10.05.2010 | Railway Technology International

Innovative Heatable Composites Based on Carbon Fibre Structures

The demand for advanced materials, such as composites, in high-performance structural applications or for design aspect has been rapidly increasing in the last ten years. Even as properties such as durability and functionality are given importance, the materials also have the potential to reduce mass. Glass or carbon fibres are just that; stronger, lighter, and high performing in applications such as automotive and aerospace. The fibres are characterized by very high elastic modulus, and when mixed with a matrix such as resin, fibre reinforced plastic (FRP) is formed. The combination of properties such as low density, high strength, and high stiffness has made composites the most preferred material in a variety of largescale commercial applications. To satisfy the needs of composites structures, industrial progress is necessary. New ways of producing and manufacturing composites is an ambitious technology challenge. The determinant cost driver is defined by high cycle times and enormous energy consumption. This is caused by the characterization of today’s resins, and the production processes. Normally for that, complex tooling with integrated oil or water heating, or a number of large ovens are used.

FROM AIRCRAFT TO RAILWAY

One step toward more industrialized production is achieved by self heating composite tooling. Instead of using high cost tooling, a more efficient way is reached by composite tooling with an integrated carbon fibre heating structure. As an alternative of heating large blocks or alloy or steel, only the surface of the part or tooling is heated up. Therefore less energy and a lower cycle time is used. For the evaluation of this production technology, a praxis test was already realised. For about one year, industrialised production of the Airbus A330 Wing Spoiler with this innovative concept was successful, and showed a lot of advantages.

CARBON FIBRE HEATING ELEMENTS

The key innovation feature in existing products on the market is achieved through an active and adaptive textile carbon fibre heating structure. The development and production of carbon fibre heating structures is based upon a technology developed at the German Leibniz Institute of Polymer Research in Dresden (Germany). The Tailored Fibre Placement Technology (TFP) allows embroidery of specialized carbon fibre rovings to a customized geometry on a textile layer. The ohmic resistance and an applied electrical potential on the rovings generate a local, definable heating output. Quite a three-dimensional form and complexity is, as a result, achievable.

INNOVATIVE, ACTIVELY HEATABLE COMPOSITE TOOLING

Adequate and homogeneous heating of the FRP part is needed to reach the desired structural component quality, which is combined with a huge time and energy effort, especially for the RTM process (resin transfer moulding), where resin is pressed into dry preformed fabrics. Therefore a preforming step for the dry fabrics is necessary. They can be formed in shape with the help of a thermoplastic component, which has to be activated. The activation of this component parts needs temperatures from 100°C (212°F) up to 180°C (360°F), and cause a continuous heating and cooling of the tooling, which wastes a lot of production time and energy as a result.

Active heatable composite tooling offers the ability to design this process more efficiently and cost effectively. It is a manufacturing system for the production of composite parts on an industrial scale. The integration of these electrical heating elements is almost possible in every composite mould. Only a CAD 2D projection of a complex geometry is needed and the electrical potential that the customer requires for the heating process. The complete heating and cooling process of the tool can be computer-recorded, so that no desire of the quality management department will be unsatisfied.

FUNCTIONAL COMPOSITE STRUCTURES

Also designated functions can be added to composite structures. Thus the utilization is not only limited to tooling or mould products. A conceivable application are de-icing structures for safety-critical parts, or heating structures for the interior. The end-user receives a plus of comfort by so-called panel heating. Health monitoring rounds up the field of application, and allows the permanent surveillance of vital structures.

Potential applications:

• Self heating composite tooling for preforming or vacuum infiltration

• Compaction and forming of prepreg or preform layer

• Thermoforming of prepreg or perform layer

• Electric de-icing and anti-icing systems for composite structures

• Electric heatable flexible membranes

The advantages of this technology are listed in the following overview.

Cost Saving Potential

• Low thermal mass reduces cycle time and saves energy.

• Simplified process reduces the workload in the production of complex composite components.

• Reduction of in-house transportation. No transportation of tools to oven and back is necessary.

• Process cost reduction up to 40 per cent in comparison with traditional preforming.

Time-Saving Potential

• Active heating of the composite tool with integrated heating structure (heat rate up to 20 K per min).

• Low thermal mass and direct heating result in very short heating and cooling.

• Lightweight for easy handling of the form tools without mechanical assistance, such as lifting equipment.

• Reduction of cycle time up to factor five in comparison with the traditional process of ovens and metal tooling.

Quality Improvement

• Homogeneous temperature of the formative tooling surface.

• Similar expansion coefficients of the composite form and the composite structure.

• Free geometry of the heating structure.

• Local definable heating output per area allows discontinuous thickness and notches of the structure.

• Out of-Autoclave Technology.

Carbon fibre heating elements in composites underlie a possible temperature range up to 180°C (360°F). Therefore heating rates of about 20 K/min are possible to reach shortest cycle times. Stepless temperature adjustment is useful. A customised adaption of the wattage from about 100 up to 10.000 W/m2 can be installed and controlled by a programmable control unit.

SUMMARY

The use of carbon fibre heating elements enhances the field of application for fibre composites. The company Qpoint Composite GmbH offers on the one hand innovative composite tooling for the production of composite parts, and on the other hand, an extension of the functionality of existing composite parts.

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