In industries where lightweight design is of importance, such as the aerospace and the automotive industry, fiber reinforced polymers (FRP) are employed due to their high stiffness and strength in combination with low weight. Mostly glass and carbon fibers are chosen to provide the stiffness and strength in the final compound, resulting in glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) materials. A wide variety of polymer resin systems are used for the matrix material, depending on the particular specifications of the application [22]. Thermoset resin systems show a comparably low viscosity and, thus, are often selected for an enhanced impregnation of the fiber bed during part manufacture [23]. In composite processing with thermoset resin an unlinked mixture of resin and hardener is introduced in the fiber bed either prior (so called prepreg) or after the part fiber layup (injection processes) is performed. Subse-quently, in most cases an exothermic cross-linking of the chemical monomers is conducted at elevated temperatures. Heat introduction is defined by the process technology chosen and either performed indirectly over the air (autoclave, oven) or directly into the tool (fluid heating or resistive heating devices). Research has also been conducted on direct heat introduction via resistive heating of the part’s fiber bed [24–26] or with help of a microwave [27, 28]. However, both procedures are rarely applied in the industry to date due to the challenge to get a constant cure temperature over the part dimensions for applications with industrial complexity level.
Especially resistive heating of the tool offers the potential of reduced cycle time, reduced thermal lag between set-point and part temperature as well as reduced energy consumption in combination with moderate additional requirements for tool manufacturing [2, 13, 29].
These tools are either used as stand-alone in, for instance, resin transfer molding (RTM) [12], in Out-of-Autoclave manufacturing processes, or within an autoclave to locally support regions with low convective heat transfer in shadow zones of the air stream. Figure 1-1 shows resistively heated tools of two different tool manufacturers. The aluminum tool produced by TCXTM(see Figure 1-1 (b) and (c)) depends on a conductor material, which is applied on the tool backside and isolated from the tool material itself, to introduce heat into the system [30]. In case of the CFRP tools produced by Qpoint Composites GmbH the conducting material is embedded in the tool laminate itself, if required [1]. Usually, the spacing of conducting paths is designed to gain constant power introduction in
1
the area and a homogeneous temperature distribution on the part face of the tool.
This technology enables the division of a designated heating area into several independent heating zones. The heat introduction in each zone is controlled by a thermocouple in combination with an external control unit, enforcing the set-point temperature cycle at the location of the thermocouple. However, the resulting tool temperature field in one heating zone can be inhomogeneous and, thus, can deviate significantly from the set-point temperature at locations afar from the control thermocouple in the considered zone [31].
a) b)
c)
Figure 1-1Resistively heated tools: (a) CFRP tool for a full-scale helicopter rotor blade from Qpoint Composites GmbH [1], (b) bottom side of an aluminum tool with a TCXTM heating element [2], (c) TCXTMheating element in service [3].
Regarding the choice of tool material, traditional options for composite manufac-turing are steel and aluminum thanks to good machinability and stiffmechanical response. As the thermal expansion discrepancy between the metallic tool and the CFRP part adds to residual stress build-up and spring-in effects [32], In-var is used in some cases where dimensional fidelity during the temperature ramp is of high importance [33]. However, these metallic tooling concepts lead to high costs of the tool manufacturing, high tool weight and low energy ef-ficiency in service life due to the high thermal tool mass. Hence, novel tooling concepts based on CFRP as tool material are subject to increasing industrial
inter-est. However, thermal sensitivity of CFRP tools are a major concern and hinders their widespread application to date. The in-plane heat conductivity of CFRP tool material is by one magnitude lower than steel and by two magnitudes lower than aluminum [34]. This can potentially lead to large inhomogeneities in the temperature field during operation, if the thermal design is insufficient.
Given that resin cure is driven by temperature [35], a uniform temperature field in the part during the cure cycle is sought in composite processing to gain homo-geneous cure within the part. Especially for thick laminates this is a significant challenge. Low thermal conductivity of the compound in combination with the exothermic reaction energy of the curing resin may lead to a local temperature overshoot in the laminate [36–38]. If this overshoot reaches a critical level, the temperature development in the part’s interior is almost solely dominated by the autocatalytic reaction resulting in very high local temperatures [39], which pose the risk of material degradation [40]. On the contrary, if the temperature cycle leads to gelation of the part’s surface well ahead of its interior, voids and non-uniform fiber volume fractions can occur [41, 42]. In general, thermal gradi-ents and accompanying cure gradigradi-ents increase the internal stresses in the lami-nate [5, 43, 44]. Hence, the thermal history of the curing part is understood to be a key parameter regarding the development of internal stresses in cross-linked polymers and, thus, potential defects such as process-induced microcracks or delamination [45–48].
Although CFRP tools in general are susceptible to temperature inhomogeneities, resistively heated CFRP tools enable an adjustment of the heat introduction to account for different part materials and thicknesses via the inclusion of multi-ple independent heat zones. Hence, multi-zonal resistively heated tools offer the thermal flexibility to adjust the tool heat introduction onto the local requirements of the part areas. Thus, they have the potential of providing a homogeneous tool temperature distribution while maintaining the above-stated advantages of CFRP material. In addition, the thermal flexibility of these tools provide the possibility of adjusting the temperature cycles in different part regions, poten-tially resulting in an overall improved cure behavior in complex parts with large discrepancies in material and thickness.
However, the thermal design of these multiple heat zones is conducted based on experience to date. Especially for complex parts the optimal allocation of heat zones is highly challenging, given that a variety of influencing factors such as the thermal behavior of tool and part, exothermic resin reaction, and environmental effects such as convection, has to be considered. In-depth process comprehen-sion is required which, in most cases, can only be offered by process simulation.
Therefore, the capability to appropriately model temperature controlled
resis-tively heated tools in a cure simulation platform has to be developed to enable a simulation-assisted thermal dimensioning of these tools.