Kutz M. Handbook of materials selection

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1066 COMPOSITES FABRICATION PROCESSES

Either ambient temperature or elevated temperature cure resins may be used.

The characteristics required of the resin depend on the chosen method of inﬁl-

tration. The resin may be preimpregnated into the reinforcement either before,

during, or after preform manufacture. Alternatively the resin may be placed in

the mold with the preform as a liquid or sometimes as a ﬁlm or sheet. A further

possibility is that the resin is injected into the closed mold; this is resin transfer

molding (RTM) and is discussed in a later section. Any of the common ther-

mosetting resins may be used; UPE and vinyl-ester systems are the most com-

mon. To minimize the molding time, a fast curing system is desirable, subject

to full inﬁltration and control of the exotherm during cure.

Press molding is widely used with precompounded reinforcements such as

prepreg, sheet and bulk molding compounds (SMC, BMC), and also with ther-

moplastic matrix systems, e.g., glass-mat/thermoplastic (GMT), these are dis-

cussed in later sections.

4.3 Ambient Cure Press Molding

In this version of the process low pressures and unheated tooling is used with

a very fast curing resin formulation. A typical sequence of operations is that the

preform is loaded into the open mold, a carefully metered quantity of the resin,

which must be of low viscosity and have been just mixed with the initiator, is

poured or sprayed into the mold. The mold is then closed and the consolidating

pressure applied. This forces the liquid resin to completely inﬁltrate the preform.

Vents are often provided to allow a little excess resin to escape, but the process

depends on the hydrostatic pressure generated in the resin to effect the inﬁltra-

tion. The mold remains closed and under pressure until the resin is sufﬁciently

cured. This may take anywhere from 2 to 20 min depending of size and com-

plexity of the molding. The fast curing resin generates a considerable exotherm,

so that the molding is heated, often to more than 100

⬚C, during the cure process.

When the cure is judged to be adequate, the press is opened and the (hot) part

is removed. The next preform may then be inserted and the process repeated.

The attraction of this process is that the capital investment of press and tooling

is relatively low and the production rate quite fast. Simple composite tooling is

adequate for short runs. Surface ﬁnish may be enhanced by using a quick gelling

sprayed gel coat. The process is not generally suitable for large moldings, e.g.,

⬎2m

projected area, and control of dimensions and porosity is relatively poor.

It is very suitable for moderate runs of commodity items where high mechanical

performance is not a requirement.

4.4 Elevated Temperature Cure Press Molding

In this case metal tooling incorporating internal heaters must be used and, gen-

erally, more substantial presses and higher consolidating pressures are used. The

cure temperatures are typically 80–160

⬚C for UPE and vinyl ester (VE) systems

and up to 250

⬚C for epoxy and some high-temperature resins. The general pro-

cedure is similar to that described above for ambient cure. However, there is

much more ﬂexibility as a fast curing (at ambient temperature) resin is not

necessary. Resins are formulated to cure in 2–20 min at the relevant cure tem-

peratures. They then have reasonably long pot lives at shop temperature. Pre-

forms are again preferred and the resin applied as liquid or ﬁlm. The press is

5 AUTOCLAVE PROCESSING OF PREPREG 1067

Vacuum manifold

Nitrogen pressure line

Temperature & cure

monitoring connections

Opening door

Gas circulation fan

Heaters

Laminates on tooling

Fig. 16 Industrial autoclaves vary in size from less than 1 m diameter to more than 4 m, and

in some cases more than 10 m in length. They are substantial pressure vessels. The interior is

pressurized with nitrogen and may be heated, typically up to 250⬚C. A vacuum manifold and

monitoring connections are provided to attach to the individual tools.

closed, and, as the resin is heated, it ﬁrst becomes less viscous so that inﬁltration

is accelerated. The application of pressure is often programmed during this pe-

riod, typically a few minutes. When the charge has been heated and the full

pressure applied, the resin is allowed to cure. Again, consideration must be given

to dissipation of the heat generated during cure, but generally conduction to the

metal tooling is quite effective in preventing hot spots and burning. Surface

ﬁnish may, again, be enhanced by use of a gel coat, and a further option is to

slightly open the mold after gelation of the main charge and to inject a further

quantity of pigmented resin between the workpiece and the mold wall, sufﬁcient

to form a surface layer 0.5–1.0 mm thick. The pressure is then reapplied while

this injected gel coat is cured. At the completion of the cure, the press is opened

and the hot part is ejected. Average times for the complete cycle are 10–20 min.

This is much faster than can be achieved with contact molding, and there is the

additional beneﬁt of closer tolerances, better surface ﬁnish, and better quality

control. The negative aspects are higher investment in tooling and equipment

and size limited to the sizes of the available presses, e.g., 2

⫻ 1 m plan form.

5 AUTOCLAVE PROCESSING OF PREPREG

5.1 Autoclave

An autoclave, shown schematically in Fig. 16, is a pressure vessel capable of

being internally pressurized with a gas, which can also be independently heated.

This is in contrast to medical autoclaves, which are usually steam operated.

Modern industrial autoclaves are often very large, commonly up to 3 m internal

diameter and 4–6 m in length. They are pressurized with nitrogen gas typically

to pressures of up to 5 bars. The use of nitrogen, rather than air, avoids any

chance of combustion of hot resin or other ﬂammable materials. The contained

gas is mechanically circulated and may be heated to temperatures of up to 400

⬚C,

although

⬇200⬚C is a more common upper limit. There are sometimes also

1068 COMPOSITES FABRICATION PROCESSES

means for cooling the interior. The autoclave with its associated pressurization,

heating and control equipment constitutes a considerable capital investment, typ-

ically in excess of $1 million. This has a signiﬁcant inﬂuence on the cost of

autoclave processed parts. It also occupies a lot of factory space. For these

reasons autoclave processing is mainly used for the manufacture of premium

products in high-performance composites.

5.2 Prepreg

Prepreg is the common name for preimpregnated warp sheet: It was brieﬂy

described in Section 1.2. It consists of a uniaxial array of ﬁber tows or roving,

formed by butting up a large number of individual tow ends (i.e., ﬁber from

many spools). The web thus formed may be from 300 mm (tape) to 2000 mm

or more, (broadgoods) wide. It is supported on a carrier ﬁlm, usually silicone-

treated kraft paper, which has been precoated with a layer of resin. A second

layer of ﬁlm, also coated in resin, is applied to form a sandwich, which is passed

between a number of heated rolls and then, often, through chilled rolls. This

thoroughly impregnates the ﬁber web and produces a uniform material. The

objective is to incorporate exactly the correct proportion of resin into the prepreg,

e.g., to form a composite with V

⫽ 0.6. Sometimes a small excess of resin is

incorporated so that some may be bled off during cure, but modern practice is

not to bleed (see subsequent discussion) but to use so-called net resin materials.

Standard prepreg is manufactured to be 0.125 mm (

⬇10 mils) thick after proc-

essing. Alternative thinner prepreg is sometimes available, mainly for space ap-

plications and a woven form with a thickness of 0.25 mm per ply is becoming

very popular. Prepregs are manufactured from several grades of carbon ﬁber,

aramid, E-glass, S2-glass, silica (‘‘quartz’’), and some ceramic ﬁbers. Carbon

and aramid are the most widely used. The resin used in prepreg is formulated

to be pasty at shop temperature with a controlled degree of tack (stickiness).

This allows the sheets to be accurately tailored and then placed, by hand or

machine. Too much tack renders it very difﬁcult to handle (it sticks to every-

thing!). Too little tack makes it more difﬁcult to locate on the tool. The prepreg

is cured by heating, and pressure is used to effect consolidation (see Section

2.2). The prepreg is supplied in continuous rolls, typically 600–1000 mm wide

and up to 100 m long. It is usually necessary that the prepreg be stored in a

cold room (

⬍⫺15⬚C) to maximize storage life.

5.3 Tooling

The most common form of tooling is to use single-sided molds with vacuum

bag consolidation. The tools are usually made from composites, with low-

temperature carbon ﬁber/epoxy being very popular. These tools are relatively

cheap, robust, and have thermal expansion characteristics that match that of

carbon ﬁber composites. Since the process is very widely used in the aerospace

industry, where one side of the molding will be the aerodynamic surface, it is

common practice to deﬁne this shape at the mold surface. Features such as

stiffeners and ribs are then built up on the inside surface of the molding using

auxiliary mold ﬁxtures. These can become very complex but allow for the man-

ufacture of large intricate moldings, which confer the beneﬁts of parts consoli-

dation.

5 AUTOCLAVE PROCESSING OF PREPREG 1069

5.4 Cutting the Prepreg

Prepreg may be hand or machine cut. The latter is adopted for all but the smallest

operations. The prepreg rolls must ﬁrst be withdrawn from cold storage and

allowed to equilibrate to shop temperature. This may take up to 24 h. The roll

must not be removed from its protective bag or unrolled while it is cold, oth-

erwise moisture may condense from the atmosphere onto the cold prepreg. This

would lead to water absorption and processing problems, e.g., porosity. Cutting

should be carried out in a clean room with temperature and humidity controlled

[20

⬚C and 50% relative humidity (RH) is common]. For hand cutting shears, a

Stanley knife, or a cutting wheel may be used. It is usual to make templates for

each ply from card, plastic, or sheet metal to provide a cutting guide. Each

template must also indicate the ﬁber orientation (e.g., 0

⬚,90⬚, Ⳳ45⬚) so that the

template may be correctly positioned on the roll of prepreg. When the width of

the part exceeds the width of the prepreg, it may be necessary to cut several

pieces to make up a single ply. Many moldings are of varying thickness. This

involves ply drops, which are preferably located toward the center thickness so

that there are no steps on the outside surfaces. It will be apparent that the

logistics of laying up a complex laminate of, say, 48 plies, with a speciﬁed

conﬁguration, (e.g., the quasi-isotropic [0

⬚,90⬚, Ⳳ45⬚]s), will be quite complex.

Each ply will require a drawing, part number, and template. The plies must then

be cut from the roll of prepreg, with due regard to minimizing wastage; each

ply must be identiﬁed and then stacked to form a kit of plies in the correct order

for the lay-up operation. If necessary positional index marks must be placed on

the cover ﬁlm of each ply to facilitate precise positioning. These are the argu-

ments for using an automated cutting system.

The automatic prepreg cutter consists of a ﬂat bed, typically 2 m wide by 10

m long. The prepreg sheets are unrolled along the bed, butting two or more

together if a wider strip is required and, sometimes, more than one layer. They

are retained in place by suction from under the bed. The cutter head is mounted

on a gantry that moves along the bed in the length direction and may also

traverse in the width and vertical directions. The cutting method may be a re-

ciprocating knife, ultrasonic knife, or water jet. The cutting operation is con-

trolled by a computer that is linked to the CAD system. Thus, each ply is deﬁned

in the software, so neither drawings or templates are required. The software also

incorporates a nesting program that adjusts the cutting sequence to minimize

waste. The software and the machines have been developed from those used in

the garment industry. A small automatic prepreg cutter is shown in Fig. 17. The

cutting operation is initiated and the cutter automatically cuts all the plies deﬁned

on the bed. There is also a printing head that numbers each piece and can print

index marks that are used to facilitate precise lay-up. At the end of the run the

cut plies are gathered and collated, either by hand or by a robotic device that

automatically collects and collates the plies into kits ready for lay-up. Although

the cost of such equipment is high, the beneﬁts in terms of precise cutting, speed,

elimination of hand operations, waste minimization, and improved quality as-

surance are very considerable.

5.5 Hand Lay-up

The tool is prepared and a release agent and/or release ﬁlm applied across the

molding surfaces. The prepreg kit is delivered to the lay-up location. At this

1070 COMPOSITES FABRICATION PROCESSES

Fig. 17 View of a small automatic prepreg cutting machine. This shows how the plies have

been positioned to minimize wastage. (Photograph by courtesy of the Eastman Machine Co.,

Rochester, NY, Model M9000)

0°

+45°

-45°

90°

Balanced, symmetric,

Quasi-isotropic [0, ± 45,90]

laminate formed from 8 prepreg plies

Fig. 18 Illustration of the necessary prepreg ply shapes showing ﬁber orientations

for laying up a quasi-isotropic laminate.

stage each piece of prepreg is still protected by the plastic or paper ﬁlm on each

side. The pieces are taken in the correct sequence, the protective ﬁlm removed

from the mold-face side, and the piece carefully positioned on the tool. Once

its correct location is conﬁrmed, It may then be partly consolidated by use of

hand, brush, or roller. Special care is taken to avoid any wrinkling or pockets

of entrapped air. (The tack allows the ply to be repositioned, if necessary, before

consolidation but not once it has been pressed down.) The outer protective ﬁlm

may then be removed and the next ply positioned. A typical lay-up sequence is

depicted in Fig. 18. It is usually necessary to further consolidate, or debulk, after

5 AUTOCLAVE PROCESSING OF PREPREG 1071

Fig. 19 Large tape laying machine. Both the tape laying head and the tooling are manipulated

to form a large aircraft fuselage component. (Photograph of the Viper tape layer

courtesy of Cincinnati Machine Co., OH)

every 12, or so, plies have been laid. This is done by placing a vacuum bag over

the mold, evacuating and usually warming to 50–70

⬚C for a period of the order

of 60 min, using an oven or radiant heaters. This softens the resin and allows

the pack to consolidate under the pressure exerted across the bag. The next set

of plies may then be placed until the lay-up is complete.

In the case of smaller moldings the prepreg pieces may be handled quite

easily and positioned by eye. If larger pieces,

⬎2 m, are to be placed, some

form of mechanical assistance may be required. Manipulators with suction pads,

which operate on the back cover ﬁlm, are convenient. For precision placement

index marks must be printed on the back-cover ﬁlm and a computer-controlled

laser projector, set above the lay-up location, used to project reference spots, a

red spot, or cross onto the back of the laminate. Each ply is lined up with these

reference spots and may be precisely located. The back-cover ﬁlm is then re-

moved and the computer set for the next ply.

5.6 Automated Lay-up

This is more difﬁcult to accomplish than automated cutting and collating. In fact

no fast, reliable method has yet been developed for laying precut plies onto the

mold. The only system is the automatic tape laying machine. This may be based

on a ﬂat bed, e.g., 10 m

⫻ 2 m, with moving gantry and a tape laying head

with 3–5 independent axes of motion. Prepreg tape, 50–600 mm wide, is dis-

pensed into the moving head where it is unrolled, the end trimmed, the backing

foil removed, and the tape laid onto the mold surface and rolled down to partially

consolidate. The most sophisticated tape laying machines can lay the tape onto

a complex tool or mandrel that is independently manipulated. This provides a

great deal of ﬂexibility in the shapes and lay-ups that can be handled. A pho-

tograph of one of these machines laying an aircraft fuselage component is shown

in Fig. 19. Instead of complete plies the layers are formed by rows of tape,

oriented according to the requirements of the speciﬁed conﬁguration. There is

minimal waste, just the portion lost when the tape ends are trimmed to ﬁt the

1072 COMPOSITES FABRICATION PROCESSES

45° ply formed by tape laying

Fig. 20 Single ﬂat 45⬚ ply as laid up by an automated tape laying machine. The ends of each

strip of tape must be accurately trimmed to the proﬁle of the ply.

ply perimeter. Tape laying allows conformation to surfaces with a higher degree

of double curvature than sheet reinforcements because small displacements may

be accommodated between the strips of tape, allowing small gaps or slight over-

lap. A representation of a tape-laid ply is shown in Fig. 20. An alternative

strategy is to use a separate machine to prepare the tape. This cuts the tape to

the required lengths, trims the ends to the correct angles, and discards the waste.

The prepared lengths of tape are transferred to a dispensing cartridge that is

loaded into the tape laying machine. This eliminates the need for the laying head

to trim the tape and is therefore faster. It also eliminates problems due to faulty

end trim and the possibility of the small waste offcuts being transferred to the

molding. The tape preparation equipment does not need the complex motions

of the laying head and is, thus, cheaper and faster. While tape laying machines

are automatic and require little manual intervention, they are not fast. Laying

speeds are only about 1 m/s, not allowing for turnaround time at the end of

each traverse. This is a consequence of the inertia due to the mass and size of

the moving parts. Tape laying machines are large and again represent a consid-

erable capital investment. Although initially troublesome, the equipment is now

well developed and reliable and is considered to be more economic than hand

lay-up for moderate runs of large moldings. Nevertheless automated cutting cou-

pled with hand lay-up is still the most common practice.

5.7 Cure Monitoring Sensors

Temperature and cure monitoring sensors are commonly incorporated into all

larger moldings or a proportion of smaller parts. These consist of ﬁne wire

thermocouples and/or proprietary cure sensors. These are smart chips that mea-

sure the changes in dielectric properties of the resin and can be used to assess

the progress of the cure. They may be incorporated into parts of the molding

that will be eventually discarded, but they are so small that they are unlikely to

compromise the performance of the component.

5.8 Tool Preparation for Curing

When the lay-up process is complete and the molding has been sufﬁciently

debulked, the mold must be ﬁnally prepared for cure in the autoclave. There are

several variants adopted by different operators. The most common is: The pe-

5 AUTOCLAVE PROCESSING OF PREPREG 1073

Cork peripheral dam

Stack of prepreg plies

Resin permeable membrane

Bleeder ply

Vacuum connection

Breather ply

Gas permeable membrane

Vacuum bag

sealed to

tool face

Tooling

Fig. 21 Typical arrangement of prepreg laminate and auxiliary materials for autoclave molding.

The perimeter of the laminate is sealed with a cork dam that prevents outward resin ﬂow. A

bleeder ply is set above a resin-permeable release membrane to absorb excess resin. The

breather ply ensures a continuous open passage for the vacuum application. The whole assem-

bly is covered with a vacuum bag sealed to the tool face. The vacuum connection is shown

through the bag, but an alternative is to connect through the tooling.

riphery of the molding is sealed to prevent in-plane resin bleed. This is often

done by ﬁxing a self-adhesive cork dam to the tool face, around the edge of the

part. A semipermeable release membrane is then placed over the part. This is

most usually a ﬁne woven glass fabric that has been coated in PTFE (Teﬂon).

This will allow liquid resin and gases to pass through but will not stick to the

cured part. It also imparts a ﬁne woven texture to the back surface of the mold-

ing. A layer of porous material is then placed on top. This may be woven glass

fabric, glass or polyester mat, or even blotting paper. It serves to absorb any

excess resin bled from the curing laminate. The thickness of this bleeder layer

is adjusted according to the quantity of resin to be bled. In many current oper-

ations, zero bleed or net resin systems are used, in these cases the bleeder may

be omitted altogether and an impervious release membrane used. However, many

consider it desirable to use some bleed. On top of the bleeder is placed a further

perforated ﬁlm, usually polyester or PTFE, which allows gas, but not resin, to

pass. There is then a further layer of porous fabric, usually a polyester felt,

termed the breather. This maintains a continuous gas-permeable layer over the

molding. The ﬁnal layer is the vacuum bag, a polyamide (nylon) or elastomeric

ﬁlm, which is sealed to the mold surface, outside the cork dam. This arrangement

is depicted in Fig. 21. A vacuum connection is made, either through the bagging

ﬁlm or through the mold, so that a vacuum may be applied to the breather ply.

When this is done, atmospheric pressure will act on the vacuum bag and press

the pack onto the molding surface. The continuity within the breather and the

permeable membranes allows gases and volatiles to be extracted from the entire

pack under the bag. The vacuum is maintained while the mold is moved into

the autoclave, when the autoclave vacuum system is attached.

5.9 Autoclave Operation

The autoclave is loaded with as many molds as it will accommodate. This might

be just a single large piece, e.g., a major wing component, or several smaller

1074 COMPOSITES FABRICATION PROCESSES

250

270

290

310

330

350

370

390

410

430

450

0 50 100 150 200 250 300

TIME - min

TEMP - K

INITIAL HEATING RAMP

DWELL

SECOND HEATING RAMP

HOLD

COOL DOWN

VACUUM

APPLIED

PRESSURE

RAMP

FULL PRESSURE APPLIED

Fig. 22 Typical temperature proﬁle for a prepreg cure cycle. The initial heat-up rate is con-

trolled, typically 5–10 K / min. The vacuum under the bag is normally maintained during heat-up.

A dwell is incorporated to assist temperature equalization in the laminate and to extend the pro-

cess window. The vacuum system is typically vented to atmosphere at this time and the con-

solidating pressure applied in the autoclave. When consolidation is completed, the temperature

is raised to the cure temperature. When cure is complete, the autoclave is allowed to cool

down to ambient temperature, with the pressure still applied. Finally the pressure is reduced so

that the autoclave may be opened and the parts removed.

panels. Each is connected to the vacuum system and temperature and cure sen-

sors connected to the central monitoring point. The autoclave may then be closed

and the cure cycle initiated. There are three controllable inputs to the cure cycle:

vacuum, pressure, and temperature. A typical cycle is depicted in Fig. 22 and

will now be described and discussed.

The autoclave is ﬂushed out with nitrogen, to eliminate air, and may then be

pressurized. The cycle commences with the vacuum line operating and the au-

toclave at ambient temperature and pressure. The heating cycle is started. As

the resin is warmed, it softens allowing further consolidation to occur under the

pressure induced by the vacuum. Eventually the resin becomes quite ﬂuid and

consolidation can be completed. At this stage the internal pressure is usually

increased and the vacuum lines opened to atmospheric pressure. The consoli-

dation is maintained by the difference between atmospheric pressure and the

pressure inside the autoclave. The reason for this is that with the vacuum applied,

the absolute pressure inside the molding is that of the vacuum. The autoclave

pressure is carried by solid-to-solid contact between the reinforcement ﬁbers,

with the liquid resin effectively ﬂoating in the interﬁber spaces. Any gas or vapor

porosity, effectively a bubble in the resin, will tend to dilate under these con-

ditions. By opening the space under the vacuum bag to atmospheric pressure,

this porosity will contract. A further alternative is to pressurize both the auto-

clave and the space enclosed under the bag, always maintaining a higher pressure

in the autoclave body, so as to maintain consolidation. This positive pressure

reduces pores more effectively and can even eliminate porosity by forcing air

and volatiles into solution in the liquid resin. It should be noted that the concept

5 AUTOCLAVE PROCESSING OF PREPREG 1075

200

250

300

350

400

450

500

0 50 100 150 200 250 300

TIME - min

TEMPERATURE - K

DEGREE OF CURE x 10

VISCOSITY

ARBITRARY UNITS

TEMPERATURE

CYCLE

PROCESSING WINDOW

Fig. 23 Shows the cure and viscosity proﬁles for a typical epoxy system under the cure cycle

described in Fig. 20. Note how the period of low viscosity is extended by incorporation

of the initial dwell at 400 K.

of sucking away volatiles with the vacuum is ﬂawed, as it would be virtually

impossible for a bubble to pass through the tightly packed prepreg ﬁbers and

hence out through the bleeder and breather plies to the vacuum pump.

The point at which the autoclave pressure is applied can be critical. If the

resin is very ﬂuid, too much may be squeezed out of the laminate. However, in

most systems the opposite problem dominates. This is to ensure complete con-

solidation and elimination of porosity before the resin gels. This can be exac-

erbated by the fact that the prepreg temperature will not be uniform. The outside

is heated ﬁrst while the interior remains cooler. Likewise the viscosity will be

lower in the hotter regions, until the cure advances and the viscosity rises sharply

to the gel point. The problem is to create a processing window where the resin

is sufﬁciently ﬂuid to allow consolidation to proceed to completion. This may

be accomplished by introducing dwell intervals in the heating cycle, as shown

in Fig. 22. The dwell or hold allows the temperatures to equilibrate in the mold-

ing and can be used to control the time at which gelation occurs. Another con-

sideration is the cure exotherm, which can result in local overheating, even in

quite thin laminates, e.g., 10 mm. This is because heat ﬂow into and out of the

laminate is very inefﬁcient due to the insulating characteristics of the bleeder

and breather plies in particular. A viscosity and cure proﬁle is shown superim-

posed on the heating cycle in Fig. 23. Note that initially the resin viscosity drops

as the laminate heats up. The dwell serves to extend the time that the resin

remains at low viscosity, allowing consolidation and elimination of porosity to

proceed. When the temperature is ramped up to the ﬁnal cure temperature, the

resin gels and viscosity increases sharply. At this stage the exotherm is most

intense and local overheating can result. The heating rate must be controlled to

prevent this overheating and, if necessary, a further temperature dwell is intro-

duced. The processing window is the interval when the viscosity lies below an

arbitrary level where effective consolidation will occur at the pressure employed.