ER PT
A
CHRapid-Prototypin
g
Processes and
Operations
° This chapter describes the technologies associated with rapid prototyping,
sharingthe characteristics of computer integration,production withoutthe use of traditionaltools and dies, and the ability torapidly produce asingle part on demand; they all have the basic characteristics of producing individual parts
layer bylayer.
° The chapter discusses the (nonmetallic and metallic) materials used in rapid prototyping and describes the commercially important rapid-prototyping
technologies.
U These processes includefused-deposition modeling, stereolithography, multijet
modeling, polyjet modeling, three-dimensional printing, and selective laser sintering.
° The chapterends with a description of the revolutionary practiceof applying
rapid-prototypingtechniques tothe production oftooling (rapid tooling) that
can be used in othermanufacturingprocesses.
Typicalparts made: A widevariety ofmetallic and nonmetallic partsfor product
design analysis, evaluation and finished products.
Alternative processes: Machining,casting, molding, and fabricating.
20.l
Introduction
In the development of anew product, there is invariably a need to produce a single example, or prototype, ofa designed part or systembefore allocatinglarge amounts of capital to new production facilitiesor assembly lines. The main reasons for this need are that the capital cost is very highand production tooling takes considerable time toprepare. Consequently, aworking prototype is needed for design evaluation
and troubleshooting before a complex product or system is ready to be produced
and marketed.
A typical product development process was outlined in Fig. 1.3 in the General
Introduction. Aniterative processnaturally occurswhen (a)errors are discovered or
(b) more efficient or better design solutions are gleaned from the study of an earlier
generation prototype. The main problem with this approach, however, is that the
20.l Introduction 525 20.2 Subtractive Processes 528 20.3 AdditiveProcesses 530 20.4 VirtualPrototyping 54| 20.5 Direct Manufacturing
and RapidTooling 542
EXAMPLES: 20.1 Functional Rapid Prototyping 526 20.2 CoffeemakerDesign 534 20.3 ProductionofSecond Life®Avatars 537
20.4 Fuselage Fitting for
Helicopters 538 20.5 CastingofPlumbing Fixtures 547 CASESTUDY: 20.l lnvisa|ign®Orthodontic Aligners 543 525
2 Chapter20 Rapid-Prototyping Processes and Operations
(H) (D) (C)
FIGURE 20.| Examples of parts madeby rapid-prototypingprocesses: (a) selection of parts
from fused-deposition modeling;(b)stereolithographymodelofcellular phone; and (c)selection
of parts from three-dimensional printing. Source: (a) Courtesy of Stratasys, Inc., (b) and
(c) Courtesyof 3DSystems, Inc.
productionof aprototypecanbe extremely time consuming. Tooling can take several months to prepare, andthe production of a singlecomplicated part by conventional
manufacturing operations can be very difficult. Furthermore, during the time that a
prototypeis beingprepared,facilitiesand staffstillgeneratecosts.
An even more important concern is the speed with which a product flows
from concept to a marketable item. In acompetitive marketplace, it is well known that products that are introduced before those of their competitors generally are more profitable and enjoy a larger share of the market. At the same time, there are important concerns regarding the production of high-quality products. For these reasons, there is a concerted effort to bring high-quality products to market
quickly.
A technologythat speeds up the iterativeproduct-developmentprocess consid-erably is the concept and practice of rapid prototyping
(RP)-also
called desktopmanufacturing, digital manufacturing, or solid free-form fabrication. Examples of
rapid-prototyped parts are shown inFig. 20.1.
EXAMPLE20.l Functional Rapid Prototyping
Toys are examples of mass-produced products that
have universal appeal. However, some toysare actu-ally quite complex, and the function of a computer-aided design (CAD) cannot be ensured until
prototypes are produced. Figure 20.2 shows a CAD model and a rapid-prototyped version of a water
squirt gun (Super Soaker Power Pack Back Packm
water gun), which was producedonafused-deposition modeling machine. Each component was produced separately and assembled into the squirt gun, andthe
prototype could actually hold and squirt water. The alternative would betoproduce components onCNC
millingmachinesorfabricatetheminanotherfashion,
butthis canbedone onlyat much higher cost.
By producing a prototype, interference issues and assembly problems can be assessed and corrected if necessary. Furthe; fromanaesthetic standpoint,the elaborate decorations on such a toy can be more effectivelyevaluated froma prototype than on aCAD
file and can beadjusted to improve the appeal of the
toy. Each component, having its design verified, then has its associated tooling produced, with better certainty that the tooling as ordered will produce the parts desired.
Section20.1 Introduction 52
(H) (D)
FIGURE 20.2 Rapid prototyping ofaSuper Soakerm squirtgun. (a)Fully functionaltoyproduced through
fused-depositionmodeling; (b) original CADdescription. Source:Courtesy of Rapid Models and Prototypes, Inc., and
Stratasys, Inc.
Developments in rapid prototyping began in the mid-1980s. The advantages of this technology include the following:
° Physical models of parts produced from CADdata files can be manufactured
in a matter of hours and allow the rapidevaluation of manufacturability and design effectiveness. Inthis way, rapid prototyping serves as animportant tool forvisualization and for concept verification.
° With suitable materials,the prototype can beused in subsequentmanufacturing operations to produce the final parts. Sometimes called direct prototyping, this
approachcan serve as an importantmanufacturing technology.
° Rapid-prototyping operations can be usedin some applications to produce
actu-altooling formanufacturing operations (rapid tooling, see Section20.5.1). Thus, one canobtain toolingin amatterofa fewdays.
Rapid-prototyping processes can be classified into three major groups: subtractive, additive, and virtual. As the names imply, subtractive processes involve material removal fromaworkpiecethatis largerthan the finalpart.Additive processes
buildupapartbyadding material incrementally to produce thepart. Virtualprocesses
use advanced computer-based visualization technologies.
Almost allmaterialscan be used through one or more rapid-prototyping oper-ations, as outlined inTable 20.1. However, because their properties are more suit-able for these operations, polymers are the workpiece material most commonly used today, followed by metals and ceramics. Still, new processes are being intro-duced continually. The more common materials used in rapid-prototyping opera-tions are summarized in Table 20.2. This chapter is intended toserve as a general
introduction to the most common rapid-prototyping operations, describe their ad-vantages and limitations, and explore the present and future applications of these processes.
528 Chapter20 Rapid-Prototyping Processes and Operations
TABLE 20.l
CharacteristicsofAdditive Rapid-prototyping Technologies
Layercreation Type of
Process Supplyphase technique phase change Materials
Stereolithography Liquid Liquid layercuring Photopolymerization Photopolymers (acrylates,
epoxies, colorable
resins, and filledresins)
Multijet/polyjet Liquid Liquidlayercuring Photopolymerization Photopolymers
modeling
Fused-deposition Liquid Extrusion ofmelted Solidification by Polymers (such as ABS,
modeling polymer cooling polycarbonate, and
polysulfone)
Ballistic-particle Liquid Droplet deposition Solidification by Polymers andwax
manufacturing cooling
Three-dimensional Powder Binder-droplet No phase change Ceramic, polymer,
printing depositiononto metalpowder, and
powderlayer sand
Selective laser sintering Powder Layer ofpowder Sinteringormelting Polymers, metals with
binder, metals, ceramics
and sandwith binder Electron-beammelting Powder Layerofpowder Melting Titanium and titanium
alloys,cobaltchrome
Laminated-object Solid Depositionofsheet No phase change Paper and polymers
manufacturing material
20.2
Subtractive
Processes
Makinga prototypetraditionallyhas involveda series ofprocesses using avarietyof tooling and machines, andit usually takes anywhere fromweeksto months, depend-ing on part complexity and size. This approach requires skilled operators using
material removal by machiningand Hnis/cling operations (as described in detail in
Part
IV)-one
byone-until
the prototype is completed. To speed the process,subtractive processes increasingly use computer-based technologies such as the following:
° Computer-based draftingpackages, which can produce three-dimensional
rep-resentationsofparts.
° Interpretation software, which can translate the CADfile into a format usable bymanufacturing software.
° Manufacturing software, which is capable of planning the operations required to producethe desiredshape.
° Computer-numerical-control(CNC) machinery with thecapabilities necessaryto
produce the parts.
When aprototype is required only for the purposeof shape verification,a soft
material(usuallya polymeror awax) is used as the workpieceinorder to reduce or
avoid any machining difficulties. Thematerialintended forusein the actual
applica-tionalso canbe machined, but this operationmay bemore time consuming, depend-ing on the machinability of the material. Depending on part complexity and
Section20.2 Subtractive Processes 529
TABLE 20.2
Mechanical PropertiesofSelected Materials for Rapid Prototyping
Tensilestrength Elastic modulus Elongationin
Process Material (MPa) (GPa) 50mm (%) Notes
Stereo- Somos 7120a 63 2.59 2.3-4.1 Transparent amber; good
general-lithography purpose materialforrapid
prototyping
Somos 9120a 32 1.14-1.55 15-25 Transparent amber; goodchemical
resistance;goodfatigueproperties;
usedforproducing patternsinrubber
molding
WaterClear Ultra 56 2.9 6-9 Optically clear resin withABS-like
properties
WaterShed 11122 47.1-53.6 2.65-2.88 3.3-3.5 Optically clear withaslight green
tinge; mechanicalpropertiessimilarto
those ofABS;usedfor rapid tooling
DMX-SL 100 32 2.2-2.6 12-28 Opaque beige; goodgeneral-purpose
material forrapid prototyping
Polyjet FC720 60.3 2.87 20 Transparentamber; good impact
strength, good paint adsorption and
machinability
FC830 49.8 2.49 20 White,blue, or black;good humidity
resistance;suitable for
general-purpose applications
FC930 1.4 0.185 218 Semiopaque,gray, or black; highly
flexiblematerialused forprototyping
ofsoftpolymersorrubber
Fused- Polycarbonate 52 2.0 3 White;high-strengthpolymer
suit-deposition able forrapid prototyping and
modeling generaluse
ABS-M30i 36 2.4 4 Availablein multiplecolors, most
commonly white;a strong and
durable material suitableforgeneral
use; biocompatible
PC 68 2.28 4.8 White; good combinationof
mechan-icalproperties and heatresistance
Selectivelaser DuraformPA 43 1.6 14 White; produces durable heat-and
sintering chemical-resistant parts; suitablefor
snap-fitassemblies and sandcasting
orsiliconetooling
Duraform GF 27 4.0 1.4 White;glass-filledform ofDuraform
PAhasincreasedstiffnessandis
suitableforhighertemperature
applications
SOMOS 201
-
0.015 110 Multiple colors available;mimicsmechanicalproperties ofrubber
ST-100c 305 137 10 Bronze-infiltrated steelpowder
Electron- Ti-6Al-4V 970-1030 120 12-16 Can beheat treated byHIPtoobtain
beam up to 600-MPafatigue strength
melting
machining capabilities, prototypes can be produced in a few days to a few weeks. Subtractive systems can take many forms; theyare similar inapproachto the manu-facturing cells described in Section 39.2. Operators may or may not be involved, although the handling ofparts isusually ahumantask.
0 Chapter20 Rapid-Prototyping Processes andOperations
20.3
Additive
Processes
Additive rapid-prototyping operations all build parts inlayers, and as summarized
in Table 20.1, they consist of stereolitliograpliy, Multiiet/polyiet modeling, fused-deposition modeling, ballistic-particle manufacturing, three-dimensional printing, selective laser sintering, electron-beam and laminated-object manufacturing. In order tovisualize the methodology used, it is beneficial to think of constructing a
loaf of bread bystacking and bonding individual slices on top of each other. All of the processes described in this section buildparts slice by slice. The main difference between the various additive processes lies inthe method of producing the individ-ual slices, which are typically 0.1 to 0.5 mm thick and can be thicker for some systems.
All additive operations require elaborate software. As an example, note the solid part shown in Fig. 20.3a. The first step is to obtaina CAD file description of the part. The computer then constructs slices of the three-dimensional part
(Fig. 20.3b). Each sliceis analyzed separately, and a set of instructions is compiled in
order to providethe rapid-prototyping machine with detailed informationregarding the manufacture ofthe part. Fig. 20.3d shows thepaths of the extruder in one slice, usingthe fused-deposition-modeling operationdescribed inSection 20.3.1.
This approach requires operator input in the setup of the proper computer
files and in the initiation of the production process. Following that stage, the
machines generally operateunattended and provide a rough part after a fewhours. Thepartis then subjected toa series ofmanual finishingoperations (suchas sanding
and painting) inorder to completethe rapid-prototyping process.
It should be recognized that the setup and finishing operations are very labor
intensive and that the production time is only a portion of the time required to
obtain a prototype. In general, however, additive processes are much faster than subtractive processes, taking as little as a few minutes to a few hours to produce a
part.
20.3.l
Fused-deposition Modeling
In the fused-deposition-modeling (FDM) process (Fig. 20.4), a gantry
robot-controlled extruder head movesintwo principal directions overa table,which canbe raised and loweredasneeded. A thermoplastic filament is extruded through the small
orifice of a heated die. The initial layer is placed on afoam foundation byextruding the filament ataconstant rate while the extruder head follows apredetermined path
(see Fig.20.3d). When the first layer is completed,the table is lowered so that
subse-quent layers can besuperimposed.
Occasionally, complicated parts are required, such as the one shown in Fig. 20.5a. This part is difficultto manufacture directly, because once the part has been constructed up to height a, the next slice would require the filament to be placed in a location where no material exists beneath tosupport it. The solution is
to extrude a support material separately fromthe modeling material, as shown in
Fig.20.5b.Note thatthe use of such support structuresallows all of the layersto be
supported by the material directly beneath them. The support materialis produced
with a less densefilament spacing on a layer, so it is weaker than the model
materi-al and can be broken off easily afterthe part is completed.
The layers in an FDM model are determined by the extrusion-die diameter, which typically ranges from 0.050 to 0.12 mm. This thickness represents the best achievable tolerancein the vertical direction. In the x-yplane, dimensional accura-cy can be as fine as 0.025
mm-as
long as a filament can be extruded into the fea-ture. A variety of polymers are available for different applications. Flat wire metalSection20.3 Additive Processes
é
fi
(H) (D) SideView ModelModel
'/2:
Su ort \yii
pp\
'Q
Z
,&»
ea/2
,~%v §§§!fé”'
/‘
/
Support A (C) (Cl)FIGURE 20.3 The computational steps in producing a stereolithography (STL) file.
(a) Three-dimensional descriptionof part. (b) Thepart is divided intoslices. (Only 1 in 10 is l d. (d) A set of tool directions is determined to
shown.) (c) Support material is p anne
manufacture each slice. Alsoshown is theextruder path at sectionA-Afrom(c) fora
fused-deposition-modeling operation.
deposition uses ametal Wireinstead of a polymer filament, butalso needs a laserto heat and bond the deposited Wire to build parts.
Closeexamination of an FDM-producedpart will indicatethat a stepped sur-face exists on oblique exterior planes. If this surface roughness is objectionable, a
heated tool can be used to smooth the surface, the surface can be hand sanded, or h' ). However, the over-acoating can be applied (oftenin the form of a polis mg Wax
d unless care is taken in these finishing
all tolerances are then compromise
32 Chapter20 Rapid-Prototyping Processes andOperations Thermoplastic filament z ,V X
Heated build head gi Efggfgdffde'
movesin
x-y
plane minutes--~___f~e;» Table "-.,,_A_ V' movesin z-direction Fixtureless Filamentsupply
2
(H) (D)FIGURE 20.4 (a) Schematic illustration of the fused-deposition-modeling process. (b) The
FDM 900mc, a fused-deposition-modeling machine. Source:Courtesy ofStratasys, Inc.
Ceiling within
Desired part Gussets Island an arch Ceiling
(H) (D) (C) (Ol) (G)
FIGURE 20.5 (a) A part with a protruding section that requires support material.
(b)-(e) Common support structures used in rapid-prototyping machines. Source: After P.F.
Jacobs, Rapid Prototypingca” Manufacturing: Fundamentals of Stereolithography. Society of
ManufacturingEngineers, 1992.
Although someFDMmachines can beobtained for around$20,000, others can cost as muchas$300,000. Themain differences between themare the maximumsize
of the parts that can be produced and the numbers and types of materials that can be used.
20.3.2
Stereolithography
A common rapid-prototyping
process-one
that actually was developed prior tofused-deposition
modeling-is
stereolit/aography (STL). This process (Fig. 20.6) isbased on the principle of curing (hardening) a liquid photopolymer into a specific shape. Avat containingamechanismwherebya platformcan be lowered and raised
is filled with a photocurable liquid-acrylate polymer. The liquid is a mixture of acrylic monomers, oligomers (polymer intermediates), and a photoinitiator (a
Section20.3 Additive Processes 533 At its highest position (depth a in Fig.20.6), a
shallow layer of liquid exists above the platform. A
lasergenerating an ultraviolet (UV) beam is focused
upon a selected surface area of the photopolymer
and then moved around inthe x-y plane. The beam \ I’
curesthat portion ofthe photopolymer (say,a ring- x
;
shaped portion) and thereby produces a solid body. V
The platform is then lowered sufficiently to cover
thecured polymerwith anotherlayer of liquid poly-mer, andthe sequence is repeated. The process is
re-peated until level b in Fig.20.6 is reached. Thus far, we havegenerated acylindrical partwith a constant
wall thickness. Note that the platform is now low-ered by a vertical distance ab.
UV lightsource Formed part Platform motion UV curable liquid Liquid surface c Platform
3
| |FIGURE 20.6 Schematic illustration ofthe stereolithography
At level b, the x-y movements of the beam
define a wider geometry, so we now have a flange-shapedportion thatis beingproduced overthe
previ-ously formed part. After the proper thickness of the liquid has been cured, the process is repeated, producing another cylindrical section between levels I9 and c.
Note that the surrounding liquid polymer is still fluid (because it has not been ex-posed tothe ultraviolet beam) andthat the part has been produced fromthe bottom up inindividual “slices.” The unused portion oftheliquid polymer can beused again to make anotherpart or another prototype.
Note that the term “stereolithography,” as used to describe this process, comes from the facts thatthe movements are three-dimensional and the process is
similar tolithography (see Section 28.7), in which the image to beprinted on aflat surface is inkreceptive and the blank areas are ink repellent. Note also that, like FDM, stereolithography can utilize a weaker support material. In
stereolithogra-phy, this support takes the form of perforated structures. After its completion,
the part is removed from the platform, blotted, and cleanedultrasonically and with
an alcohol bath. Then the support structure is removed, and the part is subjected
to a final curing cycle in an oven. The smallest tolerance that can be achieved
in stereolithography depends on the sharpness of the focus of the laser; typically,
it is around 0.0125mm. Oblique surfaces also can be ofvery high quality.
Solid parts can be produced by applying special laser-scanning patterns to speed up production. For example, by spacing scan lines instereolithography, vol-umes or pockets of uncured polymer can be formed within cured shells. When the
part is later placed in a postprocessing oven, the pockets cure and a solid
part forms. Similarly, parts that are to be investment cast will have a drainable
honeycomb structure which permits a significant fraction of the part to remain uncured.
Total cycle times in stereolithography range from a few hours to a
day-withoutpostprocessing such as sanding and painting. Depending on their capacity, the costof the machines is inthe range from $100,000 to $400,000. Thecost ofthe
liquid polymer is on the order from$80 per litre. The maximum part size that can
be produced is 0.5 >< 0.5 >< 0.6 m..
Stereolithography has been used with highly focused lasers to produce parts
with micrometer-sized features. Theuse of optics required to producesuch features necessitates thinner layers and lower volumetriccure rates. Whenstereolithography
is used to fabricate micromechanical systems (see Chapter 29), it is called
microstereolithography.
process.
Chapter20 Rapid-Prototyping Processes and Operations
20.3.3
Multijet/Polyjet Modeling
TheMultijetModeling (MJM) or Polyjetprocess is similar to inkjet printing, where
print heads deposit the photopolymeron the build tray.Ultraviolet bulbs, alongside the jets, immediately cure and hardeneach layer, thuseliminating the needfor any
postmodelingcuring that is needed instereolithography. The result is a smooth sur-face of thin layers as small as 16 ,um that can be handled immediately after the process is completed. Two differentmaterialsare used: One material is used for the
actual model, while asecond, gel-like resin is used for support, such asthese shown
inFig. 20.5. Each materialis simultaneously jetted and cured, layer bylayer. When
the model is completed, the support material is removed with an aqueous solution.
Build sizes are fairly large, withan envelope ofup to 500 >< 400 >< 200 mm. These
processes have capabilities similar to those of stereolithography and use similar resins (Table 20.2). The main advantages are the capabilities of avoiding part
cleanup and lengthy postprocess curing operations, and the much thinner layers produced, thus allowing for better resolution.
EXAMPLE202 Coffeemaker Design
Alessi Corporation is well known for its high-end kitchen products. Although itmakes products out of
a wide range of materials, it is best known for its highly polished stainless~steel designs. Anexample is
the Cupola coffeemaker,amarket favoritethatwas to be redesigned from the bottom up while preserving the general characteristics of the established design.
Alessi engineers used Multijet modeling to
produce prototypesofcomponents ofthecoffeemaker,
as shown in Fig. 20.7. The prototypes allowed engineers to evaluate the ease and security of mechanical assembly, but a significant effort was expended on the design of the coffeemaker’s lip in
order to optimize the pouring of coffee. A large number of lip prototypes were constructed and evaluated to obtain the most robust and aesthetically pleasing design. The ability to compare physical prototypes to the existing product was deemed essential to evaluatingthe designs. Afterafinal design was selectedfromthe numerous prototypes produced,
it was found that a 5-6-week time savings was achieved in product development. The time savings
Lszgzsi”Mig;
QT*
7 ,;::£l{£%l gif.1S??EZ????§Z2i“‘Vi
FIGURE 20.7 Coffeemaker prototypes produced through
Multijetmodeling and finalproduct. Source: CourtesyAlessi
Corporation and3D Systems,Inc.
translated into cost savings, as well as assuringtimely market launch ofthe redesignedproduct.
Source:CourtesyAlessiCorporationand3DSystems,Inc.
20.3.4
Selective
LaserSintering
Selective laser sintering (SLS) is a process based on the sintering of nonmetallic
or (less commonly) metallic powders selectively into an individual object. The
Section 20.3 AddntiveProcesses Galvanometers I I Sintering laser E Laser Optics gp
Environmental-§`
COHUOI Unit Process-control +*
computer C 'n~~ I'
Wh
-W
FIGURE 20.8 Schematic illustrationofthe selective-laser-sintering process.
Source:After C.Deckard andP.F. McClure.
chamber is equipped withtwo cylinders:
I. A powder-feed cylinder,which is raised incrementally to supply powder tothe
part-build cylinderthrough a roller mechanism.
2. A part-build cylinder, which is lowered incrementally as the part is being
formed.
First, a thin layer of powder is deposited in the part-build cylinder. Then a
laserbeamguided by aprocess-control computer using instructions generatedbythe three-dimensional CADprogram of the desired part is focused onthat layer, tracing and sinteringa particular cross section into a solid mass. Thepowder inother areas remains loose,yet it supports the sinteredportion. Anotherlayer ofpowder is then
deposited; this cycle is repeated again and again until the entire three-dimensional
part has been produced. The loose particles are shaken off, and the part is recov-ered. The part does not require further
curing-unless
it is a ceramic, which has tobe firedto develop strength.
A variety ofmaterials can be used inthis process, including polymers (such as
ABS, PVC, nylon, polyester, polystyrene, and epoxy), wax, metals, and ceramics with appropriate binders.It is mostcommon touse polymers because ofthe smaller,
less expensive, and lesscomplicated lasers required for sintering. With ceramics and metals, itis common to sinter only a polymerbinderthat has beenblended with the
ceramic or metal powders. The resultant part can becarefully sinteredin a furnace and infiltrated with another metal if desired.
20.3.5
Electron-beam
Melting
A process similar to selective laser sintering and electron-beam welding (see Section 30,6), electron-beam melting uses the energy source associated with an electron beam to melt titanium or cobalt-chrome powder to make metal
proto-types. The workpiece is produced in a vacuum; the part build size is limited to
Chapter20 Rapid-Prototyping Processes andOperations
from an energy standpoint (compared with 10-20% efficiency for selective laser sintering),so that the titanium powder is actually melted and fully dense parts can be produced. A volume build rateof upto 60 cm3/hr can beobtained, with individ-ual layer thicknesses of 0.05
0-0.200
mm. Hot isostatic pressing (Section 17.3.2) also can be performed on parts to improve their fatigue strength. Although applied mainly to titanium and cobalt-chrome to date, the process is being developed for stainless steels, aluminum, and copper alloys.20.3.6
Three-dimensional Printing
In the three-dimensional-printing (3DP) process, a printhead deposits an inorganic binder material onto a layer of polymer, ceramic, or metallic powder, as shown in
Fig.20.9. A piston supporting the powder bed is lowered incrementally, and with
each step, a layeris deposited and then fusedbythe binder.
Multijet modeling and polyjet processes (described in Section 2O.3.3) are sometimes referred to as three-dimensional printingapproaches, because they oper-atein a similar fashion to ink-jet printers but incorporate a third (thickness) direc-tion. However, three-dimensional printing is most commonly associated with
printing a binderonto powder.
Three-dimensional printingallowsconsiderable flexibility in the materials and binders used. Commonly used powder materials are blends ofpolymers and fibers,
foundry sand, and metals. Furthermore, sincemultiple binder printheadscan be
in-corporated into one machine, itis possible toproduce full-colorprototypes by hav-ing different-color binders (see Example 20.3). The effect is a three-dimensional analog toprinting photographs using three ink colors on an ink-jet printer.
A common part produced by 3DP from ceramic powder is a ceramic-casting shell (see Section 11.2.4),in which an aluminum-oxideor aluminum-silica powderis
fused with asilica binder. The molds have tobepostprocessed intwosteps: (1) curing at around 150°C and (2) firing at 1000° to 1500°C.
Theparts producedthroughthe 3DPprocess are somewhat porous and there-fore may lack strength. Three-dimensional printing of metal powders can also be
|1>
|_*
Holler mechanism,/-Powder
BinderI!!
_;;fi¢==Eli*
ET”
Elle
if ~;;2;;;,,.
1. Spread powder 2. Print layer 3. Piston movement
A..
4. Intermediatestage 5. Last layer printed 6. Finished part
FIGURE 20.9 Schematic illustrationofthe three-dimensional-printingprocess.
Binder deposition
Section20.3 Additive Processes 537
infiltrating metal, permeates into P/M
,
li/licrostructure detail \\\
is
powderQ
,§;!i§§*¥*
§¢
Bidderget-3d.§§e2§Z@»,
ii-eg;
e~.;,,;';.¥;a1; Metalpowder
¥s#;4¥éE;dEd§='
‘
*seed
artist’
Particlesare loosely sintered; infiltratedby
Binderis burned off lower-melting-pointmetal
(21) (bl (C)
FIGURE20.|0 Three-dimensional printingusing(a)part-build, (b) sinter,and (c) infiltration
steps to produce metal parts. (d) An example of a bronze-infiltrated stainless-steel part
produced through three-dimensional printing. Source: Courtesy of Kennametal Extrude Hone.
combined with sintering and metal infiltration (see Section 17.5) to produce fully
dense parts, using the sequence shown in Fig.20.10. Here, the part is produced as before by directing the binder onto powders. However, the build sequence is then
followed by sintering to burn off the binder and partially fuse the metal powders, just as in powder injection molding described in Section
17.33.
Common metals used in SDP are stainless steels, aluminum, and titanium. Infiltrating materials typi-cally arecopperand bronze, which provide goodheat-transfercapabilitiesas well as wear resistance. This approach represents an efficient strategyfor rapid tooling(seebelow).
In a related ballistic-particle manufacturing process, a stream of a material (such asplastic, ceramic, metal, orwax) is ejectedthrough a smallorifice at a surface (target) using an ink-jet type mechanism. A powder is not involved; the material deposited bythe ink-jet mechanism is usedto buildthe prototype.The ink-jet headis
guided by athree-axis robotto produce three-dimensional prototypes.
EXAMPLE20.3 Production ofSecond Life® Avatars
SecondLife® and World ofWarcraft® areexamplesof
virtual worlds accessed through a website and are enjoyed by millions of people worldwide. To
participate, users create an “avatar” thatdepictstheir
alter ego in the fictional world. Many modern computer games (such as Rock Band 2) also allow users to producevery detailed avatars, witha unique appearance and unique personalities. Avatars contain
38 Chapter20 Rapid-Prototyping Processes andOperations
(H) (D)
FIGURE 20.lI Rapid-prototypedversions of user-defined characters, or avatars, produced from geometric
descriptions withinpopularwebsitesor games.(a) SecondLife®avatar,asappearsona computerscreen (left)
and after printing (right); (b) anavatar known as “Wreker” fromWorld of\X/arcraft®.Source: Courtesy
Z Corporation, Figure Prints and Fabjectory,Inc.
three-dimensional geometry data that describes their appearance, which can be translated to a file format
suitable forrapid prototyping.
Avatars canbe printed infull colorto a 150-mm high figurine with Z-Corp SpectrumZ51O orZPrinter
EXAMPLE20.4 Fuselage Fitting for Helicopters
Sikorsky Aircraft Company needed to produce a
limited number of the fuselage fittings shown in Fig. 20.12a. Sikorsky wanted to produce the forging dies by means of three-dimensional-printing tech-nologies. A die was designed using the CAD part
description. Forging allowances were incorporated
and flashingaccommodated bythe die design. The dies were printed using a three~ dimensional printer produced by ProMetal and are shown in Fig. 20.12b. The dies were made by
producing 0.178-mm layers with stainless-steel
powderas the workpiecemedia. Thetotal time spent
in the 3DP machine was just under 45 hours. This
450three-dimensional printers (Fig2O.11). Users can order their avatar prototypes on the web, which are then printed and shipped tothe user within days.
was followed bycuring of the binder (10 hours, plus
5 hours for cooldown), sintering (40 hours, plus 17 hours forcooldown), and infiltration (27 hours, plus 15 hours for cooldown). The dies then were finished and positioned in a die holder, and the part
was forged in an 800-ton hydraulic press with a die
temperature of around 300°C. An as-forged part is
shown in Fig. 20.12c and requires trimming of the flash beforeitcan be used.Thedies were produced in
just over six
days-compared
withthe many months required for conventionaldie production.Section 20.3 Additive Processes 45 mm Flangethickness = 3 mm internal radii= 5mm External radii= 10 mm (6) |-|,_»_ r\r_s.___|,___.; (D) (C)
FIGURE 20.l2 Afittingrequired fora helicopterfuselage. (al CADrepresentation with added dimensions.
(b) Diesproduced by three-dimensional printing. (c)Final forged workpiece. Source:CourtesyofKennametal Extrude Hone.
20.3.7
Laminated-object Manufacturing
Lamination implies a laying dovvn of layers that are bonded adhesively to one another. Severalvariations of laminated-object manufacturing(LOM) are available. The simplest and least expensive versions of LOM involve using control software and vinyl cutters to produce the prototype. Vinyl cutters are simple CNC machines
that cut shapes from vinyl or paper sheets. Each sheet then has a number of layers and registrationholes, which allow proper alignment and placement onto a build fixture. Figure 20.13 illustrates the manufacture ofa prototype by laminated-object
manufacturing with manual assembly. Such LOM systems are highly economical and are popular in schools and universities because ofthe hands-on demonstration
of additive manufacturingand production of partsby layers.
LOM systems can be elaborate; the more advanced systemsuse layersofpaper or plastic with a heat-activated glue on one side to produce parts. The desired shapes are burned into the sheet with a laser, and the parts are built layer by layer
(Fig. 20.14). Onsome systems, the excess material must be removed manually once
the part is completed. Removal is simplified by programming the laser to burn
perforations in crisscrossed patterns. The resulting grid lines make the part appear
as if it had beenconstructed from gridded paper (With squares printed on it, similar
0
.,,»-""‘”'"il
/ “
(3) (D)
' ~ff%';§"2i
.3
""'h"'W""""QWWY""""""' if,f:r:¢§-wf .'..f::;:;. 53? »
#513555 22SSS2¥fSf~»--» »».m=»¢ ‘:.‘¢ '» ~
Wilfif ‘WSE ¥¥*fFi?¥?f> §€§§"$'=2szasaz2a z;a°;z2ee'§s22
¢;»¢f;~ ffm;
;>¢m;»=»~-ff ,f;¢:<.1.¢s,
,»»,»»»"`___ »»»,,_..`.`.,, W_`__`_»»_...._.
'P iliii
11 ‘¥’,'*Ei::$¥1i%1,§:€?il:f§5€¥§§§§`M13 $."§ $'.§5f*¢?£`»¢¢:,f?§?$3f§1:i§fli,335 'tx' '4
::;;:;,»~~:~~:~.:»¢~~~¢:fav :favfc1: :mmf,v==:e<::::::::;- :;::;:::::<¢;;;;g;;;;;;;;; §;;;;;
2E2€EF§i<%¢
’** “*" VP" Iliff "“;;§;§' il‘¢~` '~1;‘ §esa@2§i€22211e§si;ee¢;2="'z§§ss2siam;
.,.~ ~ V '»~'»»»»~~~~~~1»fff=¢¢f;;¢m=<<fff¢;=¢
.. ~ "
?§€?$iiééi§i€§i?Eii‘?Ei-Eiiiiiiiis .5:§?§5?i?5,?,E,.§..iz‘§§§.E?§€'iE'.§.i":iiE’:
it
,ffffmwin HM W X,
.£\x;;3%;;§s¢,,4,;izazgjiin ,K £3 1 :vi ¢L/ki,ww 522;i/;1
>, iam H M
mf* ’¥¢¢.‘§»» it ,¢°=>W~s1,§%¢;_» »@ ‘ @eez¢%3§2 a2.ff;§;¥sweee; gg,5,,,,;&,, -i mag, gaeasasisssz
=°f ¢:fss=§;;zea5zza%ssazef§e=:sz;@ ¢,~ ~<,=¥2=2222.§222f '¢;1¢=L==>;;me1mi;zz§sm¢ ffiwlifhé .it 2
to
~-~ .. .933- -, ,_.... ...M _,,
(C) (di
FIGURE 20.|3 Production of a prototype through laminated-object manufacturing
(a) Layers are obtainedfroma vinyl cutter; (b) layers are manually stackedto form thepart
(c) completed laminated assembly; (d) final part prototype. Source: Courtesy P.Barraclough
Boxford Ltd.
Laser
Optics
X-Ypositioning qi
device
Layer Oumne Laminating roller
andcrosshatch
wi d
p Sheet materialPartblock
V:
PlatformT@|<9'UP roll eaa_ Material
supply roil
(ai (b)
FIGURE 20.|4 (a) Schematic illustration of the laminated-object-manufacturing process
Section20.4 Virtual Prototypmg
20.3.8
Solid-ground
Curing
This process is unique in that entire slices of a part are manufactured at one time.
As a result, a large throughput is achieved, compared with that from other
rapid-prototyping processes. However, solid-groundcuring (SGC) is amongthe most ex-pensive processes; hence, its adoption has been much less common than that of
othertypes of rapid prototyping,and new machines are not available. BasicallY, the method consists of the following stePs:
I. Oncea slice is created bythe computer software, a maskof the slice is printed
ona glass sheet by an electrostaticprintingprocesssimilar to that used inlaser printers.A mask is required because the area of the slice where the solid
mate-rial is desired remainstransparent.
2. While the mask is being prepared, a thin layer of photoreactive polymer is
deposited on the work surfaceand is spread evenly.
3. The photomask is placed over the work surface, and an ultravioletfloodlight
is projected through the mask. Wherever the mask is clear, the light shines
throughtocure the polymer and causes the desiredslice to be hardened. 4. The unaffected resin (still liquid) is vacuumed offthe surface.
5. Water-soluble liquid wax is spread across the work area, filling the cavities
previously occupied bythe unexposed liquid polymer. Sincethe workpiece is
on a chilling plate and the workspace remains cool, the wax hardens quickly.
6. The layer is thenmilled to achieve the correct thickness and flatness.
7. This process is
repeated-layer
bylayer-until
the partis completed.Solid-ground curing has the advantage of a high production rate, because entire slices are produced atonce and two glass screens are used concurrently. That
is, while one mask is being used to expose the polymer, the next mask already is
being prepared, and it is ready as soonas the milling operationis completed.
20.3.9
Laser-engineered
Net Shaping
More recent developmentsinadditivemanufacturingprocessesinvolve theprinciple of usingalaserbeamto melt and deposit metalpowder or
wire-again,
layerbylayer-over a previouslymolten layer. The patterns of deposited layers are controlled by a
CADfile. Thisnear-net-shaping process is calledlaser-engineerednetshaping(LENS,
atrade name)andis based on the technologiesoflaserweldingand cladding. The heat
inputandcooling arecontrolled preciselytoobtain afavorablemicrostructure. The deposition process is carried out inside a closed area inan argon environ-ment to avoidtheadverseeffects ofoxidation (particularlyonaluminum). Itis suitable
fora wide varietyofmetals and specialtyalloys for thedirect manufacturingofparts, including fullydense tools andmolds. Also,it canbe used forrepairing thin and
deli-cate components. Thereare other, similarprocessing methods using lasers, including controlled-metal buildup(CMB)andprecision-metal deposition (PMD,atrade name).
20.4
Virtual Prototyping
Virtual prototyping is a purely software form of prototyping that uses advanced graphics and virtual-reality environments to allow designers to examine a part.In a
way, this technology is used by common, conventional CAD packages to render a
part sothatthe designer can observe and evaluateitas itis drawn. However,
Chapter20 Rapid-Prototyping Processes andOperations
The simplest forms of such systems use complex software and three-dimen-sional graphics routines to allow viewers to change the view of the parts on a
com-puter screen. More complicated versions will usevirtual-reality headgear andgloves with appropriate sensors to let the user observe a computer-generated prototype of the desired partin acompletely virtualenvironment.
Virtual prototyping has the advantage ofaffording an instantaneous rendering of parts forevaluation, but the more advanced systems are costly. Because familiar-ity with software interfaces is aprerequisite to their application, these systems have very steep learning curves.Furthermore, manymanufacturingand design
practition-ers prefer a physical prototype to evaluate, rather than a video-screen rendering. They often perceive virtual-reality prototypes to be inferior to mechanical
prototypes, even though designers debug as many or more errors in the virtual environment.
There have been some important examples of complicatedproducts produced without any physical prototype whatsoever (paperless design). Perhaps the best
known example is the Boeing 777 aircraft, for which mechanical fits and interfer-ences were evaluated on a CAD system and difficulties were corrected before the first production model wasmanufactured (seeSection 38.5).
20.5
Direct
Manufacturing
and
Rapid
Tooling
While extremely beneficial as a demonstration and visualization tool,
rapid-proto-typing processes also have been used as a manufacturingstep inproduction. There are two basic methodologies used:
I. Direct production of engineering metal, ceramic, and polymer components or
parts byrapid prototyping.
2. Production of tooling or patterns by rapid prototyping for use in various
manufacturing operations.
Not only are the polymer parts that can be obtained from various
rapid-prototyping operations useful for design evaluation and troubleshooting, but occa-sionally these processes can be used to manufacture parts
directly-referred
to as directmanufacturing. Thus, the component is generated directly to a near-net shape from a computer file containing part geometry. The main limitations to the wide-spread use of rapid prototyping for direct manufacturing, or rapid manufacturing, are as follows:° Raw-material costs are high, and the timerequired to produce each partis too long to be viablefor large production runs. However, there are many applica-tions in which production runs are small enough to justify direct
manufactur-ing through rapid-prototyping technologies.
° The long-term andconsistent performance of rapidlymanufactured parts (com-pared withthemoretraditional methodsofmanufacturingthem) maybe suspect, especiallywith respecttofatigue, wear, andlifecycle.
Much progress is being made to address theseconcerns to make rapid
manufactur-ing a more competitive and viable option in manufacturing. The future of these processes remains challenging and promising, especially in view of the fact that rapid manufacturing is now being regarded as amethod of producing a product on demand. Customerswill be able to order a particular part, which will be produced
Section20.5 DirectManufacturing and RapidTooling
CASE
STUDY
20.l
lnv|sal|gn®
Orthodontic
Allgners
Orthodontic braces have been available to straighten
teeth for more than 50 years. The braces involve metal, ceramic, or plastic brackets that are bonded adhesively to teeth with fixtures for attachment to a
wire, which then forces compliance on the teeth and straightens them to the desired shape within a few years. Conventional orthodontic braces are a well~
known and successful technique for ensuring long-term dental health. However, there are several
drawbacksto conventionalbraces, includingthe facts
that (a) they are aesthetically unappealing; (b) the sharp wires and brackets can be painful; (c) they trap
food leading topremature tooth decay; (d) brushing and flossing teeth are far more difficult and less effective with braces in place; and (e) certain foods must be avoided because they will damagethe braces. One solution is the Invisalign system, made by
AlignTechnology,Inc. Itconsists ofaseries ofaligners, each ofwhichthe person wears forapproximatelytwo weeks. Each aligner (see Fig. 20.15) consists of a
precisegeometrythat incrementally moves the teeth to the desired positions. Because the aligners can be removedfor eating, brushing, andflossing, mostofthe drawbacks of conventional braces are eliminated. Furthermore, since they are produced from a
transparent plastic, the aligners do notseriously affect the person’s appearance.
t.
at
W__5i.;c:;;;;; '\y,5j
0-~~=: ,... .e=*
The Invisalign product uses a combination of advanced technologies in the production process, shown in Fig. 20.16. The treatment begins with an
orthodontist or a general dentist creating a polymer impression of the patient’s teeth (Fig 20.16a). These impressionsthen are usedto createathree-dimensional CAD representationof the patient’s teeth, as shownin Fig. 20.16b. Proprietary computer~aided design software thenassistsin the developmentofa treatment strategyfor movingthe teethin an optimal manner.
Once the treating doctor has approved the
treatment plan and it has been developed, the
computer-based information is used to produce the aligners. This is done through a novel application of
stereolithography. Although a number of materials are available for stereolithography, they have a
characteristic yellow~brown shade to them and therefore are unsuitable for direct application as an
orthodontic product. Instead,the Alignprocessuses a
stereolithography machine that produces patterns
of the desired incremental positions of the teeth (Fig. 20.16c). A sheet of clear polymer is then
thermoformed (see Section 19.6) over these patterns
toproduce thealigners, which are sent to the treating orthodontist. With the doctor’s supervision, patients are instructedto change the next setofaligners every
two weeks.
(H) (D)
FIGURE 20.l5 (a) An aligner for orthodontic use, manufactured by a combination of
rapid tooling and thermoforming. (b) Comparison of conventional orthodontic braces
withthe use oftransparentaligners. Source: CourtesyofAlign Technology,Inc.
544 Chapter 20 Rapid-Prototyping Processes andOperations Z., a f Q Q, tt* =~ ,H 'ri .ww .. x, .,, ;_f `“` ;»»»4 'V-¢1, ». ‘5’~ Qt ;£¢ -~ ‘Z v _QQ “ "zz..; _ N -:K #ag..J §~., ;W,--,,_‘ ~*' ~a~ . ea-~ QW (D) (C)
FIGURE 20.l6 The manufacturing sequence forInvisalign orthodontic aligners. (a) Creationof a
polymerimpressionofthe patient’s teeth. (b) Computer modeling to produce CAD representations
of desired tooth profiles. (c) Production of incremental models of desired tooth movement.
An aligner isproducedby thermoforming a transparentplastic sheet againstthismodel.
Source: CourtesyofAlignTechnology,Inc.
The Invisalign product has proven to be very quickly and inexpensively allows this orthodontic popular for patients who wish to promote dental treatment tobeeconomicallyviable.
health and to preserve their teeth long into their lives.
The use of stereolithography toproduce accurate tools Source: Courtesyof Align Technology,Inc.
20.5.I
Rapid
Tooling
Several methods have been devised for the rapid production of tooling (RT) by means ofrapid-prototyping processes. The advantages to rapid tooling include the following:
I. The high cost of labor and short supply of skilled patternmakers can be overcome.
2. There is a majorreduction in leadtime.
3. Hollow designs can be adopted easily so that lightweight castings can be pro-duced more easily.
4. The integral use of CAD technologies allows the use of modular dies with base-mold tooling (match plates) and specially fabricated inserts. This modu-lar technique can furtherreduce tooling costs.
5. Chill- and cooling-channel placement in molds can be optimized more easily,
Section20.5 DirectManufacturing and Rapid Tooling
6. Shrinkage due tosolidification or thermal contraction can be compensated for
automatically through software toproduce tooling of the proper size and, in
turn, toproduce the desired parts.
The mainshortcoming ofrapid toolingis the potentially reduced toolor pattern life
(compared to thoseobtained from machined tool and die materials, such as tool steels or tungsten carbides).
The simplestmethod ofapplying rapid-prototyping operationsto other manu-facturing processes is inthe direct production ofpatterns or molds. As an example, Fig. 20.17 shows an approachfor investment casting. Here, the individual patterns
are made in a rapid-prototyping operation (in thiscase, stereolithography) and then used aspatterns inassembling atree for investment casting. Notethatthis approach
requires a polymer thatwill completely melt and burn from the ceramic mold; such polymers are available for all forms of polymer rapid-prototyping operations. Furthermore, asdrawninCADprograms, theparts are usuallysoftware modified to account for shrinkage, and it is thenthat the modified part is producedinthe
rapid-prototyping machinery.
As another example, 3DP can easily produce a ceramic-mold casting shell (Section 11.2.2) or a sand mold (Section 11.2.1) in which an aluminum-oxide or
*P
-6
‘
l
sl
_“1¢J-;|fQ,
;,='fi
-§|_»=#1_1-__»
3`
it
-T.
=
-.
si*
irici'
-a' T40 » i fO
A _ bl'eg
»O
"-o
O v
->
f
Il
-» .P
_i
,I ._ _r
G
10
i iO
,,..1. Pattern creation 2.Tree assembly 3. Insertintoflask 4. Fill with investment
Crucible Heat Molten metal ,
!
- ”-
Grinding 6 spatter Workpiece 65.Waxmelt-ouifburnout 6. Fillmold with metal 7. Cool 8. Finish
FIGURE20.I7 Manufacturingsteps for investment castingwithrapid-prototyped waxparts
as blanks. This methodusesa flask forthe investment, but a shell methodalso can be used.
Chapter 20 Rapnd-Prototyping Processes andOperations
aluminum-silica powder is fused with a silica binder. The molds have to be post-processed in two steps: curing at around 150°C and then firing at 1000°-1500°C. Another common application of rapid tooling is injection molding (see
Sect-ion 19.3), in which the mold or, more typically, a mold insert is manufactured by
rapidprototyping. Molds forslipcasting ofceramics (seeSection 18.2.1) also canbe
producedin this manner. To produce individual molds, rapid-prototyping processes are used directly, but the molds will be shaped with the desired permeability. For example, in fused-deposition modeling, this requirement mandates that the fila-ments be placed onto the individual slices with a small gap between adjacent filaments. The filaments are thenpositioned atright angles inadjacent layers.
The advantage of rapid toolingis the capability to produce a mold or a mold insert that can be used to manufacture components withoutthe time lag (typically several months) traditionally required forthe procurement of tooling. Furthermore,
the design is simplified, becausethe designer need only analyze aCAD file ofthe de-sired part; software thenproducesthe tool geometry and automaticallycompensates for shrinkage.
In additiontothe straightforwardapplication of rapid-prototypingtechnology to toolor pattern production, other rapid-tooling approaches based on rapid-proto-typing technologies have been developed.
Room-temperature vulcanizing (RTV) molding/urethane casting can be per-formed by preparing a pattern of a part by any rapid-prototyping operation. The
pattern is coated with a parting agent and may or may not be modified to define mold parting lines.Liquid RTVrubber ispoured overthepattern, andcures (usually within a few hours) to produce mold halves. The mold is then used with liquid urethanes in injection molding or reaction-injectionmolding operations (see
Sect-ion 19.3.1). One main limitation of this approach is a lesser mold life,because the
polyurethanein the mold causes progressive damage and the mold may be suitable foras few as 25 parts.
Epoxyoraluminum-filled epoxy moldsalso can beproduced, but mold design then requires special care. With RTV rubber, the mold flexibility allows it to be peeledoffthe curedpart. With epoxy molds, thehigh stiffnessprecludes thismethod
ofpart removal, and mold design is more complicated. Thus, drafts are needed, and
undercuts and other design featuresthat can be produced by RTV molding must be avoided.
Acetal clear epoxy solid (ACES) injection molding, alsoknownas directAIM, refersto the use of rapid prototyping (usually stereolithography) to directly produce molds suitable for injection molding. The molds are shells with an openend to allow filling witha materialsuch as epoxy,aluminum-filled epoxy, or a low-melting-point metal. Dependingon the polymerused in injection molding, moldlife may be asfew
as 10 parts, although a fewhundred parts per mold arepossible.
Sprayed-metal tooling. In this process, shown in Fig.20.18, apattern is
creat-ed through rapidprototyping. A metal sprayoperation (see Section 34.5) then coats the pattern surface with a zinc-aluminum alloy. The metal coating is placed in a
flask and potted with an epoxy or an aluminum-filled epoxy material. In some ap-plications, cooling lines can be incorporated into the mold before the epoxy is
ap-plied. The pattern is removed; two such mold halves are then suitable for use in
injection-molding operations. Mold life is highly dependent on the material and
temperatures used, and can vary from a few to thousandsof parts.
Keltool process. In the Keltool process, an RTVmold is produced based on a
rapid-prototyped pattern,as describedearlier. The mold is then filledwith amixture of powdered A6 tool steel (Section 5.7), tungsten carbide, and polymerbinder, and
W
Section 20.5 DirectManufacturing andRapid Tooling
Aluminumpowder- Q
f
®|\/|era| filled epoxy
spray
Alignmenttabs Flask
Pattern C0al'n9
(H) lb) (C)
Finished mold half
Molded part
Pattern
Second mold half
(Ol) (G)
FIGURE 20.18 Production of tooling for injection molding by the sprayed-metal tooling
process. (a) A pattern and baseplate are prepared through a rapid-prototyping operation;
(b) a zinc-aluminum alloy is sprayed onto the pattern (see Section 34.5); (C) the coated
baseplateand pattern assembly areplaced togetherin a flaskand backfilledwith aluminum-impregnated epoxy; (d) after curing, the baseplate is removed from the finished mold; and
(e) a second moldhalf suitable forinjection moldingisprepared.
metallurgy) is fired to burn offthe polymer and fuse the steel and the tungsten-car-bide powders. The tool is then infiltrated with copper in a furnace to produce the final mold. The mold can subsequently be machinedor polished toattain asuperior
surface finish and good dimensional tolerances. Keltool molds are limited in size to
around 150 >< 150 >< 150 mm, so, typically, amold insert suitable for high-volume
molding operations is produced. Depending on thematerial and processing
condi-tions, mold life can range from 100,000 to 10 millionparts.
EXAMPLE 20.5 Casting ofPlumbing Fixtures
A global manufacturer of plumbing fixtures and accessories for baths and kitchens used rapid tooling to transform its development practice. Une of the company’s major product lines is decorative water
faucets produced from brass castings that are subse-quently polished to achieve the desired surfacefinish. The ability to produceprototypesfrom brass is essen-tial to quickly evaluate designs and identify process-ingcomplications thatmay result.
A new faucet design was prepared in a CAD program; the finished productis shown in Fig. 20.19.
As part of the product development cycle, it was desired toproduce prototypes ofthe faucetto confirm the aesthetics of the design. Since such faucets are typically producedbysand casting, itwasalso desired to validate the design through a sand-casting process followed by polishing. This approach allowed evaluation of the cast parts in terms of porosity and other castingdefects, and also wouldidentify process-ing difficulties that might arise in the finishing stages.
A sand mold was produced first, as shown in Fig. 20.20. The mold material wasa blend of foundry 547
548 Chapter20 Rapid-Prototyping Processes andOperations
FIGURE 20.l9 A new faucet design, produced by casting from
rapid-prototypedsand molds.
FIGURE20.20 Sandmoldsproduced through three-dimensional printing.
sand, plaster, and other additivesthat were combined to provide strong molds with good surface finish(see
also Section 11.2.1). A binder was printed onto the sand mixture toproduce the mold. The mold could be produced as one piece, with an integral core (see
Figs. 11.3and 11.6), but inpractice,itis often desired
to smoothen the core and assemble it later onto core prints. In addition, slender cores may become damaged as support powder is removed from the
mold, especially for complicated casting designs. Therefore, the core is produced separately and
assembled intothe two-part mold.
Using 3D printing, the operation produced
brass prototypes of the faucets in five days, which
included the time required for mold design,
printing, metal casting, and finishing. The actual
printtime of the mold was just under three hours,
and the material cost was approximately $280. The
production of pattern plates for sand casting is, in
general, too expensive for producing prototypes,
but would cost over $10,000 and add several
months to the lead time. The incorporation of 3D
printing into the design process provided new capabilities that confirmed the design aesthetics and function, as Well as manufacturing robustness
and reliability.
SUMMARY
Rapid prototyping has grown into a unique manufacturing discipline within the past two decades. As a physical-model-producing technology, it is a useful tech-nique for identifying and correcting design errors. Severaltechniques have been developed for producing partsthrough rapid prototyping.
Fused-deposition modeling consists of a computer-controlled extruder through
which a polymer filament is deposited to produce apart slice by slice.
Stereolithography involves a computer-controlled laser-focusing system that
cures a liquid thermosetting polymercontaininga photosensitive curing agent. Multijet and polyjet modeling use mechanisms similar to ink-jet printer heads to eject photopolymers to directly build prototypes.
Laminated-object manufacturing uses a laser beam or vinyl cutter to first cut the slices on paper or plastic sheets (laminations). Then itappliesan adhesive layer if
necessary, and finally it stacks the sheets to produce the part.
Three-dimensional printing uses an ink-jetmechanism to deposit liquid droplets of the liquid binder ontopolymer, metal, or ceramic powders. The related process of ballisticparticle manufacturingdirectlydepositsthe build material. Usingmultiple
printheads,three-dimensional printing canalso produce full-color prototypes. Selective lasersintering usesa high-powered laser beam to sinter powders or coat-ings on the powders in adesiredpattern. Selective lasersintering has beenapplied to polymers, sand, ceramics, and metals. Electron-beam meltinguses the power of an electron beamto melt powders and formfully dense functional parts.
Rapid-prototyping techniques have made possible much faster product develop-menttimes, and they arehavinga major effect onother manufacturing processes. When appropriatematerials are used, rapid-prototyping machinery can produce blanks for investment casting or similar processes, so thatmetallicparts can now be obtained quickly and inexpensively, even for lot sizes as small as one part. Such technologies also can beapplied toproducing molds for operations (such as injection molding, sand and shell mold casting, andeven forging), thereby signif-icantly reducing the lead time between design andmanufacture.
KEY
TE
RMS
ACES Electron-beammelting Photopolymer
Additive processes Free-formfabrication Polyjet
Ballistic-particle Fused-deposition modeling Prototype
manufacturing Keltool Rapid tooling
Desktop machines Laminated-object RTVmolding/urethane
Direct AIM manufacturing casting
Directmanufacturing Multijetmodeling Selectivelaser sintering
BIBLIOGRAPHY
Bibliography 549
Solid-ground curing
Sprayedmetal tooling
Stereolithography
Subtractive processes
Three-dimensional printing
Virtualprototyping
Beaman, ].]., Barlow,].W, Bourell, D.L., and Crawford, R., Chua, C.K., andFua, L.K.,Rapid Prototyping: Principles and
SolidFreeform Fabrication, Kluwer, 1997. Applicationsin Manufacturing, Wiley, 1997.
Bennett, G. (ed.), Developments in Rapid Prototyping and Gebhardt, A., Rapid Prototyping, I-Ianser Gardner
550 Chapter20 Rapid-Prototyping Processes and Operations
Hilton, P.D., and ]acobs, P.F., Rapid Tooling: Technologies and Industrial Applications,CRC Press,2000.
Kamrani, A., and Nasr, E.A. (eds.), Rapid Prototyping: Theoryand Practice, Springer, 2007.
Noorani, R.I., Rapid Prototyping: Principles and
Applications,Wiley,2006.
REVIEW
QUESTIONS
Pham, D.T., and Dimov, S.S., Rapid Tooling: The
Technologies and Applications of Rapid Prototyping
and Rapid Tooling, Springer-Verlag,2001.
Wood, L., Rapid Automated Prototyping: An Introduction,
Industrial Press, 1993.
20.I. What is the basic difference between additive
manu-facturingand rapid prototyping?
20.2. Whatis stereolithography?
20.3. What is virtual prototyping, and how does it differ
from additivemethods?
20.4. Whatis fused-deposition modeling?
20.5. Explainwhat ismeant by rapid tooling.
20.6. Why are photopolymers essential for
stereolitho-graphy?
QUALITATIVE
PROBLEMS
20.7. Explain what each of the following means: (a) 3DP,
<b)Loivi, <¢> STL, (d) SGC, (Q) FDM, and (f> LENS.
20.8. What starting materials can be used in
fused-deposi-tionmodeling? Inthree-dimensional printing?
20.9. What are the cleaning and finishing operations in
rapid-prototypingprocesses? Why are they necessary?
20.l0. Examine a ceramic coffee cup and determine in
which orientation you would choose to produce the part if
you were using (a) fused-deposition manufacturing or (b)
laminated-object manufacturing.
20.1 I. How would you rapidly manufacture tooling for
injection molding? Explain any difficulties that may be
encountered.
20.|2. Explain the significance of rapid tooling in manu-facturing.
20.I3. List the processes described in this chapter that are
bestsuitedfor theproduction ofceramic parts. Explain.
20.I4. Few parts in commercial products today are directly
manufactured through rapid-prototyping operations.
Explain.
QUANTITATIVE
PROBLEMS
20.I5. Can rapid-prototyped parts be made of paper?
Explain.
20.l6. Careful analysis of a rapid-prototyped part indicates
thatit ismade upoflayers witha distinctfilamentoutline
vis-ible on each layer. Is the material a thermoset or a
thermo-plastic? Explain.
20.17. Why are the metalparts in three-dimensional
print-ingoften infiltrated by anothermetal?
20.18. Makea listofthe advantagesand limitationsof each
ofthe rapid-prototyping operations described in thischapter.
20.l9. In making a prototype of a toy automobile, list the
post-rapid-prototyping finishing operations that you think
wouldbenecessary. Explain.
|]20.20.
Using an approximate cost of $160 per litre forthe liquid polymer, estimate the material cost of a
rapid-pro-totyped rendering of atypical computer mouse.
|]20.2
I. The extruderhead in afused-deposition modelingsetup has adiameter of 1.25 mm and produceslayers thatare
0.25 mm thick.If the extruder head and polymer extrudate
velocities areboth 50 mm/s, estimate theproduction timefor
thegeneration of a 38-mmsolid cube. Assumethat there is a
10-second delay between layers as the extruder head is
moved over awire brush for cleaning.
|]20.22.
Using the data for Problem 20.21 and assumingthat the porosity forthe support material is 50%, calculate the production rate for making a 100-mm high cup with an
outside diameterof 90 mm and a wall thickness of 4 mm.
Considerthe cases (a)withthe closed endup and (b)withthe
closed end down.
20.23. InspectTable20.2 and compare thenumericalvalues
given with those formetals and other materials, as can be
Synthesis, Design, and Projects 55|
SYNTHESIS, DESIGN,
AND
PROIECTS
20.24. Rapid-prototyping machines represent a large
capi-tal investment; consequently, few companies can justify the
purchase of their own system. Thus, service companies that
produce parts based on their customers’ drawings have
be-come common. Conduct an informal survey of such service
companies, identifytheclasses ofrapid-prototypingmachines
thatthey use, and determine the percentageuse of each class.
20.25. Qne of the major advantagesof stereolithographyis
thatitcan use semitransparent polymers,so that internal
de-tailsofparts can readilybediscerned. List and describe
sever-alparts in which this featureis valuable.
20.26. A manufacturing technique is being proposed that
usesa variation of fused-deposition modeling in which there
aretwopolymer filaments thatare melted and mixed prior to
being extrudedto make the part. What advantagesdoes this
methodhave?
20.27. Identify the rapid-prototypingprocesses described in
thischapter that canbeperformed with materialsavailable in
your home or that you can purchase easily at low cost.
Explain howyou would go aboutit. Considermaterials such
as thin plywood, thick paper, glue, and butter, as well as the
use of various tools and energy sources.
20.28. Design a machine that uses rapid-prototyping
tech-nologies to produceice sculptures. Describeits basic features,
commenting on the effect of size and shape complexity on your design.
20.29. Because of relief of residual stresses during curing,
long unsupportedoverhangs in partsmade by
stereolithogra-phytend to curl. Suggestmethods ofcontrolling or
Machining
Processes
and Machine
Tools
RT PA Parts manufactured by the casting, forming, and shaping processes described in
Parts II andHI, including manyparts made by near-netor net-shape methods, often require further operations beforethe productis ready foruse. Consider, for example, the following featureson partsandwhether theycould beproducedbythe processes discussed thus far:
° Smooth and shiny surfaces, such as the bearing surfaces of the crankshaft
shown inFig. IV1.
° Small-diameter deep holes in a part such as the injector nozzle shown in Fig. IV2.
° Parts with sharp features, a threaded section, and specified close
dimensional tolerances, such as the partshown in Fig. IV3.
° A threaded hole or holes on different surfaces of a part for mechanical assemblywith other components.
° Special surface finishes and textures for functional purposes or for appearance.
A briefreview will indicatethat noneof the forming and shaping processes described thusfaris capableofproducingpartswithsuch
spe-cificcharacteristics and thatthe partswill require further
manufactur-ing operations. Machining is a general term describing a group of
processesthat consistof the removal of material and modification of the surfacesofaworkpiece afterithas beenproducedby various meth-ods. Thus, machining involves secondary and#iris/Qingoperations.
The very wide variety of shapes produced by machining can be
seenclearly in an automobile, as shown in Fig.IV4. It also should be
recognized that some parts may be producedtofinal shape (net shape) and athigh quantities by forming and shaping processes, such as die
casting and powder metallurgy. However, machining may be more economical, provided that the number of parts required is relatively
smallor the material and shape allowthe partsto be machined at high rates and quantities and with high dimensional accuracy. A good
example is the production of brass screw-machine parts on multiple-spindleautomatic screw machines.
Before After
FIGURE l\Ll A forged crankshaft before
and after machining the bearing surfaces.
The shiny bearing surfaces of the part on
the right cannot be made to their final
dimensions and surface finishbyany ofthe
processes described in previous chapters
ofthis book. Source: Courtesyof
Wyman-Gordon Company.
