Kutz M. Handbook of materials selection

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1086 COMPOSITES FABRICATION PROCESSES

1. Preform loaded into transferrable

insert

2. Preform and insert placed in mold

3. Mold closed - resin injected

4. Cured part demolded

insert recycled.

Preform

Insert

Fig. 29 Productivity in the RTM process may be enhanced by use of a system of transferable

mold inserts. These are auxiliary tools, usually thin sheet metal construction. The preforms are

built up on these inserts, which are ﬁnally placed into the RTM molds. Several inserts may be

rotated, so that preform assembly and demolding may be carried out concurrently with molding.

cesses utilize fully automatic techniques that are usually computer controlled.

Labor requirements are low but the rate of build also tends to be quite low.

9.2 Filament Winding

The principle here is that the ﬁber roving is laid onto a rotating mandrel by a

dispensing head, which may simply reciprocate or execute more complex mo-

tion. In the simplest version a cylindrical mandrel is rotated and the dispensing

head traverses forward and backward parallel to the axis of the mandrel. This

results in a simple structure laid at angles of

⫹

␪

and ⫺

␪

to the mandrel axis

(Fig. 30). This produces a tubular structure. The angle of the lay-up is controlled

by the relative speeds of the rotation and the traverse, and it is convenient to

wind at angles from near 90

⬚, the hoop direction, to about 25⬚. Clearly it is not

easy to wind close to the axial direction with this arrangement, but alternative

machine conﬁgurations allow this to be done, e.g., the race track winder (Fig.

31). A number of other conﬁgurations are also possible so that virtually any

desired conﬁguration can be attained on axisymmetric shapes. By incorporating

extra degrees of freedom in the winding head, nonaxisymmetric shapes may also

be wound. However, the main output of commercial ﬁlament winders are cylin-

drical tubes and pressure vessels. In the case of tubes there is a transition region

at each end when the traverse reverses. This is a ring of thicker section where

the angle changes from

⫹

␪

to ⫺

␪

and vice versa. This is usually trimmed off.

For pressure vessels a cylindrical section blends into hemispherical ends. This

enables the winding to be carried over the end as the traverse changes direction.

This provides an ideal ﬁber conﬁguration for pressure applications. The cylin-

9 FILAMENT WINDING AND TOW PLACEMENT 1087

Rotating mandrel

Resin bath

Fiber supply creel

Fiber feed

Traversing feeder head

Showing winding a simple

⫾⍜

array onto cylindrical

mandrel

Fig. 30 Basic ﬁlament winding process. Impregnated ﬁber tows are wound onto a rotating

mandrel. Control of rotation and traverse speeds determines the angle of wind. A Ⳳ

␪

pattern is

built up as the traverse direction is alternated. This results in a basket weave structure.

2. Dispenser reciprocates and mandrel rotates faster for

angle or hoop winding

“Racetrack”

Mandrel rotates about its axis

1. Roving dispenser circulates around racetrack whilst

mandrel rotates slowly for near axial winding.

Fig. 31 Setup illustrated in Fig. 30 is not suitable for laying tow oriented closer than about 20⬚

to the mandrel axis. Zero winding angles may be laid with a race-track machine. This is suitable

for axial, hoop, and angled windings according to the relative speeds of the

race-track dispenser and the mandrel rotation.

drical portion is wound at Ⳳ55⬚, which gives twice the strength in the hoop

direction compared with the axial, an ideal balance. The ﬁbers are wound uni-

formly in all directions over the hemispherical ends giving isotropic properties.

An alternative is to wind a spherical vessel. To wind hollow pressure vessels it

is necessary to devise a means of removing the mandrel. Various techniques

have been adopted: inﬂatable mandrels, plaster and salt mandrels, which are

leached out with hot water after the part has been cured, and expanded polysty-

rene, which can be collapsed by heating or use of hydrocarbon solvents.

Filament winding is ideally suited to the manufacture of axisymmetric hollow

shapes, but, as indicated above, some nonaxisymmetric shapes can also be

formed. There are some restrictions. The winding path needs to comply quite

1088 COMPOSITES FABRICATION PROCESSES

w - ribbon width

␪

x = w / cos

␪

y = w / sin

␪

Winding

angle

Fig. 32 Illustrates the concept of ribbon width. The ribbon is normally built up by laying

several impregnated tows simultaneously. The axial and hoop coverage are then

simple functions of the winding angle.

closely to a geodesic path (great circle path) on the curved surface, otherwise

the winding tension will lead to lateral slippage of the wound rovings. Friction

may allow some small deviations from the ideal path. Another restriction is that

it is generally not possible to wind into concave surfaces. These restrictions do

not apply to winding thermoplastic matrix systems, which are effectively welded

into place as laid (see Section 9.3).

Filament winding usually uses continuous dry rovings that are impregnated

with resin by passing through a trough of liquid resin immediately prior to

deposition. The resin content is controlled by the resin viscosity and by passing

the wet tow through a calibrated eye, which squeezes out any excess resin. This

is a simple and effective technique. After winding, the resin is allowed to cure

either at room temperature or at elevated temperature. An oven or radiant heater

may be used. It is sometimes desirable to rotate the mandrel during cure to

prevent any excess resin running to the bottom of the winding.

As the roving is laid down on the mandrel, it will be in the form of a ﬂat

ribbon, and to obtain a uniform winding it is necessary that complete coverage

be obtained with a integral number of ribbons, without overlap or gaps. It is

common practice to combine several rovings to form a wider ribbon. This ribbon

is then wound as an open helix along the length of the mandrel. The next run

will be in the reverse direction and will consist of a helix of opposite angle. It

must be arranged that the width of the tape, w, is related to the circumference

of the mandrel,

␲

d, so that nw/sin

␪

⫽

␲

d (Figs. 32 and 33). The number of

double runs necessary for complete coverage is then equal to n. It is also nec-

essary that the start point for each run be positioned so that each ribbon is

perfectly butted to its neighbors. The ribbon width must be chosen so that a

logical sequence of positions may be realized. For instance if n

⫽ 7, there must

be 7 start points around the circumference at each end of the mandrel. The

sequence for winding may then be 1, 2, 3, 4, 5, 6, 7; 1, 3, 5, 7, 2, 4, 6; or 1, 4,

7, 3, 6, 2, 5. Each of these results in a slightly different variation of the basket

weave structure that characterizes ﬁlament winding.

Filament winding may also be carried out using preimpregnated tows or with

prepreg tape. It is also possible to wind ﬁbers preimpregnated with thermoplastic

resins. In this case local heat is applied to melt the thermoplastic, and pressure

9 FILAMENT WINDING AND TOW PLACEMENT 1089

a b c d e f a

1. Single pass produces an open helix

2. Ribbon width must be arranged so that an integral

number of passes produces a complete cover -

6 in this illustration.

Fig. 33 Illustrates how a full cover is achieved by laying a number of ribbons successively so

that they each butt up to each other. The ribbon width and the start position for each pass

must be carefully synchronized to achieve this effect.

Thermoplastic impregnated

fiber tow

Hot gas jets

Cold

compaction

roller

Melt zone

Rotating winding

mandrel

Fig. 34 Thermoplastics impregnated tows may be ﬁlament wound by thermally welding them

into place at the point of application. The principle is illustrated in the ﬁgure. This technique can

also be adapted for tow placement technologies.

is applied to consolidate at the point of application (Fig. 34). This is essentially

welding the tape or roving into position as it is laid.

Tubular components are widely used for transport of ﬂuids at moderate pres-

sures and also for drive shafts. In the latter case the conﬁguration is modiﬁed.

Layers at

Ⳳ45⬚ are used to carry torsion loads, and others at 0

for bending.

Machinery for simple ﬁlament winding is relatively inexpensive, it can be op-

erated automatically with manual attention only necessary to change mandrels

when a winding run is completed. The range of sizes is wide, varying from less

than 10 mm diameter up to more than 3 m. Lengths are limited by the length

of mandrel that can be supported without undue sag. Machines have been de-

1090 COMPOSITES FABRICATION PROCESSES

veloped, with reciprocating mandrels and in situ cure, that allow for the manu-

facture of continuous piping.

9.3 Tow Placement

Tow placement technology is a variant of ﬁlament winding where impregnated

tows (rovings) are precisely placed onto the mold surface using a robotic tow

placement head. The basic technique is to place the tows into/onto a stationary

mold, which may be ﬂat or have complex curvatures. Further part shape possi-

bilities may be generated by moving the mold as well as the placement head. A

standard tow placement machine is similar in many respects to a tape laying

machine with the placement head operating from a gantry moving over a ﬂat

bed on which the tool is mounted. Other machines are essentially robots that

bear the tow placement head and can be manipulated with 3–7 degrees of free-

dom according to the complexity of the surfaces to be generated.

The tow or roving is normally fed dry into the placement head, where it is

impregnated with resin prior to being placed in position. Pressure is applied at

the point of placement to bond the laid tow to the previously laid layers or to

the mold surface if it is the ﬁrst layer. The alternative is to use preimpregnated

tow, which is supplied in spools and stored at low temperature, like prepreg. It

is also important that the impregnated tow have a degree of tack, so that it

remains in place once laid. One strategy is to warm the impregnated tow im-

mediately prior to placement; this softens the resin. After placement it cools, the

resin viscosity increases sharply, and it will be effectively stuck in place. This

strategy is also the basis for tow placement using preimpregnated thermoplastic

matrix tows. In thermoset systems the completed laminates are then cured using

conventional autoclave or vacuum bag and oven cure technologies.

Tow placement is a more versatile technique in terms of shapes that may be

generated than either ﬁlament winding or tape laying. There is no requirement

that the placement head follow a geodesic path or that the path needs to be

straight. Tows can be effectively laid into concave surfaces. The normal strategy

is to lay the tows in layers to build up the required laminate conﬁguration. The

tows are cut at the end of each traverse and the head reset for the next run. The

limitations of tow placement are the high acquisition costs of the placement

equipment and a relatively low rate of production. The maximum placement

speed is currently of the order of 1 m/s. If the tow weight were 2500 tex, a

heavy tow, this translates to a placement rate of about 9 kg/h. This does not

allow for inevitable losses due to the need to manipulate the head each time a

run is completed. In spite of these disadvantages, tow placement is seen as one

of the more cost-effective techniques for laying up high-performance structures

for aerospace and similarly performance-dominated applications.

10 PULTRUSION

10.1 Principle

This is a versatile process for the manufacture of prismatic sections. In this

respect it differs from most composites technology in that a semiﬁnished product

rather than a component is manufactured. The concept is very simple; continuous

reinforcement is impregnated with liquid resin and then drawn through a long,

10 PULTRUSION 1091

Fiber supply

creels

Resin bath

Heated pultrusion die

Cured section

pulled from die

Pultrusion speed 0.1 - 5 m/min

Max. die dimension ~1.5 m

Fig. 35 In the basic pultrusion process, resin impregnated tows are pulled through a heated

die, from which they emerge as fully cured solid sections.

Impregnated

reinforcement from

resin bath

Pultrusion die

Pair of reciprocating pullers

Cut-off saw

Dry

reinforcement

Resin injected

into die entry

Water inlet

Water outlet

Caterpillar puller

Cooled inlet section

Fig. 36 Two alternative techniques for pulling pultruded sections are illustrated. The reciprocat-

ing puller is more easily adapted to different section shapes and is generally the more versatile.

Also illustrated is the alternative resin injection technology, where resin is injected

into a cooled entry section of the die, rather than the reinforcement being impregnated

by passing through a bath of resin.

heated, shaped die. During its passage through the die, the resin is cured and a

solid section emerges. The continuous emerging section is then cut off into

convenient lengths for transportation and conversion into the ﬁnished product.

Slender sections may be coiled so that very long lengths can be manufactured.

This principle is illustrated in Figs. 35 and 36. The traction to pull the material

through the die is applied to the emerging cured section, either a caterpillar or

more usually a pair of reciprocating pullers are used. These are operated by a

chain drive in a manner analogous to ‘‘hand-over-hand’’ pulling a rope. One

puller is clamped to the pultrusion and engages in the drive chain. It pulls the

product for a short distance, e.g., 1–2 m, while the second puller is returned to

its start position. Then the second puller is engaged and the ﬁrst released and

returned. The pullers are easily adapted to different pultruded proﬁles by simply

1092 COMPOSITES FABRICATION PROCESSES

changing the shaped clamping pads. Caterpillar traction is suitable for handling

ﬂat products but is generally not as versatile as the reciprocating system. The

reinforcement may be impregnated by passing it through a bath of resin, as with

ﬁlament winding, or by injecting resin into the entry portion of the die. These

points are all illustrated in Figs. 35–38.

Solid and hollow sections may be manufactured and virtually any external

proﬁle that can be contained in a window approximately 2

⫻ 1 m. Pultrusion

speeds are quite low, seldom more than 2–3 m/min. The speed is dictated by

the length of the die and the necessary cure time, which is temperature depen-

dent. In practice die temperature and pulling rate are adjusted so that the section

is sufﬁciently cured when it emerges from the die. The process may be speeded

up by preheating the charge as it enters the die or by partly curing in the die

and then postcuring. There are shape restrictions, which are discussed below.

There are also several options in choice of reinforcement and some possibilities

for making curved sections and accommodating section changes along the length

of the product. There is considerable interest in pultruding composites with ther-

moplastic matrices. In this case the reinforcement must either be preimpregnated

or consist of one of the commingled forms, the resin in the form of ﬁbers or

strips of tape, interspersed with the reinforcement ﬁbers. The charge must be

heated to melt the matrix before or during entry into the ﬁrst section of the die,

which is hot. The charge then passes through a hot consolidation stage and

ﬁnally through a cooled exit stage so that the matrix has solidiﬁed by the exit

point. This is a developing technology and is not yet standard practice.

10.2 Reinforcement Options for Pultrusion

The basic requirement is that the reinforcement is continuous or at least in

sufﬁciently long lengths that batches may be joined to maintain the continuous

operation of the pultruder. The most obvious feedstocks are continuous rovings.

The logistics of the process warrant some consideration. Individual packages

(cheeses) of roving are generally supplied in weights of up to

⬇25 kg. For a

typical roving density of 2500 tex, this translates to a continuous length of 10

km, sufﬁcient to keep the process running for about 100 h. One E-glass roving

of 2500 tex corresponds to approximately 1 mm

cross-sectional area when

molded. Thus, to pultrude a hollow section 100 mm

and of 3 mm wall thickness,

i.e., cross-sectional area of approximately 1200 mm

, 1200 individual roving

packages, or creels, would need to be set up to feed into the die. Setting up the

machine is therefore a signiﬁcant logistical exercise. But, having set up the

process, 10 km of the section could be manufactured in a single campaign, with

very little supervision being required beyond maintaining the ﬂow of resin and

disposing of the product. One problem with using just roving as feedstock is

that the product is composed entirely of uniaxial ﬁbers. This may be acceptable

for a limited range of products but some cross reinforcement is usually required.

Alternative forms of feedstock are tapes of any of the standard sheet forms

of reinforcements. Chopped strand and continuous random mats are widely used

together with woven and braided fabrics. These are invariably prepared as rolls

of tape, trimmed to widths to suit the part being manufactured. These may be

folded as they enter the die to form angle, channel, Tee and H sections, ribbed

tubes, and a wide range of special sections, as illustrated in Fig. 37. Typical

cross sections for pultruded products are shown in Fig. 38. Pultrusions may be

10 PULTRUSION 1093

Three strips of

fabric tape form

outer layers of

Tee section

Core formed

from uniaxial

tows

Fig. 37 Pultruded sections may be assembled from a variety of tow and tape reinforcements.

This is illustrated for a Tee section formed from uniaxial tows in the core and three fabric tapes.

These could be of either woven fabric or random mat.

Rods, strips & tubes

Angles, tees, channels

Complex hollow sections

Fig. 38 Selection of section shapes suitable for pultrusion. The hollow sections are formed

using a blind mandrel. There is no requirement for draft (taper) on the ﬂanges of Tees and

angle sections and corners can be quite sharp.

manufactured in a similar range shapes as extrusions in aluminum alloys, and

they are designed, in as far as is possible, so that they merely need to be cut to

length to form the ﬁnal component. The main principle for design of a section

is to avoid having a wide range of thicknesses in a section, as this leads to

difﬁculties in achieving uniform cure. Wall thicknesses of less than 1 mm are

generally avoided, and simple proﬁles on the inside of hollow sections are pre-

ferred.

10.3 Process Variations

As noted in Section 10.1, for thermoset matrices the speed of the process is

dictated by the need to adequately cure the resin by the time the product exits

the die. Higher speeds may be achieved by operating the die at higher temper-

1094 COMPOSITES FABRICATION PROCESSES

Resin injection

pultrusion die

Resin injection

Tubular section

pultrusion

Pair of braiding wheels

winding RH & LH angled

layers

Unidirectional

tows

Blind mandrel to form hollow core of tube

Fig. 39 Further possibilities are to ﬁlament wind, using braiding wheels, onto core sections

formed from uniaxial or fabric reinforcements. The linear nature of the process allows for any

number of operations before the section enters the die.

atures or by use of longer dies, so that the residence time is adequate to ensure

cure. In practice these options are of limited use as the cure proﬁle of most

resins becomes unmanageable, due to excessive exotherm, if a temperature

higher than the normal range cure temperatures is used. Also in longer dies the

friction is greater and the pull traction needs to be higher. Heat transfer from

the hot die to the charge is inefﬁcient due to the low thermal conductivity of

the charge. So, it is advantageous to preheat the charge, i.e., resin and reinforce-

ment, at the point of entry into the die. This can be effected by microwave or

dielectric heating according to the characteristics of the resin and reinforcement.

Vinyl-ester and UPE resins can be heated by both methods, glass reinforcement

is inert and does not interfere, but carbon is an electrical conductor and may

interfere with these processes. Induction heating has been used with carbon but

is not established practice because the vast majority of commercial pultrusions

are made with glass reinforcement. Preheating must be done before the charge

enters the metal die.

Most pultrusions consist of simple prismatic sections produced from passage

through a single die. However, the linear nature of the pultrusion process makes

it adaptable to multistage processing. For example, a tubular section could be

formed from uniaxial glass rovings, then overwrapped using a pair of winding

wheels to produce a

Ⳳ

␪

outer layer, before passing through the curing die (Fig.

39). It is also possible to produce curved pultruded sections by using a recip-

rocating die in the form of an arc of a circle. A related process, termed Pul-

forming, allows for the manufacture of parts with nonuniform cross section, by

replacing the die by a series of clamshell molds carried by a pair of caterpillar

conveyers.

10.4 Summary

Pultrusion is a versatile process and, although limited to the production of fairly

simple cross-section prismatic shapes, it can be highly cost effective. It is most

suited to the production of long runs of standardized sections. The capital costs

12 SHEET AND BULK MOLDING COMPOUNDS 1095

of equipment and tooling are relatively modest and the equipment, once set up,

will run with minimal supervision. A number of construction systems have been

developed that are based on the use of pultruded sections, often assembled by

interlocking. These are widely used for the construction of corrosion-resistant

industrial walkways, and one system has been developed for construction of

bridges and associated structures.

11 CONTINUOUS LAMINATION

This is a specialized operation for the production of ﬂat and proﬁled sheet ma-

terials, mainly transparent or translucent panels for the construction industry.

Rolls of reinforcement, usually CSM or CRM, are fed through a series of rolls

where liquid resin is incorporated. The impregnated material then passes through

a curing section, which is typically a heated double-belt press. The material is

cured during passage through the press, and the continuous section is cut to the

required lengths by a ﬂying saw or shears. If transparent sheeting is required,

careful control of the choice of resin and reinforcement, especially the choice

of binder on the reinforcement, is necessary, so that refractive indices of the

glass and cured resin match and the porosity content is low. There is a danger

that weathering in service results in disbonding of the resin/glass interfaces,

which then reveal the ﬁber bundles and lower the transparency.

12 SHEET AND BULK MOLDING COMPOUNDS

12.1 General Principles

Sheet molding compound (SMC) and bulk molding compound (BMC) [also

dough molding compound (DMC) in Europe] are both precompounded rein-

forcements intended for fast processing by compression, transfer, and injection

molding. As its name implies, SMC is supplied in sheet form; usually in the

form of rolls, typically 1–2 m wide and about 5 mm thick. Bulk molding com-

pound is similar in constitution but supplied in bulk form, usually as a thick

‘‘rope,’’ 20–50 mm diameter. The compounds consist of reinforcing ﬁbers, al-

most always E-glass, and usually random discontinuous, ﬁllers, and the resin,

which is usually either UPE or vinyl-ester. The resins are specially formulated

with a thickening agent so that they thicken to a stiff pasty consistency over a

period of maturation of about 72 h after the initial compounding operation. This

renders the compounds dry, nonsticky, and easy to handle. The SMC has a

leathery consistency and may be cut with shears or blanked into shapes.

The compounds are cured by the application of heat and pressure. Initially

the heat softens the resin so that the compound may be consolidated and molded

into the die cavity by the applied pressure. The continued heating cures the

resin. The whole process cycle may be completed in 2–20 min according to the

section thickness and precise formulation.

12.2 Sheet Molding Compounds

Standard SMC consists of chopped strand E-glass blended with resin and ﬁllers

supplied in continuous rolls. It is manufactured on a continuous belt. A lower

cover ﬁlm is coated with the liquid resin/ﬁller blend, glass rovings are then

chopped to lengths of 20–75 mm, and allowed to settle on the moving belt; a