Composites- Pultrusion and Pre-Preg

Question 1

Principle of pultrusion

Answer 1

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

Question 2

Steps in pultrusion process

Answer 2

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

Question 3

Materials used for pultrusion

Answer 3

Any but glass fibre and unsaturated polyester most common

Question 4

Reinforcement features in pultrusion

Answer 4

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

Question 5

Advantages of pultrusion

Answer 5

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

Question 6

Disadvantages of pultrusion

Answer 6

Restricted geometry (straight with constant geometry).
Medium capital investment
Mould materials often expensive (tool steel) and can limit part dimensions.
Relatively slow process

Question 7

Costs of pultrusion

Answer 7

Equipment intermediate
Mould intermediate 
Labour low
Material low (for glass+poly)
Cycle time long

Question 8

Properties from pultrusion

Answer 8

ff 0.2-0.5 (fabrics), 0.65 (rovings)
fv low to inter
Mechanical poor (fabric) to good (rovings)
Quality consistency good

Question 9

Structural pultruded products

Answer 9

Civil engineer applications with constant cross-section. Like rods, beams, piles. Increasingly applied in new and replacement structures, repairs and upgrades of existing structures

Question 10

Comparing pultruded products to metal for civil applications

Answer 10

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

Question 11

What is pre-preg?

Answer 11

Continuous fibres (unidirectional, woven fabrics, etc) pre-impregnated with an uncurled resin

Question 12

How is pre-preg generally used?

Answer 12

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

Question 13

General pre-preg production

Answer 13

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

Question 14

Solution pre-preg production

Answer 14

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

Question 15

Hot melt pre-preg production

Answer 15

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

Question 16

Pre-preg lay-up

Answer 16

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)

Question 17

Applications of pre-preg lay-up

Answer 17

Typically high performance. Like aerospace, automotive, sports and motorsport, defence

Question 18

Advantages of pre-preg lay-up

Answer 18

Best properties available (high ff, low fv).
Versatile (any geometry, mould in holes and fixings).
Can be automated (but very expensive)

Question 19

Disadvantages of pre-preg lay-up

Answer 19

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

Question 20

Pre-preg lay-up costs

Answer 20

Equipment high
Mould high
Labour high
Material high
Cycle time long

Question 21

Properties from pre-preg lay-up

Answer 21

ff 0.7 for unidirectional (less fabrics)
fv low
Mechanical properties good
Quality consistency intermediate

Question 22

Automated lay-up

Answer 22

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

Question 23

How does film stacking work?

Answer 23

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)

Question 24

Other features of film stacking

Answer 24

High ff. systems can be supplied pre-interleaved. Another example of out-of-autoclave technology

Question 25

Composite processing choices

Answer 25

See lecture 10 page 22