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Pre-preg/moulding compoundsVacuum bagAutoclaveDiaphragm moulding Compression moulding.
John Summerscales
Pre-preg (usually epoxy matrix)• Reinforcement is pre-impregnated
with B-staged resino A-stage:
soluble and fusible o B-stage:
swollen but not dissolved by a variety of solvents o 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• finite life: there will be a use before dateo if out-of-date should not be used for
applicationswhich may result in injury, loss or damage.
• out-life:o 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)
o cure at ambient temperature
• low temperature systems:o cure at ~60ºC, out-life typically 3 months
• medium temperature systems:o cure at ~120ºC, out life typically 6 months,
• high temperature systems:o cure at ~180ºC, out-life typically one year.
Out-life increases with cure temperatureNB: the above times are indicative,
check the manufacturers’ recommendation
Pre-preg
• key considerations include:o drape
formability to complex curvatureso tack
stickinesso 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 componentsusually by compression moulding
• Compounds marketed in three major forms:o bulk moulding compound (BMC) o dough moulding compound (DMC) o sheet moulding compound (SMC)
Vacuum bag (VB)
• VB procedures as in lecture C5,but reinforcement now pre-impregnated
Compressibility of fabrics
• Quinn and Randall:o Vf = a + b √P i.e. P Vf
2
• Toll and Månson o P = kE(Vf
n – 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 withpipework to allow a vacuum to be maintained in the bagged work-piece.
• temperature control is normally byo gas- or electric-heating o proportional-integral-derivative (PID) controller
Autoclavesimages 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 technologyo 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.
Diaphragm forming (DF)
• autoclave technique used solely for thermoplastic matrix composites
• laminate laid up flat between 2 diaphragmso superplastic aluminium sheets, oro high-temperature polymeric films
• diaphragms are clamped in a frameo the laminate is not clamped
• laminate is formed over mould tool usingheat, vacuum and pressure in the autoclave.
Diaphragm forming (DF)
• disadvantageso diaphragms are normally a disposable itemo rubber membranes can be used
for limited production runs
o considerable literature on(avoiding) wrinkling of the reinforcement
Compression moulding
• two matched (usually steel) mould halvesmounted in a (normally hydraulic) press
• movement limited to one axisnormal 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
o rubber-block moulding, oro hydroforming (pressurised liquid)
substituting the male mould half
moulding
XXXX
Compression moulding: materials
Several materials suitable:• prepreg continuous fibres in epoxy resin • prepreg short fibres in polyester resin
o sheet moulding compound (SMC)o dough moulding compound (DMC)o bulk moulding compound (BMC)
• prepreg short fibre in a thermoplastic matrixo continuous random orientation filament
glass mat thermoplastics (GMT)