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

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

Pre-preg finite life: there will be a use before date out-life: if out-of-date should not be used for applications which may result in injury, loss or damage.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

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

Pre-preg key considerations include: drape formability to complex curvatures tack stickiness debulk every few layers subject the stack to vacuum in temporary bag or a vacuum table.

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

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

Compressibility of fabrics Quinn and Randall: Vf = a + b √P i.e. P  Vf2 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)

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

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 with pipework 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

Autoclaves images from Google search “autoclave + image”

Autoclave

Autoclave temp./pressure cycle

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

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

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 using heat, vacuum and pressure in the autoclave. 

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

Compression moulding two matched (usually steel) mould halves mounted in a (normally hydraulic) press movement limited to one axis normal to the plane of the mould

Compression 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 moulding X 

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 filament glass mat thermoplastics (GMT)