Kutz M. Handbook of materials selection

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1096 COMPOSITES FABRICATION PROCESSES

second top cover ﬁlm, also resin coated, is positioned on top of the ﬁrst and the

whole sandwich passed through a set of kneading rolls that homogenize the

mixture. On exit from the machine the strip is coiled into rolls of convenient

size and stored in the maturation room. Before maturation the resin would be

liquid and the compound soft and sticky. After maturation the resin is pasty and

nonsticky. The material is typically 1–2 m wide and upward of 5 mm thick.

There are a number of variants of the basic SMC, also called SMC-R, for

random. A standard formulation is 35 wt % E-glass, 40 wt % ﬁller and 25 wt %

resin (Fig. 40a): This allows a very good surface ﬁnish to be achieved but lacks

stiffness and strength due to the low glass content (35 wt %

⬇ 18 vol % ⫺ V

⬇ 0.18). Higher mechanical properties are realized by using 65 wt % glass and

no ﬁller (Fig. 40b), but the surface ﬁnish is inferior. The ﬁllers are mostly ﬁne

chalk powders, often wollastonite, and other processing aids such as soaps, lu-

bricants, and low proﬁle additives are also included. The low proﬁle additives

are waxes or low-melting-point thermoplastics that migrate to the surface during

cure and improve the surface ﬁnish. Instead of chopped strand, continuous strips

of CRM may be fed into the SMC machine, again with the choice of high glass/

no ﬁller, or low glass/high ﬁller. The CRM conveys some molding and property

characteristics, which will be discussed further. Another option is to manufacture

the compound from woven fabric. The ﬁnal alternative is to make the SMC by

winding continuous rovings and apply the resin onto a large-diameter drum,

giving a near uniaxial lay-up (e.g.,

Ⳳ5⬚). This is a batch process. When the

required thickness is achieved, a top-cover ﬁlm is applied and the sheet cut

across to remove it from the drum. Sheets of up to 10 m long by 2 m wide may

be produced this way. The material was originally developed for the manufacture

of vehicle leaf springs, where a near uniaxial lay-up and high V

is required.

For all SMC materials the most common resin systems are the unsaturated poly-

esters and vinyl-esters. Only these materials allow for the convenient maturation

thickening process. Vinyl-ester resins give somewhat better performance and are

considered premium materials. Epoxy resins are rarely used in SMCs.

12.3 Molding SMC

All the SMC materials discussed are compression molded in heated metal tool-

ing, typically at 120–180

⬚C. The main differences result in the differing ﬂow

potential of the different formulations. SMCs made with chopped strand ﬂow

extensively when molded, more so when the ﬁber loading is lower and the glass

chopped into shorter lengths. It is normal practice to promote ﬂow by loading

pieces of SMC onto the open tool so as to cover only about 50% of the projected

area of the mold. There is no attempt to tailor the pieces to the shape of the

part, but an optimum pattern must be determined and the charge accurately

weighed, to ensure efﬁcient and reproducible mold ﬁlling. When the mold is

closed, initially under light pressure, the resin in the charge softens and as the

pressure is increased the charge ﬂows to ﬁll the mold cavity (Fig. 41). Quite

detailed features such as ribs and bosses may be formed, but there is some

danger that the resin will bleed through the reinforcement, so that the remote

details of the molding may contain insufﬁcient reinforcement (Fig. 42). This

tendency is controlled by varying the ﬁll pattern and modifying the pressure

cycle.

12 SHEET AND BULK MOLDING COMPOUNDS 1097

GLASS FIBER FILLER RESIN

STANDARD S.M.C.

PERCENTAGE

(a)

GLASS FIBER FILLER RESIN

HIGH STRENGTH S.M.C.

PERCENTAGE

(b)

Fig. 40 Two alternative SMC formulations are illustrated in the two bar charts. (a) Standard

SMC incorporating 35 wt % of random chopped E-glass ﬁber bundles, with 40 wt % of ﬁller

(chalk) and 25 wt % resin. This gives good surface ﬁnish but only moderate stiffness. (b) This

alternative formulation contains 65 wt % of chopped glass with 35 wt % of resin. It molds less

readily to complex shapes and has an inferior surface ﬁnish, but much greater stiffness and

strength. It is widely used for hidden structural parts.

1098 COMPOSITES FABRICATION PROCESSES

Charge placed in open, hot mold

to cover ≈ 50% projected area

Mold closed, pressure applied

Charge flows to extremities of cavity

Part ejected hot when cure complete

2 - 20 min.

Fig. 41 When molding SMC the charge is placed so that it covers about 50% of the projected

area of the mold face. As the mold is closed, under pressure, the charge is heated, the resin

softens, and is forced to ﬂow to ﬁll the mold cavity.

Direction of flow

Less reinforcement penetrates

to extremities

SECTION OF SMC MOLDING

WITH STIFFENING RIBS

Fig. 42 When forming parts incorporating features such as blade stiffeners, there is a ten-

dency for less reinforcement to ﬂow into these extremities. This can result in weak regions.

The problem is countered by careful placement of the charge and by control of the

molding parameters—especially the mold closing speed.

Materials with higher ﬁber content and those manufactured with continuous

random mat will not ﬂow as extensively as the standard SMC. It is necessary

to tailor the charge more carefully to ensure good molding performance. It is

especially important, when using the high ﬁber content materials that adequate

ﬁber content is realized in all parts of the molding, as these materials are chosen

on account of their mechanical performance. An advantage of the CRM rein-

forcement is that the reinforcement will not tear during ﬂow, leaving ﬁber-

denuded zones. This can be a problem with discontinuous strand materials.

SMCs made with woven materials and the uniaxial material described above

cannot ﬂow to any signiﬁcant extent in-plane; they must, therefore, be carefully

tailored to the shape of the part, and features such as molded ribs are not a

possibility. One compromise is to charge the mold with a mixture of woven and

random feedstock, e.g., to stiffen a panel while still being able to mold some

out-of-plane features. These are highly cost-effective materials and much inge-

nuity is exercised to exploit their engineering potential.

Once the charge is heated and full pressure applied to the press, curing occurs

quite rapidly. According to the size, wall thickness, and complexity of the mold-

13 GLASS MAT THERMOPLASTICS 1099

ing, cycle times as low as 2 min may be achieved. A cycle time of 10–20 min

is more common so that 15,000–30,000 parts/year may be produced from a

single press working on a 24-h, 3-shift day. The parts are ejected from the mold

while hot. The tools may therefore be operated at constant temperature through-

out a molding campaign. This is compatible with the needs of the automobile

industry, although much slower that metal pressing. However, assembly and

ﬁnishing costs are likely to be much less when SMC is used.

12.4 Bulk Molding Compound

This is a similar formulation to SMC except that the compound is normally

prepared in a ribbon mixer. The reinforcement is always chopped strand, usually

rather shorter than that used in SMC, e.g., 5–20 mm. Standard formulations are

typically

⬇35 wt % glass, ⬇35 wt % ﬁller and the remainder resin. Fillers and

processing aids are similar to those used with SMC. The material is extruded as

a rough cylinder, matured, and then packed in bags or bulk containers to await

ﬁnal processing.

For compression molding the procedure is similar to SMC. A weighed piece

of BMC is placed in the open mold and the molding cycle initiated. BMC is

generally used for smaller parts than SMC, so faster processing is the norm.

BMC may also be processed by injection molding, which is becoming the

dominant process. Specially developed machines are used, which resemble those

used for thermoplastics in some respects. The basis of the machine is a screw

pump, which both rotates and reciprocates. The charge is fed into the feed end

of the screw by a mechanical stuffer, as the BMC cannot be gravity fed. The

charge is transported forward by the screw through the barrel, which is heated

to a temperature of usually 60–100

⬚C, by means of a water or steam jacket. This

is sufﬁcient to soften the resin in the BMC but not to initiate cure. A reservoir

of plasticized material is built up in front of the screw, which retracts under

controlled backpressure to accommodate the forward ﬂow of BMC. The screw

is then driven forward to inject the accumulated charge into the hot mold. Pres-

sure is maintained while the charge cures, typically 1–5 min. The rate-limiting

step is usually the time needed to heat the charge from barrel temperature to

cure temperature and for the cure to be completed. This may be speeded up by

using a very fast injection rate. This heats the charge by the work of shear

induced in the charge and eliminates the heat-up time. Injection molding of BMC

requires high pressures and a very robust molding machine and tooling, which

are more costly than the thermoplastics equivalents. Production can be com-

pletely automatic and very high production rates achieved. Fine detail can be

incorporated into the designs and the technique is widely utilized in the auto-

motive and domestic appliance industries, where BMC parts often replace parts

traditionally fabricated from metal castings.

13 GLASS MAT THERMOPLASTICS

13.1 Principles

Thermoplastics are always solids at room temperature, therefore, processing

methods must be adapted to accommodate the necessary melt–freeze cycle.

Glass mat thermoplastics (GMTs) are the thermoplastics equivalent of SMC.

1100 COMPOSITES FABRICATION PROCESSES

They are preimpregnated materials in sheet form. The processing cycle consists

of heating the feedstock to above the melting point of the thermoplastic matrix

and then stamping or pressing the hot material between cold tools. The hot

compound ﬂows to ﬁll the mold cavity and then cools rapidly in contact with

the cold tool faces, and the melt solidiﬁes. Process cycles can be as short as

30 s.

13.2 Reinforcements and Matrices

In principle any sheet form reinforcement may be used. The terminology GMT

implies glass ﬁber mat reinforcement, and this is the standard commercial prod-

uct, but it is possible to use both aramid and carbon ﬁbers, either alone or

hybridized with glass. Likewise formats other than mat, such as uniaxial (i.e.,

similar to prepreg) or woven, may be used. The standard commercial materials

are mostly based on the use of continuous random E-glass mat, CRM, although

some use chopped strand. This choice is driven by economics and optimization

of the ﬂow process when the material is stamped. The matrices in commercial

materials are either polypropylene (PP) or a polyamide (PA), usually PA 6, 11,

or 12. Other thermoplastics have been used and uniaxial and woven materials

using PEEK (poly-ether-ether-ketone) and other aromatic thermoplastics are

manufactured for high-performance applications. They are discussed in the fol-

lowing section. The choice of PP and PA is based on their relatively low melt

temperatures, their melt rheology, and performance. Molten thermoplastics are

very viscous and to ensure adequate ﬂow, the ﬁber fraction and ﬁller loadings

are lower than for typical SMCs. This inevitably means that the mechanical

properties, especially stiffness, will also be lower. This is compensated by greater

toughness, better environmental performance under some conditions, faster proc-

essing, and potential recyclability.

13.3 Processing

The feedstock is most often supplied a sheets, 0.5–2 m wide, 1–2 m long, and

typically 5–10 mm thick. They have been manufactured by rolling a stack of

alternate layers of glass mat and polymer ﬁlm through heated rolls. The glass

mat is not usually completely impregnated. Blanks of a suitable size are cut

from the sheets. It is not common practice to tailor the blanks to the molding,

but simply to cut a set of blanks, ensuring that the total weight of the charge is

correct.

The cut blanks are then heated, usually in a belt feed, tunnel oven, heated by

circulating hot air, or by radiant heaters. The heating process must be carefully

controlled because the material has low thermal conductivity and too ﬁerce an

application of heat could lead to local degradation at hot spots. When the blanks

emerge from the oven, the matrix is completely melted. There is often consid-

erable exfoliation, and the 5-mm-thick blanks swell up to 20 mm thick, or more.

Although the matrix is molten, the blanks may still be handled as the glass mat

holds everything together. This is one reason why CRM is preferred. The hot

blanks are then placed onto the bottom surface of the molding tool in a prede-

termined pattern (cf. SMC). In this condition the exfoliated blanks do not chill

excessively. The press is then closed and the charge is squeezed between the

tool faces, forcing the material to consolidate and ﬂow to the extremities

of the mold cavity. Quite high degrees of draw are possible. Once consolidated

14 HIGH-PERFORMANCE THERMOPLASTIC MATRIX COMPOSITES 1101

GMT blanks

Tunnel oven

Hot GMT blanks ready

for stamping

Stamping press with cold, matched dies

Transfer to press

Stamped GMT part

Fig. 43 Glass mat thermoplastic (GMT) forming process is illustrated. Cut blanks of thermo-

plastic impregnated glass mat are heated by passing through a tunnel oven. The hot blanks are

then stamped between hot dies. This forms the part and chills the material making

for a fast process cycle.

the charge is rapidly chilled by contact with the cold tooling, and the tool may

be opened and the part ejected. The process is illustrated schematically in Fig.

43. The speed of mold closure is critical. It must be sufﬁciently fast that ﬂow

to the tool extremities occurs before the charge is chilled below the freezing

point of the matrix, hence the use of the term stamping rather than pressing.

The speed of the process renders it particularly attractive for mass production

industries. Fine detail may be incorporated into the moldings, so that a single

GMT part can replace an assembly of pressed metal. In general the surface ﬁnish

is not as good as low-proﬁle SMC and inadequate where a ‘‘Class A’’ ﬁnish is

required, e.g., external panels for automobiles.

14 HIGH-PERFORMANCE THERMOPLASTIC MATRIX COMPOSITES

14.1 General

The development of the high-performance aromatic thermoplastics such as

PEEK PES (poly-ether-sulfone), and PPS (poly-phenylene-sulﬁde) provides ma-

terials with mechanical and elevated temperature properties that can compete

with epoxy resins for composites matrices. There are potential processing ad-

vantages, and it has been demonstrated that carbon ﬁber combined with these

matrices provides composites with superior toughness and resistance to some

environmental hazards. There has, therefore, been a period of intensive devel-

opment of these high-performance thermoplastic based materials for aerospace

and similar applications.

14.2 Feedstock

In conformity with aerospace practice, which is built around use of prepreg, the

thermoplastic systems are produced as prepreg-like impregnated tape, as ﬁne

1102 COMPOSITES FABRICATION PROCESSES

woven impregnated tape, and as impregnated tow. The normal reinforcement is

carbon ﬁber as it is not generally cost effective to use lower performance ﬁbers

with such expensive matrices. The only exceptions would be where speciﬁc

physical properties, such as radar transparency, were required.

The tape materials are produced in thicknesses comparable with that of con-

ventional prepreg, e.g., 0.125–0.25 mm, much thinner than GMT. Impregnated

tow is typically a single tow of 6000 or 10,000 carbon ﬁbers. The sheets are

stiff and boardlike at room temperature and cannot be draped at all without

heating. The tows may be coiled.

14.3 Processing Principles

The principles for processing high-performance thermoplastic composites are

similar to those for GMT. The material must be heated to a temperature that

melts the matrix; it can then be shaped and must be chilled in the formed shape

to freeze the matrix. There are, however, a number of additional considerations.

First, the continuous ﬁber, high V

, feedstock material will not ﬂow to any

extent when molded. Its drape possibilities are similar to those of prepreg, except

that the thermoplastic form must be heated before it can be draped at all. A

second consideration is that the melting points of some of the matrices approach

400

⬚C, so that any tooling that needs to be heated to the melt temperature must

be steel. Other materials can be used where the hot feedstock is stamped between

cold dies. A third point is that some of the thermoplastics are semicrystalline,

e.g., PEEK, and their properties are strongly sensitive to the degree of crystal-

linity. This means that the cooling rate from the melt temperature to below the

crystallization temperature must be controlled. Finally, when using high-

performance materials, very close control of V

, ﬁber architecture, and dimen-

sions is required so that optimum performance is achieved. These factors

combine to make processing a more complex operation. The simple preheat and

cold stamp process used for GMT is sometimes adequate but, in general, these

prepreg-like feedstocks require a more extended period for consolidation while

hot.

14.4 Autoclave and Press Processing

Panels of moderate single curvature and low complexity may be molded using

either an autoclave or a press. The thin sheets of feedstock are cut to the required

proﬁle and stacked in sequence, as for prepreg. The difference is that the material

has no tack to retain each ply in position. This may be accomplished by thermal

tacking. A soldering iron or similar implement is used to spot weld the sheets

together as they are laid. When the laminate has been completed, it is placed

on the tool and covered with a vacuum membrane. Owing to the high processing

temperature, this may need to be a high-temperature stable polymer or elastomer,

or even aluminum foil. The assembly is then subjected to vacuum and placed

in the autoclave. The temperature and pressure may then be raised so that the

matrix melts, typically in the range 250–400

⬚C, and the pressure consolidates

the laminate. This process can take longer than for thermoset systems due to

the higher viscosity of the molten thermoplastic. Once consolidation has been

completed, the whole assembly must be cooled, under pressure, until the matrix

has solidiﬁed. Furthermore the cooling rate must be controlled through the crit-

15 COMMINGLED THERMOPLASTIC MATRIX COMPOSITES 1103

ical crystallization region. This may entail incorporation of water cooling in the

tooling. The autoclave may then be opened and the parts demolded. This is

clearly a rather time-consuming procedure.

An alternative method is to use a press tool with means of heating and cool-

ing. The tooling may be either single-sided, working against an elastomer

counter tool, or double-sided matched tooling. The choice depends on part com-

plexity and process temperature. The lay-up procedure is similar to that de-

scribed above, the laminate is then placed into the tooling, which is heated, and

pressure is applied in a controlled manner. The full pressure and temperature

are held while consolidation is effected and then cooled down under pressure.

In favorable cases the stacked laminate may be preheated in an oven, the hot

laminated transferred to hot tooling, pressed, and consolidated. The press is then

opened and the hot part transferred to another set of cold tooling in a separate

press, where pressure is applied while the part cools. Auxiliary tooling can be

used to facilitate the transfer from the hot to the cold press tools. This method

is much faster as it is not necessary to heat and cool the molds during each

cycle. It is also more thermally efﬁcient. The disadvantage is that two sets of

tools are required, although the cold tools do not need to be made from high-

temperature stable materials.

14.5 Diaphragm Forming

Diaphragm molding is a variation of the vacuum forming process used for simple

thermoplastics. The stacked laminate is held between a pair of deformable dia-

phragms, and then, after heating, formed into a single-sided tool by vacuum,

gas pressure, or a combination of these. The diaphragm materials need to be

stable at the processing temperature but the shaped tool remains cool. Dia-

phragms may be high-temperature stable polymer or elastomer ﬁlms, suitable

for short service at temperatures of up to about 300

⬚C or superplastic aluminum

alloy for higher temperatures. The process is reasonably fast but the cost of the

disposable diaphragms is a signiﬁcant added cost.

14.6 Economic and Performance Implications

There has been much speculation, over the past 20 years, that high-performance

thermoplastics matrix composites could challenge the dominant position held by

carbon/epoxy thermoset systems. This has not happened! The initial prospect

of fast, cheap processing proved to be something of a myth and material costs

have remained high. The ease of processing of the low V

GMT systems does

not apply with high V

, continuous uniaxial ﬁber laminates, which are probably

even more difﬁcult to process than the thermosets. There are applications where

thermoplastics systems are more cost effective, mainly for less complex shapes,

and it is undisputed that the thermoplastic matrix composites offer a combination

of stiffness and toughness that cannot easily be matched with thermoset systems.

The two classes now operate side by side with thermosets the senior partner.

15 COMMINGLED THERMOPLASTIC MATRIX COMPOSITES

15.1 Principle

Several strategies have been developed to deliver thermoplastic matrix compos-

ites feedstock in a more drapable format than the prepreg-like materials dis-

1104 COMPOSITES FABRICATION PROCESSES

Thermoplastic fibres

Reinforcement fibres

Biaxial co-woven fabric

Uniaxial co-woven fabric

Fig. 44 Co-woven fabrics are woven from a combination of tows of reinforcement

(e.g., E-glass or carbon ﬁber) and tows of thermoplastic ﬁbers (e.g., polypropylene or

polyamide). Various uniaxial and biaxial reinforcement geometries are possible.

On hot pressing the thermoplastic component melts and ﬂows to form the matrix.

cussed in the previous section. This is accomplished by the combination of the

reinforcement ﬁbers with the matrix in a thin foil, tape, ﬁber, or powdered form.

These materials may be supplied as mingled tows or in sheet formats. When the

material is subjected to heat and pressure, the thermoplastic component melts

and is inﬁltrated into the reinforcement by the action of the pressure. The key

to the viability of these materials is that the inﬁltration distances are very short,

due to the ﬁnely aggregated state of the feedstock. There are three main types

of product in this category, co-woven fabrics, co-mingled (also known as com-

mingled) tows, which may in turn be woven into fabrics, and ﬁber/powder com-

binations in tow form, again with the possibility of being woven.

15.2 Co-woven Fabrics

These are simply fabrics woven from a combination of tows of reinforcement

ﬁbers and tows of thermoplastics ﬁbers. A prerequisite is that the matrix material

is available or can be drawn into a suitable ﬁbrous form. This is no problem

with a range of commodity thermoplastics that are used as textiles. These include

polypropylene, several types of polyamide (nylon), and polyesters. Some of the

higher performance thermoplastics, e.g., PEEK, may also be drawn into ﬁbers,

but as there is no established mass market for textiles in these materials costs

are likely to be much higher. It is also possible, but not extensively practiced,

to use ribbons of thermoplastics tape rather than tows for the thermoplastic

component. The two materials are combined in proportions corresponding to the

required ﬁnal V

. The most usual arrangement is simply to alternate the tows of

both warp and weft (ﬁll) between the two materials, shown in Fig. 44. The linear

densities of the tows are chosen to give the correct proportions of reinforcement

to matrix. The resulting product is a highly drapable fabric. It may be handled

in the same manner as any other reinforcement fabric. These materials have no

natural tack but may be retained in position during lamination by spot welding

using a soldering iron or similar device or by use of spots of a compatible

15 COMMINGLED THERMOPLASTIC MATRIX COMPOSITES 1105

Commingled tow

Reinforcement fiber

Thermoplastic fiber

Fig. 45 Commingled tows are formed from an intimate, one-to-one, combination of reinforcing

ﬁbers and textile ﬁbers. These materials are now available commercially. They may be woven

into fabrics or used directly in tow placement technologies. Their main advantage over co-

woven materials is that the ﬂow path is much shorter and processing therefore much faster.

adhesive. Once assembled, the laminate is consolidated by heat and pressure.

This may be carried out in an autoclave with vacuum bag, in which case the

mold/laminate assembly must be cooled to freeze the matrix before demolding.

The alternative is to heat the laminate separately, e.g., in an oven, and then press

between cold tools. The problem is to ensure that full inﬁltration is affected.

The problem with co-woven fabric is that the amount by which it must be

consolidated is greater than with conventional preimpregnated materials, because

the interstices between the ﬁbers, both reinforcement and matrix ﬁbers, and

between the woven tows are open, i.e., ﬁlled with air. This space, typically 30%,

must be eliminated to achieve a pore-free laminate. Another factor is that the

mean inﬁltration distance is of the order of one tow width, e.g.,

⬇1 mm, while

this is quite short, a signiﬁcant time interval is necessary for the very viscous

liquid thermoplastic to fully inﬁltrate the tow. For this reason there has been

intensive development of the co-mingled ﬁber systems discussed below.

15.3 Commingled Tow Materials

In these materials reinforcement ﬁbers are combined with thermoplastics ﬁla-

ments within a single tow (Fig. 45). The result is a textilelike tow that may be

woven or placed directly. The advantage of this system is that inﬁltration dis-

tances are only of the order of the ﬁlament diameter—a few microns. The most

successful commercial system is a combination of E-glass and polypropylene

ﬁbers, marketed under the name Twintex by the French company Vetrotex. This

material is available in a variety of woven fabrics that are easily draped and in

favorable circumstances may be processed by preheating the preform and press-

ing between cold dies. More complex shapes and thicker sections may require

hot pressing followed by transfer to cold tools for chilling. It is also possible to

ﬁlament wind or to use tow placement techniques where the tow and the work-

piece are heated locally at the point of application of the tow, so that it is