Vacuum infusion molding principle
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Vacuum bag infusion – step by
step
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Vacuum bag infusion
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Vacuum infusion with semi-rigid shell
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Careful resin flow rate regulation to avoid air
entrapment
RESIN FLOW
VOIDS
RESIN FRONT
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Resin infusion possibilities
From a
centre point
towards the
periphery
SLOWEST!
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Resin infusion possibilities
From the edge
MEDIUM FAST!
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Resin infusion possibilities
Infusion from
the pheriphery
FASTEST!
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Flexible, semiflexible or rigid
mould?
• Vacuum bag infusion (flexible bag): suitable
for small production volumes, large size products
and lower tolerance demands
• Vacuum infusion with semi-stiff shell:
suitable for medium production volumes, medium
product size and medium tolerance demands
• Vacuum infusion/RTM with stiff (solid)
moulds: suitable for large production volumes,
small size products and high tolerance demands
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Blades for wind mills
• Length 30 - 70 m
• 20 years life length
• Lay up of two separate
halves which are glued
together
• Filament winding
• Unsaturated polyester, vinyl
ester, epoxy resin
• Glass fibre, carbon fibre
• Stiffness and fatigue
properties are important
• Denmark major producer
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Polytec, Sweden
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Ambulance
Modular construction design
possible
• Parts are
manufactured
separately, and
joined by
adhesives
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Compression molding
• A premade compound is formed by
pressure in a closed mold
• Crosslinking is initiated by heating
• Cost effective method for long and very
long series
• SMC: sheet molding compounds
• BMC: bulk molding compounds
• Automotive and electrical industry most
important application areas
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SMC manufacture
Shelf life: 3 - 4 months
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SMC prepreg manufacture – step by
step
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Application of resin onto plastic
support film
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Addition of cut fibres
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Ready SMC is covered by
second support film
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Schematic of compression molding
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SMC press
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Compression molding - process
conditions
•
•
•
•
Pressure:
Temperature:
Time:
Molds:
plated
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20-50 kg/cm2
145 - 160 ºC
1 - 5 minutes
steel, chrome-
Volvo V70 Tailgate
Benefits with composite
compared to steel:
→ Reduced tooling need
→ Styling freedom
→ Integration capability
→ Weight reduction
compared to steel
→ Technology step
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Comparison composite/metal series length
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V70 Tailgate
M =10,3 kg (structure
only)
BMC
t=3.5, 20% glass
SMC
t=2.5 (gen.
Surfaces) 2.54(stressed areas)
, 25% glass
SMC
t=2.5, 25% glass
Reinforcement
Directional fibres
Glass fiber
carpet
Steel plate
Theft/heat
protection
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Production volumes – manufacturing process
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Reinforcements
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Fibre types
• Glass fibre: relatively good strength,
medium stiffness (E= 70 GPa),
transparent, cheap
• Carbon fibres: very good strength, high
stiffness (E=200-300 GPa), black, very
expensive, electrically conducting
• Natural fibres: flax, hemp, sisal, wood
• Aramid fibres (Kevlar): very good
tensile strength, yellow, hard to process,
expensive
• Special fibres: polyethylene fibres, boron,
ceramics, basalt
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Fibres, yarns and rowings
• An assembly of collimated glass
fibres is called a yarn, (tow,
strand), and a group of yarns is
called a rowing
• The yarns and rowings are
twisted, which simplifies
handling, but makes resin
impregnation more difficult
• The fibre thickness varies
typically between 3-25 µm
(commonly 10-20 µm)
• Linear densities are given by the
TEX number
• A rowing has a TEX of minimum
300
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3
TEX   4 10 d N
2
  density
N  number of fibers
d  fiber thickness
 TEX  g
km

•
•
•
•
•
•
Characteristics for glass fibres
Based on SiO2 with added oxides of calcium, boron,
sodium, iron or aluminium
Depending on composition different glass types are
defined:
– A-glass (Alkali glass)
– E- glass (Electrical glass)
– C-glass (Chemically resistant glass)
– S-glass (High strength glass)
Characteristic properties are high strength, good
tolerances for high temperatures and corrosive
environments
Transparency and no colour are advantages compared
to other fibres
Disadvantages are low stiffness, moisture sensitivity
and abrasiveness
Low cost has been the most critical factor when
promoting their use
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Composition and properties for glass
fibres
A glass
C glass
E glass
S glass
weight%
72
64,5
55
65
Al2O3 + Fe2O3
weight-%
2
4
4,5
25
CaO
weight-%
10
13,5
21,5
-
MgO
weight-%
2
3
0,5
10
Na2O + K2O
weight-%
14,5
10
<1
-
-
5
7,5
SiO2
B2O3
weight%
Tensile strength
GPa
3,1
3,3
3,6
4,6
Modulus
GPa
72
70
75
80
ºC
700
690
850
990
g/cm3
2,45
2,45
2,54
2,48
Softening point
Density
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Manufacturing process for carbon fibres
Polyacrylonitrile (PAN) is the most common precursor for
carbon fibres
The strength of the fibres are due to orientation and
stretching of the C-C bonds
Strength can be increased by graphitisation at 1500 ºC
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Carbon fibre production
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Textile reinforcements
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Classification of reinforcements
1.
2.
3.
4.
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Short
Unidirectional
2D
weaves/Planar
interlaced
3D/Fully
integrated
Different reinforcement types
• Chopped strand mat
• Continuous strand
mat
• Woven fabrics,
diaxial
• Woven fabrics,
multiaxial
• Stitched fabrics
• Braided fabrics
• Knitted fabrics
• Combinations
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Chopped strand mats and
continuous strand mats
• Non-woven structures
• Surface weights 150 - 900 g/m2
• Made from chopped or continuous yarns, bound
together chemically, mechanically or by heating
• Emulsion binders and polyester powder binders
are most common
• Good drapability
• Surface veils (surface eights 10-50 g/m2) are
used to get a wanted surface finish
• Mats made from other fibres are commonly
named non-wovens
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Woven fabrics = interlacing of 2 or
more yarn systems
• Characterised by the
crimp
• Lower crimp improves
formability and resin
permeability
• Crimp also reduces
stiffness
plain
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basket
twill
satin
Benefits with woven fabrics
• Good drapability
• Low manufacturing costs due to
combination of two layers
• Good impact resistance
• Lower stiffness due to crimp
• Better compression strength
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The mechanical properties for
weaves depend on:
•
•
•
•
Type of fibre
Weave structure
Stacking and orientation of fibres
Yarn twist
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Braided fabrics
•
Circular braiding is used for tubes or ropes
•
Biaxial
•
Triaxial
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Braided reinforcements
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Knitted fabrics
• Made by knitting
• Loose and flexible
weaves are
produced
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Stitched fabrics (noncrimp)
• Fibre layers are
stiched together
into one structure
• The stiching is
done by sewing
• Noncrimp fabrics
offer a rapid and
precise lay-up of
multilayered
reinforcement
• Different fibre
types can be
combined, sunh as
comingled fabrics
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Spread tow fabrics by Oxeon,
Sweden
Non-crimp fabric
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Combinations
• Combination of different mats
stitched together
• Ex: Combiflow mat:
• Porous flow layer for better mould
filling, used in resin injection
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Parabeam – 3 D fabric
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The interphase/interface in
composites
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Long term durability of composites
• Depends on the state of the resin, which may undergo:
–
–
–
–
Physical ageing
Environmental degradation
Changes in fibre-matrix interaction
Matrix stress state, due to processing, thermal and fatigue
cycling, mechanical loads
• Microcracking is the first sign of damage, which can initiate:
– Fiber fracture
– Interface debonding
– Delamination
• The microcrack can be a pathway for moisture, chemicals,
microorganisms, soil which then can lead to degradation
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Fibre-matrix interphase
• The three-dimensional boundry between the
fiber and matrix
• It is critical for the control of composite
properties, as fibre-matrix interaction occurs
through the interface
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Interaction at fiber-matrix interface
a) Micromechanical
interlocking
b) Electrostatic (dipole)
interaction
c) Chemical bonding
d) Chain entangling
e) Transcrystallisation
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Interphasial region in composites
= the region of the matrix which is influenced
by the fibre
interphase
fibre
matrix
Fibre diameter
interface
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Interphase in composites
• The interphase = a
three dimensional
region near the fiber
with properties
different compared to
the fiber and the
matrix
a) composite 3a
c) composite 4a
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b) composite 7a
d) composite 8a
Transversal fracture in composites
Transversal fracture at low
elongation (< 0.2%) due to poor
adhesion between the fibre and
the resin
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Transversal fracture at high
elongation (> 0.6%) due to strong
adhesion between the fibre and
the resin
Fibre surface treatments
• Surface oxidation; electrolytical, gases or
liquid chemicals
• Surface coating by organic/inorganic
chemicals (sizing agents)
• Polymer grafting onto fibre surface
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Surface treatment of glass
fibres
• Surface
treatment by
sizeing
• Treatment
composition:
– Film forming
polymer (PVA)
– Lubricant
– Coupling agent
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Some effects due to surface treatment
• Fibre protection during shipment, handling
and processing
• Binding of indivcidual filaments together to
ensure easier handling
• Lubrication during processing
• Reduce static electricity
• Improve chemical bonding to the matrix
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Remark
• If the surface treatment is not properly done,
it can be detrimental to the bulk mechanical
properties, and the interface properties can
vary
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Non-destructive testing (NDT)
• For identification of defects without destroying the
object
• Used for quality control and for in-service inspection
• Delaminations, failed adhesive bonds, voids,
incorrect reinforcement orientation, variations in
fiber content
• Based on differences in physical or mechanical
properties, due to the defect
• Comparative methods -> qualitative information
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Ultrasonic inspection
NDT methods
• Transmission of sound
waves through the
specimen
• 0.5 - 75 MHz sound
• Pulse-echo or through
transmission
• Coupling mediums
(water, oil, gels) for
efficient transfer of
sound wave into the
component
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NDT methods
Acoustic emission
• Detection of microscopic failures by recording
the sound of the event
• Fiber fractures and matrix microcracks
• In combination with mechanical loading
• Semi-NDT
• Careful interpretation of data necessary
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Acoustic emission
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NDT methods
Other methods
• Radiography (X-ray, -ray)
• Computer aided tomography: defects can be
located
• Thermographic inspection: based on
differences in thermal diffusivity
• Vibrational inspection: ¨coin tapping¨
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End of part 2
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