Method for inserting a connecting element into a sandwich structure

Description

[0001] The invention relates to a method for producing a sandwich structure with an embedded connecting element.

[0002] Sandwich structures are frequently used, particularly in lightweight construction, as used in the aviation industry, for example. These structures feature a relatively soft core sandwiched between two force-absorbing, rigid cover layers, which connects the two cover layers in a shear-resistant manner. Depending on the core used, these structures can be very rigid while remaining lightweight. The core often has a honeycomb structure, familiar to those skilled in the art.

[0003] A wide variety of materials are possible for both the cover layers and the core. In the aviation industry, and particularly for the interior fittings of commercial aircraft cabins, sandwich structures with two glass fiber cover layers impregnated with phenolic resin and a core made of an aramid honeycomb structure impregnated with phenolic resin are often used. To create an appealing visual impression, such sandwich structures are covered with veneer, at least on the surfaces visible when installed. For example, if the impression of high-quality solid wood furniture is to be conveyed, wood veneer is first glued to the sandwich structure and then coated with a high-gloss varnish.

[0004] Sandwich structures are typically semi-finished products in the form of sandwich panels. To connect sandwich panels to other components, internally threaded bushings – so-called inserts – are usually inserted into the sandwich panel at the designated connection points. For this purpose, a blind hole is milled into the sandwich panel, starting from one of the two cover layers, and the insert is secured with adhesive so that the internal thread can serve as a connection point.

[0005] The other top layer is continuous in the area of ​​a blind hole intended for an insert, meaning it is not pierced by an opening. The same applies to any veneer applied to it.

[0006] However, the other cover layer regularly exhibits surface indentations, known as insert pulls. These insert pulls involve the other cover layer being drawn in by a few micrometers toward the core of the sandwich panel. Even if the indentations are not visible to the naked eye, they are clearly visible in the form of distorted light reflections on high-gloss lacquered and polished surface veneers applied to a sandwich structure. Reworking the areas affected by insert pull, which regularly requires repainting, is complex and costly.

[0007] To prevent the occurrence of insert pull, it is known in the art to replace the core of the sandwich structure with a rigid filler during production of the sandwich structure in the areas where inserts are to be provided, into which an insert can then be inserted in the manner described. Although insert pull no longer occurs with such sandwich structures, the weight is increased compared to sandwich structures without such filler. Furthermore, the production of such sandwich structures is complex and therefore not suitable for one-off production or small series.

[0008] Document US 4,428,705 A describes a two-part insert for sandwich structures, in which a first expansion element, for example made of plastic, is inserted into a blind hole created in the sandwich structure and extending to the opposite cover layer. A metallic connector with an internal thread is pressed onto this expansion element, causing the expansion element to expand and engage laterally with the core layer of the sandwich structure. The flush alignment of the metallic connector with the top side of the sandwich structure is ensured during the pressing onto the expansion element.

[0009] The document DE 10 2014 108547 A1 discloses a fastening insert for a sandwich structure in which the problem of insert pull is avoided by providing an air gap between the fastening insert or its adhesive and the cover layer of the sandwich structure which is not perforated in the area of ​​the fastening insert.

[0010] Documents US 4 093 491 A and GB 1 353 236 A relate to a fastening insert for sandwich structures, which is provided on its outer surface with a solid adhesive that, after insertion, is liquefied by the application of heat in order to establish a material flow connection between the fastening insert and the sandwich structure after subsequent resolidification. Document US 4 093 491 A discloses, in particular, a tool suitable for this purpose.

[0011] Document CN 102 753 767 A proposes various embodiments of inserts for honeycomb sandwich structures, which engage in various ways with the honeycomb structure and are bonded to its walls. The inserts can protrude from the sandwich structure after insertion into it.

[0012] Document JP H06 126869 A discloses an insert with a circumferential flange whose diameter is larger than the diameter of the opening in the cover layer of a sandwich structure. With this flange, the insert rests flat against the cover layer from the inside or outside.

[0013] Document CN 102 256 766 A relates to a two-part insert for sandwich structures made of thermoplastic material, in which the two parts of the insert are melted in situ by mechanical oscillation, at least in some areas, whereby thermoplastic material can also penetrate into the core layer in order to exert an anchoring effect.

[0014] The document DE 29 11 058 A1 discloses an insert that can be produced cost-effectively by casting or compression molding and that can be glued flush into a sandwich panel with resin.

[0015] The document EP 3 303 567 B1 shows a vehicle structure in which two components are joined together in a form-fitting manner using a tenon joint, the tenon joint having such play that there is sufficient space for filling with resin to create an additional material bond.

[0016] The invention is based on the object of creating a method for producing a sandwich structure with a connecting element embedded therein, in which the disadvantages of the prior art no longer occur or only occur to a reduced extent.

[0017] This problem is solved by the method according to the main claim. Advantageous further developments are the subject of the dependent claims.

[0018] Accordingly, one aspect of the description relates to a sandwich structure with two cover layers and a core arranged therebetween, which core has at least one connecting element for connection to further components, wherein the connecting element comprises an insert which is embedded in the sandwich structure on a cover layer and whose insertion depth is smaller than the thickness of the core of the sandwich structure, and wherein the insert or a compensating element arranged on the insert is machined flush with the outer side of the cover layer over which the insert is embedded or protrudes from it.

[0019] The invention relates to a method for inserting a connecting element into a sandwich structure with two cover layers and a core arranged therebetween, wherein the connecting element comprises an insert, comprising the steps: a) making a blind hole starting from a facing layer of the sandwich structure; b) inserting the fastener into the blind hole and securing the fastener therein so that it protrudes from the facing layer; and c) finishing the fastener by grinding so that it is flush with the facing layer or protrudes from the facing layer by a predetermined height.

[0020] The "thickness of the core of the sandwich structure" corresponds to the distance between the two cover layers. The "inlet depth" refers to the depth to which the insert is embedded in the sandwich structure.

[0021] The invention is based on the finding that insert pulls occur particularly when an insert is positioned at a distance from the cover layer through which the insert is embedded in the sandwich structure. If a screw inserted into the internal thread of the insert is tightened while the sandwich structure is in contact with another component, this can cause deformation of the sandwich structure, which can ultimately lead to insert pull of the other cover layer.

[0022] To prevent this, the invention provides that the connecting element comprising an insert is machined flush with the outer side of the cover layer into which it is embedded, or protrudes from the cover layer. This prevents a relative movement between the connecting element and the sandwich structure, and thus insert pull, from occurring when a screw inserted into the insert is tightened.

[0023] A reworked fastener is understood to be a fastener that, while inserted into the cover layer, is reworked until it is flush with the cover layer. This can be done, for example, by adding or removing material. According to the invention, this is done by grinding. A fastener that has been reworked within the meaning of this invention can be recognized by the fact that it is precisely adapted to the plane of the cover layer. This can also be recognized by the structural changes to the fastener that occur during the addition or removal process.

[0024] According to the invention, the connecting element is reworked by grinding.

[0025] The connecting element is initially positioned so that it protrudes from the cover layer. The connecting element can then, if necessary, be ground flush with this cover layer. In other words, the connecting element that initially protrudes above the cover layer is ground down until it is flush with the cover layer. The cover layer itself is not damaged or ground down during this process. Whether a connecting element was positioned flush with the cover layer or ground flush can be easily determined retrospectively, for example based on grinding marks and / or the shape of the edges of the connecting element on the ground surface. Instead of grinding flush, it is also possible for the connecting element to protrude or remain protruding from the cover layer through which it is embedded in the sandwich structure. In this case, too, insert pull on the other cover layer is effectively avoided.

[0026] If the connecting element is not ground flush, it preferably protrudes by less than 6 mm, more preferably by less than 3 mm, from the covering layer over which it was embedded. A protruding connecting element can also be ground, for example, to achieve a desired protrusion height and / or parallelism of the ground surface of the connecting element with the covering layer.

[0027] The connecting element can comprise only an insert. However, it is also possible for the connecting element to comprise a compensating element arranged on the insert and not overlapping the cover layer. "No overlap" in this context means that the compensating element does not overlap the cover layer in a projection perpendicular to the cover layer. In other words, the compensating element is located entirely within the perimeter of the insert in this projection. If a compensating element is provided, it can be machined flush with the cover layer, preferably ground to a smooth finish, or it can protrude beyond the cover layer. In the latter case, it is also possible for the insert itself to already protrude beyond the cover layer, but a greater height of the protrusion is achieved by an additional compensating element. The compensating element can be arranged on the insert after it has been embedded in the sandwich structure.In this case, the insert can first be introduced into the sandwich structure according to the prior art and only then does it become a connecting element according to the invention by applying the compensating element.

[0028] The compensating element can be made of filler that hardens after being applied to the insert, or it can be a prefabricated component that is in contact with the insert or, preferably, is integrally connected to the insert. The prefabricated component can, for example, be ring-shaped and similar to a washer. If the compensating element is a prefabricated component, it is preferred if it is made of a material that is corrosion-resistant compared to the insert material.

[0029] In a preferred embodiment, a metal sheet covering both the connecting element and the cover layer of the sandwich structure can be provided, wherein the space between the insert and the covering metal sheet is filled with the hardening filler. The covering metal sheet makes it easy to create a boundary flush with the cover layer for the hardening filler serving as a leveling element. This embodiment thus allows for simple and precise post-processing of the leveling element, and thus of the connecting element, flush with the cover layer.

[0030] By using a suitable sheet metal, it is also possible to distribute the force introduction in the area of ​​the connecting element over a larger area of ​​the sandwich structure, so that load peaks that could potentially lead to damage to the sandwich structure can be avoided.

[0031] The core of the sandwich structure is preferably a honeycomb core, e.g., made of aluminum or fiber-reinforced plastic. With such a core, particularly lightweight sandwich structures can be achieved. It is also preferred if the insert is made of metal. A metal insert typically achieves a high load-bearing strength of the internal thread.

[0032] The sandwich structure according to one aspect of the description is particularly advantageous in cases where a glossy surface layer, such as a high-gloss varnished wood veneer, varnish, foil, leather or other textile, is applied to the cover layer over which the connecting element is not embedded.

[0033] For an explanation of the method according to the invention, reference is made to the above explanations.

[0034] The invention will now be described by way of example using advantageous embodiments with reference to the accompanying drawings. In the drawings: Figures 1a-d: a first embodiment of a method according to the invention for producing a first sandwich structure according to one aspect of the description; Figures 2a-e: a second embodiment of a method according to the invention for producing a second sandwich structure according to one aspect of the description; and Figure 3: a third embodiment of a method according to the invention for producing a third sandwich structure according to one aspect of the description.

[0035] In Figure 1a-d a method for producing a first sandwich structure 1 is shown.

[0036] Figure 1ashows the initial state of the sandwich structure 1. The sandwich structure 1 comprises two cover layers 2, 3, which are firmly connected to each other via a core 4 designed as a honeycomb core. Both the cover layers 2, 3 and the core 4 are made of fiber-reinforced plastic. A high-gloss varnished wood veneer 3' is arranged on the cover layer 3 shown below in Figure 1a.

[0037] In a first step, a blind hole 5 is produced by milling, starting from one cover layer 2 of the sandwich structure 1, so that the other cover layer 3 remains completely intact ( Figure 1b ).

[0038] Subsequently, a connecting element 6 consisting of an insert 7 with an internal thread 8 is placed in this blind hole 5 and secured therein with adhesive 9. In contrast to the prior art, however, the insert 7 is arranged so as to protrude beyond the cover layer 2 of the sandwich structure 1. The inlet depth of the insert 7 is less than the thickness of the core 4, so that a distance exists between the insert 7 and the other cover layer 3 ( Figure 1c ).

[0039] After the adhesive 9 has hardened, the connecting element 6 is ground down in such a way that it only protrudes a predetermined height above the cover layer 2 ( Figure 1d ). This grinding can usually still be easily detected after the actual process, for example by means of grinding marks or the absence of the originally intended bevel on the edge of the upper side of insert 7 (cf. Figure 2c). It is of course also possible for the specified height to be zero, i.e. for the connecting element 6 to be ground flush with the covering layer 2.

[0040] By securing the connecting element 6 according to the invention flush with or protruding beyond the cover layer 2, an insert pull on the other cover layer 3 in the area of ​​the connecting element 6 can be effectively avoided.

[0041] In Figure 2 a second embodiment of a method according to the invention for producing a sandwich structure 1 according to one aspect of the description is shown.

[0042] Figure 2a shows the Figure 1aidentical initial state of the sandwich structure 1. The sandwich structure again comprises two cover layers 2, 3 made of fiber-reinforced plastic, which are firmly connected to each other via a honeycomb core 4 - also made of fiber-reinforced plastic. A high-gloss varnished wood veneer 3' is arranged on one cover layer 3.

[0043] Again, in a first step, starting from the cover layer 2 of the sandwich structure 1, which is not provided with the wood veneer 3', a blind hole 5 is produced by milling, whereby the other cover layer 3 and the wood veneer 3' arranged thereon remain completely intact ( Figure 2b ).

[0044] Subsequently, an insert 7 is inserted into this blind hole 5 in a known manner as part of the connecting element 6 and secured with adhesive 9. The insert 7 is sunk into the sandwich structure 1, ie the side of the insert 7 accessible from the outside is arranged so as to recede behind the cover layer 2. However, the insertion depth of the insert 7 is also smaller than the thickness of the core 4 ( Figure 2c ).

[0045] Subsequently, a separate component as a compensating element 10 with a shape comparable to a washer is connected to the insert 7 to complete the connecting element 6, preferably by means of a material bond - in this case by gluing. The compensating element 10 has a through-hole 11 so that the internal thread 8 of the insert 7 remains accessible. At the same time, the compensating element 10 is non-overlapping with the cover layer 2( Figure 2d ).

[0046] Finally, the connecting element 6, in particular the separate compensating element, is ground down in such a way that it only protrudes a predetermined height above the cover layer 2 ( Figure 2e ). It is of course also possible for the specified height to be zero, i.e., for the connecting element 6 to be ground flush with the cover layer 2. Here, too, insert pull on the other cover layer 2 in the area of ​​the connecting element 6 can be effectively avoided.

[0047] In Figure 3 a third embodiment of a method according to the invention for producing a sandwich structure 1 according to one aspect of the description is shown.

[0048] As in the previously explained embodiments, the insert 7 is first fixed in the blind hole 5 of the sandwich structure 1 with adhesive 9. However, the insert 7 does not extend as far as the cover layer 2, so the insert 7 is arranged recessed behind the cover layer 2.

[0049] The inventive reworking of the connecting element 6, so that it is flush with the cover layer 2, is carried out in this case by applying a hardening filler 12; this acts as a leveling element 10. To avoid the laborious task of adding filler—which would also be conceivable—a sheet 13 is provided covering the cover layer 2. The sheet 13, together with the insert 7, forms a filling space that is filled with filler 12. The filler is distributed in the thus-formed filling space such that it forms a leveling element 10 that is flush with the cover surface 2. In this procedure, the sheet 13 has a filling opening for filling the filling space with filler.

[0050] Alternatively, after inserting the insert 7, the blind hole 5 can be filled with filler 12. Preferably, slightly more filler 12 is applied than the filler space can accommodate. The sheet metal 13 is then positioned over the blind hole 5 and the cover layer 2, so that the excess filler 12 is pressed out of the filler space. This procedure also ensures that the connecting element 6 is reworked flush with the cover layer 2.

[0051] To ensure that the internal thread 8 remains accessible, a through opening 11 is preferably provided.

Claims

1. Method for inserting a connecting element (6) into a sandwich structure (1) which has two outer layers (2, 3) and a core (4) in between, wherein the connecting element (6) comprises an insert (7), having the following steps: a) producing a blind hole (5) starting from an outer layer (2) of the sandwich structure (1); b) introducing the connecting element (6) into the blind hole (5) and fastening the connecting element (6) therein, so that it protrudes from the outer layer (2); and c) reworking the connecting element (6) by sanding, so that it is flush with the outer layer (2) or protrudes from the outer layer (2) by a predefined height.

2. Method according to Claim 1, characterized in that the connecting element (6) is sanded down to a height of less than 6 mm, preferably less than 3 mm above the outer layer (2).

3. Method according to either of the preceding claims, characterized in that introducing the connecting element (6) into the blind hole (5) and fastening the connecting element (6) therein comprises the following sub-steps: - introducing the insert (7) into the blind hole (5) and fastening the insert (7) therein; and - arranging a compensating element (10) on the insert (7) such that, in a perpendicular projection through the outer layer (2), the compensating element does not overlap the outer layer (2) and protrudes therefrom.

4. Method according to Claim 3, characterized in that to mount the compensating element (10), filling compound is applied to the insert (7) and cured, or a separate component is preferably fastened to the insert (7) in an integral bond.

5. Method according to one of the preceding claims, characterized in that a glossy surface layer, preferably a wood veneer (3') coated with high-gloss varnish, is mounted on the outer layer (3) from which the blind hole (5) to be produced does not start.