This article, the last in the
series, examines the different composite production methods.
Spray Lay-up
Fibre is chopped in a
hand-held gun and fed into a spray of catalysed resin directed at the mould. The
deposited materials are left to cure under standard atmospheric conditions.
Materials Options:
Resins: Primarily
polyester.
Fibres: Glass roving
only.
Cores: None. These have to be
incorporated separately
Main Advantages:
-
Widely used for many
years.
-
Low cost way of quickly depositing
fibre and resin.
-
Low cost
tooling.
Main Disadvantages:
-
Laminates tend to be very resin-rich
and therefore excessively heavy.
-
Only short fibres are incorporated
which severely limits the mechanical properties of the laminate.
-
Resins need to be low in viscosity
to be sprayable. This generally compromises their mechanical/thermal
properties.
-
The high styrene contents
of spray lay-up resins generally means that they have the potential to be more
harmful and their lower viscosity means that they have an increased tendency to
penetrate clothing etc.
-
Limiting airborne styrene
concentrations to legislated levels is becoming increasingly difficult.
Typical Applications:
Simple enclosures,
lightly loaded structural panels, e.g. caravan bodies, truck fairings, bathtubs,
shower trays, some small dinghies.
Wet Lay-up/Hand Lay-up
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.
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 means 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 etc.
-
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.
Typical Applications:
Standard wind-turbine
blades, production boats, architectural mouldings.
Vacuum Bagging
This is basically an
extension of the wet lay-up process described above where pressure is applied to
the laminate once laid-up in order to improve its consolidation. This is
achieved by sealing a plastic film over the wet laid-up laminate and onto the
tool. The air under the bag is extracted by a vacuum pump and thus up to one
atmosphere of pressure can be applied to the laminate to consolidate it.
Materials Options:
Resins: Primarily epoxy
and phenolic. Polyesters and vinylesters may have problems due to excessive
extraction of styrene from the resin by the vacuum pump.
Fibres: The consolidation
pressures mean that a variety of heavy fabrics can be wet-out.
Cores: Any.
Main Advantages:
-
Higher fibre content laminates can
usually be achieved than with standard wet lay-up techniques.
-
Lower void contents are achieved
than with wet lay-up.
-
Better fibre wet-out due to pressure
and resin flow throughout structural fibres, with excess into bagging
materials.
-
Health and safety: The
vacuum bag reduces the amount of volatiles emitted during cure.
Main Disadvantages:
-
The extra process adds cost both in
labour and in disposable bagging materials
-
A higher level of skill is required
by the operators
-
Mixing and control of
resin content still largely determined by operator skill
Typical Applications:
Large, one-off cruising
boats, racecar components, core-bonding in production boats.
Filament Winding
This process is primarily
used for hollow, generally circular or oval sectioned components, such as pipes
and tanks. Fibre tows are passed through a resin bath before being wound onto a
mandrel in a variety of orientations, controlled by the fibre feeding mechanism,
and rate of rotation of the mandrel.
Materials Options:
Resins: Any, e.g. epoxy,
polyester, vinylester, phenolic.
Fibres: Any. The fibres
are used straight from a creel and not woven or stitched into a fabric form.
Cores: Any, although
components are usually single skin.
Main Advantages:
-
This can be a very fast and
therefore economic method of laying material down.
-
Resin content can be controlled by
metering the resin onto each fibre tow through nips or dies.
-
Fibre cost is minimised since there
is no secondary process to convert fibre into fabric prior to use.
-
Structural properties of
laminates can be very good since straight fibres can be laid in a complex
pattern to match the applied loads.
Main
Disadvantages:
-
The process is limited to convex
shaped components.
-
Fibre cannot easily be laid exactly
along the length of a component.
-
Mandrel costs for large components
can be high.
-
The external surface of the
component is unmoulded, and therefore cosmetically unattractive.
-
Low viscosity resins
usually need to be used with their attendant lower mechanical and health and
safety properties.
Typical Applications:
Chemical storage tanks
and pipelines, gas cylinders, fire-fighters breathing tanks.
Pultrusion
Fibres are pulled from a creel
through a resin bath and then on through a heated die. The die completes the
impregnation of the fibre, controls the resin content and cures the material
into its final shape as it passes through the die. This cured profile is then
automatically cut to length. Fabrics may also be introduced into the die to
provide fibre direction other than at 0 degrees.
Although pultrusion is a
continuous process, producing a profile of constant cross-section, a variant
known as ‘pulforming’ allows for some variation to be introduced into the
cross-section. The process pulls the materials through the die for impregnation,
and then clamps them in a mould for curing. This makes the process
non-continuous, but accommodating of small changes in cross-section.
Materials Options:
Resins: Generally epoxy,
polyester, vinylester and phenolic.
Fibres: Any.
Cores: Not generally
used.
Main
Advantages:
-
This can be a very fast, and
therefore economic, way of impregnating and curing materials.
-
Resin content can be accurately
controlled.
-
Fibre cost is minimised since the
majority is taken from a creel.
-
Structural properties of laminates
can be very good since the profiles have very straight fibres and high fibre
volume fractions can be obtained.
-
Resin impregnation area
can be enclosed thus limiting volatile emissions.
Main
Disadvantages:
-
Limited to constant or near constant
cross-section components
-
Heated die costs can be
high.
Typical Applications:
Beams and girders used in
roof structures, bridges, ladders, frameworks.
Resin Transfer
Moulding (RTM)
Fabrics are laid up as a
dry stack of materials. These fabrics are sometimes pre-pressed to the mould
shape, and held together by a binder. These ‘preforms’ are then more easily laid
into the mould tool. A second mould tool is then clamped over the first, and
resin is injected into the cavity. Vacuum can also be applied to the mould
cavity to assist resin in being drawn into the fabrics. This is known as Vacuum
Assisted Resin Injection (VARI). Once all the fabric is wet out, the resin
inlets are closed, and the laminate is allowed to cure. Both injection and cure
can take place at either ambient or elevated temperature.
Materials Options:
Resins: Generally epoxy,
polyester, vinylester and phenolic, although high temperature resins such as
bismaleimides can be used at elevated process temperatures.
Fibres: Any. Stitched
materials work well in this process since the gaps allow rapid resin transport.
Some specially developed fabrics can assist with resin flow.
Cores: Not honeycombs,
since cells would fill with resin, and pressures involved can crush some foams.
Main Advantages:
-
High fibre volume laminates can be
obtained with very low void contents.
-
Good health and safety,
and environmental control due to enclosure of resin.
-
Possible labour
reductions.
-
Both sides of the
component have a moulded surface.
Main
Disadvantages:
-
Matched tooling is expensive, and
heavy in order to withstand pressures.
-
Generally limited to smaller
components.
-
Unimpregnated areas can
occur resulting in very expensive scrap parts.
Typical Applications:
Small
complex aircraft and automotive components, train seats.
Other Infusion
Processes - SCRIMP, RIFT, VARTM etc.
Fabrics are laid up as a
dry stack of materials as in RTM. The fibre stack is then covered with peel ply
and a knitted type of non-structural fabric. The whole dry stack is then vacuum
bagged, and once bag leaks have been eliminated, resin is allowed to flow into
the laminate. The resin distribution over the whole laminate is aided by resin
flowing easily through the non-structural fabric, and wetting the fabric out
from above.
Materials Options:
Resins: Generally epoxy,
polyester and vinylester.
Fibres: Any conventional
fabrics. Stitched materials work well in this process since the gaps allow rapid
resin transport.
Cores: Any except
honeycombs.
Main
Advantages:
-
As RTM above, except only one side
of the component has a moulded finish.
-
Much lower tooling cost due to one
half of the tool being a vacuum bag, and less strength being required in the
main tool.
-
Large components can be
fabricated.
-
Standard wet lay-up tools may be
able to be modified for this process.
-
Cored structures can be
produced in one operation.
Main
Disadvantages:
-
Relatively complex process to
perform well.
-
Resins must be very low in
viscosity, thus comprising mechanical properties.
-
Unimpregnated areas can occur
resulting in very expensive scrap parts.
-
Some elements of this
process are covered by patents (SCRIMP).
Typical Applications: Semi-production small yachts, train and truck body panels.
Prepregs Autoclave
Fabrics and fibres are
pre-impregnated by the materials manufacturer, under heat and pressure or with
solvent, with a pre-catalysed 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
degrees 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 pressurised 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.
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 minimised in
unidirectional tapes since there is no secondary process to convert fibre into
fabric prior to use.
-
Resin chemistry can be optimised 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 optimised, complex
lay-ups can be readily achieved.
-
Potential for automation
and labour saving.
Main
Disadvantages:
-
Materials cost is higher for
pre-impregnated 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.
Typical Applications:
Aircraft structural
components (e.g. wings and tail sections), F1 racing cars, sporting goods such
as tennis racquets and skis.
Low Temperature Curing
Prepregs Oven
Low Temperature Curing prepregs are
made exactly as conventional prepregs but have resin chemistries that allow cure
to be achieved at temperatures from 60-100 degrees C. At 60 degrees C, the
working life of the material may be limited to as little as a week, but above
this working times can be as long as several months. The flow profiles of the
resin systems allow for the use of vacuum bag pressures alone, avoiding the need
for autoclaves.
Materials Options:
Resins: Generally only
epoxy.
Fibres: Any. As for
conventional prepregs.
Cores: Any, although
standard PVC foam needs special care.
Main
Advantages:
-
All of the advantages 1-6 associated
with the use of conventional prepregs are incorporated in low-temperature curing
prepregs.
-
Cheaper tooling materials, such as
wood, can be used due to the lower cure temperatures involved.
-
Large structures can be
readily made since only vacuum bag pressure is required, and heating to these
lower temperatures can be achieved with simple hot-air circulated ovens, often
built in-situ over the component.
-
Conventional PVC foam core materials
can be used, providing certain procedures are followed.
-
Lower energy cost.
Main
Disadvantages:
-
Materials cost is still
higher than for non-preimpregnated fabrics.
-
An oven and vacuum bagging system is
required to cure the component.
-
Tooling needs to be able to
withstand above-ambient temperatures involved (typically 60-100 degrees C).
-
Still an energy cost
associated with above-ambient cure temperature.
Typical Applications:
High-performance
wind-turbine blades, large racing and cruising yachts, rescue craft, train
components.
Resin Film Infusion
(RFI)
Dry fabrics are laid up
interleaved with layers of semi-solid resin film supplied on a release paper.
The lay-up is vacuum bagged to remove air through the dry fabrics, and then
heated to allow the resin to first melt and flow into the air-free fabrics, and
then after a certain time, to cure.
Materials Options:
Resins: Generally epoxy
only.
Fibres: Any.
Cores: Most, although PVC
foam needs special procedures due to the elevated temperatures involved in the
process.
Main
Advantages:
-
High fibre volumes can be
accurately achieved with low void contents.
-
Good health and safety
and a clean lay-up, like prepreg.
-
High resin mechanical
properties due to solid state of initial polymer material and elevated
temperature cure.
-
Potentially lower cost
than prepreg, with most of the advantages.
-
Less likelihood of dry
areas than SCRIMP process due to resin travelling through fabric thickness only.
Main Disadvantages:
-
Not widely proven outside the
aerospace industry.
-
An oven and vacuum
bagging system is required to cure the component as for prepreg, although the
autoclave systems used by the aerospace industry are not always required.
-
Tooling needs to be able to
withstand the process temperatures of the resin film (which if using similar
resin to those in low-temperature curing prepregs, is typically 60-100 degrees
C).
-
Core materials need to be
able to withstand the process temperatures and pressures.
Typical Applications:
Aircraft radomes and
submarine sonar domes.