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

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• finite life: there will be a use before dateo if out-of-date should not be used for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

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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)