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Microwave heating is a new, fast-process technology for heating carbon-fibre-reinforced plastics and their components. The main advantage of microwaves is that the heating is selective and instantaneous. Here, we examine the influence of microwave radiation on the materials, preparatory to setting up an industrial process.
(Published on April 2006 – JEC Magazine #24)
BY MATTHIAS MEYER, GERMAN AEROSPACE CENTER, INSTITUTE OF COMPOSITE STRUCTURES AND ADAPTIVE SYSTEMS
With the market growing for carbon-fibre reinforced plastics, production processes need to be improved to reduce cycle times and to improve the quality assurance process, especially where aerospace applications are concerned.
The use of electromagnetic radiation (in particular microwave radiation) to heat products is well known. Heating with microwaves is very efficient, as it reduces process times and saves on energy.
So far, there are no applications for microwave heating in the production of CFRP structures. As the process chain for CFRP materials contains a lot of thermal steps, the integration of microwave heating would be an advantage over conventional technologies. Due to the low thermal conductivity properties of CFRP materials, these thermal process steps are a “bottleneck” for production. Processes need to be optimised, especially for aerospace applications, using high-temperature thermoset plastics.
When conventional high-temperature heating is used, it is not possible to avoid thermal lags. Microwave heating is an alternative that can eliminate such disadvantages, as it is quite independent of thermal conductivity. Microwave heating can be used for operations such as the preforming of dry fibres, resin preheating and composite curing. The use of a universal microwave furnace operating at a frequency of 2.45GHz makes it possible to develop process technologies that, for equal material properties, are more advantageous in terms of cycle times and energy savings.
The fundamental work is done in a 1m2 8 kW furnace with a temperature- controlled system (fig.1) that uses three pyrometers and four fibre-optic temperature sensors. Microwave-specific process parameters are measured by a directional coupler, which indicates the emitted and reflected microwave energy. In this sense, it is an instrument for indicating process quality.
This equipment serves to report all fundamental process parameters, making it possible to develop qualified production processes.
Using injection technology to produce composite parts is state-ofthe- art. Special resins with a high viscosity at room temperature are used for high-performance parts. For the injection, it is necessary to heat the resin to 80°C. The work at hand involves the use of Hexcel’s RTM6 resin. Using conventional techniques would require more than four hours to heat up a single 10kg batch; because of the low thermal conductivity, it would be impossible to reduce this time without risk of a thermal overshoot of the resin. Too much heat causes locally hot areas where an exothermic reaction could take place, destroying the whole batch.
To qualify microwave heating, the neat resin was examined. For mechanical and thermal examinations, the testing shows the neat resin properties. Microwave-heated RTM6 exhibits the same properties as the conventionally heated material. The non-critical influence of microwave radiation on RTM6 provided the possibility for an industrial application in the form of a special microwave resin preheating system. This system (fig.2) is designed to heat up 10kg RTM6 batches within 12 to 15 minutes from room temperature to 80°C, homogenously.
To demonstrate the performance of this system, a temperature measurement was done for one resin batch. A measurement matrix shows the temperature distribution on 8 points and three levels within the batch. The batch was heated quickly within 15 minutes to 80°C, and the temperature held for 8 hours. Figure 3 shows the absolute homogenous temperature distribution inside the system.
The second application of microwave radiation heating is a preform technology using binder-fixed fibre fabrics. Dry carbon fibres have a low thermal conductivity perpendicular to the layer surface. The binder technology includes the impregnation of the fibres with a thermoplastic or thermoset system that is used as a tackifier over a thermal activation.
The thermal activation of the binder uses conventional methods like infrared heating, hot air or ironing. The simplest way to drape fibres into a mould is to iron them. Other non-manual ways use vacuum-bag draping technologies like infrared or hot-air heating. Vacuum-bag preforming involves a mould with a flexible diaphragm that follows the shape of the mould and drapes the fibres between the mould and the diaphragm into the right geometry
This technology enables the lay-up of several sheets. Because the number of sheets is limited by the thermal conductivity compared to an economic cycle time, microwave heating is used to establish thermal conductivity independence. Tests show that fast and homogenous heating of about 50 layers of a carbon fibre fabric is possible, with consolidation in under 20 minutes. In comparison, the conventional technology requires about 50 minutes to achieve the same result.
Extensive material testing was carried out prior to the transfer of the technology to an industrial application, to find out if there was any influence on the strength of the fibre or the sizing. Tensile strength and shear strength tests were carried out. No abnormalities were detected on the material. The high shear properties obtained with microwave preforming make it an appropriate method with respect to the fibre/matrix interface.
Microwave heating was also shown to be suitable for the composite curing step at the end of the CFRP production chain. As before, no abnormalities were detected, and it is reasonable to expect that microwave radiation has no influence on the material, either. At any rate, it will be necessary to demonstrate the material behaviour under a new processing technology.
Building up the material specimens requires the use of previously qualified tools to show a specific influence of the microwave radiation. Injection technology was used to build up CFRP specimens in this way; the specimens were then cured at 180°C. The same tool and the same temperature cycle were used to build a conventional reference specimen. Thermal analysis (such as differential scanning calorimetry) and mechanical testing were done. The degree of cure and the glass transition temperature were higher with the microwave-produced specimens than with the conventional reference specimen. This was due to the direct temperature management, which does not involve the thermal lag that occurs with conventional processes. Another result was that the microwave process was about 30% faster than the conventional one, for the same material quality. Because of the energy intensity, among other things, long curing cycles should be avoided in industrial systems like high-volume autoclaves. For this reason, Scholz Maschinenbau and the German Aerospace Center have been working together on a project to develop an energy-saving production facility. To that end, the world’s first microwave autoclave (figure 4) was built and patented.
The advantages of microwave processing will be transferred to the new autoclave technology