METHOD FOR MANUFACTURING A COMPONENT FROM A FIBER COMPOSITE MATERIAL

Description

[0001] The invention relates to a method for producing a (three-dimensional) component from a fiber composite material by forming a (two-dimensional) thermoplastic organic sheet.

[0002] In the context of the invention, organic sheet means a plate-shaped (consolidated) semi-finished product consisting of fibers embedded in a matrix of thermoplastic. The fibers can be in the form of continuous fibers or long fibers, for example in the form of a fiber fabric or fiber layup. The fibers can be, for example, carbon fibers, glass fibers, or aramid fibers. Such organic sheets are used as fiber composite materials for the production of components (e.g., lightweight construction) for aerospace engineering (e.g., aircraft construction) and for vehicle technology (e.g., automobile construction). Due to the thermoplastic fiber matrix, such organic sheets can be (hot-formed) in a similar way to metal sheets, so that in practice, methods of sheet metal processing are used for the processing of organic sheets or the production of components from such organic sheets.

[0003] For example, DE 10 2011 115 730 A1 describes a method for forming thermoplastic semi-finished fiber sheets with oriented fibers into three-dimensionally shaped thermoplastic semi-finished products with defined degrees of orientation. The semi-finished fiber sheet, formed as an organic sheet, is heated by a heating device to a temperature below the softening temperature of the thermoplastic. The semi-finished fiber sheet is positioned on a molding module that reproduces the three-dimensional shape. A fluid is then fed into the molding chamber, thereby pressing the heated semi-finished fiber sheet against the molding module and thereby forming it into the three-dimensionally shaped thermoplastic semi-finished product.

[0004] Furthermore, EP 2 561 978 A2 describes a method and a device for compression molding and curing composite materials. A device comprising an elastic layer and a mold is used to produce a cured composite material. The elastic layer is arranged on a rigid support. The mold, in turn, is arranged on the elastic layer and the support and comprises an inner surface which, together with a portion of the elastic layer, defines the molding chamber. A prepreg is arranged within the molding chamber, which is molded into the composite and subsequently cured. The molding takes place by introducing a fluid which acts on the elastic layer in such a way that it expands and presses the prepreg into the mold.

[0005] Furthermore, EP 0 584 017 A1 describes a press for forming an article from a composite material with reinforcing fibers in a polymer matrix. Forming takes place between an elastic membrane and a mold that determines the shape of the component to be formed. The press has a lower platen and an upper platen, with the upper platen serving to generate a pressing force. Heating and cooling means are integrated into the mold; for example, the mold has channels through which a heating or cooling fluid can be passed. An elastic membrane is attached to the upper platen, onto which pressure is exerted by the introduction of a fluid in order to produce the molded part. The generated pressure deforms the membrane and presses the inserted material into the mold.

[0006] Further processes for processing organic sheets or components made of such organic sheets are described, for example, in DE 10 2013 105 080 A1, DE 10 2011 111 233 A1 and DE 10 2011 111 232 A1.

[0007] DE 37 27 926 A1 also discloses a process for producing molded parts using flat and / or preformed sheets by hot pressing. This involves a heatable mold cavity with a cover and a release film. The release film offers greater resistance to deformation than the semi-finished product made of fiber-reinforced thermoplastic, so the film determines the drawing behavior and the quality of the result. During forming, the release film, due to its higher strength, serves as a transmitter of uniform elongation properties and prevents uncontrolled expansion of the thermoplastic semi-finished product. The release film should preferably be made of an aluminum alloy. Alternatively, a release film made of rubber or plastic can be used.

[0008] Furthermore, WO 90 / 03880 discloses a process for producing fiber-reinforced plastic products. For this purpose, sheets consisting of a thermoplastic and reinforcing fibers are produced, heated to their softening temperature, and then pressed into a mold. Forming is preferably carried out in a porous vacuum mold using a barrier layer that prevents air from being drawn through the sheet. Alternatively, a mold consisting of an upper and a lower part is used.

[0009] Alternatively, DE 198 59 798 C1 describes the production of molded bodies from fiber composite materials using the so-called prepreg process. In this process, thin layers of fibers embedded in partially cured resin are laminated until a preform of the molded body is created. This preform is then cured by heating under mechanical pressure while simultaneously applying a vacuum to remove air bubbles from the preform. This typically takes place in an autoclave, in which the preform rests on a negative mold and is covered by a flexible membrane. The flexible membrane is sealed off from the negative mold. A layer of fabric material is arranged between the preform and the membrane to absorb excess resin and create a vacuum zone, the so-called vacuum bag. The vacuum bag area is connected to a vacuum source.

[0010] Based on this, DE 198 59 798 C1 describes a method for producing molded bodies from fiber composite materials, which is based on an RTM process. A fiber mat is applied to a rigid negative mold, and the fiber mat is covered with a flexible membrane. The membrane is sealed all the way around the fiber mat relative to the negative mold, and the thus sealed space between the negative mold and the membrane is evacuated, and a static overpressure is applied to the back of the membrane facing away from the negative mold. A quantity of liquid resin is then injected into the space between the negative mold and the membrane at an injection pressure that is greater than the overpressure on the back of the membrane. The resin is heated by the heated negative mold under the influence of the overpressure on the back of the membrane and is at least partially cured.The excess pressure on the back of the membrane is then released, and the molded body, with the fiber mat embedded in the at least partially cured resin, is demolded. The negative mold can be continuously heated, and the membrane can be cooled on its back.

[0011] Similar processes, in which a membrane press is used and a resin is injected into the mold cavity, are described, for example, in EP 1 420 940 B1 or DE 694 09 618 T2.

[0012] DE 40 40 746 A1 describes a method for compacting a composite material body with a structure of uncompacted layers of reinforcing fibers embedded in a matrix in a membrane press.

[0013] EP 2 070 678 A1 also discloses a mold and a method for producing preforms and fiber-reinforced plastics using the mold within an RTM process. The mold has two plates that form a cavity. Integrated into the mold are lines through which a heating medium is conducted, which serves to heat the mold and, accordingly, the material used. Furthermore, a rubber cord is attached to a lower platen, which serves as a membrane for deforming the material used. This rubber membrane covers the material used and presses it against the mold after the interior has been evacuated to generate pressure.

[0014] The invention is based on the object of creating a method for producing (lightweight) components made of fiber composite materials with high quality and high stability.

[0015] To achieve this object, the invention teaches a method for producing a component from a fiber composite material by forming a thermoplastic organic sheet in a membrane press according to claim 1.

[0016] The invention is based on the finding that three-dimensional fiber composite components with high stability and high precision can be economically produced from organic sheets in a membrane press. Such organic sheets are available as (two-dimensional) plate-shaped, consolidated semi-finished products and can be excellently formed into three-dimensional structures in the membrane press using pressure and heat, which can be used, for example, in aircraft construction, vehicle construction, or the like. In contrast to conventional prepreg processes, the process does not use only partially cured mats, but rather consolidated semi-finished products in the form of organic sheets, thus eliminating the need to inject liquid resins or the like into the press.In this case, an organic sheet is used as a prefabricated semi-finished product consisting of several layers of organic sheet that are put together and optionally bonded together before being placed in the press. In this way, very stable components can be manufactured that can also have a certain thickness or wall thickness. Nevertheless, within the scope of the invention, perfect forming is achieved in the membrane press because a (highly) elastically stretchable membrane is clamped into the press, which, with the interposition of the organic sheet, is elastically stretched and stretched over the mold. By applying negative pressure on the one hand and positive pressure on the other, perfect forming then takes place, with the highly elastic membrane stretching considerably and conforming perfectly to the desired contour or, with the interposition of the organic sheet, to the contour of the mold.By applying negative pressure on the one hand and (very high) positive pressure on the other, consolidated organic sheets can be formed into components with complex structures and small radii, allowing, for example, U-shaped profiles with and without undercuts to be produced flawlessly. High pressures in the membrane press ensure proper venting of the workpiece, preventing the formation of pores or allowing them to be removed. Overall, the manufactured components are characterized by very high surface quality and high stability.

[0017] This allows, for example, highly stable, lightweight components for aircraft construction, such as wings or wing parts, to be manufactured. For example, profiles can be produced that can be used as components of landing flaps.

[0018] Organic sheets with carbon fibers, glass fibers, and / or aramid fibers are preferred. High-temperature-resistant thermoplastics, such as polyetheretherketone (PEEK) or polyphenylene sulfide (PPS), are particularly preferred as thermoplastics. Alternatively, polypropylene (PP), polyamide (PA), or polyurethane (TPU) can also be used, depending on the requirements and application.

[0019] During production, it is advisable to heat the organic sheet before and / or after insertion into the press to optimize the forming process. It is advisable to heat the respective organic sheet to a temperature above the glass transition temperature. Depending on the organic sheet or the thermoplastic material, it may be advisable to heat the organic sheet to a temperature above 180 °C, e.g., more than 200 °C.

[0020] Alternatively or additionally, it is expedient to heat the mold or at least its surface facing the organic sheet before and / or during forming. Here, it may also be expedient to heat the mold or its surface to a temperature above the glass transition temperature of the thermoplastic, for example, to a temperature of more than 180 °C, e.g., more than 200 °C.

[0021] In addition, it is particularly advantageous, alternatively or additionally, if the pressure medium with which the membrane is subjected, for example a compressed gas, is also heated in order to optimize the heat input and improve the hot forming.

[0022] According to the invention, not only is a negative pressure applied to the side of the membrane facing the mold, but the side of the membrane facing away from the mold is also subjected to an overpressure, whereby an overpressure of at least 10 bar, for example at least 20 bar, can be generated particularly preferably. According to the invention, high pressures are therefore used to account for the fact that consolidated organic sheets are being processed or formed.

[0023] Instead of using a vacuum bag as is common with membrane presses for processing prepregs or injecting resin, the highly elastic membrane is stretched over the mold. According to the invention, the membrane is elastically pre-tensioned and attached to the upper part of the press and is then stretched over the mold as the press closes.

[0024] In principle, rubber membranes can be used. Considering the fact that high-temperature-resistant plastics are preferred, the invention recommends the use of a membrane made of a highly elastic and simultaneously temperature-resistant material, for example, silicone or silicone-based. Existing silicone membranes can be used that have an elongation at break of at least 500%, preferably at least 600%. The membrane preferably has a thickness of at least 1 mm, particularly preferably at least 2 mm.

[0025] As already described, the organic sheet used is a prefabricated semi-finished product consisting of several organic sheet layers or a plurality of organic sheet layers, which are combined and optionally bonded to one another before being placed in the press. It is within the scope of the invention to combine the organic sheet layers individually and press them together. However, the organic sheet layers are preferably bonded to one another beforehand (in a desired arrangement), for example by welding and / or gluing, with an intimate bond then subsequently occurring during forming in the membrane press. Alternatively, it is within the scope of the invention to combine the individual organic sheet layers in a pre-press to form a uniform organic sheet.

[0026] A large number of layers can be used, for example, at least five layers, preferably at least ten. For highly stable components (e.g., for aircraft construction), more than twenty layers can be combined to form an organo sheet.

[0027] It is within the scope of the invention to use individual layers with different fiber orientations or to stack the individual layers in such a way that their fibers do not run parallel, but at a predetermined angle. In this way, particularly stable organic sheets and corresponding components can be produced. The properties and geometry of the component can be excellently influenced by the selection and arrangement of the individual layers. For example, it is possible to provide individual layers in different sizes to form an organic sheet with a thickness that varies across the surface. This way, for example, workpieces with a greater thickness or wall thickness are created in areas where there are more layers than in other areas. Similarly, it is possible to arrange the individual layers in such a way that a desired edge geometry of the component is created when the individual layers are shifted relative to one another during the forming process.For example, if the individual layers are arranged flush in the unformed state, a beveled edge geometry can be created through forming. Conversely, a straight edge geometry can be achieved through a beveled arrangement of the individual layers in the edge area. For example, it may be desirable to produce components with beveled edges to provide improved joining surfaces for further processing.

[0028] The method according to the invention can be carried out using a press for producing a component from a fiber composite material using a method of the type described. Such a press is constructed as a membrane press, which has a lower press part on which a mold is arranged, and an upper press part, which has a pressure housing that can be sealed against the lower press part. Furthermore, a membrane is provided, which can be tensioned over the mold.

[0029] The press has at least one press cylinder acting on the upper press part and / or the lower press part. The press also has a vacuum pump, which can be used to generate a negative pressure on one side of the diaphragm, for example, the underside, and a positive pressure pump, which can be used to generate a positive pressure on the other side of the diaphragm.

[0030] The press can be designed so that the mold or the lower part of the press is heatable and thus equipped as a heating device. Furthermore, the press is designed so that the pressure medium applied to the membrane is heatable, for example, by arranging a heating device in the area of ​​the pressure medium supply.

[0031] The membrane is elastically pre-tensioned and attached to the upper part of the press, for example to the pressure case.

[0032] The invention will be explained in more detail below with reference to a drawing which merely represents an exemplary embodiment. Fig.1 a membrane press in a simplified representation, Fig. 2 the object according to Fig. 1 in another functional position, Fig. 3 a membrane press for carrying out the method according to the invention Fig. 4 the press according to Fig. 3 in another functional position, Fig. 5 shows a forming process of a multi-layer organic sheet in a first embodiment, and Fig. 6 shows a forming process of a multi-layer organic sheet in a second embodiment.

[0033] The figures show a membrane press 1 for producing a component from a fiber composite material. In such a membrane press, a component from a fiber composite material is produced by forming a thermoplastic organic sheet 2. In the exemplary embodiment, the membrane press 1 has a lower press part 3 which is designed as a press table, on which a mold 4 is arranged as a negative mold of the component to be produced. The press 1 also has an upper press part 5 which has a hood-like pressure box 6 which can be sealed against the lower press part 3. For this purpose, the lower circumferential end face 7 of the pressure box 6, which can be placed on the press table, is provided with a circumferential seal 8. A press cylinder 9 acts on the upper press part 5, wherein in the exemplary embodiment the piston 10 of the press cylinder 9 is connected to the pressure box 6, so that the pressure box 6 can be pressurized with the cylinder 9 orwhose piston 10 is pressed against the press base 3. Furthermore, the diaphragm press 1 is equipped with an elastically expandable diaphragm 11, which can be tensioned over the mold 4. Furthermore, a vacuum pump 12 is provided, which in the exemplary embodiment is connected to the press base 3. Furthermore, an overpressure pump 13 is provided, which in the exemplary embodiment is connected to the press top 5 or to the pressure case 6.

[0034] To form an organic sheet 2, this organic sheet is placed on the mold 4 and the membrane 11 is stretched and prestressed over the mold 4 with the organic sheet 2 interposed.

[0035] The organic sheet is formed to form the component by applying a negative pressure to the membrane 11 on the side facing the mold 4 via the vacuum pump 12 and applying an overpressure to the membrane 11 on the side facing away from the mold 4 using the overpressure pump 13, so that the organic sheet 2 conforms to the mold 4 to form the component.

[0036] It is provided that the organic sheet 2 is heated before being inserted into the press 1. Furthermore, it is preferably provided that the mold 4, or at least its surface facing the organic sheet 2, is heated before and / or during the forming process. Finally, it is expedient if the pressure medium with which the membrane is subjected to overpressure is heated. For this purpose, a heating device 14 is indicated in the figures. The heating devices for heating the organic sheet and for heating the mold are not shown.

[0037] Fig. 1 shows an embodiment of such a membrane press, in which the membrane 11 is attached to the press base 3 and stretched over the mold 4. Fig. 1 shows the press after the organic sheet 2 has been placed on the mold 4 and the membrane 11 has been tensioned over the mold 4 with the organic sheet 2 interposed. Furthermore, after the organic sheet 2 has been inserted and the membrane 11 has been tensioned, the upper press section 5 has been lowered onto the lower press section 3 and sealed. The negative pressure can be generated with the vacuum pump 12 before and / or after the upper press section is lowered. After the upper press section 5 has been lowered against the lower press section 3 and sealed, the interior of the pressure case 6 is pressurized. It can be provided that the pressure generated by the cylinder 9, with which the membrane press is closed as the internal pressure builds up, is gradually increased and consequently adjusted as the internal pressure builds up. Fig. 2 shows the press after the overpressure and the negative pressure have been built up with formed organic sheet 2.

[0038] The Fig. 3 and4 show an embodiment of a membrane press for carrying out the method according to the invention, in which the membrane is not attached to the lower press part 3, but to the upper press part 5, namely to its pressure case 6, and is elastically prestressed ( Fig. 3 ). After placing the organic sheet 2 on the mold 4, the pressure case 6 is lowered and the membrane is stretched over the mold with the organic sheet 2 interposed ( Fig. 4 After the press has been closed, both negative pressure and positive pressure are built up, thus forming the organic sheet 2 and producing the component.

[0039] The organic sheet 2 consists of several individual organic sheet layers 2a, which are combined to form the organic sheet 2 and formed in the press. The geometry of the layers 2a can be adjusted to one another in such a way that the individual layers 2a shift against each other during the forming process, changing the edge geometry of the component. This option is available in the Fig. 5 und 6 According to Fig. 5 The individual layers 2a are combined to form an organo sheet 2 with straight edges. During the forming process, the individual layers shift relative to each other, creating a component with beveled edges.

[0040] In contrast, Fig. 6 an embodiment in which the individual layers 2a of the organic sheet 2 do not lie flush on top of one another, but form oblique edges, so that in the course of the forming process a component with straight edges without bevels is created.

Claims

1. Method of manufacturing a component made of a fibre-reinforced composite material by forming a thermoplastic organic component (2) in a membrane press (1), wherein the membrane press comprises a press lower part, onto / on which a mould is arranged, and wherein the membrane press comprises a press upper part which has a pressure case that can be sealed against the press lower part, wherein as the workpiece, at least one organic sheet (2) is placed onto or on the mould, wherein, as prefabricated semi-finished product, the organic sheet (2) consists of several organic sheet layers (2a) which are placed together before insertion into the press (1), wherein an elastically expandable membrane (11) is stretched over the mould (4) with the interspersion of the organic sheet (2), wherein the membrane is fastened to the press upper part in an elastically pretensioned manner and wherein the organic sheet (2) is formed, forming the component, in that a negative pressure is applied to the side of the membrane (11) facing the mould and an overpressure is applied to the side facing away from the mould, so that the organic sheet (2) adapts to the mould.

2. Method according to claim 1, wherein before and / or after being placed in the press (1), the organic sheet (2) is heated, for example to a temperature of more than 180 °C.

3. Method according to claim 1 of 2, wherein the mould (4) or at least its side facing the organic sheet (2) is heated before and / or during the forming, for example to a temperature of more than 180 °C.

4. Method according to any one of claims 1 to 3, wherein pressure medium acting on the membrane (11) with overpressure is heated.

5. Method according to any one of claims 1 to 4, wherein the overpressure is at least 10 bar, preferably at least 20 bar.

6. Method according to any one of claims 1 to 5, wherein the organic sheet layers (2a) are connected to each other before being placed in the press (1).

7. Method according to claim 6, wherein the individual layers (2a) have different fibre orientations.

8. Method according to claim 6 or 7, wherein individual layers (2a) are of different sizes, forming an organic sheet with a thickness varying over its area.

9. Method according to any one of claims 1 to 8, wherein the membrane (11) is made of silicone.

10. Method according to any one of claims 1 to 9, wherein the membrane (11) has a thickness of at least 1 mm, preferably at least 2 mm, and / or an elongation at break of at least 500 %, preferably at least 600 %.

11. Method according to any one of claims 6 to 10, wherein during the course of forming, the individual layers (2a) are displaced in relation to each other, changing the edge geometry of the component.