Kutz M. Handbook of materials selection

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1076 COMPOSITES FABRICATION PROCESSES

Once the resin is sufﬁciently cured, the heaters are switched off and the autoclave

cooled down to ambient with the pressure maintained. When sufﬁciently cool,

the pressure is reduced, the interior of the autoclave ﬂushed with air, the doors

opened, and the molds removed. The whole process is likely to have extended

over 12–24 h. It will be clear that it is no simple matter to set the cure cycle.

It should also be appreciated that the cost of materials, labor, and other added-

value items making up an autoclave load is very high, so that faulty treatment

must be strenuously avoided. For this reason it is common practice to use com-

puter simulation packages, similar to that used to produce Fig. 23, to optimize

the cycle, having regard to the possible different thicknesses of the components

being processed in the same run.

5.10 Performance, Productivity, and Economics

Autoclave processed prepreg is acknowledged to offer the best mechanical per-

formance (stiffness and strength) of all composites systems. This is because a

high V

is achieved, with precise control of ﬁber architecture, good dimensional

control, and high molding quality. The process is also very versatile in the variety

of component forms that may be produced, and it is often possible to gain from

parts consolidation, with consequent savings in assembly and inspection costs.

The downside is that the feedstock is the most expensive format for all ﬁber

types, capital equipment and/or labor costs are high, and the process cycle is

very time consuming. It would be very difﬁcult to achieve a production rate of

more than 125 parts/annum from a single tool set, i.e., one part every two

working days. In the context of the aerospace industry, this may be acceptable

but this is less likely to be the case in other industries. Even within the aerospace

industry, there is intense pressure to develop more cost-effective techniques that

maintain structural efﬁciency and quality at acceptable levels. Foremost among

the contenders are RTM and resin ﬁlm inﬁltration (RFI) and also some possi-

bilities with thermoplastics matrices. These are all discussed in subsequent sec-

tions.

6 OTHER PROCESSING OPTIONS FOR PREPREG

Although prepreg is mainly processed in the autoclave, there are some other

options. One of the most straightforward is to use a vacuum bag, as in the

autoclave process, but to cure in an oven or by application of radiant heat to the

mold. A number of low-temperature curing resin systems are available, allowing

cure at temperatures as low as 60

⬚C. These systems are also widely used for the

manufacture of tooling for autoclave and other processes. Another option, useful

for very large moldings, e.g., hull moldings for yachts, is to construct a tem-

porary enclosure around the mold, from tarpaulin or plastic sheet and heat with

hot-air blowers. Prepreg may also be used alone or with other reinforcements

for contact molding or press molding. Simple moderately curved shapes may be

hot press molded using an elastomeric counter tool. For nearly ﬂat panels a

simple block of elastomer (e.g., a soft rubber) may be used, while for more

complex shapes a tailored counter tool is used. These may be manufactured

from castable polyurethane compounds.

7 RESIN FILM INFILTRATION 1077

Vacuum bag

Resin film

Dry reinforcement

Thick resin tile

Consolidating pressure

1. Alternate plies of reinforcement

and resin film

1a. Resin tile under stack of reinforcement

2. Fully consolidated reinforcement

Fig. 24 An illustration of the principle of resin ﬁlm inﬁltration (RFI). In 1 the reinforcement plies

are alternated with resin ﬁlm. With this arrangement the inﬁltration distance is only the thickness

of a ply. The alternative system, 1a, uses a thicker resin layer (tile) placed under the whole lami-

nate. The ﬂow distance in this case is the whole thickness of the laminate. This system may be

satisfactory for thinner laminates and when the resin melt viscosity is sufﬁciently low.

Resin tile may be a more economic form than resin ﬁlm.

7 RESIN FILM INFILTRATION

7.1 Basic Principles

The principle of resin ﬁlm inﬁltration (RFI) has been used for many years, but

the nomenclature RFI is comparatively recent. The concept is to use a dry re-

inforcement and to interleave it with resin, nominally in the form of a thin ﬁlm.

Under pressure and heat the resin viscosity is reduced so that it inﬁltrates through

the reinforcement and ultimately cures. Processing conditions must be set so that

the resin completely inﬁltrates the reinforcement and porosity is eliminated. The

principle is illustrated in Fig. 24. Clearly more densely woven and thicker re-

inforcements will be less permeable and require a longer process window. The

resin systems also need to be selected to give an adequate window of low vis-

cosity before gelation. The advantages of this technique is that the dry reinforce-

ments are a much cheaper feedstock than prepreg, the resin ﬁlm may be

accurately placed to ensure the correct ﬁber/resin proportion, and the inﬁltration

distance is very short, just the thickness of a single layer of the reinforcement.

This may be compared with the situation in resin transfer molding (RTM) where

the inﬁltration lengths may be of the order of 1 m (Section 8).

7.2 RFI with Autoclave Cure

The tooling and general setup are similar to that used for prepreg. The reinforce-

ment is typically lightweight woven or noncrimp fabric, usually of 200–600 g/

. The layers of reinforcement are laid on the mold and interlayered with the

resin ﬁlm. This is usually supplied on a paper backing similar to that used for

prepreg. This is dispensed to give the required ﬁnal V

. Not all resin systems

1078 COMPOSITES FABRICATION PROCESSES

are suitable for the production of ﬁlm, and in some cases thin cast slabs of resin

(tiles) are used. The completed lay-up is covered with release membrane,

breather, and vacuum bag and processed in essentially the same manner as pre-

preg. The economic advantages over prepreg stem from the use of the cheaper

fabric feedstock. The build rate is also faster than for prepreg due to the higher

areal density of the fabrics used. The resin cost is high due to the need to

manufacture it in ﬁlm or tile form. The overall V

achieved is likely to be

somewhat lower than with prepreg, and there is some loss of ﬂexibility in de-

signing the laminate conﬁguration. This results in slightly lower structural efﬁ-

ciency, but this is usually more than compensated by the lower cost. Productivity

may be marginally better than with prepreg because of the faster build rate.

7.3 RFI with Press or Oven Cure

Instead of the autoclave the molding may be cured by vacuum bagging and oven

cure. Clearly this does not allow as much consolidation pressure as in the au-

toclave, but very satisfactory results may be obtained with some resin/reinforce-

ment combinations. Radiant or hot-air heating may also be used. Parts of simple

shape and moderate size may also be hot press cured using two part matched

tools or elastomer counter tools. There are considerable economic advantages

with these methods compared with the autoclave process, although there is

sometimes some performance loss due to lower V

and higher porosity levels.

Productivity can be signiﬁcantly higher due to the elimination of the lengthy

autoclave process.

7.4 Other RFI Options

The term RFI implies the use of a resin ﬁlm, but this option is relatively expen-

sive. A viable alternative is to apply the resin, as a liquid, by spraying onto each

layer of reinforcement as it is placed on the tooling. The resin may be brieﬂy

preheated to lower its viscosity for spraying. It will then thicken as it is cooled

by contact with the cold reinforcement. A novel development has been the in-

troduction of semiimpregnated reinforcements. These materials consist of two

layers of dry reinforcement with a layer of pasty resin sandwiched in between

(see Fig. 4). This material has a number of advantages. It is dry to handle and

may be easily cut to shape and placed in the mold. It also has a unique dead

handling characteristic. It is much less springy than normal dry reinforcements,

and, when draped over details on the tooling it tends to hold its shape. This

facilitates lay-up. It can be manufactured from all conventional forms of rein-

forcement, random mat, woven cloth, and noncrimp fabric, and the additional

cost is quite modest. It is manufactured in areal densities of up to 5000 g/m

which allow fast build rates, in E-glass fabrics. In combination with low-

temperature cure resins, it is a very attractive for the manufacture of large mold-

ings, using a vacuum bag with radiant or hot-air heating. This method has been

used for the production of wind turbine blades of over 25 m length.

8 RESIN TRANSFER MOLDING

8.1 Basic Principles

Resin transfer molding (RTM) is a relatively new term for a process that has

been used in various forms for many years and under various names. Essentially

8 RESIN TRANSFER MOLDING 1079

it involves inﬁltration of dry reinforcement in a closed mold. The variants depend

on the type of tooling, the method for inducing the resin to inﬁltrate, and the

resin cure temperature. The following are the main variants:

RIM Reaction injection molding

RRIM Reinforced reaction injection molding

SRIM Structural reaction injection molding

RTM Resin transfer molding

VARI Vacuum-assisted resin injection

VARTM Vacuum-assisted resin transfer molding

SCRIMP Seeman composites infusion molding process

Some of these are discussed in detail in this and later sections. There follows a

brief description of each.

1. RIM. This term is generally used for moldings manufactured by injecting,

at high pressure, a reactive blend of precursor chemicals that rapidly cure. The

classic system uses polyurethane chemistry and approximately equal proportions

of an isocyanate and a polyol are reacted to form a polyurethane. The resin is

not reinforced, so this is not a composite. The cured polyurethane may be for-

mulated to be hard and rigid, a ﬂexible elastomer, or either rigid or ﬂexible

foam. Cure time is of the order of 1 min and molds are unheated, but there is

some heating due to the cure exotherm. Alternative resins may be based on

polyester or polyamide chemistry, but these not widely used. While RIM is not

a composite system, similar chemistry and process principles may be used for

composites manufacture.

2. RRIM. This is a reinforced version of RIM. The same polyurethane chem-

istry is used but very short chopped, or milled, glass ﬁbers are dispersed in the

polyol component. The molding procedure is similar to that for RIM, but the

cured material will be reinforced with the glass ﬁbers. The ﬁbers must be very

short so that they can be pumped through the injection system, and for similar

reasons the ﬁber loading must be quite low. Fibers cannot be dispersed in the

isocyanate component because they initiate chemical reactions that would reduce

the pot life of the isocyanate and interfere with the cure process. The total V

in the ﬁnal molding is likely to be less than 0.15, and this, combined with the

very short length, means that the degree of property enhancement is low. How-

ever, the reinforcement is sufﬁcient to improve the dimensional stability of the

moldings and is most useful for soft components: e.g., automobile interior mold-

ings and front-end moldings designed to protect pedestrians in the event of

collision.

3. SRIM. This a true RTM process. Similar urethane technology is used but

dry reinforcement is placed in the mold and, after mold closure, the resin is

injected. It inﬁltrates the reinforcement and cures in about 1 min. A schematic

of the basic principle is shown in Fig. 25. This process is widely used for the

manufacture of automobile and truck dashboards. The resin formulation is de-

signed to provide some protection to the occupants in case of accident. Rigid

1080 COMPOSITES FABRICATION PROCESSES

1. Preform placed in mold 2. Mold closed resin injected

Fig. 25 Illustrates the principle of both SRIM and RTM. The dry reinforcement, usually as a

preform, is placed in the open mold. The mold is then closed and the liquid resin injected into

the cavity to fully inﬁltrate the reinforcement. The difference between the two processes is that

in SRIM a rapidly injected polyurethane resin curing in 1–2 min is normally used. In RTM, con-

ventional UPE and epoxy resins with substantially longer processing times are used.

resin formulations may also be used. The reinforcement is most often a fairly

open random chopped strand. This provides high permeability so that the resin

may be rapidly injected. If the permeability is too low, pressure gradients may

build and distort the reinforcement. Fiber loadings of 0.1–0.25 are common, and

this provides a useful level of property enhancement, especially dimensional

stability at slightly elevated temperatures.

4. RTM. This is similar in principle to SRIM. The reinforcement is preplaced

in the mold and the resin injected after mold closure. In RTM a much higher

may be achieved and consequently the lower permeability of the reinforce-

ment means that much slower inﬁltration over a longer period is the rule. Lower

pressures allow for use of less robust and cheaper tooling. Cure times are seldom

less than 20 min and often extend to several hours. Variants involve the type of

tooling, inﬁltration pressure, and cure temperature. These are discussed at length

below.

5. VARI. Has been used for many years. It is simply RTM in which a vac-

uum is applied to the cavity so that the resin is forced into the mold by atmos-

pheric pressure. Preevacuating the cavity helps prevent air being entrapped in

the molding, a hazard when only a positive pressure is applied.

6. VARTM. This is a more recent nomenclature for VARI; it is identical. It

is probably a reinvention.

7. SCRIMP. This is also a recent term for a essentially well-established

variant of RTM. SCRIMP is a patented proprietary process, but some aspects of

the principle are used in other nonproprietary processes. This uses a single-sided

tool with a vacuum bag. A vacuum drawn between the bag and the tool induces

resin ﬂow into the reinforcement by atmospheric pressure. The novelty of the

SCRIMP process is the use of a sacriﬁcial mesh, illustrated in Fig. 26, between

the lay-up and the bag. This acts as a permeable feeder ply, allowing resin to

ﬂow rapidly over the entire area of the molding and then to ﬂow transversely

through the thickness of the reinforcement to complete the inﬁltration. Earlier

practice was to incorporate layers of CSM on top of or between layers of denser

reinforcement to accomplish similar ends.

8.2 Basic Process Details

The principal variables of the process are the type of tooling, the method of

resin transfer, and the cure temperature. There is also the question of size and

8 RESIN TRANSFER MOLDING 1081

Vacuum lines

Resin injected

through vacuum bag

Vacuum bag

Permeable

distributor

membrane

Bag sealed

to mold

Fig. 26 In the SCRIMP process, a single-sided tool is used and a permeable distributor

membrane is incorporated. The membrane allows the resin to ﬂow rapidly over the entire sur-

face of the molding, so that the inﬁltration distance is only the laminate thickness.

This results in much faster inﬁltration than conventional RTM and is especially suited

to moldings of large surface area.

required production rate. RTM is used for manufacture of parts ranging in weight

from less than 1 kg to over 1 ton and this has a profound effect on the type of

tooling that is viable and to both transfer pressure and cure temperature. An

important feature of the process is that the mold is sealed and consequently

vapor emissions are much reduced compared with contact molding.

Tooling may be composite or metallic. Most large molds are composite,

smaller tools may be composite for short runs but metal for long runs. If high

pressure or hot cure is used, it is generally necessary to use metal tooling.

Conversely high pressure is not appropriate for very large parts due to the need

for very robust tooling.

Most RTM is carried out with matched tooling in which the cavity is precisely

deﬁned. In the case of large molds the two halves may be bolted or clamped

together for the molding operation. Smaller molds may be mechanically opened

and closed, e.g., in a press. The SCRIMP process and variants use one-sided

tooling sealed with a vacuum bag. This is much cheaper than matched tooling

but does not result in a fair surface on both sides of the molding. Furthermore,

pressure inﬁltration is not an option, only vacuum may be used. This can lead

to higher levels of porosity than if pressure is used. It can be difﬁcult to efﬁ-

ciently inﬁltrate moldings of high surface area, which are made up from high

reinforcements, owing to the combination of long inﬁltration paths and low

in-plane permeability. This problem is overcome in the SCRIMP process by

using a high-permeability mesh ply, under the vacuum bag covering the whole

molding area. A permeable peel ply is placed between this mesh and the top

plies of the lay-up. This may be a layer of PTFE-treated glass cloth, similar to

that used under the resin bleed ply in autoclave molding. The resin inﬁltrates

rapidly through the mesh and then passes through the peel ply and into the lay-

up. Although the permeability of these layers may be quite low, the inﬁltration

path is very short, just the thickness of the molding, so that complete inﬁltration

is attained in a reasonable time. After cure the peel ply and mesh are stripped

from the surface of the molding and discarded.

1082 COMPOSITES FABRICATION PROCESSES

Uninfiltrated pocket

Resin injection gate

Fig. 27 Uncontrolled ﬂow in RTM processes can lead to the formation of unwetted pockets as

illustrated in this schematic. These may be avoided by proper positioning of the inlet gates

and by control of the ﬂowrate.

The transfer of resin into the tool may be assisted by vacuum, gravity, or

positive pressure or a combination of these. Vacuum is simple to engineer, and,

provided the tooling is sufﬁciently robust, there need be no problems due to

mold distortion. The resin is forced into the mold by atmospheric pressure.

Preevacuating the mold before opening the resin port removes most of the air

and vapor from the mold interior and helps prevent noninﬁltrated areas. It is

often necessary to restrict the ﬂow of resin to prevent racetracking. Streams of

resin ﬂow through the mold sealing off pockets of reinforcement that then never

ﬁll (Fig. 27). The general strategy is to admit the resin at the bottom of the mold

and to vent around the upper periphery. The riser tubes lead through resin traps

to a vacuum manifold. If the resin ﬂow is correctly throttled, the mold ﬁlls

uniformly from the bottom, and ﬁlling is indicated by the appearance of resin

in the transparent risers. These are then shut off, pinched, to stop further resin

ﬂow and prevent resin getting into the vacuum system. Further assistance may

be provided by arranging a positive head of resin above the inlet port and also

above the highest point of the mold so that there is gravity assistance. Low-

pressure assistance may be arranged by use of a pressure pot, where the resin

container is subjected to an overpressure of gas, usually nitrogen, at pressures

of up to about 5 bars. This may be used in conjunction with preevacuation, but

once all the risers have been pinched off a positive hydrostatic pressure is gen-

erated within the mold cavity. This ensures complete ﬁlling and will reduce

porosity by collapsing bubbles and forcing vapors into solution in the resin.

Higher pressures may be achieved by the use of a positive displacement resin

pump. Pressures of up to 10 bars are commonly used. The pump generates a

constant ﬂowrate and the pressure does not build up until the mold is full. Again

preevacuation is beneﬁcial.

The next consideration is cure temperature. Ambient curing systems are

widely used, and these must be formulated so that the resin viscosity remains

8 RESIN TRANSFER MOLDING 1083

sufﬁciently low, typically ⬍100 Pa/s, for the time interval necessary for full

inﬁltration to occur. The consequence of this is that gel time will normally need

to be at least double the inﬁltration time and that the time to demoulding will

be of the order of 5 times the inﬁltration time. The cure process may be speeded

up by the application of heat after the mold has been ﬁlled. Small molds may

be placed in an oven and large molds may be heated by radiant heaters or blown

hot air. With composite tooling the maximum cure temperature is unlikely to be

much more than 100

⬚C. If metal tools are used and buried heaters are used, the

molds may be heated directly to higher cure temperatures, typically 120–180

⬚C.

Electric cartridge heaters, circulating hot oil, and steam heating can be success-

fully used for mold heating. In general with thermoset resins the parts may be

ejected hot. To avoid having to cool the molds between cycles, it is advantageous

to keep them continuously at the cure temperature. The reinforcement is placed

into the hot, open mold, and after closure the resin is injected. The danger with

this system, especially if there are long ﬂow paths, is that the resin should gel

before inﬁltration is complete. A further strategy is to preheat the resin to a

temperature lower than the cure temperature, e.g., 50–70

⬚C. This lowers its vis-

cosity allowing faster inﬁltration. Clearly great care must be exercised to for-

mulate the resin and control the temperatures so that full inﬁltration is ensured

and cure is achieved in the shortest reasonable time, so that productivity is

maximized.

8.3 Low-Pressure, Ambient Cure RTM

This is the longest established version of the process. In most cases two-part

matched tooling is used. For short runs composite molds are used, but the sup-

porting structure needs to be more robust than for ordinary contact molding.

According to the size and complexity of the part, the reinforcement is either laid

directly into the mold or a preform is separately made up and simply placed in

the open mold. The mold is then closed and the resin inﬁltrated into the rein-

forcement. There are several strategies for resin inﬁltration: The attraction of the

system is that very large moldings can be economically made this way with

relatively cheap and simple tooling. Using matched tools means that dimensions,

especially thickness, is more precisely deﬁned and all surfaces are fair. A gel

coat may be used to enhance surface ﬁnish, or color, if required in the same

way as with contact molding. In large moldings, e.g., the hull of a 10-m sailing

yacht, the reinforcement is laid in the mold. This may take several days. A

variety of inserts, such as foamed sheet to form sandwich sections or timber

pads for reinforcing mounting points, may be incorporated into the reinforce-

ment. Pressure inﬁltration is not usually an option with large moldings but a

combination of preevacuation with gravity-assisted inﬁltration, i.e., maintaining

a hydrostatic head of resin, can be used.

For smaller moldings it is common to use preforms, as this reduces the length

of time that the molds are being prepared. Composite tooling is adequate for

short runs, but metal is preferred for larger numbers of moldings. The tools may

be mounted in a press, which facilitates opening and closing and also enables

the injection system to be permanently connected. Higher pressures may be used

and, since resin ﬂow paths will be much shorter, inﬁltration time will be faster,

so that faster curing resins may be used with, consequently faster, mold turn-

around time. A combination of low pressure, 1–5 bars, with vacuum is typical.

1084 COMPOSITES FABRICATION PROCESSES

When positive pressure is used, the risers over the vent ports may be closed

once the resin has inﬁltrated, so that a positive hydrostatic pressure is generated

in the cavity. This ensures complete ﬁlling and reduces porosity by collapsing

air or vapor bubbles.

8.4 High-Pressure RTM

This is applicable to fairly small components, area ⬍2m

. In this case metal

tooling is used and hot cure is normal. The molds are normally operated in a

press, facilitating opening and closing and providing the means to hold the tool

closed against the injection pressure. Pressures of 5–10 bars are common, and

the mold cavity is preevacuated. Preforms are the preferred form of reinforce-

ment when cycle times of

⬇15 min are possible with small components.

8.5 Manufacture of Preforms for RTM

Many laminating processes may be speeded up by the use of preforms. This

enables the whole lay-up process to be completed before the mold is loaded.

Effectively preform manufacture and molding can be carried out concurrently.

This can often double the rate of production from a molding station.

The main objectives in preform manufacture are to assemble the necessary

layers of reinforcement and then to consolidate them sufﬁciently so that they

may be placed in the tooling. The problem is excessive bulk, which might ob-

struct mold closure. The consolidation is effected by precompressing and the

use of binders, which may be heat set. The choice of type and proportion of

binder is vital for successful preform manufacture. Many reinforcements are

available already treated with a suitable binder system. In other cases it may be

necessary to apply binder during preform manufacture, usually by spray or as a

powder. The binders may consist of a similar resin to that used for the matrix

or either a thermoset or thermoplastic that is compatible with the system being

used. The binders are either B-staged thermosets (solid or pasty at ambient

temperature) or thermoplastics with softening temperatures of about 100

⬚C.

When the reinforcement is preheated and then compressed and allowed to cool

under pressure, the binders melt and then freeze, stabilizing the preform in the

compressed state. The sequence of operations is to lay up the required reinforce-

ments in an auxiliary half mold. This is then heated by radiant heaters, blown

hot air, or sometimes microwave to soften the binder, before being compressed,

either using a vacuum bag or between cooled shaped tooling. The principle for

building up details using this technique is shown in Fig. 28.

Simple random preforms are manufactured by blowing chopped strand onto

a mesh shape on a vacuum box. This is similar to the spray-up process described

in Section 3. When the preform is of the required thickness (the weight is usually

checked), it is consolidated as described above. The periphery may also be

trimmed after consolidation. The ﬁnished preforms are sufﬁciently robust to be

handled with care. They are delivered to the molding station for ﬁnal processing.

More complex preforms may be constructed from random mat, woven, and

noncrimp fabrics and from braided or knitted materials. In some cases it is

necessary to heat set individual plies before they are assembled on the preform

(e.g., to form stiffening ribs—Fig. 28). Such preforms may require several hours

9 FILAMENT WINDING AND TOW PLACEMENT 1085

Angle sections formed by bending

strips of fabric containing a

thermosoftening binder

Flat reinforcement and preformed angle sections assembled

to form a stiffening rib. Heat tacked into place.

Fig. 28 Complex detail may be built into a preform by using thermoformable reinforcements.

These incorporate a thermoplastic binder so that they may be heat-set before assembly to form

the preform. This is illustrated by a blade stiffener built up from thermoformed tapes.

of hand work to manufacture and a sufﬁcient number of parallel preform man-

ufacture cells must be set up to match the molding rate of the ﬁnal tool.

Another strategy is to use an auxiliary tool (or a number of such tools) on

which the preform is assembled and preconsolidated. The whole auxiliary tool

complete with preform may then be loaded into the main tool. Effectively the

auxiliary tool is a thin shell that accurately matches the bottom half of the main

tooling. It needs to be much less robust than the main tooling, just sufﬁcient to

allow it to be handled between the preform assembly cell and the molding

station. It is a much more economical option than having duplicate molds. The

auxiliary shells may be composite or even formed sheet metal. They may be

regarded as disposable, being used to produce a short run of moldings before

being replaced, while the wear and tear on the main tool is reduced and its

useful life extended. A schematic layout using auxiliary tooling is illustrated in

Fig. 29.

9 FILAMENT WINDING AND TOW PLACEMENT

9.1 General Concepts

These are two related processing concepts in which continuous ﬁber roving is

directly formed into a component. This is attractive because continuous roving

is usually the cheapest format of the reinforcement ﬁber, as there is no down-

stream process cost after ﬁber manufacture. The basic principle is that dry ﬁber

rovings are impregnated with resin and laid down on a tool or former in the

required sequence to generate the desired reinforcement conﬁguration. The use

of single rovings facilitates impregnation by the matrix resin and also the con-

tinuous uniaxial ﬁber arrangement gives the possibility of achieving very high

ﬁber fraction in the ﬁnal product; V

⫽ 0.8 is typical. The rovings are maintained

very straight and in a precise lay-up. All these features contribute to the reali-

zation of excellent mechanical properties and structural efﬁciency. Both pro-