7/29/2019 Composites Notes UNit 5 n
1/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
1 DEPARTMENT OF AERONAUTICAL ENGINEERING
UNIT-5 MANUFACTURING PROCESS OF COMPOSITES
1. Autoclave Molding
Figure 3.6 shows the vacuum bag lay-up sequence for a typical epoxy matrix prepreg composite.
Different lay-up sequences can be used for other types of prepregs.
1. Thoroughly clean the aluminum plate (10) using acetone or a detergent. Then apply mold-release agent
to the top surface of the aluminum plate twice.
2. Lay one sheet of Teflon film (1) and the peel-ply (2) (nonstick nylon cloth) on the aluminum plate. The
Teflon film is used to release the lay-up from the aluminum plate, and the peel-ply is used to achieve the
required surface finish on the laminate. Note: There should be no wrinkles or raised regions in the peel-
ply, and its dimensions should be identical to those of the laminate.
3. Place the prepreg stack (3) on the plate, being sure to keep it at least 50 mm from each edge. Note: Do
not cover up the vacuum connection in the plate.
4. Place a strip of the corkrubber material (9) along each edge of the panel, making sure that no gaps
exist and a complete dam is formed around the laminate. The dam around the lay-up prevents lateral
motion of the panel, and minimizes resin flow parallel to the aluminum plate and through the edges of the
laminate (9).
5. Completely encircle the prepreg stack and dam with bagging adhesive making sure that the adhesive
material is adjacent to the dam. The purpose of the adhesive material is to form a vacuum seal.6. Place a peel-ply (4) and a ply of Teflon-coated glass fabric (5) (with the same dimensions as the panel)
on top of the prepreg stack. The purpose of the Teflon-coated glass fabric is to prevent the bleeder sheets
(6) from sticking to the laminate.
7/29/2019 Composites Notes UNit 5 n
2/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
2 DEPARTMENT OF AERONAUTICAL ENGINEERING
7. Place the proper number of glass bleeder sheets (6) (e.g., style 181 glass cloth with the same
dimensions as the prepreg stack) over the Teflon-coated fabric (5). The bleeder sheets absorb the excess
resin from the laminate.
8. Place a sheet of perforated Teflon film (7) (0.025 mm) over the bleeder material. The Teflon film,
perforated on 50 mm centers, prevents excess resin from saturating the vent cloth (8).
9. Place a porous continuous-vent cloth (8) (e.g., style 181 glass cloth) on top of the lay-up. Extend the
cloth over the vacuum line attachment. Make sure that the vacuum line is completely covered by the vent
cloth. The vent cloth provides a path for volatiles to escape when the vacuum is applied and achieves a
uniform distribution of vacuum.
10. Place nylon bagging film over the entire plate, and seal it against the bagging adhesive. Allow enough
material so that the film conforms to all contours without being punctured.
11. Place the plate in the autoclave and attach the vacuum line (Figure 3.7). An autoclave is generally a
large pressure vessel equipped with a temperature- and pressure-control system. The elevated pressures
and temperatures, required for processing of the laminate, are commonly achieved by electrically heating
a pressurized inert gas nitrogen). The use of an inert gas will reduce oxidizing reactions that otherwise
may occur in the resin at elevated temperatures, and will prevent explosion of evolving volatiles.
12. Turn on the vacuum pump and check for leaks. Maintain a vacuum of 650 to 750 mm of mercury for
20 min and check again for leaks.
13. After closing the autoclave door, apply the pressure and initiate the appropriate cure cycle (see
example shown in Figure 3.8). As the temperature is increased, the resin viscosity decreases rapidly and
the chemical reaction of the resin begins. At the end of the temperature hold, at 127C in Figure 3.8, the
resin viscosity is at a minimum and pressure is applied to squeeze out excess resin. Thetemperature hold
controls the rate of the chemical reaction and
prevents degradation of the material by the exotherm. The pressureis held constant throughout the cure
cycle to consolidate the plies until the resin in the laminate is in its glassy state at the end of the cooling
phase. The vacuum should be checked throughout the cure cycle. The vacuum is applied to achieve a
uniform pressure on the laminate and draw out volatiles created during the cure. Loss of vacuum will
result in a poorly consolidated laminate.
14. After the power is turned off to the autoclave, maintain pressure until the inside temperature has
dropped to about 100C.
15. Carefully remove the laminate from the aluminum plate. Gently lift it in a direction parallel to the
main principal direction of the laminate.
16. Clean the aluminum plate, and store it for future use.
7/29/2019 Composites Notes UNit 5 n
3/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
3 DEPARTMENT OF AERONAUTICAL ENGINEERING
2 . Resin Transfer Molding of Thermoset Composites
Resin transfer molding (RTM) of composite laminates is a process where inthe dry-fiber preform is
infiltrated with a liquid polymeric resin and the polymer is advanced to its final cure after the
impregnation process is complete. An extensive review of the resin transfer molding process can be found
in Reference [12]. The process consists of four steps: fiber preform manufacture, mold filling, cure, andpart removal. In the first step, textile technology is typically utilized to assemble the preform. For
example, woven textile fabrics are often assembled into multilayer laminates that conform to the
geometry of the tool. Braiding and stitching provide mechanisms for the creation of three-dimensional
preform architectures. Typically, a thermosetting polymer of relatively low viscosity is used in the RTM
process. There have been applications for thermoplastic polymers, but they are rare. Pressure is applied to
7/29/2019 Composites Notes UNit 5 n
4/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
4 DEPARTMENT OF AERONAUTICAL ENGINEERING
the fluid polymer to inject it into a mold containing the fiber preform, and the mold may have been
preheated. The
flow of the fluid through the fiber preform is governed by Darcys Law [12], wherein the velocity of the
flow is equal to the product of the pressure gradient, the preform permeability, and the inverse of the
polymer viscosity.
Clearly, the lower the polymer viscosity, the greater the flow rate, and the greater the permeability, the
greater the flow rate. Note also that because the fiber preforms typically exhibit different geometries in
the three principal
directions, permeability is a tensor and exhibits anisotropic characteristics. That is, for a given pressure
gradient, the flow rates in three mutually orthogonal directions will differ. Flow through the thickness of a
fiber preform that
contains many layers of unidirectional fibers will be quite different than flow in the planar directions. In
addition, the permeability of the preform depends on the fiber volume fraction of the preform. The greater
the volume fraction, the lower the permeability. It is important to vent the mold to the atmosphere to
remove displaced gases from the fiber preform during the mold filling process. Otherwise trapped gases
will lead to voids within the laminate. After the polymer has fully impregnated the fiber preform, the third
step occurs: cure. This step will begin immediately upon injection of the polymer into the mold and will
occur more rapidly if the mold is at an elevated
temperature. As the cure of the polymer advances to the creation of a cross-link network, it passes
through a gelation phase wherein the polymer viscosity increases and transforms the polymer into a
viscoelastic substance, where it possesses both viscous and elastic properties. As this process proceeds
and the cross-link network continues to grow, the instantaneous glass transition temperature of the
polymer increases. Finally, vitrification of the polymer occurs when its glass transition temperature
exceeds the laminate temperature. Should gelation or vitrification (or both) occur prior to completion of
mold filling and preform impregnation, the resulting laminate will not be fully impregnated. The viscosity
of most polymers is highly dependent on temperature and polymer cure kinetics are controlled by
temperature as well. Therefore, heat transfer phenomena must be managed for successful RTM processes.
Heat transfer between the polymer and the fiber preform, and between tool, preform, and polymer, as well
7/29/2019 Composites Notes UNit 5 n
5/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
5 DEPARTMENT OF AERONAUTICAL ENGINEERING
as exothermic heat generation during the cure of the polymer, are three such phenomena that influence
the].
process [12
3. Vacuum-Assisted Resin Transfer Molding (VARTM) Processing
Both open-mold approaches, where one surface is bagged with a flexible film, and closed-mold
approaches to resin transfer molding are practiced. An example of open-mold RTM, vacuum-assisted
resin transfer molding (VARTM) is a common method employed as an alternative to autoclave use. In
VARTM, atmospheric pressure is utilized to achieve consolidation and impregnation by vacuum bagging
the laminate in the same way as discussed in Section 3.1.1. An inlet for the polymer is located at one or
more points in the tool or bag, and vacuum outlets are located some distance away. The vacuum pump
creates a pressure gradient of approximately 1 atm within the bag, which is sufficient for the impregnation
of laminates large in size and complex in geometry. For processes in which final cure occurs after the
mold is filled, completion of the cure can be carried out in an oven while atmospheric pressure is
maintained on the impregnated laminate. The VARTM procedure for a representative flat 61.0 30.5 0.64
cm panel (Figure 3.9) is described in the following steps:
1. Tool surface.
The tool is a flat aluminum plate with planar dimensions sufficient to accommodate the proposed
composite panel.
7/29/2019 Composites Notes UNit 5 n
6/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
6 DEPARTMENT OF AERONAUTICAL ENGINEERING
First, clean the metal tool surface using sandpaper and acetone. On the cleaned surface, create a 71 30.5
cm picture frame using masking tape. Apply several coats of release agent to the metal surface inside of
the masked frame. Remove the masking tape.
2. Bagging tape.
In place of the masking tape, apply a 1.3-cm-wide silicone bagging tape to the bare metal surface. The
silicone tape
should again form a 71 30.5 cm frame. Add a strip of the tape, 5 cm in length, to the outer edge of the
length of the frame at either end. These two strips will provide an added adhesive surface for attachment
of the inlet and outlet tubing. Leave the paper backing on the silicone tape to protect it during the
remainder of the lay-up procedure.
3.Preform.
Place the fiber preform stack on the coated tool, inside the tape frame. A 5.1-cm gap should exist between
the silicone tape and both edges of the preform to allow room for tubing. No gap should exist between the
silicon tape and fiber preform along the panel width to avoid providing a flow pathway outside the
perform to the vacuum port.
4.Release cloth.
Cut one layer of porous release film to 66 30.5 cm, and place it on top of the preform. Place the cloth so
that it completely covers the preform and allow 5.1 cm in length to overhang and contact the coated metal
surface at the injection side of the lay-up.The release film will allow the composite laminate to release
from the distribution media. Cut a second piece of release cloth to 5.1 30.5 cm, and place it on the tool
surface at the vacuum side of the preform.
This patch of cloth provides a clear path for the vacuum.
5.Distribution media.
Cut one to six layers of highly permeable distribution media, e.g., biplanar nylon 6 mesh to dimensions of
63.528.0 cm and stack them above the Armalon release clot h. Place the layers of media so that a 2.5-
cm gap exists on the top of the perform at the vacuum end. This gap will force the resin to fill through the
thickness rather than be drawn directly into the vacuum port. The length of this gap will vary with the
desired thickness of the composite panel. A 1.3-cm gap should exist between the media and the sides of
the preform. This will help prevent resin flow outside the preform. A 5-cm length of the media will
overhang the preform at the resin inlet end of the lay-up.
6.Distribution tubing.
Place a 28.0-cm length of distribution tubing across the width of the laminate at points 2.5 cm in front of
the preform (inlet) and 2.5 cm away from the preform (vacuum). On the inlet side, place the tubing on top
of the distribution media that overhangs the preform. At the vacuum side, place the tubing on the 5 30.5
7/29/2019 Composites Notes UNit 5 n
7/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
7 DEPARTMENT OF AERONAUTICAL ENGINEERING
cm piece of release cloth. Spiral-wrap, 18-mm-diameter conduit is an ideal choice for the distribution
tubing because it allows the resin to flow quickly into the distribution media and preform in a continuous
line across the width. A plastic tube with holes at 2.5-cm intervals also works well. Attach a 13-mm
portion of the spiral tubing to both the inletsupply tubing and the vacuumtubing using Kapton tape
(E.I. duPont de Nemours and Co.). Embed the free end of the spiral tubing in a 2.5-cm-diameter roll of
the silicone bagging tape, and then afix it to the strip of bagging tape forming the frame of the laminate.
7.Resin supply and vacuum tubing.
Use flexible plastic tubing (vinyl or Teflon, depending on temperature requirements) approximately 1.5 m
in length to supply resin and draw vacuum on the laminate. Tape one end of the tube to the distribution
tubing inside of the
bag. At a point just past this taped interface, wind one layer of silicone vacuum tape twice about the outer
surface of the tubing. This 2.5-cm-long sleeve of vacuum tape on the tube should match the tape frame
and added strips that exist on the tool surface. Attach the taped tubes to the tool at these locations and
place two more 7-cm-long strips of tape on top of the tool and tape sealant to form a smooth, airtight joint
when the bagging film is in place. Clamp the free end of the resin supply tubing to ensure a temporary
airtight seal. Connect the free end of the vacuum tubing to a resin trap, which catches any resin that might
be pulled into the tube on its way to the vacuum pump.
8.Vacuum bag.
With the laminate complete and the tubing in place, the part can be bagged using an appropriate film.
Take care to
eliminate creases in the bag and ensure an airtight seal with the tool surface and silicone bagging tape.
Once bagging is complete, the laminate should be fully evacuated to 762 mmHg using the vacuum pump.
Leaks can be detected by using either a listening device or by clamping the vacuum line and using a
vacuum gauge. Even a small leak in the system may result in voids and poor consolidation of the final
composite part.
9.Resin degassing.
Before infiltration can occur, the resin must be degassed to remove any air bubbles that were introduced
during
mixing. Perform degassing separately in a vacuum chamber; degassing can typically require 1 to 4 h,
depending on the resin viscosity. All air bubbles must be removed prior to infiltration. Contain the resin
in a bucket.
10.Resin infiltration.
With the bagged laminate under full vacuum, submerge the clamped end of the resin supply tubing in the
degassed resin bucket. Remove the clamp while the tube end is submerged to prevent any air entering the
7/29/2019 Composites Notes UNit 5 n
8/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
8 DEPARTMENT OF AERONAUTICAL ENGINEERING
tube and the part ahead of the resin. With the tube clamp removed, the resin flows through the supply
tubing and into the distribution tubing. The spiral distribution tubing allows the resin to spread quickly
across the width of the lay-up as it enters the distribution media. The distribution media provides the path
for the resin to flow quickly down the length of the preform and then through the laminate thickness.
11.Completion of infiltration.
The flow-front of resin through the part can be viewed through the bagging film. Halt the flow of resin
when the preform is fully infiltrated, as evidenced by resin beginning to enter the vacuum distribution
tubing. Stop the resin flow by first clamping and severing the resin supply tubing and then clamping and
severing the vacuum tubing. Again, these clamps must provide an airtight seal, because any leaks during
cure will result in poor consolidation of the part. It is recommended that a second envelope bag be used to
pull vacuum on the part during cure. Finally, place the vacuum-sealed part in an oven, and heat it
according to a cure cycle prescribed by the resin supplier.
Manufacturing Processes
Introduction
Taking composite materials as a whole, there are many different material options to choose from in the
areas of resins, fibres and cores, all with their own unique set of properties such as strength, stiffness,
toughness, heat resistance, cost, production rate etc.. However, the end properties of a composite part
produced from these different materials is not only a function of the individual properties of the resin
matrix and fibre (and in sandwich structures, the core as well), but is also a function of the way in which
the materials themselves are designed into the part and also the way in which they are processed. This
section compares a few of the commonly used composite production methods and presents some of the
factors to be borne in mind with each different process, including the influence of each process on
materials selection.
Comparison of Processes
Spray Lay up
7/29/2019 Composites Notes UNit 5 n
9/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
9 DEPARTMENT OF AERONAUTICAL ENGINEERING
Description
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:
i) Widely used for many years.
ii) Low cost way of quickly depositing fibre and resin.
iii) Low cost tooling.
Main Disadvantages:
i) Laminates tend to be very resin-rich and therefore excessively heavy.
ii) Only short fibres are incorporated which severely limits the mechanical properties of the laminate.
iii) Resins need to be low in viscosity to be sprayable. This generally compromises their
mechanical/thermal properties.
iv) 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.
(v) 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.
7/29/2019 Composites Notes UNit 5 n
10/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
10 DEPARTMENT OF AERONAUTICAL ENGINEERING
Wet Lay-up/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 ccomplished 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:
i) Widely used for many years.
ii) Simple principles to teach.
iii) Low cost tooling, if room-temperature cure resins are used.
iv) Wide choice of suppliers and material types.
v) Higher fibre contents, and longer fibres than with spray lay-up.
Main Disadvantages:
i) 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.ii) 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.
7/29/2019 Composites Notes UNit 5 n
11/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
11 DEPARTMENT OF AERONAUTICAL ENGINEERING
iii) Limiting airborne styrene concentrations to legislated levels from polyesters and vinylesters is
becoming increasingly hard without expensive extraction systems.
iv) 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.
Description
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:
i) Higher fibre content laminates can usually be achieved than with standard wet lay-up techniques.
ii) Lower void contents are achieved than with wet lay-up.
iii) Better fibre wet-out due to pressure and resin flow throughout structural fibres, with excess into
bagging materials.
iv) Health and safety: The vacuum bag reduces the amount of volatiles emitted during cure.
Main Disadvantages:
i) The extra process adds cost both in labour and in disposable bagging materials
ii) A higher level of skill is required by the operators
7/29/2019 Composites Notes UNit 5 n
12/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
12 DEPARTMENT OF AERONAUTICAL ENGINEERING
iii) 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 techniques
Description
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:
i) This can be a very fast and therefore economic method of laying material down.
ii) Resin content can be controlled by metering the resin onto each fibre tow through nips or dies.
iii) Fibre cost is minimised since there is no secondary process to convert fibre into fabric prior to use.
iv) 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:
i) The process is limited to convex shaped components.
ii) Fibre cannot easily be laid exactly along the length of a component.
iii) Mandrel costs for large components can be high.
iv) The external surface of the component is unmoulded, and therefore cosmetically unattractive.
7/29/2019 Composites Notes UNit 5 n
13/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
13 DEPARTMENT OF AERONAUTICAL ENGINEERING
v) 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
Description
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 . Although pultrusion is a continuous process, producinga 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:
i) This can be a very fast, and therefore economic, way of impregnating and curing
materials.
ii) Resin content can be accurately controlled.
iii) Fibre cost is minimised since the majority is taken from a creel.
7/29/2019 Composites Notes UNit 5 n
14/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
14 DEPARTMENT OF AERONAUTICAL ENGINEERING
iv) Structural properties of laminates can be very good since the profiles have very straight fibres and
high fibre volume fractions can be obtained.
v) Resin impregnation area can be enclosed thus limiting volatile emissions.
Main Disadvantages:
i) Limited to constant or near constant cross-section components
ii) Heated die costs can be high.
Typical Applications:
Beams and girders used in roof structures, bridges, ladders, frameworks.
Resin Transfer Moulding (RTM)
Description
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.
7/29/2019 Composites Notes UNit 5 n
15/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
15 DEPARTMENT OF AERONAUTICAL ENGINEERING
Cores: Not honeycombs, since cells would fill with resin, and pressures involved can crush some foams.
Main Advantages:
i) High fibre volume laminates can be obtained with very low void contents.
ii) Good health and safety, and environmental control due to enclosure of resin.
iii) Possible labour reductions.
iv) Both sides of the component have a moulded surface.
Main Disadvantages:
i) Matched tooling is expensive, and heavy in order to withstand pressures.
ii) Generally limited to smaller components.
iii) 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.
Description
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.
7/29/2019 Composites Notes UNit 5 n
16/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
16 DEPARTMENT OF AERONAUTICAL ENGINEERING
Main Advantages:
i) As RTM above, except only one side of the component has a moulded finish.
ii) Much lower tooling cost due to one half of the tool being a vacuum bag, and less
strength being required in the main tool.
iii) Large components can be fabricated.
iv) Standard wet lay-up tools may be able to be modified for this process.
v) Cored structures can be produced in one operation.
Main Disadvantages:
i) Relatively complex process to perform well.
ii) Resins must be very low in viscosity, thus comprising mechanical properties.
iii) Unimpregnated areas can occur resulting in very expensive scrap parts.
iv) Some elements of this process are covered by patents (SCRIMP).
Typical Applications:
Semi-production small yachts, train and truck body panels.
Prepregs
Description
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 C. This allows the resin to initially reflow and eventually to cure. Additional
7/29/2019 Composites Notes UNit 5 n
17/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
17 DEPARTMENT OF AERONAUTICAL ENGINEERING
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:
i) Resin/catalyst levels and the resin content in the fibre are accurately set by the materials manufacturer.
High fibre contents can be safely achieved.
ii) The materials have excellent health and safety characteristics and are clean to work with.
iii) Fibre cost is minimised in unidirectional tapes since there is no secondary proc- ess to convert fibre
into fabric prior to use.
iv) Resin chemistry can be optimised for mechanical and thermal performance, with the high viscosity
resins being impregnable due to the manufacturing process.
v) The extended working times (of up to several months at room temperatures) means that structurally
optimised, complex lay-ups can be readily achieved.
vi) Potential for automation and labour saving.
Main Disadvantages:
i) Materials cost is higher for preimpregnated fabrics.
ii) Autoclaves are usually required to cure the component. These are expensive, slow to operate and
limited in size.
iii) Tooling needs to be able to withstand the process temperatures involved
iv) 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.
7/29/2019 Composites Notes UNit 5 n
18/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
18 DEPARTMENT OF AERONAUTICAL ENGINEERING
] Description
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 C. At 60 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:
i) All of the advantages ((i)-(vi)) associated with the use of conventional prepregs are incorporated in low-
temperature curing prepregs.
ii) Cheaper tooling materials, such as wood, can be used due to the lower cure temperatures involved.
iii) 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.
iv) Conventional PVC foam core materials can be used, providing certain procedures are followed.
v) Lower energy cost.
Main Disadvantages:
i) Materials cost is still higher than for non-preimpregnated fabrics.
ii) An oven and vacuum bagging system is required to cure the component.
iii) Tooling needs to be able to withstand above-ambient temperatures involved
(typically 60-100 C).
iv) 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.
7/29/2019 Composites Notes UNit 5 n
19/19
626701 -Composites Materials and Structures 2011-12 Sathyabama University
19 DEPARTMENT OF AERONAUTICAL ENGINEERING
Description
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.
Cores: Most, although PVC foam needs special procedures due to the elevated temperatures involved in
the process.
Main Advantages:
i) High fibre volumes can be accurately achieved with low void contents.
ii) Good health and safety and a clean lay-up, like prepreg.
iii) High resin mechanical properties due to solid state of initial polymer material and elevated
temperature cure.
iv) Potentially lower cost than prepreg, with most of the advantages.
v) Less likelihood of dry areas than SCRIMP process due to resin travelling through fabric thickness only.
Main Disadvantages:
i) Not widely proven outside the aerospace industry.
ii) 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.
iii) 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, istypically 60-100 C).
iv) Core materials need to be able to withstand the process temperatures and pressures.
Typical Applications:
Aircraft radomes and submarine sonar domes.
Note:For netting analysis refer class notes