FRERES, Patrick (17 rue des Vergers, Echternach, Echternach, L-6488, LU)
BOTTLER, Oliver (Valdenaire Ring, 160, Konz, 54329, DE)
FRERES, Patrick (17 rue des Vergers, Echternach, Echternach, L-6488, LU)
| Claims 1 . Process for manufacturing a hollow body of fibre composite plastic, in particular a hollow cylindrical tube such as a roller tube of fibre plastic composite, including: providing a preform assembly comprising: a self-supporting elongated soluble core that has a lateral surface between two end surfaces and is manufactured based on a shaping material that is dispersible in liquid; and a braid of dry reinforcing fibres braided onto said elongated soluble core so as to surround said lateral surface of said soluble core; impregnating the reinforcing fibres of said braid with curable matrix material in an infusion process, wherein matrix material is sucked, by action of reduced pressure, into an infusion space that is delimited by a vacuum film that surrounds the preform assembly; partially or fully curing the matrix material; and removing the soluble core by dissolution with a liquid. 2. Process according to claim 1 , wherein said preform assembly comprises a flow promoter, preferably a fibre-reinforced flow promoter, that surrounds said lateral surface of said core and onto which said braid of dry reinforcing fibres is braided so as to surround said lateral surface of said core. 3. Process according to claim 1 or 2, comprising round braiding of a round braid of reinforcing fibres around the soluble core, preferably by a radial braiding technique or by a 3D-rotational braiding technique. 4. Process according to claim 1 , 2 or 3, wherein in a given braided layer of the braid the co-rotating and counter-rotating braided threads respectively have approximately the same flexural strength and preferably consist of the identical fibre material. 5. Process according to any one of claims 1 to 4, wherein a low viscosity matrix material with a viscosity of <600 mPa s is sucked in. 6. Process according to any one of claims 1 to 5, comprising braiding of a plurality of braided layers arranged on top of one another on the soluble core, wherein one or more external braided layers consist of a fibre material different from that of the inner layers and/or wherein at least one external braided layer has a fibre orientation with a larger braid angle than have the inner braided layers. 7. Process according to any one of the previous claims, comprising enveloping the preform assembly with a semi-flexible, gas-permeable shape-processing aid, which is preferably impermeable to matrix material, in order to improve the surface quality of the completed hollow body. 8. Process according to any one of the previous claims, wherein the preform assembly comprises dry, non-impregnated reinforcing fibres, which are optionally provided with auxiliaries, said fibres enclosing the soluble core in the circumferential direction. 9. Process according to any one of the previous claims, wherein the soluble core consists of a hardened liquid-dispersible shaping material, in particular of a shaping material that comprises an inorganic filler, in particular quartz sand, and a water-soluble binder, preferably an inorganic binder such as a potassium silicate, a polyborate, a polyphosphate, a metal oxide or a mixed-type binder comprising a mixture of a polyacid with a silicone or an ester. 10. Process according to any one of the previous claims, wherein the matrix material is sucked into the infusion space by the action of vacuum of <20 mbar and wherein partial curing respectively full curing of the matrix material is preferably carried out under this vacuum of <20 mbar and optionally under the supply of heat. 1 1 . Process according to any one of the previous claims, wherein the core is enclosed by a sealing film that is impermeable to the matrix material before the reinforcing fibres are braided around the soluble core. 12. Process according to any one of the previous claims, wherein a plastic resin from the group of the thermosets is used as the matrix material, especially an epoxy resin, cyanurate resin, cyanate ester resin, polyester resin, phenolic resin, vinyl ester resin, acrylic resin, polyimide resin, benzooxazine resin or silane or a resin mixture of at least two of the aforementioned resins. 13. Process according to any one of the previous claims, wherein glass fibres, carbon fibres, boron fibres, Aramid fibres, ceramic fibres, metal fibres, natural fibres or a combination of at least two of the aforementioned fibre types, preferably in the form of endless rovings, are used as the reinforcing fibres. 14. Process according to any one of the previous claims, comprising enclosing the preform assembly of reinforced fibres and soluble core with a resin-permeable separation layer; putting a resin distribution medium and/or a flow promoter onto the separation layer or, in case of enveloping the preform assembly that is enclosed with a separation layer by means of a shape-processing aid, putting a resin distribution medium and/or a flow aid onto the shape- processing aid; sealing the assembly comprising at least the preform assembly, the separation layer, optionally the shape-processing aid and the resin distribution medium and/or the flow aid in a vacuum bag; and then impregnating the reinforcing fibres with curable matrix material. 15. Process according to any one of the previous claims, wherein the preform assembly has a longitudinal axis that is vertically oriented when the reinforcing fibres are impregnated with matrix material in the infusion process and wherein the matrix material is preferably sucked into the infusion space in the direction opposite to the gravitational force. |
COMPOSITE
Technical Field
[0001] The present invention generally relates to composite materials and in particular to a process for manufacturing hollow bodies of fibre-reinforced plastic.
Prior art
[0002] A fibre composite material, as is known, is a multiphase or mixed material of at least two main components (bedding matrix and reinforcing fibres). A resin from the group of the thermosets (also curable plastics, synthetic resins), the elastomers or the thermoplastics is used as the bedding matrix. Various types of materials can be used as fibres (e.g. hydrocarbon fibres (also called carbon fibres), glass fibres, Aramid fibres, etc.).
[0003] In the context of the present invention, hollow bodies are components with a closed circumference, i.e. components that are circumferentially closed around the longitudinal axis and which define a hollow body (such as e.g. pipes, rollers, mandrels or the like). Hollow bodies of fibre-reinforced plastic (FRP), i.e. FRP components, have many uses in industry. Thus, rotationally symmetrical round bodies of FRP are used for example as light weight structural elements in the aviation and automotive industries, as containers for material storage or as pipes for material transport in the logistics industry, or as shafts or axles in mechanical engineering. Hollow components of FRP with non-developable geometry also find diverse applications, for example as structural hollow components in the aviation or automotive industries.
[0004] Various processes are known for manufacturing hollow bodies of FRP. Fibre winding (filament winding) is a widespread and to a large extent automated process, in which reinforcing fibres are wound over a positive (male) core, wherein the core geometry defines the internal cavity of the component (hence positive core). Depending on the core geometry, reusable cores, releasable cores of a soluble medium or lost cores that remain in the component can be used. Filament winding is a preferred manufacturing technique for shaped articles such as containers, tubes, axles and shafts (see Neitzel, Mitschang (Ed.) Handbuch Verbundwerkstoffe, Munich: Carl Hanser, 2004. - ISBN 3-446-22041 -0). For automated thermoset winding, rovings originating from a reel are impregnated in an impregnation device or immersion device with liquid reactive resin and then controllably wound by means of a feed eye onto the core. The fibre lay-up thus wound on the core is subsequently cured in an oven or autoclave, usually under continuous rotation about the component axis. For thermoplastic winding on the other hand, pre-impregnated rovings are fused onto the core at the lay-on point by means of a suitable heating device. Both of these types of filament winding processes imply a relatively high equipment cost, for example inter alia immersion equipment moveable with the mobile and adjustable feed eye or precise heating systems and correspondingly complex process controls.
[0005] Another known, highly automated direct process is the so-called "tape- fibre placement" process, which is possibly also suitable for manufacturing hollow components. In contrast to the filament winding process that can realise geometries with undercuts only in the case of rotational symmetry, the tape-fibre placement process also allows more complex, even multi-curved (left and right curved) geometries to be manufactured in fully automatic manner. For tape placement, the semi-finished part is accurately contour-layered by means of tape layers (portal robots) with a fully automated placement head. Also, both thermoplastic tapes as well as fibre tapes impregnated with thermoset resins can be used for tape laying, wherein thermoset resins are then typically cured in the autoclave.
[0006] The abovementioned processes are characterised by their very high degree of automation. Consequently, in particular, but not exclusively due to the automated resin impregnation and subsequent autoclave curing (for thermosets) and the automated in situ consolidation (for thermoplastics), the processes imply a high equipment cost and corresponding investment costs. Accordingly, the acquisition of such equipment is mainly economically justifiable only for high-quantity component production runs.
[0007] Another known approach for manufacturing hollow FRP articles such as hollow tubular parts uses sleevings made of braided prepregs, i.e. pre- impregnated "free standing" braids. The article "Prepreg braid and its uses" (Sainsbury-Carter, J. B.; Manufacturing Technology International, London: Sterling Publications Ltd., 1990, ISSN: 0950-4451 ) recommends, for complex parts or when outside surface fidelity is critical, using a split female mould, for curing the prepreg with an internal pressure bladder as mandrel (male form) for defining the inside surface. The moulding of prepreg braids requires fairly high pressure involves practical difficulties due to distortion of the prepreg sleeves during manufacture and, as for the aforementioned approaches also involves comparatively high investment cost.
[0008] A further process, in which in particular the impregnation of the fibres is carried out in another way, is incidentally described in the European Patent application EP 1 745 908, which relates to a water-dispersible supporting core. A reduction in investment costs is to be expected for this process in comparison with the abovementioned prior art processes. In the process according to EP 1 745 908, dry fibres on a water-dispersible supporting core are impregnated with a heat-curable plastic, for example an epoxy resin, once the already dry fibres have been deposited onto the core. The plastic can be cured following the RTM process or following the vacuum injection process. In other words, a preassembled water-dispersible supporting core wound with reinforcing fibres according to EP 1 745 908 is firstly placed between both female moulding tools, i.e. for example the upper and the lower tooling of a heated press. The reinforcing fibres on the supporting core are then impregnated with the heat-curable plastic, which is injected into the cavity between both of the tooling parts, in which the supporting core is located. According to EP 1 745 908, in the RTM process the heat-curable plastic can be compression moulded into the cavity under a pressure of 50 bar, in particular more than 80 bar or even 100 bar and more, whereas in the vacuum injection process the heat-curable plastic is injected with the aid of a vacuum into the cavity between both of the female moulding tools with the assembled male supporting core located therein. According to EP 1 745 908, after curing of the resin, demoulding takes place and the supporting core is flushed with water such that the fibre-reinforced hollow structural component is formed.
Object of the Invention
[0009] Having regard to the abovementioned prior art, the object of the present invention is to propose a cost-efficient, particularly low investment cost process for manufacturing a hollow body of fibre plastic composite.
[0010] This object is achieved by means of a process according to claim 1.
General description of the invention
[0011] The process for manufacturing a hollow body of fibre plastic composite according to claim 1 includes the following steps:
providing a preform assembly comprising a braid, preferably a braid of the "round braid" type, of dry reinforcing fibres (i.e. non-impregnated fibres) braided around a self-supporting elongated positive core made of soluble, liquid-dispersible material so as to circumferentially engirdle i.e. enclose the lateral surface of the core along at least a major portion of the core length;
impregnating the reinforcing fibres with liquid, curable matrix material in the infusion process according to the vacuum bag (moulding) principle, wherein, by means of reduced pressure, the matrix material is sucked into a technically vacuum-tight vacuum film surrounding the perform assembly; partial or full curing of the matrix material using the self-supporting positive core as exclusive positive moulding tool that entirely defines the shape of the fibre plastic composite to be produced; and
removing the soluble core by dissolution with a liquid.
[0012] The resin infusion process is a cost-efficient process with a low degree of automation, in particular in comparison with the RTM resin injection process with closed, tightly sealable negative (female) mould halves. Moreover, it should be noted that, with the presently proposed Rl process, all in all, no negative (female) moulds are needed (also no one-piece supporting mould), as the shaping function is integrally assumed by the self-supporting internal positive (male) core. The Rl process according to the vacuum bag (moulding) principle additionally enables rapid and cost-efficient modifications of the component geometry, as only the shape of the core needs to be modified to this effect. However, due to the absence of injection overpressure, higher demands are placed on the impregnation of the fibre assembly. Using a self-supporting soluble core in combination with a braid of dry fibres facilitates transportation and storage of the perform assembly. Accordingly, resin infusion and braiding need neither be carried out on the same site nor with short time-lag.
[0013] In contrast to other manufacturing possibilities for fibre assemblies on a supporting core (e.g. an orientated fibre packet produced solely by winding or enveloping the core with flat woven fibre mats) it has emerged that the use of a suitably chosen fibre braiding enables a reliable application of the vacuum bag infusion process, although the braiding technique implies inter alia lower fibre volume densities. The production of fibre bends in the interweaving strands (threads) of the braid creates inherent flow channels for the resin in the infusion process. The presence of flow channels is a decisive requirement for vacuum infusion in order to adequately and homogeneously saturate (i.e. fully impregnate) the fibre packet, which is impeded by the vacuum compaction. In comparison to this, fibre packets produced by filament winding or by tape-fibre placement and which consist of a dense stacking of purely mono-directionally laid fibres are not or only inadequately homogeneously saturated with resin, due to a lack of free volume or cavities in the fibre packet, particularly in the vacuum compacted state.
[0014] In comparison with the known filament winding technique with pre- impregnated fibres, the deposition of dry fibres by braiding has the further advantage that the installation does not need to be equipped with an impregnation bath, and any adjustment of the impregnation of the fibres with reactive resin as a function of the variable winding speed (e.g. when tapering the cross section of the core), is obviated. As an expendable product, the use of a soluble positive (male) core for the shaping process enables additional savings. Compared with the use of RTM mould halves or of reusable cores, laborious cleaning is not required. In addition, compared to the RTM process, the proposed process obviates the effort of positioning the core in place inside the closable RTM mould halves.
[0015] Preferred embodiments of the inventive process are defined in dependent claims 2-15. Further details and advantages of the invention can be discerned from the following detailed description of possible embodiments of the invention.
Brief Description of the Figures
[0016] A preferred development of the invention will now be described in more detail below with the help of the accompanying drawings, in which:
FIG.1 shows a schematic representation of a process set up for carrying out a process according to the invention;
FIG.2 shows a schematic cross-sectional representation of a second process set up for carrying out a second embodiment of the process according to the invention.
Identical reference signs are used to identify identical or similar elements throughout the drawings. Description of a preferred development of the invention
[0017] The process set up, schematically represented in FIG.1 for illustrating the invention, includes a soluble positive core 10 (shown by dotted line). The soluble positive core 10 defines the inner contour (i.e. the shape of the inner surface) of the FRP-hollow body and possesses a correspondingly conjugated outer surface that depends on the type of component. The positive core 10 thus provides a "male" moulding tool. In order to manufacture a FRP-roller tube to be used in a printing press for example, the positive core 10 has a simple circular cylindrical outer surface. Accordingly, the positive core 10 of FIG.1 has two substantially opposite end faces (front faces) between which extends a cylindrical convex lateral surface, i.e. the shell surface of the core 10. More complex geometries with curvatures and/or diameter variations in the component are also possible, for example non-developable and/or multi-curved and/or undercut i.e. not purely convex surfaces of the positive core 10. In other words, the lateral surface of the core 10, which primarily defines the shape FRP-hollow body to be produced, need not be cylindrical nor strictly convex.
[0018] The positive core 10 is manufactured on the basis of a liquid-dispersible i.e. dissolvable shaping material. Accordingly, the positive core 10 consists essentially of a liquid-dispersible, compacted and cured shaping material that includes sand, especially quartz sand, as the filler and a water-soluble binder, e.g. an organic binder such as starch. The production of these types of liquid- soluble supporting cores is known per se and therefore will not be mentioned here in more detail. Reference may be made to the patent documents EP 1 745 908, DE 195 34 836 or US 5 262 100, which comment on the production of such cores. Due to the use of an Rl process without high pressure (see further below) it should be noted that, whilst the core 10 is self-supporting (see further below), the compressive strength of the positive core 10 can be designed to be relatively low, for example in the range of 2-10 bar, thereby enabling a more cost-efficient production and more cost-efficient material choice for the soluble positive core 10.
[0019] The soluble positive core 10 is externally circumferentially sealed tightly with a, preferably technically vacuum-tight sealing film (not shown), that is impermeable for the matrix material used in the process, for example by a thermoplastic film that is suitable for the required curing temperature (see below).
[0020] A fibre braid 12 is braided onto the soluble positive core 10, more specifically, as seen in FIG.1 , the braid 12 is braided around the positive core 10 so as to engirdle, i.e. surround the lateral surface of the positive core 10. The fibre braid 12 is manufactured by known suitable braiding techniques, preferably by round braiding techniques such as radial braiding or by 3D-rotational braiding techniques. Reference may be made, for example, to the patent application WO 2004/057082, which discloses a device for braiding onto a core. The process parameters to be used for the production of the fibre braid 12 such as braiding angle, braiding fibre tension, number of braiding fibres, etc., are predetermined according to the intended use of the FRP-hollow body, such that the fibre orientations are matched to the expected loading of the FRP-hollow body. The braiding angle (angle of the fibres to the longitudinal axis of the component) in the fibre braid 12 is generally between 10° and 70°. In order to improve the mechanical properties, filler threads, especially 0° filler threads, can be braided in.
[0021] The fibre braid 12 constitutes the actual reinforcing fibres (e.g. of glass fibres and/or carbon fibres) of the FRP-hollow body to be manufactured. The fibre braid 12 consists of dry (i.e. not pre-impregnated with matrix material) FRP-reinforcing fibres. The fibres, from which the fibre braid is produced, can however be optionally provided with process auxiliaries (for example binders for the shaping process). The fibre braid 12 consists of braided layers arranged one over the other (individual layers of the braid) and as a round braid (forming a "core braid"), surrounds the positive core 10 completely in the circumferential direction with respect to the longitudinal axis 14 of the FRP-hollow body or of the positive core 10. The braided fibres in the fibre braid 12, i.e. the reinforcing fibres in the FRP-component to be produced, consist of endless fibre rovings (also called filament yarns), for example glass fibres, carbon fibres, boron fibres, Aramid fibres, ceramic fibres, metal fibres, natural fibres or of a combination of at least two of these fibre types. In case a subsequent additional treatment by machining the surface of the hollow body is intended, one or more external ("lost") braided layers of the fibre braid 12 are preferably provided from more favourably priced and/or more easily processable fibre material with respect to the inner layers. Alternatively or in addition, at least one external braided layer can have a fibre orientation with a larger braid angle, such that a lesser quantity of fibre material is sacrificed during the further treatment and/or a higher surface quality can be obtained in this way by the further treatment.
[0022] In order to produce flow channels in the fibre arrangement circumferentially surrounding the positive core 10, individual braided layers in the fibre braid 12 each have co-rotating and counter-rotating threads with approximately the same flexural strength (modulus of elasticity on bending or flexural modulus). By this means, free interstices (clearances) are produced in the braided layer at the fibre crossovers in the fibre braid 12 due to the direction changes of the braided fibres, i.e. by means of the undulation of the braided threads. This is in contrast to the uncrossed unidirectional (UD) fibre arrangements that are laid down in the pure filament winding process but also in contrast to the round braiding process known from the patent application WO 2005/098117 for fibre-composite semi-finished products which precisely aims to avoid such free interstices in order to increase the fibre volume density. In the present process on the other hand, co-rotating and counter-rotating braided threads of preferably identical fibre material are used in each braided layer of the fibre braid 12, wherein variations in the fibre material between different braided layers are however not excluded.
[0023] The present process specifically produces free spaces in the fibre braid 12 and thereby creates inherent "flow channels" in the fibre braid 12 for an improved and more regular impregnation with matrix material during the subsequent resin impregnation in the Rl process. The average spacing between parallel co-rotating braided threads and/or parallel counter-rotating threads within at least one individual braided layer of the fibre braid 12 is preferably in the range 2-20% of the braided fibre cross section (when braiding single fibres as braided threads) or of the roving width (when braiding rovings as threads) such that a minimum volume of the interstices for the resin impregnation is guaranteed, without the fibre volume density being reduced too much in the hollow body being manufactured.
[0024] The braided fibre braid 12 and the sealed soluble positive core 10 that is enclosed in the circumferential direction by the fibre braid 12, together form a so-called "preform" assembly. A suitable resin-permeable separation layer, such as for example a perforated separation film known per se (not shown in more detail), is arranged around the perform assembly prepared as described above. A peel ply, for example, is also suitable as a resin-permeable separation layer. Subsequently, a suitable resin distribution medium or flow promoter 15, known per se, (shown schematically) is laid onto the resin-permeable separation film in order to promote the subsequent impregnation of the fibre braid 12 with resin. In order to further improve the resin flow behaviour inside the process installation, additional resin distribution strips can be deposited onto the resin distribution medium or the resin distribution medium can be laid in multiple layers, for example in two layers.
[0025] In order to improve the surface quality or surface finish in regard to unevenness and to improve the regularity of the wall thickness in the FRP- hollow body to be produced, the use of a semi-flexible component clamp as a shape-processing aid (not shown) is advantageous. Such a shape-processing aid is provided between the resin-permeable separation film and the resin distribution medium or flow aid, i.e. put directly onto the separation film. The shape-processing aid can, for example, be provided in the form of two perforated sheet metal shells that can interlock with one another under vacuum. A perforated silicone sheath or tube made of suitable material can also be used. The shape-processing aid is clamped around the preform assembly with a suitable clamping force and is required to be shape-giving but gas-permeable. Moreover, by adding a matrix material barrier in the form of a semi-permeable membrane (e.g. a so-called VAP membrane) on the inside of the shape- processing aid, a subsequent cleaning of matrix material from the shape- processing aid can be avoided. The shape-processing aid is retained, being clamped around the component, as a jacket during the subsequent resin infusion and is only removed from the component once the resin has been cured.
[0026] As can be seen in the schematic layout in FIG.1 , a spiral hose 16 is provided as the inlet pipe on the lower end of the preform assembly in order to ensure a uniform supply of resin, particularly in the circumferential sense. The spiral hose 16 is connected to a resin inlet pipe 18, in which is provided a shut- off device 19 for stopping resin supply, for example in the form of clamping tongs or a shut-off valve. The resin inlet pipe 18 is connected to a resin container 20, which stores the liquid, low viscosity resin, preferably a resin with an infusion viscosity < 600 mPa s. Another spiral hose 22 is provided for delivering a circumferentially uniform outflow of the resin at the opposite end of the preform assembly. The spiral hose 22 is connected to a vacuum line 24, for example a vacuum tube, with a device 25 for stopping resin supply, for example in the form of clamping tongs or a shut-off valve. The vacuum line 24 leads into a resin trap 26 that is connected with a vacuum pump 28 in a gastight manner over a vacuum line, in which a device for blocking the vacuum 27 is provided, which protects the pump from excess resin.
[0027] The above-described assembly consisting of preform assembly, i.e. sealed positive core 10 with surrounding fibre braid 12, separation film, optional shape-processing aid, resin distribution medium or flow aid and provided at both ends with spiral hoses 16, 22, is then enclosed with a vacuum bag 30 made of flexible, pliant and technically gastight vacuum film. The vacuum bag 30 is required for putting the preform assembly under reduced pressure in order to carry out the Rl process, but itself has no shape-processing function. As a result of the vacuum infusion process that is used, the surrounding atmosphere, which works against the supporting action of the positive core 10 and thus presses the fibre braid 12 onto the positive core 10, acts as a negative mould. The external contour or the external surface is determined exclusively by the surface of the positive core 10 together with the nature (especially thickness) of the fibre braid 12. Expensive metallic negative moulding tools, as are used for the moulding press in the RTM process, are therefore not required in the present process.
[0028] As is apparent from FIG.1 , a suspension device 32 is provided on one front end of the preform assembly, for example in the form of a suitable hook that is screwed into the positive core 10. A suitable vacuum sealing tape is used in order to seal the point of passage of the resin inlet tube 18, the vacuum line 24 and the suspension device 32 and to tightly seal the vacuum bag 30. The assembly that is preferably already under reduced pressure and enclosed in a gastight manner with the vacuum bag 30 is vertically suspended by means of the suspension device 32 from a gibbet 34, i.e. such that the longitudinal axis 14 is aligned vertically. It follows that the positive core 10 has a self-supporting dimensionally stable structure.
[0029] After the material process set up, the process sequence will be discussed below in more detail. According to the invention, the preform assembly as described above is subjected to a resin infusion process in the vacuum bag 30. To this effect, the set up sealed in the vacuum bag 30 is firstly subjected to an technical vacuum in that the vacuum pump 28 is switched on and the closing devices 25, 27 are opened. On reaching a predetermined reduced pressure < 20 mbar (abs), e.g. <0.5 mbar (abs), the set up is suspended vertically by means of the suspension device 32 onto the gibbet 34.
[0030] The resin infusion operation is then begun by opening the shut-off device 19. It should be noted that on impregnating the reinforcing fibres with matrix material, the matrix material is sucked in from bottom to top, i.e. according to a resin infusion direction 36 opposite to gravity, into the infusion space i.e. into the resin-tight enclosed space between the sealed positive core 10 and vacuum bag 30. A regular resin distribution is guaranteed in the axial, circumferential as well as in the radial directions by the corresponding course of the resin front, because local resin accumulations are avoided. A suitable matrix material is a plastic resin from the group of the thermosets, especially an epoxy resin, cyanurate resin, cyanate ester resin, polyester resin, phenolic resin, vinyl ester resin, acrylic resin, polyimide resin, benzooxazine resin or silane or a resin mixture of at least two of the cited resins.
[0031] After having checked (e.g. visually) that the fibre braid 12 has been completely saturated, the resin supply is cut off, i.e. the shut-off device 19 is closed. The preform assembly saturated with matrix material is held under vacuum (< 20 mbar (abs) e.g. <0.5 mbar (abs)) throughout the whole of the subsequent curing phase. The partial or full curing (pretempering) can be carried out at room temperature or at higher temperature depending on the matrix material used, for example in a heated room or by suitable heating agents.
[0032] The vacuum is removed after curing the matrix material or resin. The vacuum bag 30, the resin distribution means 15, the optionally used shape- processing aid and the separation film (the latter facilitates the removal) are then removed such that only the waste positive core 10 remains in the manufactured, cured hollow body of fibre plastic composite.
[0033] In order to remove the positive core 10, it is dissolved out of the interior of the FRP-hollow body with water, e.g. by means of a hose (running water) or by dipping in water (still water).
[0034] After cleaning, drying and optionally required subsequent post-curing (tempering or post-curing e.g. in an oven), the fibre composite hollow body is completed as an intermediate product for further treatment (e.g. turning to a roller tube) or as a finished product (e.g. as a conduit) for use.
[0035] FIG.2 illustrates an alternative process set up for a variant of the process. Aspects that are not detailed below are identical or analogous to those set out above. As seen in FIG.2, the preform assembly also comprises a self- supporting soluble core 10 of elongated cylindrical shape with a lateral shell surface between its two end faces. As set out above, the core 10 is manufactured using on a suitable shaping material that is dispersible in liquid. A braid 12 of dry reinforcing fibres is also braided onto the positive core 10 and circumferentially surrounds the lateral surface of the core 10 along most of its length. As in FIG.1 , the core 10 is fully enveloped by a sealing film, which is impermeable to matrix material, before the braid 12 is braided around the core 10. In addition, as illustrated in FIG.2, the perform assembly includes an inner flow promoter 46 which is arranged on the sealing film and circumferentially surrounds a length of the core 10 in excess of the portion engirdled by the braid 12. The flow promoter 46 is thus sandwiched in between the braid 12 and the core 10 during the braiding operation. Preferably, the flow promoter 46 is of a suitable fibre-reinforced type, which are known per se. A fibre-reinforced flow promoter 46 is intended to form part of the hollow body of fibre composite plastic that is to be produced. In other words, a fibre-reinforced inner flow promoter 46 remains part of the hollow body to form its internal surface. Alternatively, a non-reinforced inner flow promoter 46 may be used and removed after curing, e.g. using an interposed peel ply. In the embodiment of FIG.2, a peel ply 44 surrounds the perform assembly to ease removal of a surrounding second outer flow promoter 15, as described above, after curing. Alternatively, a fibre-reinforced outer flow promoter 15 may be provided, without interposed peel ply, directly onto the outer surface of the braid 12 in order to form part of the cured intermediate product. In the latter case, the outer flow promoter 15 may then form part of the finished hollow body. A main advantage of using both an inner and an outer flow promoter 15, 46 lies in that hollow bodies of increased wall thickness can be reliably manufactured because complete impregnation of the braid 12 in the radial direction is warranted. In practice, the use of an additional inner flow promoter 46 has proven beneficial for producing hollow parts having a wall thickness >6mm. Furthermore, the braid 12 may have a reduced thickness thus enabling material and braiding cost savings. Using a fibre-reinforced outer flow promoter 15 enables additional savings.
[0036] As further seen in FIG.2, a vacuum bag 30 sealingly encloses the perform assembly, which comprises the core 10, the inner flow promoter 46, the braid 12, together with the outer flow promoter 15 and the interposed peel ply 44. To this effect, the vacuum bag 30 is sealing fixed at opposite ends onto the sealing film on the core 10 by means of vacuum sealing tape 48. A first resin feed inlet 36 and a second resin feed inlet 38, e.g. in the form of spiral hoses or perforated hoses, which both surround the outer flow promoter 15 and the longitudinally protruding portion of inner flow promoter 46 are provided at opposite sides adjacent to the ends of the braid 12. A resin outlet 40 surrounds the outer flow promoter 15 and is located substantially midway between the resin feed inlets 36, 38. As will be appreciated, the latter configuration allows reducing the resin infusion time substantially by half. In a manner analogous to FIG.1 , the resin feed inlets 36, 38 are connected to a resin supply whereas the resin outlet 40 is connected to a vacuum pump and a resin trap. Infusion and curing are carried out as described in relation to FIG.1 except that the set up including the preform assembly is supported in a horizontal position, e.g. by means of supporting studs 50 fixed to the self-supporting core 10.
[0037] The above-described process enables intermediate products or finished products to be manufactured for various applications. Some preferred examples are given below for the choice of various materials and process parameters.
Examples of material and process parameters:
- positive core:
o hardened core based on readily available quartz sand dispersed with an inorganic binder, e.g. an LK binder (available from Laempe GmbH, Schopfheim, D-79650 Germany).
o hardened ready-to-use filler-binder mix such as Aquacore 6001 (available from Aero Consultants AG, Uster, CH-8606, Switzerland);
• any other suitable binders that have cohesive properties for forming adhesive bonds between grains of inorganic filler may be used, in particular an inorganic binder such as a potassium silicate, a polyborate, a polyphosphate, a metal oxide or a mixed-type binder comprising a mixture of a polyacid with a silicone or an ester,
- fibre braid:
o 11 layers with braid angle: ± 15°; 90°; ± 15°; ± 15°; 90°; ± 15°; or
o 11 layers with braid angle: ± 75°; 90°; ± 15°; ± 15°; 90°; ± 75°; or
o 12 layers with braid angle: ± 15°; 90°; 0°; ± 15°; ± 15°; 0°; 90° ± 15°; or
o 11 layers with braid angle: ± 15°; ± 15°; 90°; 0°; 90°; ± 15°; ± 15; or
o 12 layers with braid angle: 0°; 90°; 0°; ± 45°; 90°; 90°; ± 45°; 0°; 90°; 0°.
- fibres:
o HT carbon fibre 24 K; or
o HT carbon fibre 12 K; or
o HT carbon fibre 6K; or
all available for example from Toray Carbon Fibers America, Inc. (Decatur, AL 35602, USA), from Toho Tenax Europe GmbH (Wuppertal, D-42103, Germany), or from Mitsubishi Chemical Corp. (Tokyo 108- 0014, Japan)
o fibreglass, e.g. S-glass or E-glass fibres
- matrix material:
o 2 component epoxy resins:
• Larit L285 with curing agent H287 (available from Lange+Ritter GmbH, Gerlingen; D-70839, Germany); or • RenLam RY113 with curing agent HY98 (available from Bodo Mόller Chemie GmbH, Offenbach/Main, D-63069, Germany); or
• Epikote RIM 935 with curing agent RIM H937 (available from Hexion Specialty Chemicals B.V., Rotterdam, NL-3194 DH Hoogvliet; Netherlands) or
• Toolfusion 1A with curing agent Toolfusion 1 B (available from Airtech Europe S.A., L-4562 Differdange, Luxemburg)
o 1 component resin:
• EC-RI-FST (available from EURO-COMPOSITES S.A., L-6401 Echternach, Luxemburg); or
• Epsilon 99100 (available from Henkel Corp. Aerospace Group, Bay Point, CA 94565, USA)
- Rl process parameters:
o resin infusion: at room temperature
o vacuum: <1 mbar (abs)
o pretempehng at 50 0 C (under vacuum) between 2-6 hours depending on the resin type used, then detaching the component from the set up;
o post-curing the component, depending on resin type, stepwise 1 -
2 hours in tempering steps of 20 0 C, up to 180 0 C
[0038] Finally, several advantages of the proposed process may be noted:
virtually no investments required for production infrastructure;
the process is a closed process, i.e. the process is physiologically favourable for the work area and personnel; virtually all 3D-hollow body geometries can be manufactured (the geometry is solely limited by the braiding technique used);
theoretically there is no limitation to the component length but at most only to the component diameter (e.g. by the braiding machine used).
high fibre volume contents can be realised, thereby enabling a low component weight;
by means of the infusion technique with vacuum bag (in contrast to the pressurised resin injection), a broader spectrum of resin systems can be used, thereby enabling additional improvements in the material properties.
List of reference numerals:
FIG.1 36 resin infusion direction
10 soluble positive core FIG.2
12 fibre braid 10 soluble positive core
14 longitudinal axis 12 fibre braid
resin distribution medium / (outer) resin distribution
15 15 flow promoter medium / flow promoter
16 spiral hose 30 vacuum bag
18 resin inlet pipe 36 first resin feed inlet
19 shut-off device 38 second resin feed inlet
20 resin container 40 resin outlet
22 spiral hose 44 peel ply
24 vacuum line inner resin distribution
46 medium / flow promoter
25 shut-off device
48 vacuum sealing tape
26 resin trap
50 supporting stud
27 vacuum closing device
28 vacuum pump
30 vacuum bag
32 suspension device
34 gibbet
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