Contact moulding: spray and hand lamination techniques

John Summerscales

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

byo discontinuous fibres (ηl <1),

o random fibre orientation (ηo =3/8), and

o no significant consolidation pressure (low Vf).

Spray techniques: equipment

• pumpo generally air-operated positive displacement

type o catalyst pumps may be piston or peristaltic

• choppero cott wheel moves the fibres through the

chopper against the cutter wheel with embedded blades

• spray head assembly (gun)o integrates the chopper and resin mixing.

Spray techniques: mixing• common forms of mixing are:

o external mixing: typically 4 air-driven nozzles in a square pattern around the chopper with two nozzles dispensing resin and two nozzles dispensing catalyst.

o airless external mixing: catalyst and resin fed from pressure pots with mixing by stream impingement.

o air-driven internal mixing: air, catalyst and resin are mixed within the head before ejection.

o airless internal mixing: high pressure mixed in head

o 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 dropletsso significant vapourisation of styrene

• CFA “Controlled Spraying” to reduce vapouro spray gun pressure calibrationo overspray containment flanges o operator training

• potential for automation by robots

Spray techniques: advantages

• low equipment and tooling costs.• inexpensive materials

o 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:o short fibre spray head travels on that axiso 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

o aramid, carbon or especially glasso chopped strand mat, woven-, knitted-, stitched-

or bonded-fabrics, singly or in combination.

• brushes used to distribute the resino stippling is used to wet chopped strand mato brushing is used for fabric reinforcements

• rollers used to work air bubbles out.

Hand lamination

• lay-up should be continuouso no break of >24 hours for polyester resin.o extended breaks may necessitate cleaning

and/or roughening the surface before continuing.

o 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)

o volatile.o low thermal and mechanical properties.

• high shrinkage and potential exothermo especially in resin rich areas/thick components.

• draining of resin from vertical surfaceso 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 too ensure uniform vacuum pressureo 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, excepto higher fibre volume fractions/lower void

contents.o better fibre wet-out due to pressure/resin flow.o heavier fabrics can be wet-out.o volatile organic compounds are largely contained

during the curing stage.o the additional consolidation pressure can help

the reinforcement to conform to tight curvatures.o improved mechanical properties consequent

upon the higher fibre volume fraction.

Vacuum bagging: disadvantages

• as for hand lamination, excepto higher labour skills for the bagging stage.o low production rates due to bagging stageo joining bagging film for large items.o mould tool must be vacuum tight.o costs for consumables and equipment.o the vacuum pump may strip volatiles from

resino consumable material compatibility with resin

• consolidation pressure limited to 1 atm.