Polymers & Composites Flashcards
(80 cards)
1
Q
3 criteria for material selection
A
material, function, process
2
Q
What’s an Ashby diagram?
A
For material selection. Young’s modulus against density
3
Q
What are polymers?
A
Long chain of organic molecules e.g. C, H, O, S, N
Most have 1 long dimension (backbone chain) & 2 smaller dimensions
High molecular weight 10^4-10^6 (inorganics 10^2)
Low density & cheap
4
Q
What is a plastic?
A
Any synthetic organic solid that’s mouldable e.g. polymers
Plastic = polymer(s) + additive(s)
UK plastics industry 2% GDP, £17.5 billion
38% packaging, 22% construction, 6% electrical, 7% fun/house, 7% transport, 2% medical, 18% others
5
Q
How to synthesise addition polymers?
A
Chain growth
AA + A = AAA
Always involves a monomer which is added to polymer backbone, requires catalyst
6
Q
How to synthesise condensation polymers?
A
Step growth
A + B = AB
Reactions condense out a small molecule, often water
Monomers react with each other to form dimers which react with dimers/monomers to form oligomers
Reaction involves any molecule (monomer, dimer, oligomer, polymer)
7
Q
What are thermoplastics & what does thermoplastic processing do?
A
Melts, recyclable
Linear, branched, few crosslinks
Moulds pre-synthesised polymers
Simple but requires someone else to synthesise
- heat, homogenise, force into mould, cool
8
Q
What are thermosets & what does thermoset processing do?
A
Don’t melt but degrade at high T
Highly crosslinked, 3D network
Uses monomers & synthesises polymer during moulding
More complicated but lower viscosity so ideal for composites
- heat resin till pourable, mix with curing agent & additives, cast into mould, cure (cure schedule - controlling heat & time)
9
Q
Chemical classification
A
- Structure - homopolymer, copolymer, linear, branched, crosslinked, 3D network
- Tacticity - if backbone has side groups (isotactic has higher crystallinity than atactic)
- Elastomers - reversible crosslink network e.g. natural rubber (polyisoprene), nitrile rubber (copolymer of butadiene & acrylonitrile)
- thermoset/thermoplastic
10
Q
Physical classification
A
- crystallinity - amorphous (non-crystalline), semi-crystalline (never 100% crystalline due to chain end imperfections), dependent on processing (slow cool leads to higher crystallinity as more time for molecules to organise) & structure (large side groups lead to lower crystallinity)
- primary phase transition (solid/liquid)
- secondary phase transition (glassy solid/rubber solid)
11
Q
Common thermoplastic polymers
A
1) Polyethylene Terphthalate (PET)
2) High Density Polyethylene (HDPE)
3) Polyvinyl Chloride (PVC)
4) LDPE
5) Polypropylene (PP)
6) Polystyrene, PS
7) Others (polyamides, polycarbonates, ABS, polyurethanes, PTFE, PMMA)
1-6 everywhere & cheap, 90% by volume of world plastic consumption
12
Q
Polymer bonding
A
Responsible for thermo-mechanical properties
Stiff, strong covalent bonds (350 kJ/mol)
Compliant, weaker van der Waal interactions (3-10 kJ/mol), hydrogen bonding requires specific chemistry & ion-ion interactions are rare
13
Q
Polymer processing, strength order & considerations
A
Polymer melts are shear thinning (pseudoplastic) as random alignments become more aligned
Apply force > flow (T, pressure, strain)
Plastics stiffer & stronger in flow direction
Strength order: LDPE, HDPE, PP, ABS, PA6, PMMA
Mechanical, thermal, environmental, electrical, appearance, hazards, manufacture, economics
Thermoplastic polyurethanes (TPU) has wide range of properties & applicaitons
Poly(methyl methacrylate) (PMMA) similar to polycarbonate
Poly(tetrafuloroethylene) (PTFE) - very low coefficient of friction so non-stick
14
Q
Industrial thermoplastics
A
- Commodity - PE, HDPE, LDPE, PS, PP, PVC
- Engineering - polyesters (PET), PA, ABS, PC
- Specialty - PUR, PTFE, PMMA
15
Q
PE
A
Polyethylene, 2. HDPE, 4. LDPE
- HDPE - UTS 15 MPa, E = 1 GPa (linear, 55-75% crystalline), semi-rigid, moisture/chemical resistance, gas impermeable, packaging, containers, films, pipes
- LDPE - UTS 5 MPa, E = 0.3 GPa (branched, 30-55% crystalline, flexible, moisture/chemical resistance, gas impermeable, food bags, squeezy bottles, films)
- Ultra-high molecular weight PE, 3500 MPa, 130 GPa (highly aligned molecules, 95% crystalline, very high specific strength, fishing lines, chopping boards, ballistic armour)
- linear low density, crosslinked
16
Q
PP
A
5., stronger & stiffer than PE, 25 MPa, 2 GPa
Side group methyl increases stiffness
Opaque, cheap, good melting point, chemical & moisture resistant
Brittle at rtp, adding impact modifiers e.g. rubber particles for toughness but lower stiffness
Improve stiffness with glass fibre but costly & reduces ductility
Commonly isotactic, 40-70% crystalline
Packaging, containers, labelling, pipes, textiles & fibres, lab equipment, automotive parts, reusable items
17
Q
PS
A
6., stronger & stiffer than PE & PP, 30 MPa, 2.5 GPa
Transparent, thermally stable, chemically resistant, cheap
Commonly atactic, amorphous
Brittle, add impact modifiers for high impact PS or co-polymerise for ABS
Rigid packaging, household applications, disposable cups, yoghurt pots, medical devices
Expanded PS
18
Q
PVC
A
- Stronger & stiffer than PE & PP, 30 MPa, 2.5 GPa
Side group chlorine reduces crystallinity so amorphous
Commonly atactic, cheap
Less commonly injection moulded as it degrades at 200 releasing HCl gas (removed with thermal stabilisers)
Rigid so plasticised to improve flexibility
Unplasticised PVC - window & door frames, pipes, gutters, credit cards
Plasticised PVC - wire coatings, shoes, inflatables, bouncy balls
19
Q
Polyesters
A
- PET most common
Excellent strength & stiffness, 80 MPa, 4 GPa
Thermally stable, gas & liquid impermeable
Semi-crystalline/amorphous
Drink bottles, oil/detergent bottles, packagin films, automotive parts
Backbone phenyl ring imparts very good mechanical properties
20
Q
Polyamides (nylons)
A
Nylon 6, Nylon 6.6
Stronger & stiffer than PE & PP
Thermally stable, tough, wear resistant, semi-crystalline
Gears, bearings, textiles, fibres, packaging, films, coatings, automotive parts
Extensive intermolecular hydrogen bonding between amid groups leading to good properties
21
Q
Polycarbonates & ABS
A
- Bisphenol-A polycarbonate, stronger & stiffer than PE & PP, tough, thermally stable, transparent, health concerns (safety glass, CDs, windows, medical device, large water bottles)
- ABS - co-polymer adds toughness via butadiene groups to brittle polystyrene (lego, car dashboards, keyboards)
22
Q
Thermosetting polymers
A
- unsaturated polyesters are common matrix for glass fibre composites
- epoxy resins are common matrix for carbon fibre composites
- thermosetting polyurethanes have wide properties & applications, often foamed
- phenolic resins (Bakelite) have low flammability
- Bismaleimides (BMI) & cyanate esters are high T processing & applications, expensive
23
Q
Thermoplastic recycling
A
- Primary - commercial scrap - regrind, heat & reuse, relatively efficient
- Secondary - post consumer - sort by hand/automatic systems monitor chemical signatures, shred, separate by density/induced charges e.g. PET negative & PP positive, inefficient
- Tertiary - controlled degradation, depolymerise into useful products e.g. fuels, oils, gases
- Quaternary - controlled degradation, burn to generate electricity
24
Q
Themoset recycling
A
ground/chipped & used as a filler e.g. tyers
25
Define polymer, polymer blend, mixture & plastic
- pure system
- mixture of 2/more pure polymers
- pure polymer + anything else
- mixture of polymer(s) & additive(s)
26
Why are plastics not economically viable to recycle?
Mixtures of polymers, additives & other materials
27
Mechanical additives
- fillers increase volume at low cost but add weight e.g. chalk (calcium carbonate for stiffness, whiteness), kaolin for coupling, talc (magnesium hydroxide/silica for stiffness, impact strength)
- reinforcements short (E-glass) or long fibres (e-glass, carbon, Kevlar) but affects processability
28
Appearance additives
- colorants with dyes (soluble, compatible with polymer) or pigments (insoluble, incompatible with polymer) e.g. metal oxide pigment powders, anthraquinone yellow dye
- black (carbon black) & white (TiO2)
- special effects - Al for metallic lustre, Mica coated with TiO2 for pearlescent, fluorescein
29
Heat & light additives
- heat stabilisers/anti-oxidants e.g. dibutyl tin oxide in PVC to prevent HCl formation (radical scavenger), hindered poly-phenols (in fruit & veg)
- light/UV stabilisers - block (e.g. carbon black, chalk, TiO2), retransmit e.g. benzophenone
30
Flame & electrical additives
- flame retardants e.g. halogenated compounds (Cl, Br phased out for environmental & health reasons), aluminium trihydroxide
- anti-static - provide conductive path away from surface e.g. gycerol monostearate, carbon black
31
Processing additives
- curing & crosslinking - react monomers to form polymers/link polymer chains e.g. dicumyl peroxide
- coupling/compatibilising - surface modification of fibres/fillers for better adhesion e.g. organo-silanes for treatment of glass fibres, stearic acid which is amphiphilic (both hydrophobic & hydrophilic)
- plasticising - enhance flexibility e.g. dioctyle phthalate with PVC, benzoate esters (safer)
- blowing agents - cellular foam within polymer e.g. pentane, azdicarbonamide, expanded polystyrene used CFCs but now CO2
32
Toughening & anti-bacterial & biocides additives
- absorb more energy e.g. rubber, thermoplastic particles, copolymerise with tough polymer ABS
- resistance to attack by micro-organisms e.g. organic copper compounds
33
Polymer blending
- Miscible polymer blends are rare - like dissolves like, controlled by Gibbs free energy of mixing & viscosity (negative change in Gibbs free energy = enthalpy change - T*entropy of mixing) where enthalpy term dominates, has 1 glass transition temperature
- Immiscible blends are phase separated & more common with 2 glass transition temperatures
34
Polymer-additive mixtures process
- dispersive & distributive mixing achieved with high shear forces & stretching/dividing/reorienting mixture
- compounding done with batch mixers & continuous mixers to form homgeneous bled/concentrate/masterbatch
- feed, mix, filter (screens before die remove contaminants & control flow), pellets (strands solidify in water bath)
- continuous mixing uses single (relies on good coefficient of friction between extruder & polymers with open channel back up length of screw)/twin screw extruders > heat, high shear, mix
35
Extrusion (stages, special points, screw types, dies)
- most thermoplastics, relatively efficient, polymer orientation
- rotating Archimedes tool steel screw
- feed, melting & compression (channel depth decreases), metering
- breaker plate reduces spiral motion & screen pack filters out contaminants. Both control flow into die
- venting section allow evolved gases to escape
- PVC type screw is gradual for amorphous polymers which progressively soften through a glass transition
- Nylon type screw is sudden increase in thickness for semi-crystalline polymers with sharp melting point
- die should maintain uniform velocity with minimal pressure drop
- die swell caused by velocity unification, viscoelastic relaxation > eliminate with non-orthoganal aperture/longer/tapered die
- residual stress eliminated using constant wall thickness
- can co-extrude with reinforcement/lining e.g. gardn hosepipe
- guttering, skirting, conduit, cladding, window & door frames, deals, gaskets, strips, rods & bars
36
Film blowing/lay-flat
1) plastic forced through ring/annular die
2) tube pulled vertically by nip rolls
3) air inflates tube into a bubble
4) frost line is where geometry has stabilised & polymer started to solidify into cylindrical films
- plastic bags, packaging, slit into continuous film
- Polymer must have good melt strength e.g. branched PE
Polymer orientation mostly biaxial (extrude then inflate)
37
Film & sheet extrusion
1) plastic forced through slot die onto chill roll (cools & draws molten plastic)
- makes films & sheets (> 0.5mm)
- coathanger die (polymer melt forced to spread out along manifold, up onto land & out through lip)
- multi layer films use extra extruders e.g. crisp packets
- polymer orientation uniaxial (poorer quality but cheaper than film blown)
38
Film & sheet extrusion coating & 2 e.g.s
- extruded film coated onto non-polymeric substrate e.g. paper, card, textiles, metals
E.g. 1 - layer of LDPE coated onto thin card to make milk cartons - prevent leakage, adhesive layer
E.g. 2 - layer of LDPE is extrusion coated onto foil to make yoghurt pot lids - adhesive layer
39
Wire & cable extrusion/over-jacketing
- Insulating coating using tubular die
- LDPE or plasticised PVC
- high T applications use cross-linked PE or polyamides
40
Fibre spinning
- many fibres (d<0.1 mm) extruded through spinneret die & drawn using godet rolls to stretch
- polyesters, polyamides, polypropylene
- woven into fabrics, clothing, soft furnishings, tyres, composites
- drawing stage critical to properties - unidirectional orientation
- monofilaments (d>0.1 mm) made same way for bristles, fishing lines, racket strings, sewing thread, rope, nets, carpets
41
Non-woven fabrics
- Spinneret>fibres fall randomly onto mat > embossing roll
- cheaper as no weaving
- not drawn so not stiff/strong
- very absorbent materials for nappies, sanitary products
- Rayon (cellulose fibre) was popular but now polypropylene/PET
42
Injection moulding
- complex, high quality, quick, high pressure 100 MPa, high initial investment, moderate scrap e.g. sprues, runners, flash which is reused
- most thermoplastics & thermosets
- reciprocating screw, closed 2 part cavity mould
1) Injection - charge enters metering zone, forced into cavity mould, pressurised at constant pressure, check valve prevents backflow
2) Packing - maintain pressure
3) Cooling - gate freezes over, density increases, prepare next charge
4) Ejection
- outer skin more densely packed
- short shot, flash, sink marks, weld lines, parting line, gate mark, ejector pin marks
- surface treated tool steel
- polymer orientation influenced by fountain flow
- mould has sprue, runner, gate
- co-injection moulding, foam core (surf boards), injection compression moulding (partially fill then use clamping system for reduced distortion e.g. CDs, smartphone covers)
43
Injection moulding equation of state
Spencer-Gilmore: (Pe + Pi)(v-v0)=83.1T
Pe is external pressure, Pi is cohesive energy density, v0 is specific volume at absolute 0
For PMMA (Pa, m^3/kg, K): (10^-3Pe + 215,800)(10^3v-0.734)=83.1T
44
Thermoforming
- Softened plastic sheet forced over convex/concave mould
- since secondary process, double orientation
- 2-sided objects of limited complexibility e.g. lids, trays, blister packs, food packaging, auto panels, fridge liners
- thermoplastics
- polymer orientation - not true biaxial orientation > anisotropy & weaker side walls
- vacuum/pressure - clamp to form seal, apply vacuum & pressure, cool, eject
- mechanical - plug assisted for thick-walled/deep draw parts
- inflate before vacuum to control wall thickness, use rounded corners
- cheaper then injection mould for short runs & lower pressure so cheaper mould materials (short run - wood, resin, hard plastic, Zn/Al alloys for medium runs)
45
Blow moulding
- molten/softened plastic tube blown from inside into 2-part mould
- hollow parts, thin walls, little scrap
- High melt strength thermoplastics (PE, PET)
- moulds rounded corners & thickened bases, vents so air escape & not scorch. Cheaper, medium pressure so cheaper tooling & mould.
1) Extrusion, EBM - extrude parison > close mould (pinch off) > inflate with blowing pin, cool, eject. Milk bottles, low polymer orientation. Extrusion quicker than inflate/cool/eject so multiple moulds on rotating wheel. For large moulds, parison can tear so use accumulator to collect polymer melt & extrude quicker
2) Injection, IBM - injection mould preform, soften & load into blow mould, inflate with blowing pin, cool, eject. Medical/food transparent bottles with precise threaded necks, medium polymer orientation (inflation - higher stiffness, lower permeability, better clarity). No pinch-off, 2 moulds.
3) Stretch, SBM - softened pre-form mechanically stretched with blowing tube. PET pressurised bottles. High levels of polymer orientation (biaxial from stretching & inflation). Best stiffness & clarity, worst permeability. Allows use of high molecular weight polymers.
46
Rotational moulding
- Plastic flows onto inside of rotating mould in stations
- low pressure, 0.1 MPa
- Hollow, large, seamless parts, varied walk thickness, with/out openings
- Used with PVC in plastisol form (suspension of fine polymer powder in liquid plasticiser)
- Road furniture, storage tanks, refuse bins, flotation buoys, toys
- no polymer orientation
1) Charge, heat & rotate with gravity (not centrifugal), cool, de-mould
- multilayer moulding e.g. petrol tank (nylon for chemical resistance, crosslinked PE for strength), foamed interior (small particles to sinter to mould 1st for solid outer layer, large particles sinter to outer layer & contain blowing agent), integrated parts e.g. metallic nuts & bolts
47
What are composites & why make them
- made from 2/more constituent phases which are separate w. different properties (tailor these)
- Fibre reinforcements bear most tension load, add tensile modulus, strength, creep resistance, thermal stability & can be particulate reinforcements e.g. nanotubes, lightweight, expensive
- Matrix is thermosetting polymer which binds reinforcements & maintains shape - add toughness, shear, compressive for energy absorption, bending & buckling. Processing depends on viscosity, cure T, safety
- lightweight e.g. 25% or 50% in civil aircraft
48
Fibre composite structure
- lamina/ply - 0.1-1mm thick layer of fibres in a matrix
- long/continuous fibres are unidirectional, bidirectional (weaving), multidirectional
- short/discontinuous fibres are unidirectional/random (poorer properties than continuous)
- laminates parallel (0/0/0/0 for high anisotropy) or cross-ply laminate (0/90/90/0). Or hybrids e.g. interply (each lamina contains a 1 type of fibre), intraply (each lamina contains 2/more types of interwoven fibres), fibre-metal laminates e.g. GLARE (glass laminate aluminium reinforced epoxy)
- Sandwich panels - core material (foam/honeycomb) with 2 thin composite skins, better bending for little extra weight
49
Fibre reinforcements - glass fibres
White E or S-glass
- random 3D network of tetrahedra linked by oxygen atoms (amorphous & isotropic), inorganic oxides
- Insulating, low tensile & fatigue, absorbs water, high density
- Matrices: unsaturated polyesters, vinylesters, epoxies
- Automotive, military, construction, storage tanks, electrical equipment, baths, sporting goods
50
Carbon fibres
Black, high performance & expensive
- Hexagonal carbon atoms, ordered graphitic carbon on skin
- high tensile & conductivity, moisture resistance, low density, low strain & impact resistance, expensive
- matrix: epoxy
- high performance automotive, aerospace, sporting goods
51
Aramid fibres
Yellow highly crystalline aromatic polyamides. Kevlar 29 tough for bullet proof armour. 49 high modulus for composites.
- phenyl rings stiffness, hydrogen bonds
- high tensile strength to weight ratio, low thermal expansion, *impact* & corrosion resistant, hard to process, poor matrix compatibility, poor in compression, susceptible to moisture & UV radiation, expensive
52
Polyethylene fibres
White highly crystalline polyolefins
- Ultra-high molecular weight PE
- highly aligned molecules for high tensile strength, low density & moisture absorption, impact resistant. Low service temperature (property loss above 80-90), poor adhesion with matrices
- Ballistic & blast protection, sporting goods
53
Natural fibres
Brown from agricultural plants e.g. hemp, flax, jute, cellulose fibres in lignin matrix
- biodegradable, high strength to weight ratio than E-glass, acoustic damping, cheap
- low strength, m.p., moisture absorption, degrade above 200
- interior panels automotive
54
Fibre treatments
- protect, improve wettability & bonding (remove contaminants, add chemically reactive groups, mechanical interlocking)
- glass (organo-silanes/sizing), carbon (oxidative or non-oxidative with reactive coatings), Kevlar & UHMWPE (oxidise surface, plasma treat to etch/add functional groups)
55
Matrices types
- thermoset - epoxies, unsaturated polyesters, vinylesters, phenolics, BMI, cyanate esters
- thermoplastic - polyketones (PEEK), high strain to failure
- metallic - Al, Ti, Mg alloys
- ceramic, Al2O3, SiO2, Al2O3, SiC, concrete, carbon (high density & tensile modulus)
56
Thermoset vs thermoplastic matrices
- setA - low viscosity reactants, adhesive, thermal stability, chemical resistance
- D - limited storage life, long cure, low strain to failure
- plasticA - high impact strength, fracture, strain, good storage, low fabrication
- D- high melt viscosity, low creep resistance, low thermal stability
57
Thermoset matrices
- unsaturated polyesters cured with reactive monomer e.g. styrene & initiator e.g. MEKP
- good mechanical/thermal, easy cure, cheap. Thermally unstable, shrinkage, health concerns
- Epoxy resins - unstable epoxide rings opening with curing agent (amine/anhydride). Increased cross link density increases Tg, modulus, thermal stability & decreases strain to failure, fracture toughness
- good mechanical/thermal, low shrinkage, chemical resistance & adhesion. Expensive, health concerns, elevated cure T
- Vinylester - compromise between above, easy to process but shrinkage & low adhesion
- Phenolic resins - high T applications (char layer ablates heat)
- BMI, cyanate ester - high T applications (200-300C)
58
Thermoplastic matrices: polyketones
- Poly ether ether ketone, PEK, PEKK
- backbone pheny rings excellent mechanical
- high T stability, solvent resistance, fracture toughness, low water absorption
- expensive & hard to process (400C)
59
Common composite materials
- glass fibres + unsaturated polyester - low cost, moderate properties
- Long carbon fibres + epoxy resin matrix - moderate cost, high properties
- glass fibre + epoxy resin - possible but not as common
60
Mechanical properties of composites (W10, p29)
E1 = ff*Ef + (1-ff)Em
where E1 is tensile modulus in parallel, ff is fibre volume fraction, Ef is fibre moduli, Em is matrix moduli
E2 = (Ef*Em)/((1-ff)Ef+ff*Em)
E2 is for series. Inaccurate & Halpin-Tsai used instead.
v12 = ff*vf+(1-ff)vm
Where v12 is major poisson's ratio
v12/E1 = v21/E2
v21 is stiffer fibre dominated axial direction
61
Wet lay-up
liquid resin applied to reinforcement layer
2 types: hand & spray
62
Hand lay-up
Fibre mat in mould is manually infused with resin
1) Apply release agent to mould - fluorinated polymers, PVA, wax
2) Apply gel coat for surface finish to mould - protection, stain resistance
3) Lay fibre mat into mould
4) Apply matrix & work in - stipple with bush (contact) & roll with roller (consolidate)
5) Repeat till desired thickness
6) Cure
- high T cure expensive but better properties
- use vacuum bag & pressure (breather cloth evacuates air)
- any materials but glass fibre & unsaturated polyester common
- vehicles, boats, construction parts, windmill blades, pipes, storage tanks, bathroom interiors
- A: cheap, versatile, simple, prototyping & short runs, simple moulds (composite, plaster, wood)
- D: poor properties (low volume fibre fraction 0.2-0.3 for mats & 0.4 for fabrics), labour intensive, slow, quality varies, 1 moulded surface, safety issues
63
Spray lay-up
Chopped glass fibre & resin sprayed onto mould & rolled
- high labour cost but less than hand
- equipment more expensive
- messy & only for short fibres
- poor mechanically but better quality consistency
64
Compression moulding for composites
1) Apply weighed charge (moulding compound) to mould
2) Close mould & heat to cure
- press & mould expensive, high throughput
- materials: soft & pliable (25% unsaturated polyester, 20% chopped E glass fibre, 55% filler e.g. CCO3, MgO), sheet moulding compound (reduced reorientation), bulk moulding compound with blocks & ropes (increased reorientation of fibres)
- cars, trucks, buses bodies & interior panels, containers, electrical housings
- A: any geometry, moulds fixings, 2 good surfaces, can be automated (expensive), quality consistency
- D: poor properties, can't mould holes, medium capital investment, size limited. 0.1-0.2 BMC, 0.2-0.3 SMC
65
Filament winding
Wind continuous reinforcements onto rotating mandrel > rotational symmetry, hollow, convex, enclosed/tubular
1) Strands/tows of continuous fibres impregnated in resin bath
2) Wound onto mandrel using traversing carriage
- if tension too low, poor compaction. Too high, tows migrate in towards mandrel giving resin poor areas internally
3) Cure
- multi-axial (2axes tubes, 3 enclosed vessels, 4+ complex shapes controlled by computer), hoop (slow, excellent circumferentially), helical (fast, excellent torsionally & bending), polar (entire mandrel rotated to create enclosed vessel, excellent longitudinal properties), hoop-helical-hoop
- remove mandrel with release agents, tapered, plaster, collapsible, wind directly
- any materials but glass fibre & unsaturated polyester common
- pressure vessls, fuel & storage tanks, drive shafts & pipes, launch tubes
- A: any size, good inner surface, relatively automated, good quality consistency
- restricted geometry, poor outer surface, not versatile, medium capital investment (less than compression), intermediate cycle time
- 0.5-0.6 helical/polar, 0.6-0.7 hoop
66
Liquid moulding & strategies
Liquid resin transferred into closed mould & cured
- unreinforced resin + dry fibre preform in mould e.g. RTM, VARTM
- short fibre reinforced resin + empty mould e.g. injection moulding of short fibre filled plastics
67
Resin transfer moulding, RTM
1) Liquid resin injected into closed mould containing dry preformed reinforcements
2) Cure
- any fibre/resin combo
- reinforcements are hand laid/preform (tightly packed dry fibres aiming for 0.7 ff but 0.5-0.6)
- resins must be low viscosity
- curing at rtp or elevated
- moulds are 2-part rigid cavity at lower pressure than injection moulding. Al mould
Car, truck, bus, train body & interior panels, yachts, ships, aircraft interior panels, propellors, construction
- A: versatile, good surfaces, relatively automated, good quality consistency
- D: movement of fibres as resin injected, medium-large capital investment, expensive tooling, specific low viscosity resins
68
Vacuum assisted RTM
1) 1 part open mould with vacuum bag
2) Run resin through till mould filled - high operator skill level, not easy to avoid voids
3) Heat 1 side so typical for thinner gauge parts
- high quality finish on 1 surface, 0.4-0.5, inexpensive equipment, longer cycle time than RTM
69
RTM preforms
- wind, weave, stitch, braid, knit
- non-crimp fabrics - fibres in unidirectional sheets & stitched, 25% stronger in tension, better shear properties due to increase fibre-fibre contact, no resin rich areas so higher ff, chopped strand mat hybrids reduce cost
- overbraiding - braid fibres over reciprocating core for excellent properties, delamination resistant, crimping issues, high equipment cost, high-end applicaitons
- 3D weaving - computer controlled weaving - highly optimised, very expensive, highly delamination resistant, excellent properties in & out plane, crimp issues, fibres in z-direction not load bearing
70
Pultrusion
1) Continuous reinforcements drawn through liquid resin bath
2) draw into die
3) cut to length
- any materials esp. glass fibre & unsaturated polyester
- unidirectional/multiaxial for longitudinal/transverse properties, surface veils improve appearance
- control resin viscosity
- A: any length, good surfaces, true unidirectional possible, relatively automated, good quality consistency
- D: restricted geometry, medium capital investment, expensive mould materials, slow
- 0.2-0.5 fabrics, 0.65 rovings (straight fibres)
- rods, beams, plies
- compared to metal, more expensive, quicker construction, less maintenance, longer service life (lighter, corrosion resistant)
71
Pre-impregnated fibres (pre-preg)
1) Continuous fibres pre-impregnated with uncured resin
2) Cut to size
3) Assemble layers on mould to required thickness
4) Freeze & store
- uses high viscosity resins - reduce with solvent/hot melt (gel prematurely)
Solution pre-preg:
1) Dissolve resin in solvent
2) After bath, squeeze out excess & evaporate solvent off
3) Apply release film for spooling/handling
Hot melt/film transfer pre-preg:
1) melt resin & form stable film
2) force resin & reinforcement together
3) Apply release film for spooling/handling
- carbon or glass fibre in epoxy
- B-staged matrix - easy to handle partially cured resin, flat sheets in bulk
- additives are toughening agents e.g. thermoplastic which dissolves/disperses
- pre-preg plies unidirectional 0.125-0.25 mm thick
- curing T 80-180C
- curing pressure - vacuum bag & autoclave
- only high performance: civil & military aerospace, automotive, sports, defence
- best properties, versatile
- very expensive, only 1 good surface, limited size, properties are skill-dependent, long cycle time, 0.7
72
Automated lay-up
Reduce part cost, time, & uniformity but expensive
- Also called automated tow/fibre placement/tape laying
- robot lays down narrow strip <8mm of unidirectional pre-preg on mould/mandrel
73
Film stacking
Reinforcement fabrics stacked with:
- thin films of frozen thermoset resin
- thin films of thermoplastic polymer
- lay-up dry fibre preform
- alternate with resin film
- cure in hot press/autoclave (heat + pressure)
- high ff
- systems can be supplied pre-interleaved
74
Composite processing: relatively cheap prototype
wet lay-up
75
Composite processing: very large part which doesn't need extreme properties
spray lay-up
76
Composite processing: 1000s of relatively small, simple parts
compression moulding
77
Composite processing: hollow/enclosed tbe
filament winding
78
Composite processing: 100s of medium-large good performance parts
RTM
79
Composite processing: Part constant cross section
Pultrusion
80
Composite processing: ultimate performance & huge budget
Pre-preg
