Dipl.-Ing. Matthias Meyer

MICROWAVE TECHNOLOGIES FOR COMPOSITE PROCESSING

M. MEYERGerman Aerospace Center

Institute of Composite Structures and Adaptive Systems

Summary

The application of microwave heating is examinedfor the production of carbon fiber-reinforced polymercomponents. Three methods for the thermal treat-ment of the composite polymers are identified. Thepreconditioning of the resin at the laboratory level inpreparation for an ensuing injection is examined aswell as the preforming of bonded, semi-finishedmaterials and the curing of components. Thesuitability of the microwave process is demonstratedby representative material parameters. These inputdata are used to implement the technology withindustrial standards, which is demonstrated by aresin heating device and a microwave autoclave.

Microwave Technology for the CFRP Production

In view of increasing industrial production, theoptimization of current processes is necessary forthe manufacture of carbon fiber-reinforced plastics.In addition to shortening cycle times, a quality-assured production process is of primaryimportance in aerospace applications.The use of electromagnetic radiation, particularlymicrowaves, to heat up products has been knownfor a long time. The heating of products usingmicrowaves is characterized by a selctivly efficientintroduction of heat into the product, which shortensthe entire process and reduces the necessaryenergy expenditure.Heating carbon-fiber reinforced plastics withmicrowaves is not yet a very common process.Because of its many thermal process steps, theprocess chain for the manufacture of CFRPcomponents is suitable for examining theapplication of microwaves. Due to the material’s lowthermal conductivity, the thermal process steps area bottle neck in the manufacture of components. Anoptimization of the process, particularly in aero-space-related applications, is necessary due to theuse of high-temperature duroplasts. In addition toothe curing of structures, additional areas ofapplication are also the preforming of bonded, semi-finished materials and the preconditioning of resins.

The use of a universal microwave oven with aworking frequency of 2.45GHz gave rise to aprocess technology that has advantages comparedto the state of the art.The prospect arises of applying the microwaveheating process of CFRP materials also to fiber-reinforced thermoplastics and low-temperaturethermosets. The focus of the work is on thedevelopment of the processes used in the field ofaerospace. In a first step in the microwave heatingprocess for the preconditioning of resin, thepreforming and component curing processes aredeveloped using already qualified molds and proc-essing tools. Pre-qualified processing tools are usedfor the identification of a possible influence of themicrowaves on the material. In order to examinethis more closely, material samples are made andparameters are examined that are stronglydependent on the temperature of the material. Me-chanical as well as thermal tests are carried out.Heating up materials using microwaves is alsocalled dielectric heating [1]. Compared toconventional heating methods by means of forcedconvection, a supporting medium like air, as used inmany industrial processes, is not necessary to heatproducts. The microwaves interact directly with theproduct to be heated, making the heating aselective process and keeping the surrounding areacold. Carbon fiber reinforced-plastics are heated bythe induction of eddy currents via the so-called skineffect [10] in which is heating occurs via the fiberand the orientation polarization of the matrix. Themicrowaves penetrate the material, which is why theprocess is called volumetric heating.Microwave heating has already found its way in themanufacture of CFRP and fiber glass-reinforcedstructures at the laboratory level [7-9]. Statementson this method describe an influence of microwaveheating on the material properties of the matrix andcomposite. However, it must be noted that themethod described here was carried out under non-constant circumstances, which is why these resultsremain inconclusive. What all methods have incommon is the application of microwaves to speedup the process. Processes with constant boundaryconditions seem to particularly advantageous for theapplication of microwave heating. This is the reason

Dipl.-Ing. Matthias Meyer

why the continuous flow-heating used duringinfiltration [11] to decrease the viscosity was exam-ined. Further tests are based on a comparison ofconventional with microwave-based methods inwhich, among other things, the material propertiesdominated by the matrix are examined more closely(2-5). The results presented there come from anunadapted technology. For this reason, the materialsamples produced by microwaves have a lowermaterial performance than their conventionalcounterparts. When manufacturing CFRP compo-nents with microwaves, the boundary conditionsthat result from the physics of microwaves must betaken into consideration. The potential advantagesof microwave heating can only be achieved with anadapted technology.Particular advantages come about within theprocess due to this particular form of heat. Theintroduction of the temperature at the component isdirectly influenced by the emission of microwaves,which enables a very precise tempering of thecomponent. Direct heating of the material via micro-waves eliminates dead times and inertia during theprocess that were previously unavoidable due to theheating of molds and atmospheres. The introductionof temperature directly at the component has advan-tages regarding quality assurance since the reactiontime in the process regulation is low due to the coldoven atmosphere. Minimizing the inertia in theprocess can lead to a maximizing of heatinggradients within the manufacturing process, whichsaves time.In addition to the skin effect, the polarizationproperties of a material, which are characterized bya complex relative permittivity, are particularlydecisive for with microwave heating [2].

''' εεε i+= (1)

ε := complex relative permittivityε’ := real part (stored share)ε’’ := imaginary part (share of loss)

Molecules with a dipolar character are aligned in thealternating electric fields of the microwaves. Duringthis so-called polarization, heat occurs due to lossmechanisms in the material. A permanent dipole ispresent as long as the centers of charging of thenegatively loaded electrons of the molecules do notcoincide with the positively loaded center ofcharging of the atomic core. The dipoles in an alter-nating field are aligned out of place. This phasedisplacement is described with the loss angle andhas the effect of a thermal loss. This effect isexploited for heating with microwaves.

The dielectric loss factor tanδ is the measure for thisphase displacement and the argument for thecomplex dielectricity number [3].

'''tan

εεδ = (2)

The dielectric loss factor is the operating parameterfor polymers to test their suitability for microwaveheating.The properties necessary to heat the materialdepend on influencing factors. Viscosity,temperature and degree of polymerization are someof the factors for duromeres. Alignment of themolecules in the alternating field is better feasiblewith declining viscosity than when in a solid con-dition so that, in the end, the entire molecule isaligned in the direction of the alternating field sincethe dipoles are locally set on the molecule. ��Theviscosity of the matrix system therefore alsoinfluences the dielectric loss and, with that, thecomplex relative permittivity. The effect describedhere is called a relaxation. The relaxation was de-scribed for the first time through the Debye model.In this model relaxation times of the dipoles τd occurthat describe the alignment time frames of theindividual dipoles. Therefore, a complete relaxationis possible as long as the imaginary share of thedielectric number reaches a maximum and the lossangle is maximized as well [1]. The following ap-plies:

2)(1'

d

URU ωτ

εεεε+

−+=

(3)

20 )(1

)(''d

dUR

ωτωτεε

ωεσε

+−

+=(4)

ε0 := electrical field constantεr := relaxed dielectric numberεu := unrelaxed dielectric numberε d := relaxation time of the dipolesσ := conductivity

The frequency of the alternating field is anadditional factor that influences the relaxation. Theviscosity primarily has an influence on the mobilityof the load carrier but if the frequency of thealternating fields is located in the area of therotation frequency of the dipoles, the polarizationdeclines since the dipoles no longer reach theorientation distribution.

Dipl.-Ing. Matthias Meyer

If a lossy dielectricum is exposed to an alternatingfield, the electromagnetic wave penetrates thedielectricum but is damped in the process. Energy isreleased and a term is defined for the performancedifference ∆P [5,6].

)e-P(1P -2ax=∆ (5)

a := damping constantsx := path in the field direction of the

dielectricum

The damping constant a is given by:

1)'²²

²1(2

''−+=

εωχεµωa (6)

µ’ := real part of the complex permeabilityχ := conductivity of the dielectric

The conductivity of the dielectric results from theimaginary part of the complex dielectric constants[4].

''0 εωεχ ⋅= (7)

The depth of penetration in a dielectricum, however,is relevant for the processing. The penetration depthis defined as the depth with which the performancedifference of 1/e was realized. It is also dependenton the wave length of the alternating electrical fieldand shows that the penetration depth increases withdecreasing frequency but also that the releasedheat decreases in the dielectricum. The wave lengthof the microwave of 2.45 GHz is a good compro-mise.

δεπλυ

tan''20= (8)

The penetration depth of the microwave in theproduct to be heated is a measure for an evendistribution of the heat in relation to the wallthickness of the component. If a component has agreater wall thickness than the penetration depth ofthe microwave in the component material, the influ-ence of the inner heat conduction in the componentincreases again.Here it becomes clear that the process andmethods can only be successfully carried out ifcertain dielectric boundary conditions are taken intoconsideration. The dielectrically constant surface,i.e. a surface that is evenly heated in dependence

on the field distribution is particularly necessary forCFRP with its very dielectrically different output com-ponents. Excess temperatures that cannot bemeasured are critical in a microwave process.These excess temperatures are caused by“hotspots” and “arcings”.Hotspots are the result of an inhomogeneous fielddistribution in the process area and also as theresult of materials with different dielectric properties.Based on the carbon fiber and a matrix, the matrixis more strongly heated in the microwave field thanthe fiber. This is the reason why the accumulationsof resin, so-called resin-rich volumes, have to beavoided at the component since these areas causea momentum that can damage the component.Arcings are electric arcs caused by high localpotential differences.

Basic Laboratory Research

Basic research takes place in a 1m3 universalmicrowave oven, as seen in Fig. 1. The microwaveoven has an overall performance of 8kW and is adevice that is controlled by the temperature. Threepyrometers and four fiber-optical thermocouples areavailable for temperature measurement. Microwave-specific process parameters are recorded by a direc-tional coupler that measures the emitted andreflected microwave performance and thereforeenabling an assessment of the effectiveness of themicrowave process.

Fig. 1 Universal microwave oven 2.45GHz 8kW

The use of microwave heating is applicable for theprocess steps of the preforming, the pre-conditioning of the resin and for the curing ofcomponents. Components cured with microwave

Dipl.-Ing. Matthias Meyer

heating takes place for prepregs as well as in theinjection process. Because of the laboratoryfurnishing with a multi-mode-atmosphere microwaveoven, the injection method without using anautoclave is first looked at here.The injection method for fiber-reinforced compositecomponents is the state of the art for manycomponents. The injection of a matrix systemdepends on its viscosity. The lower the viscosity,the easier it is to carry out an injection process andthe duration of the injection is shorter as well. Theviscosity of a matrix system depends on the tempera-ture and degree of polymerization whereby thedegree of polymerization, in turn, depends on thetemperature and time.For this reason, the resin is heated to a pre-definedtemperature in preparation for an enusing injectionin order to obtain optimum properties with regard tothe flow characteristics and processing duration.The heating of individual resin containers greatlydepends on the shape of the container and takes alot of time with today’s level of technology. Batchesranging from 10 to 20 kg are common in aerospaceapplications.Due to the low inner heat conduction of resinsystem the containers are stored in a preheatedoven for up to four hours. It takes this long for theentire container to be adequately and evenlyheated. It is hardly possible to shorten the processwith today’s technology. A too high temperature ofthe oven in which the container is heated results inan inhomogeneous heating of the container. Suchhigh temperatures on the container walls couldinevitably result in an exothermal reaction of thesystem. This excessive temperature cannot becompensated by inner heat conduction of thecontainer. The temperature curves depicted in Fig.2 are a reference point measurement that was car-ried out in a standard 10kg container. Heating tookplace in a convection oven that was pre-heated to80°C. An even temperature can be attained in theentire container only after 8 hours at a constanttemperature, which is not acceptable for industrialapplication.

0,00

20,00

40,00

60,00

80,00

100,00

0,00

1,11

2,22

3,34

4,45

5,56

6,67

7,78

8,89

10,00

Zeit [h]

Tem

pera

tur [

°C]

Sensor 7 Sensor 1 Sensor 3-Mitte Sensor 2Sensor 5-Mitte Sensor 6 Sensor 8 Sensor 4-Mitte

Fig. 2 Conventional heating of a resin container

Still, the heating process needs to be shortened inorder to guarantee the availability of injectable resinand to provide a degree of flexibility in theproduction process. The disadvantage of heatingwith today’s technology, in addition to the avail-ability, is the reduced processing time for the resinsystem.One way to shorten the heating process is by usingmicrowaves. The dielectric properties of epoxyresins make them particularly suitable for beingheated with microwaves.In order to verify the suitability of microwave heatingfor the preconditioning of epoxy resin, RTM6 fromHexcel in this case, a test based on a conventionalprocessing method was carried out. Since in thiscase only the matrix did not undergo a qualifiedprocess, the material parameters qualified by thematrix have to be determined. The remainingenthalpy, viscosity, shear strength and shearmodule are selected. Material samples are made forthis purpose that are cured under identical condi-tions. The only difference between the samples isthe pre-treatment of the matrix. One is convention-ally heated in a convection oven for 4 hours and theother by microwaves for 25 minutes. Thetemperature of both matrix systems is 80°C afterbeing heated up.

0

2

4

6

8

10

12

14

0 30 60 90 120 150 180 210 240time [min]

visc

osity

η [P

as]

4h 80°C conventional 25min 80°C microwave

120°C isothermal

Preconditionig:

Fig. 3 RTM6 viscosity curve

The rheological analysis in Fig. 3 clearly shows anuncritical influence of the microwaves on the

Dipl.-Ing. Matthias Meyer

viscosity. The viscosity of the matrix system shouldbe reduced as much as possible for an injectionfollowing the heating of the resin. Due to the lowrise in viscosity, RTM6 heated with microwaves isjust as suited for the injection process as con-ventionally heated RTM6. The time framenecessary to carry out an injection is increased withmicrowave-heated resin.In order to examine the influence on the degree ofpolymerization and remaining enthalpy, RTM6 pre-heated with microwaves and conventionally pre-heated RTM6 are cured at 180°C and then evalu-ated by means of a DSC analysis. The curing of indi-vidual samples takes place under the sameconditions so that the only difference in the processis the pre-heating of the matrix. As seen in Fig. 4,there is no significant influence of the microwaveheating on the RTM6 resin system on the enthalpyremaining in the system. The measured remainingenthalpy is at the same level as the referencesamples (conv1, conv2).

3,8

5,0

3,53,2

4,3 4,2

0

1

2

3

4

5

6

MW 1 MW 2 MW 3 MW 4 CONV 1 CONV 2

Enth

alpy

afte

r cur

ing

[%]

Fig. 4 Enthalpy after curing cycle

A possible influence of the chemical structure of theresin system, which has an effect on the boundarylayer between the fiber and matrix, is examined onthe basis of the shear strength and shear module ofa conventionally cured composite. In order toresearch the chemical bonding properties of thematrix on the carbon fiber, the shear properties of acomposite component are examined according toan internal norm from Airbus: AITM 1.0007. The com-ponent is manufactured by using the microwaveresin heating (MW) and a reference (REF) heatedwith the currently used conventional heatingmethod. The individual material samples are curedunder identical conditions. The test is carried outwith a HTA 6K fiber that is weaved to a fabric. Theresults shown in Fig. 5 show no direct influence ofthe microwave heating neither on the shear strengthnor on the shear module, which is not shown here.Neither can a change be determined in the structure

of the matrix using an infrared spectroscopy, whichis not shown here either.

97,7 96,5 94,05 97,27 95,02 93,3

0

20

40

60

80

100

120

MW-01 MW-02 MW-03 MW-04 MW-05 REF

Schu

bfes

tigke

it τ z

[MPa

]

Fig. 5 Shear strength of a carbon fiber-reinforced compositewith a 57% fiber volume content

This is the evidence that heating RTM6 withmicrowaves is a suitable method. This methodsaves a considerable amount of processing time,which is necessary for the preparation of individualresin batches.The amount of process time saved in the preformingprocess is also examined.Preforming using the binding technology is alsopossible with the microwave heating process inorder to minimize the effect of material-specificproperties like poor heat conduction in the dry fiberpreform. In the binding technology, the fiber pliesand weaves are partially impregnated with athermoset or thermoplastic polymer. Impregnationtakes place with polymer non-woven materials,polymer powder or filaments. The binder melts orconnects by means of a thermal activation. Thematrix system binds the binder as a component inthe chemical structure or the elements physicallyremain in the composite.Today, fiber plies are thermally activated withconventional heating methods. The most simplemethod is the use of heating plates or irons withwhich the semi-finished fiber is treated and drapedin a mold. Other methods make use of convectionovens and infrared facilities in which flexible mem-branes are used. The membranes press the semi-finished fiber into the desired contour using avacuum and activation takes places with anadditional heating device by means of convectionand heat radiation. This way it is possible to laydown single or several fiber plies at the same time.However, the introduction of heat into the product isnot very efficient. The heating is directly related tothe temperature of the convection oven, whichpassively heats the fibers and binder. It isnecessary to heat the oven atmosphere and themold, which is why only a fraction of the energy can

Dipl.-Ing. Matthias Meyer

be used to heat the product. This is where themicrowave technology provides a solution since thecarbon fibers are selectively heated. Heating takesplace solely in the preform and not in the periphery.However, integrating microwave heating into thepreform process is not a trivial matter since therequirements of the process including the precisemolding of the fiber preform and the application ofthe vacuum deformation must be taken intoconsideration. An at least one-sided transparency ofthe mold must be guaranteed in microwave heatingso that the microwaves can penetrate the product.

In addition to the process technology requirements,the influence of the microwave heating on the fibersthemselves needs to be researched. The fiber beingresearched here is a HTA 6K fiber that is woven toa Cramer 445 fabric. A possible effect of themicrowave heating on the boundary layer betweenthe matrix and fiber needs to be researched that,among other things, is influenced by the size of thefibers. In addition, the tensile strength of the fiber isexamined in order to eliminate any structuralchanges in the fiber.The basic suitability of microwave heating for thepreform process is shown in the following work. Theprocess is reproduced with a simple setup. Thesetup consists of 8 plies of carbon fiber-reinforcedfabric and is symmetrically laid down on analuminium sheet. The fibers are consolidated over avacuum with the aid of a membrane and thenheated with microwaves. The preform process takes20 minutes. Fig. 6 shows the influence of themicrowave heating on the stiffness of the fiber orthe structure of the sizing. The values derived frommicrowave preforming are compared with those de-rived from conventional methods such as the ironingof individual fiber plies and a conventionalconvection oven. Microwave heating was carriedout at 2 temperature levels. The fibers were heatedup to 90°C in the first test and to 120°C in thesecond. This, however, only had an influence on thehaptics of the of the dry preform that, after treatmentat 120°C, showed greater stiffness than the otherpreforms. But it did not have a severe influence onthe mechanical parameters. The results providedhere were determined according to an AITM and sta-tistically secured. Microwave heating is appropriatefor the preforming. The advantages of this processare savings in time and energy. The referenceprocess took 30 to 50 minutes in order to achievethe same degree of strength as with dry fiberpreforms.The tension and shear properties are partiallyshown in Fig. 7 and 8. Even in additional tests no

influence of microwave heating on the mechanicalproperties of the composite was determined.

99,994,1 96,1 94,5

0

40

80

120

MW 120°C MW 90°C IRON CON 90°C

shea

r str

engt

h [M

Pa]

Fig. 6 Shear strength for CFRP fabrics

77,287 74,33 70,313 71,154

0

20

40

60

80

100

MW 120°C MW 90°C IRON CON 90°C

E-M

odul

us [G

Pa]

Fig. 7 E-module for CFRP fabrics

This proves that the microwave heating ofindividual carbon fiber-reinforced polymercomponents is a suitable process. These resultsnow need to be applied to the manufacture of CFRPcomponents. The curing of structures via microwaveheating remains to be tested at the end of thethermal manufacturing chain of CFRP structures.The effect of selective and volumetric heating isalso exploited in the curing of composite structures.Injected components as well as prepreg structurescan be cured in a microwave field. However, onlythe curing of injected components is explained inthe following since the basic laboratory equipmentconsists of an atmospheric pressure microwaveoven. Both the dielectric properties of prepregs andan injected fiber setup are similar.The process chain for the manufacture of a carbonfiber-reinforced composite structure ends with thecuring of structures. The curing is the mostdemanding process since the highest temperaturesare generated at low tolerances in connection withthe vacuum strength of the production setup. Knowl-edge of the distribution of the temperature over thecomponent is necessary to guarantee the quality ofthe component later on. The maximum temperaturegenerated in the process is 180°C. The height ofthe temperature generated by microwaves dependson the power density of a microwave chamber that

Dipl.-Ing. Matthias Meyer

chamber that occurs from the quotient of maximummicrowave performance and chamber volume. Thepower density was 8 kW/m³ in the following tests. Inaddition to the maximum temperature, the homoge-nous distribution of the temperature over thecomponent surface and in the component volume isdecisive. The temperature distribution is dependenton an appropriate coupling of the microwaves in thesystem, i.e. the number of microwave sources andthe antenna form. The facility used for the tests isequipped with slotted wave guide antenna. Ahomogenous temperature field is also necessary forthe process control since the microwave process isone controlled by the temperature. The maximummeasured temperature is compared with thesetpoint value and the microwave performance isaccordingly emitted. The temperature measurementtechnology is integrated into the processing tool inaccordance with a series process.Figure 8 shows an example of a temperature curve.The temperature is recorded at two measurementareas. Fluctuations in temperature are caused bythe inertia of the temperature measurement in thetool and not by the microwave facility. The tempera-ture distribution in the entire tool is known from pre-tests. The critical areas are selected asmeasurement points for the process control in orderto achieve a sufficient temperature distribution.

0

20

40

60

80

100

120

140

160

180

200

0:00 0:30 1:00 1:30 2:00 2:30 3:00 3:30 4:00 4:30Prozesszeit

Tem

pera

tur [

°C]

Sollwert Faseroptik 1 Faseroptik 2

Fig. 8 Temperature distribution in a microwave process

Knowledge of a possible influence of the microwaveheating on the material is necessary in order toacheive a qualified curing of carbon fiber-reinforcedpolymers. After the processing setup is adapted to asecure processing in the microwave field, a stan-dard curing cycle for the RTM6 resin system fromHexcel is carried out first. The influence ofshortening the curing time and of faster processingby increasing the heating gradients are researchedwithin the processing parameters prescribed withinthe resin system. The parameters to examine the

microwave process are the curing times and theheating rate. Curing at 180°C is shortened from90min to 60min for the MW sample (1hour at180°C) and the heating rate of 1°C/min ismaintained. The MW sample (3°C per min) isheated to 3°C/min and curing is maintained at180°C/90min. Despite the faster processing, thereare no considerable changes in the shear propertiescompared to the microwave reference process andautoclave reference (Fig. 10). The finished samplesare different only in the manufacturing process andidentical with regard to fiber orientation and fibervolume content.

104,8 101,5 104,2

88

0

20

40

60

80

100

120

Ref MW MW (1h bei180°C)

MW (3°C proMinute)

Referenz Autoklav

Schu

bfes

tigke

it τ z

[MPa

]

Fig. 9 Shear strength with material samples cured bymicrowaves

32223391 3270 3243

0

1000

2000

3000

4000

Ref MW MW (1h bei180°C)

MW (3°C proMinute)

Referenz Autoklav

Schu

bmod

ul G

z [M

Pa]

Fig. 10 Shear module with material samples cured bymicrowaves

The enthalpy remaining in the system that ischaracteristic for the degree of curing, Fig. 11, ismeasured by means of a differential scanningcalorimetry (DSC). The enthalpy remaining in thesystem is lower for all microwave processes than forconventional references. Accordingly, the degree ofcross-linkage of the samples cured with microwavesis higher than the conventional reference. The glasstransitional temperature for the samples cured withmicrowaves is correspondingly higher than theconventional reference.

Dipl.-Ing. Matthias Meyer

0 50 100 150 200

MW1 h3°C

Konv.

0 10 20 30Spez. Enthalpie: [J/g]

TG [°C]0 50 100 150 200

MW1 h3°C

Konv.

0 10 20 30Spez. Enthalpie: [J/g]

TG [°C]

Fig. 11 DSC Test for cured structures; specific enthalpyin the foreground; glass transition temperature TG in thebackground

This, however, is not a specific effect that can beascribed to the use of microwaves but rather thedirect temperature control at the component itself isresponsible for the improved material performance.If the microwave process shown in Fig. 8 iscompared with the temperature control in Fig. 12, itbecomes clear that the conventional process islimited by the thermal inertia.

0

30

60

90

120

150

180

0:00 2:00 4:00 6:00 8:00 10:00

Tem

pera

tur [

°C]

Soll-Temperatur Ist-Temperatur

Fig. 12 Temperature control in a conventional process

The processing of carbon fiber-reinforced polymercomponents using microwave heating is possible forthe process presented here. Microwave heatingdoes not negatively influence the material propertiesof the composite later on. The process is shortenedin either case with a precise control of thetemperature and a direct heating of the material.The potential of the microwave process has notbeen exhausted since the curing of the compositefiber components and the preforming have not yetbeen implemented in the industry. However, theprimary goals of saving processing time and energyare being met.

Implementation in the industry

Exhaustion of the full potential microwave heatinghas to offer is only possible with an adapted facilitytechnology. The effect described above can evenbe increased with the use of a special facility. Thefacility depicted in Fig. 13 was especially developedfor the heating of RTM6.

Fig. 13 Microwave resin warming facility

The facility warms 10kg of RTM6 from roomtemperature to 80°C within 12 minutes. Comparedto the temperature distribution shown in Fig. 2, thetemperature distribution in Fig. 14 clearlydemonstrates the performance spectrum of thefacility. The temperature distribution within thecontainer is sufficiently homogenous so that it canbe assumed that the viscosity of the entire containeris constant.The temperature in the container was maintainedfor over 8 hours to demonstrate the stability andprecision of the process in the facility.

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

80,00

90,00

0 1 2 3 4 5 6 7 8time [h]

tem

pera

ture

[°C

]

Sensor 1Sensor 2Sensor 3Sensor 4Sensor 5Sensor 6Sensor 7Sensor 8Setpoint Value

Fig. 14 Temperature distribution in a 10kg RTM6container on several levels

The application of the component curing is looked atmore closely in the following. In view of CFRPstructures that are constantly increasing in size, theprocessing facilities also have to be adapted to therequirements of the components. The state-of-the-art autoclave technology for the manufacture ofprepreg components will most likely not bedismissed in the next few years. The energyexpenditure required for the operation of anautoclave is enormous. The facilities used todayhave a loading diameter of more than 8 meters anda length of over 30 meters.

Dipl.-Ing. Matthias Meyer

0

1

2

3

4

5

6

0 500 1000 1500 2000 2500 3000

Autoklavvolumen [m³]

Ener

gie

[GJ]

GesamtenergieBauteilspezische Energie

Fig. 15 Energy results of a convection oven

Looking at Fig. 15 it becomes clear that only aminimum amount of energy expended in theprocess is actually used for the component itself.Due to selective heating, microwave heating has theadvantage of saving energy and therefore operatingcosts. For this reason, Scholz Maschinenbau andthe German Aerospace Center DLR / Institute ofComposite Structures and Adaptive Systems haveentered into a cooperation agreement for thedevelopment of a microwave autoclave at theindustrial level.This comprises launching the microwaves in thepressure area of the autoclave as well as thedevelopment of a cost-effective temperaturemeasurement system. The design of a microwaveautoclave is depicted in Fig. 16. A microwavesystem can be upgraded for an existing facility butis not discussed in more detail here.

Fig. 16 View of a microwave autoclave

At present, the plant technology, Fig. 17, is beingrun-in in a first step and typical patterns of thefacility are being tested.

Fig. 17 Microwave autoclave at DLR/FA in the test phase

AcknowledgementsThe institute would like to thank its partners in theindustry. Scholz Maschinenbau GmbH was agenerous supporter in the realization of the“Microwave Autoclave” project. CTC GmbH Stadeadvanced the topic of the microwave processing ofCFRP materials in the past and still continues to doso. Our reliable partner in the microwave industry isthe Fricke und Mallah GmbH Company who, with itsexpertise, provides us with valuable support in ourresearch.

References

[1] Peter Elsner; Dielektrische Charakter-isierung des Aushärtungsverlaufs Po-lymerer Harze; Stuttgart; 1992

[2] Einführung in die Festkörperphysik; VerlagR. Oldenburg; 10. Auflage 1993; S. 415 ff.

[3] M. Rudolph, H. Schaefer; Elektro-thermische Verfahren; Springer Verlag;Berlin Heidelberg New York; 1989

[4] W. v. Münch; Werkstoffe der Elek-trotechnik; Teubner; Stuttgart; 1993

[5] Püschner; Wärme durch Mikrowellen;Philips technische Bibliothek; 1964

Dipl.-Ing. Matthias Meyer

[6] Roger Meredith; Handbook of industrialmicrowave heating; IEE Power Series 25

[7] Mijovic, Fischbain, Wijaya; Comparison ofkinetics in thermal and microwave fields;macromolecules 1992, 25

[8] Wei, Hawley; kinetics modelling and time-temperature-transformation diagram ofmicrowave and thermal cure of epoxyresins; polymer engineering and science,1995, vol. 35

[9] Wei, Hawley, DeLong; comparison ofmicrowave and thermal cure of epoxyresins; polymer engineering and science,1993, vol. 33

[10] Meinke, Grundlach; Taschenbuch derHochfrequenztechnik; Vierte Auflage;Springer Verlag 1986

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[12] C. Nightingale, R. J. Day; flexural andinterlaminar shear strength properties ofcarbon fibre/epoxy composites curedthermally and with microwave radiation;Composites Part A 33 (2002)

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