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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 FRONT
VOIDS
RESI
N F
LOW
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Resin infusion possibilities
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From a centre point towards the periphery
SLOWEST!
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Resin infusion possibilities
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From the edge
MEDIUM FAST!
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Resin infusion possibilities
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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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AmbulancePolytec, Sweden
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 monthsMSK 20120213
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: 20-50 kg/cm2
• Temperature:145 - 160 ºC• Time: 1 - 5 minutes• Molds: steel, chrome-
plated
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Volvo V70 TailgateBenefits with composite compared to steel:→ Reduced tooling need→ Styling freedom→ Integration capability→ Weight reduction
compared to steel→ Technology step
Comparison composite/metal series length
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V70 Tailgate
Steel plate
Theft/heat protection
Reinforcement
Directional fibres
BMC
t=3.5, 20% glass
SMC
t=2.5, 25% glass
SMC
t=2.5 (gen. Surfaces) 2.5-
4(stressed areas) , 25% glass
Glass fiber carpet
M =10,3 kg (structure only)
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
TEX 4 103d2N
densityN number of fibersd fiber thickness
TEX g km
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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
Composition and properties for glass fibres
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A glass C glass E glass S glass
SiO2 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 -
B2O3 weight-% - 5 7,5
Tensile strength GPa 3,1 3,3 3,6 4,6
Modulus GPa 72 70 75 80
Softening point ºC 700 690 850 990
Density g/cm3 2,45 2,45 2,54 2,48
Manufacturing process for carbon fibres
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Polyacrylonitrile (PAN) is the most common precursor for carbon fibresThe strength of the fibres are due to orientation and stretching of the C-C bondsStrength can be increased by graphitisation at 1500 ºC
Carbon fibre production
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Textile reinforcements
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Classification of reinforcements
1. Short2. Unidirectional3. 2D
weaves/Planar interlaced
4. 3D/Fully integrated
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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 andcontinuous 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 basket twill satin
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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
Spread tow fabrics by Oxeon, Sweden
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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 bondingd) Chain entanglinge) Transcrystallisation
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Interphasial region in composites = the region of the matrix which is
influenced by the fibre
matrix
fibreinterphase
Fibre diameter
interface
Interphase in composites
• The interphase = a three dimensional region near the fiber with properties different compared to the fiber and the matrix
c) composite 4a d) composite 8a
a) composite 3ab) composite 7a
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Transversal fracture in composites
Transversal fracture at low elongation (< 0.2%) due to poor adhesion between the fibre and
the resin
Transversal fracture at high elongation (> 0.6%) due to strong adhesion between the fibre and
the resin
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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
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
• 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
NDT methods
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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
NDT methods
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Acoustic emission
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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¨
NDT methods
End of part 2
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