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
• use before date: there will be a finite life
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if out-of-date should not be used for applications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 ~3 months
• medium temperature systems:
o
cure at ~120ºC, out life ~6 months,
• high temperature systems:
o
cure at ~180ºC, out-life ~12 months.
… above temperatures may be ±20ºC
Out-life increases with cure temperature
NB: the above times are indicative,
check the manufacturers’ recommendation
Pre-preg
• key considerations include:
o
drape

o
… and conformability

o
ability to be (manually) formed to complex curvature
tack

o
natural ability to follow complex curvature
stickiness
debulk

the laminate stack is subject to vacuum
every few layers in temporary bag or vacuum table.
Vacuum bag (VB)
• VB procedures as in lecture C5 (hand-lay),
but reinforcement now pre-impregnated
Pearce compressibility curves
• 625 gsm plain weave E-glass
• one (black) or five (red) layers
http://dx.doi.org/10.1016/0956-7143(95)93709-S
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
• greater levels of consolidation require
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 must be 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 applied outside 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: 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)
Compression:
moulding compounds
• normally unsaturated polyester resin matrix
• normally short fibre reinforcement
• normally supplied "just in time" for
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
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
Any questions ?
Summary of lecture
•pre-preg/moulding compounds
•vacuum bag
•autoclave
•diaphragm moulding
•compression moulding