Prepregmoulding compounds Vacuum bag Autoclave Diaphragm moulding Compression

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: 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Pre-preg • use before date: there will be a finite life o if out-of-date should not be used for applications which may result in injury, loss or damage.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 • (not ambient cure) (not normally prepreg) • 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 v o … and conformability v o ability to be (manually) formed to complex curvature tack v o natural ability to follow complex curvature stickiness debulk v the laminate stack is subject to vacuum every few layers in temporary bag or vacuum table.

Tube rolling (mandrel wrapping) • a technique where pre-preg is formed onto a tapered mandrel and consolidated using shrink-wrap. • most often used to make fishing rod blanks • Kilwell Fibretube (NZ) technical information • You. Tube video (Mister Ataraxia)

Tube rolling ~ micrographs solid unidirectional tip inserted in rolled blank evidence of voids Courtesy of Gregory Benson, 2016

Vacuum bag (VB) • VB procedures as in lecture C 5 (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 Vf 2 • Toll and Månson o P = k. E(Vfn – Vfon) k = power-law coefficient v E = elastic modulus of fibres (normal to plane!) v Vf = fibre volume fraction v Vfo = limiting fibre volume fraction, below which P=0 v n = power-law exponent v • also Freundlich equation (see C 8 webpage)

Toll and Månson exponents • Fibre k. E Vfo % n • • • 13 420 4500 820 260 700 500 115 100 500 8 15 1. 45 2 3 Wool Planar spun glass roving fluffy glass roving straight glass roving graphite roving Mat 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] 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 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 • 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. Hydroclave extremely high costs and elevated risk o pressurized with liquid water at high temperature o only for exceptional requirements o v curing of ablative composite rocket engine nozzles and missile nosecones

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

Diaphragm forming (DF) • Solvay Applications Centre are developing robotic pick-and-place of dry fibre, or prepregs, combined with diaphragm forming and use of matched press tooling to produce complex 3 D components at rates required by the automotive industry. • the polymeric diaphragms serve a secondary role as post-production protection/delivery packaging

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 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. . pressclave/hydroclave • diaphragm moulding • compression moulding