Polymer matrix composites

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
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
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 CH 3 H H CH 3 CH 3 H

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, Al 2 O 3 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  10 3 to 10 5

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.

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 ) ‏
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 T g.
Generally, amorphous thermoplastics are less chemically resistant.

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 T g , polymeric materials changes a hard solid to soft, tough ( leather like) solid. Over a temperature range around T g .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 T m .
Variation of Tensile modulus with temperature for Amorphous and Semi crytaline thermoplastic.

Thermoplastic polymer have higher strain-to-failure.

Types of Thermoplastics

COMPARISON OF THE THREE POLYMER CATEGORIES

Thermoplastics Vs Thermosets

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 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.

Hand layup products:

SPRAYUP 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 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
Prepreg is the composite industry’s 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.

PREPREG PROCESS
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.
The final prepreg product is a thin tape consisting of continuous and aligned fibers embedded in a partially cured resin.

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%

PREPREG
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.
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 .

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.

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.

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

Copyright Joseph Greene 2001

Filament Winding

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
The cost is about half that of tape laying
Productivity 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.

Filament winding

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:
g

Kevlar component

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

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
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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.

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

RTM Products:

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.

Autoclave Mol u ding

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

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 machine

Injection Molding Machine

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.

Polymer matrix composites

by Ramesh Balasubramanian

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Polymer matrix composites Presentation Transcript