Composites- Filament Winding and Liquid Moulding Flashcards
How does filament winding generally work?
Wind continuous reinforcements onto a rotating mandrel to form parts with rotational symmetry. These can be hollow, convex, enclosed or tubular
Wet filament winding
Strands or tows of continuous fibres are collected together and passed into a resin bath. Fibres are impregnated with liquid resin and then wound onto the mandrel using a traversing carriage. Can also use woven tapes which aren’t unidirectional to reduce anisotropy
Tape winding
Alternative to wet filament winding where pre-preg tape is wound onto a mandrel. Similar to automated tape laying
Types of resin bath
Drum: fibres go between compaction roller and large impregnation roller, large is partly in the resin tub which then wets the fibres, then goes over another impregnation roller, resin strippers remove excess resin, placement unit guides wetted fibres onto mandrel.
Dip: same as drum but no compaction rollers and impregnation rollers fully submerged in resin tub.
For both then cure
What does resin viscosity control in filament winding?
Resin pickup and fibre wetting
How is precise fibre placement controlled for filament winding?
By resin bath speed, mandrel rotation speed, angle between resin bath and mandrel
Importance of tension on tows during winding
Breaking system after tow spools. Provides compaction. Too low means poor compaction. Too high means tows migrate in towards mandrel giving resin poor areas internally and resin rich areas externally.
Number of axes for filament winding
2 axes gives tubes
3 axes gives enclosed vessels
4+ axes gives more complex shapes and requires computer control
Hoop winding
Slow speed and nearly 90° angle. Tows laid side by side. Excellent circumferential properties
Helical winding
Faster speed with varying angle. Tows laid with gaps which meet after several revolutions and overlap. Excellent torsional and bending properties
Polar winding
Entire mandrel rotated to give enclosed vessel. Excellent longitudinal properties
Ways of removing mandrel after curing
Release agents on mandrel Polished tapered mandrel (gives tapered part) Plaster mandrel broken up when done Collapsible mandrel Wind directly onto finished part
Materials used and applications for filament winding
Any materials but cheaper glass fibre and unsaturated polyester common.
Pressure vessels, fuel and storage tanks, drive shafts and pipes, launch tubes
Advantages and disadvantages of firmament winding
+ve: any size, good inner surface, relatively automated, quality consistency good
-ve: restricted geometry, poor outer surface, not that versatile (hard to wind in holes, fixings), medium capital investment
Costs of filament winding
Equipment intermediate Mould low to inter Labour low Material low Cycle time inter to long
Properties of part from filament winding
ff 0.5-0.6 (helical/polar) or 0.6-0.7 (hoop)
fv low to inter
Mechanical good
Quality consistency good
What is liquid moulding?
Generic term for when liquid resin is transferred into a closed mould and cured. Generally thought of as an intermediate process in terms of cost, volume and properties
Two strategies for liquid moulding
Unreinforced resin and dry fibre preform in mould (RTM and VARTM).
Short fibre reinforced resin and empty mould (injection moulding of short fibre filled plastics)
Why are parts made from pre-preg very expensive?
Mostly due to the autoclave. So recent investment in out-of-autoclave processes like RTM
How does resin transfer moulding (RTM) work?
Liquid resin injected into a closed mould containing dry preformed reinforcements. You close the mould with reinforcements in then pump liquid resin in and cure. Moulds are 2 part rigid cavity like injection moulds but lower pressures used so cheaper Al can be used.
Requirements for RTM materials
Can use any typical fibre/reinforcement combination. Reinforcements hand laid or as preform. Performs very tightly packed dry fibres aiming for ff around 0.7. Resins must be low viscosity as needs to infiltrate entire volume of cavity and all the fibres need wetting.
Curing for RTM
Either at room temperature or elevated temperature. Directly through mould or indirectly in an oven
Applications for RTM
Automotive (car, truck, bus, interior panels)
Marine (components for yachts, ships)
Aerospace (aircraft interior panels, propellers)
Construction
Advantages of RTM
Versatile as any geometry and can mould in holes, fixings (overhangs difficult as mould must come apart) Any size Good surfaces Can be relatively automated Quality consistency very good
Disadvantages of RTM
Movement of fibres as resin is injected.
Medium to large capital investment.
Mould tooling can be expensive.
Requires specific low viscosity resins (RTM resins)
Vacuum assisted RTM (VARTM)
Similar to RTM but vacuum helps draw resin into mould. Mould is 1 part open mould with vacuum bag on top. Run resin through until mould filled. Higher operator skill level and not easy to avoid voids. Heating from one side of mould so typically for thinner gauge parts. Cheaper than RTM
RTM vs VARTM
RTM high quality finish both surfaces VARTM only one.
Heat from both sides RTM
Lower void content RTM
Higher ff for RTM
Expensive equipment RTM with 2 part mould and press and resin pumps. VARTM inexpensive equipment with 1 part mould and vacuum pump
RTM costs
Equipment low to inter Mould inter Labour inter Material low Cycle time low
Properties of parts from RTM
ff 0.5-0.6 (fabrics)
fv low
Mechanical good
Quality consistency good
VARTM costs
Equipment low Mould low Labour inter Material low Cycle time inter to long
Properties of parts from VARTM
ff 0.4-0.5 (fabrics)
fv low to inter
Mechanical inter to good
Quality consistency good
What are RTM preforms?
Dry fibres arranged into correct shape, size, thickness, fibre orientation prior to resin injection. Most traditional textile/fabric processes can be used to produce them
Ways of making preforms
Winding (dry filament winding for simple hollow preforms)
Weaving (woven fabrics can be stacked)
Stitching (woven or non-woven fabrics can be stitched together)
Braiding (for tubular and other complex hollow preforms)
Knitting (for complex geometries)
Problem of crimp
Problem with traditional woven fabrics. When fibre bends one side is in tension and the other in compression. This induces flaws in fibres which reduces their strength.
Non-crimp fabrics
Straight, un-bent fibres laid in flat unidirectional sheets and stitched together. Used to make preforms for RTM and variants. Better than woven fabrics as hot to 25% stronger in tension, better shear properties due to increased fibre-fibre contact, no resin rich areas mean higher ff. Can have multiaxial reinforcement (uni, bi, tri, quadraxial). Chopped strand mat (CSM) hybrids reduce cost
Overbraiding
Braid fibres over a reciprocating core. Get excellent properties, delamination resistant. Does suffer from usual crimping issues and high equipment cost. Used to make preforms for RTM and variants. Used for high end applications
3D weaving
Used to make RTM preforms. Layers joined together as part of weaving process. Highly delamination resistant, excellent properties in and out of plane. Still suffers from crimp and fibres in z-direction might not be load bearing. Can have computer controlled weaving. This can make any shale and is highly optimised but is very expensive
Delamination
Where individual laminae separate from each other
