularly suitable for a part 1, where the graining is at right angles to the pin axis, or for an anchoring in a chipboard, whose surface is at right angles to the pin axis. The strength of such an anchor-ing results more particularly from the self-closure obtained between the wooden part and the joining pin.

Fig. 5 shows another embodiment of the predetermined, first anchoring point 31 on a joining pin 3 and a corresponding bore end 41. It is a first anchoring point having substantially the same construction as the second anchoring point of fig. 2. Bore 4 has a step-like cross-sectional reduc-tion 43, on which is mounted the pin in its first position. If this joining pin is pressed into the bore, more particularly a pressure is built up radially in the vicinity of the bore end and the plasticized material is pressed into the wood, more particularly transversely to the pin axis.

Fig. 6 shows a part 5 made from a random material, which is fixed to a part 1 with the aid of pin-like joining elements 3, which are anchored in said part 1 e.g. made from wood and in accordance with the method of the inven-tion. The part 5 is a fitting (e.g. a hinge part), e.g. made from plastic.
The two joining pins 3 are shaped onto the part 5 or are joined thereto in some other appropriate way and are driven in the described way into bores of part 1 and anchored therein. Here again, as mentioned in conjunction with fig. 1 concerning a joining pin head, the part 5 can have a random shape and, even after producing the anchoring, can be appropriately mounted on the joining pin or pins 3.

Fig. 7 shows the production of a connection of a wooden part 1 with a fitting part 6, e.g. made from metal, by means of an inventive anchoring of a joining element 3.3 in part 1 and the fixing of the fitting part 6 to the anchored joining element 3.3. The joining element 3.3 has a two-step, first anchoring point 31 and is introduced into a bore 4.3 with a step-like, narrowing base. On pressing the joining element into the bore and during the simultaneous plasticizing of the anchoring point 31, bore 4.3 and joining element 3.3 act as explained in conjunction with fig. 4. A corres-ponding "two-step" anchoring 10 is formed.

The joining element 3.3 of fig. 7 has at its outside, facing the predeter-mined anchoring point, as the means for fixing a further part an internal thread 34, into which is screwed the fitting part 6 after anchoring the joining element 3.3 in part 1.

Fig. 8 shows the result of a further, exemplified embodiment of the inven-tive method, namely an anchoring of a joining element 3.4 in a part 1, which is a lightweight constructional component with cavities 11. The closed bore necessary for the method according to the invention and into which is introduced the joining element 3.4, is in this case a through bore 4.4 through one of the outer layers 1.1 of part 1. This through bore 4.4 is closed by a further element, e.g. by an inner layer 1.2 or option-ally by the facing, outer layer 1.3, in such a way that between the through bore 4.4 and the element closing it opens a cavity area 11.1 extending substantially at right angles to the bore axis or is produced by the pres-sure of the joining element 3.4 on the bore-closing element, e.g. by a corresponding deformation of the inner layer 1.2.

The joining element 3.4 is inserted in the bore 4.4 and is positioned by the bore-closing element (e.g. inner layer 1.2). The joining element 3.4 is then pressed against the bore-closing element and simultaneously the plastics material is plasticized in the area of this element and is pressed into the cavity area 11.1, existing or produced between the outer layer 1.1 and the bore-closing element, so that a macroscopic anchoring 10 is obtained.

As has been stated, the method according to fig. 8 is particularly suitable for lightweight constructional applications where, in place of solid . . .

material, use is made of thin, board-like material carried by a support structure (indicated by the two laths 20). The outer layers 1.1 and 1.3 are e.g. thin solid wood boards or coated chipboards. The element closing the bore 4.4 can e.g. be a plastic or metal inner layer 1.2 extending from a lath 20 to a neighbouring lath 20 or over the entire surface of the first layer, or can be a differently shaped element locally and specifically integrated for this purpose into the cavity 11.

The joining element 3.4 according to fig. 8 is e.g. suitable for fixing fittings to lightweight structural components.

Thermoplastic materials for use in joining elements advantageously have a high mechanical strength, particularly a high tensile strength and a high modulus of elasticity. Polyamides, polycarbonates or polyester carbonates are particularly suitable. For increasing the strength the plastics mater-ial of a joining element can e.g. also contain glass or carbon fibres.
Further thermoplastics suitable for joining elements are acrylonitrile-butadiene-styrene, styrene-acrylonitrile, polymethylmethacrylate, polyvinyl chloride, polyethylene, polypropylene and polystyrene.

An exemplified joining pin for joining two wooden parts, as shown in fig.. 2, e.g. has the form shown in fig. 2 and is made entirely from acrylonitrile-butadiene-styrene. It has a smaller, circular cross-section with a dia-meter of 8 mm and*a larger, circular cross-section with a diameter of 10 mm.
It is 60 mm long and is centrally provided with the cross-sectional reduc-tion. The corresponding bore is 40 mm deep and has the step corresponding to the cross-sectional reduction of the pin at a height of 30 mm. The pin is inserted in the bore and is pressed into it for 5 sec. with a pressing force of 2000N and ultrasonic excitation with an amplitude of approximately 44 um. Subsequently the pin end is flush with the wood surface.

Claims

1. A method for anchoring a joining element in a part consisting of porous material, the joining element having a distal end and a proximal end and including a thermoplastic material at least at said distal end, the method comprising:
forming a bore in the part, the bore having an inner closed end and being matched to the shape and dimensions of the joining element to support insertion of the joining element within the bore;

inserting the joining element in the bore such that the distal end of the joining element is disposed against the inner closed end of the bore;

applying pressure to the proximal end of the joining element to force the joining element deeper into the bore, the pressure being applied approximately along a central axis of the bore and producing an increase of pressure of the distal end of the joining element against the inner closed end of the bore; and, during the application of pressure, applying vibration energy to the joining element to cause the thermoplastic to plasticize at the distal end of the joining element, the pressure causing the plasticized thermoplastic material to flow into pores of the part beyond the inner closed end of the bore in the axial direction of the bore, thereby forming a macroscopic anchoring connection between the part and the distal end of the joining element.

2. A method according to claim 1 including joining a second part made of a porous material to the first mentioned part with the joining element, wherein the joining element is a joining pin having a reduction in diameter intermediate the ends thereof forming a first shoulder comprising the thermoplastic material, wherein the step of forming a bore includes forming a portion of the bore through the second part and into the first part to the inner closed end, the bore in the second part having a reduction in diameter forming a second shoulder matching the first shoulder of the joining pin, and the step of positioning includes inserting the joining pin into the first and second parts so that the first and second shoulders are in contact with each other, the contacting first and second shoulders forming a second macroscopic connection between the second part and the joining element.

3. A method according to claim 1 including joining a second part made of a porous material to the first mentioned part with the joining element, wherein the joining element is a joining pin, wherein the step of forming a bore includes forming a portion of the bore through the second part and into the first part to an inner closed end, and wherein the joining pin has an enlarged head portion on the proximal end thereof.

4. A method according to claim 1 including fixedly attaching the joining element to a second part.

5. A method according to claim 4 wherein the step of fixedly attaching is performed after positioning the joining element in the bore.

6. A method according to claim 5, wherein for fixedly attaching the second part, the joining element comprises at its proximal end an internally threaded opening for receiving an attachment of the second part.

7. A method according to claim 1 wherein the step of applying vibration energy includes ultrasonically exciting the joining element to cause the thermoplastic to plasticize.

8. A method according to claim 1 wherein the joining element consists entirely of thermoplastic material capable of being plasticized.

9. A method according to claim 1 wherein the part comprises wood or a woodlike material.

10. A method according to claim 1 wherein the part comprises at least one of sandstone, porous ceramic, burnt brick or concrete.

11. A method according to claim 1, wherein the distal end of the joining element is shaped with a point.

12. A method according to claim 1, wherein the distal end of the joining element is flat or concave.

13. A method according to claim 1, wherein the thermoplastic material is selected from the group consisting of polyamide, polycarbonate, polyester carbonate, acrylonitrile-butadiene-styrene, styrene-acrylonitrile, polymethylacrylate, polyvinyl chloride, polyethylene, polypropylene and polystyrene.

14. A method according to claim 1, wherein the joining element further comprises another material that is different from the thermoplastic material.