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UNIT II POLYMER MATRIX COMPOSITES: 9
Types Processing Thermo sensing matrix composites Hand layup and
sprayup techniques filament winding, pultruion, resin transfer
moulding, auctoclave moulding thermoplastic matrix composites
Injection moulding, film stacking diaphragm forming thermoplastic
tape laying. Glass fibre/polymer interface. Mechanical properties
Fracture. Applications.
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Classification based on MatricesComposite
materialsMatricesPolymer Matrix Composites (PMC)Metal Matrix
Composites MMC)Ceramic Matrix Composites
(CMC)ThermosetThermoplasticRubber
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What is a polymer? Poly mer many repeat unit
repeatunitrepeatunitrepeatunitExamples of polymers:
A polymer is a large molecule (macromolecule) composed of
repeating structural units typically connected by covalent chemical
bonds
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Polymer Matrix Composite (PMC) is the material consisting of a
polymer (resin) matrix combined with a fibrous reinforcing
dispersed phase.
Polymer Matrix Composites are very popular due to their low cost
and simple fabrication methods.
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Discontinuous phase - ReinforcementContinuous phase -
MatrixPolymer(Matrix)Composite (Matrix +
Reinforcement)ReinforcementsPrincipal load bearing
member.Matrixprovides a medium for binding and holding the
reinforcements together into a solid.protects the reinforcement
from environmental degradation.serves to transfer load from one
insert (fibre, flake or particles) to the other.Provides finish,
colour, texture, durability and other functional properties.
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Classification of PolymersLinear polymer - Any polymer in which
molecules are in the form of chains.Thermoplastic polymers - Linear
or branched polymers in which chains of molecules are not
interconnected to one another.Thermosetting polymers - Polymers
that are heavily cross-linked to produce a strong three dimensional
network structure.Elastomers - These are polymers (thermoplastics
or lightly cross-linked thermosets) that have an elastic
deformation > 200%.
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Molecular chain configurations:
a.Linearb.Branchedc.Crossed linkedd.Ladder
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Various forms: discontinuous (chopped), continuous or woven as a
fabric Principal fiber materials in FRPs are glass, carbon, and
Kevlar 49. Less common fibers include boron, SiC, Al2O3 and steel.
Glass (in particular Eglass) is the most common fiber material in
today's FRPs; its use to reinforce plastics dates from around 1920.
Thermosetting resins are the most widely used polymers in PMCs.
Epoxy and polyester are commonly mixed with fiber
reinforcement.
The most widely used form is a laminar structure, made by
stacking and bonding thin layers of fiber and polymer until the
desired thickness is obtained.
Fibers in PMCs
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Polymerisation:This is the process of joining monomers into
gaint chain like molecules.
Methods of Polymerisation:Condensation polymerisationAddition
polymerisationDegree of polymerization = No of monomer units in a
chain 103 to 105
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ThermosetsThermoset materials are usually liquid or malleable
prior to curing, and designed to be molded into their final
form.
Has the property of undergoing a chemical reaction by the action
of heat, catalyst, ultraviolet light, etc., to become a relatively
insoluble and infusible substance.
They develop a well-bonded three-dimensional structure upon
curing. Once hardened or cross-linked, they will decompose rather
than melt.
Thermoset materials are generally stronger than thermoplastic
materials due to this 3-D network of bonds, and are also better
suited to high-temperature applications up to the decomposition
temperature of the material.
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Thermosets are made by mixing two components (a resin and a
hardener) which react and harden, either at room temperature or on
heating.
The resulting polymer is usually heavily cross-linked, so
thermosets are also called as network polymers.
The cross-links form during the polymerisation of the liquid
resin and hardener, so the structure is almost always
amorphous.
On reheating the crosslinks prevent true melting or viscous flow
so the polymer cannot be hot-worked. Further heating just causes it
to decompose.
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Thermosets
Extensive cross-linking formed by covalent bonds.Bonds prevent
chains moving relative to each other.
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Types of Thermosetting plastics
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Epoxy:
Epoxy is a polymer that contain an epoxide group in its chemical
structure.Example: DGEBA (Diglcidyl Ether of Bisphenol A )
Charecteristics of Epoxy:
Better Moisture ResistenceLow shrinkageGood adhersion with
Reinforcement
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Polyester:
A condensation reaction between a glycol and an unsaturated
dibasic acid results in polyster. This contains a double bond C=C
between its carbon atoms.Example: poly ethylene terephthalate
(PET). Charecteristics of Polyester:
CheapResistance to variety of chemicalsAdequate moisture
resistance
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Thermosetting plastics - applications
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ThermoplasticsIn thermoplastic polymer, individual molecules are
linear in structure with no chemical linking between them.
They are held in place by weak secondary bond (intermolecular
force), such as van der Walls bonds and hydrogen.
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Some thermoplastics normally do not crystallize, they are termed
as"amorphous" plastics and are useful at temperatures below the
Tg.
Generally, amorphous thermoplastics are less chemically
resistant.
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Thermoplastics (80%)
No cross links between chains.Weak attractive forces between
chains broken by warming.Change shape - can be remoulded.Weak
forces reform in new shape when cold.
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Reasons for the use of thermoplastic matrix
compositesRefrigeration is not necessary with a thermoplastic
matrix.Parts can be made and joined by heating.Parts can be
remolded, and any scrap can be recycled.Thermoplastics have better
toughness and impact resistance than thermosets.Shorter fabrication
time.Can be recycled.
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UNIQUE CHARACTERISTIC OF THERMOPLASTICNear to glass transition
temperature Tg, polymeric materials changes a hard solid to soft,
tough ( leather like) solid. Over a temperature range around
Tg.Near this temperature, the materials is also highly
viscoelastic.When load is applied it exhibit Elastic
deformation.With increasing temperature polymer changes into
rubberlike solid undergoing deformation on external load.Further
increasing the temp both amorphous and semicrystallline
thermoplastic achieve highly viscous state and attain the melting
temp Tm.
Variation of Tensile modulus with temperature for Amorphous and
Semi crytaline thermoplastic.
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Thermoplastic polymer have higher strain-to-failure.
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Types of Thermoplastics
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COMPARISON OF THE THREE POLYMER CATEGORIES
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Thermoplastics Vs Thermosets
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Functions of Matrix
Holds the fibres together.
Protects the fibres from environment.
Distributes the loads evenly between fibres so that all fibres
are subjected to the same amount of strain.
Enhances transverse properties of a laminate.Improves impact and
fracture resistance of a component.
Helps to avoid propagation of crack growth through the fibres by
providing alternate failure path along the interface between the
fibres and the matrix.
Carry inter-laminar shear.
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Desired Properties of a Matrix
Reduced moisture absorption.
Low shrinkage.
Low coefficient of thermal expansion.
Good flow characteristics so that it penetrates the fibre
bundles completely and eliminates voids during the
compacting/curing process.
Must be elastic to transfer load to fibres.
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Reasonable strength, modulus and elongation (elongationshould be
greater than fibre).
Strength at elevated temperature (depending on application).
Low temperature capability (depending on application).
Excellent chemical resistance (depending on application).
Should be easily processable into the final composite shape.
Dimensional stability (maintains its shape).
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Effect of Temperature on ThermoplasticsDegradation temperature -
The temperature above which a polymer burns, chars, or
decomposes.Glass temperature - The temperature range below which
the amorphous polymer assumes a rigid glassy structure.The effect
of temperature on the modulus of elasticity for an amorphous
thermoplastic.
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Stress-strain behavior of different polymer matrices
Thermoplastic polymersThermosetting polymersNotice to the range of
ultimate strains of different polymers
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Comparision of various polymers as matrix materials
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Limitations of PMC Low maximum working temperature.High
coefficient of thermal expansion- dimensional
instabilitySensitivity to radiation and moisture.Processing
temperature are generally higher than those with thermosets.
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Polymer Processing Forming Processes for Thermosetting matrix
composites:
Hand layup and sprayup techniques.Filament
winding.Pultrusion.Resin transfer moulding.Autoclave moulding.
Forming Processes for Thermoplastic matrix composites:
Injection moulding.Film stacking.Diaphragm forming.Thermoplastic
tape laying.
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Hand Layup Hand layup process: Gel coat is applied to open
mold.
Fiberglass reinforcement is placed in the mold.
Base resin mixed with catalysts is applied by pouring and
brushing.
Layup is made by building layer upon layer to obtain the desired
thickness.
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Hand LayupThe most popular type of Open Molding is Hand Layup
process. The Hand Layup is a manual, slow, labor consuming method,
which involves the following operations: The mold is coated by a
release anti-adhesive agent, preventing sticking the molded part to
the mold surface.The prime surface layer of the part is formed by
applying gel coating.A layer of fine fiber reinforcing tissue is
applied.Layers of the liquid matrix resin and reinforcing fibers in
form of woven fabric, rovings or chopped strands are applied. The
resin mixture may be applied by either brush or roll.The part is
cured (usually at room temperature).The part is removed from the
mold surface.The disadvantages of the Hand Layup method are: low
concentration of reinforcing phase (up to 30%) and low
densification of the composites (entrapped air bubbles).
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Hand LayupAdvantages:
Widely used. Low tooling cost.Custom shape. Larger and complex
items can be produced.Potential Problems:
Labour intensive.Low-volume process.Styrene emission.Quality
control is entirely dependent on the skill of labourers.
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Hand layup products:
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Hand layup products:
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In Sprayup process liquid resin matrix and chopped reinforcing
fibers are sprayed by two separate sprays onto the mold surface.
The fibers are chopped into fibers of 1-2 (25-50 mm) length and
then sprayed by an air jet simultaneously with a resin spray at a
predetermined ratio between the reinforcing and matrix phase. The
Sprayup method permits rapid formation of uniform composite
coating, however the mechanical properties of the material are
moderate since the method is unable to use continuous reinforcing
fibers. SPRAYUP
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A spray gun supplying resin in two converging streams into which
roving is chopped.Automation with robots results in high rate of
production.Labor costs are lower.SPRAYUP
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Sprayup process:
In Sprayup process, chopped fibers and resins are sprayed
simultaneously into or onto the mold.
Applications are lightly loaded structural panels, e.g. caravan
bodies, truck fairings, bathtubs, small boats, etc
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Hand and Spray LayupIn both the cases the deposited layers are
densified with rollers.
Catalysts and Accelerators are used. * Catalyst - substance
added to the gel coat or resin to initiate the curing process. *
Accelerator - A compound added to speed up the action of a catalyst
in a resin mix.
Curing at room temperature or at a moderately high temperature
in an oven.
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Advantages of Hand Layup and SprayupTooling cost is
low.Semiskilled workers are easily trained.Design
Flexibility.Molded-in inserts and structural changes are
possible.Sandwich constructions are possible.Large and Complex
items can be produced.Minimum equipment investment is necessary.The
startup lead time and the cost are minimal.
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Disadvantages of Hand Layup and SprayupLabor Intensive.Low
volume process.Longer curing times.Production uniformity is
difficult.Waste factor is high.
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PREPREG
Prepreg is the composite industrys term for continuous fiber
reinforcement .Pre-impregnated with a polymer resin that is only
partially cured.Prepreg is delivered in tape form to the
manufacturer who then molds and fully cures the product without
having to add any resin.This is the composite form most widely used
for structural applications.
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Manufacturing begins by collimating a series of spool-wound
continuous fiber tows. Tows are then sandwiched and pressed between
sheets of release and carrier paper using heated rollers
(calendering). The release paper sheet has been coated with a thin
film of heated resin solution to provide for its thorough
impregnation of the fibers.PREPREG PROCESSThe final prepreg product
is a thin tape consisting of continuous and aligned fibers embedded
in a partially cured resin.
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PREPREG Prepared for packaging by winding onto a cardboard core.
Typical tape thicknesses range between 0.08 and 0.25 mm Tape widths
range between 25 and 1525 mm. Resin content lies between about 35
and 45 vol%
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PREPREGThe prepreg is stored at 0C (32 F) or lower because
matrix undergoes curing reactions at room temperature. Also the
time in use at room temperature must be minimized. Life time is
about 6 months if properly handled.Both thermoplastic and
thermosetting resins are utilized: carbon, glass, and aramid fibers
are the common reinforcements.Actual fabrication begins with the
lay-up. Normally a number of plies are laid up to provide the
desired thickness.The layup can be by hand or automated.
Easily obtained with epoxies.
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*Filament WindingFilament Winding method involves a continuous
filament of reinforcing material wound onto a rotating mandrel in
layers at different layers. If a liquid thermosetting resin is
applied on the filament prior to winding the, process is called Wet
Filament Winding. If the resin is sprayed onto the mandrel with
wound filament, the process is called Dry Filament Winding. Besides
conventional curing of molded parts at room temperature, Autoclave
curing may be used.
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*Filament WindingFilament Winding ProcessFor Round or
Cylindrical partsA tape of resin impregnated fibers is wrapped over
a rotating mandrel to form a part. These windings can be helical or
hooped. There are also processes that use dry fibres with resin
application later, or prepregs are used.Parts vary in size from 1"
to 20Winding directionHoop/helical layers Layers of different
materialHigh strengths are possible due to winding designs in
various directionWinding speeds are typically 100 m/min and typical
winding tensions are 0.1 to 0.5 kg.
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Copyright Joseph Greene 2001*Filament WindingDemoldingTo remove
the mandrel, the ends of the parts are cut off when appropriate, or
a collapsible mandrel (e.g., low melt temperature alloys ) is used.
Curing in done in an Autoclave for thermoset resins (polyester,
epoxy, phenolic, silicone) and some thermoplastics (PEEK)Fibers are
E-glass, S-glass, carbon fiber and aramids (toughness and
lightweight) .Inflatable mandrels can also be used to produce parts
that are designed for high pressure applications, or parts that
need a liner, and they can be easily removed.AdvantagesGood for
wide variety of part sizes Parts can be made with strength in
several different directions Very low scrap rate Non-cyclindrical
parts can be formed after winding Flexible mandrels can be left in
as tank linersReinforcement panels, and fittings can be inserted
during winding Due to high hoop stress, parts with high pressure
ratings can be made Disadvantages Viscosity and pot life of resin
must be carefully chosen NC programming can be difficult Some
shapes can't be made with filament winding Factors such as filament
tension must be controlled
Copyright Joseph Greene 2001
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Filament Winding
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Filament winding - applicationspressure vessels, storage tanks
and pipesrocket motors, launch tubesLight Anti-armour Weapon
(LAW)Hunting Engineering made a nesting pair in 4 minutes with ~20
mandrels circulated through the machine and a continuous curing
oven.drive shaftsEntec the worlds largest five-axis filament
winding machine for wind turbine bladeslength 45.7 m, diameter 8.2
m, weight > 36 tonnes.
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FILAMENT WINDING CHARACTERISTICSThe cost is about half that of
tape layingProductivity is high (50 kg/h).Applications include:
fabrication of composite pipes, tanks, and pressure vessels. Carbon
fiber reinforced rocket motor cases used for Space Shuttle and
other rockets are made this way.
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Filament winding
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Filament winding - winding patternshoop (90) - girth or
circumferential windingangle is normally just below 90 degreeseach
complete rotation of the mandrel shifts the fibre band to lie
alongside the previous band. helicalcomplete fibre coverage without
the band having to lie adjacent to that previously laid. polardomed
ends or spherical componentsfibres constrained by bosses on each
pole of the component.axial (0) beware: difficult to maintain fibre
tension
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Filament winding patternshoop:helical:
polar:
g
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Kevlar component
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Filament wound pressurebottles for gas storage
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PultrusionDescription:
Pultrusion is a process where composite parts are manufactured
by pulling layers of fibres/fabrics, impregnated with resin,
through a heated die, thus forming the desired cross-sectional
shape with no part length limitation.
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PultrusionPultrusion is an automated, highly productive process
of fabrication of Polymer Matrix Composites in form of continuous
long products of constant cross-section. A scheme of the process is
presented on the picture:
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PultrusionPultrusion process involves the following operations:
Reinforcing fibers are pulled from the creels. Fiber (roving)
creels may be followed by rolled mat or fabric creels. Pulling
action is controlled by the pulling system.Guide plates collect the
fibers into a bundle and direct it to the resin bath.Fibers enter
the resin bath where they are wetted and impregnated with liquid
resin. Liquid resin contains thermosetting polymer, pigment,
fillers, catalyst and other additives.The wet fibers exit the bath
and enter preformer where the excessive resin is squeezed out from
fibers and the material is shaped.The preformed fibers pass through
the heated die where the final cross-section dimensions are
determined and the resin curing occurs.The cured product is cut on
the desired length by the cut-off saw.
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PultrusionPultrusion process is characterized by the following
features: High productivity.The process parameters are easily
controllable.Low manual labor component.Precise cross-section
dimensions of the products.Good surface quality of the
products.Homogeneous distribution and high concentration of the
reinforcing fibers in the material is achieved (up to 80% of roving
reinforcement, up to 50% of mixed mat + roving
reinforcement).Pultrusion is used for fabrication of Fiber glass
and Carbon fiber reinforced polymer composites and Kevlar (aramid)
fiber reinforced polymers.
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PultrusionManufacturingFibers are brought together over rollers,
dipped in resin and drawn through a heated die. A continuous cross
section composite part emerges on the other side.
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production of constant cross-section profiles
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*PultrusionDesign Hollow parts can be made using a mandrel that
extends out the exit side of the die. Variable cross section parts
are possible using dies with sliding parts. Two main types of dies
are used, fixed and floating. Fixed dies can generate large forces
to wet fiber. Floating dies require an external power source to
create the hydraulic forces in the resin. Multiple dies are used
when curing is to be done by the heated dies.
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Very low scrap. Up to 95% utilization of materials (75% for
layup). Rollers are used to ensure proper resin impregnation of the
fiber. Material forms can also be used at the inlet to the die when
materials such as mats, weaves, or stitched material is used. For
curing, tunnel ovens can be used. After the part is formed and
gelled in the die, it emerges, enters a tunnel oven where curing is
completed. Another method is, the process runs intermittently with
sections emerging from the die, and the pull is stopped, split dies
are brought up to the sections to cure it, they then retract, and
the pull continues. (Typical lengths for curing are 6" to 24")
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Copyright Joseph Greene 2001*Materials Most fibers are used
(carbon, glass, aramids) and Resins must be fast curing because of
process speeds. (polyester and epoxy)Processingspeeds are 0.6 to 1
m/min; thickness are 1 to 76 mm; diameters are 3 mm to 150mm double
clamps, or belts/chains can be used to pull the part through. The
best designs allow for continuous operation for production.diamond
or carbide saws are used to cut sections of the final part. The saw
is designed to track the part as it moves.these parts have good
axial properties.Advantagesgood material usage compared to layup
high throughput and higher resin contents are
possibleDisadvantagespart cross section should be uniform.Fiber and
resin might accumulate at the die opening, leading to increased
friction causing jamming, and breakage.when excess resin is used,
part strength will decrease void can result if the die does not
conform well to the fibers being pulled quick curing systems
decrease strength
Copyright Joseph Greene 2001
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PultrusionAdvantages:Minimal kinking of fibres/fabrics Rapid
processing Low material scrap rateGood quality control Potential
Problems:Improper fibre wet-outFibre breakage Inadequate cureDie
jamming Complex die design
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Pultrusion -characteristicsseek uniform thickness in order to
achieve uniform cooling and hence minimise residual stress.hollow
profiles require a cantilevered mandrel to enter the die from the
fibre-feed end. continuous constant cross-section profilenormally
thermoset (thermoplastic possible)impregnate with resinpull through
a heated dieresin shrinkage reduces friction in the diepolyester
easier to process than epoxytension control as in filament
windingpost-die, profile air-cooled before grippedhand-over-hand
hydraulic clampsconveyor belt/caterpillar track systems.moving
cut-off machine ("flying cutter"). The solid laminate will be cut
to the desired lengthInside the metal die, precise temperature
control activates the curing of the thermoset resin.
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Shapes such as rods, channels, angle and flat stocks can be
easily produced.Production rate is 10 to 200 cm/min.Profiles as
wide as 1.25 m with more than 60% fiber volume fraction can be made
routinely.No bends or tapers allowed (continuous molding cycle)
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Pultrusion -applicationspanels beams gratings ladderstool
handles - ski poles kiteselectrical insulators and enclosureslight
poles - hand rails roll-up doors450 km of cable trays in the
Channel Tunnel
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Pultrusion ApplicationsAdvanced Composite Construction
Systemcomponents: plank ............... and connectors
used in Aberfeldy and Bonds Mill Lock bridges
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Resin Transfer MoldingIn the RTM process, dry (i.e.
non-impregnated ) reinforcement is pre-shaped and oriented into
skeleton of the actual part known as the preform which is inserted
into a matched die mold.The heated mold is closed and the liquid
resin is injectedThe part is cured in mold.The mold is opened and
part is removed from mold.
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Resin Transfer MouldingClose mold low pressure process.
A dry preform is placed in a matched metal die.
A vaccum pulls the Low viscosity resin through a flow medium
that helps impregnate the preform.
Resin may also be forced by means of a pump.
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Resin Transfer Moulding
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RTMTransfer Molding (Resin Transfer Molding) is a Closed Mold
process in which a pre-weighed amount of a polymer is preheated in
a separate chamber (transfer pot) and then forced into a preheated
mold filled with a reinforcing fibers, taking a shape of the mold
cavity, impregnating the fibers and performing curing due to heat
and pressure applied to the material. The picture below illustrates
the Transfer Molding Process. The method uses a split mold and a
third plate equipped with a plunger mounted in a hydraulic press.
The method combines features of both Compression Molding -
hydraulic pressing, the same molding materials (thermosets) and
Injection Molding ram (plunger), filling the mold through a sprue.
Transfer Molding cycle time is shorter than Compression Molding
cycle but longer than Injection Molding cycle. The method is
capable to produce very large parts (car body shell), more
complicated than Compression Molding, but not as complicated as
Injection Molding.
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RTMTransfer Molding process involves the following steps:
The mold cavity is filled with preformed reinforcing fibers.A
pre-weighed amount of a polymer mixed with additives and fillers
(charge) is placed into the transfer pot.The charge may be in form
of powders, pellets, putty-like masses or pre-formed blanks. The
charge is heated in the pot where the polymer softens. The plunger,
mounted on the top plate, moves downwards, pressing on the polymer
charge and forcing it to fill the mold cavity through the sprue and
impregnate the fibers.The mold, equipped with a heating system,
provides curing (cross-linking) of the polymer (if thermoset is
processed). The mold is opened and the part is removed from it by
means of the ejector pin.If thermosetting resin is molded, the mold
may be open in hot state cured thermosets maintain their shape and
dimensions even in hot state. If thermoplastic is molded, the mold
and the molded part are cooled down before opening. The scrap left
on the pot bottom (cull), in the sprue and in the channels is
removed. Scrap of thermosetting polymers is not recyclable.
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Advantages of RTMLarge complex shapes and curvatures can be made
easily.High level of automation.Layup is simpler than in manual
operations.Takes less time to produce.Fiber volume fractions as
high as 60% can be achieved.Styrene emission can be reduced to a
minimum.Cost effective High volume process for large-scale
processing.
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Disadvantages of RTM
Mold design is complex and requires mold-filling analysis. Fiber
reinforcement may "wash" or move during resin transfer.
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Resin Transfer MouldingAdvantages:Low skill labour required Low
tooling costLow volatile emissionRequired design tailorability
Potential Problems:Control of resin flowKinking of fibres
Criticality in mould design
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RTM Products:
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Autoclave mouldingAutoclave Curing is a method in which a part,
molded by one of the open molding methods, is cured by a subsequent
application of vacuum, heat and inert gas pressure. The molded part
is first placed into a plastic bag, from which air is exhausted by
a vacuum pump. This operation removes air inclusions and volatile
products from the molded part. Then heat and inert gas pressure are
applied in the autoclave causing curing and densification of the
material. Autoclave Curing enables fabrication of consistent
homogeneous materials. The method is relatively expensive and is
used for manufacturing high quality aerospace products.
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Autoclave mouldingAn autoclave is a closed vessel (round or
cylindrical) in which processes occur under simultaneous
application of high temperature and pressure.
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Copyright Joseph Greene 2001*AutoclaveAn oven that allows for
high pressures to be used.Composites cure under heat and pressure
provides a superior part because the voids are reduced due to the
pressure.ProcessThe part is placed in the pressure vessel, and
heated, pressure is applied simultaneously. Vacuum bagging can be
used in an autoclave.Thermoset composites are
crosslinked.Thermoplastics are melted.AdvantagesThe pressure helps
bond composite layers, and remove more voids in the matrix.Very
large parts can be made with high fiber loadings.Properties are
improved.Many different parts can be cured at the same
time.DisadvantagesAutoclaves are expensive
Copyright Joseph Greene 2001
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a) Autoclave process to make a laminated compositeb) Prepregs of
different orientations stacked to form a laminated
compositea)b)Higher fiber volume fractions (60 65%) can be
obtained
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Autoclave process- CharcteristicsVery high quality
productGenerally prepregs are usedChopped fibres with resin can
also be usedHybrid composites can be producedHigh fibre volume
fractions can be obtainedsimultaneous application of high
temperature and pressure helps in, * Consolidating the laminate. *
Removing the entrapped air. * Curing the polymeric matrix.
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Autoclave Moluding
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Injection moulding Injection Molding is a Closed Mold process in
which molten polymer (commonly thermoplastic) mixed with very short
reinforcing fibers (10-40%) is forced under high pressure into a
mold cavity through an opening (sprue). Polymer-fiber mixture in
form of pellets is fed into an Injection Molding machine through a
hopper. The material is then conveyed forward by a feeding screw
and forced into a split mold, filling its cavity through a feeding
system with sprue gate and runners. Screw of injection molding
machine is called reciprocating screw since it not only rotates but
also moves forward and backward according to the steps of the
molding cycle. It acts as a ram in the filling step when the molten
polymer-fibers mixture is injected into the mold and then it
retracts backward in the molding step. Heating elements, placed
over the barrel, soften and melt the polymer. The mold is equipped
with a cooling system providing controlled cooling and
solidification of the material.
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Injection moulding The polymer is held in the mold until
solidification and then the mold opens and the part is removed from
the mold by ejector pins. Injection Molding is used mainly for
thermoplastic matrices, but thermosetting matrices are also may be
extruded. In this case curing (cross-linking) occurs during heating
and melting of the material in the heated barrel. A principal
scheme of an Injection Molding Machine is shown in the picture
below. Injection Molding is highly productive method providing high
accuracy and control of shape of the manufactured parts. The method
is profitable in mass production of large number of identical
parts. One of the disadvantages of the method is limited length of
fibers decreasing their reinforcing effect.
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Injection moulding
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Injection moulding machineThe injection molding machine
comprises of:The plasticating and injection unit: The major tasks
of the plasticating unit are to melt the polymer, to accumulate the
melt in the screw chamber, to inject the melt into the cavity and
to maintain the holding pressure during cooling.The clamping unit:
Its role is to open and close the mold, and hold the mold tightly
to avoid flash during the filling and holding. Clamping can be
mechanical or hydraulic.The mold cavity: The mold is the central
point in an injection molding machine. Each mold can contain
multiple cavities. It distributes polymer melt into and throughout
the cavities, shapes the part, cools the melt and ejects the
finished product.
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The Injection MoldThe mold consists Sprue and runner
systemGateMold cavityCooling system (for thermoplastics)Ejector
system
Features of injection moldingDirect path from molding compound
to finished productProcess can be fully automatedHigh productivity
& quality
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Injection molding machine
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Injection Molding Machine
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INJECTION MOLDINGThermoplastics : Polystyrene,PE, PP, ABC,
PC,PMMA etc
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Injection Molding CycleInjection molding involves two basic
steps:Melt generation by a rotating screwForward movement of the
screw to fill the mold with melt and to maintain the injected melt
under high pressure
Injection molding is a cyclic process:Injection: The polymer is
injected into the mold cavity. Hold on time: Once the cavity is
filled, a holding pressure is maintained to compensate for material
shrinkage. Cooling: The molding cools and solidifies. Screw-back:
At the same time, the screw retracts and turns, feeding the next
shot in towards the frontInjection molding is the most important
process used to manufacture plastic products. It is ideally suited
to manufacture mass produced parts of complex shapes requiring
precise dimensions.It is used for numerous products, ranging from
boat hulls and lawn chairs, to bottle cups. Car parts, TV and
computer housings are injection molded.
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Thermosets : Unsaturated polyester resin, Phenol formaldehyde
etc
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Reaction injection mouldingReaction injection moulding (RIM) -
Two reactive ingredients are pumped at high speeds and pressures
into a mixing head and injected into a mold cavity where curing and
solidification occur due to chemical reaction.
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Reinforced reaction injection moldingReinforced reaction
injection moulding (RRIM) - similar to RIM but includes reinforcing
fibers, typically glass fibers, in the mixture .
Advantages: similar to RIM (e.g., no heat energy required, lower
cost mold), with the added benefit of fiber reinforcement.
Products: auto body, truck cab applications for bumpers,
fenders, and other body parts
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Film stackingStack of laminate consists of fibers, impregnated
with insufficient thermoplastic matrix, and polymer films of
complementary weight to give the desired fiber volume fraction in
the end product. These are then consolidated by simultaneous
application of heat and pressure.Generally, a pressure of 6-12 MPa,
a temperature between 275 and 350 C, and dwell times of up to 30
mins are appropriate for thermoplastics such as polysulfones and
polyetheretherketone (PEEK).
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DIAPHRAGM FORMING This process involves the sandwiching of
freely floating thermoplastic prepreg layers between two diaphragms
.
The air between the diaphragms is evacuated and thermoplastic
laminate is heated above the melting point of the matrix.
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DIAPHRAGM FORMING
Pressure is applied to one side, which deforms the diaphragm and
makes them take the shape of the mold. The laminate layers are
freely floating and very flexible above the melting point of the
matrix, thus they readily conform to the mold shape.
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DIAPHRAGM FORMINGAfter the completion of the forming process,
the mold is cooled, the diaphragms are stripped off, and the
composite is obtained.The diaphragms are the key to the forming
process, and their stiffness is a very critical parameter.
For very complex shapes requiring high molding pressures, stiff
diaphragm are needed. At high pressures, a significant transverse
squeezing flow can result, and this can produce undesirable
thickness variations in the final composite.
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DIAPHRAGM FORMING ADVANTAGES:Components with double curvatures
can be formed. Compliant diaphragm do the job for simple
components.
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Thermoplastic tape laying (Automated Layup)In this method layers
of prepreg (reinforcing phase impregnated by liquid resin) tape are
applied on the mold surface by a tape application robot.Cost is
about half of hand lay-up.used for thermoset or thermoplastic
matrix.limited to flat or low curvature surfaces. Extensively used
for products such as airframe components, bodies of boats, truck
,tanks, swimming pools and ducts.
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Automated tapelaying machine (photo courtesy of Cincinnati
Milacron). Automated tapelaying machines operate by dispensing a
prepreg tape onto an open mold following a programmed path .Typical
machine consists of overhead gantry to which the dispensing head is
attached The gantry permits xyz travel of the head, for positioning
and following a defined continuous path.
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Interfacial bonding
Good bonding (adhesion) between matrix phase and dispersed phase
provides transfer of load, applied to the material to the dispersed
phase via the interface. Adhesion is necessary for achieving high
level of mechanical properties of the composite. There are three
forms of interface between the two phases: Direct bonding with no
intermediate layer. In this case adhesion (wetting) is provided by
either covalent bonding or van der Waals force.Intermediate layer
(inter-phase) is in form of solid solution of the matrix and
dispersed phases constituents.Intermediate layer is in form of a
third bonding phase (adhesive).
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The Interface
There is always an interface between constituent phases in a
composite material. For the composite to operate effectively, the
phases must bond where they join at the interface.
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Reinforcement-Matrix InterfaceThe load acting on the matrix has
to be transferred to the reinforcement via. Interface.The
reinforcement must be strongly bonded to the matrix if high
stiffness and strength are desired in the composite materialsA weak
interface results in low stiffness and strength but high resistance
to fracture.A strong interface produces high stiffness and strength
but often low resistance to fracture, i.e. brittle behavior
2 types of failure at interface
1) Adhesive failure - failure occur at interface2) Cohesive
failure failure occur close to the interface (either at the fiber
or matrix)
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Once the matrix has wet the reinforcement, bonding will
occur.For a given system, more than one bonding mechanism may exist
at the same time.The bondings may change during various production
stages or during services.
Interfacial bonding
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Types of interfacial bonding at interfaceMechanical
bondingPhysical bondingChemical bondingMechanical BondingIt is a
simple mechanical keying or interlocking effect between the
fiber-matrix phases.When the matrix shrinks radially on cooling
over the reinforcement leads to a griping action of the matrix on
the fiber.
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Physical BondingThese kind of bonding involves weak secondary or
vander waals forces, dipolar interactions and hydrogen bonds.These
type of bonding mechanism is of low significance because of its low
magnitude.The bond energy lies in the range of 8-16 kJ/mol.
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Chemical bondingDissolution Bonding: This bonding is of short
range and occurs at an electronic scale. This type of bonding is
hindered by the presence of impurities on the fiber surface and
also gas or air bubbles at the interface.
Reaction Bonding: This bonding is due to the transport of the
molecules, atoms or ions which diffuse to the interface.
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InterphaseIn some cases, a third ingredient must be added to
achieve bonding of primary and secondary phases Called an
interphase, this third ingredient can be thought of as an
adhesive
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Another Interphase
Interphase consisting of a solution of primary and secondary
phases
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APPLICATIONS OF PMCsPolymer composites are used to make very
light bicycles that are faster and easier to handle than standard
ones, fishing boats that are resistant to corrosive seawater and
lightweight turbine blades that generate wind power efficiently.
New commercial aircraft also contain more composites than their
predecessors. A 555-passenger plane recently built by Airbus, for
example, consists of 25 percent composite material, while Boeing is
designing a new jumbo aircraft that is planned to be more than half
polymer composites.
Polymer Matrix Composites (PMCs) are used for manufacturing:
secondary load-bearing aerospace structures, boat bodies, canoes,
kayaks, automotive parts, radio controlled vehicles, sport goods
(golf clubs, skis, tennis racquets), fishing rods, bullet-proof
vests and other armor parts, brake and clutch linings.
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