Processing of Polymer Matrix Composites (THERMOSETS) Flashcards
(20 cards)
What is the general principle of processing polymer matrix?
- Thermosets starts as fairly low-viscosity liquids.
- This allows easier coating of all fibres.
- Allows high fibres volume fractions to be used.
- Amendable to continuous fibre composite.
What are the hand lay up processes?
Simplest process
Positive former or negative mould made.
- Wood, thermoplastic, aluminium, steel.
- Finished parts: male mould and female mould.
What is the prep for the hand lay-up process?
- Mould release agent / layer applied.
- Gel - coat applied to give smooth and impermeable surface.
- Resin applied and allowed to partially - cure.
What is the hand lay-up process?
- Then the layers of fabric placed on mould (mostly commonly woven rovings, but stitch-bonded and chopped strand mat used.
- Resin applied onto the fabric using brush.
- Roller used to (try to ) remove trapped air bubbles.
- Next layer applied.
What is the benefit of hand lay-up process?
- Layers built up to give desired thickness.
- Cured in air
-Normally at room temp.- Elevated temp can be used to
give faster / more complete
cure (post -cure).
- Elevated temp can be used to
- Low equipment and mould cost.
- Slow, labour intensive, fibre volume fraction limited, voids a problem.
What are some of the quality of hand lay up process:
- Most commonly used with glass-reinforced polyesters (though not limited to this).
- Typically for small numbers of components.
- Quality depends on skill of operator.
What are the properties of spray up process?
- Similar to hand lay-up.
- Faster, slightly more automated version.
- Chopped fibres (normally glass) mixed with resin and sprayed onto mould.
- Faster, cheaper and lower-quality than hand lay up.
- Used for ‘fibreglass’ roofs, backing for baths etc.
What is vacuum / pressure bag?
- A method to improve quality of hand lay - up.
- Method almost the same.
- Membrane placed over component and pressure applied above, or vacuum below.
- Sucks or pushes entrapped air out.
- Allows higher fibre volume fraction.
What is the benefits / downside of vacuum / pressure bag?
- Still slow.
- Cost increases somewhat.
- Quality is improved.
What is autoclave moulding?
- An autoclave is used to give a heated and pressurised environment.
- Can be used to compress lay-up together and squeeze out entrapped air.
- Elevated temperature allows more rapid cure.
- Capital cost increase.
- Widely used for carbon-fibre epoxy composites.
What is Resin transfer moulding (RTM)?
- Sometimes also called “resin infusion moulding”.
- Reinforcement placed in closed mould.
- Liquid resin injected in under (quite low) pressure or sucked through with vacuum.
- Can mix 2 -component resin just before injection
- Reinforced reaction injection
moulding.
What is the advantages/disadvantages compared to hand lay-up / vacuum bagging?
- Higher Vf so better quality.
- Less release of volatiles to the atmosphere.
- It is slower.
- Requires more consumable.
What is pre-pregs?
- Used extensively with epoxy resin, especially with carbon fibres.
- Fibres pre -impregnated with resin.
- Commercially provided as uncured, hand able sheet of unidirectional material.
- Allows good wetting of fibres.
- Can be laid up into laminated and then cured (often autoclave).
How do we make larger numbers of complex mouldings?
By using moulding compounds:
- Sheet moulding compound (SMC)
Up to 30 mm thickness
- Bulk (or dough) moulding compound (BMC/DMC).
Thicker
What are the combination of the chopped glass fibres, uncured resin, fillers and catalyst?
30% to 50% fibres.
30% resin (polyester / poly vinyl ester)
20% to 40% filler (calcium carbonate).
How does compression moulding work?
TS/Reinforcement charge is loaded compression and curing, part is ejected and removed.
What are some properties of compression moulding?
- Placed into matched mould.
- High pressure applied.
- Mould heated to cure.
- Faster cycle times.
- Automated and reproducible.
- Capital cost higher.
- Moulds need to be robust (tool steel / Cr plated).
What are the benefits / downside of compression moulding?
- Simpler
- Process itself generally limited to simpler part geometries due to lower flow capabilities of TS material.
- Mould must be heated, usually by electric resistance, steam, or hot oil circulation.
How does pultrusion work?
- Continuous fibres passed through tank of uncured resin.
- Then into shaping die.
- Then heated to cure.
What does pultrusion do?
- Produce continuous length of constant cross-section.
- Sheets, rods, tubes.
- Fibres unidirectional along main axis.
