Method for producing multi-dimensional rigid foam parts by means of connection of puzzle blocks

Abstract

The invention relates to a method for producing a multi-dimensional rigid foam part.

Description

Technical Field

[0001] The invention relates to a method for producing multidimensional rigid foam parts. Background Art

[0002] Rigid foam, such as the product name Rohacell ® The polymethacrylimides sold are used, in particular, as core materials for sandwich structures with covering layers consisting of fiber composite materials. They are usually milled to the desired core geometry. The size of the finished core is limited here by the size of the starting panels, which usually do not exceed a length of 2.50 m. If larger core structures are to be produced, the connection surfaces between the core segments are configured as butt joints. This means that the ability to position the segments precisely relative to each other cannot be guaranteed. In addition, slippage of the segments relative to each other and relative to the mold or the already applied covering layers is possible. Air gaps may also form between the segments. Due to the occurrence of local stress points, this can constitute a mechanical weak point in the finished component. In addition, this gap may be filled with resin during the curing of the covering layers, which can lead to dents on the component surface caused by excessive shrinkage, local thermal damage caused by the exothermic curing process, and impaired dielectric properties of the component (for example in radomes that exhibit increased absorption at this point).

[0003] If a core with complex geometry, such as sharp curvatures or large thickness changes, is to be inserted into the mold as a complete mechanical solid connection, it must be milled from the solid or produced by a thermoforming process. The milling of the solid generates very high losses and is therefore generally uneconomical. The economic feasibility is increased if the core before milling is divided into individual segments by a butt joint. However, the above-mentioned disadvantages of the butt joint connection occur.

[0004] Jigsaw puzzle-piece connections in cardboard and flexible foam structures are obtained by die cutting. This method has only very limited applicability for rigid foams, since the tolerances that can be achieved in the connection are not small enough.

[0005] Milling is not used as a production process for joining jigsaw puzzle pieces in flexible foams, since the foams cannot be processed accurately enough during milling. In addition, the temperature resistance of many common flexible foams is so low that the friction heat generated during milling leads to further softening of the foam, resulting in increased chipping by the milling machine. Due to the porous structure of the foams, cooling by coolants and lubricants is not advisable, since they cannot be easily removed from the cells again.

[0006] DE102012024167 describes the detachable connection of interlocking panels arranged in facets. DE102012024167 discloses a method for producing complex rigid foam parts by milling, wherein the complex assembly is divided into a plurality of individual parts, and the plurality of individual parts are milled into a puzzle piece shape. In order to realize a dome, for example, the panels are overlapped two-dimensionally over their entire side length, and each connection forms an interlock at least twice locally. In the interlock, the panels are directly butted linearly on the one hand and - protected to prevent the edges from sliding against each other - butt joints, and on the other hand, the obviously fixed or movable protrusions as edge parts (which are arranged in the same position in the cross section, but offset from each other in the top view of the larger planar arrangement (outside)) are hooked to each other instead of penetrating each other with relative lugs. As a protrusion on the edge of the panel, the fixed edge part is kept between or beside the flaps that can be folded and rotated, and the adjacent panels can be abutted against it with their movable flaps.

[0007] The panel material used is tough because for each lug a "living hinge" is formed as a line joint in the remaining covering layer after another covering layer and to a large extent also the lightweight core of lightweight material has been cut along the fold line. Summary of the invention Technical issues

[0008] Therefore, in the context of the prior art discussed, an object of the present invention is to provide a method of joining large-volume multi-dimensional rigid foam parts that prevents the component segments from sliding relative to each other, yet enables the joined rigid foam parts to be handled in a manner comparable to a one-piece rigid foam part.

[0009] Other issues not explicitly discussed here may be inferred from the prior art, the description or the claims. Solution

[0010] The object is to produce by milling a product having a glass transition temperature T of at least 100° C. measured in accordance with DIN EN ISO 11357-2 (publication date: 2020-08). g A method for producing a multidimensional rigid foam component of a polymer, the method being characterized in that,

[0011] - Multi-dimensional shaping of flat component segments by thermoforming,

[0012] - milling said multi-dimensionally formed component segments into jigsaw puzzle piece shapes,

[0013] - Assembling the multidimensional component segments to obtain a multidimensional rigid foam part.

[0014] Surprisingly, it has been found that a voluminous rigid foam component assembled from jigsaw-piece-shaped component segments results in a very stable construction. Dividing the multidimensional rigid foam component into several jigsaw-piece-shaped component segments enables complex constructions to be achieved.

[0015] It has been found that the use of component segments in the shape of jigsaw pieces results in considerable savings in material, and therefore in costs and resources.

[0016] Multidimensional rigid foam parts are parts that contain two or more dimensions.

[0017] In particular, it has been found that it is also possible to produce two-dimensional or three-dimensional curved rigid foam parts.

[0018] Compared to the prior art, such as EP 2380747, which discloses two-dimensional flat components that are always constant in one dimension, for example the height, the multidimensional rigid foam component according to the invention has a variable expansion in all three dimensions (especially also the height) both within the individual component segments and between the component segments. Unlike known geodesic structures or meta-geodesic structures, the rigid foam component according to the invention is used to assemble continuously curved structures in a simple manner.

[0019] A geodesic dome is a convex irregular polyhedron. Euler's polyhedron theorem applies to geodesic domes: number of corners + number of faces - number of edges = 2.

[0020] Usually, the geometric transformation of a dodecahedron or an icosahedron is performed into a geodesic dome bounded by pentagons or triangles, respectively. However, it is also possible to convert all Platonic solids or even any polyhedral geometry into a geodesic structure by subdividing into triangles accordingly. A geodesic half dome consists of triangles joined together to form pentagons and hexagons, so that the angles or side lengths of the triangles are different.

[0021] Furthermore, multidimensional components can be characterized by the fact that the component does not form a continuous surface but has significant openings. A particular field of application is the production of support structures similar to rod arrangements, which have so far consisted of individual rods which were freely positioned on a carrier surface, in particular a carrier film, and to which a stable connection was obtained only by adhesive bonding. In this context, a rod arrangement refers to a spatial support structure consisting of two or more elongated rods connected to one another. The butt joints of the rods are produced by gluing with a resin, which is also used for bonding to the carrier surface or carrier foil.

[0022] First, the rods are glued together to prevent slipping. Second, the rods are secured to the carrier surface.

[0023] Rigid foams are preferably used for the process according to the invention.

[0024] The rigid foam panel used according to the invention has a glass transition temperature T measured according to DIN EN ISO 11357-2 (publication date: 2020-08) of at least 100° C., preferably at least 140° C. g The DIN EN ISO 11357-2 standard for plastics describes differential scanning calorimetry (DSC) - Part 2: Determination of glass transition temperature and activation energy of glass transition.

[0025] The polymer is selected from polyethersulfone, polyphenylsulfone, polyetheretherketone (PEEK), polysulfone, polyurethane (PU), polyethylene terephthalate (PET), preferably poly(meth)acrylimide (P(M)I), polymethyl(meth)acrylate, poly(ether)imide (P(E)I) and mixtures and copolymers thereof.

[0026] The term "poly(meth)acrylimide" (P(M)I) is to be understood hereinafter as meaning polymethacrylimide (PMI), polyacrylimide (PI) or mixtures thereof. This applies analogously, for example, to polymethyl(meth)acrylate. Thus, the term polymethyl(meth)acrylate is to be understood as meaning not only polymethyl methacrylate (PMMA), but also polymethyl acrylate and mixtures of the two.

[0027] The material used for the multidimensional rigid foam component is preferably P(M)I, particularly preferably PMI. Such P(M)I foams are also called rigid foams and exhibit a particular strength.

[0028] The production of P(M)I foams is described, for example, in EP 3221101. P(M)I foams are usually produced in a two-step process: a) production of a cast polymer, and b) foaming of this cast polymer.

[0029] The cast polymer is produced by first producing a monomer mixture containing (meth) acrylic acid and (meth) acrylonitrile as main components, preferably in a molar ratio of 2:3 to 3:2. In addition, other comonomers may be used, such as esters of acrylic or methacrylic acid, styrene, maleic acid or itaconic acid or their anhydrides, or vinyl pyrrolidone. However, the proportion of comonomers should not be greater than 30% by weight. It is also possible to use a small amount of crosslinking monomers, such as allyl acrylate. However, this amount should preferably be at most 0.05 to 2.0% by weight.

[0030] The mixture for the copolymerization further contains a blowing agent which decomposes or evaporates at a temperature of about 150° C. to 250° C. to form a gas phase. Below this temperature, polymerization occurs, so that the cast polymer contains a latent blowing agent. The polymerization is advantageously carried out in a block form between two glass plates. In order to produce a foamed sheet, the cast polymer is subsequently foamed at a suitable temperature in a second step according to the prior art. The production of these P(M)I foams is in principle known to the person skilled in the art and can be found, for example, in EP 1 444 293, EP 1 678 244 or WO 2011 / 138060. Examples of PMI foams include in particular ROHACELL from Evonik Industries AG, Germany. ® series. Acrylimide foams can be considered similar to PMI foams in terms of production and processing. However, due to toxicological reasons, these are clearly less preferred than other foam materials.

[0031] The density of the rigid foam material can be selected relatively freely. The P(M)I foam can be, for example, 20 to 320 kg / m 3 , preferably 25 to 250 kg / m 3 It is particularly preferred to use a density of 30 to 200 kg / m 3 PMI foam between . Measured according to DIN EN ISO 845 (publication date: 2009-10).

[0032] The rigid foams described herein are produced in the form of foam blocks. Depending on the polymer used and the density achieved, these foam blocks have different thicknesses. The thickness of the foam blocks is preferably in the range of 50 mm to 200 mm, particularly preferably between 55 mm and 165 mm. Preferably, foam panels are first cut from the foam blocks. The thickness of these foam panels is between 5 mm and 150 mm, particularly preferably between 10 mm and 90 mm.

[0033] Various methods are known for cutting rigid foam blocks and panels. Cutting is preferably performed using a laser, a band knife or a band saw.

[0034] The flat component segments thus obtained are hot-formed before being milled into the shape of puzzle pieces. In principle, 2 methods can be used for this purpose.

[0035] A first thermoforming method for planar component segments comprises the following steps:

[0036] - Inserting the planar component segments into the forming die,

[0037] - heating the forming mould in an air circulating oven to a temperature between 180 and 230°C,

[0038] - Removal of multi-dimensionally formed component segments from the forming die.

[0039] The second thermoforming method for the planar component segments comprises the following steps:

[0040] - heating the planar component segments to 180 to 230°C in an oven, a hot press or using infrared radiation,

[0041] - inserting the heated planar component segment into the forming die,

[0042] - close the forming mould and shape the component segments using pressure or vacuum,

[0043] - Remove multidimensional component segments from forming dies.

[0044] In these thermoforming processes, it is necessary to prevent or minimize cooling of the planar component segments between the heating step and the insertion into the forming die. To this end, the planar component segments can be encapsulated with a material selected from nonwovens, fabrics or films before thermoforming.

[0045] One-piece and two-piece forming dies are known to those skilled in the art as forming dies. Depending on the type of die, pressure or vacuum is applied to the forming step.

[0046] After cooling of the multi-dimensionally formed component segments, they can be fed into a milling process in the form of jigsaw puzzle pieces.

[0047] Milling using a CNC machine is particularly preferred when machining the rigid foam blocks into component segments in the form of puzzle pieces.

[0048] When the milled jigsaw puzzle pieces are connected, a press fit can be provided, which enables a secure connection of the individual component segments. In addition, this prevents the formation of air gaps and thus the penetration of resin therein during subsequent joining processes.

[0049] An alternative embodiment is a clearance fit. In this case, a defined minimum gap between the jigsaw puzzle pieces is milled. The resin as a binder penetrates into the gap of the joint resulting in a stable bond between the component segments and at the same time to the carrier surface or cover layer.

[0050] The shapes of the puzzle pieces can vary greatly. Preferably, the interlocking parts of the puzzle stone are realized as mortise and tenon joints. Preference is given to using round mortise and tenon shapes. In order to achieve a displacement-free form fit, the tenon is milled in the form of a rod with a semicircular head (lollipop shape), with the corresponding counterpart serving as the mortise. Such a mortise and tenon joint results in a particularly displacement-free connection both for multi-dimensionally shaped components and for planar components. The resulting form fit is particularly preferred for large-volume components.

[0051] It has been found that this approach prevents dents and thickness variations between component segments. A wide variety of geometries with smooth surfaces can be achieved.

[0052] It has been found that cutting rigid foam parts into puzzle piece shapes has several advantages over joining straight cuts together. The puzzle piece connection is a much more stable connection as it is very resistant to torsion and does not necessarily require glue points due to the interlocking connection.

[0053] Another object is to provide high volume rigid foam parts.

[0054] This object is achieved by a method in which the component segments are joined by a puzzle piece connection before insertion into a mold.

[0055] Bulky means that the dimensions of the rigid foam part exceed the maximum usable dimensions of the starting material in at least one direction.

[0056] Common manufacturing methods can only provide rigid foam blocks with limited sizes as starting material. Typically, the maximum size of a rigid foam block is 250×125×15 cm.

[0057] If, for example, aircraft wings (eg with a length of 5000 cm) or rotor blades are to be manufactured therefrom, these massive rigid foam parts must be produced by juxtaposing two or more rigid foam blocks.

[0058] The jigsaw puzzle connection ensures an interlocking connection of the individual component segments, which allows for easy handling during positioning in the mold. Slipping of the component segments relative to each other is prevented. Furthermore, a uniform force transmission within the core is ensured during use in the finished sandwich structure.

[0059] It is possible to use rigid foam blocks of different thicknesses as starting material for component segments which have greatly different thicknesses in the finished core. This allows the material utilization to be optimized.

[0060] In one variant, rigid foam blocks of different densities are combined so that locally varying stresses in the rigid foam components are taken into account.

[0061] The method according to the invention is also suitable for producing rigid foam parts which are suitable for producing sandwich components.

[0062] For this purpose, the rigid foam part can be covered by at least one outer layer. The covering material can comprise wood, metal, decorative film, composite material, prepreg, woven fabric or other known materials.

[0063] This makes it possible to realize sandwich materials, for example, in lightweight construction. Alternatively, the outer layer can simply be just a protective film, or a decorative layer, which can be removed again.

[0064] For example, there may be a rigid foam component with a thermoplastic or cross-linked covering layer. The prior art discloses various methods for producing composite components.

[0065] The rigid foam part is particularly preferably glued, sewn or welded to the covering material. "Welding" here means that the heating of the assembly causes a cohesive connection (bonding) between the rigid foam part and the covering material.

[0066] The multidimensional rigid foam parts produced by the method according to the invention are used for the construction of spacecraft or aircraft, shipbuilding, rail vehicle construction or vehicle construction, in particular in the interior or exterior thereof. This can include the multidimensional rigid foam parts produced by the method according to the invention, and also the sandwich components realized therefrom. More particularly, due to their low flammability, the rigid foam parts according to the invention can also be installed in the interior of the vehicle. In addition, the material is used in wind energy technology, for example in wind turbines, in particular in rotor blades.

[0067] Further possible applications are in the sports and leisure industry, such as large-sized sports equipment, for example skis or surfboards, and in medical technology for medical technology components, such as X-ray or CT tables.

[0068] The sandwich components with the multidimensional rigid foam parts produced according to the invention are also particularly suitable for incorporation into the aircraft exterior. "Exterior" does not only mean filling in the outer skin of the aircraft, but also in particular in the aircraft nose, tail area, wings, outer doors, control surfaces or rotor blades.

Claims

1. A method for producing a multidimensional rigid foam part by milling and wherein the multidimensional rigid foam part has a glass transition temperature T of at least 100° C., measured in accordance with DIN EN ISO 11357-2 (published on August 2020). g The method is characterized in that - Multi-dimensional shaping of flat component segments by thermoforming, - milling said multi-dimensionally formed component segments into jigsaw puzzle piece shapes, - Assembling the multidimensional component segments to obtain a multidimensional rigid foam part.

2. The method for producing a multidimensional rigid foam component according to claim 1, characterized in that The thermoforming of the planar component segments comprises the following steps: - Inserting the planar component segments into the forming die, - heating the forming mould in an air circulation oven to a temperature between 180 and 230 °C, - Removal of multi-dimensionally formed component segments from the forming die.

3. The method for producing a multidimensional rigid foam component according to claim 1, characterized in that The thermoforming of the planar component segments comprises the following steps: - heating the planar component segments to 180 to 230°C in an oven, a hot press or using infrared radiation, - inserting the heated planar component segment into the forming die, - close the forming mould and shape the component segments using pressure or vacuum, - Remove multidimensional component segments from forming dies.

4. The method for producing a multidimensional rigid foam component according to claim 3, characterized in that The planar component segments are encapsulated with a material selected from a nonwoven, a fabric or a film prior to thermoforming.

5. The method for producing a multidimensional rigid foam part according to claim 1, characterized in that The mortise and tenon joints of the puzzle pieces are milled into a lollipop shape.

6. The method according to claim 1, characterized in that The polymer is selected from polymethacrylimide (PMI), polyetherimide (PEI) and polymethyl methacrylate (PMMA).

7. The method according to claim 1, characterized in that Production of particularly large rigid foam parts by joining together jigsaw puzzle pieces.

8. The method according to claim 1, characterized in that Produce two-dimensional or three-dimensional curved rigid foam parts.

9. The method according to claim 1, characterized in that Production of rigid foam parts for sandwich components.

10. Use of the rigid foam parts produced according to claim 1 for the production of articles for the aerospace industry, shipbuilding, wind energy, the sports and leisure industry, preferably large-sized sports equipment such as skis or surfboards, vehicle construction, in particular electric vehicles, and medical technology.