Composites- Manufacturing 2 Flashcards
Why are monomeric reactants a safety concern before they are cured?
Epoxy resins are skin sensitisers and often carcinogens.
Amine curing agents are often carcinogens.
Solvents such as styrene are dangerous
The two categories of techniques used for manufacturing thermosetting polymer composites
Liquid resin impregnation routes.
Consolidation of pre-prepared composite materials: consolidation of ore-pregs, compression moulding of moulding compounds
General features of liquid resin impregnation routes
Low-viscosity resin is impregnated into the reinforcement as part of the processing technique. More control over the constituent phases but also more that can go wrong (e.g voids).
Examples of liquid resin impregnation routes
Wet lay-up: hand and spray.
Pultrusion
Filament winding
Resin transfer moulding (RTM)
Hand lay-up: how it works and applications
Concept is that fibre mats of fabrics are laid down by hand and resin is manually infused into the fibres. 1. apply release agent to one-sided mould. 2. apply optional gel coat. 3. place prepared fabric/mat into mould. 4. add prepared resin and work in with a brush for contact (maybe be impractical for large parts) and a roller for consolidation. 5. Repeat 3 and 4 until desired thickness. 6. Cure. Used for lorry trailers, boats, storage tanks, baths, low performance aircraft, architectural mouldings
Hand lay-up: advantages and disadvantages
Advantages: cheap, versatile (any geometry and size), simple (easy to add fixings and holes), good for prototypes or short runs, simple moulds and mould materials (wood, MDF, plaster, etc).
Disadvantages: poor properties (low ff, high fv), labour intensive, slow, quality consistency varies, properties highly skill-dependent (control of fibre orientation), only one moulded surface, resin fumes.
Spray lay-up: how it works and applications
Concept is similar to hand lay-up but glass fibre rovings are fed to cutter which sprays chopped fibres and resin onto the mould. 1. Apply release agent to one-sided mould. 2. Apply optional gel coat. 3. Spray chopped fibres and prepared resin onto mould. 4. Work in using a roller for consolidation. 5. Repeat 3 and 4 until desired thickness. 6. Cure. Applications are similar to hand lay-up
Spray lay-up: advantages and disadvantages
Advantages: cheap, versatile (any geometry, size), good for very large parts like boat hulls, simple (easy to add fixings, holes), good for prototypes or short runs, simple moulds (wood, MDF, plaster, etc).
Disadvantages: poorer properties than hand lay-up (fibres randomly oriented and short), labour intensive but more automated than hand, slow but faster than hand, only one moulded surface, messy, fumes from resin.
What are resin baths used in and what types are there?
Used in continuous composite processing as a continuous fibre wetting system is required. Used for pultrusion and filament winding. Can have a drum which brings the resin to the fibre, or a dip which brings the fibre to the resin
How do drum and dip resin baths work?
Drum: fibres go between a compaction roller and a larger impregnation roller (in the resin tub). Then go past another smaller impregnation roller. Through resin stripper. Wound onto mandrel.
Dip: fibres go through a roving guided down into the resin tub which where it goes through impregnation rollers. Comes back out and wound onto mandrel. Both have placement unit before mandrel
Pultrusion: how it works and applications
Concept is continuous fibre rovings are pulled through a resin baths and then a heated shaping die. 1. Assemble reinforcements (primarily unidirectional, also fabrics/mats). 2. Draw fibres through a resin bath (drum or dip). 3. Draw through a performer than die and cure. 4. Cut to length. Between die and cut-off saw is caterpillar pull-off. Archives constant cross-section. Used for construction beams and piles for buildings, bridges, ladders, etc
Pultrusion: advantages and disadvantages
Advantages: any length, good surfaces, excellent fibre alignment (true unidirectional possible), once process is stable is relatively automated, good quality consistency.
Disadvantages: restricted geometry (straight with constant cross-section), medium capital investment, mould materials often expensive (tool steel) and can limit part dimensions (not length), relatively slow
Concept of filament winding
Continuous fibre rovings are drawn through a resin bath and are then wound onto a rotating mandrel
Filament winding process
- Assemble reinforcements (primarily unidirectional, also fabrics/mats). 2. Draw fibres through a traversing resin bath (drum/dip). 3. Wind onto mandrel. 4. A breaking system is used to control fibre tension. 5. Fibres are laid side by side (hoop winding) or overlapping (helical). 6. Cure. 7. Remove from mandrel (mechanically collapsible mandrel for large parts, polished metal or disposable mandrel materials for smaller parts).
What needs precise control in filament winding?
Often computer control over:
Resin bath speed
Mandrel rotation speed
Fibre-mandrel angle
Applications of filament winding
Chemical storage tanks, pipelines, gas cylinders
Filament winding: advantages and disadvantages
Advantages: any size, good inner surface, relatively automated, quality consistency is good.
Disadvantages: restricted geometry (rotational symmetry), poor outer surface (hoop wound skin can alleviate this), not particularly versatile (hard to wind in holes, fixings, etc), medium capital investment, mandrel materials can be expensive and limit part dimensions
Resin transfer moulding (RTM): how it works and applications
Concept is liquid resin is pumped into a two-part closed mould which contains the preformed reinforcements. 1. Place reinforcement into mould and close. 2. Pump prepared low-viscosity liquid resin into mould. 3. An optical vacuum can be used to help draw the resin through the mould (vacuum-assisted RTM). 4. Cure (often through a metallic mould). Used for small complex aircraft and automotive components, train seats, etc
RTM: advantages and disadvantages
Advantages: versatile (any geometry, can mould in holes, fixings, etc). Any size. Good surfaces. Can be relatively automated. Quality consistency is very good.
Disadvantages: movement of fibres as resin is injected. Medium to large capital investment. High pressure so mould materials are expensive (tool steel). Requires specific low viscosity resins (RTM resins)
Two general features of consolidation of pre-prepared materials methods
As combination step has been done elsewhere the cost is proportionately higher.
You have less control over the constituent phases.
Pre-preg lay-up: how it works and applications
Concept is continuous fibre reinforcements are pre-impregnated with resin and then cut and laid up similar to hand lay-up. 1. Cut pre-preg sheets to required shapes and sizes (can be automated). 2. Assemble layers on a mould to required thickness (can be automated). 3. Vacuum bag. 4. Cure (autoclave for best results, hot press is alternative). Typically used to high performance aerospace, automotive, sports.
Pre-preg lay-up: advantages and disadvantages
Advantages: best properties available (high ff, low fv). Versatile (any geometry, can mould in holes, fixings, etc). Can be automated (but very expensive).
Disadvantages: very expensive (equipment, moulds, labour, materials). One good surface (this effect can be minimised). Size limited (autoclave, mould, etc). Properties are skill-dependent
Compression moulding: how it works and applications
Concept is a moulding compound is pressed into the shape of the internal cavity in a two-part mould. 1. Apply weighed charge to mould in a hot press. 2. Close mould and apply pressure. 3. Apply heat to cure. Used for car body panels, electrical housings, etc.
Compression moulding: advantages and disadvantages
Advantages: any geometry. Versatile (can mould fixings, but…). Two good surfaces. Can be automated (but very expensive). Good quality consistency.
Disadvantages: poor properties, versatility (can’t easily mould holes). Medium capital investment. Size is limited (hot press, mould, etc).
