Comprehensive Composite Materials || Composite Processing and Manufacturing—An Overview

2.16Composite Processing andManufacturingÐAn OverviewJ.-A. E. MAÊ NSON, M. D. WAKEMAN, and N. BERNET

Ecole Polytechnique FeÂ deÂ rale de Lausanne, Switzerland

2.16.1 INTRODUCTION 2

2.16.2 PROCESS DRIVEN DESIGN 2

2.16.2.1 The Interaction of Materials and Processes 22.16.2.2 Design Flexibility and Manufacturing Restrictions 22.16.2.3 Quality Issues During Manufacture 3

2.16.3 DRIVING FORCES FOR COMPOSITES 3

2.16.3.1 Aerospace Industry 42.16.3.2 Transportation Industry 52.16.3.3 Mechanical Industry 62.16.3.4 Other Industries 8

2.16.4 A MATERIALS PERSPECTIVE TO COMPOSITE PROCESSING 8

2.16.4.1 Matrices 82.16.4.2 Reinforcement 9

2.16.5 PERFORMANCE±COST±PRODUCTION RELATIONS 10

2.16.5.1 Material Conversion Routes 102.16.5.2 Shape Complexity vs. Intrinsic Stiffness 112.16.5.3 Resin Viscosity vs. Reinforcement Aspect Ratio 112.16.5.4 Orientation Control vs. Reinforcement Aspect Ratio 122.16.5.5 Shape Complexity vs. Annual Volume 13

2.16.6 PROCESS WINDOWS 13

2.16.6.1 Thermosets 132.16.6.2 Thermoplastics 152.16.6.3 Impregnation 162.16.6.4 Manufacturing Induced Internal Stresses 17

2.16.7 EMERGING TECHNIQUES 17

2.16.7.1 Resin Infusion Processes (RIFT) 172.16.7.2 Engineered Preforms 182.16.7.3 Large Injection Molding Technology 202.16.7.4 Integrated Processing 212.16.7.5 Reactive Thermoplastics 222.16.7.6 Robotic Lay-up of Thermoset Tows 24

2.16.8 LIFE CYCLE ENGINEERING, COST AND IMPLEMENTATION 26

2.16.8.1 Life Cycle Engineering 262.16.8.2 Cost Modeling 272.16.8.3 Composites ImplementationÐReason for Change 28

2.16.9 REFERENCES 29

1

2.16.1 INTRODUCTION

This chapter provides an overview to com-posite manufacturing, where the subsequentchapters in Volume 2 will consider individualconversion techniques and processing princi-ples in detail. After this introductory chapter,the first section discusses impregnation andconsolidation phenomena and describes differ-ent fiber architectures for composites. Pro-cessing techniques are then grouped intothermoset and thermoplastic based composites,with process fundamentals and conversiontechniques considered separately. The final sec-tion considers the joining of composites, theselection of novel manufacturing routes, andthe life cycle engineering of composites forboth distinguishing classes.Chapter 2.16, Composite Processing andManufacturingÐAn Overview, MaÊ nson, Wake-man, BernetChapter 2.17, Impregnation and ConsolidationPhenomena, SastryChapter 2.18, Composite Preforming Techni-ques, VerpoestChapter 2.19, Processing Principles for Thermo-set Composites, Boogh, MezzengaChapter 2.20, Prepregging and Autoclaving ofThermoset Composites, Seferis, Hillermeier,BuehlerChapter 2.21, Open Mold Techniques for Ther-moset Composites, Cripps, Summerscales,SearleChapter 2.22, Compression Molding of SMCs,Revellino, Saggese, GaieroChapter 2.23, Liquid Molding of ThermosetComposites, Advani, SozerChapter 2.24, ContinuousMolding of ThermosetComposites, SohlChapter 2.25, Processing Principles for Thermo-plastic Composites, Michaeli, KoschmiederChapter 2.26, Compliant Mold Techniques forThermoplastic Composites, Mallon, O'BradaighChapter 2.27, Compression Molding of Thermo-plastic Composites, Wakeman, RuddChapter 2.28, Liquid Molding of ThermoplasticComposites, BourbanChapter 2.29, Continuous Molding of Thermo-plastic Composites, GibsonChapter 2.30, Injection Molding Based Techni-ques, BrooksChapter 2.31, Joining of Composites, Gillespie,Bourban, TierneyChapter 2.32, Manufacturing Process Selectionfor Composite Components, Rudd, JohnsonChapter 2.33, Life Cycle Engineering of Com-posites, Leterrier

As an introduction to composite manufac-turing, this chapter discusses the interactingnature of material constituents and the subse-

quent conversion process. After a brief reviewof driving forces for composites in a wide spec-trum of industries followed by some designconsiderations, the manufacturing processesare considered under a number of themes.These include how different processing techni-ques vary with respect to part size and complex-ity, fiber volume fraction, fiber aspect ratio,and reinforcement orientation control togetherwith matrix viscosities and required impregna-tion times. Having taken a broad overview ofdifferent conversion processes, a brief look atsome examples of novel processing techniquesthat have surmounted previously limiting man-ufacturing boundaries is then taken to showhow advancements in manufacturing processescan introduce new opportunities. Tools forassessing quantitatively the economics of dif-ferent composite processing techniques andmaterial classes are examined from a cost mod-eling perspective. The life cycle implications ofcomposites are then reviewed and issues of theimplementation and change to composites aresummarized as a conclusion.

2.16.2 PROCESS DRIVEN DESIGN

2.16.2.1 The Interaction of Materials andProcesses

Composite materials offer a diverse range ofproperties suited to an equally wide range ofapplications, offering the design engineer aplethora of opportunities for many differentend uses. Applications vary significantly insize, complexity, loading, operating tempera-ture, surface quality, suitable production vo-lumes, and added value. The expanding choiceof raw materials, in terms of reinforcement type(concentration and fiber architecture) togetherwith matrix material (subsets of both thermo-plastic and thermoset polymers), followed bymany subsequent final conversion processesgives impressive flexibility. These variablesoften interact to create for the uninitiated anoften confusing material and process ªsystem.ºHence the initial choice of fiber and matrixtype, together with the subsequent processingroute, control the properties of the final moldeditem.

2.16.2.2 Design Flexibility and ManufacturingRestrictions

Due to their special features, such as highspecific strength and rigidity, composite mate-rials are increasingly being used to replace met-allic components. Composite manufacturing

Composite Processing and ManufacturingÐAn Overview2

techniques offer the ability to produce geome-tries that are often difficult to achieve withconventional metallic materials (e.g., a desir-able thickness variation is easily obtained). Incontrast, composite conversion processes oftenimpose restrictions on the part shape and thereinforcement configuration (e.g., pultrusionyielding long-fiber composite profiles but ofconstant cross-section). To understand how re-inforcement and matrix components can becombined successfully into a viable final pro-duct, knowledge is needed of both the consti-tuent materials and of how to manipulate theseto suit the various shaping possibilities offeredby different molding processes. The final con-version process often creates the compositematerial as the component is made. For exam-ple, resin transfer molding (RTM) uses a semi-finished product, a fiber preform, which is thenimpregnated with resin to create the materialsystem and the final component in the sameoperation. This is in contrast to steel stamping,where for example the preproduct with a gen-erally full set of mechanical properties is usedand then reshaped. The composite processingtechnique therefore must be chosen at the out-set of part design due to the influence of theconversion route on mechanical properties.

2.16.2.3 Quality Issues During Manufacture

The final conversion process can first createboth the composite and the component (e.g.,RTM), or second, complete impregnation togive the final composite and molded item(e.g., SMC compression molding). For bothcases the influence of processing on part qualitymust be considered. This is not simply per-formed for a particular combination of fiber,matrix, and processing techniques, but is also afunction of component geometry. Quality pro-

blems arising from the manufacturing stage areboth general to composite materials but alsoprocess and material specific. Typical issuesinclude: general consolidation quality, matrixbased porosity, intrabundle voids (or dryspots), high levels of internal stress and post-molding warpage, either directly or shortlyafter processing, outgassing of solvents, fibersor fiber bundles migrating to the surface, chan-ging fiber architectures, and polymer degrada-tion. This can result in quality deviation fromone part to the next, which is an undesirableproduct trait for applications. Knock-downfactors are therefore required in the designprocess, with the manufacturing phase oftenincurring the biggest penalty. Figure 1 showsgeneralized knock-down factors from a moreconservative design approach. Clearly, themaximum added value cannot be gained fromthe raw materials and processing facility if thefull potential of the composite is not exploited.An understanding of the underlying effects,through study of material/process/propertyrelations, is an essential step towards reliableprocesses. Application of this knowledgetogether with process optimization techniques(e.g., injection molding) can help to create arugged process and reduce knock-down factors.

2.16.3 DRIVING FORCES FORCOMPOSITES

Common driving forces for the use of com-posite materials include the ability to saveweight, increase mechanical properties, reducethe number of elements in a component, ob-tain a unique combination of properties, andto increase shaping freedom. Increasingly,composites are being used for the abovewhile also achieving a reduction in part cost.Many of these driving forces, together with the

Figure 1 Examples of knock-down factors affecting the design value of composites.

Driving Forces for Composites 3

manufacturing cycle, often offset the higherraw material costs of the composite constitu-ents to produce a commercially viable endproduct. The criteria on which composite ma-terials are selected for a particular applicationare naturally dependent on the industrial sec-tor for which they are intended, as illustratedin Figure 2 (Gysin, 1991). For example, aero-space has traditionally been driven by perfor-mance, where longer cycle times and increasedscrap levels were tolerated, whereas highvolume applications, typified by the automotiveindustry, require rapid and highly automatedtechniques but where the full potential of com-posites in terms of mechanical properties isseldom reached.

2.16.3.1 Aerospace Industry

A leading role in the development of bothcomposite materials and processing technologyhas been taken by the aerospace industry. Thehigh specific stiffness and strength of the rein-forcement offered the potential for reduced fuelconsumption and increased range with passen-ger aircraft and increased performance (range,turn rates, stealth) for military aircraft. Theability to tailor thermal expansion togetherwith the low material density also made materi-als attractive for space applications. A substan-tial research effort was therefore made by theaerospace industrial, governmental, and aca-demic communities to develop this materialclass. The main driving forces for the aerospace

industry are therefore primary weight reductionby using a material with higher specific mech-anical properties (mechanical property/den-sity), facilitating secondary weight savings,leading to considerable additional weight re-duction (Figure 3). The strong demand forweight saving in aerospace applications, aswell as the lower sensitivity of this industry toproduction rates and material costs, has led tothe development of finely-tuned high-perfor-mance processing techniques and materials.Reinforcement orientations are specified tosuit the principle stresses in the componentand prepregs are placed carefully to maintainthe desired orientations. The stresses carried inaerospace structures are often high, requiringcontinuous fibers for maximum strength andcreep resistance. This limits processing techni-ques to those where fiber lengths are maintainedin highly controlled directions and where stiff-ness is achieved through the intrinsic propertiesof the composite. Flow molding techniqueswhere the reinforcement volume fractions andorientation control are reduced are generallyunsuitable. Aerospace applications are typifiedby planar geometries, as shown in Figure 4(NASA, 2000; Airbus, 2000), requiring specificcomposite materials and conversion techniquesto enable the manufacture of such structures.

The first composite primary structure toenter production on a commercial aircraft wasthe all composite rudder introduced in 1985 forthe A300/A310 (Anon, 1999). This consisted ofa single-piece rudder assembled from a hollowtriangle of three honeycomb sandwich panels

Figure 2 Composite materials selection criteria as a functional of industrial sector (after Gysin, 1991).


with carbon-fiber/epoxy skins. This replaced itsmetal counterpart without design changes tothe aircraft, reducing 2000 parts (including fas-teners) for the metal system to less than 100 forthe composite system with a 20% weight savingand an overall cost saving, despite the higherraw material cost. Combined with other designchanges, the composite rudder and vertical finlead to reduced fuel consumption, illustratingthe commercial reality of composites logic illu-strated in Figure 3. Hence traditional aerospacedriving forces for weight reduction, which yieldimproved range and reduced operational costs,are increasingly being extended to the addi-tional requirement of total cost savings.

2.16.3.2 Transportation Industry

The transportation industry represents a po-tentially large application area for fiber rein-forced composites and is driven by a complexset of interacting driving forces. If the needs ofthe aerospace industry have led to the develop-ment of advanced composites, then the needs ofthe automotive industry have dominated thedevelopment of engineering composites, withincreased shape complexity and a strong em-phasis on decreasing system cost. Decreasingcost must be considered while maximizing qual-ity, functionality, and return on manufacturinginvestment together with meeting legislative re-quirements for safety, emissions, and recycling.In the automotive industry, weight increasesfrom improved safety, refinement, and func-tionality interact with the vehicle mass reduc-tions necessary to improve fuel economy andreduce emissions (Rudd et al., 1997). Studieshave shown that up to 40% of a vehicle's fuelconsumption is related to factors attributable tovehicle inertia losses (Harrison, 1997). Addi-tionally, the location of weight reduction willhave an influence on the safety, comfort, andmaneuverability of the vehicle, as illustrated inFigure 5. By reducing the weight away from thecenter of gravity, the handling of the vehicle canbe significantly improved. Figure 6 (OwensCorning, 2000) shows how composite materialshave been used commercially as a means forreducing fuel consumption and cost.

Driving forces in the automotive industryhave changed, as viewed by original equipmentmanufacturers (OEMs), evolving from the1980s (corporate average fuel economy, emis-sions, alternative fuels, and electric vehicles) tothe 1990s (cost-effectiveness, weight reduction,government initiatives, low-volume vehicles) tothe current increased demands of return oncapital, safety, and niche products (Johnson,

Figure 3 Aerospace driving forces (after MaÊ nson,1994).

Figure 4 Examples of planar aerospace structures.


1999). These are needed to drive developmentsin the twenty-first century.

Developments in the automotive industryhave not only been focused on material perfor-mance, as in the aerospace industry, but also onthe constraints imposed by manufacturing tech-niques for large series and by the need to reduceproduction costs. For composite materials tocompete with steels and alloys, higher valuemust be added and cost reductions made inmanufacturing cycles to balance the higherraw material cost. In the automotive industry,the current use of composites is dominated byrelatively low fiber volume fractions and ran-domly oriented fiber architectures, includingSMC (100 k tonnes per year-US) and GMT(77 k tonnes per year-US) (Johnson, 1999) forexterior panels, underbonnet parts, and semi-structural subsurface parts, respectively. RTMand SRIM (10 k tonnes per year) (Johnson,1999) exhibit slow but steady growth for nicheproducts and some increasingly structuralitems. While GMT and injection molding tech-nologies are relatively mature, there is interestin dedicated continuous fiber architectures(weaves, knits, etc.). The improved propertiesarising from the higher and aligned fiber con-tent offers potential in structural areas.

As two examples of how composites haveprovided cost-effective solutions within theboundary of these complex driving forces, aSMC cross-car beam from a pick-up truckand an injection molded inner door moduleare briefly examined. The 6 kg cross-car beam(Figure 7) (Owens Corning, 2000) uses a two-piece bonded part to replace 25 steel parts whileimproving dashboard sag and NVH properties.This is produced at 940 thousand parts per yearwith a cycle time of 60 s, showing how the

advantages of composites can be used econom-ically where the parts count reduction and addi-tional performance pay for the higher rawmaterial cost (Owens Corning, 2000). Figure 8(Delphi, 2000) shows the inner door moduleproduced from 30% short glass fiber reinforcedPBT/PC alloy. A reported system cost reduc-tion of 5±10%was achieved while saving weightand offering assembly advantages. Here theability to form complex shapes with high designbased stiffness, together with the incorporationof many additional features in the same piece,offers maximum added value to the raw mate-rials through a flexible manufacturing processand the increased design freedom that thisoffers.

2.16.3.3 Mechanical Industry

The mechanical industry is currently under-taking large efforts to apply advanced compo-site materials to specific applications. Here, thevolumes and application demands can generallybe considered as lying between those of theaerospace and automotive industries. Whileproduction volumes and part needs have notin many cases been sufficient to justify an in-dependent composites strategy, knowledge in

Figure 5 Transportation driving forces (afterMaÊ nson, 1994).

Figure 6 Composite use in an automotive applica-tion.


design, analysis, and manufacture from theaerospace and automotive industries has beensuccessfully applied to this market sector. Themechanical industry, which may be exemplifiedby advanced machine elements in high-speedreciprocating or rotational applications, has itsmain reason for weight reduction in improve-ment of operation rates and efficiency, as wellas in convenience and handling (Gysin, 1991;MuÈ ller, 1992). These are shown in Figure 9.

Machine elements in high-speed textile andpacking machines, pumps, energy conversionequipment, and lightweight hand-machinesare some examples of applications in whichadvanced composite materials are presentlybeing used or evaluated. These are typicallysmall and are manufactured in series of somedozens to several thousand pieces. Here, theadvantages of composites in terms of a long-term high stress fatigue resistance, tribological

Figure 7 SMC cross-car beam molding.

Figure 8 Injection molded automotive door module.

Figure 9 Mechanical industry driving forces (after MaÊ nson, 1994).


properties, and corrosion resistance in aggres-sive environments can be exploited. As an ex-ample, a wheel in a band-weaving machine isbriefly taken. This is driven between a transi-tion of 0±3608 at a frequency of 10Hz, where areduction in component mass will reduce theaxial torsional moment to meet this accelera-tion profile (Gysin, 1990). Where the high spe-cific properties of composites enable themachine to work faster by reducing the bottle-neck in the whole machine, relatively expensivematerials can improve the efficiency of themachine with the critical component only form-ing a small fraction of the total machine cost.

2.16.3.4 Other Industries

Branches such as the leisure, building, andmarine industries are also significant users ofcomposites, in a diverse range of materials,processes, and applications with a wide rangeof driving forces.

The leisure industry, and particularly thesports industry, has many applications wherethe introduction of polymer composites has hadconsiderable impact on the product perfor-mance. As this industry undergoes frequentproduct replacement, several generations ofcomposite applications have been in use and asuitable range processing techniques have beendeveloped and refined. This includes applica-tions such as ball game rackets and clubs, bi-cycle frames, fishing rods, and water sportsboards, which are well known and in wide-spread use. Both specialized low-volume andhighly automated mass production techniquesare used with a variety of materials, generally incontinuous fiber form. Here driving forces areagain complex, being driven both by perfor-mance and user image with a strong marketinginfluence.

In the building industry, considerable pro-blems from aging concrete bridge structures hasled to advanced carbon fiber based compositesbeing used as a repair material, to delay boththe high rebuild costs and the considerableinconvenience that this produces.

The marine industry produces large struc-tures such as hulls, superstructures, and funnelswhere composites reduce metal forming opera-tions, offer increased performance, and specificproperties in applications such as mine sweeperhulls where steel is not suitable. Manufacturingtechniques here are biased towards large struc-tures where a higher level of manual work issuitable and production volumes lower. Anexample of a large marine structure wherehealth and safety issues together with increased

performance have been strong driving forces isthe production of a 55.2m, 470 tonne displace-ment, minehunter craft by Vosper Thornycroft(Anon, 1998). The vacuum infusion techniquereduced costs and offered greater efficiency inproduction with fewer layers of a heavier(6000 gm72) fabric needed compared to the35 separate plies of 800 gm72 woven rovingglass used in hand lamination. Componentweight was reduced by the ability to achievehigher fiber volume fractions and laminatestrength increased by reducing void contentsfrom 5% for hand lamination down to lessthan 1%. This is discussed further in Chapter2.21, this volume.

2.16.4 A MATERIALS PERSPECTIVE TOCOMPOSITE PROCESSING

The composite constituents and the form inwhich they are used have a profound influenceon the manufacturing process. A brief overviewis therefore given prior to a discussion of com-posite manufacturing routes from a generalperspective.

2.16.4.1 Matrices

Both thermoplastic and thermoset resins arecurrently used as matrices for composites, andthese materials are described separately inChapters 2.19 and 2.25, this volume. Eachtype of polymer offers advantages and disad-vantages with respect to processability and ser-vice performance, as generalized in Table 1(Hancox, 1988; Johnston and Hergenrother,1987).

In general, the cross-linked structure of ther-moset polymers provides potential for a higherstiffness (E-modulus) and service temperaturesthan thermoplastics. An improved surfacefinish is also more readily achieved for thermo-set-based composites. On the other hand,toughness and elongation to break may be con-siderably higher for thermoplastic resins. Thischaracteristic is particularly interesting for ap-plications where impact strength is a majorrequirement. From a processing point of view,the high melt viscosity of thermoplastic resinsmakes impregnation of the fiber reinforcementproblematic. For example, the viscosity of con-ventional thermoplastics under melt processingconditions is generally in the range of 50±2000 Pa.s, which contrasts with thermosets intheir noncross-linked state where the viscosityseldom exceeds 50 Pa.s. As a consequence, ther-moplastic composites generally require higher


temperatures and pressures to ensure sufficientflow during processing. This can limit the suit-ability of thermoplastic-based composites forlarge structures where the heating step and thepressure required for consolidation becomeschallenging.

Thermoplastic-based composites are gener-ally suited to lower cycle times and hence highervolume production due to the nature of thematrix, where the often time consuming chemi-cal reaction (cross-linking) is not required dur-ing processing (see Chapter 2.25, this volume).The matrix viscosity is lowered by heating, en-abling impregnation, with subsequent removalheat after impregnation, consolidation, andforming to cool the finished component. Heattransfer thus forms the principle boundary con-dition for such materials, with a correspondingpotential for lower conversion costs. For exam-ple, the injection molding of composite com-pounds requires heating of the pellets before themolten material is forced into a cold tool, whereheat transfer occurs rapidly resulting in a lowcycle time. The shelf life of thermoplastics is alsoan advantage where B-staged prepregs of epoxyand carbon, for example, have a limited shelf life

prior to processing and often require subam-bient storage. Thermoplastic composites alsooffer increased recyclability, as they can be post-formed and/or reprocessed by the applicationof heat and pressure.

2.16.4.2 Reinforcement

The reinforcement phase uses stiff fibers in aconsiderable variety of fiber architectures(Figure 10), giving impressive flexibility to thematerial system as a whole. This is described inmore detail in Chapter 2.18, this volume. Arange of manufacturing processes have beendeveloped, which are often suited to differentfiber architectures, which in turn define, to alarge extent, the composite and hence compo-nent properties. The efficiency of the reinforce-ment increases with its aspect ratio, reaching amaximum level for continuous fibers (Hill,1965; Hashin, 1965; Halpin and Kardos, 1976;Folkes, 1982). By increasing the reinforcementlength, the impact strength of the compositemay also be improved, but the possibility to

Table 1 Potential for commercial resinsÐproperty/process characteristics.

Property Thermoset Thermoplastic

Modulus High MediumService temperature High MediumToughness Medium HighViscosity Low HighProcessing temperature Low HighRecyclability Limited Good

Figure 10 Characterization of reinforcing materials: properties, shape, configuration (after MaÊ nson, 1992).

A Materials Perspective to Composite Processing 9

flow during processing to create complex shapesis generally reduced considerably (Gibson et al.,1982; Gibson, 1985; Silverman, 1987). Thecontrol of fiber orientation allows a selectivereinforcement of the part, or the tailoring of thematerial to the load situation, developinganisotropic properties (advantageously).

The two extremes consist of discontinuous,random fibers on the one hand, and continuous,unidirectional layers on the other. The formerhas low but almost isotropicmechanical proper-ties and due to the low fiber fraction, the dis-continuous fiber length and random orientationhas reduced mechanical properties. The latterhas high mechanical properties in the fiber di-rection, but is very anisotropic, often limitingforming potential. Textile based preforms (in-cluding weaving, knitting, and braiding) fallbetween these extremes to give a broad rangeof fiber architectures by using a wide range ofconventional and specifically developed textileprocesses. Such preforms offer considerably im-proved drapability over unidirectional layersand improved mechanical properties over dis-continuous random materials. Intrinsic proper-ties, orientation, aspect ratio, configuration,and concentrations, as shown in Figure 10,therefore characterize the reinforcing fiberarchitecture of the composite.

2.16.5 PERFORMANCE±COST±PRODUCTION RELATIONS

2.16.5.1 Material Conversion Routes

Figure 11 gives an overview of the potentialconversion routes for polymer composites,from fiber and matrix to finished part, in

terms of flow capability and drapability (MaÊ n-son, 1992). Techniques where impregnationoccurs during final processing rely on a lowmatrix viscosity and a permeable fiber bed.Here drapable fiber structures can be fashionedinto a preform, enabling relatively complexshapes to be molded when the resin is intro-duced into the mold. Preimpregnation techni-ques, such as hot melt or solutionimpregnation, offer the advantage that thefibers are already fully impregnated before thestacking and shaping operations. However, thepreforms made by these techniques may havereduced drapability, as is usually the case whenusing reinforcement preimpregnated with ther-moplastic resin, restricting the application ofthese preform types to flat or simply-curvedparts. This problem can be avoided with ther-moplastic prepregs where nonisothermal tech-niques are used. Here the preconsolidatedmaterial is preheated in an oven and formingthen occurs in cold tools, such that the toolsdrape the prepreg to the desired geometry whilethe matrix is still in the melt phase, which thencools rapidly under pressure to give the con-solidated component. The alternative isother-mal technique can be used to process flexibletextile based prepregs (typically thermoplasticbased). Here the material is draped while coldinto a cold tool. The tool is then closed andheated with the application of pressure to con-solidate the material, followed by cooling of thecomponent before the tool is opened again andthe part released. In this case, intimate minglingof the fibers and the matrix, for example, byfiber hybridization or powder coating methods,allows processing times and pressures to begreatly reduced in comparison with meltimpregnation techniques.

Figure 11 Alternative manufacturing routes: from resin and fiber to finished part (after MaÊ nson, 1994).


2.16.5.2 Shape Complexity vs. IntrinsicStiffness

Where a stiffness-driven design is required,the component stiffness is composed of a com-bination of both the intrinsic properties of thereinforcement and the design shape of the part.A general relationship, which is illustrated inFigure 12, exists between the design complexitythat can be achieved with a process and thereinforcement content and degree of organiza-tion. Hence, if a component with many inte-grated functions and stiffening features isneeded, flow based techniques may be bettersuited whereas a highly loaded structurerequires continuous fibers, processed accord-ingly. Injection molding, for example, can beused to process parts from composite com-pounds giving a high degree of shape complex-ity and hence component stiffness. However,the reinforcement is limited to short fibers oflow volume fraction, resulting in reduced mate-rial stiffness, impact properties, and creep resis-tance. Tape-placement, on the other hand, hasthe advantage of providing high intrinsic stiff-ness, but is restricted to simple generally planarshapes using more costly raw materials.

In between these extremes, numerous fiberarchitectures combine a higher reinforcementcontent, often in the form of drapable textilestructures, but without the forming complexityoffered by bulk flow processes where the rein-forcement moves with the flowing matrix.Resin transfer molding, for example, offersthe ability to produce complex components,but where ribs and thickness changes are re-quired, substantial manipulation of differentreinforcement forms is needed to produce the

complex preform. Hence, different processingroutes are suited to different types of applica-tions in terms of the desired shape complexityand operational load case. To overcome thelimitations of these techniques, a synergy ofthe intrinsic stiffness of aligned fiber structuresand the geometrical stiffness of bulk compositeflow processes would give increased design flex-ibility and add increased value to the raw ma-terials. An increasing number of processes areattempting to combine different materials andprocesses to gain the advantages of design andintrinsic stiffness, with such processing avenueslocated towards the upper right-hand corner ofthe diagram in Figure 12.

2.16.5.3 Resin Viscosity vs. ReinforcementAspect Ratio

The constituent materials of a polymer±composite give many possibilities for pro-cessing. Figure 13 shows a map of variouscomposite processing methods in terms ofmaterial characteristics (Lee and MaÊ nson,1991; MaÊ nson, 1994). The viscosity considera-tion generally forces processes into thermoplas-tic and thermoset based techniques, but withequivalent reinforcement aspect ratios existingfor both matrix types. In the low reinforcementaspect ratio range are materials that can beprocessed by extrusion and injection molding.These include mineral and fiber-filled thermo-plastics and thermosets, such as reinforcedreaction injection molding (RRIM) and rein-forced thermoplastics (RTP). Fiber-reinforcedcomposites with intermediate aspect ratios

Figure 12 The interaction of processing technique, design stiffness, and intrinsic stiffness (after MaÊ nson,1992).

Performance±Cost±Production Relations 11

include preforms with flow capability, based onboth thermosets (SMC, BMC) and thermoplas-tics (TPS or GMT), which are designed to beprocessed by compression molding. Discontin-uous fiber-reinforced thermoformable sheet isalso in this area of the map. Processes usingengineered fabrics, such as RTM and SRIM orthe stamping of continuous fiber reinforcedthermoplastics, move towards the higher aspectratio area. In the highest aspect ratio region arecontinuous fiber composites formed by handlay-up and tape lay-up of prepregs, filamentwinding, and pultrusion. For advanced thermo-plastic composites, solvent impregnation, pow-der coating, or special melt impregnationmethods are usually used to ensure sufficientprepreg quality (Gibson and MaÊ nson, 1992).

2.16.5.4 Orientation Control vs.Reinforcement Aspect Ratio

The fiber volume fraction and the fiber or-ientation influence the strength and stiffness ofa composite part. These can be controlled byusing an appropriate conversion route andFigure 14 represents a map of the level ofcontrol over reinforcement that can be reachedin various composite processes (Lee and MaÊ n-son, 1991; MaÊ nson, 1994). While the processingof short aspect ratio reinforcement compositesis easier, less control of fiber orientation ispossible. For example, the fiber orientation ininjection molded materials is determined by theflow kinematics during the injection process,which in turn is influenced by the gate and

Figure 13 Composite processing map: resin viscosity, and reinforcement aspect ratio (after MaÊ nson, 1994).

Figure 14 Composite processing map: potential for control of reinforcement orientation and aspect ratio(after MaÊ nson, 1994).


design of the mold. In hand lay-up of contin-uous fiber prepreg, on the other hand, fiberorientation can be very precise, and is deter-mined by the stacking sequence of the laminate.Orientation control can be further increased togive not only fibers in the general plain of theapplication, but by using textile processingtechniques such as knitting, but also throughthe thickness of the component. Fully 3-D pre-forms can also be used such as simple braids ormore complex fiber architectures.

2.16.5.5 Shape Complexity vs. Annual Volume

A switching of position occurs when the axesof the processing map are changed to partgeometry complexity and annual volume.Whereas injection molding was located in thelower left corner of Figure 14, the change ofaxes moves this to the top right-hand corner ofFigure 15. Hence the number of parts to beproduced (assuming one tool set and processingmachine used at the maximum rate as definedby the cycle time) and the part complexitydesired will influence the selection of a suitablemanufacturing process (Lee andMaÊ nson, 1991;MaÊ nson, 1994). For a small productionvolume, a process that requires lower invest-ment in tooling and equipment costs, but longercycle times and more intensive labor, may beappropriate. This includes processes such ashand lay-up, variants of resin transfer molding(RTM), or polyester fiberglass (FRP) spray-up.As the production volume increases and greaterpart complexity is required, an increased degreeof automation and highly reactive resin chemis-tries can be used to push techniques such asRTM and SRIM towards a medium volume

level (50 k per year). Where flow of the matrixthrough the fiber bed is required, extensiveautomation is needed to produce complex pre-forms with varying part thicknesses anddetailed features. Such techniques still do notmatch the forming complexity obtainable frominjection and compression molding techniques,but extend the capabilities of liquid moldingtechniques to higher volumes. For the highestvolumes, it may become more economicallycompetitive to use processes such as injectionmolding or compression molding of GMT orSMC, assuming that the operating loads of theapplication can be met with these materials. Ageneral tendency exists for the lowest cycle timeprocesses to use bulk flow materials injected orplaced into a steel tool (offering high shapecomplexity) or preimpregnated products forstamping (3-D planar complexity). Pultrusionalso enables products to be produced at highvolumes, but with limited shape complexity.Shaped dies for thermosets and postformingfor thermoplastics helps to move the shapecomplexity up for pultruded products.

2.16.6 PROCESS WINDOWS

2.16.6.1 Thermosets

A typical thermal cure history for a thermo-set composite is shown in Figure 16, which istypified by a prepregging and autoclave pro-cessing sequence. The prepreg is first producedby impregnating the fiber bed with matrix resin,where the resin viscosity will drop during heat-ing which initiates the first stage of cross-linking. With increased cure the viscosityincreases, but the reaction is controlled enabling

Figure 15 Composite processing map: ability for complex shaping and annual production volumes (afterMaÊ nson, 1994).

Process Windows 13

the preimpregnatedmaterial to be used later in afinal conversion step. To inhibit final cure of thematerial and hence stop the viscosity increasing,the prepreg is stored at a subambient tempera-ture. For final processing, the material is heatedto reinitiate the reaction, upon which the visc-osity drops until cross-linking increases and thematerial gels whereupon the viscosity becomesinfinite. Often a two-step final cure is used tocontrol warpage and void content. The part isthen cooled and removed. In some cases a finalpostcure is performed in order to obtain ulti-mate properties.

In general, the process is performed within atool holding the fiber assembly in the presence

of resin. Pressure and heat are applied to ensureproper impregnation of the fiber network bythe resin. Heating is necessary to reduce theinitial matrix viscosity (easing impregnation)and to initiate the curing reaction. As cureprogresses, the resin solidifies by forming irre-versible cross-links. When the curing reaction iscompleted, the composite part is cooled andfinally removed from the mold.

For thermoset composites, the properties ofthe final material are intimately related to thetime±temperature path of the cure cycle.Figure 17 shows a schematic isothermal time±temperature±transformation (TTT) diagramassociated with the cure of a thermoset polymer

Figure 16 Typical thermal cure history for a thermoset composite.

Figure 17 Isothermal time±temperature±transformation (TTT) diagram associated with the cure of athermoset polymer.


(Enns and Gillham, 1983; Pang and Gillham,1990). At a temperature between Tg

gel and Tg?,

where processing generally takes place, thepolymer first gels and then vitrifies. Isothermalcure between Tg

gel and the glass transition tem-perature of the reactants, Tg8, causes the poly-mer to vitrify simply by an increase ofmolecular weight. Below Tg8, gelation is notpossible.

2.16.6.2 Thermoplastics

As illustrated in Figure 18, a typical pro-cessing cycle for a composite made from ther-moplastic matrix consists of heating the

constituents above the melting temperature ofthe resin (Tm), applying a chosen pressure for aselected period of time to ensure appropriateconsolidation, and finally cooling the materialwhile maintaining the pressure. From a manu-facturing point of view, considering equipmentinvestment cost and production rate, it is desir-able to process the part at the lowest possiblepressures and with the shortest possible cycletimes. Shortening cycle times requires exposingthe composite to high heating rates prior tofinal shaping and high cooling rates duringthe solidification stage of the processing cycle.

Optimal processing conditions, in terms ofpressure and cooling rate, may be defined by aprocessing window, as represented in Figure 19.The lower limit for the cooling rate is fixed byeconomic constraints, since long cycle times arecostly. Furthermore, if the polymer is exposedto excessively high temperatures over anextended time, thermal degradation mechan-isms become increasingly important (and unde-sirable). At high cooling rates, a considerablethermal gradient is imposed over the thicknessof the composite part, which leads to bothmorphological skin/core effects and thermalskin/core stresses. Relations between differentdefect mechanisms, often governed by the inter-nal stress build-up (voiding, microcracking)and cooling rates, have been observed. Anincreased forming pressure, corresponding tothe upper right boundary of the diagram, maysuppress this defect initiation. The formingpressure upper boundary will be governed bythe practical limit of forces applied by the pressforming equipment, as well as by the increasedcost for a mold capable of withstanding these

Figure 18 Typical processing cycle for a thermoplastic composite.

Figure 19 Generic processing window for thermo-plastic composite: pressure and cooling rate (after

MaÊ nson, 1994).

Process Windows 15

high forces. High forming pressures may alsocause damage to the fiber bed as well as leadingto resin starvation due to a high degree of resinbleed from the mechanically locked fiber bed.

The consolidation of thermoplastic matrixcomposites consists of three major steps, inti-mate contact, autohesion, and fiber impregna-tion, as shown in Figure 20. Intimate contact isrequired between the surfaces of successiveplies, which are initially rough with gaps con-sequently existing between the plies. Hence, theapplication of heat and pressure causes viscousdeformation of the contact points and thedegree of intimate contact is increased. Thesecond step occurs after the two surfaces havecome into contact with a bonding processoccurring at the interface. This is primarily anautohesion process where segments of macro-molecules diffuse and interpenetrate across theinterface, with the fracture stress of the bondincreasing proportionally to the fourth root oftime. The third step depends on the initialimpregnation state of the composite. Here,impregnation of the fiber bed is either fullyaccomplished or completed during the finalstages of compaction.

The morphology and mechanical propertiesof semicrystalline thermoplastic composites areinfluenced by the cooling conditions appliedduring the solidification stage of the processingcycle. An increase in cooling rate usually resultsin a lower degree of crystallinity, smaller crys-tallite sizes, a higher interlaminar fracturetoughness, and a lower transverse elastic mod-ulus (MaÊ nson et al., 1990; Ye and Friedrich,1993; Curtis et al., 1987; Berglund, 1987).

2.16.6.3 Impregnation

Darcy's law is commonly used to describe thefactors influencing impregnation of the fiber

bed by the matrix resin. A low polymer viscos-ity facilitates impregnation of the small-diameter (5±20 mm) reinforcing fibers at therequired fiber contents (generally higher than50% by volume). A factor often limiting awider use of thermoplastics in composites hasbeen their relatively high melt viscosity (typi-cally 50±2000 Pa.s) compared to conventionalthermosets (typically 1±50 Pa.s). Although ahigh resin molecular weight, and thus highviscosity, provides improved mechanical prop-erties, a low viscosity is desirable from a pro-cessing point of view. The influence of the resinviscosity on the impregnation rate can be de-monstrated by Darcy's law (see Chapter 2.17,this volume), describing laminar flow of fluidsthrough homogeneous porous media (Schei-degger, 1974). The form of Darcy's law for1-D flow is given as

v � edL

dt� ÿKp

ZqPqz

�1�

where v is the superficial velocity of the fluid, eis the porosity of the porous medium, L is theflow distance in the z direction, t is the time, Kp

is the permeability of the porous medium, Z isthe viscosity of the fluid, and P is the pressure.By integrating Equation (1) twice, and assum-ing that the permeability remains constant dur-ing infiltration of the fluid, the time necessaryto fully impregnate the porous medium can beestimated as

t � eZL2

2Kp�Pa ÿ Po� �2�

where Pa designates the applied pressure and Po

the atmospheric pressure. As can be seen fromEquation (2), a solution to overcome the pro-blem associated with the high viscosity inherentin thermoplastic resins is to minimize the flowdistance for impregnation, L. This can be

Figure 20 Consolidation steps for thermoplastic composites.


achieved through the utilization of preformsthat have an intimate mixing of matrix andreinforcement (e.g., powder-coated tows orcommingled yarns). Capillary pressure canalso play a role in the spontaneous wetting offibers.

2.16.6.4 Manufacturing Induced InternalStresses

Section 2.16.2 discussed the effects of manu-facturing induced defects on design knock-down factors. Processing conditions have adirect influence on the build-up of internalstresses in composites, and hence on the finalcomponent quality (Chapman et al., 1990;Lawrence et al., 1992; Eom, 1999). For exam-ple, rapid cooling of the composite surface atthe end of the processing cycle induces tempera-ture gradients through the part thickness, lead-ing to internal stress build-up fromheterogeneous and anisotropic shrinkage ofthe material. Nonisothermal solidification ofsemicrystalline thermoplastic composites also

gives rise to internal stresses due to crystallinitygradients. For thermoset composites, nonuni-form resin shrinkage is also a cause of internalstress. Due to changes in stiffness and thermalexpansion between plies, internal stresses mayalso be generated as a result of the chosenstacking sequence. On a different scale, internalstresses can result from the mismatch of ther-mal expansion between matrix and fiber.Figure 21 (MaÊ nson and Seferis, 1992) showsthe three different levels at which internal stres-ses may develop in a composite laminate.

Prediction and measurement of process--induced internal stresses are important sincethese stresses may have undesirable effects onthe performance of the end product, reducingstrength and initiating cracks or delaminations,thereby leading potentially to premature failureof the composite. Furthermore, internal stressesmay affect the dimensional stability of the partby generating postprocessing distortions. Post-treatments like annealing can be a solution tothe problem of relaxing internal stresses incomposite components, but the time and costsinvolved can be prohibitive. Methods proposedfor the experimental determination of internalstress fields in composites include the processsimulated laminate method (MaÊ nson andSeferis, 1987; MaÊ nson, 1992), the successivegrooving technique (Sunderland et al., 1995;Sunderland, 1997), and the embedding of fiberoptic sensors (Barret, 1995; Lawrence et al.,1996).

2.16.7 EMERGING TECHNIQUES

The previous sections have given an overviewof the interacting nature of material and pro-cess and have shown how these inter-relation-ships operate. Chapters 2.20, 2.21, 2.22, 2.23,2.24, 2.26, 2.27, 2.28, 2.29, and 2.30, this vol-ume give detailed descriptions of the individualtechniques. A selection of novel techniques arediscussed below which are opening interestingperspectives for the next generation of polymercomposite components.

2.16.7.1 Resin Infusion Processes (RIFT)

Hand lamination has offered a suitablemethod for producing large, low-volumeproducts, but has generally suffered frominconsistent mechanical properties, operatorvariability, and increasingly from legislationconcerning the permitted levels of volatile or-ganic compounds (VOC). An alternative tech-nique based on resin infusion (RIFT) uses a

Figure 21 Levels of internal stress in a cross-plylaminate exposed to high cooling rates: (a) fiber±matrix interface (heterogeneity); (b) interlaminar(anisotropy); (c) skin-core (nonisothermal solidifi-

cation) (after MaÊ nson, 1992).

Emerging Techniques 17

variant of the RTM process where a closedmold system is still used but with one toolface typically replaced by a flexible film. Emis-sions are therefore reduced, together with thecost of extraction and treatment. The capitalequipment required for the RIFT process issimply the mold tool and a vacuum pump.

The use of only one tool reduces costs andwith virtually no size limitations, large andsmall components, as well as complex, multi-dimensional reinforced parts, can be produced.Resin infusion processes can be used for avariety of thermoset matrices and most conven-tional woven or stitched fabrics. It offers theadvantages of relatively low tooling costs forhigh-performance components with more con-sistent properties than wet-laid components.Higher fiber volume fractions together with amore uniform microstructure and minimal voidcontent lead to improved mechanical perfor-mance compared with hand lamination.

To produce a component, the dry fabric ispositioned in the mold and enclosed in a va-cuum bag, with a peel ply and a flow mediumoften placed between the laminate and the bag-ging film. The vacuum removes air from the dryfiber bed (hence minimizing trapped air) andthen draws resin from a container (usually at

atmospheric pressure) into the reinforcement.The flow of resin into the carrier layer floodsone surface of the component and impregna-tion then proceeds by through-thickness flowmore or less simultaneously across the wholecomponent. Flow results only from the vacuumdrawn under the film and any gravity effects.The flow front in the reinforcement also pushesany residual air towards the vacuum port.

A typical application is shown in Figure 22(SCRIMP, 2000), with the process describedfurther in Chapter 2.21, this volume.

2.16.7.2 Engineered Preforms

With conventional textile processes, a rangeof fabrics, knits, and braids can be produced,but the possibilities of fiber arrangement tai-lored to the application are limited. Addition-ally, where liquid molding processes are used, afiber preform is needed that must be cut toshape. Hence a significant amount of wasteresults, which has a strong adverse effect onsystem economics. To overcome these limita-tions, new textile processes have been devel-oped, typified by a tailored fiber placement

Figure 22 Example of resin infusion processing.


system and a robotic fiber placement preform-ing system. The first of these addresses theability to place fibers in the desired orientationwhile the second focuses on eliminating wastewhile also offering a degree of tailored proper-ties. These are briefly reviewed as examples ofcurrent work in this field.

A special characteristic of fiber reinforcedcomposites is their anisotropic properties,where the maximum mechanical properties liealong the fiber direction. A textile process hasbeen developed (Gliesche, 1997) that uses tai-lored fiber placement to produce reinforcingstructures with stress field aligned fiber orienta-tions for the production of reinforcing struc-tures, thereby linking textile design to stressanalysis. The preform thus has locally varyingfiber orientations and quantities. This process isbased on embroidery technology, with the pro-cessing principle shown in Figure 23 (Gliesche,1997). A roving (e.g., carbon or glass fibers) isfixed using stitches on to a base fabric or non-woven material (typically a thin glass fabric).Between the stitches the base material can bemoved by numeric control in theX,Y direction.The roving is placed on the base material by zig-

zag stitches (often with a polyester needle yarn)either side of the roving. Figure 24 (Gliesche,1997) shows the working unit of the fiber place-ment machine. The main advantage, comparedto common textile technologies, is the ability toarrange reinforcing fibers in every direction ofthe reinforcing area from an angle of 08 to 3608.Accumulation of fibers can be achieved bystitching several times across the same area.

A robotic fiber placement preforming system(P4Ðprogrammable powder preform system)was developed to advance preform technologyfor the automotive industry to meet the lowcost and low scrap preforming requirementsfor high-volume liquid molding processes(50 k units per year) (Chavka and Dahl, 1999).The P4 system consists of three main processingsteps: glass deposition, consolidation, and pre-form demolding, illustrated schematically inFigure 25 (Owens Corning, 2000). The firststep consists of chopping glass roving which isapplied along with powdered thermoplasticbinder to a preform screen. Robotic control ofspray-up routines offers repeatability and partconsistency. Two choppers are used, one for thesurface veil (12mm fibers, deposition rate of

Figure 23 Fiber placement with stress field aligned fiber orientations (after Gliesche, 1997).

Figure 24 Working unit of fiber placement machine.


40±400 gmin71) and a second for structuralreinforcement (15±90mm fibers, depositionrate of 200±3000 gmin71). Positive airflowthrough the screen holds the chopped fiber onthe surface during the entire spray-up routine.The tool is then closed and the preform com-pressed to the desired thickness. Hot air is thendrawn through the screen in order to melt thebinder, after which ambient air is again drawnthrough the preform to freeze the binder. Thetool is opened and the finished preformdemolded. Net shape preforms, with minimalloft, are produced via robotic control of glassdeposition, essentially eliminating glass fiberoverspray and the need for preform trimmingoperations. The cost of textile processes is elimi-nated (i.e., weaving processes) and scrap reduc-tion also occurs. A degree of orientationcontrol is also possible through an orientingdevice in the cutter head. This process is parti-cularly suited to component geometries such apickup truck boxes, with the preforms impreg-nated by rapid liquid molding processes such asSRIM, with a typical preforming cycle of 4min.

2.16.7.3 Large Injection Molding Technology

Another example that has pushed the bound-aries of conventional process characterization isthe large injection molding body technology(LIMBT) program (see Chapter 2.30, this

volume) (Argeropoulos et al., 1999; http://www.paragondie.com/, 2000). In the past, thelogic for using compression molding based pro-cesses with materials like glass mat thermoplas-tics (GMT) (see Chapter 2.27, this volume) andsheet molding compound (SMC) (see Chapter2.22, this volume) was that the injection mold-ing facilities and tools required to cope with thehigh clamping and injection pressures becamecost prohibitive as size increased. SMCs andGMTs have therefore been used for large auto-motive subcomponents such as front-endstructures.

In an attempt to reduce the overall weight ofautomotive body structures while also reducingmanufacturing costs, DaimlerChrysler and itssupply chain have developed technology forproducing large short fiber reinforced injectionmolded body components in a process intendedfor volumes of greater than 100 000 bodies fromone set of tools and equipment. The use of aninjection molding based system offered a quick,accurate, and clean manufacturing process to-gether with ease of assembly. Four large bodycomponents bonded together, using 15% glassfiber reinforced PET, PP/PS, or PP/PMMApolymer, formed a semistructural body systemin four pieces as opposed to over 80 steel stamp-ings. A steel chassis was used as a structuralcarrier. The four components are cooled in fix-tures prior to adhesive bonding. In mold coat-ing techniques were investigated to try andeliminate conventional painting. An outcome

Figure 25 Robotic fiber placement preforming system (after Owens Corning, 2000).


was the CCV (Chrysler composite vehicle)shown in Figure 26 (Paragon, 2000; Argero-poulos et al., 1999). To produce the moldings,tooling weighing 145 152 kg (160 tons) per dieand capable of withstanding 1034 bar(15 000 psi) injection pressure was required. Ahot runner mold system with a 78.3MN(8800 ton) clamp pressure, two platen injectionmolding machine (Figure 27) (Husky, 2000)was required to produce components with anominal cycle time of 3min.

2.16.7.4 Integrated Processing

To further improve manufacturing flexibilityand to provide added value to composite andpolymer products, a novel integrated proces-sing methodology has been developed (MaÊ nsonet al., 1995; Bourban et al., 1998; Wakemanet al., 1999a, 1999b). This technique enables theintegration of several material transformationsteps, adding value through the process fromfiber impregnation to placement and final con-solidation of composite preforms, performed inan integrated manufacturing cell (Figure 28).With the integration of several polymers andreinforcement forms and their associated pro-

cesses, the properties and functions of a partcan thus be optimized. Synergy effects areachieved by combining flow molding andintrinsically stiff materials, possible by control-ling interfacial conditions such that a solid sub-strate can be overinjection or compressionmolded. Nonisothermal conditions betweenthe two subcomponents results in heat transferfrom the molten polymer to the solid substratewhere local melting of a thin layer occurs. Dur-ing the cooling phase of semicrystalline materi-als, crystallization occurs from the remainingcrystals around the newly created solid andliquid interface and propagates through theoriginal interface, such that full fracture ener-gies can be reached within standard moldingcycles (see Chapter 2.31, this volume).

Design stiffness and high functionality resultfrom the flow molding materials while the in-trinsic stiffness of aligned fiber structures pro-vides localized increases in properties. Forinstance, commingled yarns can be placed ro-botically in the part area where stiffness andstrength are required, while additional polymeris overinjected to provide complexity of shapeand geometrical stiffness, together with an im-proved surface finish. The UD fibers provideenhanced structural performance and creep re-sistance, with an example shown in Figure 29

Figure 26 Large injection molding technologyÐChrysler composite vehicle.


for a generic hook. Another example is thecombination of aligned fiber thermoplasticsheet stamping with overinjection molding,facilitating the combination of the intrinsicstiffness of sheet forming with the ability toinclude a high degree of integrated functionalityvia the overinjected polymer (Figure 30).Another flow molding material, GMT, hasbeen locally reinforced with robotically placedwire frame performs, avoiding the potentiallyinefficient use of structural materials (Wake-man et al., 1999a, 1999b) (see Chapter 2.27,this volume).

2.16.7.5 Reactive Thermoplastics

While thermosetting resins have been usedwidely in liquid molding processes, the highmelt viscosity of thermoplastics has requiredalternative conversion routes. However, liquidmolding processes have also been adapted toreactive thermoplastic systems (see Chapter2.28, this volume). Generally, thermoplasticshave viscosities ranging from 50 and 2000 Pa.sat processing temperatures, while monomersystems are of the order of mPa.s. Fiber bedimpregnation time is therefore reduced, with a

Figure 27 Injection molding cell for Chrysler composite vehicle.

Figure 28 Integrated processing manufacturing cell (after Wakeman, 1999b).


subsequent decrease in pressure and also thepotential to reduce cycle times. Impregnation ofthe fiber bed during the final processing step isfollowed by polymerization of the monomerand cooling of the final part.

Potential systems include ªCyclicsº technol-ogy and anionically polymerized laurolactam.ªCyclicsº technology (http://www.cyclics.com/,2000) uses a low-viscosity thermoplastic resin(prepolymer) that reacts in the presence of heatand a catalyst to increase its molecular weightvia a ring-opening polymerization reaction.This technology has been demonstrated in thepast for polybutylene terephthalate (PBT) andpolycarbonates. More recent work on cyclicesters, that have properties similar to PBT,has been performed in the USA. Reactive pro-cessing for polyamide 12 has been patented by

EMS Chemie (Leimbacher and Schmid, 1998)which uses anionically polymerized laurolac-tam with an optional powder coating stage toproduce a fiber reinforced part. This differsfrom traditional anionic polymerization wheretwo volumes of lactam preblended with catalystand activator are combined, but which have ashort pot life and slowly polymerize in thetanks. To overcome this problem, a liquid acti-vator system was developed containing bothactivator and catalyst (Figure 31) (EMS Che-mie, 2000). Both the liquid activator and themolten laurolactam can be stored indefinitely.A processing window for this material systemfor RTM is shown in Figure 32 (EMS Chemie,2000). This is bound by the increase of viscosityduring polymerization at high temperatures(limiting impregnation), equipment limitations

Figure 29 Integrated processing of unidirectional tow with overinjection molding.

Figure 30 Integrated processing of thermoplastic prepreg sheet and overinjection molding.


regarding flow rates, monomer melting pointand long cycle times at lower temperatures, andthe elimination of porosity.

2.16.7.6 Robotic Lay-up of Thermoset Tows

Predominately in the aerospace industry,where large often planar structures are pro-duced, parts were initially made by hand layingprepregs into a tool followed by final autoclave

consolidation and cure (see Chapter 2.20, thisvolume). Such procedures required highlyskilled operators to ensure that the high qualityrequired together with the accurate and consis-tent fiber orientations needed were maintained.

Technology developed in the 1970s auto-mated the placement of generally carbon/epoxy prepreg using large gantry systems witha robotic head moving over the part tool. Thehigh machine cost limited applications to theaerospace industry, with, for example, lay-up ofskins of the F-16 horizontal stabilizer in 1978.

Figure 31 Liquid molding of thermoplasticsÐreactive processing of polyamide 12 (after EMS Chemie,2000).

Figure 32 Illustrative processing window: thermoplastic RTM of polyamide 12 (after EMS Chemie, 2000).


This system laid-up tape on a flat surface withsubsequent transfer to the final contoured tooland hand draping to the mold geometry. Laterdevelopments have improved the process, withan example of a CNC tape laying system for flatand low contour structures shown in Figure 33(Cincinnati, 2000). Such 10 axis gantry-typetape laying machines, for example, to be usedfor Eurofighter Typhoon production, enableheating and placement of thermoset tapes75±150mm wide along curved paths, with12.8m of X-axis longitudinal travel and 4.2mof Y-axis traverse. Geometries with rise and fallangles of up to 158 are possible. Once the tape islaid, cutters shear the end of the tape to the partoutline. The prepreg backing material is alsoremoved as the head traverses the tool surface.

Filament winding is capable of placing fibersprecisely and dramatically reducing the manu-facturing cost of large complex-shaped thermo-set composite parts in comparison with handlay-up. However, this process is generally lim-ited to shapes of revolution, requires a mandrel,and has a nominally constant wall thicknessand is sometimes problematic due to fiber ten-sions created by the winding head. Recent ef-forts have been made to develop a machine thatcombines the features of automated tape lami-nation with filament winding. The followingadditional features were also required (Benja-min, 1998): ability to position and compact

continuous fibers on a wide range of largecomplex shapes, reduced fiber tension on arotating mandrel, ability to vary fiber band-width, high fiber placement speeds, accuratefiber placement, modular design, and the abilityto heat or cool the fibers to control tack.

Cincinnati (2000) has developed a seven-axisfiber placement system for highly contouredsystems, such as cowls, ducts, fan blades, fuse-lage sections, pressure tanks, nozzle cones,spars, and ªCº channels. This combines theadvantages of tape laying and filament windingwith computer control to break down conven-tional boundaries of the two processes. Up tothirty-two 3.2mm wide tows can be indepen-dently dispensed, clamped, cut, and restartedfor lay-up of curved, convex, concave, or com-pound contoured surfaces. Using 32 tows, alay-up bandwidth of 101mm is achieved. Abuilt-in compaction roller in the process lami-nates the tows onto the lay-up surface. Due tovarying tack properties between different com-posite materials systems, the fiber placementsystem head is able to cool the tow duringcut/clamp and restart applications and also toapply heat during lay down to improve lamina-tion and surface consolidation. Such systemsare also claimed to reduce scrap by up to 65%.

Tows can be placed at up to 45m per min(machine dependent), with an accuracy of+1.3mm, offering productivity and quality

Figure 33 Gantry type automated prepreg placement machine.


improvements over manual placement. TheCincinnati Viper 3000 system has 16m of hor-izontal Z- axis and 5m of X-axis travel. Amandrel rotates on a horizontal axis with partdiameters of 5.5m possible. Monolithic one-piece structures can be produced, reducingparts counts compared with traditional multi-part assemblies and also offering the potentialfor reduced overall cost. An example of a heli-copter body section in Figure 34 (Cincinnati,2000) shows this capability, where fibers areplaced at optimum angles. Raytheon are pro-ducing all-composite fuselage structures inthree main sections using such technology.

2.16.8 LIFE CYCLE ENGINEERING,COST AND IMPLEMENTATION

2.16.8.1 Life Cycle Engineering

Growing environmental concern coupledwith a responsible attitude to material use re-quires a life cycle plan (which Chapter 2.33, thisvolume addresses in detail) for material andprocess systems, particularly for high volumeor mass applications. Conservation of the rawmaterial value must be considered, as well asthe impact of the material on the environment

during manufacturing, service, and disposal.Figure 35 presents a schematic illustration ofthe change in value of a polymer compositeduring its life cycle. It is not only service thatcauses a loss in the restorable value of thematerial, but also improperly selected materialcombinations and/or processing conditions.For example, painting of a polymer part oftenlimits successful recovery of the material, sinceseparation becomes difficult. The problem isexuberated for thermoset polymers, where recy-cling the polymer as a matrix material is limitedby nonreversible cross-links. For compositesincluding continuous fibers, maintaining fiberlength while also recovering the matrix materialis a difficult task. Thermoplastics offer anadvantage here, but maintaining fiber lengthsis problematic. Several techniques exist basedon tertiary recycling (recovery of the monomer)which in the future could open up opportunitiesfor recovering the polymer and the fibers asindividual constituents.

Initially driven by performance, life-cycleconsiderations are increasingly becoming prom-inent features in the design of composite-basedproducts, with a gradual increase of recyclingefforts, and growing interest for durability ana-lyses. The issues of loop-closing, resource effi-ciency, waste reduction, and life-extension are

Figure 34 Automated prepreg placement machine for contoured geometries.


to be seen as many facets of the life-cycleengineering concept, developed as an integratedmethod to design, manufacture, use, andrecover materials and products for optimalresources turnover. Effort is still required tolink product design to composite science andtechnology, and to environmental science:ªrecyclableº does not necessarily implyªrecycled,º and ªrecycledº does not necessarilyimply ªenvironment-friendly.º

The wide spectrum of polymer compositeapplications implies that an optimal loop-clos-ing strategy has to be defined for each particu-lar case. Considerable efforts have beendevoted to improve durability analyses, andalso to develop reliable recycled and renewablecomposites. In the latter two fields, these effortshave been accompanied by the development ofnumerous eco-design tools. Issues such as ma-terial quality, reliability, and durability,whether for virgin or recycled composites,should be enhanced in eco-design practices,similar to life-cycle assessment. Such integra-tion in the design stage would eventually offer asound framework within which more durablecomposite materials may be developed.

As an example of how composite materialscan offer a potential lower life-cycle cost, theuse of carbon fiber based composites in theautomotive industry can be briefly examined.If a reduction in vehicle mass (specifically body-in-white) could be achieved through novelmaterials and processing technology, then acorresponding reduction in engine powerwould occur (needing lighter and smaller drivesystems) thereby reducing the degree of struc-ture required to hold the engine and hence thetotal cost of lightweight materials, as illustratedin Figure 36. Carbon fiber composites, consid-ered by several OEMs, have shown a 67%weight reduction over steel body-in-white struc-

tures, while a study by the Rocky MountainInstitute (RMI, 2000; Hypercar, 2000) showeda 585 kg vehicle yielding a fuel consumption of0.8±2 liters per 100 km, compared with 3 litersper 100 km for a VW Lupo variant (Whitworth,1998) and 8.3 liters per 100 km for a 2.0 literFord Scorpio (Harrison, 1997). Compositematerials would be highly synergetic in redu-cing vehicle energy use and pollutant produc-tion while increasing transportation efficiency.Recycling issues would become of fundamentalimportance here, and workable systems forpractically accomplishing this would be needed.

As a further example of improving transpor-tation system efficiency, synergy between fuelcell technology and composites can be consid-ered. Daimler Chrysler will have invested morethan $1.4 billion in fuel cell technology by thetime the first fuel cell vehicles come to market,about the same as an entire line of profitableChrysler vehicles (DaimlerChrysler, 2000). Thisfuel cell power train currently costs 10 timesthat of an equivalent internal combustion sys-tem; vehicle weight savings would reduce powertrain output and hence cost. Figure 36 thereforeshows how the use of advanced compositescould offer a cost-effective solution.

2.16.8.2 Cost Modeling

To confirm the potential of any innovativeprocessing route for cost-effective manufactur-ing of composite components, comparative costanalyses must be conducted, which requires theuse of a proper cost estimation methodologyand the availability of reliable cost information.Several models and approaches evaluatingmanufacturing costs for composite productshave been presented in the literature (Bernet

Figure 35 Life cycle engineering of composites and polymersÐchange in value of a polymer compositeduring its life cycle (after MaÊ nson, 1994).

Life Cycle Engineering, Cost and Implementation 27

et al., 2000). These can be summarized as com-parative techniques (Silverman and Forbes,1990; Walls and Crawford, 1995; Bader,1997), process-oriented cost models (Gutowskiet al., 1991; Karbhari and Jones, 1992; Mayeret al., 1997), parametric cost models (Foley andBernardon, 1990; Wang and Gutowski, 1990;Gutowski et al., 1994), and process flow simu-lations (Lee and Hahn, 1995; Jones, 1997; Liet al., 1997; Olofsson and Edlund, 1998; Evanset al., 1998; Kendall et al., 1998). An under-standing of how these techniques work and oftheir general suitability is of importance forrealistically modeling a particular process.Comparative techniques rely on historicaldata for a similar part using a standard manu-facturing process. As a result, comparative costestimating techniques are generally unsuited tonovel materials and processes. Unlike compara-tive techniques, process-oriented cost modelsare adaptable to new processes. Furthermore,these models enable identification and quanti-fication of part cost drivers, and are sensitive toimprovements in manufacturing processes cur-rently under development. Nevertheless, theyrequire an exhaustive understanding of themanufacturing process and a detailed definitionof the part. Parametric models, often supportedby computer-based spreadsheet programs, havethe advantage of offering great flexibility inmaking changes to the model, as well as allow-ing easy manipulation of process and economicfactors for sensitivity studies. However, a lim-itation of the parametric model approach isrelated to the assumption that each step in themanufacturing process operates independentlyfrom another, such that large buffers exist be-tween the steps (different manufacturing loca-tions, batch processing, etc.). This discrepancy

with reality often results in an underestimationof the manufacturing cost (Kendall et al., 1998).The obstacles encountered with the parametricmodeling technique are overcome by processflow simulations, which simulate the dynamicsof a manufacturing process with a repre-sentation of the interactions between the differ-ent manufacturing operations. Process flowsimulations include commercial codes such asExtend2, Profit Cue2, or Witness2. Benefitsderived from implementing a process flowsimulation model include the ability to predictthe cycle time and the capacity of the process, inaddition to the manufacturing cost. Thesesimulation-based cost forecasting tools arealso suitable for process improvement studies,identifying bottlenecks, and achieving a betterdistribution of personnel and raw materialson the shop floor. Where Six-Sigma qualitymethods are incorporated into the model, anestimate of repair costs resulting from manu-facturing defects can also be achieved.

2.16.8.3 Composites ImplementationÐReasonfor Change

Going beyond a cost model to assess theimplementation of composite materials into aparticular industry or application is a logicalstep towards using the material and processsystem to its full advantage. While the advan-tages of a composite solution to a problem maybe obvious or well established, the commercialmotivation and rationale including assessmentof hidden costs and risk is a complex interactingissue. The potential benefits that a new compo-site material and processing technology might

Figure 36 Driving forces for weight reduction in automotive applications (after Wakeman, 1999b).


offer compared with other solutions can beexamined on a cost basis by determining theratio of advantages over costs (MaÊ nson et al.,1999):

RFC � DadvDcost

�3�

where RFC is reason for change, Dadv is theadvantage variation between the existing andthe suggested technology or part, and Dcost isthe associated cost variation. In order to makeRFC an efficient decision tool for economicand technical feasibility, advantages and costsneed to be considered. For example, a lowertotal manufacturing cost may be the objectiveor increased properties may be desired togetherwith a longer product life. The advantages willdepend on the application but include mechan-ical performance, added functionality, processsimplification, and parts count reduction. Costsinclude both direct costs together with produc-tion reorganization, logistics, and personnel.

The advantage/cost ratio is consideredagainst the risk of introducing the particulartechnology and here the increased advantagesor performance must outweigh the costs for thegiven component, but only if the risk is accep-table. Many of the costs associated with intro-ducing a new technology are hidden but mustbe part of the consideration. Quantification ofsuch parameters remains a challenge and toolsare needed to enable a decision to be maderegarding the readiness of a particular technol-ogy, where the advantages may be evident, butthe risk will reduce with time or a certain levelof investment. Risk would ideally be mini-mized, but with competitive companies work-ing in similar fields, risk needs to be assessed sothat the technology can be introduced atthe correct time, balancing the compositesystem advantages with risk and marketcompetitiveness.

Advantages, cost, and risk need to be con-sidered not only from the technology sphere(including mechanical performance, processa-bility, durability and reliability, quality assur-ance, life cycle performance, etc.), but also fromthe cost (including raw materials, tooling, pro-ductions, labor, logistics, etc.) and strategyspheres (including engineering base, customersegments, technology updating, production andassembly location, supplier relations, etc.).Hence, a desire to use composites in a newapplication will have wide-ranging implica-tions. These are beyond the choice of the ma-terial and process system examined from aconventional technical cost modeling ap-proach, but reach into the strategy sphere.Here, a whole new supply base together with

new production facilities and a plant locationmay be required, together with different per-sonnel and a new engineering skill base.

Where a supply chain exists for a product,the RFC should be positive for all members sothat the supply chain can work together tominimize the total manufacturing cost as awhole while building the health of each com-pany involved. The RFC for different suppliersin the chain will have different advantages, cost,and risk depending on the role, service, processstep, component, or subcomponent that theyare involved in.

As part of maximizing the success of imple-menting a composite component, an approachis needed which considers the interacting natureof design, material, manufacture, and econom-ics. This approach in itself helps to reduce ormanage risk. A methodology suited to compo-site materials consists of several steps:

(i) identification of unique advantages (cost/performance/functionality),

(ii) full support of design team,(iii) concurrent engineering approach (pro-

duction),(iv) full use of specific advantages,(v) replacement of application ripe for

change,(vi) check: are unique advantages not avail-

able at the same price with competition?Due to the fact that composite part perfor-

mance, cost, processing, and the material sys-tem are so highly interlocked, the above list canhelp to approach composite part design andprocess selection with the global perspectivenecessary to maximize the product's ownchance of profitable implementation.

ACKNOWLEDGMENTS

The authors wish to thank and acknowledgethe work, advice, and feedback of their collea-gues at the Laboratoire de Technologie desComposites et PolymeÁ res (LTC) that hasmade this chapter possible. They wish to ex-press their particular thanks to Paul Sunder-land (dimensional stability), Yves Leterrier (lifecycle engineering), Nicolas Weibel (RFC), andP.-E. Bourban (reactive thermoplastics) fortheir willing information exchange.

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