Composites- Pultrusion and Pre-Preg Flashcards
Principle of pultrusion
Continuous reinforcements are drawn through a liquid resin bath then shaped and compacted in a die. Similar to thermoplastic extrusion with slower throughput. Part has a constant cross section
Steps in pultrusion process
Assemble the reinforcements from rovings. They are drawn through guides into the resin bath (drum or dip type). Drawn through the preformer into the die. Caterpillar pull-off provide motion to the system. Cut to length. Resin viscosity must be carefully controlled
Materials used for pultrusion
Any but glass fibre and unsaturated polyester most common
Reinforcement features in pultrusion
Unidirectional rovings (bundles of straight fibres) for longitudinal properties. Multiaxial woven tapes, fabrics and mats for transverse properties. Surface veils can be used to improve final appearance
Advantages of pultrusion
Any length.
Good surfaces.
Excellent fibre alignment (true unidirectional possible).
Once process is stable, relatively automated (only start-up and shut-down are manual).
Good quality consistency
Disadvantages of pultrusion
Restricted geometry (straight with constant geometry).
Medium capital investment
Mould materials often expensive (tool steel) and can limit part dimensions.
Relatively slow process
Costs of pultrusion
Equipment intermediate Mould intermediate Labour low Material low (for glass+poly) Cycle time long
Properties from pultrusion
ff 0.2-0.5 (fabrics), 0.65 (rovings)
fv low to inter
Mechanical poor (fabric) to good (rovings)
Quality consistency good
Structural pultruded products
Civil engineer applications with constant cross-section. Like rods, beams, piles. Increasingly applied in new and replacement structures, repairs and upgrades of existing structures
Comparing pultruded products to metal for civil applications
Pultruded more expensive. Excellent specific strength and stiffness. Panels and sections easily pre-fabricated. Cheaper cranage (lighter parts). Quicker construction with less downtime. Less maintenance. Longer service life as lighter and corrosion resistant
What is pre-preg?
Continuous fibres (unidirectional, woven fabrics, etc) pre-impregnated with an uncurled resin
How is pre-preg generally used?
It is a B-staged matrix (partially cured resin, produced in flat sheets, rolled up, frozen). Defrost before use. Cut shapes to size, assemble layers on a mould to required thickness, cure
General pre-preg production
Typically resins used are high viscosity at RT. Reduce viscosity by dissolving in solvent (solution method) but is discouraged by environmental legislation. Or heat resin (hot melt method) so it starts to gel prematurely (not so hot to cure). Can add toughening agents like thermoplastic which dissolves and disperses. Or can extruder fine fibres of thermoplastic which are mingled with reinforcement
Solution pre-preg production
Feed in continuous reinforcement. Dissolve resin in a solvent. Draw fibres through resin bath. Squeeze out any excess and meter thickness. Evaporate solvent off in oven. B-stage resin but retain tack. Apply release film for spooling/handling. Freeze and store
Hot melt pre-preg production
Feed in continuous reinforcement. Melt resin and form stable film. Force resin and reinforcement together. Meter thickness. N-stage resin but retain tack. Apply release film for spooling/handling. Freeze and store
Pre-preg lay-up
The plies are unidirectional and 0.125-0.25mm thick or woven with wider thickness range. Complex stacking sequences of ply angles to balance laminate stresses, shrinkage and shearing effects. Cure at elevated temperature (80-180C). Can use vacuum bag to improve consolidation (remove volatiles and surface voids and improve ff). Vacuum bag and autoclave for best results (good ff improvement, compresses internal trapped voids)
Applications of pre-preg lay-up
Typically high performance. Like aerospace, automotive, sports and motorsport, defence
Advantages of pre-preg lay-up
Best properties available (high ff, low fv).
Versatile (any geometry, mould in holes and fixings).
Can be automated (but very expensive)
Disadvantages of pre-preg lay-up
Very expensive (for autoclave).
Equipment, moulds, labour, materials expensive.
Only one good surface (effect can be minimised).
Size limited by autoclave and mould.
Properties skill-dependent
Pre-preg lay-up costs
Equipment high Mould high Labour high Material high Cycle time long
Properties from pre-preg lay-up
ff 0.7 for unidirectional (less fabrics)
fv low
Mechanical properties good
Quality consistency intermediate
Automated lay-up
Aims to reduce per-part cost, lay-up time, increase uniformity. All involve a robot (arm or gantry) laying a narrow strip (8mm or less) of unidirectional pre-preg on a mould or mandrel. Very expensive equipment
How does film stacking work?
Reinforcement fabrics stacked together. Use thin films of frozen thermoset resin, thin films of thermoplastic polymer. Lay-up dry fibre preform. Interleave with resin. Cure in hot press or autoclave (heat+pressure)
Other features of film stacking
High ff. systems can be supplied pre-interleaved. Another example of out-of-autoclave technology
Composite processing choices
See lecture 10 page 22
