Contact moulding: Material MutahirMaterial Mutahir John Summerscales

Spray techniques Hand lamination Vacuum bagging Outline of lecture Spray techniques Hand lamination Vacuum bagging

Spray techniques Image from the Gurit Guide to Composites

Spray techniques hand-held ‘gun’ feeds a stream of chopped fibres into a spray of catalysed liquid resin. stream projected onto the mould tool. deposited materials left to cure under standard atmospheric conditions. fibres normally settle onto the tool in a random (quasi-planar) orientation composite mechanical properties limited by discontinuous fibres (ηl <1), random fibre orientation (ηo =3/8), and no significant consolidation pressure (low Vf).

Spray techniques: equipment pump generally air-operated positive displacement type catalyst pumps may be piston or peristaltic chopper cott wheel moves the fibres through the chopper against the cutter wheel with embedded blades spray head assembly (gun) integrates the chopper and resin mixing.

Spray techniques: mixing common forms of mixing are: external mixing: typically 4 air-driven nozzles in a square pattern around the chopper with two nozzles dispensing resin and two nozzles dispensing catalyst. airless external mixing: catalyst and resin fed from pressure pots with mixing by stream impingement. air-driven internal mixing: air, catalyst and resin are mixed within the head before ejection. airless internal mixing: high pressure mixed in head two-pot system: high pressure resin and catalyst streams arranged to impinge around 150 mm ahead of the gun

Spray techniques: health & safety traditionally a stream of fine droplets so significant vapourisation of styrene CFA “Controlled Spraying” to reduce vapour spray gun pressure calibration overspray containment flanges operator training potential for automation by robots

Spray techniques: advantages low equipment and tooling costs. inexpensive materials cheapest form of fibre is continuous roving fast deposition rates. low labour costs (re. hand-lamination). versatile part shape/laminate configuration, including thickness variations. potential for automation with robots.

Spray techniques: disadvantages H&S for volatile organic compounds (VOC). cost of extraction and treatment of VOC. low production rates when hand-controlled. quality is dependent on operator expertise. difficult to avoid trapped air in the part. dimensional inconsistency in/between batches. only one moulded surface. limited physical properties waste, especially when overspray is significant

Centrifugal casting normally a cylindrical mould rotating with the axis of symmetry horizontal two modes: short fibre spray head travels on that axis fabric reinforcement laid dry and resin sprayed good external surface can easily produce resin-rich inner surface

Hand lamination

Hand lamination fibres placed on mould and wetted by resin. fibres may be of any material aramid, carbon or especially glass chopped strand mat, woven-, knitted-, stitched- or bonded-fabrics, singly or in combination. brushes used to distribute the resin stippling is used to wet chopped strand mat brushing is used for fabric reinforcements rollers used to work air bubbles out.

Hand lamination lay-up should be continuous no break of >24 hours for polyester resin. extended breaks may necessitate cleaning and/or roughening the surface before continuing. for epoxy resins, use of a layer of peel-ply: additional benefit of removing any resin by-product that may have formed at the surface through reaction with the air. peel-ply is removed from the surface just prior to restarting lamination and leaves a clean textured surface.

Hand lamination: advantages I accumulated experience over many years standardised training exists e.g. City and Guilds Hand Laminators Certificate short lead times for component production minimal equipment and tooling costs. relatively inexpensive materials. wide choice of suppliers and material types. versatility of part shape, size and laminate configuration.

Hand lamination: advantages II design changes can be easily effected. cost-competitive for individual items and short production runs of complicated shape. higher fibre volume fractions than for spray. scope for a wide range of physical and mechanical properties in the laminate resin-rich composites can produce high-quality corrosion resistant components. sandwich construction (foam or balsa-wood cores) and inserts are readily incorporated.

Hand lamination: disadvantages I H&S legislation/regulation/voluntary codes cost of extraction and treatment of VOCs. high labour content. product quality and variability dependent on operator expertise potential for resin mixing errors. dimensional inconsistency. low production rates due to cure times of room temperature resins.

HLU: disadvantages II low viscosity resins (low molecular weight) volatile. low thermal and mechanical properties. high shrinkage and potential exotherm especially in resin rich areas/thick components. draining of resin from vertical surfaces may require addition of thixotropic agents. only one moulded surface. low fibre volume fraction/high level of voids high levels of waste

Vacuum bagging NB: this process is fundamentally different to resin infusion (see lecture C7), even though both use a bag !

Vacuum bagging: consumables I Peel-ply: sacrificial open weave fibreglass or perforated heat-set nylon ply to provide the textured and clean surface necessary for further lamination or secondary bonding. Release film: (perforated) sheet to prevent adhesion. Bleeder cloth: non-structural fabric to absorb excess resin/reactants from the laminate. Breather cloth: porous material to provide a gas flow path over the laminate to ensure uniform vacuum pressure permit the escape of air, reactants, moisture and volatiles

Vacuum bagging: consumables II Bagging film: membrane which permits a vacuum to be drawn within the bag. Tacky tape: adhesive strip used to bond the bag to the tool and provide a vacuum seal. Caul plate: tool placed on the laminate inside the bag to define the second surface. Edge dams: Profile to define component edge Intensifiers: hard rubber profiles incorporated in the bag to consolidate the laminate at sharp radii.

Vacuum bagging: equipment Breach unit: connector to permit a vacuum to be drawn through the bagging film. Vacuum pipes: link between the breach unit and the vacuum pump. Resin trap: container in the vacuum line to collect any excess resin before it can damage the vacuum pump. Vacuum pump: generally a high-volume pump (absolute vacuum is rarely required) suitable for continuous running. For slow-curing epoxy resins need 24h operation. Pressure gauges: generally clock-type or digital gauges attached via a breach unit connection.

Vacuum bagging: breach unit, pressure gauge and hose

Vacuum-bagging: advantages as for hand-lamination, except higher fibre volume fractions/lower void contents. better fibre wet-out due to pressure/resin flow. heavier fabrics can be wet-out. volatile organic compounds are largely contained during the curing stage. the additional consolidation pressure can help the reinforcement to conform to tight curvatures. improved mechanical properties consequent upon the higher fibre volume fraction.

Vacuum bagging: disadvantages as for hand lamination, except higher labour skills for the bagging stage. low production rates due to bagging stage joining bagging film for large items. mould tool must be vacuum tight. costs for consumables and equipment. the vacuum pump may strip volatiles from resin consumable material compatibility with resin consolidation pressure limited to 1 atm.