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

1
Pre-preg/moulding compoundsVacuum
bagAutoclaveDiaphragm moulding Compression
moulding.

• John Summerscales

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Pre-preg (usually epoxy matrix)

• Reinforcement is pre-impregnated
• with B-staged resin
• 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 gt better quality
• expensive relative to dry reinforcements

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Pre-preg

• use before date there will be a finite life
• if out-of-date should not be used for
  applicationswhich may result in injury, loss or
  damage.
• out-life
• time outside cold storagewill reduce its useful
  life
• normal to allow the material to warmto ambient
  temperature before use as condensation may form
  on cold material 

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Pre-preg systems

• cold-cure (not normally prepreg)
• cure at ambient temperature
• low temperature systems
• cure at 60ºC, out-life 3 months
• medium temperature systems
• cure at 120ºC, out life 6 months,
• high temperature systems
• 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

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Pre-preg

• key considerations include
• drape
• natural ability to follow complex curvature
• and conformability
• ability to be (manually) formed to complex
  curvature
• tack
• stickiness
• debulk
• the laminate stack is subject to vacuum every
  few layers in temporary bag or vacuum table.

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Vacuum bag (VB)

• VB procedures as in lecture C5 (hand-lay),but
  reinforcement now pre-impregnated

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Pearce compressibility curves

• 625 gsm plain weave E-glass
• one (black) or five (red) layers

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Compressibility of fabrics

• Quinn and Randall
• Vf a b vP i.e. P ? Vf2
• Toll and Månson
• 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
  P0
• n power-law exponent
• also Freundlich equation (see C8 webpage)

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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

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Autoclave

• only possible to apply1000 mbar pressure with a
  vacuum bag
• greater levels of consolidation requirean
  autoclave advanced pressure cooker
• autoclave is a pressure vessel withpipework to
  allow a vacuumto be maintained in the bagged
  work-piece.
• temperature control is normally by
• gas- or electric-heating,
• proportional-integral-derivative (PID) controller

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Autoclavesimages from Google search autoclave
image
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Autoclave
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Autoclave temp./pressure cycle
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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 designedto permit
  circulation of heated air
• VB consumables may be a thermal barrier

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Pressclave

• pressclave alternative relatively inexpensive
  technology
• normally a hinged frame with an elastomeric
  membrane so vacuum can be drawn under the
  membrane via perforations in the pressclave
  baseand external pressure applied outside
  membrane.

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Diaphragm forming (DF)

• autoclave technique used solely for thermoplastic
  matrix composites
• laminate laid up flat between 2 diaphragms
• superplastic aluminium sheets, or
• high-temperature polymeric films
• diaphragms are clamped in a frame
• the laminate is not clamped
• laminate is formed over mould tool usingheat,
  vacuum and pressure in the autoclave. 

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Diaphragm forming (DF)

• disadvantages
• diaphragms are normally a disposable item
• rubber membranes can be used for limited
  production runs
• considerable literature on(avoiding) wrinkling
  of the reinforcement

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Diaphragm forming (DF)

• Solvay Applications Centre are developingrobotic
  pick-and-place of dry fibre, or
  prepregs,combined with diaphragm formingand use
  of matched press toolingto produce complex 3D
  componentsat rates required by the automotive
  industry.
• the polymeric diaphragms serve a secondary
  roleas post-production protection/delivery
  packaging

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Compression moulding

• two matched (usually steel) mould halvesmounted
  in a (normally hydraulic) press
• movement limited to one axisnormal to the plane
  of the mould

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Compression moulding materials

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

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Compressionmoulding compounds

• normally unsaturated polyester resin matrix
• normally short fibre reinforcement
• normally supplied "just in time" for production
  of composite componentsusually by compression
  moulding
• Compounds marketed in three major forms
• bulk moulding compound (BMC)
• dough moulding compound (DMC)
• sheet moulding compound (SMC)

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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
• rubber-block moulding, or
• hydroforming (pressurised liquid)
• substituting the male mould half

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Any questions ?

• Summary of lecture
• pre-preg/moulding compounds
• vacuum bag
• autoclave
• diaphragm moulding
• compression moulding