Question

1) What are the differences between the wet lay-up approach and the prepreg approach in hand lay-up? Include both process differences and differences in the final product. Make a list.

• Differences  

• Similarities

Answer 1

Wet/Hand Lay-up

Description:

Resins are impregnated by hand into fibres which are in the form of woven, knitted, stitched or bonded fabrics. This is usually accomplished by rollers or brushes, with an increasing use of nip-roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath of resin. Laminates are left to cure under standard atmospheric conditions.

Materials Options:

• Resins: Any, e.g. epoxy, polyester, vinylester, phenolic
• Fibres: Any, although heavy aramid fabrics can be hard to wet-out by hand.

• Cores: Any.

Typical Applications:

Standard wind-turbine blades, production boats, architectural mouldings.

Main Advantages:

• Widely used for many years.
• Simple principles to teach.
• Low cost tooling, if room-temperature cure resins are used.
• Wide choice of suppliers and material types.
• Higher fibre contents and longer fibres than with spray lay-up.

Main Disadvantages:

• Resin mixing, laminate resin contents, and laminate quality are very dependent on the skills of laminators. Low resin content laminates cannot usually be achieved without the incorporation of excessive quantities of voids.
• Health and safety considerations of resins. The lower molecular weights of hand lay-up resins generally mean that they have the potential to be more harmful than higher molecular weight products. The lower viscosity of the resins also means that they have an increased tendency to penetrate clothing.
• Limiting airborne styrene concentrations to legislated levels from polyesters and vinylesters is becoming increasingly hard without expensive extraction systems.
• Resins need to be low in viscosity to be workable by hand. This generally compromises their mechanical/thermal properties due to the need for high diluent/styrene levels.

Prepregs

Description:

Fabrics and fibres are pre-impregnated by the materials manufacturer, under heat and pressure or with solvent, with a pre-catalyzed resin. The catalyst is largely latent at ambient temperatures giving the materials several weeks, or sometimes months, of useful life when defrosted. However to prolong storage life the materials are stored frozen. The resin is usually a near-solid at ambient temperatures, and so the pre-impregnated materials (prepregs) have a light sticky feel to them, such as that of adhesive tape. Unidirectional materials take fibre direct from a creel, and are held together by the resin alone. The prepregs are laid up by hand or machine onto a mould surface, vacuum bagged and then heated to typically 120-180°C. This allows the resin to initially reflow and eventually to cure. Additional pressure for the moulding is usually provided by an autoclave (effectively a pressurized oven) which can apply up to 5 atmospheres to the laminate.

Materials Options:

• Resins: Generally epoxy, polyester, phenolic and high temperature resins such as polyimides, cyanate esters and bismaleimides.
• Fibres: Any. Used either direct from a creel or as any type of fabric.
• Cores: Any, although special types of foam need to be used due to the elevated temperatures involved in the process.

Typical Applications:

Aircraft structural components (e.g. wings and tail sections), F1 racing cars, sporting goods such as tennis racquets and skis.

Main Advantages:

• Resin/catalyst levels and the resin content in the fibre are accurately set by the materials manufacturer. High fibre contents can be safely achieved.
• The materials have excellent health and safety characteristics and are clean to work with.
• Fibre cost is minimized in unidirectional tapes since there is no secondary process to convert fibre into fabric prior to use.
• Resin chemistry can be optimized for mechanical and thermal performance, with the high viscosity resins being impregnable due to the manufacturing process.
• The extended working times (of up to several months at room temperatures) means that structurally optimized, complex lay-ups can be readily achieved.
• Potential for automation and labour saving.

Main Disadvantages:

• Materials cost is higher for preimpregnated fabrics.
• Autoclaves are usually required to cure the component. These are expensive, slow to operate and limited in size.
• Tooling needs to be able to withstand the process temperatures involved.
• Core materials need to be able to withstand the process temperatures and pressures.