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EFFECT OF THE BAGGING CONFIGURATION ON THE PROPERTIES OF VACUUM-BAGGING-ONLY
(VBO) - OVEN CURED IN COMPLEX-SHAPED COMPOSITES
MUHAMMAD HAFIZ BIN HASSAN
UNIVERSITI SAINS MALAYSIA
2014
EFFECT OF THE BAGGING CONFIGURATION ON THE PROPERTIES OF VACUUM-BAGGING-ONLY (VBO) - OVEN CURED IN COMPLEX-SHAPED
COMPOSITES
by
MUHAMMAD HAFIZ BIN HASSAN
Thesis submitted in fulfilment of the requirements for the degree of
Master of Science
June 2014
i
ACKNOWLEDGEMENTS
Bismillahirrahmanirrahim. In the name of Almighty Allah S.W.T., the Most
Gracious and the Most Merciful, the grateful thank is only belong to Him for bestowing
me this valuable opportunity in attaining this great experience in life and endlessly thank
for His precious blessings. Thank as well to His Messenger, Prophet Muhammad S.A.W.
for His grand inspirations and pure bond of love.
This research project would not have been possible without the support of many
personnel. I would like to express my gratitude to my superb sporting supervisor, Dr. Abdul
Rahim Othman who was abundantly helpful and offered invaluable assistance, support and
guidance. Deepest gratitude is also due to the member of the supervisory committee,
Associate Prof. Dr. Shahrul bin Kamaruddin. Also, I am deeply grateful and sincere thanks to
all of the technicians involve in completing my research including they are from School of
Material and Mineral Resources, Aerospace and needless to mention, School of Mechanical
Engineering.
During this work, special thanks to all my beloved friends, seniors and juniors who
always had been there whenever I needed help. Not forgetting, my warm thanks to few of my
special friends who always supporting me as my back bone, and never gave me untiring help
during my difficult moments. In addition, I would like to express my deepest gratitude to my
sponsorship. As one of the RLKA scholar, appreciation is given to Kementerian Pendidikan
Malaysia (KPM) and Universiti Sains Malaysia as being my financial aid throughout my
studies.
I owe my loving thanks to my beloved parents, Mr. Shuhaini bin Ramli and Madam
Masidah binti Mat Saad. Without their encouragement and understanding, it would have been
impossible for me to finish this work. My special gratitude is due to my fiancé for their loving
support, understanding and endless love, through the duration of my studies. Lastly, I offer
my regards and blessings to all of those who supported me in any respect during the
completion of the project.
Best wishes, MUHAMMAD HAFIZ BIN HASSAN
FEBRUARY 2014
ii
TABLE OF CONTENTS
ACKNOWLEDGEMENTS ............................................................................................. i
LIST OF TABLES .......................................................................................................... vi
LIST OF FIGURES ....................................................................................................... vii
LIST OF ABBREVIATIONS ........................................................................................ xi
CHAPTER 1 INTRODUCTION .................................................................................. 1
1.1 Composite in general ........................................................................................... 1
1.2 Autoclave manufacturing ..................................................................................... 3
1.3 Out of Autoclave manufacturing- Vacuum Bag Only- Oven Cure .................... 5
1.4 Problem statement................................................................................................ 8
1.5 Objectives .......................................................................................................... 11
1.6 Scope of the study ................................................................................................ 9
CHAPTER 2 LITERATURE REVIEW .................................................................... 12
2.1 Fabrication of composite materials .................................................................... 12
2.1.1 Wet Resin ................................................................................................... 15
2.1.2 Infusion/Liquid Moulding .......................................................................... 15
2.1.3 Prepreg (Pre-Impregnated) ......................................................................... 17
2.2 Complex-shaped laminate structure................................................................... 18
2.3 Effect of processing parameters on manufacturing defects ............................... 21
iii
2.4 Correlations between lay-up processing parameters and manufacturing defects
……………………………………………………………………………...22
2.4.1 Effect of stacking sequence ....................................................................... 22
2.4.2 Effect of mould design ............................................................................... 24
2.4.3 Effect number of plies ................................................................................ 27
2.5 Correlations between vacuum bagging processing parameters and manufacturing
defect .................................................................................................................. 28
2.5.1 Effect of ply debulking .............................................................................. 28
2.5.2 Effect of bagging configuration ................................................................. 29
2.5.3 Effect of intensifier/ caul-sheet application ............................................... 33
2.5.4 Effect of edge breather application ............................................................ 35
2.6 Correlation between curing processing parameters and manufacturing defects
……………………………………………………………………….……..37
2.6.1 Effect of staggering stage for curing process ... ………………………......37
2.6.2 Effect of two stage curing .......................................................................... 38
2.7 Physical properties evaluation ........................................................................... 42
2.8 Void content analysis ......................................................................................... 42
2.9 Thickness measurement in complex-shaped laminate ....................................... 44
2.9.1 Point to point measurement ....................................................................... 44
2.9.2 Degree of uniformity (DOU) ..................................................................... 45
iv
2.10 Mechanical properties evaluation ...................................................................... 46
2.11 Literiture findings .............................................................................................. 49
CHAPTER 3 RESEARCH METHODOLOGY ........................................................ 52
3.1 Introduction ........................................................................................................ 52
3.2 Design of experiment (DOE) ............................................................................. 55
3.3 Materials ............................................................................................................ 57
3.3.1 Prepreg material ......................................................................................... 57
3.3.2 Consumable materials ................................................................................ 57
3.4 Laminate preparation ......................................................................................... 58
3.4.1 Lay-up process ........................................................................................... 60
3.4.2 Vacuum bagging process ........................................................................... 61
3.4.3 Oven curing process ................................................................................... 66
3.5 Measurement of Resin mass loss (RML) ........................................................... 67
3.6 L-shaped laminate performance measurement .................................................. 67
3.6.1 Physical properties evaluation ................................................................... 68
3.7 Analysis of variance (ANOVA) ........................................................................ 72
3.8 Mechanical properties evaluation ...................................................................... 73
CHAPTER 4 RESULTS AND DISCUSSION ........................................................... 75
4.1 Introduction ........................................................................................................ 75
4.2 Initial study .... …………………………………………………………………75
v
4.2.1 Effect of processing parameters on the physical properties of L-shaped
laminate ……………………………………………...……………… …………75
4.2.2 Effect of the void content on mechanical properties of L-shaped laminate
……………………………………………………………....…………99
4.3 Finding of the initial study ...... ……………………………………………….101
4.4 Analysis of extended study .............................................................................. 102
4.4.1 Analysis of void content .......................................................................... 102
4.4.2 Analysis of thickness variation ................................................................ 108
CHAPTER 5 CONCLUSIONS AND FUTURE WORK ....................................... 112
5.1 Conclusions ...................................................................................................... 112
5.2 Future work ...................................................................................................... 114
REFERENCES ............................................................................................................. 116
vi
LIST OF TABLES
Table 2. 1: Cronology of vacuum assited technique from 1950 to 1995. .......................... 14
Table 2. 2: Summary of correlation between processing parameters toward manufacturing
defect ................................................................................................................. 41
Table 3. 1: Level of control factors for selected processing parameters 56
Table 3. 2: The design of experiment using 2 full factorials design .................................. 56
Table 3. 3: Level of control factors for selected processing parameters used for extended
study .................................................................................................................. 56
Table 3. 4: The design of experiment using 2 full factorials design for extended study ... 57
Table 3. 5: Detail of consumables materials ...................................................................... 58
Table 3. 6: Details of the bagging configuration for initial study ...................................... 61
Table 3. 7: Details of VBO type for the extended study .................................................... 63
Table 3. 8: Natural rubber compound for rubber intensifier .............................................. 65
Table 4. 1: Percent of the deviation of void content between the flat and the corner reagion
for all bagging configurations 76
Table 4. 2: Summary of the ANOVA for void content ...................................................... 83
Table 4. 3: Percent of the deviation of thickness between the flat and the corner reagion
for all bagging configurations ........................................................................... 92
Table 4. 4: Summary of the ANOVA for degree of uniformity (DOU) ............................ 97
vii
LIST OF FIGURES
Figure 1. 1: Comparison between the Autoclave and the vacuum bagging process ............ 7
Figure 1. 2: Two ply of Engineered Vacuum Channels (EVaCs) in OOA prepreg .............. 7
Figure 2. 1: Fibre wrinkling of L-shaped laminate at the corner region ............................. 20
Figure 2. 2: Longitudinal and shear stresses generated by the compaction of a curvilinear
fibre bed for convex and concave tool configurations. The original shape is
shown by the dotted lines ................................................................................. 23
Figure 2. 3: Example of convex and concave tool to produce L-shaped composite laminate
.......................................................................................................................... 25
Figure 2. 4: Reaction stress at the corner for convex and concave mould configurations .. 26
Figure 2. 5: Macro and micro void formation ..................................................................... 32
Figure 2. 6: Breather application for surface wrinkle at corner reagion ............................. 33
Figure 2. 7: Type of edge breather application in vacuum bagging – oven cure process ... 36
Figure 2. 8: Two stage curing application to control the resin flow out from laminate....39
Figure 3. 1: Methodology of research study ......................... Error! Bookmark not defined.
Figure 3. 2: Detailed dimensions of a complex-shape laminate .......................................... 59
Figure 3. 3: Convex mould and prepreg laminate .............................................................. 60
Figure 3. 4: Schematic of OOA L-shaped laminate composite manufacturing .................. 62
Figure 3. 5: Position of the air breathing for a VBO configuration in the bagging
configuration .................................................................................................... 63
Figure 3. 6: Types of air breathing system for (A) 244 type for initial and extended study
and (B)109type for extended study .................................................................. 64
Figure 3. 7 : The application of aluminium intensifier in bagging configuration ............... 65
viii
Figure 3. 8: Cure cycle of Cycom 7668/7781-1 prepreg laminates in oven ........................ 66
Figure 3. 9: Resin loss flow out from the laminate at the transverse direction after the
curing process .................................................................................................. 67
Figure 3. 10: Typical cut sample for physical and mechanical evaluation ......................... 68
Figure 3. 11: Locations of the L-shaped laminate for thickness measurement ................... 69
Figure 3. 12: Position of coupon size for burn off test for each segment ............................ 70
Figure 3. 13: Thermogravimetric machine and data analysis for 41mg specimen of
fibreglass .......................................................................................................... 70
Figure 3. 14: Flow chart of image processing using image matching ................................. 72
Figure 3. 15: Four-point bending test setup ......................................................................... 74
Figure 4. 1: Void content distribution of the L-shaped laminate for different bagging
configurations 77
Figure 4. 2: Void content value at the corner region of L-shaped laminate for different
bagging configurations ..................................................................................... 79
Figure 4. 3: Illustration of entrapped air bloked at the transverse direction by PTFE film
application. ....................................................................................................... 79
Figure 4. 4: Percentage of improvement for individual and combined processing
parameters to the void content reduction ......................................................... 80
Figure 4. 5: Percentage of influence factor towards processing parameters for the void
content value .................................................................................................... 84
Figure 4. 6: The main effects plot of the processing parameters towards the void content at
the corner region .............................................................................................. 85
ix
Figure 4. 7: General factor interaction plot for combination of two processing parameters
towards void content ........................................................................................ 86
Figure 4. 8: Histogram plot of the Resin mass loss for each panel based on different
bagging configurations ..................................................................................... 88
Figure 4. 9: Graph void content at corner region towards resin mass loss .......................... 89
Figure 4. 10: Picture for SEM and Binary image for all processing parameters ................. 91
Figure 4. 11: Thickness measurement of the L-shaped laminate for different bagging
configurations .................................................................................................. 94
Figure 4. 12: Pressure distribution along L-shapedcomposite laminate during curing
process .............................................................................................................. 94
Figure 4. 13: Bar plot of degree of uniformity towards different bagging configurations .. 96
Figure 4. 14: Percentage of influence factor towards processing parameters for the degree
of uniformity values ......................................................................................... 97
Figure 4. 15: The main effects plot of processing parameters towards degree of uniformity
.......................................................................................................................... 98
Figure 4. 16: General factor interaction plot for combination of two processing parameters
towards Degree of uniformity .......................................................................... 99
Figure 4. 17: Graph of CBS and Maximum interlaminar stresses of the composite
laminated structure to the bagging configuration .......................................... 101
Figure 4. 18: Graph of (a) CBS and (b) Maximum radial stresses of the composite
laminated structure to the void content .......................................................... 101
Figure 4. 19: Void content distribution through the L-shaped laminate for different bagging
configurations ................................................................................................ 103
x
Figure 4. 20: Void content at a corner region for different bagging configurations ......... 104
Figure 4. 21: The main effects plot of processing parameters towards void content ........ 105
Figure 4. 22: General factor interaction plot for combination of two processing parameters
towards void content ...................................................................................... 106
Figure 4. 23: Pictures of SEM and Binary image for all processing parameters in extended
study ............................................................................................................... 107
Figure 4. 24: Thickness measurement of the L-shaped laminate for different bagging
configurations ................................................................................................ 108
Figure 4. 25: Histogram of the Degree of uniformity (DOU) to the bagging configurations
........................................................................................................................ 110
Figure 4. 26: The main effects plot of processing parameters towards Degree of uniformity
........................................................................................................................ 110
Figure 4. 27: General factor interaction plot for combination of two processing parameters
towards Degree of uniformity ........................................................................ 111
xi
LIST OF ABBREVIATIONS
ANOVA : Analysis of variance
ASTM : American Society for Testing and Materials
CBS : Curve beam strength
DOE : Design of experiment
DOU : Degree of uniformity
DT : Destructive test
GRP : Glass Reinforced Plastic
ILSS : Inter-laminar shear strength
ILTS : Inter-laminar tensile strength
MIS : Maximum inter-laminar stress
NDT : Non destructive test
OOA : Out of Autoclave
PTFE : Polytetrafluoroethylene
RML : Resin mass loss
RTM : Resin Transfer Moulding
SEM : Scanning electron microscopy
UTM : Universal testing machine
VBO :
Vacuum Bagging Only
xii
Kesan daripada Konfigurasi Beg ke atas sifat-sifat Vakum Beg Sahaja- Pengawetan
Ketuhar dalam Komposit Berbentuk Kompleks
ABSTRAK
Dengan keperluan dalam industri aeroangkasa yang semakin meningkat untuk
mengeluarkan komponen komposit yang lebih besar dalam masa yang sama, kos dapat
dikurangkan, teknologi pembuatan di luar-penggunaan-autoklaf mungkin memberi
jawapan untuk permasalahan tersebut. Walau bagaimanapun, kerja-kerja pembangunan ke
atas proses tersebut yang menggunakan bahan-bahan pra-pembentukan laminat untuk
mencapai kandungan udara terperangkap yang minimum tanpa dikenakan tekanan luar
yang tinggi dari autoklaf telah terbukti ia adalah cabaran utama. Kajian penyelidikan ini
memberi tumpuan khusus kepada proses vakum beg. Dalam kajian awal, lapan
konfigurasi vakum beg yang terdiri daripada tiga parameter utama pemprosesan, iaitu
penggunaan pelapik filem (PTFE), pengudaraan di pinggir laminat dan penggunaan
pemberat telah dicadangkan untuk meningkatkan kualiti komposit laminat berbentuk L.
Spesimen seterusnya dinilai berdasarkan tahap keseragaman dan tahap kandungan udara
terperangkap. Keputusan kemudiannya dinilai menggunakan kaedah analisis varians
untuk menentukan peratusan pengaruh bagi setiap proses parameter dalam peningkatan
kualiti stuktur laminat. Keputusan eksperimen menunjukkan bahawa penggunaan PTFE
telah menyumbang 37.84 % kepada pembentukan udara terperangkap yang ketara di
bahagian lengkuk laminat. Sebaliknya, penggunaan pengudaraan di pinggir laminat dan
pemberat berjaya mengurangkan tahap kandungan udara terperangkap sebanyak masing-
masing 19.64% dan 29.41%. Untuk respon tahap keseragaman, penggunaan pengudaraan
xiii
di pinggir laminat dan pemberat hampir mencapai nilai sasaran (1) masing-masing dengan
nilai sebanyak 4.4% dan 6.9%. Walau bagaimanapun, penggunaan PTFE telah
menunjukkan hasil yang paling buruk dengan mengurangkan nilai tahap keseragaman dari
nilai sasaran sebanyak 17.8 %. Di samping itu, untuk bahan ini, kandungan udara
terperangkap yang meningkat daripada 3.85 % kepada 6.64%, penurunan sebanyak 60%
untuk nilai kekuatan rusuk lengkuk dan kekuantan maksimum antara laminat
diperhatikan. Kemudian , penyiasatan yang berterusan telah dijalankan untuk kajian yang
selanjutnya ke atas kesan jenis pemberat (aluminium dan getah asli ) dan luas ketumpatan
pengudaraan di pinggir laminat (109 g/m2 dan 244 g/m2 ). Hasil kajian mendapati bahawa,
apabila kekakuan pemberat dan luas ketumpatan pengudaraan di pinggir laminat
meningkat, kandungan udara terperangkap di kawasan lengkuk stuktur laminat juga
meningkat sebanyak masing-masing 13.48 dan 4.36%. Sebaliknya, peningkatan dalam
kekakuan pemberat telah mencapai hampir kepada nilai sasaran ( 1) dengan nilai sisihan
daripada 1.44 %. Peningkatan luas ketumpatan pengudaraan di pinggir laminat
menunjukkan hasil yang paling teruk dengan pengurangan nilai tahap keseragaman oleh
2.92 %.
xiv
Effect of the Bagging Configuration on the Properties of Vacuum Bagging Only
(VBO)-Oven Cured in Complex-Shaped Composites
ABSTRACT
With the growing needs of the aerospace industry to produce larger composite
components at reduced costs, Out-of-Autoclave (OOA) manufacturing technology is
perhaps the answer for the problem. However, the development work of the OOA process
using prepreg materials to achieve minimum void content without high external pressure
from the autoclave proved to be a major challenge. The research study focused
specifically on the vacuum bagging process. In the initial study, eight bagging
configurations which consist of three main processing parameters, including the use of
release film (PTFE) , edge breather and the intensifier were proposed to improve the
quality of the L-shaped laminate composites. The specimens were assessed based on
degree of uniformity (DOU) and void contents levels. The results were subsequently
evaluated using the analysis of variance (ANOVA) to determine the percentage of
influence of the processing parameters to the quality improvement. The experimental
results indicated that the use of PTFE has contributed 37.45% to the significant void
formation at the corner section. In contrast, the edge breather (VBO) and intensifier
successfully reduced the void content level by 19.64% and 29.41%, respectively. For
DOU response, the use of VBO and intensifier almost achieved the target value (1) with
the value of 4.4% and 6.9% respectively. Nevertheless, the PTFE application has shown a
worst result by reducing the DOU away from the target value by 17.8%. In addition , as
the void content increased from 3.85% to 6.64% for this material, a decrease of 60% in
xv
the CBS and MIS value is observed. Subsequently, an extended investigation has been
carried out to extended study the effect of different types of intensifier (aluminium and
natural rubber) and VBO areal density (109 g/m2 and 244 g/m2). It has been observed that,
as the intensifier stiffness and the areal density of VBO type increased, the void content at
the corner region of laminate also increased by 13.48 and 4.36% respectively. On the
other hand, the increase in the intensifier stiffness has achieved close to the target value
(1) with the deviation value of 1.55%. The increase in the areal density of VBO type
provided the worst result with a reduction of the DOU value by 2.37%.
1
CHAPTER 1
INTRODUCTION
1.1 Composite in general
Advanced composite materials have been increasingly used in a wide range of
industrial applications due to the excellent properties such as high specific strength,
specific stiffness and fatigue characteristics. Especially in civil aviation aircraft, the
weight proportion of the composite materials in relation to the whole weight of the
airplane has significantly increased for the past 30 years. According to Soutis (2005a),
the significant reduction in weight without comprising the mechanical strength that can
be achieved by this materials, provides the main driver for the increasingly use of carbon
fibre reinforced plastic in high-performance applications over the conventional metal
alloys. Composite materials were initially limited to secondary structures such as
movable wing components (rudder, flaps, and spoilers), but with the rapid development
in testing and inspection's techniques nowadays, composite materials are used also for
primary structures such as complete empennage, wings and fuselage on many modern
aircraft (Soutis, 2005b, Soutis, 2005a).
Primary structure is a critical load-bearing structure of an aircraft. If this
structure is severely damaged or failed, the aircraft could experience fatal accident to the
passengers. On the other hand, secondary structure acts as a structural element, mainly
to provide enhanced aerodynamics features to the assembly (Christos, 2013). In aircraft
industry, level of void content for the primary structure should be less than 1% whilst
void content less than 5 % is acceptable for secondary structure.
2
The structural composite parts could be typically categorized into three main
groups prior to the assembly process, based on their shapes and geometries, which are
flat laminate, sandwich and complex-shaped laminate (Callister, 2003). The application
of flat laminates and sandwich composite structures are wide used mainly in secondary
structures due to the wide-range of advantages and potentials, which will lead to
extended weight and cost saving. Hence, there are many researches that propose the
utilization of the flat laminate and sandwich composite structures in the primary
applications (Herrmann et al., 2005). Most developments in the research work are
focused mainly on the flat laminate, and sandwich composite structure rather than the
complex-shaped laminates (Long, 2005). It was established that, the manufacturing
defects were easy to form at the complex section. Thus, the region of high stress
concentration is easily developed, and subsequently, increasing the tendency of laminate
to fail.
The fabrication of composite materials in the aircraft industry, where the resin is
combined with the fibre materials, is available via three methods (Joshi, 2009); wet resin
lay-up, resin infusion and pre-impregnating processes (Karlsson and TomasÅström,
1997). Due to the low material cost, wet resin lay-up may still be used in the fabrication
of the composite components for the general aviation industry, however this process has
reduced in its application over the recent years. The advanced composite materials used
for commercial and military aircraft industries focuses on the later two, especially on the
pre-impregnating process that will be discussed in detail in the next section.
3
1.2 Autoclave manufacturing
Autoclaves are pressure vessels with heating capability; in essence they are
pressurized ovens. They are equipped with vacuum systems into which the vacuum
integrity of the vacuum bag bagged composite laminate is maintained during the cure
cycle. The integrity of the vacuum is essential to ensure the trapped volatiles, moisture
and air are removed prior to the cross-linking of the thermosetting resin.
The curing pressures are generally in the range of 50 to 100 psi (350 to 700 KPa)
and cure cycles normally involve long hours where the time required for pressurization
and de-pressurization are accommodated. Autoclave molding is a modification of
vacuum bag molding, however it is superior to vacuum bagging processing methodology
as this advanced process produces denser, void free composite parts as a result of the
pressure used during the curing. It is widely used methodology in the aircraft industry to
fabricate high strength to weight ratio parts; this method is able to accommodate higher
temperature matrix resins such as epoxies and thermoplastics.
For the resin to achieve full cure to develop the required physical and mechanical
properties, cross-linking reaction must be chemically initiated through the application of
heat and completed to form a solid and rigid cured matrix. In the autoclave process, high
pressure and heat are applied to the curing resin through the autoclave atmosphere. An
additional atmospheric pressure is applied via the vacuum bag used on the curing parts,
where it is also served to protect the laminate from the autoclave gases. The autoclave
cure cycle for a specific application is usually determined empirically; this is applicable
especially to thick laminates and laminates with complex shapes. As a result, several
cure cycles may be developed for a single material system in order to account for these
4
variables that will affect the cure in producing good quality composite parts (Yan, 2006,
Yang and Lee, 2002).
Typically, autoclave cure cycle is a two-step process; first, vacuum and pressure
are applied while the temperature is ramped up to an intermediate level and held there
for a short period of time. The heat reduces the resin viscosity, allowing it to flow and
making it easier for trapped air, moistures and volatiles to escape. Also, the resin begins
wetting the fibres at this stage.
In the second ramp up, the temperature is raised to the final cure temperature and
is held for a sufficient length of time to complete the cure reaction. During this step, the
viscosity continues to drop, but the pre-set temperature and hold time stabilize the
viscosity at a level that permits adequate consolidation and fibre wetting, while avoiding
excessive flow and subsequent resin starvation. These control factors are also required
for slowing the reaction rate, which prevent excessive heat generation from the
exothermic polymerization process.
Although autoclave process is well established, besides offering excellent
reliability and part quality, autoclave processing requires high capital investment
(Buehler and Seferis, 2000), maintenance and energy costs. In addition, part size is
limited to the size of the autoclave. Like any manufacturing processes, autoclave curing
methodology has its advantages and disadvantages, as summarised in the following
Table 1. 1.
.
5
Table 1. 1: Advantages and disadvantages of autoclave process
ADVANTAGES
High fibre content laminates can be achieved, thus highest strength to weight ratio is achievable for the cured laminates. Pressurized curing reduces voids within the resin.
With modern monitoring and control technologies, pressure and heat can be controlled very closely to achieve the customized curing that meets individual component complexity requirement.
DISADVANTAGES
High equipment investment cost. High operating cost typically applicable to the machinery that generates compressed air or the alternative nitrogen gas supply, which serve as pressurized air supply. High on the associated consumable costs, energy usage and waste generated.
As the continuous feeding of parts for curing is not possible until the autoclave curing cycle is completed, autoclave curing process usually becomes the bottle neck for the composite part manufacturing.
1.3 Out of Autoclave manufacturing- Vacuum Bag Only- Oven Cure
In the drive for a low manufacturing cost and the solution for large component
size constraint, a considerable amount of efforts have been put into the areas, focusing
on the capability of composite parts manufacturing moving away from the high-cost and
size constrained autoclave process (Campbell, 2003, Hernández et al., 2011). Out-of-
autoclave (OOA) composite component manufacturing processes have emerged as
alternate processing techniques to autoclave processing in the 90s.
Vast majority of advance composite structures in production today are still cured
using autoclaves (Hernández et al., 2011). With autoclave curing method, the autoclave
pressure compresses the trapped air, moisture and volatiles within the resin of the
6
composite material into smaller size at micro level and extendedly diluted into the resin
before the cross-linking phase of resin. With proper process management, autoclave
curing process is able to produce cured laminate with very low voids (porosity) content.
OOA process means the absent of the pressure, and therefore the method of removing
trapped air, moisture and volatiles will depend solely on the vacuuming through the air
path ways within the vacuum bag system. Figure 1. 1 shows the difference between the
cure cycles of autoclave and vacuum bag only cure.
With the availability of the OOA prepreg materials, out-of-autoclave curing
processes especially VBO- oven curing and automated-tape-placement (ATP) are the
newer alternatives processing techniques for aircraft composite parts manufacturing
(Kratz and Hubert, 2011, Lukaszewicz and Potter, 2011). VBO prepregs are
manufactured by hot melt process, therefore the solvent content is negligible, and
volatiles released by the evaporating solvent are therefore negligible.
On the other hand, the resin content of a VBO material is optimized with its
reactivity to allow the prepreg material to be cured at a lower temperature, where lower
temperature will prevent the emission of any volatiles. Modern hot melt pre-pregging
process is designed with Engineered Vacuum Channels (EVaC); the prepreg is
engineered for air removal using partial impregnation (Sequeira Tavares et al., 2011,
Centea and Hubert, 2011) and is also identified as semipreg as shown in Figure 1. 2.
7
Figure 1. 1: Comparison between the Autoclave and the vacuum bagging process (Hexcel, 2005)
Figure 1. 2: Two ply of Engineered Vacuum Channels (EVaCs) in OOA prepreg (Wysocki et al., 2009)
1st ply
2nd ply
Wet portion
EVaCs
Dry portion
