IIT | Instituto de Investigacion Tecnológica - GRADO …mediante “puntos de soldadura” usando...
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ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA (ICAI) GRADO EN INGENIERÍA ELECTROMECÁNICA Especialidad Mecánica MICROWAVE PROCESSING OF CARBON- FIBRE/PEEK Autor: José Antonio López Cabezas Director: Richard Day Madrid Junio 2016
Transcript of IIT | Instituto de Investigacion Tecnológica - GRADO …mediante “puntos de soldadura” usando...
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ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA (ICAI)
GRADO EN INGENIERÍA ELECTROMECÁNICA
Especialidad Mecánica
MICROWAVE PROCESSING OF CARBON-
FIBRE/PEEK
Autor: José Antonio López Cabezas
Director: Richard Day
Madrid
Junio 2016
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AUTORIZACIÓN PARA LA DIGITALIZACIÓN, DEPÓSITO Y DIVULGACIÓN EN RED DE
PROYECTOS FIN DE GRADO, FIN DE MÁSTER, TESINAS O MEMORIAS DE
BACHILLERATO
1º. Declaración de la autoría y acreditación de la misma.
El autor D. José Antonio López Cabezas
DECLARA ser el titular de los derechos de propiedad intelectual de la obra:
Microwave processing of Carbon-Fibre/PEEK , que ésta es una obra original, y que ostenta la
condición de autor en el sentido que otorga la Ley de Propiedad Intelectual.
2º. Objeto y fines de la cesión.
Con el fin de dar la máxima difusión a la obra citada a través del Repositorio institucional de la
Universidad, el autor CEDE a la Universidad Pontificia Comillas, de forma gratuita y no exclusiva,
por el máximo plazo legal y con ámbito universal, los derechos de digitalización, de archivo, de
reproducción, de distribución y de comunicación pública, incluido el derecho de puesta a disposición
electrónica, tal y como se describen en la Ley de Propiedad Intelectual. El derecho de transformación
se cede a los únicos efectos de lo dispuesto en la letra a) del apartado siguiente.
3º. Condiciones de la cesión y acceso
Sin perjuicio de la titularidad de la obra, que sigue correspondiendo a su autor, la cesión de
derechos contemplada en esta licencia habilita para:
a) Transformarla con el fin de adaptarla a cualquier tecnología que permita incorporarla a
internet y hacerla accesible; incorporar metadatos para realizar el registro de la obra e
incorporar “marcas de agua” o cualquier otro sistema de seguridad o de protección.
b) Reproducirla en un soporte digital para su incorporación a una base de datos electrónica,
incluyendo el derecho de reproducir y almacenar la obra en servidores, a los efectos de
garantizar su seguridad, conservación y preservar el formato.
c) Comunicarla, por defecto, a través de un archivo institucional abierto, accesible de modo
libre y gratuito a través de internet.
d) Cualquier otra forma de acceso (restringido, embargado, cerrado) deberá solicitarse
expresamente y obedecer a causas justificadas.
e) Asignar por defecto a estos trabajos una licencia Creative Commons.
f) Asignar por defecto a estos trabajos un HANDLE (URL persistente).
4º. Derechos del autor.
El autor, en tanto que titular de una obra tiene derecho a:
a) Que la Universidad identifique claramente su nombre como autor de la misma
b) Comunicar y dar publicidad a la obra en la versión que ceda y en otras posteriores a través
de cualquier medio.
c) Solicitar la retirada de la obra del repositorio por causa justificada.
d) Recibir notificación fehaciente de cualquier reclamación que puedan formular terceras
personas en relación con la obra y, en particular, de reclamaciones relativas a los derechos
de propiedad intelectual sobre ella.
5º. Deberes del autor.
El autor se compromete a:
a) Garantizar que el compromiso que adquiere mediante el presente escrito no infringe ningún
derecho de terceros, ya sean de propiedad industrial, intelectual o cualquier otro.
b) Garantizar que el contenido de las obras no atenta contra los derechos al honor, a la
intimidad y a la imagen de terceros.
c) Asumir toda reclamación o responsabilidad, incluyendo las indemnizaciones por daños, que
pudieran ejercitarse contra la Universidad por terceros que vieran infringidos sus derechos e
intereses a causa de la cesión.
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d) Asumir la responsabilidad en el caso de que las instituciones fueran condenadas por infracción
de derechos derivada de las obras objeto de la cesión.
6º. Fines y funcionamiento del Repositorio Institucional.
La obra se pondrá a disposición de los usuarios para que hagan de ella un uso justo y respetuoso
con los derechos del autor, según lo permitido por la legislación aplicable, y con fines de estudio,
investigación, o cualquier otro fin lícito. Con dicha finalidad, la Universidad asume los siguientes
deberes y se reserva las siguientes facultades:
La Universidad informará a los usuarios del archivo sobre los usos permitidos, y no
garantiza ni asume responsabilidad alguna por otras formas en que los usuarios hagan un
uso posterior de las obras no conforme con la legislación vigente. El uso posterior, más allá
de la copia privada, requerirá que se cite la fuente y se reconozca la autoría, que no se
obtenga beneficio comercial, y que no se realicen obras derivadas.
La Universidad no revisará el contenido de las obras, que en todo caso permanecerá bajo
la responsabilidad exclusive del autor y no estará obligada a ejercitar acciones legales en
nombre del autor en el supuesto de infracciones a derechos de propiedad intelectual derivados
del depósito y archivo de las obras. El autor renuncia a cualquier reclamación frente a la
Universidad por las formas no ajustadas a la legislación vigente en que los usuarios hagan uso
de las obras.
La Universidad adoptará las medidas necesarias para la preservación de la obra en un futuro.
La Universidad se reserva la facultad de retirar la obra, previa notificación al autor, en
supuestos suficientemente justificados, o en caso de reclamaciones de terceros.
Madrid, a 30 de Mayo de 2016 .
ACEPTA
Fdo………………………………………………
Motivos para solicitar el acceso restringido, cerrado o embargado del trabajo en el Repositorio
Institucional:
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ESCUELA TÉCNICA SUPERIOR DE INGENIERÍA (ICAI)
GRADO EN INGENIERÍA ELECTROMECÁNICA
Especialidad Mecánica
MICROWAVE PROCESSING OF CARBON-
FIBRE/PEEK
Autor: José Antonio López Cabezas
Director: Richard Day
Madrid
Junio 2016
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PROCESADO DE FIBRA DE CARBONO / POLIÉTER-ÉTER-CETONA
(CFR/PEEK) APLICANDO MICROONDAS
Autor: López Cabezas, José Antonio
Directores: Day, Richard
Entidad Colaboradora: Airbus Advanced Composite & Developement Centre
Glyndwr University
1. Introducción
Los termoplásticos de alto rendimiento como el PEEK (Poliéter-Éter-Cetona) alcanzan
incluso mejores propiedades cuando son combinados con fibra de carbono. La
combinación de la fibra de carbono y el PEEK (CFR-PEEK) es un material compuesto
avanzado con aplicaciones principalmente aeronáuticas, médicas o estructurales.
El procesado de éste material es una de sus principales limitaciones. Los métodos
usados actualmente para fabricar piezas de este material permiten altas velocidades de
producción, pero necesitan equipo especializado y específico. Además, como este
material no es muy flexible comparado con otros materiales compuestos, especialmente
a temperatura ambiente, las geometrías creadas con esto métodos han de ser simples.
El procesado mediante microondas de materiales compuestos tiene varias ventajas
(especialmente con microondas de frecuencia variable, VFM) como un calentamiento
uniforme de la pieza, la utilización de un equipo general que puede ser usado para
varios propósitos y tiempos de fabricación reducidos. Aplicar éste método al CFR-
PEEK es algo que nadie ha intentado antes públicamente y que podría mejorar los
métodos de producción no sólo de ese material, si no de los materiales compuestos de
matriz termoplástica en general.
En este proyecto ha sido estudiado el procesado mediante microondas de éste material.
Investigando y llevando a cabo una gran cantidad de test para encontrar un método
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adecuado y óptimo, analizando los resultados y procedimientos y estudiando la posible
aplicación de este método a procesos de producción reales.
2. Metodología
Las primeras semanas de laboratorio fueron usadas para aprender a manejar
adecuadamente todo el equipo disponible. Las muestras hechas para los test constan de
8 capas de CFR-PEEK unidireccional, siguiendo esta configuración que es la más usada
comúnmente: 90º, 0º, 45º, -45º, -45º, 45º, 0º, 90º. Las capas tienen que ser unidas
mediante “puntos de soldadura” usando un soldador de estaño. El tamaño de las
muestras es de 7x18mm.
Una de las limitaciones del procesado mediante microondas del CFR-PEEK son los
materiales usados para preparar la muestra. La temperatura necesaria para procesar el
CFR-PEEK (Aproximadamente 380 ºC) es más alta que el punto de fusión de
prácticamente todos los materiales usados para la producción de materiales compuestos.
Pero como la temperatura alcanzada en la mayoría de los experimentos no llega hasta
esta temperatura, y como la temperatura tenía que ser medida y para ello eran necesarios
otros materiales, éstos fueron usados igualmente.
La base de la muestra usada para insertarla dentro de la cavidad, está hecha de un vidrio
cerámico conocido y registrado como Neoceram ™. Dos planchas de este material
fueron encargadas a medida para éste propósito.
Los microondas usados constan de varias partes: Un ordenador con un programa para
controlar el proceso, un generador de onda, un amplificador, la cavidad y un termómetro
conectado al ordenador midiendo la temperatura de la muestra. Han sido usados dos
tipos distintos de procesado por microondas, frecuencia fija y frecuencia variable. Para
el proceso con microondas variable han sido probados dos modos: controlado
manualmente y controlado por ordenador.
El proceso con VFM requiere que se escojan y se fijen las frecuencias a las cuales va a
trabajar. Para cada frecuencia el comportamiento del material es diferente. Se han
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intentado escoger las frecuencias con más baja reflexión para conseguir la máxima
absorción de potencia por parte del material.
3. Discusión y resultados
Siguiendo los procedimientos descritos anteriormente, multitud de métodos diferentes
han sido probados para procesar el CFR-PEEK, cambiando todos los parámetros
posibles para conseguir un buen procesado. Se distinguen 4 variables de métodos
distintos, así que han sido agrupados en 4 grupos, ordenados cronológicamente.
3.1 Primer método, cavidad grande, VFM controlado manualmente.
La muestra fue precalentada en un horno. Con este método las frecuencias deben ser
escogidas como un rango y no frecuencias específicas. Lo que significa que el
microondas usará todas las frecuencias comprendidas en ese rango aleatoriamente. El
rango escogido fue 7-7.5 GHz. La máxima temperatura alcanzada fue 244ºC y la
muestra estaba completamente doblada después de este proceso.
El procesado de CFR-PEEK con microondas necesita mucha potencia. La cinta usada
para aplicar presión estaba desintegrada después del proceso. Actualmente no es posible
encontrar una cinta de presión que se distribuya comercialmente que pueda soportar la
temperatura requerida para el procesado de éste material.
3.2 Segundo método, frecuencia fija.
El microondas de frecuencia fija disponible en el laboratorio usa la frecuencia 2,45 GHz
como la mayoría de ellos. La reflexión (SWR) del CFR-PEEK a esta frecuencia es
bastante alta, la potencia no puede ser absorbida eficientemente. Además, el programa
del microondas de frecuencia fija funciona estableciendo unos parámetros, y éste regula
el proceso a partir de un modelo teórico. Es posible lo diferente que es éste modelo
teórico con el material estudiado.
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En éste grafico se puede observar como la temperatura alcanzada por el modelo teórico
era de 350ºC mientras que la temperatura real de la muestra es de 51ºC. Otras pruebas
fueron realizadas, cambiando todos los parámetros disponibles, obteniendo resultados
similares. Ésta frecuencia fija, la más común, es inútil para procesar CFR-PEEK. Otros
experimentos han sido realizados con el VFM usándolo para una sola frecuencia,
obteniendo resultados significativamente mejores.
3.3 Tercer método, cavidad pequeña, VFM controlado manualmente
El procedimiento seguido en estos test ha sido el mismo usado para el primer método
pero con una cavidad más apropiada al tamaño de la muestra. Los mejores resultados
para este método han sido obtenidos usando un rango de frecuencias muy pequeño
(6.752 - 6.917 GHz). Elegir un rango de frecuencias pequeño permite el uso de mucha
más potencia que un rango de frecuencias grande con picos de alta reflexión.
3936,12; 350,01
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Real temperature
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Temperature (C)
Power (W)
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3.4 Cuarto método, VFM controlado con ordenador.
Éste es el método más completo y el que tiene un más alto potencial para ser usado en
procesos reales de fabricación. A pesar de las limitaciones del equipo utilizado, éste ha
sido el método más usado y con el que mejores resultados se han obtenido. Éste método
permite usar varias frecuencias exactas, elegidas por el usuario y elegir la potencia
aplicada a cada una de ellas. El programa va cambiando entre ellas ordenadamente
varias veces por segundo.
Frequency Reflection Power (dB) 4.4.6.1(a) 4.4.6.2 (b) 4.4.6.3 (c) 4.4.6.4 (d) 4.4.6.5 (e) 4.4.6.6 (f)
6.8239 1.119 * * * * * * 7.0641 1.104 -2 -1.4 -0.9 -0.4 0 0 7.8046 1.035 -2 -1.3 -0.7 -0.4 0 0 8.0318 1.109 -2 -1.4 -0.8 -0.4 0 * 8.2799 1.0202 -2 -1.2 -0.6 -0.4 0 *
El gráfico enseña juntos todos los diferentes test que constituyen un solo experimento,
durante los experimentos se produjeron diversos puntos calientes, alcanzando
temperatura suficiente para fundir la matriz. Elegir las frecuencias con la reflexión más
baja es un factor determinante.
5930; 277,91
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Test 4.4.6 Temperature
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4. Conclusiones
El CFR-PEEK es un material con excelentes propiedades, ideal para múltiples
aplicaciones. El procesa con microondas de este material ha demostrado consumir
mucha potencia. Este material reacciona bien a las microondas, especialmente a
relativamente bajas temperaturas, pero la temperatura necesaria para procesar este
material es muy alta, entorno a los 380ºC. Mientras más alta es la temperatura, mucha
más potencia necesita el material para subir esta temperatura. Esto demuestra que la
tangente de pérdidas del CFR-PEEK es baja.
La principal limitación de los experimentos ha sido la falta de potencia del equipo
utilizado. Aunque la potencia hubiera sido la necesaria, los materiales necesarios para
preparar la muestra no hubiesen suportado esa temperatura. En un proceso de
producción los materiales usados en contacto con el CFR-PEEK, deberían ser
cerámicos, ya que los metales reflejan las microondas y prácticamente ningún polímero
puede aguantar las temperaturas necesarias.
Las mejores frecuencias para este proceso varían de un experimento a otro, pero
siempre están en el rango 6,5-8,5 GHZ, Así que el equipo usado debería cubrir este
rango de frecuencias.
Para consolidar el laminado es necesaria tanto temperatura como presión, debido a que
los métodos de vacío y las cintas de presión no son compatibles con este proceso por las
altas temperaturas, la presión no ha podido ser reproducida en el laboratorio.
Hacer test a las muestras no tiene sentido ya que el laminado no está consolidado y las
capas están separadas. Se han hecho algunos test de calorimetría diferencial de barrido
(DSC), pero se han omitido debido a la inutilidad y poca coherencia de los resultados.
El procesado mediante microondas de materiales compuesto es relativamente nuevo y
no está muy extendido. Este proyecto abre las puertas para poder aplicar este método al
CFR-PEEK y otros materiales compuestos con matriz termoplástica.
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MICROWAVE PROCESSING OF CARBON-FIBRE/PEEK
Author: López Cabezas, José Antonio
Directors: Day, Richard
Collaborating Entity: Airbus Advanced Composite & Developement Centre
Glyndwr University
1. Introduction
High-performance thermoplastics like Polyether ether ketone (PEEK) achieve even
better properties when combined with Continuous Carbon fibres. The combination of
CF with PEEK is a high-performance advanced composite with excellent properties,
suitable for lots of high demanding applications including medical, structural and
aeromechanical.
Processing CFR-PEEK is the main limitation of this material. Methods available and
used to process CFR-PEEK like hydroforming and matched die forming allows high
production rates, but they need expensive specific equipment, and, since the material is
solid and not very flexible at ambient temperatures, the geometries that can be created
with this methods are very simple.
Microwave processing of composites materials has advantages (especially Variable
Frequency Microwaves, VFM) like uniform heating, more general equipment and also
reduced processing times. Applying this method to CFR-PEEK is something that
nobody has tried before publically and can improve processing methods not only for
this material, but for all thermoplastic-matrix composites in general.
In this project has been studied the microwave processing of this material. Researching
and Performing lots of test to find a correct and optimum method, analysing the results
and procedures and studying the application of this method to real industry processes.
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2. Experimental procedures (Methodology)
The first weeks were spent in learning about the equipment available in the laboratory.
The samples made for the tests consist of eight layers of unidirectional CFR-PEEK pre-
preg. Following this common configuration: 90º, 0º, 45º, -45º, -45º, 45º, 0º, 90º. Layers
have been joined together by welding spots using a soldering iron. The size of the
samples is 7x18mm.
One of the limitations of the CFR-PEEK microwave processing is the materials used to
prepare the simple. Temperature necessary to process CFR-PEEK (380ºC
approximately) is higher than the melting point of all materials (polymers) commonly
used to process composite materials. Since the temperature were not high enough in
most cases, and since it was necessary to record the temperature and try to apply some
pressure to the sample, different materials have been used.
The base of the sample inside the cavity was made of a glass-ceramic material
trademarked and commonly known as Neoceram ™. Two plates were purchased with
similar size of the samples.
The microwave machines used are formed of several parts: the computer running the
program that controls the process, the wave generator, the amplifier, the cavity and the
thermometer. Two different microwave processing have been used, single frequency
and variable frequency. Variable frequency microwave, as expected, worked better with
this material. Two methods were used with VFM, manually controlled and program
controlled.
The VFM process requires to set the frequencies that are going to be used, for each
frequency the material’s behaviour is different. It has been tried to choose the lowest
reflection frequencies to achieve the maximum absorption of power by the material.
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3. Results and discussion
Following the procedures described before, lots of different methods have been tried to
microwave CFR-PEEK, changing parameters to achieve a good microwave processing:
the materials used to prepare the sample, the material of the base, the way of the
pressure is applied, the microwave cavity, the microwave machine, the frequencies
used, the power used, the time, the ramp-rate, the program, et cetera. The experiments
were divided in 4 methods, chronologically ordered.
3.1 First method, big cavity, manually controlled VFM
The sample was pre-heated in an oven, with this method the frequencies have to be set
by a range, which means the microwave uses all the frequencies in the range set,
swapping between them randomly. The range set was 7-7.5 GHz. The maximum
temperature reached was 244ºC and the sample was completely bent after the process.
CFR-PEEK microwave processing is very power demanding, in a big cavity
microwaves cannot concentrate properly to heat up a small simple. Shrink tape was
used to apply pressure and it was completely disintegrated after the process. Nowadays
is not possible to find a commercial shrink tape which can withstand the temperature
required.
3.2 Second method, single frequency
Single frequency microwave facilities available in the laboratory uses the common
2,45GHz frequency. The reflection (SWR) of CFR-PEEK at this frequency is high, and
the power cannot be absorbed efficiently. Furthermore, the software of single frequency
microwave works setting the parameters of a theoretical model, it is possible to see how
different the behaviour of this material is compared with a common CFR-Thermoset
composite:
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The temperature reached by the theoretical model was 350ºC while the real temperature
of the sample were stabilised at 51ºC. Another tries were performed changing all the
parameters available, but the results were similar. Single frequency microwaves at this
frequency are useless to process CFR-PEEK. Another experiments with the VFM
facilities has been performed with a single frequency, obtaining better results.
3.3 Third method, small cavity, manually controlled VFM
The procedure followed in these tests were the same used in the first method but with a
more appropriated cavity size. The best results were obtained with a small range of
frequencies (6.752 - 6.917 GHz). Choosing a low-reflection small range allows the use
of much more power than a big range with high reflection peeks.
3936,12; 350,01
3936,12; 51,17950
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Theoric temperature
Real temperature
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Temperature (C)
Power (W)
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3.4 Fourth method, computer controlled VFM.
This is the most complete method and it has the highest potential. Besides some
limitations of the equipment, it was the most used method. This method allows to use
several exact frequencies, and choose the power applied for each one. The program
swaps between them periodically and orderly several times per second. For example:
Frequency Reflection Power (dB) 4.4.6.1(a) 4.4.6.2 (b) 4.4.6.3 (c) 4.4.6.4 (d) 4.4.6.5 (e) 4.4.6.6 (f)
6.8239 1.119 * * * * * * 7.0641 1.104 -2 -1.4 -0.9 -0.4 0 0 7.8046 1.035 -2 -1.3 -0.7 -0.4 0 0 8.0318 1.109 -2 -1.4 -0.8 -0.4 0 * 8.2799 1.0202 -2 -1.2 -0.6 -0.4 0 *
The graph shows the different tests that form one experiment. Several hot points were
created during the experiments, reaching the enough temperature to melt the matrix.
Finding the lowest-reflection frequencies is a determinant factor.
5930; 277,91
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Test 4.4.6 Temperature
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4. Conclusions
CFR-PEEK is a material with exceptional properties and ideal for multiple applications.
The microwave processing of the material has demonstrated to be very power-
demanding. The material reacted well to microwaves, especially at relatively low
temperatures, but the temperature needed to process this material is very high, around
380ºC. The higher the temperature of the material is, the higher power is needed to
elevate this temperature. This demonstrates it is a low loss-tangent material. The
principal limitation of the tests was the microwave facilities available were not powerful
enough to process the material.
Even If the power would had been enough, the materials used to prepare the sample
cannot afford these high temperatures. In a production process the materials that should
be used are ceramics or metallic, because other polymers (films and tapes) cannot afford
the high temperatures needed. Metallic materials reflect microwaves, so they should not
be used during microwave processing.
The best frequencies to process the material (low reflection ones) vary from one test to
another, but the best frequencies always were between 6.5 to 8.5 GHz so the equipment
used should cover this range of frequencies.
To consolidate a laminate it is required both, pressure and temperature. Since the
available vacuum methods and shrink tapes are not compatible with this process
because of the high temperature needed, it was not possible to reproduce this pressure in
the laboratory. The pressure should be applied after the heating.
Testing the samples has no point when the laminate is not consolidated. Differential
Scanning Calorimetry (DSC) tests have been made but omitted because of the
uselessness of the results.
Single frequency and variable frequency microwaves have been tested. In theory and
based on the research made, variable frequency microwaves can perform a uniform
heating of a sample, penetrating inside the material and heating not only the surface but
the inside.
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Microwave processing of materials is relatively new and not very extended. This project
opens the gates to applying this method to CFR-PEEK and other thermoplastic
materials.
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ACKNOWLEDGEMENTS
I would like to express my very great appreciation to my project supervisor, Professor
Richard Day, who gave me the freedom to explore on my own, but always leaded
me in the right way and helped me with his vast knowledge about composites.
My deepest gratitude to Mr. Ian Winnington, who helped me not only academically,
but emotionally. His patience and support helped me overcome frustration moments.
My great appreciation to professor Jesús Ramón Jiménez Octavio, who always
solved all my doubts kindly, quickly and comprehensively.
Most importantly, none of this would have been possible without the love and support
of my family. My father, my mother, my sister and my grandparents, who have been
a constant source of love, concern, support and strength all these years.
I am also grateful to Airbus Composite centre and Glyndwr University, for giving me
all the facilities and sources of knowledge. And, of course, I am very grateful to ICAI
and all its professors for giving me the knowledge and the experience about
engineering, completely necessary to face this project properly.
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TABLE OF CONTENTS
ACKNOWLEDGEMENTS........................................................................................ 1
TABLE OF CONTENTS .......................................................................................... 2
1. INTRODUCTION .............................................................................................. 5
1.1 Aims of the project...................................................................................... 6
2. REVIEW OF LITERATURE .............................................................................. 7
2.1 Classifying CFR-PEEK and comparison between Thermoplastic Matrix
Composites (TPMC) and Thermosets Matrix Composites (TSMC). .................... 7
2.1.1 The Reinforcement .................................................................................. 7
2.1.2 The matrix ............................................................................................... 8
2.2 The Matrix : common properties and PEEK structure .............................. 11
2.3 Crystallinity of the matrix .......................................................................... 12
2.4 Microwaving composite materials ............................................................ 13
2.5 Keys of manufacturing Thermoplastics composites like CFR-PEEK ........ 15
2.6 Conventional manufacturing methods for CFR-PEEK .............................. 17
3. EXPERIMENTAL PROCEDURES ................................................................. 18
3.1 Project timeline and analysis of task ............................................................ 18
3.1.1 Brief Analysis of timeline tasks .............................................................. 18
3.1.2. Project timeline ..................................................................................... 20
3.2 Introduction to laboratory ............................................................................. 21
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3.3 Making the samples ..................................................................................... 22
3.4 Preparing the samples for microwaving. .................................................. 24
3.5 Materials used ............................................................................................. 25
3.5.1 Materials to prepare the sample ............................................................ 25
3.5.2 Material of the base ............................................................................... 26
3.6. Microwaves used ........................................................................................ 29
3.6.1 General information ............................................................................... 29
3.6.1 Single frequency microwave ................................................................. 32
3.6.2 Variable frequency microwave (VFM) ................................................... 33
4. RESULTS AND DISCUSSION ....................................................................... 38
4.1 First method, big cavity, manually controlled VFM ....................................... 39
4.1.1 Procedure and results ........................................................................... 39
4.1.2 Discussion ............................................................................................. 42
4.2 Second method, single frequency ................................................................ 43
4.2.1 Procedure and results ........................................................................... 43
4.2.2 Discussion ............................................................................................. 44
4.3 Third method, small cavity, manually controlled VFM .................................. 45
4.3.1 First test, procedure and results ............................................................ 45
4.3.2 Second test, procedure and results ....................................................... 47
4.3.3 Discussion ............................................................................................. 48
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4.4 Fourth method, computer controlled VFM. ................................................... 50
4.4.1 First attempt, results and discussion ..................................................... 51
4.4.2 Second attempt, results and discussion ................................................ 53
4.4.3 Third attempt, results and discussion. ................................................... 55
4.4.4 Fourth attempt, results and discussion .................................................. 58
4.4.5 Fifth test, results and discussion ........................................................... 60
4.4.6 Sixth attempt, results and discussion. ................................................... 62
5. CONCLUSIONS ............................................................................................. 69
6. RECOMMENDATIONS .................................................................................. 72
References ............................................................................................................ 73
Figures ............................................................................................................... 73
Text references .................................................................................................. 74
Appendixes............................................................................................................ 76
LUXTRON 712 FLUOROPTIC THERMOMETER DATASHEET .................... 76
HP872ET AMPLIFIER DATASHEET (used in single frequencymicrowave) .. 76
T82-200 WIDEBAND AMPLIFIER DATASHEET ........................................... 80
VICTREX PEEK 450G DATASHEET ............................................................. 76
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1. INTRODUCTION
In this project, microwave processing of Carbon-Fibre/Polyether-ether-ketone (CFR-
PEEK) has been studied, design and evaluated. Lots of tests have been performed,
trying to adapt microwave processing to this high-performance thermoplastic matrix
composite.
A composite material is a combination of two or more materials in order to improve
the characteristics and properties of those materials used alone. Nowadays
composites are being used more and more, and there is a huge variety of different
composites in the market for different applications and with very different properties.
CFR/PEEK is a high performance thermoplastic suitable for multiple applications.
The increasing in the use of composites is due to their big advantages compared to
others materials, and the variety of factors that can be varied in order to adapt
perfectly a material to a purpose or an application. High temperature Thermoplastic
composites like PEEK have characteristics that give them clear advantages in
structural and high-demanding applications compared to concretes and steel,
aluminium and titanium alloys. The most important is their high strength-density and
stiffness-density ratio, but also their corrosion and chemical resistance. Also they
have much higher toughness and temperature resistance than classics CFR-Epoxy
composites.
Microwave heating is a very familiar process for lots of people in the world due to
many people having a single frequency microwave at home to heat up the food.
Apart from food, microwave technologies are also used to process materials,
including composite materials. Microwave processes have several advantages in
terms of efficiency and high production rates.
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1.1 Aims of the project
CFR-PEEK is a high-performance advanced composite with excellent properties,
suitable for lots of high demanding applications. But processing this material is its
main limitation. Methods available and used to process CFR-PEEK like hydroforming
and matched die forming use infrared heating, and it cannot heat the material
uniformly, only superficially. These methods allow high production rates, but they
need expensive specific equipment, and since the material is solid and not very
flexible at ambient temperatures, the geometries capable of being created with these
methods are very simple.
The main aim of the project is to develop a microwave processing for CFR-PEEK,
studying and investigating an effective way to do it. A way that can be suitable for
companies and applicable to real industry processes. Microwave processing of
composite materials has many advantages in some aspects, so the aim of the project
is to understand how to apply microwave processing to CFR-PEEK composites and
profit from its advantages.
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2. REVIEW OF LITERATURE
2.1 Classifying CFR-PEEK and comparison between Thermoplastic
Matrix Composites (TPMC) and Thermosets Matrix Composites (TSMC).
2.1.1 The Reinforcement
There are lots of types of composites, the classification of the composite material
depends on the matrix utilized, the type of fibre used for the reinforcement, and also
the length, thickness, quantity, position and orientation of the fibres.
There are several types of fibres used to make composite materials.
Figure 2.1.1.1 Different types of fibres used in composite materials (Us department of transportation,
2012)
Also there are different types of fibre distribution typically used as the figure below
shows.
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Figure 2.1.1.2. Possible configurations of continuous and discontinuous carbon fibres. (Campbell F.
C, 2010)
The fibre used for the composite material that has been studied (CFR-PEEK)
is continuous unidirectional carbon fibre reinforcement.
2.1.2 The matrix
Relative to the matrix, this is a short scheme to have a quick idea of different types
of matrix used in composites materials and to classify PEEK matrix.
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Figure 2.1.2.1 Types of matrix used in composites materials
PEEK is classified as a Thermoplastic material, thermoplastics are high-viscosity
resins and to process those resins they have to be heated above their melting
temperature. Thermoplastics are very different to thermosets like Polyester resin or
epoxy (The most common resin used with Carbon Fibre), thermosets are low-
viscosity resins that crystallise with heat during processing. (Campbell, F.C. 2010)
Thermoplastics Matrix Composites (TPMC) can be reprocessed by reheating them
to their melting point and thermosets matrix composites (TSMC) cannot do it. So
(TPMC) can be recycled relatively easy and can have more than one life.
Furthermore, there are more differences between TPMC and TPMS. Their general
mechanical properties are quite similar, but TPMCs have some advantages
compared to TSMCs especially in toughness and environmental resistance (high
temperature, moisture, aggressive fluids, non-flammability and infinite shelf time).
Type of matrix
Polymer Matrix Composites
(PMC)
Thermoset
Thermoplastic
RubberMetal Matrix Composites
Ceramic Matrix Composites
(CMC)
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In this chart the approximate difference in specific toughness between TPMC and
other materials can be appreciated.
Figure 2.1.2.2 Chart comparing specific toughness (energy absorption) between different composites
and common structural materials. (US Department of transportation, 2012)
The manufacturing process for TPMCs is geometrically more limited than TSMCs
because they are solid at ambient conditions, and also because of the lack of
flexibility and stickiness in pre-pregs TPMCs. The layers must be heated up and
pressure has to be applied in order to consolidate the laminates.
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2.2 The Matrix : common properties and PEEK structure
Thermoplastic matrix composites are commonly used in high-performance
applications. All thermoplastic materials have aromatic rings in their molecules,
these rings give them relatively high glass transition temperatures and an excellent
dimensional stability at elevated temperatures.
“The ‘Victrex’ range of polymers from ICI provides a series of high performance
engineering materials whose origins, history and applications have been described
by Rose and Belbin and Staniland. Polyethersulphone (PES) and polyetherether-
ketone (PEEK) arc the best known representatives of this family, whose members
are based on separating rigid aromatic units:” (Cogswell, 1992)
Polyether ether ketone is a linear aromatic thermoplastic and its molecules are made
by this following unit repeated periodically
Figure 2.2.1 Chemical formula of Polyetherether ketone (PEEK) (Mallick P.K, 2007)
Thanks to its aromatic rings, continuous carbon fibre PEEK composites are known
as Aromatic Polymer Composites (APC).
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“The outstanding property of PEEK is its high fracture toughness, which is 50–100
times higher than that of epoxies. Another important advantage of PEEK is its low
water absorption, which is less than 0.5% at 238C compared to 4%–5% for
conventional aerospace epoxies. As it is semicrystalline, it does not dissolve in
common solvents.” (Mallick P.K.,2007)
2.3 Crystallinity of the matrix
Crystallinity of thermoplastic materials is a determinant factor, and if affects its
performance and behavior
PEEK’s glass transition temperature is about 144 degrees, but it changes slightly
when combined with carbon fibres. PEEK melting point usually is around 340ºC.
Conventional ways of manufacturing CF-PEEK need temperatures around 380-390
ºC. The maximum temperature for continuous use is 250ºC.
PEEK is a semi crystalline polymer and its maximum achievable crystallinity is about
48% but at normal cooling rates it usually is around 30-35%. Crystallinity can be
modified by changing the time PEEK spends in cooling from melting. The slower the
cooling is, the more crystallinity the material will have, as it has been explained below
in chapter 2.5. Also the fibres in the materials tends to increase crystallinity, the fibres
improve by acting as nucleation sites. For amorphous PEEK, the material must be
quenched. (P.K.Mallick, 2007)
Crystallinity strongly influences the material’s mechanical properties, an increase in
crystallinity is also an increase in modulus and yield strength of PEEK but is a
decrease in the strain to failure.
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Figure 2.3.1 Tensile stress–strain diagram of PEEK at different crystallinities.(Adapted from Seferis, J.C., Polym. Compos., 71, 58, 1986.)
2.4 Microwaving composite materials
Microwaves are part of the electromagnetic spectrum, microwaves frequency range
goes from 0.3 GHz to 300 GHz. The range of frequencies offered by the laboratory
equipment is from 2GHz to 8 GHz. Domestic microwaves are around 2.45 GHz. (H.S.
Ku, F. Siul, 2003)
“The material properties of greatest importance in microwave processing of a
dielectric are the complex relative permittivity ε = ε’−ε’’ and the loss tangent, tan δ =
ε’’/ε’. The real part of the permittivity, ε’, sometimes called the dielectric constant,
mostly determines how much of the incident energy is reflected at the air-sample
interface, and how much enters the sample. The most important property in
microwave processing is the loss tangent, tan δ or dielectric loss, which predicts the
ability of the material to convert the incoming energy into heat” (Metaxas A C, 1983)
(H.S. Ku, 2004)
To summarize that and make it easy to understand, the larger the value of the real
part of the complex permittivity, the more the incident energy will be reflected by a
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dielectric but the energy that enters the material will penetrate further than in a
dielectric with the same ε’’ but lower ε’.( H. S. KU, 2006)
In a Variable frequency microwave process the power absorbed by the sample and
the power reflected are a function of temperature and they are is different in each
frequency. In terms of experimental procedures, it is more helpful and practical to
use the Standing Wave Ratio (SWR), because it is what it can be measured
practically in the laboratory, even when it is not linear, but logarithmic. That
coefficient predicts the power reflected by the sample. It is commonly used in
telecommunications. (Walraven, 2006)
“The heating effect in most polymeric matrix composites is the result of dipolar
rotation and ionic conduction because these composites contain none or very small
amount of magnetic materials. Molecules that are non-polar but are asymmetrically
charged may behave as dipoles in an electric field; however, their responses to
microwaves are usually about an order of magnitude less than that of water. The
other heating mechanism is ionic conduction. The electrical field causes dissolved
ions of positive and negative charges to migrate towards oppositely charged regions.
This results in multiple billiard ball-like collisions and disruption of hydrogen bonds
in water, both of which result in the generation of heat” (Venkatesh, M. S., 2004)
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2.5 Keys of manufacturing Thermoplastics composites like CFR-
PEEK
Some processes of manufacturing thermoplastics are very similar to thermoset
manufacturing, but others are very different, this is due to some critical differences
between thermoplastics and thermosets. Thermosets pre-pregs are not sticky and
they are not as flexible as thermosets, and that makes much more difficult the use
of contoured mould surfaces.
“To overcome the problem associated with the lack of stickiness, thermoplastic pre-
preg layups are spot-welded together along the outside edges. One method of spot
welding is to use a hot soldering iron and light pressure, which causes the matrix to
melt and fuse at the edges” (P.K.Mallick , pp 441, 2007)
That is how the samples have been made, following this method to weld the layers
together temporally before microwaving.
Another critical difference is that processing thermoplastics composite materials
requires much higher temperature than processing thermosets. So the materials
used in bag-molding processes has to admit high temperatures, in this project, the
materials used also had to be microwave friendly, which makes it even harder to find
the rights materials.
But the main essential difference is that no chemical reaction occurs during the
processing of thermoplastic matrix composites but physical reactions. The individual
layers have to be consolidated to form the desired laminate, and to achieve that,
both high temperature and pressure has to be applied. Depending on the laminate
thickness and geometry the consolidation usually takes minutes or even seconds.
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After the laminate consolidation, the laminate has to be controlled cooled to avoid
residual stresses and warpages. Also a controlled cooling rate is used to control the
crystallinity of the laminate. And here is one of the most challenging points of the
project. Because as it has been explained before, controlling crystallinity,
mechanical properties can be controlled (Specially the fracture toughness, Young’s
modulus and elastic limit). As in the majority of semi crystalline polymers, the slower
the cooling-rate is, the more crystallized the laminate will be, and the faster the
cooling-rate the more amorphous the laminate will be. Studies have been made to
demonstrate this fact and it can be known approximately the relation between the
cooling-rate and the crystallinity percentage. Of course, it is only approximated
because it depends on many factors.
Figure 2.5.1 Crystallinity in PEEK thermoplastic matrix composite as a function of cooling rate. (Mallick, P.K., 2007)
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2.6 Conventional manufacturing methods for CFR-PEEK
CFR-PEEK components currently produced have a relatively simple geometry due
to the lack of flexibility of the fibres, but have a potential rapid production because of
the short processing times required by the material.
Most used and efficient methods to process PEEK are hydroforming, die forming
and thermoforming. These are processes for transforming spot welded laminates of
PEEK into three-dimensional objects, with a “non-complicated” geometry in a
relatively high production rates.
Figure 2.6.1. Forming methods for thermoplastic matrix composites: (a) matched die forming, (b) hydroforming, and (c) thermoforming (Okine.R.K, 1989)
Independently of the process followed the material need to be heated up, the use of
material quartz lamps or infrared heaters is the most common way of heating up the
material.
Once the material is at the right temperature, (Normally it is heated up until 380
Celsius degrees approximately, it follows a moulding process, the most common
ones have been named before.
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3. EXPERIMENTAL PROCEDURES
The general experimental procedures are going to be described in this chapter, but
since the process and procedures are a big part of the project, specific procedures
for each attempt and test will be described in more detail in Chapter 4 – Results and
discussion.
3.1 Project timeline and analysis of task
3.1.1 Brief Analysis of timeline tasks
Research: A lot of research is necessary to get the knowledge about
microwaves, composites in general and Carbon fibre-PEEK processing.
Without understanding the process and what this project is looking for, it is
impossible to advance.
Make the samples: To microwave the material, samples have to be made, the
samples consist of 8 layers of CFR-PEEK, welded together with welding spots
made with a soldering iron.
Prepare and microwave the samples: Before microwaving the samples, they
have to be prepared, using release film or any material suitable, and a
microwave friendly base. After that to microwave them there are lots of
parameters that can variate like the cavity used, the power, the
frequency/frequencies, time spent, ramp-rate…
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Test the samples: Testing the samples after microwave processing (with
DSC, DMA or bending test) had no point in this project, a few tests were made
but omitted because its incoherency and uselessness.
Analysis and comparison of results: The results of the tests of and the data
taken from the microwave program give us information about what we did well
or bad and why.
Investigating new techniques: After getting the information from analysis and
comparison of results, this has to be applied to next attempts, correcting
errors or improving the process.
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3.1.2. Project timeline
2015 2016
October November December January February March April
Researching about composites in general
Researching about microwave processing
Researching about CFR/PEEK
Learning how to use the Laboratory equipment
Preparing CFR/PEEK samples
Microwaving CFR/PEEK samples
Testing CFR/PEEK samples
Analysis and comparison of results
Investigating new techniques to use in microwaving
First Presentation
Interim report
Final Report
Final Presentation
Work at home
Work at Airbus Composite Centre
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3.2 Introduction to laboratory
In a laboratory there is equipment that is not user friendly and not intuitive at all. Most
of this equipment is very specific, accurate and expensive. So that, before start using
this equipment it is necessary to learn how to use it. The variable frequency
microwave, the single frequency microwave and the software related to that use, the
DSC machine, the oven, all the films, tapes and tools used to prepare the sample,
etcetera, needs time to learn how to use them properly. The first weeks spent in the
laboratory were dedicated to that purpose.
This project has an additional difficulty, there are no experimental procedures
established in terms of microwaving CRF-PEEK or other high-temperature
thermoplastic to process it, because there is no public information about someone
making something similar to that before. And the procedures used microwaving
thermosets did not worked properly with this material, getting poor results. That also
gives freedom to use the methods and procedures wanted, always based on the
research made before.
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3.3 Making the samples
Since CFR-PEEK do not degrade at ambient temperature and one sample can be
used for more than one test if the temperature reached is not very high, seven
samples of CFR-PEEK have been made (Since no high pressure is applied, high
temperatures can create tensions and cause the bending of the layers).
Each sample is made of eight layers of unidirectional CFR-PEEK pre-preg. The
direction of the layers is 90º, 0º, 45º, -45º, -45º, 45º, 0º, 90º. This is a common
orientation of the layers, this distribution is symmetric and has a similar behaviour
with efforts in all directions. That is why it is frequently used by composites
manufacturers.
Picture 3.4.1 Eight CFR-PEEK layers cut specifically to make one sample. It is possible to appreciate
the orientation of the unidirectional fibres
The CFR-PEEK layers are completely solid before processing, and it is not possible
to stick them together without applying heat. To overcome this problem, as has been
explained in section 2.5, the spot welding method have been used. Using a soldering
iron and applying light pressure, which causes the matrix to melt and fuse at the
edges (P.K.Mallick , pp 441, 2007)
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These layers are welded between them by six-eight welding points. This process is
very time consuming because the layers have to be welded one by one, point by
point. Since the CFR-PEEK melting temperature is about 380ºC each point needs
time. This process has been repeated for the seven samples.
Picture 3.4.2. Picture of the process used to weld the layers. The soldering iron heat melt the matrix
to make the welding points.
The dimensions of the samples are:
Figure 3.4.2 Dimensions of the samples made.
This measures has been designed to fit perfectly and loosely in all the microwave
cavities used. The samples could be smaller, but microwaving a bigger sample you
can determine how different parts reacted and also how uniform is the field inside
the cavity. The weight of the samples is between 22’5 and 23’5 grams.
7 mm
18 mm
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3.4 Preparing the samples for microwaving.
To make the tests, it is not sufficient to introduce the CFR-PEEK sample in the cavity
and apply power. It is necessary to prepare the sample beforehand. In a real
composite production process a lot of materials have to be used. They perform
different functions: The easy unmoulding of the sample, they can give different
surface texture to the material, the absorption of the excess of matrix during the
process, vacuum bags, etcetera. In this tests, to simulate this process, also different
materials and procedures have been used.
The materials used were mainly films and tapes. Since these tests have to be
monitored and analysed, it is necessary to record the temperature with very
expensive and accurate thermometers. Films are necessary to fix the thermometer
to the sample and prevent any possible damage caused by melted material. Also
they can apply some pressure to the sample, necessary to consolidate the laminate.
Figure 3.5.1 Basic scheme showing how the sample is prepared for the tests.
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3.5 Materials used
3.5.1 Materials to prepare the sample
For each test, the materials used have been changed, trying to test the behaviour of
the most common materials used to process composite materials.
One of the limitations of the CFR-PEEK microwave processing is, in fact, the
materials used to prepare the sample. Since most of the materials used to prepare
the samples are polymers, and CFR-PEEK’s processing temperature is higher than
the melting point of most polymers (Research made with Matweb.com), it is not easy
to find a material suitable for CFR-PEEK processing. Another added difficulty is that
the material has to be “microwave friendly”, it cannot react (absorb or reflect) a high
amount of radiation, that made the search even harder.
Since the temperature were not high enough in most cases, and since it was
necessary to record the temperature and try to apply some pressure to the sample,
different materials have been used. Common materials are not suitable for this
application as has been proven, all polymers used have been damaged.
Picture 3.5.1.1 and Picture 3.5.1.2 Materials damaged and melted because of high temperatures.
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3.5.2 Material of the base
The material used in the laboratory (and commonly in industry) to be the base of the
sample inside the cavities is Polytetrafluoroethylene, commonly known as Teflon.
This material is microwave friendly and it ignores microwaves almost completely,
which is ideal for most microwaves processing. But its melting point is 327ºC and it
can be even toxic at this temperature (microwaves101, 2016). That is not a problem for
nearly all composites materials, but it is for CFR-PEEK which required temperature
to be processed is about 370-380ºC. So Teflon is not suitable for this process.
Figure 3.5.2.1 CFR-Epoxy inside a microwave cavity using two Teflon plates as a base.
To find a good material to be the base, all unreinforced Plastics/Polymers has to be
dismissed because PEEK has one of the highest melting points (which is one of its
most important and useful characteristics), higher than anyone when combined with
carbon fiber, which is the case. (Research made using www.matweb.com, a material data-
base).
http://www.matweb.com/
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Dismissing polymers, Ceramics are a good option. Borosilicate glass, commonly
known as Pyrex (Trademarked as PYREX) is a good option to be used with
microwaves. Due to its low-thermal-expansion is suitable for high temperatures and
also microwave friendly. A Pyrex cookware was used in this project for the first test,
but it was too big to use it in smaller cavities. Pyrex is sold template, so it cannot be
cut without breaking it with the usual glass factory equipment. So it was not useful in
this project.
Figure 3.5.2.2 CFR-PEEK sample prepared to use microwaves using a Pyrex cookware as base
Finally the material used as a base was a Glass-Ceramic material (Trademarked
and commonly known as Neoceram). It is used for fire applications, it has low-
thermal-expansion and it normal service temperature is higher than 700ºC. (TGP,
2016). Two plates of 5mm thickness of Neoceram were used to make the test. The
material was purchased and cut on request in a glass factory to have similar
dimensions of the samples made before.
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Figure 3.5.2.3 Two Neoceram™ plates (centre and right) and a CFR-PEEK sample (left), the plate in
the centre has some melted polymers fastened after a few microwave tests.
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3.6. Microwaves used
3.6.1 General information
The Microwave machines used are not made of a single device. They are formed of
several parts.
Figure 3.6.1 Elements involve in the microwave used
The wave generator: The wave generator create the microwaves, set the
frequencies and can be controlled manually or by the computer. It is also used to
see the SWR graph and look for the lowest reflection frequencies. Data sheet
attached in appendixes
Cavity Amplifier
Computer
Wave generator
Thermometer
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The amplifier: This device amplify the microwaves received from the wave generator,
using the power from the electrical current. It establishes the power limitation of the
facilities. Data sheet attached in appendixes.(
The thermometer: The thermometer measures the temperature of the sample and it
is connected to the computer, which records the data. The thermometer used a
Fluoroptic thermometer (Luxtron 712)
The Cavity: The cavity is the space where the microwave radiation is applied, the
sample is placed inside the cavity to perform the tests. They are made of metal, and
their walls are thick to avoid a radiation leakage. They have a small hole to introduce
the thermometer. The cavities used are two, the project will refer to them as the
“small cylindrical cavity”, and the “big square cavity”.
The small cylindrical cavity has a length of 299 mm and a diameter of 93mm, was
the most used cavity because easy to open (with screws) and can be handled by
only one person.
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Picture 3.6.1.2 and Picture 3.6.1.3 – Pictures of the small cylindrical cavity exterior (left) and interior
with a CFR-Epoxy sample (right).
The big square cavity measurements are 170x340x470 mm. It is a big heavy cavity,
more difficult to handle. It has wheels for easier transport and it needs a hydraulic
crane to be opened.
Picture 3.6.1.4 and Picture 3.6.1.5 Interior of the big square cavity and picture of the process
necessary to open the cavity, using a hydraulic crane.
The power applied during the microwave process is controlled (either manual or
programmed) by changing the decibels of the Amplifier. Where -18 decibels (dB) is
the minimum power (nearly 0 W) and 0 dB is the maximum power of the amplifier
applied. The maximum power of the amplifier is 200 Watts in theory, but it was
proved that at 0 dB, the amplifier was giving a power higher than 300 W. The power
in watts applied is showed in the amplifier screen, but it changes in function of the
frequency, time and temperature, even when the value in decibels continues being
the same. That makes very difficult and inaccurate to record the data in watts, and it
was only recorded in some tests.
Must take into account that decibels are a logarithmic scale, they are used to state
a value as a relative unit, a logarithmic ratio between a reference (The maximum
power) and the power applied.
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3.6.1 Single frequency microwave
Single frequency microwaves, as the name says, use a single frequency to heat the
sample, by definition, this types of microwave machines are less complete and
produce more irregular heating, but the equipment needed is usually cheaper and
more than enough to process some composite materials.
The single frequency machine used in the laboratory is controlled by a computer
program, with the following inputs. The frequency of this
Sample ID
Initial soak T °C
Soak time min
Ramp rate °C/min
Final dwell T °C
Dwell time min
Table 3.6.1.1 Single frequency machine program inputs
Sample ID: Name of the file.
Initial soak temperature: Temperature of the theoretical model at the
beginning of the test (usually room temperature).
Soak time: Time before the program start applying power.
Ramp rate: Also called heating rate, is the temperature increase per minute.
Final dwell temperature: Final maximum temperature of the sample
Dwell time: Time the sample will be maintained at the maximum temperature.
The program used follows a model. And when you input a ramp rate, the program
will adjust the power applied to follow a constant ramp rate in the theoretical model
until it reach the “Final dwell T”, then it will adjust the power to maintain this
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temperature. But the theoretical model’s temperature is not the real temperature of
the sample, if the model and the sample are made of different materials, the results
could be very inaccurate.
3.6.2 Variable frequency microwave (VFM)
The VFM machine used works following the schema below, the range off frequencies
available for the amplifier goes from 2GHz to 8 GHz. The experimental procedure
using the VFM varies in function of the method used, it can be manually controlled
or computerized, using a computer program.
3.6.2.1 Manually controlled:
This method allows to control the Microwave without a computer program, giving the
possibility of changing some parameters like power applied while the process is
running. This real-time possibility allow us to change the power, adapting it to the
needs of the material, and trying to make the heating process more efficient, trying
to make the classic “logarithmic” heating graph looks closer to a “linear” heating
graph.
The problem of this method is that the data has to be recorded manually, and the
results are less accurate. But the biggest limitation of procedure followed is that the
frequencies chosen have to be in a range, the operator is able to change the size of
the range and limit frequencies, but multiples singles frequencies cannot be chosen.
The program will use all the frequencies in a range. For example, if the range is from
6 GHz to 7GHz. The program will use all the frequencies included in that range.
So, the frequency range chosen should not include high-reflection frequencies,
otherwise, if the power reflected is higher than 30% of the maximum power of the
amplifier (30% of 200W , about 60 W) the process will stop automatically for security
reasons.
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34
The wave generator can show the Standing Wave Ratio (SWR) of the different
frequencies when connected directly to the cavity, instead of the amplifier. Having a
look on the SWR, the low reflection ranges can be identified and chosen.
Figure 3.6.2.1 Picture of the wave generator display, showing the SWR graph for frequencies
between 2GHz and 8 GHz.
3.6.2.2 Computer program controlled:
With this method is the computer program who control the wave generator. That
allows to use multiple frequencies (usually between 2 and 6) and fix a different power
for each frequencies. As described in the manually controlled microwave method, to
find the best frequencies (the ones with less reflection), is necessary to connect the
wave generator directly to the cavity and look the SWR graph. The frequencies will
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35
the lowest SWR will be the ones with lowest reflection. The lowest point will be
identified and saved because they will be part of the input in the program.
Figure 3.6.2.2.1 and Figure 3.6.2.3 Wave generator display showing the SWR of a CFR-PEEK
sample between 5.5 GHz and 8 GHz. The figures shows the marker placed in two low peeks.
The inputs of the program are:
File name
Cure temperature °C
Low temperature limit °C
Number of frequencies X
Frequency 1 GHz
Power of frequency 1 dB
Frequency 2 GHz
Power of frequency 2 dB
Frequency 3 GHz
….
….
Frequency X(GHz) GHz
Power of frequency X (dB) dB
Lowest power applied dB
Table 3.6.2.2.2. Inputs of the VFM program.
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36
The frequencies entered in the program will be the ones with low reflection found
before. The “cure temperature limit” and the “low temperature limit” are not relevant
because the temperature required was not reached, so the values was fixed to
380°C for all the tests.
The VFM program will swap between the frequencies entered regularly, applying the
power entered to each frequency to heat the sample correctly.
The limitation of that method with the equipment used is that to change the power
the program should be stopped and ran again, entering again all the values. In the
meantime could be a heat loss and the data is saved in another file, recording the
time from 0 again. So the graphs showed are the addition of multiple graphs, multiple
tests. This limitation of the method can alter the results, and make more difficult to
extract conclusions.
Between different tests made in the same graph it is possible to appreciate a heat
loss, a heat gain, or even discontinuities. After a few test and some analysis it was
possible to realize that when the program is stopped, the microwave machine uses
the last frequency and power used by the program. So the microwave became single
frequency for a while. If this frequency can apply more power than the average of
the frequencies used before, there will be a heat gain (It depends on the power set
and the power reflected for this frequency). And, if the frequency can apply less
power than the average of the frequencies before, there will be a heat loss in the
time spent introducing the new values in the program.
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37
Figure 3.7.2.2.3 Imagen taken from test 4.4.6 It shows clearly a heat loss and a heat gain, depending
the las frequency used before stopping the program. Example.
Since the frequency can swap more than 10 times in a second is nearly impossible
to choose the frequency wanted. It is randomly chosen. But it helps to identify the
better frequencies.
The program also spend some time recording data before starting to apply power.
So the data recorded before starting gives information about the behavior of the
sample with the last single frequency used. In the example Marker 1 shows a heat
loss while Marker 2 shows a heat gain, depending on the frequency the program
was stopped.
Computer program controlled method was not available in the laboratory until
February when the thermometer was fixed.
1
2
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38
4. RESULTS AND DISCUSSION
Following the procedures described before, lots of tests microwaving the samples
have been made. Lots of different methods have been tried to microwave CFR-
PEEK, changing lots of parameters to achieve a good microwave processing: the
materials used to prepare the sample, the material of the base, the way of the
pressure is applied, the microwave cavity, the microwave machine, the frequencies
used, the power used, the time, the ramp-rate, the program, etcetera.
This chapter will describe all the procedures followed in each test and the results
obtained, organizing it by the method used. Following an evolution and learning from
one attempt to the next one, analysing data obtained and applying this knowledge
and the research made to evolve the method, improve the results and learn from the
material.
All data presented here has been recorded manually or computerized depending on
the machine or the method used. The results and methods are chronologically
ordered.
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39
4.1 First method, big cavity, manually controlled VFM
4.1.1 Procedure and results
Microwaving the Carbon fibre-PEEK is the difficult part. The first test of microwaving
the CFR-PEEK was very useful to realize how not to do it, and to detect lots of errors.
The results were not far from what it was expected.
It is known that processing of CFR-PEEK requires both pressure and high
temperature to consolidate the laminate, but applying both together at the same time
in a laboratory is not easy at all. The first idea was to apply pressure using shrink
tape, which as the name indicate, shrinks with temperature. The way of how to apply
pressure in the sample has been one of the issues of this project.
To prepare the sample a Pyrex cookware has been used as a base to put the sample
on, some release film for an easy separation of the CFR-PEEK after microwaving
and shrink tape to make the pressure necessary. The cavity used in the test was the
big square cavity showed before. The microwave used was the VFM, it was ran
manually because the thermometer was broken and it was necessary to run the
program, the procedure followed was the one explained before.
It is known that some materials react better to microwaves when they reach a high
temperature. It was believed that CFR-PEEK had a bad reaction to microwaves at
low temperature, so for this test the sample was pre-heated in an oven.
The sample was heated up to 250`C, this temperature is much lower than the
temperature needed to consolidate the laminate and far from its melting temperature,
so it didn’t affect the material at all. There is a high loss of heat in the time spent
taking the sample out of the oven and putting it in the microwave (The cavity used is
so heavy it needs a hydraulic crane to open it). The temperature of the sample when
it started receiving microwaves was 118’4 `C.
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40
Figure 4.1.1 Graph showing the temperature of the sample and the approximate power used in the
microwave
The VFM was set to use all frequencies in the range of 7 GHz to 7.5 GHz. This
method is not efficient, because the power reflection of some frequencies in this
range is high.
The maximum temperature reached by the sample in the microwave was 244
degrees. The power reflected was too high and the VFM stopped (it stops when the
power reflected is higher than 30% of the maximum power). The power used was
not enough to heat up the sample to the required temperature. The cavity was too
big for this purpose.
To make sure, another attempt was made with same conditions, the results obtained
were similar.
2040; 2332040; 244
0
50
100
150
200
250
300
0 500 1000 1500 2000 2500
Tem
per
atu
re(º
C )
, Po
wer
(W
)
Tíme(s)
First Test Big cavity, attempt 1
Power
Temperature
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41
Figure 4.1.2 Graph showing the temperature of the sample and the approximate power used in the
microwave.
After the tests the sample was curved and the shrink tape was broken, the shrink
tape used was not adequate for this temperature. It became very brittle and useless.
With the heat, the shrink tape curved the sample.
Figure 4.1.2 .Picture of the sample after the process. The sample is curved and the shrink tape is
broken.
1760; 243
1860; 210
100
120
140
160
180
200
220
240
0 500 1000 1500 2000
Tem
per
atu
re º
C, P
ow
er (
W)
Tíme(s)
First Test Big cavity- attempt 2
Temperature
Power
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42
4.1.2 Discussion
CFR-PEEK microwave processing is very power demanding, in a big cavity
microwaves cannot concentrate properly to heat up a small sample, and also the
heat loss around the metal walls of the cavity is higher. But the Pyrex cookware used
as a base prevented the use of smaller cavities. The Neoceram base used in
following attempts was suitable for smaller cavities.
Shrink tape is not adequate to apply pressure. Even if the shrink tape chosen was
not the proper one or the position around the sample was not suitable, with the shrink
tape the pressure applied cannot be calculated accurately and easily and it cannot
be applied to difficult geometries. It could be a good option of applying pressure for
tubular shapes but not for general processes. Nowadays is not possible to find a
shrink tape which can withstand the temperature required.
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43
4.2 Second method, single frequency
4.2.1 Procedure and results
The single microwave machine used, as it has been explained in 3.7.1 chapter, use
a model to adjust the frequency power to the values entered in the program.
The program input was:
Sample ID Singlefreq-nooven
Initial soak T 27 °C
Soak time 1 min
Ramp rate 5 °C/min
Final dwell T 400 °C
Dwell time 10 min
Table 4.2.1. Values entered in the program.
The sample was prepared using the normal procedure, and the base was the
Neoceram plate described before. Since the small Neoceram plates were
purchased, the small cylindrical cavity was available, so it was used for this test. The
graph below shows the results.
3936,12; 350,01
3936,12; 51,1795
0
50
100
150
200
250
300
350
400
0 1000 2000 3000 4000 5000
Tem
per
atu
re (
ºC)
Time (s)
2nd Test, Single frequency
Theoric temperature
Real temperature
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44
Graph 4.2.1. Graph showing the theoretical model temperature and the sample real temperature as
a function of time
The graph shows a huge difference between the theoretical temperature and the
real temperature of the sample. The test was intentionally stopped after more than
one hour running, because the temperature of the sample was already constant.
The sample reached only 51.2 ºC while the theoretical model was around 350ºC.
Another quick test was made elevating the ramp-rate to 10 ºC/min, and the results
did not improved significantly. Choosing a lower ramp-rate would not has worked
either because the temperature was already stabilized. This test could not be
finished.
4.2.2 Discussion
The model used for the program is not useful at all for CFR-PEEK. The model
imitates the behaviour of a common composite, probably a Carbon-fibre/thermoset-
matrix. But compared with the model, CFR-PEEK is much more power demanding.
All the power applied during this time only heated up the sample to 51ºC, while an
average composite would have reached 350ºC. Usual models used to process
composites do not work properly with CFR-PEEK. Frequency used by this single
frequency microwave is the common 2.45 GHz, which seems to be a high reflection
frequency looking the SWR graph in the following experiments.
The single frequency microwave machine used was not adequate, next single
frequency tests have been made with the VFM machine. (Which is also able to work
with a single frequency) The VFM machine program is much more controllable and
suitable for the material studied.
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45
4.3 Third method, small cavity, manually controlled VFM
After trying the single frequency microwave with bad results, and since the
Neoceram plates were purchased, the manually controlled method could be used
again with the small cylindrical cavity. It was thought that one of the causes of the
first method’s failure was the use of a cavity much bigger than the sample. So the
microwaves could be too dispersed to heat the sample properly. To avoid that, the
small cylinder cavity described before and the procedure explained before about
manually controlled VFM were used.
4.3.1 First test, procedure and results
For this test the frequency range used was from 6 GHz to 7.9 GHz. There were high-
reflection peeks in the range used but the test was performed anyway to analyse the
results. The Temperature and the power (dB) were recorded manually.
To perform this test, and due to the bad reaction to microwaves of the first test made
without pre-heating the sample in the oven, the sample was pre-heated in an oven
to 180 ºC. By the time the microwave process started, the sample was already at 99
ºC. The heat loss was about 81 degrees, lower than the heat loss with the big cavity,
the time spent moving the sample was shorter.
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46
Graph 4.3.1.1 Graph showing the temperature of the sample versus time.
Graph 4.3.1.1 Graph showing the power of the sample versus time. The power was changed
manually.
1320; 127,4
80
90
100
110
120
130
140
0 200 400 600 800 1000 1200 1400
Tem
per
atu
re (
ºC)
Time (s)
Test 4.3.1 Temperature
1320; -4
-12
-10
-8
-6
-4
-2
0
0 200 400 600 800 1000 1200 1400
Po
wer
(d
B)
Time (s)
Test 4.3.1 Power
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47
The power was changed manually trying to make the heating graph “linear”. The
microwave machine stopped the process because of the excess of power reflected.
The temperature reached was only 127.4ºC.
4.3.2 Second test, procedure and results
This test was similar to the first one but the sample was not pre-heated in the oven,
it was wanted to see how the reaction of the sample to microwaves at low
temperatures was. The SWR was checked again to find a shorter range of
frequencies to avoid the high reflection peeks. The range used for this test was from
6.752 GHz to 6.917 GHz.
Graph 4.3.2.1 Temperature of the sample and the approximated power applied
3000; 245
0
50
100
150
200
250
300
0 500 1000 1500 2000 2500 3000 3500
Tem
per
atu
re (
C)
Po
wer
(W
)
Time (s)
Test 4.3.2 Power (W) and Temperature (C)
Temperature (C)
Power (W)
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48
Graph 4.3.2.2 Power applied in decibels.
The temperature reached in this attempt was much higher than the one in the first
attempt. The microwave was working full power, but the test had to be stopped
because there was smoke coming out the cavity. The temperature reached at some
points of the sample was higher than expected. And some of the materials used to
prepare the sample started melting.
4.3.3 Discussion
The power reflected in the first test was so high that the maximum power that could
be applied was -4dB. With this power, the microwave machine is not even close to
being capable of heating the sample to the required temperature. The method
followed to choose the frequencies is not adequate at all and will not be used again
in this project.
In the second attempt the way the sample accepted microwaves at low temperature
was surprisingly good, the pre-heating process is not necessary. So it will not be
necessary to use it in the following attempts
3000; 0
-10
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
0 500 1000 1500 2000 2500 3000 3500P
ow
er (
dB
)
Time (s)
Test 4.3.2 Power (dB)
POWER(dB)
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49
With these two test it is possible to see that a range with low reflection even if it is
small, allows the use of much more power than a big range with high reflection
peeks. More power means higher possible temperature reached. It is very useful
especially for a high power-demanding material like CFR-PEEK.
The temperature reached in this cavity was not far away from the one reached in the
big cavity, it is believed that following this method, the result obtained could have
been the same using the small and the big cavity. But due to it is being easier to deal
with the small cavity, and also because the big cavity needs more than one person
to be used, the small cavity was used for the following attempts anyway.
The materials used to prepare the sample were not able to endure the temperature
reached in the hot points. For next tests the materials used will be different.
