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

John Summerscales

Pre-preg (usually epoxy matrix)

• Reinforcement is

pre

-im

preg

nated with B-staged resin • • • o o o

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• • • finite life: there will be a use before date o if out-of-date should not be used for applications which may result in injury, loss or damage.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 temperature NB: the above times are indicative, check the manufacturers’ recommendation

Pre-preg

• key considerations include: o o o

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: o bulk moulding compound (BMC) o o 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: o

V f = a + b √P

Toll and Månson o

i.e. P



V f 2 P = kE(V f n



– V fo n )

k = power-law coefficient  E = elastic modulus of fibres (normal to plane!)    V f V fo = fibre volume fraction = limiting fibre volume fraction, below which P=0 n = power-law exponent also Freundlich equation (see C8 webpage)

Toll and Månson exponents

•

Fibre kE

• • • • • • • • • • • • Wool 13 Wool Planar spun glass roving fluffy glass roving 420 4500 820 260 straight glass roving 700 graphite roving Mat Mat Weave Weave Weave 500 115 100 500 8 15

V fo % n

1.45

2 3

3 3 5 8.5

7 15.5

14.5

3.5

4.5

11 7 9 Reference

J Schofield, J Textile Institute, 1938 [3] CM van Wyk, J Textile Institute, 1946 [4] S Toll et al, ICCM-9, 1993 [5] YR Kim et al, Polymer Composites, 1991 [6] YR Kim et al, Polymer Composites, 1991 [6] YR Kim et al, Polymer Composites, 1991 [6] YR Kim et al, Polymer Composites, 1991 [6] R Gauvin & Chibani, SPI-43, 1988 [7] JA Quinn and Randall, FRC-4, 1990 [1] YR Kim et al, Polymer Composites, 1991 R Gauvin & Chibani, SPI-43, 1988 [7] 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 o o 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 o 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 o o superplastic aluminium sheets, or high-temperature polymeric films diaphragms are clamped in a frame o the laminate is not clamped laminate is formed over mould tool using heat, vacuum and pressure in the autoclave.

Diaphragm forming (DF)

•

disadvantages

o o diaphragms are normally a disposable item 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 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

X X X X

 moulding • •  near vertical surfaces subject to wrinkling may be resolved by the use of o o rubber-block moulding, or hydroforming (pressurised liquid) substituting the male mould half

Compression moulding: materials

Several materials suitable:

• prepreg continuous fibres in epoxy resin • • prepreg short fibres in polyester resin o o o sheet moulding compound (SMC) dough moulding compound (DMC) bulk moulding compound (BMC) prepreg short fibre in a thermoplastic matrix o continuous random orientation filament glass mat thermoplastics (GMT)