Mod 3 PMCs.ppt notes for composite materials

Polymer matrix
composites (PMC)

Polymer matrix composites (PMC)


Different molecular chain configurations: a linear,
b branched, c crosslinked, d ladder

 Thermosetting resins include polyesters, vinylesters,
epoxies and polyamides.
 Thermosetting polyesters are commonly used in fiber-
reinforced plastics, and epoxies make up most of the
current market for advanced composites resins.
 Initially, the viscosity of these resins is low; however,
thermoset resins undergo chemical reactions that crosslink
the polymer chains and thus connect the entire matrix
together in a three-dimensional network. This process is
called curing.
 Thermosets tend to have high dimensional stability, high-
temperature resistance, and good resistance to solvents.

 Thermoplastic resins, sometimes called engineering
plastics, include some polyesters, polyetherimide, polyamide
imide, polyphenylene sulfide, polyether-etherketone (PEEK),
and liquid crystal polymers.
 They consist of long, discrete molecules that melt to a
viscous liquid at the processing temperature, typically 500” to
700” F (260° to 3710 C), and, after forming, are cooled to an
amorphous, semicrystalline, or crystalline solid.
 Unlike the curing process of thermosetting resins, the
processing of thermoplastics is reversible, and, by simply
reheating to the process temperature, the resin can be formed
into another shape if desired.
 Thermoplastics, although generally inferior to thermosets in
high-temperature strength and chemical stability, are more
resistant to cracking and impact damage.

What are Elastomers Examples?
Natural rubber: These are used in the automotive
industry and in the manufacture of medical tubes,
balloons, adhesives.
Polyurethanes: These are used in the textile industry for
manufacturing elastic clothing like lycra.
Polybutadiene: These are used for providing wear
resistance in wheels of vehicles.
Silicone: These are used in the manufacture of medical
prostheses and lubricants as they have excellent chemical
and thermal resistance.
Neoprene: These are used in the manufacture of wet-suits
and in industrial belts.

Properties of Elastomers
Temperature: The specific working temperature of elastomers
vary depending on the factors like media compatibility, seal
design, and dynamic and static operation.
Low-temperature flexibility: The rate of recovery of
elastomeric material can be studied by subjecting the material
to low-temperature retraction.
Hardness: It differs from material to material. The soft
compounds deform easily and have high friction, while the
harder compounds have high resistance and low friction.
Ageing: If the elastomers are pushed beyond their ageing
resistance, they will suffer from hardening, cracking, and
splitting.
Elongation at break: This property is used for testing the
moment of rupture when the material is under tensile stress.

Hand Layup and Spray Techniques
 Hand layup and spray techniques are perhaps the
simplest polymer processing techniques.
Hand lay-up molding is the method of laying down
fabrics made of reinforcement and painting with the
matrix resin layer by layer until the desired thickness is
obtained.
This is the most time and labour consuming,
composite processing method but majority of aerospace
composite products are made by this method in
combination with the autoclave method

Hand Layup and Spray Techniques
 Hand lay-up is the oldest and simplest method used for
producing reinforced plastic laminates.
 Capital investment for the hand lay-up processes is
relatively low. The most expensive piece of equipment
typically is a spray gun for resin and gel coat application.
 Some fabricators pour or brush the resin into the molds
so that a spray gun is not required for this step.
 There is virtually no limit to the size of the part that
can be made.
The molds can be made of wood, sheet metal, plaster,
and FRP composites.

Spray-up Molding
 Spray-up molding is much less labour intensive than the
hand lay-up method by utilizing a spray gun and a fiber
cutter.
 However, only short fiber reinforced composites can be
made. A continuous fiber is fed into the cutter and
chopped.
 The chopped fiber is sprayed upon a mold with the
stream of resin mist and catalyst delivered through
separate nozzles.
 The sprayed mixture of fiber and resin soon cures on the
mold at room temperature and the product is produced.
 Because of the spraying operation, large and complex-
shaped objects can be easily made.

Spray-up Molding
Advantages:
Continuous process
Any materials can be used as mold.
Error can be corrected by re-spraying.
Disadvantages:
Slow.
inconsistency.
No control of fiber orientation.
Only one side finished.
Environmental unfriendly.

A prepreg (short for preimpregnated) is a composite that
comes with the resin already added to the reinforcement
This means that the only concern when working with prepreg
is shaping the part
 Since the resin is already mixed (resin and catalyst) there is
a limited shelf life
For the same reason prepreg must be cured in an oven or
autoclave

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

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
assure thorough
impregnation of the fibers.

The final prepreg product is a thin tape consisting of continuous
and aligned fibers embedded in a partially cured resin
 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%
Advantages:
 orientation of fibers can be changed
 consistent
 high productivity
Disadvantages:
 continuous process needs work
 limited shelf life
 delamination

 Similar to extrusion of metal parts
 Pultrusion involves pulling resin-impregnated glass strands
through a die
 Standard extruded shapes can easily be produced such as
pipes, channels, I-beams, etc.

a Schematic of the pultrusion process.
b A helicopter windshield post made of carbon fibers/vinyl ester
resin by pultrusion. The post is 1.5 m long.

Continuous fiber tows come from various creels. Mat or biaxial
fabric may be added to these to provide some transverse strength.
These are passed through a resin bath containing a catalyst. After
this, the resin impregnated fibers pass through a series of wipers to
remove any excess polymer and then through a collimator before
entering the heated die.
A thorough wet out of the rovings is very important. Stripped
excess resin is recirculated to the resin bath.
The heated die has the shape of the finished component to be
produced.
The resin is cured in the die and the composite is pulled out. At the
end of the line, the part is cut by a flying saw to a fixed length.
Typically, the process can produce continuously at a rate of 10– 200
cm/min

Advantages:
Automated processes.
High speed.
Versatile cross-sectional
shape.
Continuous reinforcement.
Disadvantages:
Die can be easily messed up.
Expensive die.
Mainly thermoset matrix

 A continuous reinforcement, either previously impregnated or
impregnated during winding is wound around a rotating
mandrel to form a composite part
 Pros: fast lay-up speed, very accurate and repeatable product,
possibility to use continuous fiber, parts can have huge size
 Cons: expensive equipment, high cost for mandrel, poor
surface finish, shape of the products limited (only cylindrical
possible), curing by heat is not easy to apply, spinning speed is
limited due to resin penetration and splashing, traveler speed
and yarn breakage.
 Examples: oxygen bottles for firemen, rocket motors, tennis
rackets, shafts

Filament winding is another very versatile technique in which a
continuous tow or roving is passed through a resin impregnation
bath and wound over a rotating or stationary mandrel.
A roving consists of thousands of individual filaments. Figure
5.2a shows a schematic of this process, while Fig. shows a
pressure vessel made by filament winding.
Successive layers are laid on at a constant or varying angle until
the desired thickness is attained. Curing of the thermosetting resin
is done at an elevated temperature and the mandrel is removed.
Very large cylindrical (e.g., pipes) and spherical (e.g., for
chemical storage) vessels are built by filament winding. Glass,
carbon, and aramid fibers are routinely used with epoxy,
polyester, and vinyl ester resins for producing filament wound
shapes.

There are two types of filament winding processes: wet
winding and prepreg winding.
In wet winding, a low viscosity resin is applied to the filaments
during the winding process. Polyesters and epoxies with
viscosity less than 2 Poise (2000 cP) are used in wet winding.
In prepreg winding, a hot-melt or solvent-dip process is used to
preimpregnate the fibers. Rigid amines, novolacs, polyimides,
and higher viscosity epoxies are generally used for this process.
In filament winding, the most probable void sites are roving
crossovers and regions between layers with different fiber
orientations.

PMC fabrication
 Thermoforming is a process of shaping
flat thermoplastic sheet which includes two stages: softening
the sheet by heating, followed by forming it in the mold
cavity.
Elastomers and Thermosets can not be formed by the
Thermoforming methods because of their cross-
linked structure – they do not soften when heated.
Thermoforming is widely used in the food packaging
industry for manufacturing ice cream and margarine tubs,
meat trays microwave containers, snack tubs sandwich packs
etc.
Thermoforming is also used for manufacturing some
pharmaceutical and electronic articles, small tools, fasteners,
toys, boat hulls, blister and skin packs.

PMC fabrication
There are three thermoforming methods, differing in the
technique used for the forming stage:
Vacuum Thermoforming
Pressure Thermoforming
Mechanical Thermoforming
The process involves shaping a preheated thermoplastic sheet by
means of vacuum produced in the mold cavity space.
The atmospheric pressure forces the soft sheet to deform in
conformity with the cavity shape.

PMC fabrication

PMC fabrication
Pressure Thermoforming

PMC fabrication
Mechanical Thermoforming

PMC fabrication
 Extrusion is a process of manufacturing long products of constant
cross-section (rods, sheets, pipes, films, wire insulation coating)
forcing soften polymer through a die with an opening.
 Polymer material in form of pellets is fed into an extruder through a
hopper. The material is then conveyed forward by a feeding screw
and forced through a die, converting to continuous polymer product.
 Heating elements, placed over the barrel, soften and melt the
polymer. The temperature of the material is controlled by
thermocouples.
 The product going out of the die is cooled by blown air or in water
bath.
 Extrusion is used mainly for Thermoplastics,
but Elastomers and Thermosets are also may be extruded. In this
case cross-linking forms during heating and melting of the material
in the extruder.

PMC fabrication
Blow Molding is a process in which a heated
hollow thermoplastic tube (parison) is inflated into a closed mold
conforming the shape of the mold cavity.
The production cycle consists of the following steps:
 The parison is extruded vertically in downward direction
between two mold halves.
When the parison reaches the required length the two mold
halves close resulting in pinching the top of parison end and
sealing the blow pin in the bottom of the parison end.
Parison is inflated by air blown through the blow pin, taking a
shape conforming that of the mold cavity. The parison is then cut
on the top.
The mold cools down, its halves open, and the final part is
removed.

PMC fabrication
Compression Molding is a process in which a molding polymer is
squeezed into a preheated mold taking a shape of the mold cavity
and performing curing due to heat and pressure applied to the
material.
The method is used mostly for molding thermosetting resins
(thermosets), but some thermoplastic parts may also be produced by
Compression Molding.
Compression Molding process involves the following steps:
A pre-weighed amount of a polymer mixed with additives and fillers
(charge) is placed into the lower half of the mold.
The charge may be in form of powders, pellets, putty-like masses or
pre-formed blanks.
The charge is usually preheated prior to placement into the mold.
Preheated polymer becomes softer resulting in shortening the molding
cycle time.
The upper half of the mold moves downwards, pressing on the
polymer charge and forcing it to fill the mold cavity.

PMC fabrication
 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.

PMC fabrication
 A thin uniform layer of the powder is spread over the surface of the build
chamber. The layer thickness is commonly less than 0.004” (0.1 mm).
 A high power laser beam scan over the layer surface according to
Computer-Aided Design (CAD) model of the fabricated part. Scanning is
performed by a mirror of the beam deflection system.
 The energy of the laser is focused in a small spot of the layer surface
heating the powder to the sintering temperature (a temperature below the
melting point but close to it). The powder particles of the layer bind to each
other and to the preceding layer due to diffusion of the material.The
powder of the layer lying beyond the scanned area remains unsintered. It
supports the sintered object in the build chamber.
 The operation of powder spreading followed by laser sintering is repeated
until the object building is completed.
 The sintered part is left in the chamber to cool down.
 The sintered model is taken out from the chamber. The excess powder is
easily removed by compressed air blasting.

Mod 3 PMCs.ppt notes for composite materials

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