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52.2.5 Autoclaves

•Basically an oven that also uses pressure.

•The part is placed in the pressure vessel, and heated, pressure is applied simultaneously. Vacuum bagging can be used to increase the heating effects.

•The heat accelerates the curing of the thermosets, or melting of the thermoplastic resins.

•The pressure helps bond layers, and remove more voids in the matrix.

•Inert gases are often injected to prevent fires.

•Although autoclaves are expensive, they produce better parts, and can process many parts at the same time.

52.2.6 Filament Winding

•Basic (Typical) Process - A tape of resin impregnated fibres is wrapped over a rotating mandrel to form a part. These windings can be helical or hooped. This continues until the part is thick enough. There are also processes that use dry fibres with resin application later, or prepregs are used.

•Parts vary in size from 1” to 20’

spool of filaments

tensioner

resin bath

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•mixtures of hoop/helical layers, and layers of different materials allow higher strengths in various direction, and resistance to impact damages.

•geodesic paths are commonly preferred with this approach.

•winding speeds are typically 100 m/min.

•typical winding tensions are about 0.1 to 0.5 kg.

•to remove the mandrel, the ends of the parts are cut off when appropriate, or a collapsible mandrel is used when the parts must remain intact. (one way to do this is with low melt temperature alloys).

•entire parts on mandrels can be cured in autoclaves when desired. A rotating mandrel will help reduce the resin flow effects caused by gravity.

•inflatable mandrels can also be used to produce pretensioned parts that are designed for high pressure applications, or parts that need a liner, and they can be easily removed.

•this method is well suited to round parts, or parts undergoing high hoop stresses.

•advantages

-can handle a wide variety of part sizes

-parts can be made with strength in several different directions

-high percentage of material usage

-forming after winding will allow non-cylindrical shapes to be made

-flexible mandrels can be left in as tank liners

-reinforcement panels, and fittings can be inserted during winding

-parts with high pressure ratings can be made

•disadvantages,

-viscosity and pot life of resin must be carefully chosen

-NC programming can be difficult

-Some shapes can’t be made with filament winding

-Factors such as filament tension must be controlled

52.2.7 Pultrusion

• Basic principle - fibers are brought together over rollers, dipped in resin and drawn through a heated die. A continuous cross section composite part emerges on the other side.

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continuous operation for production.

•diamond or carbide saws are used to cut sections of the final part. The saw is designed to track the part as it moves.

•these parts have good axial properties

•Advantages,

-good material usage compared to layup

-high throughput

-higher resin contents are possible

•Disadvantages,

-part cross section should be uniform

-fibre and resin might accumulate at the die opening, leading to increased friction causing jamming, and breakage.

-when excess resin is used, part strength will decrease

-void can result if the die does not conform well to the fibres being pulled

-quick curing systems decrease strength

52.2.8 Resin-Transfer Molding (RTM)

• Basic principle - A mold is filled with fibre, it is closed and resin is injected. The mold is often in vacuum before injection. The pressure of injection wets the fibres.

mold

fiber liner

thermoset resin

•This process was used to make car body panels.

•The fibre in the mold can be any that holds its shape during the injection. Layers are often