Kutz M. Handbook of materials selection

Подождите немного. Документ загружается.

1046 COMPOSITES FABRICATION PROCESSES

Table 2 Implications of Choice of Reinforcement Format for E-glass / Epoxy Resin

Reinforcement Format

max C

␩

max

(GPa)

max

(GPa) Applicable Processes

Uniaxial tow 0⬚ 0.80 1.00 0.80 56 5.0 Tow placement, ﬁlament

wind, pultrusion

UD prepreg 0

⬚ 0.65 1.00 0.65 47 5.0 Autoclave, RFI

Biaxial prepreg 0

⬚,90⬚ 0.60 0.50 0.30 22 5.0 Autoclave, RFI

Quasi-isotropic prepeg 0

⬚, Ⳳ45⬚,90⬚ 0.60 0.33 0.18 13 8.0 Autoclave, RFI

Balanced woven 0

⬚,90⬚ 0.50 0.50 0.25 18 4.1 RFI, RTM, contact mold

Isotropic woven 0

⬚,90⬚; Ⳳ45⬚ 0.50 0.33 0.16 12 7.0 RFI, RTM, contact mold

Noncrimp fabric 0

⬚,90⬚; Ⳳ45⬚ 0.55 0.33 0.18 13 7.5 RFI, RTM, contact mold

Orthogonal 3D x, y, z 0.40 0.33 0.13 9.4 4.0 RTM

CSM Random 2D 0.30 0.33 0.10 7.2 3.5 RFI, RTM, contact mold

CRM Random 2D 0.20 0.33 0.07 5.0 3.0 RFI, RTM, contact mold

Short ﬁber Random 3D 0.15 0.12 0.02 3.4 3.0 Injection mold

Unﬁlled epoxy resin 0.3 2.5

feedstock and may also involve selection of the basic material system, e.g.,

E-glass/polyester or carbon/epoxy.

2.2 Common Features of Composite Processing Routes

Inﬁltration

The reinforcing ﬁbers are initially produced as spools of continuous roving. This

may be treated with a size and/or binder but is otherwise dry. During the proc-

essing operations, it must be fully inﬁltrated with the matrix resin. This may be

accomplished either by some precompounding operation or during the ﬁnal

molding process. Inﬁltration of a porous medium by a liquid has been exten-

sively studied and a simple relationship proposed by Darcy is widely used:

⌬p

q ⫽ (1)

␩

⌬L

The rate of inﬁltration, q ,is proportional to the permeability, S, and the driving

pressure gradient,

⌬p/⌬L, and inversely proportional to the viscosity of the me-

dium,

␩

. Thus, increasing the applied pressure or the permeability will assist

inﬁltration while increasing viscosity will hinder. Viscosity may usually be de-

creased by raising the temperature, but this leads to faster cure in thermosets

and ultimately to degradation in thermoplastics. It is necessary to establish a

processing window where the temperature and pressure are controlled to allow

complete inﬁltration to be effected before gelation or degradation of the matrix

resin. The length L over which inﬁltration must occur is basically determined

by the geometry of the part. However, the effective inﬁltration path can be altered

by multiple gating and also in rather more subtle ways by design of the rein-

forcement.

2 BASIC PRINCIPLES FOR PROCESSING 1047

PARALLEL FLOW:

Fluid flows smoothly between

the fibres. Rate proportional

to square of diameter.

TRANSVERSE FLOW:

Path is tortuous - Rate

only

≈

1/20 of that for

parallel flow.

(a)

0.001

0.01

0.1

100

0.2 0.3 0.4 0.5 0.6 0.7 0.8

FIBER VOLUME FRACTION

RELATIVE PERMEABILITY

AXIAL FLOW

TRANSVERSE FLOW

(b)

Fig. 5 (a) This depicts ﬂow parallel and normal to a uniaxial array of ﬁbers. The tortuous ﬂow

in the normal direction results in a ﬂowrate only ⬇ of that in the parallel direction under simi-

––

lar conditions. (b) Relative permeabilities in the axial (parallel) and transverse (normal) directions

of uniaxial ﬁber arrays is plotted against ﬁber fraction, V

. Note that the transverse ﬂowrate is

only of that in the axial direction. There is a reduction of permeability by a factor of approxi-

––

mately 100 for an increase in V

from 0.3 to 0.7.

The permeability, S, is determined by the reinforcement architecture and may

be modeled using the Carman Kozeny relationship:

r (1 ⫺ V )

ƒƒ

S ⫽ (2)

4kV

Here the permeability of a uniform array (of ﬁbers) is given by the square of

the ﬁber radius, a function of V

, and the Kozeny constant, k. It can be seen that

the permeability will be lower when the ﬁbers are of smaller radius. The effect

of V

is very severe, and the permeability decreases by a factor of about 6 for

a V

increase from 0.6 to 0.7. The Kozeny constant is a measure of tortuosity:

In a uniform uniaxial array of cylindrical ﬁbers (Fig 5a), k for ﬂow parallel to

the ﬁbers is some 20 times lower than that for ﬂow normal to the ﬁbers. This

1048 COMPOSITES FABRICATION PROCESSES

effect and that of V

are further illustrated in Fig. 5b, which shows that per-

meability is reduced by a factor of 10 for an increase in V

from 0.4 to 0.6.

These two relationships allow some simple generalizations to be stated:

(i) Permeability will be higher for a coarser ﬁber array.

(ii) Permeability is much higher in the ﬁber direction than at right angles.

(iii) Permeability decreases sharply as V

is increased.

Unfortunately small-diameter ﬁbers at high V

tend to give composites with

better mechanical properties, so it is necessary to balance the aims of achieving

high mechanical properties against the requirements of ease of processing. For-

tunately there are a number of ways in which the effective permeability may be

increased without undue performance penalty. These are discussed in a later

section.

Consolidation

This is the process whereby the thickness of the section is reduced to the design

value and porosity and other defects eliminated. When dry or impregnated plies

(or tows) are laid up to form a laminate, the initial lay-up (pack) will be signif-

icantly thicker than ultimately required. This is due to the difﬁculty in obtaining

perfect contact, to entrapment of air, and sometimes due to an excess of resin

that is often deliberately incorporated. The application of pressure and/or heating

to the pack will cause it to consolidate. Ideally, any entrapped air or other

volatiles will also be eliminated together with any excess resin. Provision for

removal of resin and volatiles may be made by venting the tooling, by the use

of absorbent bleeder fabrics, and/or by the use of vacuum. The consolidation

process requires that resin ﬂows, usually in the thickness direction of the pack.

The process may be partially affected by a preconsolidation operation (debulk-

ing) prior to the main processing operation or as part of the main process.

The principles of consolidation of a simple laminate, with a bleed ply, are

illustrated in Fig 6. Once again the process requires a sufﬁcient time for the ﬂow

processes to occur and this will be affected by the applied pressure and the

temperature. Where preconsolidation is used, care must be taken not to advance

the cure of thermoset systems too far so as to compromise satisfactory ﬁnal

consolidation and cure.

Effects of Applied Pressure

Examination of the Darcy relationship (Eq. 1) indicates that the rate of inﬁltra-

tion may be increased by increasing the pressure gradient forcing inﬁltration.

While this is true, there are some limitations and implications that must be

discussed. First, much composite molding is carried out using simple tooling,

usually also composite, and a vacuum bag. This limits the pressure that may be

applied to less than one atmosphere (

⬇1 bar). Higher pressures may be applied

if an autoclave is employed, typically up to 5 bars. The limitation here is the

pressure capacity of the autoclave vessel, which may be 3 m in diameter. Higher

pressures may be exerted by press, but it should be noted that to apply a pressure

of only 10 bars, which is approximately 1 MPa, a closing force of 100 ton

䡠fis

2 BASIC PRINCIPLES FOR PROCESSING 1049

Absorbent bleeder pack

Resin permeable membrane

Resin flow

Mold surface

Consolidated plies

Excess resin

absorbed into bleeder

Prepreg plies

containing excess resin

Fig. 6 Process of consolidation involves ﬂow in the thickness direction of the laminate.

If a bleeder ply is used, excess resin ﬂows through a resin-permeable release ply and

is absorbed in the porous bleeder.

required for each 1 m

projected area of the molding. One implication of using

higher pressures is that the tooling needs to be made more robust. This means

use of steel tools, at a cost of at least ﬁve times that of the simple composite

tooling. A further problem is that the application of high compressing pressures

to the reinforcement before it is inﬁltrated, will tend to consolidate it, increase

the V

, and decrease the permeability, so that there is no gain in inﬁltration rate.

Cure of Thermosetting Resins

Thermosetting resins are cured by a chemical process that is enhanced by raising

the temperature. The detailed chemistry of this process will not be discussed,

but the basic behavior is common to all systems. The resins are usually supplied

in liquid form, typically with the consistency of a light lubricating oil. There

are usually at least two components, which must be mixed in precise proportions

prior to processing. One component is normally the resin and the other may be

designated curing agents, initiators, accelerators, or catalysts. (The term catalyst

is, however, often used loosely in the industry when the ingredients are not

technically catalysts!) Once mixed they are fully formulated and the chemical

process leading to cure is initiated. Resins may be formulated to cure at ambient

temperature (e.g., 15–25

⬚C) or at elevated temperatures (typically 50–250⬚C)

and over time intervals from a few seconds to several hours, even days. Some

systems, notably the unsaturated polyester resins, are more versatile in this con-

text than others.

The curing reaction results in the development of a dense network of chemical

bonds between the original polymer molecules. On the completion of cure the

resin is solid, cannot be melted, and cannot be dissolved in solvents without

degradation. Most of the thermosets used in composites cure to hard rigid solids,

but it is possible to formulate systems to yield softer or even elastomeric prod-

ucts. The cure process occurs in several stages. First there is often an incubation

period when there is little observable change. Then the viscosity is observed to

increase, and heat arising from the reaction(s) will be evolved. This usually

1050 COMPOSITES FABRICATION PROCESSES

results in a rise in temperature, referred to as the cure exotherm. A consequence

of this temperature rise is that the viscosity of the resin will be decreased, which

assists the processes of inﬁltration and consolidation. The next stage is gelation:

At this point the resin ceases to be liquid and transforms to a solid, initially it

is rubbery but then becomes progressively harder. The evolution of heat is gen-

erally most intense during the period around gelation. As the cure proceeds the

exotherm becomes weaker and the resin becomes fully hardened or vitriﬁed.

Raising the initial cure temperature by heating will accelerate the cure process.

Gelation will occur sooner and the exotherm will be more intense. Care must

be exercised to ensure that the exothermic heat does not lead to the resin/

laminate becoming excessively hot so that degradation occurs. This is often

observed as burnt regions in the thicker sections of a molding where heat transfer

out of the laminate is less favorable. In extreme cases the molding can com-

pletely disintegrate, or even explode, as a result of excess exotherm! It is for-

tunate that the reinforcement, which does not participate in the reactions, acts

to some degree as an internal heat sink.

Three temperature, viscosity and cure proﬁles for a laminate cured at different

temperatures are shown in Figs. 7a,7b, and 7 c. Note how the time to gelation

is reduced by raising the cure temperature. For reasons of productivity it is

generally desirable to cure as fast as possible, provided excessive exotherm can

be controlled. However, it is also necessary that the resin is at low viscosity for

a sufﬁcient time interval to allow any inﬁltration or consolidation processes to

be completed. This may be achieved by balancing the cure temperature proﬁle

and the consolidation pressure.

Processing Thermoplastic Matrix Systems

Thermoplastics, with the exception of the monomer polymerization systems, are

not cured but are processed through melt–freeze cycles. Thermoplastics are ei-

ther amorphous or semicrystalline, with degrees of crystallinity of up to about

75%. On heating to above the melting temperature, T

, amorphous polymers

soften over a range of temperatures while the semicrystalline materials melt more

sharply and generally to melts of lower viscosity. Overheating leads to degra-

dation, discoloration, and loss of properties. On cooling back through the freez-

ing temperature, the amorphous materials become gradually more rigid, but the

semicrystalline polymers freeze more sharply and recrystallize. The extent of

crystallinity is dependent on the rate of cooling through the critical region. Fast

cooling leads to a ﬁne substructure but lower overall crystallinity, while slow

cooling has the converse effect. In most cases optimum mechanical performance

of the composite requires that the crystallite size and degree of crystallinity be

controlled. It is therefore necessary to exercise control over cooling rates when

processing thermoplastic systems. A further property of amorphous thermoplas-

tics is the glass transition temperature, T

. This transition occurs below the

melting point and at T

⬍ T

the polymer behaves as a glass. It is hard, often

brittle, and does not creep signiﬁcantly under load. At T

⬎ T

the polymer is

rubbery and much softer, extendible, and creeps under load. The temperature T

effectively deﬁnes the upper service temperature for amorphous thermoplastics.

In the case of the semicrystalline thermoplastics only the amorphous portion is

affected by T

, so the effect is less severe. Semicrystalline polymers therefore

have higher useful service temperatures and in the temperature interval between

2 BASIC PRINCIPLES FOR PROCESSING 1051

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200 250 300 350 400

TIME - min

DEGREE OF CURE

380K = 107°C

400K = 127°C

420K = 147°C

GEL POINT

(a)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200 250 300

TIME - min

DEGREE OF CURE

0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0.008

0.009

0.010

RATE OF EXOTHERM

MAX RATE OF HEAT

EVOLUTION

400K = 127°C

ARBITRARY UNITS

(b)

Fig. 7 These ﬁgures illustrate the cure characteristics of a typical thermosetting resin, e.g., an

epoxy. (a) Shows the cure proﬁle at three different curing temperatures. The degree of cure is

plotted against time of cure. The cure rate is much faster at the higher temperatures. A temper-

ature increase of only 20⬚C, from 107⬚C to 127⬚C, reduces the gel time from 250 min to just

over 100 min. (b) Cure proﬁle at 127⬚C is shown together with the rate of exotherm. This peaks

soon after gelation. The exotherm rate is higher at higher cure temperatures (not shown). (c)

Resin viscosity proﬁles for the same resin at the three temperatures is shown. The viscosity in-

creases sharply after the gel point and this deﬁnes the workable range, or process window.

and T

are often quite tough. The high melt viscosity of thermoplastics means

that processes dependent on inﬁltration cannot be used. Most systems are, there-

fore, precompounded or co-mingled.

Heat Transfer

It will have become apparent from the preceding discussion that most processing

operations involve heat ﬂow, to manage heating, cooling, or to control exotherm

or crystallinity. Polymer resins are generally poor thermal conductors and are

1052 COMPOSITES FABRICATION PROCESSES

100

150

200

250

300

350

400

450

500

0 50 100 150 200 250 300

TIME - min

VISCOSITY - Pa s

420K = 147°C

400K = 127°C

380K = 107

RESIN WORKABLE UP TO THIS

VISCOSITY

PROCESS

WINDOW @ 400K

(c)

Fig. 7 (Continued )

sensitive to overheating. Unimpregnated ﬁber arrays are also very poor conduc-

tors, although some ﬁbers themselves are quite good. The materials used for

tooling may also be poor conductors. This applies particularly to composite

tooling, which is very widely used, also to plaster and concrete. The problem is

often increased by the use of vacuum bags, porous breathers, and bleed plies

over the laminate on the tool. Metal tooling is the exception, but this is only

generally used for the higher production rate processes. It follows that consid-

erable care is needed to manage the heat input and output during processing and

that the time required for heat transfer is a signiﬁcant and sometimes critical

part of most processing operations. Problems may be minimized by avoidance

of thick sections and by the incorporation of internal heating or cooling into

mold tools.

Flow Processes during Molding

In all processes some ﬂow of resin will occur, but the ﬂow or movement of the

reinforcement warrants further consideration. At one extreme there are processes

that involve vertical consolidation only with no relative movement of the rein-

forcement in other directions. The other extreme is when molding with a com-

pound, such as BMC, where the whole charge, resin plus ﬁber, ﬂows in all

directions to ﬁll the mold. In reinforcements incorporating straight continuous

ﬁbers, no movement can occur in the ﬁber direction. Any signiﬁcant movement

normal to the ﬁbers will result in splitting if there is extension or wrinkling if

there is compression. On the other hand limited shear deformation is possible.

If there is biaxial continuous reinforcement, as in a woven cloth, no stretch can

be accommodated in either direction, compression will result in wrinkling but

again some shear movement is possible.

Reinforcements containing discontinuous ﬁbers, however, are free to ﬂow to

a much greater extent. Sheet molding compound, with chopped roving reinforce-

ment, will ﬂow in three dimensions and allow features such as ribs and bosses

2 BASIC PRINCIPLES FOR PROCESSING 1053

to be molded without deliberate placement of reinforcement at these features.

Continuous random mat will also ﬂow but to a lesser degree. Clearly it is im-

portant to select reinforcement appropriate to the degree of ﬂow necessary to

form the part. See the comment in the following section concerning thermo-

plastic matrix systems.

Drape

This is related to ﬂow but is concerned with the extent to which a reinforcement

will comply to a complex molding surface. A uniaxial prepreg can only be

successfully draped on single curvature surfaces. Double curvature requires ex-

tension or contraction in the lateral direction leading to splits or wrinkles. Double

curvature panels can, however, be formed by use of narrow tapes with small

gaps or overlaps accommodating the misﬁt. Biaxial continuous ﬁber reinforce-

ment, in principle, suffers from a similar limitation but can, in fact, accommodate

some draping by tow slippage and shear. Satin and twill weaves have signiﬁ-

cantly better drape characteristics than plain weaves and lighter fabrics drape

more efﬁciently than heavy. Random mats drape very well, but this is affected

by the type of binder used. A strong binder will prevent effective drape. If the

binder is resin soluble, draping can be effected after the resin has been applied.

With CSM there is a possibility of tearing if the binder is too weak. This leads

to depletion of reinforcement in the torn areas. Continuous strand mat is resistant

to tearing, it drapes well but cannot ﬂow as much as CSM. Both CSM and CRM

are used in precompounded thermoplastic sheet systems. They cannot ﬂow in

any way until the matrix has been melted, but then behavior is similar to that

in thermosets.

2.3 Design of Reinforcements to Enhance Permeability

Following from the discussion of inﬁltration in Section 2.2, it would appear that

the objectives of obtaining high mechanical properties by maximizing V

and

beneﬁting from high permeability are mutually exclusive. Tightly packed arrays

of small-diameter ﬁbers are highly impermeable, especially in the directions

normal to the ﬁber axes. The effect of V

on permeability is shown in Fig. 5b

for axial and transverse inﬁltration. The time to inﬁltrate a 250-mm-long uniform

array of 10-

␮

m-diameter ﬁbers, at a V

of 0.5, under a pressure of 2 bars with

a resin of 0.1 Pa/s is calculated to be in excess of 200 h, i.e., practically im-

possible! However if the ﬁbers are arranged as tightly packed tows, of 1 mm

diameter and packed with a V

of 0.7 within the tows, and the tows then packed

with a packing factor of 0.7, to give an overall V

of 0.49 (⬇0.5), as shown in

Fig. 8, under similar conditions the inﬁltration time of the array is about 5 s.

Furthermore the transverse inﬁltration of the tows takes only a further 15 s

because the distance to inﬁltrate is only about 0.5 mm (half the tow diameter),

so that complete inﬁltration takes about 20 s. This is a much more realistic

prospect. The key to high permeability with high volume fraction is to provide

a network of relatively large passages through the reinforcement, between tows,

or simply by leaving some gaps between tows. The resin can then quickly in-

ﬁltrate through these large passages by axial macroinﬁltration, and then inﬁltrate

the tows by transverse microinﬁltration. These inﬁltration passages may be in-

corporated into the reinforcement during the weaving operation.

1054 COMPOSITES FABRICATION PROCESSES

MACRO FLOW:

Longitudinal flow between

fibre bundles.

MICRO FLOW:

Transverse flow into

fibre bundles.

Much slower but distance

only bundle diameter.

Fig. 8 Illustrates the principles of macro- and microﬂow in an array of ﬁber bundles. This is the

normal structure of ﬁber composites. Flow is fast through the relatively large channels between

the bundles. Final inﬁltration is accomplished by microﬂow in the transverse direction into the

tightly packed ﬁber bundle. This is much slower but the inﬁltration distance is very short.

Another method of speeding up inﬁltration is to incorporate layers of open

reinforcement, such as CSM, between layers of heavier material such as woven

roving or noncrimp fabric. The distributor ply allows the whole area of the

molding to be inﬁltrated so that the heavy reinforcement only has to be inﬁltrated

through its thickness direction. Again the key is to minimize the distance through

which tightly packed ﬁbers need to be inﬁltrated. This principle is further ex-

ploited in the SCRIMP and RFI processes discussed in later sections.

2.4 Tooling

Most composite laminates are of shell form, often with additional detail such as

stiffening ribs, bosses, and cutouts. These forms may be molded using single-

sided tools of either male or female form. The choice is usually governed by

the geometry of the piece, the logistics of the laminating process, and which

side constitutes the prime surface where dimensions and surface ﬁnish need to

be controlled. Ribs or bosses may be incorporated either by cutting these features

into the tool face or, more usually, by building up on the back surface using

additional tool pieces where necessary. Simple one-sided tools may be fabricated

using composites by taking an impression off a master form, usually termed the

plug. The basic impression is a thin shell that is then backed up with stiffeners

and supporting structure to render it sufﬁciently robust to withstand the chosen

molding process. The mold materials may be E-glass with unsaturated polyester,

vinyl ester or epoxy resin matrices, or carbon ﬁber with epoxy backed up with

timber or metallic supporting structures. The advantage of carbon ﬁber is its

2 BASIC PRINCIPLES FOR PROCESSING 1055

1. Master form - “Plug”

2. CFRP shell laminated onto plug

3. Back-up structure added4. Completed female half-tool

Prime surface

Fig. 9 Composite tool may be manufactured by laminating a CFRP shell onto a master form

or plug. This is then backed-up by a suitable structure to provide the necessary robustness for

subsequent molding operations.

greater stiffness, better thermal conductivity, and, most signiﬁcant, much lower

coefﬁcient of thermal expansion. This facilitates molding to closely speciﬁed

dimensions and is particularly relevant when the molding is also carbon ﬁber

reinforced. In contact molding, laminates are built up on the mold surface and

simply hand consolidated without application of additional pressure. Alterna-

tively the mold may be vacuum bagged so that one atmosphere of consolidating

pressure may be used. In some instances single-sided tooling may be press

consolidated using an elastomeric counter tool, but note that a more robust sup-

porting structure may be necessary. Single-sided tooling is widely used, with a

vacuum bag for autoclave cure. Some of these options are illustrated in Figs. 9–

11. Double-sided or matched tooling may also be fabricated using composites.

This is again usually used in a press, and therefore size and consolidating pres-

sure will be determined by the equipment available.

Composite tooling is suitable for short to medium production runs. Typically

a tool may be used to mold up to 100 parts before needing extensive reworking.

This ﬁgure can, however, vary very widely depending on the complexity of the

molding and the consolidating and cure conditions. Where long runs are re-

quired, the options are multiple tools or metallic tooling. Metallic tools are

inevitably much more costly than composite, but they are more robust and more

durable. A serious problem is the mismatch in thermal expansion between the

composite part and the tool, which causes difﬁculty in establishing dimensional

control. The options for metal tooling are cast-zinc alloy, cast or fabricated

aluminum alloy, cast iron, fabricated steel, and Invar. The choice depends on the

part size and complexity, surface ﬁnish requirements, and thermal expansion

considerations. Invar has the great advantage of virtually zero thermal expansion

over typical molding temperature ranges (20–150

⬚C). It is an alloy of nickel and

iron and may be fabricated using standard steel practices, the cost is much higher

than that of steel. Metal tooling is generally necessary for all high-pressure