Contact moulding: spray and hand lamination techniques

1. Contact moulding: spray and hand lamination techniques John Summerscales

2. Outline of lecture Spray techniques Hand lamination Vacuum bagging

3. Spray techniques Image from the Gurit Guide to Composites

4. 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).

5. 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 headassembly (gun) • integrates the chopper and resin mixing.

6. 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

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

8. 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.

9. 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

10. 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

11. Hand lamination

12. 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.

13. 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.

14. Hand lamination: advantages I • accumulated experience over many years • standardised training existse.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.

15. 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.

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

17. 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

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

19. 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

20. 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.

21. 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.

22. Vacuum bagging: breach unit, pressure gauge and hose

23. 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.

24. 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.