INCREASING THE SESSIBILITY OF DRY FIBER PREFORMS

Description

[0001] The present invention relates to the production of rotationally symmetrical composite components constructed from dry preforms made of reinforcing fibers and subsequently impregnated or infiltrated with liquid resin to form a resin matrix. In particular, the present invention relates to the production of fiber preforms with increased impregnability for liquid resin.

[0002] In the aerospace industry, composite components made of fiber-reinforced plastic are increasingly in demand because they offer significant weight savings compared to metal components and because their variable material properties allow them to be adapted to a wide range of load profiles. The use of dry preforms is preferred for cost reasons, as the provision of pre-impregnated prepregs is complex and can lead to storage problems.

[0003] Fiber preforms are typically produced by depositing and / or unwinding prefabricated fiber sheets or fiber bundles (rovings) onto a core or mold. One well-known method for this is the AFP process, in which dry fiber rovings are mechanically deposited and fixed along any desired path to stabilize the preforms. The finished, near-net-shape fiber preforms are then typically placed into a sealable mold or vacuum bag and impregnated or infiltrated with a curable plastic material (resin matrix).

[0004] In the manufacture of pressure vessels, wet winding with in-situ resin-impregnated rovings is state-of-the-art. Disadvantages include the process-related need for excess resin, which incurs additional costs; laminate settling, which can lead to laminate waviness, especially in thick-walled components, and which must then be avoided by gradually building up the laminate thickness with intermediate curing; and limitations due to the available pot life of the resin.

[0005] Alternatives include the resin infusion process and RTM, in which a fiber preform is first created by winding and laying dry fiber materials, which is then impregnated with a resin matrix. The final curing produces the finished component. The preform must be compact and dense to minimize settling in the thickness direction, thus preventing fiber waviness. However, such compact preforms made of wound rovings and similar semi-finished fiber products are very difficult to impregnate, especially in the thickness direction and with large wall thicknesses.

[0006] US 2014 / 0014667 A1 describes a method for producing a hollow body for a pressure vessel made of fiber-reinforced plastic. This involves using a fiber yarn (continuous fibers) that is provided with so-called spacers arranged like barbed wire along its length and circumference.

[0007] EP 1 419 875 A1 describes a reinforcing fiber substrate, a composite material obtained therefrom, and a method for its production. A spacer yarn for winding is also described here.

[0008] EP 1 491 323 A1 describes the structure of a complex of reinforcing fibers. Such a reinforcing fiber is mentioned as a fiber coated with a yarn or such a fiber yarn.

[0009] Regardless of the deposition process used to produce the fiber preforms, the achievable material thickness of both the preforms and the composite components to be produced is limited by the impregnability of the fiber preforms. The impregnability of fiber preforms with high material thickness is particularly critical in their thickness direction. Even low-viscosity resin systems with a long pot life do not penetrate to the base of the preforms, leaving dry areas that render the component unusable.

[0010] The fundamental idea of ​​the present invention was to develop ways to improve the impregnability of fiber preforms, especially with high material thicknesses and particularly in the thickness direction, compared to the state of the art. The aim was to demonstrate generally applicable measures for all materials and liquid resin impregnation methods.

[0011] The present invention accordingly relates to a method for producing rotationally symmetrical, thick-walled composite components by building up near-net-shape, dry preforms from layers of reinforcing fibers and subsequent impregnation with liquid resin and curing agent, wherein during the construction of the near-net-shape, dry fiber preforms, flow paths for the liquid resin are specifically created in the fiber preforms, preferably by building up the fiber preforms using modified, structurally modified, combined and pre-compacted rovings of reinforcing fibers by binders with plastic or resin or winding with a thread or yarn with constrictions arranged at predetermined intervals, in order to increase the impregnability of the fiber preforms in the thickness direction, that is to say essentially perpendicular to the layer planes.

[0012] By creating flow paths for the liquid resin along the thickness of the fiber preform, it can be made denser. At the same time, the resin can penetrate the fiber preform more quickly, suppressing the entrapment of air and / or reaction gases and thus the formation of voids and pores. It is understood that faster resin impregnation along the thickness direction results in faster and more complete impregnation of the entire preform, which ultimately results in time and cost savings and makes the method more suitable for mass production. The settling distances for the subsequent resin injection are reduced (<10%). This allows for much thicker components to be manufactured and improves process reliability.

[0013] Preferred embodiments of the present invention are defined in the subclaims.

[0014] An advantage of the present invention is that the fiber preforms can be constructed by dry laying and / or winding, preferably by winding, the reinforcing fiber layers, even in very thick material. By specifically creating flow paths for the resin, rapid and complete resin impregnation is still possible with high process reliability. This significantly expands the application range of the resin infusion process, which, for example, promises cost advantages over the use of prepregs due to the low material prices of the roving and resin system. Fiber and resin can be procured separately and combined as needed.

[0015] Within the scope of the present invention, various techniques for the inventive introduction of flow paths into the fiber preform during its construction were developed and tested.

[0016] The flow paths could be created by deliberately interposing thermoplastic nonwovens with a low basis weight of 5 to a maximum of 50 g / m². A polyamide nonwoven with a weight of 12 g / m² could be used here purely as an example. First, one or more layers of a fiber material are draped or wound onto a mold core. Once a certain, predetermined layer thickness has been reached, the fiber layer is fully or partially covered with a thermoplastic nonwoven, and the layer build-up to the preform continues, with repeated interposition of the nonwoven. The nonwoven acts as a spacer, i.e., a gap-forming agent, and also as a distribution medium for the resin.

[0017] The flow paths could also be created by introducing gaps directly during the laying and / or winding of the fiber layers. This can be done by laying the individual rovings, fiber layers, or fiber ribbons not overlapping or butting up, but rather with a small gap of approximately 0.5 to a maximum of 5 mm between them. This creates gaps in the overall structure of the preform, which promote the resin flow in the thickness direction without compromising the quality of the composite.

[0018] According to the invention, however, the flow paths are created by constructing the fiber preforms using modified rovings that have been structurally modified, combined, and pre-compacted by binders such as plastic or resin or by winding a thread or yarn around them with constrictions at predetermined intervals. In this way, the flow paths can be evenly distributed across the fiber preforms in a single operation. By using the modified rovings, flow paths can be generated in such a way that the impregnation process of the fiber preform is improved, even when winding at high thread tension, so that the preform has low settlement distances of less than 10% even under a pressure difference of, for example, 1 bar. Without the introduction of targeted flow paths, the preform could not be compacted to this extent, resulting in greater settlement distances, thus leading to the occurrence of undulations and poor mechanical properties.

[0019] In a further preferred embodiment of the present invention, the laying and / or winding angles for introducing the gaps and flow paths during the construction of the fiber preform are selected such that the number of intersection points is increased compared to conventional laying / winding. Here, too, the coverage density of the fiber deposition is varied in the thickness direction, allowing targeted areas to be created in which the resin can penetrate the preform even more quickly.

[0020] Such flow paths in the fiber preform can also be created by alternating circumferential and cross layers.

[0021] Roving is a bundle, strand, or multifilament yarn made of parallel, continuous fibers, such as glass, aramid, or carbon fibers. Mixed-fiber rovings also exist. As such, they have an elliptical or rectangular cross-section.

[0022] To introduce gaps or flow paths into the fiber preforms constructed from them, the rovings are structurally modified by binding them. Binding with plastic or resin gathers and precompacts the rovings, so that during winding or laying, gaps and flow paths automatically form between the rovings in the fiber structure, allowing the resin to be infiltrated to penetrate easily in the thickness direction. Depending on how many such flow paths are desired in the fiber preform, the number of twists or bindings used, or the amount of binder, can be freely varied and adapted to the application.

[0023] In one embodiment of the present invention, the flow paths during the construction of the fiber preforms are created using rovings wrapped with an additional thread or yarn. Such wrapped rovings can be suitably wrapped with the additional yarn, e.g., made of glass fiber or polyester, before covering / winding, so that constrictions are arranged on the roving at predetermined intervals. These constrictions, in combination with the wrapping yarn, form the desired flow paths in the finished preform construction. At the same time, the wrapping stabilizes the geometry of the roving, thus further enhancing the precision of the deposition or winding process.

[0024] The thread or yarn diameter and the thread or yarn material can be varied almost freely. For example, a thicker yarn with a larger diameter can create larger flow paths than a thinner yarn.

[0025] In a particularly preferred embodiment of the present invention, the roving to be unwound for the construction of the preform is wound with the yarn simultaneously while the roving is being unwound. This makes it possible to decide on a case-by-case basis which yarn material is used to wrap the roving to be used. The number of wraps per meter can also be adjusted to suit the specific requirements. The yarn can be fed to the roving via a spool, with the rotation speed being controlled, for example, by means of a compressed air or electric drive.

[0026] The yarn material can be selected so that it dissolves or does not dissolve in the resin matrix. Suitable yarn materials include glass, polyester, special polyamides such as Grilon® (ESM-Grivory), and similar materials.

[0027] The reinforcing fibers, which can also form the roving, are preferably selected from the group of glass fibers, carbon fibers, aramid fibers, and / or polyester fibers. Mixed-fiber rovings can also be used.

[0028] In principle, all liquid resin impregnation processes such as infusion, injection / RTM, preferably infusion, with all suitable resin systems, such as thermosets, e.g. epoxy resins, vinyl esters, polyesters, cyanate esters, bismaleimides, polyimides, benzoxazines, polyurethanes, phenols and isocyanurates or even thermoplastics, can be used to impregnate the fiber preforms prepared in this way for the construction of the resin matrix.

[0029] The process according to the invention is suitable for the production of all rotationally symmetrical components, in particular for pressure vessels (internal and external pressure), as well as pipes or aircraft engine components. It is particularly suitable for the production of thick-walled components, since the flow paths provided for the resin in the fiber preforms solve the problem of impregnability in a unique way. Complete, pore-free impregnation of the fiber preform is a prerequisite for the highest quality of the components. The introduced flow paths enable highly compacted fiber preforms to be impregnated, which allows for components with very high fiber volume fractions of over 60%. Above all, the risk of fiber waviness is greatly reduced or completely eliminated by reduced setting distances.Of course, the process is also suitable for the production of dry-laid two-dimensional (flat) components, whereby the fiber layers can also be built up using the AFP process.

Claims

1. Process for producing rotationally symmetrical, thick-walled composite components by building up near-net-shape, dry preforms from layers of reinforcing fibers and subsequent impregnation with liquid resin and curing, whereby during the build-up of the near-net-shape, dry fiber preforms, flow paths for the liquid resin are created in the fiber preforms by using for the build-up of the fiber preforms structurally modified, combined and pre-compacted rovings made of reinforcing fibers by binding with plastic or resin or winding with a thread or yarn with constrictions arranged at predetermined intervals, whereby during winding or laying between the rovings, flow paths and gaps are automatically introduced into the fiber build-up, which increase the impregnability of the fiber preforms in the thickness direction, i.e. perpendicular to the layer planes.

2. Process according to claim 1, characterized in that the rovings are combined to form bands of defined width.

3. Process according to claim 1 and 2, characterized in that the rovings wound with a yarn are produced by winding the roving with the yarn in-situ during build-up of the fiber preform as the roving is unwound.

4. Process according to claims 1 to 3, characterized in that the yarn used for winding is soluble or insoluble in the resin matrix to be produced.

5. Process according to claims 1 to 4, characterized in that the flow paths during construction of the fiber preform are created by alternating circumferential and cross layers.

6. Process according to one of claims 1 to 5, characterized in that the yarn is a glass, polyester, Grilon® yarn or the like.

7. Process according to any one of claims 1 to 6, characterized in that the reinforcing fibers are selected from the group of glass fibers, carbon fibers, aramid fibers and / or polyester fibers.

8. Process according to any one of claims 1 to 7, characterized in that the resin matrix is selected from the group of thermosets or thermoplastics.

9. Process according to claim 8, characterized in that the thermosets are selected from the group of epoxy resins, vinyl esters, polyesters, cyanate esters, bismaleimides, polyimides, benzoxazines, polyurethanes, phenols and isocyanurates.

10. Process according to one of claims 1 to 9 for producing rotationally symmetrical, thick-walled components with a fiber volume fraction of greater than 60%.

11. Process according to one of claims 1 to 10 for producing pressure vessels.

12. Process according to claim 11 for producing pressure vessels for use in aerospace.