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
o A-stage:
o
o
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if out-of-date should not be used for applications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:
drape
formability to complex curvatures
o tack
stickiness
o debulk
every few layers subject the stack to vacuum in
temporary bag or a vacuum table.
o
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)
o dough moulding compound (DMC)
o sheet moulding compound (SMC)
o
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  Vf2
• Toll and Månson
o
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
•
•
•
•
•
•
•
•
•
•
•
•
13
420
4500
820
260
700
500
115
100
500
8
15
1.45
2
3
Wool
Wool
Planar
spun glass roving
fluffy glass roving
straight glass roving
graphite roving
Mat
Mat
Weave
Weave
Weave
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
gas- or electric-heating
o proportional-integral-derivative (PID) controller
o
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
superplastic aluminium sheets, or
o high-temperature polymeric films
o
• 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
diaphragms are normally a disposable item
o rubber membranes can be used
for limited production runs
o
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
rubber-block moulding, or
o hydroforming (pressurised liquid)
o
substituting the male mould half
Compression moulding: materials
Several materials suitable:
• prepreg continuous fibres in epoxy resin
• prepreg short fibres in polyester resin
sheet moulding compound (SMC)
o dough moulding compound (DMC)
o bulk moulding compound (BMC)
o
• prepreg short fibre in a thermoplastic matrix
o
continuous random orientation filament
glass mat thermoplastics (GMT)