Pre-preg/moulding compounds Vacuum bag Autoclave Diaphragm moulding Compression moulding.

1. Pre-preg/moulding compoundsVacuum bagAutoclaveDiaphragm moulding Compression moulding. John Summerscales

2. Pre-preg (usually epoxy matrix) Reinforcement is pre-impregnated with B-staged resin A-stage: soluble and fusible B-stage: swollen but not dissolved by a variety of solvents C-stage:rigid, hard, insoluble, infusible safer than liquid resins mixing done by suppliers > better quality expensive relative to dry reinforcements

3. Pre-preg • finite life: there will be a use before date • if out-of-date should not be used for applicationswhich may result in injury, loss or damage. • out-life: • time outside cold storagewill reduce its useful life • normal to allow the material to warm to ambient temperature before use as condensation may form on cold material 

4. Pre-preg systems • cold-cure (not normally prepreg) • cure at ambient temperature • low temperature systems: • cure at ~60ºC, out-life typically 3 months • medium temperature systems: • cure at ~120ºC, out life typically 6 months, • high temperature systems: • cure at ~180ºC, out-life typically one year. Out-life increases with cure temperature NB: the above times are indicative,check the manufacturers’ recommendation

5. Pre-preg • key considerations include: • drapeformability to complex curvatures • tack stickiness • debulkevery few layers subject the stack to vacuum in temporary bag or a vacuum table.

6. Moulding compounds • normally unsaturated polyester resin matrix • normally short fibre reinforcement • normally supplied "just in time" for the production of composite componentsusually by compression moulding • Compounds marketed in three major forms: • bulk moulding compound (BMC) • dough moulding compound (DMC) • sheet moulding compound (SMC)

7. Vacuum bag (VB) • VB procedures as in lecture C5,but reinforcement now pre-impregnated

8. Compressibility of fabrics • Quinn and Randall: • Vf = a + b √P • Toll and Månson • P = kE(Vfn – Vfon) • k = power-law coefficient • E = elastic modulus of fibres (normal to plane!) • Vf = fibre volume fraction • Vfo = limiting fibre volume fraction, below which P=0 • n = power-law exponent • also Freundlich equation (see C8 webpage)

9. Toll and Månson exponents • Fibre kE Vfo % n Reference • Wool 13 1.45 3 J Schofield, J Textile Institute, 1938 [3] • Wool 420 2 3CM van Wyk, J Textile Institute, 1946 [4] • Planar 4500 3 5S Toll et al, ICCM-9, 1993 [5] • spun glass roving 820 8.5YR Kim et al, Polymer Composites, 1991 [6] • fluffy glass roving 260 7YR Kim et al, Polymer Composites, 1991 [6] • straight glass roving 700 15.5YR Kim et al, Polymer Composites, 1991 [6] • graphite roving 500 14.5YR Kim et al, Polymer Composites, 1991 [6] • Mat 115 3.5R Gauvin & Chibani, SPI-43, 1988 [7] • Mat 100 4.5 JA Quinn and Randall, FRC-4, 1990 [1] • Weave 500 11YR Kim et al, Polymer Composites, 1991 • Weave 8 7R Gauvin & Chibani, SPI-43, 1988 [7] • Weave 15 9 JA Quinn and Randall, FRC-4, 1990 [1]

10. Autoclave • only possible to apply~1000 mbar pressure with a vacuum bag • to achieve greater levels of consolidation,use an autoclave: advanced pressure cooker • autoclave is a pressure vessel withpipework to allow a vacuum to be maintained in the bagged work-piece. • temperature control is normally by • gas- or electric-heating • proportional-integral-derivative (PID) controller

11. Autoclavesfrom http://www.aeroform.co.uk/pics

12. Autoclave

13. Autoclave temp./pressure cycle

14. Autoclave • dwell to get correct resin viscosity • cure to achieve optimum properties • high capital cost equipment • long cycle times • economics demands high autoclave loading • mould tools designed to permit circulation of heated air • VB consumables may be a thermal barrier

15. Pressclave • pressclave: alternative relatively inexpensive technology • normally a hinged frame with an elastomeric membrane so vacuum can be drawn under the membranevia perforations in the pressclave base and external pressure is applied outside the membrane.

16. Diaphragm forming (DF) • autoclave technique used solely for thermoplastic matrix composites • laminate laid up flat between 2 diaphragms • superplastic aluminium sheets, or • high-temperature polymeric films • diaphragms are clamped in a frame • the laminate is not clamped • laminate is formed over mould tool usingheat, vacuum and pressure in the autoclave. 

17. Diaphragm forming (DF) • disadvantages • diaphragms are normally a disposable item • rubber membranes can be used for limited production runs • considerable literature on(avoiding) wrinkling of the reinforcement

18. Compression moulding • two matched (usually steel) mould halvesmounted in a (normally hydraulic) press • movement limited to one axisnormal to the plane of the mould

19. Compression moulding X X X X  moulding  • unlike VB/autoclave processes,no consolidation pressure on vertical surfaces • near vertical surfaces subject to wrinkling • may be resolved by the use of • rubber-block moulding, or • hydroforming (pressurised liquid) substituting the male mould half

20. Compression moulding: materials Several materials suitable: • prepreg continuous fibres in epoxy resin • prepreg short fibres in polyester resin • sheet moulding compound (SMC) • dough moulding compound (DMC) • bulk moulding compound (BMC) • prepreg short fibre in a thermoplastic matrix • continuous random orientation filamentglass mat thermoplastics (GMT)