polymermatrixcomposites-

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.

Classification based on Matrices
Composite
materials
Matrices
Polymer Matrix
Composites (PMC)
Metal Matrix
Composites MMC)
Ceramic Matrix
Composites (CMC)
Thermoset Thermoplastic Rubber

What is a polymer?
Poly mer
many repeat unit
C C C C C C
H
H
H
H
H
H
H
H
H
H
H
H
Polyethylene (PE)
Cl
Cl Cl
C C C C C C
H
H
H
H
H
H
H
H
H
Polyvinyl chloride (PVC)
H
H
H
H
H H
Polypropylene (PP)
C C C C C C
CH3
H
H
CH3
CH3 H
repeat
unit
repeat
unit
repeat
unit
Examples of polymers:
A polymer is a large molecule (macromolecule) composed of repeating
structural units typically connected by covalent chemical bonds

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.

Discontinuous phase - Reinforcement
Continuous phase - Matrix
Polymer(Matrix) Composite (Matrix + Reinforcement)
• Reinforcements
– Principal load bearing member.
• Matrix
– provides 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.

Classification of Polymers
 Linear 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%.

Molecular chain
configurations:
a. Linear
b. Branched
c. Crossed linked
d. Ladder

• 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 E glass) 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

Polymerisation:
This is the process of joining monomers into gaint chain like molecules.
Methods of Polymerisation:
• Condensation polymerisation
• Addition polymerisation
Degree of polymerization = No of monomer units in a chain
≈ 103
to 105

Thermosets
• Thermoset 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.

• 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.

Thermosets
• Extensive cross-linking formed by covalent
bonds.
• Bonds prevent chains moving relative to
each other.

Types of Thermosetting plastics

Epoxy:
Epoxy is a polymer that contain an epoxide group in its chemical structure.
Example: DGEBA (Diglcidyl Ether of Bisphenol A )
E
Charecteristics of Epoxy:
• Better Moisture Resistence
• Low shrinkage
• Good adhersion with Reinforcement

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:
• Cheap
• Resistance to variety of chemicals
• Adequate moisture resistance

Thermosetting plastics - applications

Thermoplastics
• In 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.

• 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.

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.

Reasons for the use of thermoplastic matrix composites
• Refrigeration 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.

UNIQUE CHARACTERISTIC OF
THERMOPLASTIC
• Near 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.

• Thermoplastic polymer have higher strain-to-
failure.

COMPARISON OF THE THREE POLYMER CATEGORIES

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.

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.

• 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).

Effect of Temperature on Thermoplastics
 Degradation
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.

Stress-strain behavior of different polymer
matrices
0
10
20
30
40
50
60
70
80
90
0 1 2 3 4 5
Strian(%)
Stress
(Mpa)
Phenolic
Polyester Epoxy
0
10
20
30
40
50
60
70
80
0 100 200 300 400 500
Strian(%)
Stress
(Mpa)
Polysulfon
Polyamid
Polyethylene
Thermoplastic polymers Thermosetting polymers
Notice to the range of ultimate strains of different polymers

Comparision of various polymers as matrix materials

Limitations of PMC
– Low maximum working temperature.
– High coefficient of thermal expansion- dimensional
instability
– Sensitivity to radiation and moisture.
– Processing temperature are generally higher than those
with thermosets.

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.

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.

Hand Layup
• The 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).

Hand Layup
Advantages:
 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.

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

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

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

Hand and Spray Layup
• In 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.

Advantages of Hand Layup and Sprayup
• Tooling 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.

Disadvantages of Hand Layup and Sprayup
• Labor Intensive.
• Low volume process.
• Longer curing times.
• Production uniformity is difficult.
• Waste factor is high.

PREPREG
۰ Prepreg is the composite
industry’s term for continuous
fiber reinforcement .Pre-
impregnated with a polymer
resin that is only partially
cured.
۰ Prepreg is delivered in tape
form to the manufacturer who
then molds and fully cures the
product without having to add
any resin.
۰ This is the composite form
most widely used for structural
applications.

۰ Manufacturing begins by
collimating a series of
spool-wound continuous
fiber tows.
۰ Tows are then
sandwiched and pressed
between sheets of release
and carrier paper using
heated rollers
(calendering).
۰ 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 PROCESS
۰The final prepreg product is a thin tape consisting of continuous and aligned fibers
embedded in a partially cured resin.

PREPREG
۰ Prepared for packaging by winding onto a cardboard core.
۰ Typical tape thicknesses range between 0.08 and 0.25 mm
۰ Tape widths range between 25 and 1525 mm.
۰ Resin content lies between about 35 and 45 vol%

PREPREG
۰ The prepreg is stored at 0°C (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.
۰ Both thermoplastic and thermosetting resins are utilized: carbon,
glass, and aramid fibers are the common reinforcements.
۰ Actual fabrication begins with the lay-up. Normally a number of
plies are laid up to provide the desired thickness.
۰ The layup can be by hand or automated.
۰
Easily obtained with epoxies.

50
Filament Winding
Filament 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.

51
Filament Winding
• Filament Winding Process
– For Round or Cylindrical parts
– A 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 20’
– Winding direction
• Hoop/helical layers
• Layers of different material
– High strengths are possible due to
winding designs in various direction
– Winding speeds are typically 100
m/min and typical winding tensions
are 0.1 to 0.5 kg.

Copyright Joseph Greene 2001 52
Filament Winding
• Demolding
– To 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.
• Advantages
– Good 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 liners
– Reinforcement 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

Filament winding - applications
• pressure vessels, storage tanks and pipes
• rocket motors, launch tubes
– Light 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 shafts
• Entec “the world’s largest five-axis filament winding machine” for wind
turbine blades
– length 45.7 m, diameter 8.2 m, weight > 36 tonnes.

FILAMENT WINDING CHARACTERISTICS
۰The cost is about half that of tape laying
۰Productivity is high (50 kg/h).
۰Applications 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.

Filament winding - winding patterns
• hoop (90º) - girth or circumferential
winding
– angle is normally just below 90°
degrees
– each complete rotation of the
mandrel shifts the fibre band to
lie alongside the previous band.
• helical
– complete fibre coverage without
the band having to lie adjacent
to that previously laid.
• polar
– domed ends or spherical
components
– fibres constrained by bosses on
each pole of the component.
• axial (0º)
– beware: difficult to maintain
fibre tension

Filament winding patterns
hoop : helical:
polar:

Filament wound pressure
bottles for gas storage

Pultrusion
Description:
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.

Pultrusion
• Pultrusion 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:

Pultrusion
• Pultrusion 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.

Pultrusion
• Pultrusion 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.

Pultrusion
• Manufacturing
– Fibers 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.

production of constant cross-section profiles

69
Pultrusion
• Design
· 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.

· 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")

• Materials
– Most fibers are used (carbon, glass, aramids) and Resins must be fast curing
because of process speeds. (polyester and epoxy)
• Processing
– speeds 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.
• Advantages
– good material usage compared to layup
– high throughput and higher resin contents are possible
• Disadvantages
– part 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

Pultrusion
Advantages:
 Minimal kinking of
fibres/fabrics
 Rapid processing
 Low material scrap rate
 Good quality control
Potential Problems:
 Improper fibre wet-out
 Fibre breakage
 Inadequate cure
 Die jamming
 Complex die design

Pultrusion -characteristics
• seek 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 profile
• normally thermoset (thermoplastic possible)
– impregnate with resin
– pull through a heated die
• resin shrinkage reduces friction in the die
• polyester easier to process than epoxy
• tension control as in filament winding
• post-die, profile air-cooled before gripped
– hand-over-hand hydraulic clamps
– conveyor belt/caterpillar track systems.
• moving cut-off machine ("flying cutter"). The solid laminate will be cut to the
desired length
• Inside the metal die, precise temperature control activates the curing of the
thermoset resin.

• 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)

Pultrusion -applications
• panels – beams – gratings – ladders
• tool handles - ski poles – kites
• electrical insulators and enclosures
• light poles - hand rails – roll-up doors
• 450 km of cable trays in the Channel Tunnel

Pultrusion Applications
• Advanced Composite Construction System
– components: plank ............... and connectors
– used in Aberfeldy and Bonds Mill Lock bridges
– http://

Resin Transfer Molding
• In 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 injected
• The part is cured in mold.
• The mold is opened and part is removed from mold.

Resin Transfer Moulding
Close 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.

RTM
• Transfer 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.

RTM
Transfer 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.

Advantages of RTM
• Large 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.

Disadvantages of RTM
• Mold design is complex and requires mold-filling analysis.
• Fiber reinforcement may "wash" or move during resin transfer.

Resin Transfer Moulding
Advantages:
 Low skill labour required
 Low tooling cost
 Low volatile emission
 Required design tailorability
Potential Problems:
 Control of resin flow
 Kinking of fibres
 Criticality in mould design

Autoclave moulding
• Autoclave 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.

Autoclave moulding
• An autoclave is a closed vessel (round or cylindrical) in which
processes occur under simultaneous application of high
temperature and pressure.

Autoclave
• An 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.
• Process
– The 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.
• Advantages
– The 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.
• Disadvantages
– Autoclaves are expensive

a) Autoclave process to make a laminated composite
b) Prepregs of different orientations stacked to form a laminated composite
a)
b)
Higher fiber volume fractions (60 – 65%) can be obtained

Autoclave process- Charcteristics
• Very high quality product
• Generally prepregs are used
• Chopped fibres with resin can also be used
• Hybrid composites can be produced
• High fibre volume fractions can be obtained
• simultaneous application of high temperature and pressure helps in,
* Consolidating the laminate.
* Removing the entrapped air.
* Curing the polymeric matrix.

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.

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.

Injection moulding machine
The 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: It’s 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.

The Injection Mold
The mold consists
• Sprue and runner
system
• Gate
• Mold cavity
• Cooling system
(for thermoplastics)
(
• Ejector system
Features of injection molding
Direct path from molding compound to finished product
Process can be fully automated
High productivity & quality

INJECTION MOLDING
Thermoplastics : Polystyrene,PE, PP, ABC, PC,PMMA etc

Injection Molding Cycle
Injection molding involves two basic steps:
– Melt generation by a rotating screw
– Forward 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 front
 Injection 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.

Thermosets : Unsaturated polyester resin, Phenol formaldehyde etc

Reaction injection moulding
• Reaction 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.

Reinforced reaction injection molding
Reinforced 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

Film stacking
• Stack 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
350º C, and dwell times of up to 30 mins are appropriate for
thermoplastics such as polysulfones and polyetheretherketone
(PEEK).

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.

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.

DIAPHRAGM FORMING
• After 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.

DIAPHRAGM FORMING
ADVANTAGES:
• Components with double curvatures can be formed.
• Compliant diaphragm do the job for simple components.

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.

Automated tape laying machine (photo courtesy of Cincinnati Milacron).
‑
Automated tape laying 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 x y z travel of the head, for positioning and following a defined
‑ ‑
continuous path.

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).

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.

Reinforcement-Matrix Interface
• The 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 materials
• A 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 interface
2) Cohesive failure – failure occur close to the interface (either at the fiber or
matrix)

• 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

Types of interfacial bonding at interface
• Mechanical bonding
• Physical bonding
• Chemical bonding
Mechanical Bonding
• It 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.

Physical Bonding
• These 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.

Chemical bonding
• Dissolution 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.

Interphase
• In 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

Another Interphase
Interphase consisting of a solution of primary and
secondary phases

APPLICATIONS OF PMCs
• Polymer 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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