CFOAM® A relatively recent advancement in the area of direct heating of composite moulds has been the use of Touchstone Research Laboratory’s CFOAM® as an electrical heating element. CFOAM® was originally developed as a coal-based carbon foam, for use as a rigid, lightweight and easily machined tooling substrate. A tool’s support structure could be machined to near net-shape from blocks of CFOAM® , with a composite mould surface applied on top of the foam structure. As Stewart indicated in his review of new mould technologies, the combination of a carbon based substrate, with a carbon tool surface, and a carbon fibre reinforced polymer (CFRP) part material is extremely attractive. As the coefficient of thermal expansion of all the materials involved are Figure 2.3-2: CFOAM® heating setup [22] 15 very low, dimensional accuracy is extremely high [19]. As the carbon foam is conductive, it was quickly realized that it could be used as a resistive heater to heat the mould surface. By isolating a layer of CFOAM® between the mould structure and tool surface, and applying a current across it, heat could be applied across the entire tool surface. An example of this setup is included in Figure 2.3-2.
This method was studied by Blacker et al., who demonstrated that using a constant cross-section thickness of carbon foam as the heater could achieve a uniform temperature distribution on the tool surface [20]. The difficulties they faced came primarily from the management of bonded joints between CFOAM® blocks, where the local conductance varied, which could create hotspots or short circuits. Despite this, the authors were able to show very minimal temperature variations on a complex tool for low ramp rates. However, while the base materials are inexpensive for this process, significant machining is required to assemble the mould. First, the structure must be assembled out of blocks of foam bonded together to near net-shape. Then a first machining pass is done to bring the blocky surface down to approximately the part geometry. A similar machining pass is done on a separate assembly of CFOAM® blocks which will become the constant thickness heating element. These two blocks, essentially a male and female version of each other, are then joined, and the part geometry is again machined into the assembly. Finally, the prepreg tooling surface is applied, which must then be machined to the final part geometry. This results in a total of four complete surface machining passes, which is an expensive process. Despite the complicated machining process, CFOAM® tools are very promising and are being investigated by a number of aerospace companies [19].
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