Flashcards
Polymers & Composites
Cards in this deck (51)
- 1What are thermosetting plastics?Plastics that, once cured, cannot be re-melted.
On heating will degrade and burn
Instead of Van der Wals chains of molecules are joined by cross linking
Have to facilitate cross linking - 2What additives can you add to thermosetting plastics?Plasticizer - Helps with toughness & moulding
Filler - Enhanced strength
Pigments / Dies - Colour
Flame retardants - 3What is a composite?A material made up of two or more constituents which do not dissolve in each other.
Properties of final material different to those of constituent materials - 4What reinforcements are available for composites?Fibre - continuous
Fibre - discontinuous - short length - no control of fibre allignment
Particulate - cheap - limited property gain
Glass
Carbon
Aramid (Kevlar) - ductile
Metal
Hemp - 5What resins are used in composite materials?Epoxy - most widely used
Polyester - 10x cheaper than epoxy
- Inferior properties
-Large shrinkage on moulding
- High moisture absorption
Other factors such as fibre direction are far more important than what resin is used - 6Composites AdvantagesTailor made properties
High stiffness and strength to weight ratio
Damage tolerant - fibre acts as barrier to cracks - 7Composites LimitationsExpensive
Labour intensive manufacture
High tooling costs
Repeat material properties difficult to obtain
Voids are a major issue
No reliable failure criteria
Experimental analysis essential
Numerical analysis is computationally heavy - 8Thermoplastic Forming Injection MouldingHopper moves material into heated barrel
Material is moved along by screw
Once the material is at the front of the screw it is injected into the mould
Material for the next injection is plasticised as the material in mould cools - 9Injection Moulding The ScrewFriction between polymer and screw heats polymer
Has 3 sections
- Feed - moves granules from hopper and compacts them
- Compression - Polymer is squeezed until plasticised and air removed
- Metering - regulates flow of material into mould
A material which changes abruptly from a liquid to a solid only requires a small compression zone - 10Thermoplastic Forming ExtrusionRods, tubing, sheets, hollow sections, constant cross sections
Hopper, heated barrel, screw, die
Versatility comes from range of dies which can be used
Mandrels / torpedos can be used for hollow sections
Product cooled immediately to preserve shape through water jets or water rollers - 11Thermoplastic Forming Extrusion Blow MouldingThick walled tube is extruded and fed vertically between two halves of the mould
When mould is firmly closed, compressed air is blown into the tube so that it takes the shape of the mould - 12Thermoplastic Forming Injection Blow MouldingSimilar to extrusion
Material injected into mould, this mould is removed during solidification and replaced by a blow mould
Compressed air inflates injection moulded shape - 13Blow Moulding Injection Advantages over ExtrusionNo trimming required
Wall thickness distribution can be controlled
Wide variety of thermoplastic materials can be used - 14Thermoplastic Forming Vacuum FormingSheet of extruded thermoplastic clamped over an open mould and heated
Space inside mould evacuated
Air outside is at atmospheric pressure and pushes sheet into mould
Cooled rapidly with water/air
Drape forming - thermoplastic sheet is mechanically pre-stretched before applying vacuum, push mould up into material. - 15Thermoplastic Forming Rotational MouldingProduces hollow, seam free components
Quantity of moulding component put into split hollow mould
Mould rotated biaxially and heated
Heat causes material to adhere to walls
Rotation and heating continues until solid coatings is formed over inside of mould
Cooled with air / water
Inexpensive mould
No application of pressure
Slow
Limited shapes - 16Thermoplastic Forming CalendaringContinuous manufacture of sheet / film
Polymer & filler blended and heated
Fed between heated rollers
May still contain coarse particles of polymer / filler
Removed by extrusion through fine mesh
Plastic dough fed into calendar
Calendar consists of 3 / 4 cast iron / steel rollers
Minimum thickness = 0.01mm
High equipment cost
very specialised - 17Thermosetting Plastics Condensation PolymerisationWater given off during reaction
Molecules join together losing small molecules as a by product
Have to consider this in mould design - 18Thermosetting Plastics Epoxy ResinsMay be cross linked with themselves through catalytic homopolymerisation
Usually co-reactants are used
- Hardeners
- Curatives
Exothermic Reaction - Can cause thermal degredation is not controlled - 19Thermosetting Plastics Compression Moulding2 heated patterns to which moulds are fixed
Hydraulic ram to provide pressure
Require additional machining to remove additional material
Can also be used to make composites with particulate (very short fibres) - 20Thermosetting Plastics Resin Transfer MouldingMaterials mixed then injected into mould
Used for composites
Better wet - out over fibres as thermosetting plastics are less viscous - 21Thermosetting Plastics Injection MouldingMaterials mixed in hopper and injected into mould
Have to ensure material doesn't cure before it reaches the mould
Quick, automated process
Mould is heated to cause solidification (in thermoplastic forming mould is cooled)
Part can be removed from mould while still hot as long as it is cured - 22Foam Expanded PlasticsStart with plastic compound
Add a 'blowing agent'
Execute curing process
Blowing agent decomposes on heating, giving off nitrogen
Other ways to achieve foam
- Blowing by in situ chemical reactions
- Low pressure release - gas expands in material
- Vapour produced from a volatile solvent incorporated into material - 23Structural FoamsAs foam flows through mould, surface cells collapse and form a skin against the wall of the mould while the internal material remains foam.
- 24Order of economically viable quantitiesInjection Mould - 10,000
Extrusion - 300m
Blow mould - 1,000
Foaming - 1,000
Rotational - 100
Thermoforming - 100 - 25What design features are difficult to deal with in plastic forming?Undercuts as the mould has to be removed
Thick areas of material will cool slower than thin sections leading to voids and distortion from stress concentrations - 26Composite Manufacture Types of fibreFilaments - very long continuous fibres
Yarn - twisted bundle of filaments
Roving - Untwisted bundles of gathered filaments
Tows - bundles of thousands of filaments
Woven fabrics - Made from filaments, yarns or rovings
Mats - continuous or chopped fibre in a random pattern
Whiskers - Fine, short, single crystal fibres - 27Composite Manufacture Open MouldingResin in a bucket and dry fabric laid into mould
Resin applied with hand roller
Improvements
- Vacuum bag for consolidation
- Curing in an oven
- Spray up - chopped up fibre & resin sprayed into mould ('Automated') - 28Composite Manufacture Pre - PregFilm of resin material
Fibres dragged through to form matting
Kept in a freezer
Cut to shape
Stacked into laminate
Cured in oven / autoclave - 29Pre - preg AdvantagesGood control of volume fraction
Lower fabrication cost
Reduced Energy Consumption
Reduced number of parts
Control of fibre content
Consistent performance and processing characteristics - 30Pre - preg Vacuum bag layersVacuum bag
Breather fabric
Release film
Bleeder fabric
Release film
Peel Ply
Pre-preg
Peel Ply
Release agent
Mould - 31Composite Manufacture Resin Film InfusionLaminate produced from stack of dry fabric
- Easier to handle
- Doesn't need to be kept in freezer
Resin film layed down
Vacuum applied and resin drawn through fabric - 32Composite Manufacture Liquid Resin InfusionDry mat stacked into laminate
Have an infusion mesh instead of breather fabric
Liquid resin drawn through fabric
Can be post cured in oven
Voids and bubbles can occur in the liquid resin
Can pre-form / press fibres prior to infusion
- Saves costs
- Better final product - 33Composite Manufacture Filament WindingContinuous strand of roving drawn through resin bath and wound around mandrel
Can vary thickness
Can have multi-axial winding machine
Volume fraction of 70% which is excellent
Can only make tubes - 34Composite Manufacture PultrusionSimilar to extrusion
Roving pulled through resin impregnator and through a forming and cutting die
Constant cross section parts - 35Adhesives spread...hence there are no sharp corners, no stress concentrations so failure of joint occurs at higher load
- 36Why do we need adhesive joining?Two perfectly smooth surfaces would bond together through intermolecular reactions when in contact
Surfaces aren't smooth hence we need to fill the gap - 37Epoxy AdhesivesVery Strong
Cold, warm and hot cure formulations available
Supplied in paste, film, powder or liquid form
Usually two part, resin and hardener system
Single part formulations have a latent hardener which is released at elevated temperature - 38Acrylic Adhesives AnaerobicSealing & locking adhesives
Cure in the presence of metal and exclusion of air
Used to lock metal fasteners - 39Acrylic Adhesives CyancacrylatesHighly reactive single part adhesive (super glue)
Harden in a thin film
Surface moisture is a catalyst
Don't like moisture once hardened - 40Surface PretreatmentsNeed to remove oxide layer
- By hand
- Shot blasting / peening
Provide some roughness for adhesive to key into
Problems occur when the surface energy of the liquid is greater than that of the solid
Al and stainless steel produce oxide layers which prevent corrosion hence are bad for adhesives - 41Design of JointsLarge band area - lowers stress
Uniform and continuous band line
Shear and compressive loading preferred
Avoid tension and peel - 42Adhesive Joining AdvantagesJoin dissimilar metals
Join can be over a large area
No stress concentrations
Permanent band
Applied at room T
- No high temperatures like in welding
Smooth finish - 43Adhesive Joining DisadvantagesTime required for curing
Band affected by moisture
Standard adhesives lose strength over 200'C - 44Failure of Adhesives Cohesive FractureFailure of adhesive
- 45Failure of Adhesives Adhesive / InterfacialGlue hasn't stuck properly to material
- 46Failure of Adhesives Defect in AdhesiveLeads to crack which jumps between bands forming a zig zag pattern
- 47Failure of Adhesives Adhesive Stronger than MetalCrack follows path of least resistance i.e. starts in adhesive and moves into metal
- 48Loading an Adhesive Joint TensileTwo adherends are pulled apart
- 49Loading an Adhesive Joint ShearAdherends are pulled in opposite directions
- 50Loading an Adhesive Joint CleavageAdherends are pulled apart at one end which begins to fail while remaining end remains attached
- 51Loading an Adhesive Joint PeelOne adherend pulled away from other which remains fixed
Peels away
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- Flashcards on Polymers & Composites, created by tom8390 on 08/01/2015.
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