Strong Parts from the Cold Furnace

Many parts of aeroplanes and automobiles are long since not made out of steel but from modern composite materials. They consist of carbon or fibreglass laminates bonded with special resins to form an extremely resilient material withstanding crash tests, elastically absorbing virbrations and which at the same time are as light as a feather. And this saves fuel. Yet so far such light-weight composites have to be produced in enormous furnaces, so-called autoclaves, under high pressures and temperatures, which uses up a lot of energy. But this could change: The microwave furnace HEPHAISTOS, developed at the Karlsruhe Institute of Technology in close cooperation with industry partners, will be able to produce light-weight composites of any shape and size for the industry and requires only about a fifth of the energy compared to the conventional production in the autoclave, as comparative measurement by EADS has shown.

“The microwaves simply heat up only those parts, which really have to be heated, such as the light-weight materials and not the furnace itself. You even can still touch the furnace, that’s how cool it stays”, explains Project Manager Dr. Lambert Feher from the KIT’s Institute for Pulsed Power and Microwave Technology. “This saves 80 percent of energy and furthermore even improves the material properties.”

At the core of this technology are microwave components as they are also used in households. They are located on the sides of hexagonal cylindrical modules measuring some two metres in diameter and one metre in length. The largest furnace of the experimental hall features three modules in a row, so that parts of a length of up to three metres can be produced in one cycle. Inside the furnace an almost homogenous microwave field is created, which can be optimised as to the shape of the part according to requirements by controlling the individual microwave radiators. “The microwaves transfer their energy target-oriented to the carbon fibres within the composite material and triggers these like antennas. This causes their surface to warm up and they conjoin fast and in a much better way with the surrounding matrix of resins”, Feher explains.

Whereas in a conventional thermal furnace the energy is transferred exclusively via thermal conduction, which is very low in carbon fibres. Therefore, the light-weight composites from the HEPHAISTOS furnace also are of an extremely good quality.

Hardness and shear strength are greater, the various layers are more closely connected with each other and absorb an impact better, at the same time they are more elastic. “Apparently, microwave technology leads towards strengthening the network between fibre mesh and resin matrix”, assumes Feher. However, what happens exactly within the material has not been understood in detail as yet. “Roughly speaking, we imagine that the microwaves lower the activating energies for certain chemical reactions and thus they are accelerated.”

During the hardening process, Feher and his team observed with a thermal camera, how the heat spreads within the usually complex shape of parts. An homogeneous warming is optimal, which can be achieved in the Hephaistos technology via a programmed control of fields – another advantage compared to conventional thermal furnaces.

Additional funding from a Federal Ministry of Education and Research project running from 2006 to 2011 enables the KIT team to put the finishing efforts on the Hephaistos technology and above all brought sufficiently strong industry partners to the project: Amongst these are the aerospace companies EADS and GKN Aero-space, the chemicals companies BASF and Hexion, the automotive manufacturer Porsche as well as the producers of composites SGL Brakes and Fritzmeier Composites. Further large Hephaistos furnaces, which can produce also very large parts for aeronautics, are in their design stage at the KIT’s experimental centre. “We have developed the technology so far to be licensed to the industry”, Feher states. And it works, too: For example, the firm GKN Aerospace installed a large-scale plant as first industrial customer already at the beginning of 2010 in Munich.

Yet in research a lot remains to be done as well: On the agenda are not just the question what exactly microwave radiation effects in the composite material but also the optimisation of the entire process. For instance, the calculations for controlling the microwave radiators still are very demanding: “Our next goal is to further refine control of the microwave fields so that we can process also very thick and asymmetrical parts”, Feher says.