Abstract
A method of fastening a second object to a first object includes: providing the first object with an attachment surface; providing the second object; placing the second object relative to the first object, with a resin composition in between the attachment surface and the second object, wherein the resin composition has a resin having a first viscosity and being in a flowable state; pressing the first and second objects against each other and causing mechanical vibration to act on at least one of the objects until the resin composition experiences a vibration induced activation, which includes at least one of reduction of the viscosity of the resin compared to the first viscosity and activation of particles dispersed in the resin. The pressing and mechanical vibration are continued or repeated until the resin has at least partially cross-linked and the viscosity of the resin is increased compared to the first viscosity.
Claims
1. A method of fastening a second object to a first object, the method comprising the steps of: providing the first object comprising a first attachment surface; providing the second object; placing the second object relative to the first object, with a resin composition in between the first attachment surface and the second object, wherein the resin composition comprises a resin, the resin composition having a first viscosity; causing mechanical vibration to act on the second object or the first object or both, until the resin composition is subject to a vibration induced activation, wherein the activation comprises at least one of reduction of the viscosity of the resin composition compared to the first viscosity, and of an activation of at least one element at least partially environed by the resin, continuing or repeating the step causing mechanical vibration to act until the resin has at least partially cross-linked and the viscosity of the resin is increased at least locally compared to the first viscosity. whereby the resin composition secures the second object to the first object.
2. The method according to claim 1, wherein the at least one element is a thermoplastic and/or elastomeric element, and wherein the mechanical vibration causes vibration energy to be absorbed by the element, whereby the element transfers heat to surrounding resin material.
3. The method according to claim 1, wherein the at least one element comprises a substance capable of undergoing a first-order phase transition at a temperature above room temperature, and wherein the vibration induced activation comprises causing at least portions of the material to undergo the first-order phase transition.
4. The method according to claim 3, wherein the material capable of undergoing the first-order phase transition is a thermoplastic material.
5. The method according to claim 3, wherein the material capable of undergoing the first-order phase transition is a phase change material.
6. The method according claim 1, wherein the resin composition comprises a plurality of the elements being the particles.
7. The method according to claim 6, wherein the particles have an at least approximately spherical geometry.
8. The method according to claim 6, wherein an average diameter of the particles is between 10 m and 100 m.
9. The method according to claim 6, wherein the particles have an elastic modulus that differs from the elastic modulus of the resin after cross linking by at most a factor 2.
10. The method according to claim 6, wherein at least the surface of the particles is capable of reacting chemically with the resin.
11. The method according to claim 6, wherein the particles comprise a polyamide polymer.
12. The method according to claim 1, wherein the at least one element at least partially environed by the resin has thermoplastic properties.
13. The method according to claim 1, wherein the at least one element at least partially environed by the resin comprises an auxiliary element, and wherein in the step of placing the second object and/or the step of causing mechanical vibration to act comprises pressing the first and second objects against each other and using the auxiliary element as a distance holder in the step of pressing.
14. The method according to claim 13, wherein the auxiliary element has thermoplastic properties and forms at least one energy directing structure.
15. The method according to claim 1, wherein the second object has a second attachment surface, wherein in the step of placing, the resin composition is between the first and second attachment surfaces, and wherein the first attachment surface or the second attachment surface or both has/have an attachment structure with at least one of attachment protrusions, attachment indentations, a macroscopic surface roughness.
16. The method according to claim 15, wherein the attachment protrusions and/or attachment indentations are undercut.
17. The method according to claim 1, wherein the at least one element at least partially environed by the resin comprises a plurality of particles dispersed in the resin.
18. The method according to claim 1, wherein the resin prior to the step of causing mechanical vibration to act is pre-polymerized.
19. The method according to claim 1, wherein the resin, at least for some time while the mechanical vibration is caused to act, is thixotropic.
20. The method according to claim 1, wherein the resin composition comprises an additive that reduces the viscosity due to the shear rate, or due to a low glass transition and/or liquefaction temperature.
21. The method according to claim 1, wherein the activation comprises a reduction of the viscosity of the resin compared to the first viscosity, and wherein the resin composition further comprises abrasive particles.
22. The method according to claim 1, wherein the resin comprises particles as the elements, the particles containing an activation component, and wherein pressing and causing mechanical vibration to act causes the activation component to be dissolved in the resin.
23. The method according to claim 22, wherein the activation component is capable of activating at least one further substance contained in the resin composition, for example the resin.
24. The method according to claim 23, wherein the activation component comprises at least one of a hardener, an initiator substance, a gas forming substance.
25. The method according to claim 22, wherein the activation component comprises a substance capable of impinging on the first attachment surface and/or a second attachment surface of the second object.
26. The method according to claim 25, wherein the activation component comprises at least one of a solvent, a primer, an etchant.
27. The method according to claim 1, comprising pressing the second object and the first object against each other while the mechanical vibration is caused to act.
28. The method according to claim 1, wherein the resin composition comprises particles capable of forming a self-stabilizing particle network.
29. The method according to claim 1, wherein a vibration power of the mechanical vibration is caused to be modulated, wherein in an initial phase the vibration power is lower than in a subsequent phase.
30. A resin composition for serving as an adhesive between a first and a second object, the resin composition comprising a resin and being activatable by mechanical vibration energy, and the resin composition further comprising at least one of; abrasive particles dispersed in the resin, wherein the resin is equipped for its viscosity being reduced upon activation by mechanical vibration energy; particles containing an activation substance, wherein the substance is capable of being dissolved in surrounding resin composition material upon mechanical vibration acting on the resin composition; thermoplastic particles dispersed in the resin; particles capable of forming a self-supporting network dispersed in the resin; an additive causing the resin composition to be thixotropic.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0108] In the following, ways to carry out the invention and embodiments are described referring to drawings. The drawings are schematical. In the drawings, same reference numerals refer to same or analogous elements. The drawings show:
[0109] FIG. 1, in section, an arrangement of a first object, a second object and a sonotrode;
[0110] FIG. 2 a development of the viscosity during the process according to an embodiment;
[0111] FIG. 3 a development of the diffusion during the process according to an embodiment;
[0112] FIG. 4 a resin composition with vesicles;
[0113] FIGS. 5 and 6 a resin composition with abrasive particles during two different stages of a process;
[0114] FIG. 7 an arrangement of relatively large first and second objects;
[0115] FIG. 8 a further arrangement of a first object, a second object and a sonotrode;
[0116] FIGS. 9-11 further resin compositions;
[0117] FIG. 12 a further arrangement of a first object, a second object, and a resin composition portion;
[0118] FIG. 13 a temperature-vs.-time diagram;
[0119] FIG. 14 a process diagram;
[0120] FIGS. 15 and 16 sections through an assembly of a first object, a second object and a sonotrode, with a resin bead being dispensed between the first and second objects;
[0121] FIG. 17 an example of a second object;
[0122] FIG. 18 a section through an arrangement with a structured particle serving as an auxiliary element;
[0123] FIGS. 19-21 top views of embodiments of structured particles;
[0124] FIG. 22 a structured particle with a guiding nipple;
[0125] FIGS. 23-25 sections illustrating measures for confining the resin composition;
[0126] FIGS. 26 and 27 sections through arrangements with an attachment structure;
[0127] FIG. 28 a section through an auxiliary element; and
[0128] FIGS. 29 and 30 alternative attachment structures.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0129] FIG. 1 shows, in section, an arrangement of a first object 1, and a second object 2, with a resin composition portion 3 therebetween. The first object in the depicted embodiment is a fiber composite part 1 hat has a structure of fibers embedded in a matrix of hardened resin. The structure of fibers is locally exposed at a first attachment surface portion of the surface of the first object, for example by matrix material being removed. The resin composition portion 3 is applied to the exposed part of the surface.
[0130] In the depicted embodiment, the first object comprises a fiber composite material at least at the first attachment surface. However, other surfaces with suitable physical (roughness, porosity) and/or chemical properties are suitable as well. Especially, suitable first object and/or second materials include metals, ceramic materials, wood or wood-based material, other plastic materials than fiber composites, etc., all with or without surface roughening.
[0131] For illustration purposes, in all depicted examples, the first object is shown to have a general flattish shape. All examples of the invention are, however, also applicable to first objects that are not flattish but have any other shape.
[0132] Also the second object may have any shape, as long as a common attachment interface comprising a first attachment surface and a second attachment surface is formed. Especially, in embodiments the second object may be a connector comprising a fastening structure for fastening a further object to the second object and thereby to the first object.
[0133] The second object may have any material suitable for the specific purpose of the second object and further for an adhesive connection with the first object via the resin. For example, the second object may comprise at least on of a metal, a ceramic, a polymer based material, for example a composite, etc. Especially, in embodiments the second object may comprise a fiber reinforced composite, especially with fiber exposed at the second attachment surface. Other surfaces with suitable physical (roughness, porosity) and/or chemical properties are suitable as well.
[0134] The second object is illustrated to have a distinct structure on a distal side thereof for example a plurality of indentations, for example channels. The distal surface of the second object forms a second attachment surface of the configuration.
[0135] The second object may for example be a fastener for fastening a further object to the first object.
[0136] For fastening the second object and the first object to each other, a sonotrode 6 is used to press the second object against the first object, with the resin composition portion 3 between the parts, while mechanical vibration is coupled via the sonotrode into the second object 2. It has been found that the mechanical vibration has a double effect: Firstly it causes the resin to become well distributed and to completely wet/interpenetrate and if applicable embed any structure on the attachment surfaces, thereby cause the resin to penetrate into such structure relatively deeply. Secondly, the mechanical vibration energy is primarily absorbed at the interface between the first object and the second object and in the resin, thereby stimulating the curing process.
[0137] In FIG. 1, the resin composition 3 is illustrated to be disposed as a portion applied to the first attachment surface, for example by an according dispensing tool immediately prior to the activation process. Alternatively, especially if the viscosity is initially very high, the resin composition could be provisionally secured to the second and/or first attachment surface in a separate step any time prior to the activation process, or could be present as separate strand or sheet of material.
[0138] According to a group of embodiments, the resin composition has a viscosity that is initially relatively high (for example, the resin composition may be pasty or rubber-like/waxy) and that is reduced as a result of the activation. FIG. 2 shows an according graph of the viscosity as a function of time. The viscosity 11 is relatively constant prior to the activation since the resin composition does not undergo any chemical transition or only a comparably slow chemical transition (for example a cross-linking) prior to the activation. After the onset 12 of the activation the viscosity firstly drops to a value at which the flowability is sufficient for the resin composition to interpenetrate structures of the first and/or second object. Thereafter, due to the initiated cross-linking, the viscosity rises again until the resin composition is sufficiently hardened to fasten the first and second objects to each other.
[0139] Generally, in embodiments, the viscosity drops by at least an order of magnitude (by at least a factor 10), and for example a plurality of orders of magnitude (by at least a factor 100) by the effect of the activation by the mechanical vibration.
[0140] The diffusion 21 of any particle or substance within the resin composition will be relatively low initially and substantially rise after the onset of the vibration, as shown in FIG. 3.
[0141] FIG. 4 shows a resin composition 3 with a resin embedding particles 71, for example vesicles filled by a substance or droplets of a substance. Since the particles are distributed within the resin, the approach according to the invention has a double effect when the substance within the particles is to be distributed in the resin: [0142] Firstly, because the substance is present in particles distributed within the resin, which particles will dissolve/disintegrate by the effect of the vibration, the necessary length of the diffusion paths for the substance to be approximately equally distributed within the composition will be lower than if the substance was present at a surface of the resin only. [0143] Secondly, as illustrated in FIG. 3, the diffusion itself will, due to the approach described in this text, be initially higher.
[0144] Examples for substances contained in the particles comprise a substance that activates the resin/resin composition itself and/or comprise a substance that impinges on the first and/or second attachment surface, as described hereinbefore.
[0145] An embodiment that uses the effect of a viscosity behavior as illustrated in FIG. 2 is depicted in FIGS. 5 and 6. The resin composition comprises, in addition to the resin, abrasive particles 77 dispensed in the resin, which resin is pre-polymerized to be in a solid/waxy state. At least some of the abrasive particles form part of the surface and come into contact with the first and/or second attachment surface at the beginning of the process. When the mechanical vibration starts impinging, the still relatively solid (high viscosity) resin composition will transmit vibration, and the abrasive particles will be held in the resin matrix and by the vibration impinge on the first/second attachment surface. Thereby, an initial phase of the vibration application becomes a preparatory step (FIG. 5).
[0146] After the resin becomes sufficiently flowable, the particles will be pressed into the interior of the composition and will remain dispensed therein. The resin composition is bonded to the then roughened surface.
[0147] FIG. 7 very schematically illustrates a possible application of embodiments of the invention. A first object 1 and a second object 2 are to be bonded to each other by an adhesive connection, wherein the first and second objects are both relatively large. In a manufacturing process, the hardening of the adhesive between the objects until the bond is sufficiently strong for further manufacturing steps may cause a significant delay. The approach according to embodiments of the invention is therefore to use the fastening method described herein at a plurality of discrete spots 81 to activate the resin at these spots. Thereby, the bond is caused to be sufficiently stable in a rapid process. The resin portions between the discrete spots 81 may harden slowly thereafter while the assembly of the first and second objects is subject to further processing steps.
[0148] FIG. 8 shows an arrangement immediately prior to the activation step. The second object 2 is a fastener having an anchoring plate 151 and a fastening element 152, here being a threaded bar, secured thereto. In the embodiment of FIG. 8, the sonotrode comprises a receiving structure cooperating with the fastening element to mechanically couple the sonotrode and the second object with each other.
[0149] The first object 1 may be of any nature. In FIG. 8, it is illustrated to be a metal sheet.
[0150] The resin composition 3 is present as a coating of the second object, in FIG. 8 of the anchoring plate thereof. If the resin composition has a comparably high viscosity, for example so that it is waxy, at room temperature, it may be essentially inactive, so that the second object may even be stored with the resin composition 3 pre-applied.
[0151] FIG. 9 depicts an example of a resin composition 3 with activatable particles 73 dispersed in the resin 72, which particles are thermoplastic. When the vibration energy impinges on the composition, the thermoplastic particles will tend to absorb mechanical vibration energy and thereby induce a heating of the surrounding resin to activate the resin. Also, the thermoplastic material may have a further function, for example by contributing to the mechanical properties of the resin composition after the activation process, for example by adding a certain ductility.
[0152] FIG. 10 shows a variant of the resin composition of FIG. 9 in which variant the thermoplastic particles 73 have a size corresponding to the final thickness of the resin composition layer. Thereby, the thermoplastic particles 73 have a double function: [0153] During the step of pressing the second object and the first object against each other, they serve as distance defining spacers. [0154] They absorb mechanical vibration energy thereby activating the surrounding resin by heat. In contrast to the embodiment, mechanical vibration is coupled directly from the second/first object into the thermoplastic particles 73, whereby the concept is independent of the vibration transmitting properties of the resin 72.
[0155] A further possible function, depending on the structure of the first and/or second object is a contribution to the anchoring as explained hereinafter referring to FIG. 26.
[0156] FIG. 11 shows an example of a resin composition 3 comprising particles 74, for example of glass or ceramic, that form, in the resin environment, a self-stabilizing particle network at least when composition 3 is compressed between the first and second objects. Thereby, when mechanical vibration is coupled into the resin composition, friction in the regions 75 between the particles generates heat, whereby the resin is activated.
[0157] Particle materials that are particularly suited for heat transmission/heat conduction comprise diamond, graphite, carbon(mono), aluminum nitride, boron nitride.
[0158] FIG. 12 is a further example of an arrangement of a first object 1, a second object 2, and a resin composition portion 3 therebetween. In the embodiment of FIG. 12, both, the first object 1 and the second object 2 are each illustrated to be a metal sheet, the sheets being arranged relative to one another so that they overlap at least in a region where the resin composition is between them.
[0159] The arrangement of FIG. 12 illustrates two measures for heat equalization, which two measures can be realized independent of each other. [0160] The first object (the distal object in the set-up illustrated) is mounted on a non-vibrating support 81, which support immediately distally of the attachment spot/attachment location (the place where the resin composition is between the first and second objects) is interrupted (opening 82) so that there is no direct contact between the support and the first object 1 at the attachment location. Thereby, the heat transfer away from the first object, which being a metal sheet is a good heat conductor, to the support is strongly reduced. In addition or as an alternative, the support could be of a poorly heat conducting but nevertheless heat resistant material, such as a wood-based, fiber based (e.g.non-woven,), paper/cardboard or high temperature polymer (for example Tm>200) material. [0161] The resin composition comprises thermoplastic and/or PCM particles 73, which are not only capable of absorbing vibration and thereby generating heat, but are also potentially capable of absorbing heat.
[0162] As discussed hereinbefore, the filler firstly brings about the effect that an overheating of the resin is prevented in that the particles absorb heat as soon as the first order phase transition temperature (the melting temperature in the discussed embodiment) is reached and as long as not all thermoplastic material has liquefied. Thereby, the temperature is stabilized. Secondly, after the energy input is switched off, the particles dissipate heat and thereby prolong the effect of the energy input. Therefore, the processing time during which the energy is coupled into the assembly can be reduced for a given curing time. Especially, the processing time may be shorter than the time it takes for the resin to sufficiently cross-link at the processing temperature (which approximately corresponds to the melting temperature).
[0163] FIG. 13 very schematically illustrates this. FIG. 13 shows the temperature 191 of the resin as a function of time, wherein at t=0 the energy input is assumed to be switched on. During an initial phase (heating interval I.sub.h), the energy input causes the temperature to rise, similarly to systems with no thermoplastic filler. As soon as the melting temperature T.sub.m has been reached, the heat absorption by the thermoplastic particles increases so that the heat input does not cause a temperature rise to further than about the melting temperature (the temperature of the resin may be slightly above the melting temperature due to temperature gradients). When the energy input stops at a certain time (t.sub.s), the temperature will fall only slightly to below the melting temperature but will thereafter be stabilized by heat from the thermoplastic particles, which dissipate heat due to the crystallization process. The interval I.sub.stim, during which the cross-linking is stimulated/accelerated by the resin being around the optimal crystallization temperature is thus considerably longer than the interval after the heating interval during which the energy impinges. This reduces the processing time, i.e. the time during which the assembly has to be treated actively.
[0164] FIG. 14 shows a possible process control by depicting the energy input (vibration power P) 195 and the pressing force F 196 as a function of time. This process is independent of the resin composition, i.e. may be an option for all resin compositions taught in this text.
[0165] The mechanical vibration input during a first stage is relatively small, with a small vibration amplitude, whereby a thixotropy and wetting effect is achieved, i.e. the first stage has the purpose of supporting the wetting process for securing an intimate contact between the resin composition and the objects to be joined. In this first stage, the energy input is sufficiently low to keep chemical reactions (especially cross-linking) at a minimum. This may especially be important for highly reactive systems, for example two-component systems intermixed in the liquid state.
[0166] Thereafter, in a second stage, the amplitude is higher, whereby the cross-linking process is accelerated. Then, the vibration is switched off.
[0167] The force in the first stage is relatively high to support the wetting process. Then, while the vibration amplitude is high, the force is for example reduced, especially to enable a vibration relative to one another of the objects to be joined, whereby the coupling of vibration into the resin is enabled.
[0168] In an optional third stage (pressure holding stage), the force may be maintained or even, as in the illustrated embodiment, raised, to compensate for a shrinking during the cross-linking phase.
[0169] Hereinafter, configurations are described that work both, as configurations for carrying out the method according to the present invention and as configurations for carrying out a method of fastening a second object to a first object with a conventional resin or other resin composition.
[0170] FIG. 15 depicts an arrangement of a first object 1, a second object 2 and a resin composition portion 3 therebetween. The second object 2, like, in FIG. 15, also the first object 1, is a relatively thin sheet-like object, for example a metal sheet. Both, the first and second objects are assumed to have relatively large in-plane (x-y)-extension, with the resin portion being applied extensively on the surface of at least one of the objects or, for example by a corresponding robot, an extended adhesive bead. The surface of the resin may be too large for the mechanical vibration to be applied extensively over the whole area covered by the adhesive, and the hardening may take place at discrete spots only. The remaining portions of the adhesive may harden thereafter at a much slower rate and/or induced by heating.
[0171] A possible challenge in this may be that depending on the stiffness of the second object 2 it may be difficult to selectively couple the vibration through the second object into the desired spot without too much vibration energy being dissipated by flowing away laterally. [0172] In embodiments, the second object is of a material (for example a membrane-like thin sheet material) that is locally sufficiently pliable to selectively couple the vibration to that portion of the resin that is immediately underneath the sonotrode that couples the vibration into the second object. [0173] In other embodiments, the second object comprises a local deformation, for example embossment that has energy directing properties.
[0174] In FIG. 15, the embossment forms a local indentation/bead 91. As shown in FIG. 16, which depicts the configuration of FIG. 15 in a section along a plane perpendicular to the section plane of FIG. 15, the indentation may optionally form a corrugation at the bottom. Thereby, a plurality of effects may be achieved: [0175] The indentation as a whole and especially the corrugation provide pronounced structures, such as edges, that have energy directing properties. Absorption of vibration energy takes place in an intensified manner at these structures. As a consequence, the hardening process sets in around these structures, as indicated by the regions 95 in FIG. 16. [0176] The structure influences the vibration behavior and may somewhat de-couple the regions in the indentation 91 from regions around the indentation 91. [0177] The indentation with the structure serves as interior distance holder when the first and second objects are pressed against each other with the resin still being flowable, thereby defining the thickness of the adhesive portion after the process
[0178] FIG. 17, depicting a second object 2 in cross section (upper panel) and in a top view (lower panel), shows a variant of a structure with an indentation (that may optionally be provided with an additional structure, similar to FIG. 16), in which variant the indented region is surrounded by an embossed groove 97 that serves as joint-like structure for making vibrations primarily of the part encompassed by the groove possible.
[0179] A further possible solution to the problem of selectively coupling vibration energy into a desired spot is illustrated in FIG. 18. This solution is based on the concept of a thermoplastic particle being present in the resin composition. In contrast to the above-described embodiments, however, the particle has a defined shape and in FIG. 18 also a defined location, and thereby serves as an auxiliary element between the first object 1 and the second object 2. The auxiliary element serves as distance holder thereby defining the thickness of the resin portion 3. When mechanical vibration energy is applied for example to the second object locally at the position of the auxiliary element 101 while the second object 2 and the first object 1 are pressed against each other, the thermoplastic material of the auxiliary element absorbs vibration energy, especially due to external and/or internal friction, and thereby is locally heated. As a consequence, heat is conveyed also to surrounding resin material 3.
[0180] In embodiments, like in FIG. 18, the auxiliary element 101 has energy directors 102, 103, for example being ridges, tips or other protrusions. FIG. 18 shows first energy directors 102 at the interface to the first object 1 to be more pronounced than second energy directors 103 at the interface to the second object to compensate for an asymmetry arising from the fact that the vibrations in the depicted embodiment will be coupled into the second object and not directly into the first object.
[0181] FIG. 18 illustrates regions around the energy directors in which regions the activation of the resin material is predominating.
[0182] FIGS. 19-21 show top views on different auxiliary elements, thereby illustrating possible auxiliary element shapes. Generally, in embodiments it may be advantageous if the auxiliary element has a shape different form a mere disk so that the lateral surfaces are larger and thereby the interface to the resin is larger. The particles 73 dispersed in the resin in accordance with previously described embodiments may also be viewed as auxiliary elements, of essentially spherical shape.
[0183] FIG. 22, again showing a section, depicts an option of providing the auxiliary element 101 with a guiding nipple 112 cooperating with a guiding hole 111 of the first object 1 to define the exact position of the auxiliary element with respect to the first object.
[0184] In embodiments, it is advantageous if the resin composition 3 can be laterally confined to a defined region between the first and second objects at least partially. FIG. 23 shows an option to do so. The first object 1 comprises a shallow indentation 111 that defines a region for the resin composition 3. Such indentation serves as a kind of pocket confining the resin. In addition or as an alternative, the edge around the indentation may serve as flow confiner stopping the sideways flow of the resin, by capillary effects/surface tension. A similar confinement could be achieved by other discontinuity, such as a circumferential ridge or groove etc.
[0185] Similarly, as illustrated in FIG. 24, in indentation can be formed by an embossed indented structure 112 instead of a local thinning as shown in FIG. 23. Such embossed structure may optionally further comprise smaller ridges/indentations, as for example shown in FIG. 16, which ridges/indentations may be present in the first and/or in the second object and may serve as energy directors and/or distance holders.
[0186] FIG. 25 illustrates an example of a circumferential embossed groove 113 that may serve as discontinuity assisting a confinement of the resin composition 3.
[0187] FIG. 26 illustrates the principle of the first attachment surface (of the first object 1) and/or the second attachment surface (of the second object 2) comprising an attachment structure, which attachment structure is different from merely plane. In the embodiment of FIG. 26, both, the first attachment surface and the second attachment surface both comprise an attachment structure, each comprising a plurality of attachment protrusions 141. The attachment protrusions may have at least one of the following functions: [0188] Attachments stabilization: by their structure, they, after hardening of the resin composition (including any auxiliary elements if applicable) provide additional stability by contributing to a positive-fit effect. The attachment structures illustrated in FIG. 26 are undercut with respect to longitudinal directions (directions perpendicular to the attachment surfaces), whereby after solidification of the resin composition 3 they secure the respective first/second object to the resin composition in a positive-fit manner. Also even if they are not undercut, they provide additional stability against shear forces. Similar effects may be achieved by other attachment structures, especially attachment indentations and/or roughness (see hereinafter). [0189] Energy directing properties: the attachment protrusions or other attachment structure may have pronounced energy directing properties, for example by forming a tip (as in the embodiment of FIG. 26) and/or an edge or similar. When such pronounced feature is in physical contact with the thermoplastic particles 73 or a thermoplastic auxiliary element, it will cause strong energy absorption at the location of such contact when vibration energy is coupled into the system, thereby causing targeted heating.
[0190] In embodiments of the kind shown in FIG. 26, where the resin composition comprises relatively few relatively large dispersed thermoplastic particles 73, a possible design criterion may be that a distance d between two neighboring attachment protrusions corresponds to at most half a diameter D or to at most a diameter D of an average particle, so that every particle is in contact with at least one attachment protrusion. Fulfilling this design criterion may especially be useful if a positive-fit effect between not only the resin and the attachment structure but especially between the thermoplastic material of the particles 73 and the attachment surface is of importance and/or if the energy directing effect of the attachment structure is important.
[0191] FIG. 27 shows an embodiment that differs by the following properties from the embodiment of FIG. 26: [0192] Instead of dispersed thermoplastic particles 73, a sheet-like auxiliary element 101 that serves as thermoplastic spacer is present. The attachment protrusions 141, the amount of resin material and the pressing force applied during the process may be adapted to each other for the attachment protrusions to penetrate into the auxiliary element 101 while locally liquefying material thereof. [0193] The first attachment surface instead of distinct attachment protrusions comprises an attachment structure in the form of a surface roughness 143. Such surface roughness will be a macroscopic roughness that is larger than a residual (microscopic) roughness that comes about when an element is manufactured for example by injection moulding. For example, the roughness (R.sub.a, arithmetic average roughness) of such roughened portion may be at least 10 m or at least 20 m or even at least 50 m or at least 100 m.
[0194] These two differences are independent of each other and do not necessarily have to be combined.
[0195] Instead of both, the first and second objects having an attachment structure, it would also be possible for just one of the objects to have such a structure.
[0196] A targeted attachment structure may for example be manufactured by a shaping process known in the art, such as laser ablation, or also a depositing process or an embossing or molding process, or in the case of surface roughness also by grinding with rough grinding means.
[0197] FIG. 28 illustrates a further embodiment of an auxiliary element 101, namely a thermoplastic mesh. Such mesh may form a ribbon. The porosity may in embodiments be about 50%, and/or it may be used as a carrier for the resin, so that placing the resin composition may comprise just placing the ribbon impregnated by the resin.
[0198] As an alternative to a mesh, also other structure impregnatable by the resin may be used, for example a cord structure or similar.
[0199] FIGS. 29 and 30 illustrate alternative shapes of attachment protrusions 141. The attachment protrusions of FIG. 29 form sharp tips so that they have good energy directing properties, whereby the energy input into the system necessary for the activation is reduced, i.e. the attachment protrusions are optimized for a penetrating into the resin composition with the dispersed particles/auxiliary element with minimal energy and time input. However, the attachment protrusions of FIG. 29 have no undercut. The embodiment of FIG. 29 is therefore suited for a quick process, for example if the required connection strength is not high or if the adhesion by the resin is particularly (sufficiently) strong.
[0200] The embodiment of FIG. 30 has attachment protrusions that do almost not have any energy directing properties but that are undercut. This embodiment may for example be suited for situations where a slow, even energy input is desired, in combination with the effect of the undercut. Other shapes with or without undercut and with or without energy directing properties are possible.
