Kutz M. Handbook of materials selection

Подождите немного. Документ загружается.

1056 COMPOSITES FABRICATION PROCESSES

Female mold Part laminated into female mold

Part laminated onto male mold

Principle of two-part closed mold

Optional vacuum bag

Fig. 10 There are several options for contact molding composite laminates. Single-sided tool-

ing of either male or female form may be used, in conjunction with vacuum bag consolidation,

if required. Enhanced shape deﬁnition and surface ﬁnish may be obtained by using

two-part tooling, normally set in a press.

Soft elastomeric counter tool

1. Laminate on male tool

2. Press closed - compressed

elastomer consolidates laminate

during cure

Fig. 11 Pressure consolidation may be effected by use of single-sided tooling with an elasto-

meric counter tool setup in a press. For parts of moderate curvature and draw a simple block

of elastomer may sufﬁce. More deeply formed shapes will require the counter tool

to be shaped. The elastomer tool is normally a much less costly option than

conventional matched tooling.

(⬎10 bars) and high-temperature (⬎200⬚C) molding processes including SMC

and BMC.

It is difﬁcult to provide any reliable estimate of tooling cost without detailed

information of the size, complexity, and molding conditions. However, the cost

of a simple one-sided composite tool is in the region of $5000–$10,000 for a

tool of 1-m

plan area. Matched tools would cost 2 –3 times as much, and

–

metallic tooling 5–10 times. These are substantial costs and contribute signiﬁ-

cantly to the cost of the product.

2.5 Rate of Build

The rate at which a part can be laid up is a vital aspect of productivity for all

laminated composite structures. This is determined by the method of lay-up and

2 BASIC PRINCIPLES FOR PROCESSING 1057

the areal density of the reinforcements. Most construction is still hand laid and

therefore the choice of reinforcement is the most important factor. The labor

costs of laminating tend to be one of the critical cost factors, and this is strongly

inﬂuenced by the number of plies that need to be laid. To maximize the build

rate and to minimize lay-up costs, a heavy reinforcement is preferable, but in-

ﬁltration, consolidation, and drape may be compromised if the reinforcement is

too heavy. In the case of thinner laminates the speciﬁed conﬁguration may re-

quire several plies to be laid at different orientations. Thus, if a quasi-isotropic

laminate is speciﬁed, there must be a minimum of 8 plies of uniaxial reinforce-

ment (e.g., prepreg) in the conﬁguration [0

⬚,90⬚, Ⳳ45⬚]

or four plies of balanced

woven fabric in the sequence [0

⬚/90⬚, Ⳳ45⬚, Ⳳ45⬚,0⬚/90⬚]. This leads to a

minimum thickness of 1 mm if standard uniaxial or woven prepreg is used. A

heavier reinforcement such as a woven glass fabric of 1250 g/m

would give a

molded thickness of 1 mm for one layer, but the only conﬁgurations would be

⬚/90⬚ or Ⳳ45⬚. Alternatively two plies of a four-layer [0⬚,90⬚, Ⳳ45⬚] noncrimp

fabric of 625 g/m

would produce a 1-mm quasi-isotropic laminate. If drape

were a problem, then a lighter-weight satin weave fabric, e.g., eight plies of 160

g/m

, might be the better choice. For thicker laminates, where drape require-

ments are less severe, heavier reinforcements may be considered.

In the aerospace industry it is common to design complex laminates incor-

porating several thickness changes, cutouts, and other features. These must all

be implemented while maintaining the basic conﬁguration, balance, and sym-

metry of the laminate. This leads to the choice of thin uniaxial or woven prepreg,

even for laminates that are very thick in their thickest regions. Laminates of

over 20-mm thickness containing 160 plies are typically speciﬁed. This imposes

a very considerable cost penalty in comparison with a simpler laminate made

up from heavier reinforcements. The enhanced performance must be balanced

against this cost penalty. There is currently considerable effort within the aero-

space industry to develop manufacturing technologies that reduce processing

costs, especially labor, while maintaining acceptable levels of performance.

An alternative approach is to use automated lay-up. This generally implies

use of computer-controlled tow or tape laying equipment. For laying tape the

machinery consists of a moving gantry with a tape laying head with 4–6 axis

positional control. This is controlled by software linked to a computer-aided

design and manufacturing (CAD/CAM) package to lay down a series of strips

of tape to comply with the speciﬁed lay-up and part geometry. This equipment

is suitable for making ﬂat or shallow curvature panels, which may be sometimes

subsequently further shaped. The equipment is costly but once set up, completely

automatic. Quality and reproducibility are very good, but production rate is not

always much higher than for hand lamination. This is due to the use of quite

narrow tape, 50–600 mm, and a laying speed of only 1–5 m/s. The economics

are generally more advantageous for the manufacture of large panels, e.g.,

⬎2

m length, where manual positioning of large sheets of reinforcement is difﬁcult.

2.6 Economic Implications of Choice of Feedstock

A wide range of feedstock options are available; these range from the raw ﬁber

tows in spool form, through the range of woven and nonwoven sheet materials,

to the precompounded materials such as prepreg and SMC. There is an added

cost associated with every operation performed on the ﬁber to convert it into

1058 COMPOSITES FABRICATION PROCESSES

Table 3 Typical Costs of Basic Fiber Rovings (2000)

Fibre Type

Typical

Weight

(tex)

Price

Range

(US$/ kg)

Approximate Annual

World Production

(ton)

E-glass roving 1,200 2– 5 4 million

Aramid roving (yarn)

(e.g., Kevlar 49)

300 10–20 10,000

High-strength carbon

(e.g., T-300)

900 5–20 18,000

Intermediate modulus carbon

(e.g., T-800)

350 100–200 ⬍2,000 (estimated)

Table 4 Typical Costs of Converted Fiber Reinforcements—US$ / kg (2000)

Reinforcement

Format E-glass

Aramid

(Kevlar 49)

Carbon

(T-300)

Carbon

(T-800)

Fiber roving 3.75 15 15 120

Chopped strand mat 5.0 N /A

N/A N/A

Woven fabric 24 43 44 108

Noncrimp fabric 38 63 65 147

Uniaxial prepreg

65 95 135 220

Sheet molding compound

7.0 N/A N/A N/A

These formats include the resin.

N/ A, not usually available in this format.

the feedstock of choice, and this must be offset against possible savings in the

downstream processing operations. If dry reinforcement is used, then the matrix

resin must be separately acquired and downstream processes must involve inﬁl-

tration of the reinforcement. The alternative would be to purchase preimpreg-

nated feedstock, such as prepreg, at a cost premium and then eliminate the need

for inﬁltration downstream. Similar arguments may be used for the use of mul-

tilayer noncrimp fabrics in place of simple cloth or mat. In this case the lay-up

process would be simpliﬁed at the cost of using more expensive feedstock.

Typical costs of some of the more widely used feedstock materials are sum-

marized in Tables 3 and 4. These values should be used with caution as actual

prices vary widely according to availability, demand, the exact speciﬁcation of

the material, and, of course, the quantity to be purchased. The likely availability

is indicated by the annual production ﬁgures. Thus E-glass is a commodity

material and is readily available in all formats, but intermediate modulus carbon

must often be specially ordered. The values quoted in Table 4 are based on

actual quotations, obtained in 1998 and adjusted for inﬂation. Close analysis

reveals several anomalies, but these are due to market forces at the time of

quotation.

3 CONTACT MOLDING

3.1 Principles

Contact molding is the original process used for the manufacture of composite

laminates; it is still very widely used and there are many modiﬁcations and

3 CONTACT MOLDING 1059

derivatives of the basic process. A single-sided tool is used, and this may be of

either male or female form. The lamination surface of the mold is usually smooth

and glossy, it is treated with suitable release agents to prevent adhesion of

the laminate. These release agents are typically waxes and either silicone or

poly(tetraﬂuoroethylene) (PTFE) suspensions. In the basic process, a layer of

sheet reinforcement is laid on, or in, the mold, and then liquid resin is applied

by brush or spray. The resin is worked by hand using brush or roller so that a

uniform spread of resin and full impregnation, wetting out, of the reinforcement

is achieved. A second layer of reinforcement may then be applied and resin

worked into that layer as before, and so on, until the required number of plies

have been laid. If a smooth glossy and/or colored surface is required, it is usual

to brush or spray a gel coat consisting of a layer of a thixotropic resin with

ﬁller and/or pigment on to the prepared mold surface, and to allow it to gel

before the ﬁrst layer of reinforcement is applied. The most usual choice of resin

is an unsaturated polyester (UPE). This is of a syrupy consistency and is for-

mulated to cure at shop temperature, e.g., 15–25

⬚C. When the resin is fully

cured, the part may be removed from the mold, the mold prepared for another

molding, and the process repeated.

3.2 Key Factors and Variations

Choice of Resin

Unsaturated polyester resins are very versatile, many variants are available, and

their processing characteristics may be further modiﬁed at the point of molding

by variation of the choice and proportions of initiator and accelerator added to

the basic resin. The initial choice of resin involves its chemical formulation,

viscosity, basic cure characteristics, and ﬁre and smoke characteristics. The main

resin types are ortho-phthallic, iso-phthallic, tere-phthallic, and bisphenol-based

resins. The ortho-phthallic resins are economic general-purpose materials; iso-

phthallic and tere-phthallic are premium grades offering superior performance

especially in wet environments. Bisphenol resins are used for superior resistance

to a number of chemical environments. Detailed recommendations are available

from the suppliers. Most resins can be supplied with ﬁre-retardant additives to

improve their behavior when exposed to ﬁre hazard. Fillers and pigments may

also be incorporated in the resin to provide self-coloring and also to improve

the surface smoothness, especially if the part is to be subsequently painted. It is

generally considered that solid ﬁllers and pigments impair the resistance of the

resin to environmental hazards, so that unﬁlled and unpigmented systems are

preferred when the component is destined to be exposed to water or chemical

hazards, e.g., the underwater areas of yachts and motor boats. Viscosity is an

important factor. Low viscosity improves the rate of wetting out and inﬁltration

but can lead to resin run-off from vertical molding surfaces. A consistency sim-

ilar to that of a medium lubricating oil is a common compromise, and there are

often additions to promote thixotropy; this property allows the resin to thin when

worked, e.g., with a brush or roller, but to thicken again when the working

operation ceases. This allows good wetting out and reduces run-off. Alternative

resin systems include the vinyl-ester and epoxy resins. The former may be pro-

cessed in essentially the same way as the UPE resin and offer superior properties

1060 COMPOSITES FABRICATION PROCESSES

under some service conditions. The epoxy resins are considerably more costly

than UPE, they are superior for higher temperature operation, and while widely

used, they are not often processed by contact molding.

Resin Formulation

The resins are formulated by blending an initiator and often an accelerator with

the basic resin. The initiator (often designated catalyst in the industry although

most are not strictly chemical catalysts) is always added just before processing.

The accelerator is often added to the resin by the supplier but may also be added

at the point of processing. Accelerators and initiators must never be mixed di-

rectly owing to the risk of a violent chemical reaction or explosion. The accel-

erator is ﬁrst thoroughly mixed into the resin, and then the initiator may be

added and also thoroughly mixed. The combination of resin, accelerator, and

initiator set the cure temperature range, the cure rate, the pot or shelf life, the

gel time, and the cure time. Unsaturated polyester resins can be formulated to

cure in times varying from less than 1 minute to over 24 h. The formulation

must ensure that the resin remains workable, i.e., liquid and not too high a

viscosity, for as long as it takes to lay and wet-out at least one layer of the

reinforcement. In the case of small moldings, which might be completely laid

up in an hour or two, it is common to formulate the resin to remain workable

through the whole laminating process, but for large moldings the laminating

operation may extend over several days and the resin is allowed to gel after each

layer has been applied. However, it is imperative that not too long a period

elapse before further layers are applied, otherwise bonding may be inadequate.

Typically, a period of up to a week is permitted. At the end of the resin working

period gelation occurs quite suddenly. The resin ceases to be liquid and, although

still quite soft for a period, any attempt to work it would result in resin fracture.

Once the laminate is complete, it is generally desirable that the resin cures in

as short a time as possible. The molding may then be removed from the tool,

demolded, and the tool may then be reused. Typically if gelation takes place

after 1 h, then demolding will be possible after about 5 h.

Cure Temperature and Cure Time

Unsaturated polyester resins with no added accelerator need to be cured at el-

evated temperatures in the range of 70–160

⬚C. The exact temperature of cure in

this range is determined by the choice of initiator, several of which will be

recommended by the resin supplier. They have storage lives of upward of 1 year

in a dark cool environment. The addition of an accelerator is required for cure

at shop temperature, when satisfactory cure may be affected at temperatures

above 15

⬚C, although 20–25⬚C is usually considered more satisfactory. Increas-

ing the proportion of initiator results in a faster cure. UPE resins can be cured

in less than 1 min, even at 20

⬚C, or the cure extended to several hours. An

alternative strategy is to formulate the resin to cure at an elevated temperature,

e.g., 50–120

⬚C. This will ensure a very long working life at shop temperature.

After the lay-up is completed, the molding must be heated to its cure tempera-

ture, and cure can be much faster than would be achieved with a low-temperature

cure. Heating may be by placing the tool in an air-circulation oven, by radiant

heaters, or by blown hot air. The need to ensure adequate working life has

3 CONTACT MOLDING 1061

already been discussed; the other critical consideration is exotherm (see also

discussion of cure of thermosetting resins above). The amount of heat released

during cure is approximately the same for most UPE resins, so the rate of heat

evolution will be a function of cure rate. Faster cure will result in a higher rise

in temperature. This will also be affected by the thickness of the molding and

by heat transfer from the laminate to the mold and the environment. Fast cures

can often be tolerated in thin laminates but in thick sections would result in

unacceptable temperatures being reached. It should be noted that exothermic

heating occurs even in cold (i.e., shop temperature) cured systems. The temper-

ature of a molding will typically be observed to rise to 50–80

⬚C. The cure is

actually accelerated by this temperature rise, and thin laminates will take longer

to cure than thicker ones where the exotherm effect is more severe. In practice

the proportions of accelerator and initiator should be adjusted for each type of

molding to optimize working time, curing time, and hence productivity.

Styrene Emissions

Unsaturated polyester resins are formulated with a proportion of styrene or other

vinyl monomer that is used to control the resin viscosity and provide the basis

for the addition cure. Styrene is highly volatile, and inﬂammable, and the vapor

is known to constitute a health hazard. Hence the permitted levels of styrene in

the working environment are strictly regulated by the health authorities. Details

vary according to location, but there will always be a requirement to provide

sufﬁcient ventilation and regulate the release of styrene vapor into the atmo-

sphere. Lamination of UPE resins onto open molds obviously constitutes an

extreme hazard. Resins are available with ﬁlm-forming additives that help limit

emissions, but there is a movement to adopt alternative processes that reduce

potential exposure.

3.3 Laminate Design for Contact Molding

The contact molding process is extremely versatile. The size of moldings varies

from less than 1 kg up to more than 500 tons, and many features can be incor-

porated into the molding. It is suitable for production runs from single parts up

to several hundred but is basically a low production rate process and is dependent

on a high labor content. Quality is also strongly dependent on the expertise of

the labor employed.

The cheapest reinforcement is chopped strand E-glass mat. It is of low V

but

high permeability and has good drape characteristics. It is therefore very widely

used for production of commodity moldings. Its disadvantage is that the me-

chanical properties are only modest, and it is therefore not ideal where structural

mass is an important consideration. It is available in a range of areal densities,

generally from about 100 to 5000 g/m

and may be treated with different pro-

portions and types of binder. Resin-soluble binders allow the mat to wet-out and

drape very easily once the resin is applied, while insoluble binders stabilize the

mat through the wet-out process. The choice of weight and binder depends on

the complexity of the molding. Superior mechanical properties are obtained if

woven or noncrimp fabrics are selected or by the use of aramid or carbon re-

inforcements. These are, of course, more costly and also less permeable. This

renders them more difﬁcult to wet-out and extends the lamination time, reducing

1062 COMPOSITES FABRICATION PROCESSES

V - Vacuum bag

B - Breather ply

M - Membrane permeable to gas

Br - Bleeder ply

Mp- Membrane permeable to resin

a. Simple lay-up

c. Vacuum bag and breather to

improve consolidation

d. As (c.) but with additional bleeder

ply to control resin content

b. Alternate plies of CSM and

woven fabric - assists wet out

CSM

Fig. 12 Alternative strategies for lay-up of contact molded laminates. (a) Simple lay-up of simi-

lar plies, mat or fabric. (b) Plies of chopped strand mat (CSM) are alternated with woven fabric.

The woven fabric has a much higher ﬁber density and continuous ﬁbers and thus gives higher

stiffness and strength than mat. The mat is more permeable and assists wet out of the fabric.

layer has been incorporated to absorb excess resin. This helps control the resin content.

productivity. An excellent general compromise is to alternate layers of CSM or

CRM with woven or noncrimp fabric. The random mats help distribute the resin

and speed up the laminating process while the woven materials enhance the

mechanical properties. Some typical lamination strategies are illustrated in Figs.

12–14. Mechanical properties, especially bending and/or torsional stiffness can

also be enhanced by designed-in features such as ribs, hollow or cored stiffeners,

and cored sandwich sections. Metal or timber inserts for local stiffening or for

subsequent attachments may also be easily incorporated. Some of these possi-

bilities are illustrated in Fig. 15.

3.4 Some Variations on Basic Contact Molding Process

Vacuum Bagging

When the lamination process is complete the laminate is covered with a ﬂexible

bagging ﬁlm, usually a polyamide or elastomer. This is sealed around its pe-

riphery to the mold and the space between tool and bag is connected to a vacuum

system. This applies a consolidating pressure of up to 1 atm (

⬇1 bar) to the

laminate, assisting consolidation, improving surface ﬁnish on the back surface,

and reducing styrene emissions. It is advantageous to place a porous breather

blanket between the surface of the molding and the vacuum bag (Fig. 12c)to

ensure that the vacuum is evenly distributed over the surface and that any wrin-

kles forming in the bag material are not impressed on the surface of the part.

Prewetting Out Reinforcement

When manufacturing large moldings, it is often convenient to prewet the rein-

forcement before laying on the mold. This may be accomplished by passing a

3 CONTACT MOLDING 1063

5-Plies of Chopped Strand Mat

Chopped Strand Mat

Alternated with Woven Fabric

Fig. 13 Further illustration of the use of alternate plies of mat and fabric.

8-Plies of uniaxial fabric stacked to give a balanced,

symmetric & isotropic lay-up

[0,90,⫾45]

␴

4-Plies of biaxial woven fabric

give a similar lay-up

Fig. 14 Uniaxial reinforcement, such as prepreg, must normally be laid up in a balanced, and

symmetric sequence. This requires eight plies to give an isotropic laminate. If balanced woven

reinforcement is used a similar effect may be realized with just four plies.

roll of reinforcement, typically 1–2 m wide, through a trough of liquid resin

and then passing it through one or more pairs of rolls, which ensure uniform

impregnation and control the ﬁnal resin contact. The wetted reinforcement is

then laid into or onto the mold and cut to length as required. This speeds up

the process and is more convenient than handling large sheets of reinforcement.

The hull moldings of larger vessels are laid this way. Rolls of woven roving are

1064 COMPOSITES FABRICATION PROCESSES

Conventional blade ribs require

complex tooling

Hat stiffener formed over foam core

Sandwich panels using end-grain

balsa wood, honeycomb or foam cores

Panels stiffened by corrugations

Fig. 15 Laminates may be effectively stiffened in a number of ways. Conventional blade stiff-

eners are effective but require more complex tooling. A simple method is to laminate extra ma-

terial over foam or timber former to form a hat section. Sandwich sections are very efﬁcient and

can be formed using end-grain balsa wood, aluminum, or aramid honeycomb and foam cores.

Corrugations may be incorporated into the tooling to provide stiffening and more complex

double and triple layers are a further possibility.

laid from gunwale down to the keel and back up to the opposite gunwale. The

layers are consolidated by hand or mechanical rolling. The largest hulls manu-

factured this way are some 85 m long and weigh upward of 250 tons.

Spray-up

The contact molding operation can be further mechanized by use of the spray-

up process. The basis of this is a spray head from which liquid resin and chopped

rovings are sprayed simultaneously onto the mold surface. Continuous ﬁber rov-

ings are fed into the spray head, which is equipped with a cutting device that

cuts them to lengths of 25–75 mm; a jet of compressed air directs the chopped

fragments onto the mold. Resin and initiator or curing agent are also fed sepa-

rately into the spray head, where they are blended in the correct proportions and

sprayed so that they wet out the chopped rovings on the mold surface. The resin/

ﬁber layer on the mold must then be consolidated by rolling. The spray head

may be directed by hand or further automated by use of a robotic manipulator.

It is convenient to program such robots by having them replicate the actions of

an expert sprayer. This is a cheap, ﬂexible, and effective programming technique.

The principal disadvantages of the system are that only random layers can be

deposited and that styrene emissions are very high. To combat the emission

problem, hand spraying must be conducted in adequately ventilated booths, but

robotic spraying could be carried out in completely sealed environments. The

random layer limitation may be partially overcome by interrupting the spray

process and laying intermediate layers of woven fabric and applying the resin

by spray, followed by a further layer of the chopped material.

3.5 Productivity and Economics

The contact molding process is attractive because it is very versatile, cheap

tooling can be used, and production runs as low as that of a single molding may

4 PRESS MOLDING 1065

be viable. Quality is dependent on the skill of the labor force and the process

is slow. It is eminently suitable for the production of very large moldings, where

long runs are unlikely. For smaller moldings there are few circumstances where

more than one part may be manufactured from a single tool set in a working

day. Ordinary composite tooling, under favorable manufacturing circumstances,

may be sufﬁciently durable to produce 100 parts before the tool requires exten-

sive rework or replacement. A run of 1000 parts per year would thus require

about ﬁve tool sets in constant operation and would consume at least 10 tools

per annum. A labor force sufﬁcient to handle this volume of work would also

be required. This means that where larger production runs are envisaged alter-

native processes that are more reliable, in terms of quality, and less dependent

on high labor content must be sought.

4 PRESS MOLDING

4.1 Basic Process

The principle of press molding is that a pair of matched tools and a press are

used to apply pressure during consolidation and cure. Within this scenario there

are several variants depending on the size of the molding, the pressure used,

and the cure temperature. The advantage of matched tooling is that the mold

cavity is more precisely deﬁned, section thickness is more accurately controlled,

and a high surface ﬁnish may be imparted on all surfaces of the molding. The

pressures used to consolidate the part vary considerably according to the pro-

jected area of the mold, and the tooling must be designed to be sufﬁciently

robust. If low pressures,

⬍5 bars, are used, then composite tooling with solid

support structure, e.g., cast-ﬁlled resin or concrete, may be adequate. At higher

pressures metal tooling is needed. Aluminum or zinc may be adequate for mod-

erate temperatures and cure temperatures, but steel will be required at the higher

end of the pressure/temperature spectrum. It should be noted that a more precise

control of dimensions and surface ﬁnish, as well as more durable tooling is

achieved if steel tooling is used. Molding surfaces may be further hard coated,

e.g., hard chromium plated, to enhance surface ﬁnish and durability.

4.2 Reinforcements and Resins for Press Molding

All conventional dry and preimpregnated reinforcements may be used in press

molding. However, it is highly desirable to minimize lay-up time in the press

because this would restrict productivity while the equipment remained idle. For

this reason it is common to preform the reinforcement so that a single entity

may be placed in the mold. The simplest preforms are made by spraying chopped

ﬁbers with a small proportion of binder onto a perforated former on a vacuum

box. The loose preform may be brieﬂy consolidated by vacuum bag or by an

auxiliary pressing operation at low pressure and moderate temperature, usually

⬍100⬚C. Fabrics may be tailored to size and preconsolidated in a similar way.

Again a small proportion of binder is necessary, and this may be incorporated

in the fabric at the point of manufacture or applied as a spray or dry powder.

In either case the preform is simply placed in the open mold, and the molding

operation is then initiated.