Composites- Wet lay-up and Compression Moulding Flashcards
General process of hand lay-up
Fibre mats or fabrics are laid down into a one-sided mould and manually infused with resin (build up in layers).
Steps of hand lay-up
Apply release agent to mould so composite doesn’t stick (fluoridated polymers, PVA, wax).
Apply gel coat for surface finish to mould (outer layer for protection, strain resistance, optional).
Lay fibre mat into mould.
Apply fibre matrix to mat and work in (stipple with brush, roll with a roller).
Repeat last 2 steps until desired thickness.
Cure
Curing temperatures for hand lay-up
Room temperature cheaper. High temperature more expensive but gives better properties
Curing pressure for hand lay-up
Use vacuum bag to improve consolidation. Use vacuum bag and extra pressure for best results
Arrangement of a vacuum bag
Bagging film enclosed mould and composite. Has a gap for the vacuum port. There is a breather cloth inside to allow all air to escape up vacuum port.
Materials and applications for hand lay-up
Can use any materials but cheaper glass fibre and unsaturated polyester is common. Automotive (vehicles, boats) Construction parts Windmill blades Pipes, storage tanks Bathroom interiors (tubs and showers)
Advantages of hand lay-up
Cheap. Any geometry, size. Easy to add fixings, holes. Good for prototypes or short runs. Simple moulds made from composite, plaster or wood
Disadvantages of hand lay-up
Poor properties (low ff, high void volume fraction fv). Labour intensive Relatively slow Quality consistency varies Only one moulded surface Safety issues
Spray lay-up
Similar to hand but chopped glass fibres (always used) and resin sprayed onto mould and rolled. Still need to cure. Reduced labour cost but more expensive equipment. Messy and only for short fibres. Low ff. Poor mechanical properties but better quality consistency
Level of costs of hand lay-up
Equipment cost low. Mould cost low Labour cost high Material cost high Cycle time long
Level of costs of spray lay-up
Equipment low to medium Mould low Labour high Material low Cycle time long
Properties from hand lay-up
ff is 0.2-0.3 (mats) or 0.4 (fabrics).
fv is intermediate
Mechanical properties poor to intermediate
Quality consistency poor
Properties from spray lay-up
ff=0.2-0.3
fv intermediate
Mechanical properties poor
Quality consistency medium
How does compression moulding work?
A moulded compound is forced to adopt the shape of a two part cavity mould. Apply a weighted charge to the mould. Close mould and apply heat to cure. Similar to compression moulding of plastics. Moulds are held in a hot press so high pressure compresses voids (occupy lower volume), curing is through mould.
What is the moulding compound?
Often 25% unsaturated polyester or vinyl ester, 20% chopped E glass fibre, 55% filler material (CaCO3 or MgO). They are soft and pliable before curing
