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Novel tooling for composites curing under microwave heating

Final Report Summary - MU-TOOL (Novel tooling for composites curing under microwave heating)

Executive Summary:
The MUTOOL project developed novel tooling for the processing of composite parts using microwaves.
The main idea for the design of the tooling was to direct the microwave radiation to the vicinity of the composite part and prevent it from heating the tool bulk.
In order to prevent microwave radiation to be absorbed by the tool bulk, ceramic materials were chosen for the production of the tool. Ceramics are relative inert to microwave radiation, absorbing only minimum amounts of energy. The specific ceramic material chosen for the fabrication of the tool was cordierite. The selection was done mainly due to the low coefficient of thermal expansion (CTE) of cordierite.
In order to direct the microwave radiation to the area of the composite part, a layer of a microwave absorbing material was applied to the surface of the tool. The microwave absorbing material of choice was ferrite. Ferrite particles are thought to have the right dielectric and magnetic properties for effective conversion of microwave radiation to heat.

The initial approach was to develop the tool using an inexpensive technology such as freeze casting and then spray the ferrite particles on the tool surface. The development work showed that freeze casting can indeed produce tools of adequate complexity. However, the mechanical performance of the tools was not satisfactory. Therefore, the final prototype tools were developed using a combination of freeze casting and slip casting technology.
The application of ferrite to the tool posed the challenge of mismatch of the thermal expansion coefficients between the ferrite and the cordierite. The technical solution for the successful application of the ferrite layer involved the development of a new enamel glaze using a mixture of ferrite and other inorganic materials. The additives in the glaze reduced the CTE values mismatch. Also, a seed layer was applied to the ceramic tool prior to the application of the glaze in order to further reduce any thermal stresses.
The application of the ferrite layer to highly complex geometries, usually found in automotive parts proved to be very challenging. For this reason, two different tools were produced: One tool for aerospace application, following the original project concept and one for automotive application were the ferrite particles were distributed in the tool bulk. Prototypes for both tools were produced and tested in real manufacturing conditions.
The new technology was presented at the recent JEC Paris trade show. The SME beneficiaries have applied for a patent on the MUTOOL technology.

Project Context and Objectives:
The main objective of the project was to use microwaves and novel ceramic tooling incorporating a microwave absorber layer for the rapid heating of composites for the processing of large polymer matrix composite (PMC) structures. The basic principle relies on a microwave absorbent layer on the tool, which absorbs microwaves and turns this into heat. The heat then transfers in conduction/convection to the composite part, saving the need to heat (and cool) the chamber and tool. The process will leads to reduced manufacturing time and cost and reduced process energy consumption.
By reducing processing time and cost, the targeted application areas for composite components can be broadened to include industries where the ability to mass-produce lighter parts at lower cost is imperative. The automotive and aerospace industry will be key beneficiaries of this development. Lower-weight and lower-cost components are essential if the fuel efficiency of existing vehicles and future hybrid/electric variants is to be improved, and lower cost PMC structures offer significant advantages.
Developing this technology within the EU ensures access to the SMEs involved in this proposal assuring them of world leading capability and know-how, beating global competition and access to the growing composites market.

The tasks leading to the achievement of the main objective described above are the following:
A) Process simulation: Modelling of the oven chamber, the tool and the composite part in order to understand the electromagnetic energy distribution to the various materials
B) Prototype tooling development. This activity breaks into three phases:
B1) Preliminary experiments for the selection of the tool manufacturing process and the ceramic materials to be used
B2) Process optimisation for the fine tuning of the tool production technology
B3) Prototype tools production
C) Production of demonstration parts using the microwave oven and the prototype tooling

Project Results:
A) Process simulation
The simulation of the process was performed sequentially. The oven chamber was initially modelled. Additional items were introduced to the simulation until the whole process was completed.
The modelling was performed using the CST Microwave studio software.

Microwave chamber simulation
The following activities were performed for the simulation of the microwave chamber:
i) Creating a hexagon curve representing the oven chamber: Creating the chamber by sweeping the curve (material: PEC, wall thickness: 2mm)
ii) Introducing slots (material: vacuum) for the WR 340 wave guides into the hexagon‘s walls - Setting of wave guides WR 340 - previously modeled and parameterized as separate project inserted into the chamber model by „copy/paste“, loss of parameterization
iii) Setting of waveguide ports onto the coupling openings of the wave guides
The simulation results are shown in Figure 1 in the attached report. The electromagnetic radiation inside the oven chamber is uniform

A1) Microwave chamber simulation with the support bench and tool
The modelling configuration can be seen in Figure 2 in the attached report. The tool is represented by a cube made of cordierite material. The support bench is made from steel.

The simulation results are shown in Figure 3 in the attached report. The field is homogenised in within the ceramic tool

A2) Simulation of the microwave chamber, support bench, tool and composite part
Several simulations were performed in order to optimise the tool design and the microwave absorbent layer. Typical simulation results are depicted in Figure 4 and Figure 5 in the attached report. The ferrite coated area absorbs effectively the microwave radiation (blue colour in the figures). A homogebeous thermal bubble around the composite can be seen.

A3) Final simulation of the prototype tool
The final simulation of the tool is presented in Figure 6 in the attached report. In this figure, the red areas represent high absorption of microwave radiation where the blue areas show minimum or no absorption of microwave radiation. The absorption of microwave radiation follows the shape of the composite demonstrator.

B) Prototype tool development
B1) Freeze casting process
The initially selected process for the development of the tool was the freeze casting process.
The process starts with a colloidal silica sol. When the sol is taken below the triple point of water the silica starts to interact with ice to form a continuous phase ahead of the growing ice crystals. The size and direction of growth of the ice crystals has a significant effect on the final morphology. Often, a freeze drying stage can be utilised in order to remove the water. Alternatively, the water may be removed by post baking or sintering.
In freeze cast alumina (FCA) fine powdered alumina is mixed with the colloidal silica with solid loading up to 80%. With the use of wetting agents the alumina powder can be evenly dispersed in the sol, such that when the mixture is frozen the alumina becomes part of the silica particle network. The mixture is thixotropic so when the sol mixture is poured into the mould a vibrating table can help in order to allow the liquid to flow properly to create a good surface.
The freezing process can be carried out at any temperature below 0°C, but smaller grain sizes are achieved at lower temperatures; -50°C to -80°C being typical.
Although freeze casting can be carried out in a domestic freezer, dedicated equipment is need to achieve the required low temperatures and near vacuum pressures to produce the highest quality parts.
A practical solution is to use liquid nitrogen for the freezing process: The tool is placed into a small pedestal in the foam box and liquid nitrogen is poured in until it covering the base, but not the tool. The lid is then placed on the box and the material is left to freeze, as shown in Figure 7 in the attached report.
The powder used need not be alumina. Many other materials have been used including: zirconia, silicon carbide, apatite powders as well as metallic powders.
Other organic solvents are also used, notably camphene, C10H16 (2,2-dimethyl-3-methylene-bicyclo[2.2.1]heptanes). This has the advantage of much higher working temperatures, but has a pungent smell and can be explosive.

B2) Preliminary studies for the process development
A number of formulations (different mixtures of raw materials) for the initial slurry for the freeze casting process have been produced. Freeze casting specimens were sintered. Their mechanical and thermal performance in the microwave has been measured. Table 1,in the attached report, lists a representative number of configurations tested, alongside comments for the performance assessment. Tests using Alumina and cordierite powder were performed. Typical specimens developed can be seen in Figure 8 in the attached report.
The impact of the amount of ferrite in the heating of the specimens has been studied. Specimens with varying amount of ferrite have been prepared and tested simultaneously, as can be seen in Figure 9 in the attached report. The dependence of the ferrite content in the measured heating rate in the samples is shown in Figure 10 in the attached report. A logarithmic relationship was established:

Heating rate = 1.5132*ln(ferrite concentration+1)+1.2384

The above relationship suggests that further loading of ferrites will have diminishing returns in the obtained heating rate of the tool.
The influence of the ferrite layer thickness was also investigated. Specimens of varying thickness where prepared. Their thermal response in the microwave was tested. The tests configuration is shown in Figure 11 in the attached report. The main conclusion from the study is that a ferrite layer of a thickness of few millimetres with a ferrite concentration higher than 20% by weight is suitable for achieving very fast heat rates and temperatures higher than 250°C.

Composites processing experiments using the produced tools were also performed. The typical configuration of the experiments can be seen in Figure 12 in the attached report.

Real specimen and the respective thermographs can be seen in Figure 13, Figure 14, Figure 15 and Figure 16 in the attached report. The thermographs reveal that the basic concept of the MUTOOL project is valid. Heat is concentrated in the area of the composite part, while the rest of the tool remains relatively cooler (being heated only through conduction from the ferrite layer).

B3) Process Optimisation
The optimisation process focused on the following items:
i) Material property measurements for the finalization of the ferrite layer concentration and thickness
ii) Assessment of the freeze casting ceramic structure
iii) Optimised way to produce and apply the ferrite layer
iv) Development of tools using the optimised process
v) Production of composite parts

The main material properties affecting the mechanical and microwave performance of the tool are the coefficient of thermal expansion and the complex dielectric constant.

The coefficient of thermal expansion (CTE) for the cordierite ceramic was measured. The measurement was done on a NETZSCH 402 dilatometer. The CTE values measured are plotted in Figure 17 in the attached report. The measured values vary between 2.5x10-6 – 3.5x10-6 1/K. They fall within the specifications set out at the beginning of the project.

Dielectric measurements were performed on the Püschner Portable Dielectric Measurement Kit (PDMK) using a coaxial sensor. Specimens were prepared by IVF. The specimens were discs with a diameter of about 65 mm and a thickness of about 8 mm. Three different combinations of cordierite/ferrite were prepared:
i) Pure cordierite
ii) 85% cordierite – 15% ferrite w/w
iii) 70% cordierite – 30% ferrite w/w
For each combination, 3 specimens have been produced.

The temperature dependent measurements were performed using an oven. The oven was preheated at 150°C. Each sample was placed in the oven for about 15 minutes. It was then manually taken out of the oven and placed on the PDMK. The measurement commenced right away. A thermal image camera was used in order to capture the specimen temperature in real time. Typical results are shown in Figure 18 in the attached report. Both the dielectric constant and dielectric loss have a linear increase in value as temperature increases.

The microstructure of the freeze cast materials showed a typical pore structure caused by the formation of ice crystals during freezing (microscopy images shown in Figure 19 in the attached report). The size and orientation of the ice crystals (pores) are probably influenced by the direction of the freezing, the speed of the freezing, the solids loading as well as the composition of the liquid to be frozen. When the microstructures of the cordierite based materials obtained from the water and the water/glycerol based suspensions are compared, there was a clear difference even though both materials were prepared under the same conditions. The addition of glycerol gave a less pronounced formation of crystals and thus also a more homogeneous material compared to when only water was used, which also contributes to an increased reliability.

A ferrite enamel glaze has been produced in order to have the required surface quality in the tool. The enamel was applied using thermal spraying. In order to optimise the spraying process, a number of tests were performed at different temperature and spraying conditions. The most successful tests and the corresponding properties of the coated samples are listed in Table 3 in the attached report.
Different thicknesses of ferrite coating have been tried. The thicknesses varied from 100 μm to 400 μm. The best properties were obtained on the thickest coating samples. The tests results are summarised in Table 4 in the attached report. The microstructure of the coating can be seen in Figure 21 in the attached report.

B3.1) Optimum glassy matrix for the enamel
Preliminary tests have been conducted using a given glassy matrix. The fired glaze shows some small crazing at the surface due to the thermal expansion mismatch between the glaze layer and the substrate.
An attempt has been made to fine tune the recipe of the glassy matrix in order to decrease this mismatch in the coefficients of thermal expansion (CTE) and consequently decrease the risk of crazing.

Three different glaze recipes were prepared, in which 25% w/w of ferrite were introduced. A first firing has been done at 1000°C. The three specimens after the first firing can be seen in Figure 22 in the attached report. Some samples resulted in mat surfaces while others produced a slightly glassy surface and some bubbles at the surface.
In order to improve the surface of the glaze, a second firing has been performed at temperature higher than 1000°C. The second firing improved the quality of the surface in the samples. Some crazing was still observed. However, the extent of the crazing is not detrimental for the MUTOOL application.
Regarding the optimum thickness of the glaze layer, it is important to know that the higher the thickness, the higher the risk of crawling and the higher the risk of crazing. As an indication, a layer thickness higher than 200-250μm will increase the tendency of the glaze to detach partly from the substrate during the firing and also maximize effects of thermal mismatch due to different thermal expansion coefficients (CTE) of the glaze and the substrate.

B4) Prototype development
A tool with and without the microwave absorbing layer has been produced using freeze casting. The process stages for the construction of the tool are described below:
i) A part sample has been supplied by the partner Microcab. The part is a real sample of a car part. It can be seen in Figure 25 in the attached report. The original manufacturing method is wet layup of polyester gel coat and chopped strand mat. The part thickness is 4 mm. The part contains a lot of complex features such as the holes and the abrupt corners.
ii) The sample part is used for the construction of the plug and the cavity tools. The construction of the plug tool starts by masking all the holes in the part and placing it to a custom made “box” in order to cast the ceramic mixture. A schematic of the process can be seen in Figure 26 in the attached report. In this figure, the tool is denoted by the blue line and the cast ceramic is shown in grey. The whole construction was place in liquid nitrogen. Once taken out from the liquid nitrogen box the part was allowed to reach room temperature. Then, the part was de-moulded (Figure 27 in the attached report) and fired for 14 hours at 400°C.
iii) The plug tool constructed in the previous step is then used for the construction of the cavity tool, as shown in Figure 28 in the attached report. The cavity tool is designated with the number “2” in the figure. A wax layer (dark grey line in Figure 28) with a thickness equal to the part thickness is applied to the plug tool surface. A custom made “box” is again constructed in order to cast the ceramic material. A similar casting, freezing in liquid nitrogen and firing at 400°C process is followed.

During the construction process, a number of practical observations were made, with regards to the application of the freeze casting process:
i) Masking the holes in the sample part. In real cases the part is manufactured without any holes and the holes are machined afterwards. In this exercise, the masking of the holes was done using wax. Some discrepancies in the surface of the part were observed in these areas.
ii) Increase the flange size for better grip proved to make easy the positioning of the sample part.
iii) Wax proved to be very good in forming the part when the cavity tool was constructed.
iv) The following compositions were tried for the plug and cavity tools: a) Pure Al2O3: This was used for the first tries in order to gain experience with the process and identify any practical and technical issues, b) Bi-layer process with individual Al2O3 and Al2O3/ferrite layers: This required a complex addition process due to the complex geometry of the part, c) Uniform Al2O3/ferrite layer

B4.1) Final tools produced
The plug and cavity tools are shown in Figure 30 in the attached report.

The final prototype tools have been developed using a combination of freeze casting and slip casting techniques. The final demonstrator tools are shown in Figure 31 and Figure 32 in the attached report.

C) Production of composite parts
C1) Initial processing of composite parts
The initial production of composite parts was performed in the tool shown in Figure 30 in the attached report. A carbon fibre prepreg (commercial name VTM® 264 FRB) was used in the manufacturing trials. The prepreg is an epoxy thermoset preform, partially impregnated with the resin.

Forming of the prepreg was performed by hand. The layup was done on the plug tool, as can be seen in Figure 33 in the attached report. Four layers of preform were consolidated. Once the layup was completed, the cavity tool was placed at the top and the part was inserted in the microwave chamber. Thermocouples were placed at various locations in order to record and control the temperature. The final assembly prior to cure can be seen in Figure 34 in the attached report. No external pressure was applied.

Microwave curing of the part was performed using the following cure cycle.
The cure cycle imposed to the part was the following:
i) Heat up from ambient temperature to 120°C at a rate of 10°C/min
ii) Dwell at 120°C for 45 minutes
iii) Natural cooling to ambient temperature (the MW oven door was opened. The tool and the part were allowed to cool by natural convection)

The control temperature, part temperature and the MW power requirements during the manufacturing process are plotted in Figure 35 in the attached report. The part follows closely the imposed cure profile. The MW power requirements reach a maximum of about 50% of the maximum power output. Power demands are higher during the heat up segment. In the isothermal segment, the average power output of the oven is down to about 10%.

A thermal image of the tool right after the end of the curing process is shown in Figure 36 in the attached report. It can be seen that the ceramic tool remains much cooler during the process (recorded temperatures in Figure 36 are in the order to 60°C). Heating is concentrated in the tool surface as expected (red contour in Figure 36). The final part is shown in Figure 37 in the attached report.

The same process was used for the production of a glass fibre composite part. The cure cycle was also the same in order to compare the power requirements in the two cases. The temperature profiles and MW power output are shown in Figure 38 in the attached report.
The imposed temperature profile is followed closely, as was the case with the carbon fibre composite part. The MW output requirements were lower compared to the carbon fibre composite part. The maximum MW power output recorded was around 30%, during the heating up of the part. The average MW power output during the dwell segment was about 15%, similar to the carbon fibre composite part. The final part can be seen in Figure 39 in the attached report.

The final demonstrator composite parts were developed using the prototype tool shown in Figure 32 in the attached report. The demonstrator design is shown in Figure 40 in the attached report. Carbon-Peek plies were cut and pre-formed on the ceramic tooling using an industrial hot air gun, as seen in Figure 41 in the attached report. The different plies were then bonded together using an ultrasonic gun in order to create the desired lay-up sequence as seen in Figure 42 in the attached report. Polyimide film (Kapton®) was used as a realise film for both the top and bottom sides of the composite part, as can be seen in Figure 43 in the attached report. Two N-type thermocouples were used in order to record the temperature evolution during cure. The thermocouples were shielded with aluminium tape and placed in direct contact to the composite as shown in Figure 43. The caul plate was then positioned on top of the composite staking and a vacuum bag was placed and sealed on the tooling as seen in Figure 44 in the attached report. The final component can be seen in Figure 45 in the attached report.

Potential Impact:
A) Automotive sector
Currently, the percentage of composite parts in a vehicle is low compared to aeronautics. However, there is a strong interest in the industry that comes from the following trends:
i) Electric cars: In order for the electric cars to extend their autonomy and offset the weight of the batteries, there is extensive use of composite structures in new cars that arrive into the market. A recent example of such development is the BMW i3 electric car.
ii) New materials that can be processed (in the case of thermoplastics) or cured (in the case of thermosets) in very short times open new areas for applications of composites.

Microwave processing of composites offers both potential for rapid processing and a field for development of material systems that can effectively be processed in short time. The technology developed in MUTOOL will enhance the uptake of microwave processing as a viable processing route as it offers a solution for low energy consumption tooling.

There is a growing interest in the Circular Economy in automotive and this is driven by alternative ownership models such as car sharing. In this space, the lifetime of the vehicle is greatly extended with recycling, re-manufacture and re-use designed in from the start. In such cases, composites can be expected to be used in areas of the vehicle which have extended life such as chassis while more recyclable materials will be employed in areas which are replaced more often.

B) Aeronautics sector
There is a continuous increase of composite parts in commercial aircraft. The current level for the latest aircraft from Airbus and Boeing is in the order to 50% and higher. The main manufacturing process utilised by the aeronautics sector is the autoclave. The main advantage of the autoclave is the application of positive pressure to the composite part. This results in high quality structures with minimum void content and optimised consolidation, which lead to high fibre volume fraction.
The major disadvantage of autoclave is the very high demand for energy. Heating of the part is done by convection/conduction mechanisms and requires all the air inside the autoclave to be heated to the process temperature. Due to the slow heating process, typical cure cycles for large composite parts can extent to many hours and even days. Furthermore, a very large percentage of the energy supplied to the autoclave is used to heat up the tool. This can render the autoclave process not financially viable for the production of very large composite parts.
Finally, the capital costs for the purchase and installation of an autoclave are very high.
The microwave process and the technology developed in MUTOOL offer an alternative route to the aeronautics market and open up opportunities for faster processing of large structures. The composite part can be heated directly (through the microwave absorbing layer in the tool surface), leaving the tool and the air inside the microwave oven relatively cool. Furthermore, ceramic based tools are much lighter than metallic ones and hence easier to handle and transport.
Although there are still technical challenges before the MUTOOL technology is readily adopted by the industry (for example tool maintenance & repair and certification of the microwave oven as an autoclave alternative) the market potential is high. At the moment, there is a lot of research and development activity in Out-of-Autoclave processing for the aeronautics sector. Microwave processing is one of these alternative processing technologies.

C) Main Dissemination activities
The project results were presented by the coordinator in the JEC Paris show in 2013 and 2014. The latest event took place in 11-13 March 2014.
Loiretech devoted 2 engineers based at the Loiretech booth for the dissemination of the MUTOOL project results. A dedicated slideshow was being displayed continuously during the exhibition. The demonstrator part (Carbon/Peek) was in display in order to prove the process feasibility.
Several customers in automotive and aeronautical industry declared their interest to the MUTOOL process.
In the aeronautical field, representatives from AEROLIA, AIRBUS and DAHER expressed strong interest.
In the automotive field, representatives from FAURECIA and PLASTIC OMNIUM showed the most interest.

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