Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999

7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999

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SITTINGAT

A COMPUTER, a designer has just com pleted a rendition of an exo tic shape for an equally exotic application

or art ist ic object . I t is now time t o create th e real thing. Sh e clicks

o n

the hard cop y comma nd, and minutes later

the object materializes beside her de sk. Is this the manufacturing environment

of

the future?

his dream portrays an ideal method of manu-

facturing, one in whic h

a

part , component, or

entire subsystem would be designed electron-

ically, relying on the power of present c omp uter-

aided design (C AD ) systems and then output

immediately at a push of a butto n. Thou gh it may seem

fanciful, th e me tho d is reminiscent of a well-established

conc ept, desktop publishing, in which print and graphics

are laid out on a com puter and automatically printed by

dot matrix, ink jet, or laser printers.

In many ways, printing is a good two-dimensional

analogy of three-dimensional manufacturing. Lettering

or drawing by han d is analogous to low-volum e manual

manufacturing meth ods using standard machines and

tools. Offset printing is analogous to high-volume auto-

m a t e d m a n u f a c t u r i n g u s i n g s p e c i a l i z e d m a c h i n e s .

Desktop printing, however, lacks an analogy in present

manufacturing. While it does not compete with offset

printing for large numbers of copies, i t is eminently

practical for small numbers. Its analogy is with an up-

and-co ming area of manufacturing under development

fo r severa l years a t the Massachuse t t s Ins t i tu te o f

T e c h n ol o g y , C a r n e g i e M e l l o n U n i v e rs i t y, a n d T h e

University of Texas at Austin, among other places.

Researchers here in Austin have been working on tech-

nologies termed solid freeform fabrication (SFF) or, as

reported in the popular press, desktop manufacturing,

rapid prototyping, and layered manufacturing.

Such processes have the potential to produce accu-

r a t e , s t r u c t u r a l ly s o u n d 3 - D r e n d i t i o n s of o b j e c t s

designed with computers and manufactured directly from

a CAD database, without part-specific tooling

or

human

intervention, and to make them available to the user in

minutes or hours [Figs. 1 and 21. The benefits include

greatly reduced prototyping cost and design time and

the ab i li ty to ach ieve , in one opera t ion , shapes tha t

would otherw ise require multiple operations or in some

cases be impossible to produce w ith standard techniques.

Automated manufacturing technologies in general

are well suited to large production numbers, but i l l suit-

ed to low-volume runs. In th e latter case, the com po-

nents are too few to adequately amortize the cost of

part-specific tooling, instead, they are typically made

by hand at much greater unit cost and longer comple-

tion times. No r is it a completely desirable low-volume

option to interface CAD systems with numerically con-

trolled (NC) machining centers-in essence, computer-

controlled m aterial removal systems-because so much

human intervention is involved in producing NC pro-

grams and setting up and supervising MC systems. In

fact, the low -volume production arena is exactly where

SFF s lashes cos t and t ime to comple t ion . Obvious ly ,

too, these new manufacturing technologies fit well into

computer-integrated manufacturing environments.

Developmen t cy cles for complex systems can be dra-

matically shortened by desktop manufacturing, which

breaks the bottleneck between model-making and pro-

totyping. In addition, the geometric information about

a part's shape that can be captured digitally in a CAD

da tabase can a l te rna t ive ly be t ransmi t ted over te le -

phone lines, radio signals, and by satell i te . Coupling

this information with an SFF machine may make large

inventories a thing of the past by enabling parts to be

produced on demand in remote locations or allowing

replacement parts to be produced on the sp ot. Again,

couple this solid freeform fabrication technology with

modern 3-D digitizing techniques in which geometric

information on existing parts can be scanned w ith lasers

or X-rays and captured, and the means to create 3-D

copy or fax machines is at hand. In fact, a 3-D fax was

i m p l e m e n t e d f o r t h e f i r st t i m e i n A u g u s t 1991 in

Austin, Texas, when a part was digitized at Scientific

Measurement Systems Inc., an Austin-based technology

company that manufactures high-power X-ray scanning

systems. Th e data was then sent over telephone lines to

the Labo ratory for Freeform Fabrication at Th e Univer-

sity of Texas, in Austin, where a selective laser-sintering

system was used to recreate the part.

Different processes

Today, about

30

technologies address the rapid cre-

ation of models, prototypes, patterns, and limited man-

ufacturing runs. Many

SFF

methods are based upon lay-

ered manufacturing. In this appro ach,

a

computer repre-

sentation of an object's geometry is decomposed into

slices of 1 - 1 0 dimensional information. A slice is a pla-

nar cross-section of an object w ith an associated small,

but finite, thickness. During processing, each slice is

physically deposited and its interior is fused to t he pre-

vious layer to create an object sequentially. While a

slice's interior region demarcates the geometry of the

object, its exterior also serves an important function in

SFF technologies. In most processes, it becomes a struc-

ture that serves as an implicit fixture for the part as well

as a suppo rting matrix upon w hich regions that o ccur in

overhangs can be constructed. The overhangs require

this support since they are not joined with the main

bod y until subsequent layers are deposited.

A

layer's sup port areas are hand led in distinctly

dif-

ferent ways by many of the process methods. In some,

RICHARD

H.

CRAWFORD

&JOSEPHJ . BEAMAN

The Universi ty

o

Texas at ustin

5


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explicit supp ort structures use the same

mater ial as the object being formed,

but are deposited in such a way as to

be

easily removed, usually manually. In

others, the ent i re exter ior of the slice

becom es a sac r i f ic i a l po r t i on of t he

layer that is subsequently removed and

discarded or recycled, of ten, the sup-

port material is different from the part

m a t e r ia l a n d c a n be can se l ec t i ve ly

r emoved by d i sso lu t i on , me l t i ng , o r

e t ch ing . S t i ll o the r p r ocess m ethods

use base materials that are the same as

the part but are in a powder form that

i n t ri n s ica l ly suppor t s t he p a r t wh i l e

remaining removable.

Solid freeform fabrication te chnolo -

gies use a computer graphic represen-

tation and simple stock material (pow-

der , l iquid, gas, sheets, an d

so

o n ) t o

fabricate complex parts. Photopolymer

s y s t e m s b ui l d s h a p e s u s i n g l i g h t t o

selectively solidify liquid ph otocura ble

resins. A number of other systems pre-

f e r powder s a s t he i r s t ock mate r i a l .

Som e examples of powder-based tech-

n i q u e s a r e s e l e c t i v e l a s e r s i n t e r i n g

[Fig. 31,

3- D p r in t i ng , and 3 - D l ase r

cladding. Lamination systems opera te

with a variety

of

feedstock from paper

sheets to metal plates. SFF deposi t ion

techniques include extrusion, ink-jets,

3 - D weld ing , gas , and p l asma sp r ay

[Table

11.

T h e c a p a b i l it i e s a n d c o s t s of

SFF

processes are rapidly changing. Cur-

rently, SFF workstations cost between

US 50 a n d 500 000. W o r k -

stations such as those from Z -C orp and

S t r at asys a r e on t he l ow en d of t h a t

spec t r um, wh i l e l a r ge r wor ks t a t i ons

f rom 3 D Systems and

DTM

Cor p . t end

toward the high e n d of the range. Parts

made b y service bureaus average

1

000

t o 2000, d e p e n d i n g o n p a rt s i z e,

m a t e r ia l , a n d p r o c e s s . T h e a v e r a g e

build volume for SFF workstations is a

c u b e of a b o u t

2 5 0

m m o n a s i d e .

Accuracy and sur face roughness vary

wide ly , depe nd in g on t h e pa r t i cu l a r

SFF technology and precursor material.

Typical numbers for dimensional errors

a r e 25 pm t o 380 pm. Surface rough-

ness

varies with the orientation

of

t he

par t for layer -based techniques.

The

b e s t r e p o r t e d s u r f a c e r o u g h n e s s is

a b o u t 100 nm o n

a

t op su r f ace, and

about 170 nm o n a side surface (assum-

ing a vertical build).

Contrast this with computer numeri-

cally controlled

(CNC)

milling, which

can eas i l y p r oduce pa r ts w i th d imen-

sional errors in th e range of 13 pm and

su r f ace r oughness

of

400 t o 800 nm.

While CNC offen

an

order

of

magni-

tude improvement in the accuracy

of

t he

part, the required accuracy of the par t

must be weighed against the time sav-

ings offered by SFE Man y examples arc

reported in the literature that document

six- to eight-week savings i n lead time

compared to conventional manufactur-

ing processes. This leads to thousands of

dollars in savings over the design cycle

of

a product.

M a n y applications

f

it is true that a picture is worth a

thousand words, then a physical model

is wor th a t housand p i c tu r es . Wh en

designers employ the latest

3-D CAD

systems, they see and under s tand the

physical nature

of

t he i r wor k - in - p r o -

g r e s s i n w a y s u n i m a g i n a b l e i n t h e

wor ld

of

or thogr aph ica l l y p r o j ec t ed

2 - D d r a w i n g s . Ye t t h e s e 3-D CA D

objects remain onl y abstractions, virtu-

al representations

of

yet- to-be- real ized

artifacts, because their world

is

without

g r av ity , f r i c t i on , o r r e l a t i ona l sca l e .

Th e virtual models viewed within this

environment rout inely defy the laws

of

physics, require interpretation , and can

engen der a false sense

of

certainty in

the design team.

In

contrast, by using the rapid cap-

abilities of solid freeform fabrication,

people of all disciplines can use their

senses

of

sight and touch to thorough-

l y c o m m u n i c a t e a n d u n d e r s t a n d t h e

basic mechanical implications of num-

erous design solut ions. Many product

deve lopmen t com pan ies a r e r ea li z ing

the benef i ts

of

solid freeform fabrica-

t ion prototypes, as the fol lowing case

studies show.

Integrat ing

electronic

and

mechanical

design

CHRISTOPHER

CAVELLO

Design Edge Inc.

ne of t he ha r des t - t o - communi -

cate aspects of a product devel-

opm ent ef for t is the integrat ion

of

elect ronic components into a mechan-

ical system. Design E dge Inc. , a prod-

uct development firm in Austin, Texas,

employs solid freeform fabrication to help

br idge

the

gap betwee n electrical engi-

neers, who

use

2 - D

CAD,

and mechan-

ical engineers and industrial designers,

w h o u s e 3 - D C A D . T h e

CAD

tools of

these discipl ines di f fer not only in the

number

of

dimensional axes (3-D vs.

2-D)

but also often cross com puter plat-

forms. Complicat ing things fur ther, the


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design activity

of

these two disciplines can

be located far apart , even when the groups

are within th e same organization.

A 2-D

board outline drawing, provided

by a mechanical engineer, is the formal

mode of comm unication with the electri-

cal engineers responsible for the layout

and componen t se lec t ion of printed-cir-

cu i t boards . The d rawing desc r ibes the

dimensions

of

the board and p rov ides a

sort

of

m a p

of

i t , to dictate the allowable

physical dimensions of an electronic com-

p o n e n t s l o c a t i o n . T h e c r e a t io n of th is

o u t l i n e d r a w i n g i s a g i v e - a n d - t a k e e x -

change be tween the board des igners and

the mechanical designers. In the process,

an electrical engineer, in an effort to meet

e lec t romagne t ic in te rfe rence

or

thermal

goals, may wish t o posit ion connectors or

other com ponents o n the circuit board in

a location the mechanical design team has

d e e m e d

off

l imi ts . Converse ly, the me-

chanical team may b e trying to reduce the

package size

or

to mee t a s ty l i s t ic goa l

that m ight require moving electrical com-

ponents from their electronically desired

locations. In either case, it is difficult

for

a

[2] Selective laser sintering was used to c reate these mo ckups o f

a

thermostats housing [ left] and

printed-circuit board [righ t]. The models help designers iden tify and resolve pot ential m echanical,

and even electrical, interf erence problems early in t he d esign cycle.

t e a m s t u c k w i t h t h e t r a d i t i o n a l 2-D o r

3-D CAD t o o l s t o see

if

a compromise

may be reached.

Solid freeform fabrication can vastly

improve design communication between

disciplines with con flicting design goals.

The physical model,

or

perhaps multiple

models in far-flung locations, is excellent

as a common tool with which to discuss

an d mediate a design concepts potential

1 D e ve l o p ed u n d e r N A S A R a p i d P ro t o t yp i n g p ro g ra m .

Technolo gy i censed f rom t he Massachuset t s inst i t u te of Technology.

3

Technology l i censed f rom the Univ ers i t y o f Texas at Aust in.

C R A W F O R D B E A M A N

OLID

FREEFORM FAB RICATION

A

N E W M A N U F A C T U R I N G I A R A I I I G M

7


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mechanical and electromagnetic interfer-

enc e i s sues . Physica l m ode ls of circuit

boards , wi th the s ign i f ican t e lec t ron ic

compo nents represented, help to visualize

d e s i g n c h a l l e n g e s .

A

p h y s i c a l m o d e l

assists all the parties in exploring, feeling,

and see i ng the ir mechan ica l impac t on

the system in a way that electrical design-

ers working in 2-DCAD may find hard to

visualize.

SFF

enables mechanical design-

ers to crea te more accurate volumetric cri-

teria in the board outline drawing because

it lets them test their designs

i n

a true

physical space. Also, beyond the design

of the circuit board, the approach i s help-

ful

in exploring t he effects

of

components

t h a t a r e v e r y h a r d t o d e s c r i b e in 3- D

CAD , such

as

flexible cables and wire har-

nesses. This is done by having t he design-

ers mock up flat designs

of

flexible cables

or by routing actual wire assemblies into

the freeform physical model with their

own hands.

T h a n k s t o t h e r a p id m o d e l i ng t e c h -

niques available today for solid freeform

fabr ica t ion , modif ica t ions to these de -

signs can be made quickly and assessed in

the most effective manner.

The

models

are often created overnight. Design Edge

CO, laser

Las er - beam

s c a n n i n g m i r r o r

Lev e l i ng r o l l e r

P o w d e r bed

C h a m b e r f o r

b u i l d i n g p a r t s

P o w d e r c a r t r i d g e

[ ] In a typ ical selective laser sintering process, a laser beam fuses particles of

powdered plastic, metal,

or

ceramic to b ui ld a th ree-dimensional object, laye; by

layer, in a chamber. The beam is activated in accordance with 3 D computer-

aided design data of t he objects geometry.

uses prototype services, which can quick-

ly provide models from its 3- D computer

files of componen t pa r t s . These p r o t o -

typ in g se rv ices supp ly phys ica l p ro to -

types made with selective laser sintering

(SLS)

o r s t e r e o l i t h o g r a p h y a p p a r a t u s

(SLA) , a n d e m p l o y f a s t I n t e r n e t f i l e -

t r a n s f e r p r o t o c o l (FTP) file e x c h a n g e

m e t h o d s t o s p e e d t h e c o m m u n i c a t i o n

process. Th e cost of these services varies

w i t h t h e q u a l i t y of f i n is h d e s i r e d a n d

with turnaround times. In a t ime crunch,

it is not uncommon for Design Edge to

h a v e t h e p r o t o t y p e s c r e a t e d o v e r n i g h t

l r E E SPEC

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FFURUARY 1999

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and delivered the next m orning, counter

to cou nter, by an airline package service.

Although it may seem expensive to the

un in i t ia ted , the numerous i te ra t ions of

physical models cost far less than do the

m i s t a k e s , m i s se d o p p o r t u n i t i e s , a n d

u n d e r - o p t i m i z e d d e s i g n s t h a t c a n b e

avoided by physical m odel reviews.

W he n Honeywel l Inc . , Minneapo l i s ,

Minn. , asked Design Edge to develop a

huge volume of thermosta t devices for the

Asian market, the design had above all to

be

r e f in e d a n d o p t i m i z e d t o m e e t : t h e

small package size required, production

volumes

of

approximately IO0

000

units

p e r m o n t h , v e r y l o w c o m p o n e n t c o s t s,

an d a long production life. The se require-

ments

led

to a very close integration of

the m echanical and electrical teams. Both

SLA a n d SLS physical models were used

to review and explore early design con-

cepts [again, Figs. an d 21. Printed-circuit

boards a nd the i r e lec t ron ic compone n ts

were modeled in to the CAD assemblies

and refined to help meet assembly, elec-

t r i ca l d e s i g n , a n d b o a r d p a n e l i z a ti o n

goals. Witho ut the aid of physical models

early on and regularly through the design

process, the communication and verifica-

141 After evaluatin g

several prototypes

using sol id freeform

fabr ication technol-

ogy, engineers wi th

M

Telecom Systems

Division, in Austin,

Texas, develo ped this

protective housing-

the FibrDome-for

optic al-fiber cables.

3M

9

C R A W F O K D B E A M A N

OLID

FKEEFOKhl FAI


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tion of concepts would not have been as

effective Th e product is now successfully

up and running in

full

production in the

People's Republic of China

Symbol Technologies Inc , Holtsville,

N Y gave Design Edge a contract to cre-

ate a han d-held laser scanning system with

s t r i ct e r g o n o m i c , f u n c t i o n a l , a n d c o s t

requirements Because

of

the organic form

of these highly integrated components, it

was a l l bu t im poss ib le to v isua lize the

i m p l i c a t io n s o f c o m p o n e n t p l a c em e n t

strategies without a physical model review

Design Edge tested various electronic lay-

outs in physical model form in order to

develop and optimize flexible circuits, con -

nector placements, and switch locations

T h e s p e e d o f o b t a i n in g s o l i d f r ee f o r m

m o d e l s e n a b l e d t h e t e a m t o r e f i n e t h e

design many times over without holding

up t he extremely aggressive schedule

W h e n m o d e l i n g e le c t r o n i c co i n p o -

nen ts and c i rcu i t boards in 3 D AD,

mechanical designers can either represent

the topographic boundaries indicated in

the board ou t l ine d rawing , o r they can

create libraries of key electronic compo-

n e n t s a n d a c t u al l y p l ac e t h e m o n t h e

board in the desired locations Th e elec-

t r ica l and mechan ica l teams mus t w ork

toge ther c lose ly to be su re tha t the 3 -D

CAD mechan ica l rep resen ta t ion o f the

electrical layout is accurate

A new method of ensuring accuracy is

the mechanical design package that can

read the da ta f rom an e lec t r ica l layou t

program and automatically create a 3-D

CAD file as the electrical engineer has d e-

signed it This can now be undertaken by

a n E-CAD s o f t w a r e m o d u l e c a l l e d

P r o / E n g i n e e r) f ro m P a r a m e t r i c T e c h -

n o l og y C o r p , W a l t ha m , M a s s , w h i c h

w o r k s w i t h e l e c t ri c a l l a y o u t p r o g r a m s

such as Allegro To profit from this capa-

b i l i ty , a 3 -C CAD l ibrary, t ied in with

bo th the 3 -D CAD program and the e lec -

trical layout design program , must be cre-

a t e d a n d m ai n ta i n ed W i t h

CAD

too ls

l ike- thes e in p lace , o rga n iza t ions can

exploit solid freeform fabrication further,

to represent their current design thinking

in a faster and more accurate fashion

Reduced time to

market and

higher quality

JERRY

D. A C K S O N ,

~

ith the growth of th e Internet, the

dema nd for bandwidth from the

hom e has soared To handle the

dema nd quickly, local telephone com panies

and various other com munication providers

need to upgrade much of their networks from

copper wire to glass f iber. O ne of the ele-

ments vital to this developm ent is a low-cost

housing to prote ct the optical-fiber cable

splices from the outside environment. The

3M Telecom Systems Division, Austin, Texas,

responded by starting he FibrDome project.

From t he begmning, the project team under-

stood that a short product development cycle

was critical for success. Th ey d eterm ined

that the best way to reduce cycle t ime was

to use solid freeform fabrication to quickly

generate prototyp es for design ev aluation.

The FibrDome Closure system consists

of a sealed, external housing to protect the

fiber splices, plus a fiber management sys-

tem that organizes the fibers without dam-

age [Fig.

4 ].

As the external housing had

already been developed, the team focused

on the fiber management system, which

had to store up to

96

splices in four sepa-

rate trays, and had to include a spare tray

for slack fiber storage. However, the most

cha l leng ing requ i rements were tha t the

fiber could not be bent with a radius less

than

38

mm a t any t ime , and any f ibe r

splice must be accessible without disturb-

ing the o ther fibers.

Once the design process started, SFF

prototypes were used for f ive purposes:

customer presentations, design form an d

fit tests, functional tests, product installa-

tion tests , and supplier communication.

For customer presentations, a 3 -D repre-

s e n t a t i o n c o m m u n i c a t e s b e t t e r t h a n a

d e s i g n o n p a p e r a n d i s a g o o d w a y t o

impress cus tomers w i th respons iveness ,

For example, after one FibrDome presen-

tation, the customer concerned requested

severa l des ign changes tha t o the r p ro -

cesses would take weeks to p ro to type .

Th e team qu ick ly made the changes on

t h e 3 - D c o m p u t e r m o d e l , g e n e ra t e d a

n e w SFF p r o t o ty p e , a n d s h o w e d t h e

d e l i g h te d c u s t o m e r t h e u p d a t e d m o d e l

just a few days after the initial request.

Solid freeform fabrication prototypes

are an excellent means of checking for

in te r fe rence p rob lems wi th mechan ica l

designs. In an earlier design, interference

occur red a t the h inge when two o f the

splice trays were pivoted. A feature on one

tray was flexing more than expected and it

was hitting the other tray.

To

identify the

exac t in te r fe rence , the h inge a rea was

modeled at four times actual size [Fig. 51.

These prototypes helped the team identify

the exact nature of the problem , and it was

rectified easily and quickly.

For most of th e prototype s for this pro-

jec t , the s te reo l i thography p rocess was

used because of i ts f inish, dimensional

accuracy, and cost. Nonetheless, in one

i n st a n ce t h e p a r t n e e d e d t o b e t e s t e d

functionally, to determine whether i t con-

formed a round an optical-fiber cable. As

luck would have it, the streng th and flexi-

bil i ty of the SLA po lymer were inade-

quate For this reason, the selective laser

sintering process was employed because it

usually overlooked is the improved com-

this case, the injection

lid freeform prototypes

was t ru ly a t remendous success fo r the

t T h e t e a m e st i m a te s

a

pp

ca

o

ns

form fabrication, in thei r

les of th e use of solid

urgical plann ing tools

E

SPECTRUM

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screen, requiring abstract interpretation.

Worse yet, noise is present in the data, as

medical imaging relies on processing sen -

s o r s i g na l s r a t h e r t h a n t h e r e l a t i ve l y

noise-free input from a keyboard and a

mouse that

CAD

designers use.

Even so, many doctors f ind that 3-D

models of

CT

o r

MRI

data allow them t o

study subtle features that are difficult to

detec t otherwise. Th e result is better pre-

operation planning.

The

models are also

u s e fu l i n e x p l a i n i n g p r o c e d u r e s t o

patients, for training physicians, and for

practicing difficult techniques. T he results

a re tha t su rgeons ca r ry ou t opera t ions

more accurately, with more confidence,

and in less time, all of which benefit the

patient tremendously.

Solid freeform

biological

par ts

W h i l e t h e a p p l ic a t i on s d e s c r i b e d

a b o v e p r o d u c e m o d e l s , m u c h o n g o i n g

research into this technology focuses on

searching for new processes and materi-

al systems that will allow the creation of

functional parts . On e promisin g area of

application is fabrication of rep lacem ent

biological parts , whose complex shapes

are well served by solid freeform tech-

nologies. Research at The University of

Texas at Austin is focused on the selec-

t ive la se r s in te r ing (SLS) p r o c e s s a n d

t h e d e v e l o p m e n t o f b i o c o m p a t i b l e

material systems that can be processed

wi th SLS. In particular, a new material

system has been created for fabricating

bone implan ts .

T h e p r o c e ss f o r d e v e l o p i n g b o n e

implants uses a calcium phosphate pow-

d e r t h a t i s c o a t e d w i t h p o l y - m e t h y l -

methacry la te

(PMMA)

in

a

spray dryer.

T h e

PMMA

acts as a binder. During the

SLS

process, the

PMMA

coating is melted

by the laser and binds the calcium phos-

phate particles to form a "gree n" part-a

part that has not reached its

full

strength

or dens i ty . (T he term der ives f rom the

color of ceramic parts prior to their firing

in a kiln .) Laser power, scan spe ed, scan

spacing, and powder bed temperature are

c a re f u ll y c o n t r o l l e d t o o p t i m i z e g r e e n

part stre ngth. The SLS-fabricated green

parts are subsequen tly infil trated with a

calcium phosphate solution, fired in a fur-

nace to remove the

PMMA

binder, and

then fired at higher temperature to sinter

the calcium phosphate powder.

P re l iminary s tud ies in an imals have

b e e n c a r r i e d o u t a t B i o M e d i c al E n t e r -

prises Inc., San A ntonio, Texas. On e set

of studies involved oral implants in dogs.

Radiograph images taken four weeks after

implan ta t ion sho wed a h igh degree o f

biocompatibili ty and bon e ingrowth. The

resul t s suggest tha t , in t ime , the spec i -

m e n s w o u l d b e c o m p l e t e l y f i l le d w i t h

mineralized bone.

Embedded electronics

LEE

E

WElSS

Carnegie Mel l on University

h a p e d e p o s i t i o n m a n u f a c t u r in g

*% r

(SD M) is

a

solid freeform fabrication

wm s rocess originally intended t o unite

the advantages of geometry decomposi

tion and material addition with the advan-

tages of processes for removing m aterials

[Table 1 1 In this meth od, individual seg

ments of a part and of sup port material

structure are deposited

as

near net-shapes

(nomina l shapes as des igned) and then

mach ined to ne t - shape befo re fu r the r

material is deposited a nd shaped

Th e rap id p ro to typ ing o f complex

shapes is clearly possible and, so, too ,

are the use of selective additive material

processing to fabricate structures from

several materials and the embedding of

p re fabr ica ted componen ts wi th in the

grow ing shapes For example, an em-

bedded electronic device can be fabri

cated by building up a nonconductive

hous ing package and s imul taneous ly

embedd ing and in te rconnec t ing e lec -

tronic components within the housing

Th e approach lends i tself to produc-

ing compact, rugged, customized com-

puter modules in small lots In particu

lar, it suits such military and industrial

applications as manufacturing mission-

specific, conformally shaped "smart" de-

vices-wearable com put ers are an ex-

ample. These last might store maps or

e q u i p m e n t d e s c r i p t i o n s , h e l p t o l o g

data, or provide communication links.

The F rogman is a waterproof com-

puter built with SDM tha t can s to re

maps for navigational aids or detailed

assembly drawings for service, mainte-

nance, or f ield operations. The graph-

ica l in fo rmat ion , which i s s to red on

PCMCIA cards [see Defining Terms,

p.

361,

is displayed o n a he ads-up dis-

play.

A

conformal ly shaped rea r su r -

face is

also

requ i red

so

tha t the un i t

c a n b e c o m f o r t a b l y s t r a p p e d t o a

diver's leg.

T h e u n i t i s b u i l t u p i n l a y e r s o f

polyurethane and sacrif icial wax. The

former is depo sited as

a

two-par t the r -

mose t . The wax can be ex t ruded wi th

a conventional hot-glue gun, or thick

layers can be poured from a h o t - m e l t

p o t . T h e i m p o r t a n t p o i n t s a r e t h a t

custom tooling is not required to man-

ufactu re the F rogman and tha t em bed-

ding facil i tates waterproofing.

CRAWFORD BEAMAN -SOLID FREEFORM FABRICATION A NEW MANUFACTURING PARADIGM

41


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The com pany is developing this material

in

the hope that

it

will on e day be approved

for use

i n

humans. Imagine a scenario

in

which , say, a patient suifers an injury to th e

le f t jaw. Today , o ra l s u rgeons scu lp t a

replacement implant by hand out of a bone

replacement material. The implant sewe s as

a scaffold upon which the bod y rebuilds the

original bone. Soo n, however, i t may be

possible to scan the right jaw, reflect

i t ,

and create a custom implant automatically.

Customized prosthet ics

Medical researchers are also develop-

ing methods for applying solid freeform

f a b r ic a t io n t o p r o s t h e t i c s . A t t h e D e -

par tmen t of Rehabili tation Medicine at

The University

of

Texas Health Science

C e n t e r i n S a n A n t o n i o , a p r o c e s s h a s

been developed for manufacturing custom

prosthetic sockets for below-the-knee am-

putees. The residual limb is first digitized

with a high-speed laser scanner.

An

inter-

ac t ive CAD sys tem then p rocesses the

da ta an d a l lows a sk i l led p ros the ti s t to

modify the geo metry to improve its fit, its

comfort, and the stabili ty

of

the socke t .

The CAD

data are then transmitted to a

CNC milling mac hine, which mills a plas-

ter mold. Finally, the plastic socket is vac-

uum formed using the mold.

The

current process is faster than the

s tandard manua l approach

of

making a

p las te r cas t

of

the res idua l l im b . S t il l ,

researchers hope that, using solid freeform

technologies, prosthetic sockets can be

produced d i rec t l y f rom the

CAD

data,

eliminating the mold fabrication step. The

San Antonio group has collaborated with

DTM Co rp. in Austin, Texas, to fabricate a

nylon socket using the SLS process.

One

advantage of using

SLS

is that the

fitting for attaching the leg pylon to the

socket can be built as part

of

the socket.

Currently, fittings are attached niechani-

cally to sockets, and they require exte n-

sive alignment for each patient. The inte-

gral socket fitting can potentially include

the patients specific alignment cha racter-

istics, resulting in a better product.

Th e San Antonio group is now collabo-

r a t i n g w i t h r e s e a r c h e r s f r o m The

University of Texas at Austin to develop

sockets with locally variable mechanical

properties. Such a socket would have areas

of relative compliance to increase comfort

near pressure points, and other areas of rel-

ative stiffness to improve the sockets load-

bearing capability and stability. T h e goal is

to allow prosthetists to design and fabri-

ca te socke ts comple te ly f rom com pute r

data, with manual intervention.

171 Intric ate

3 D

geo metr ical shapes can be

generated us ing

solid

f reeform fabr icat ion

technology. Artist Brent Collins

is

s how n w i th

the f i r s t wood sculpture he m ade jo int l y

wit h Car los Sequin and the special program

Sequin developed.

Rapid prototyp ing

o f geometr ic

scu I

p tu es

CARLO SEQUIN

University o f California a t erkeley

t is hard to make a sculpture that looks

good from every side Conceptual flaws

often cannot be corrected when discov

ered during the actual sculpting Even an

artist excellent at 3-Dvisualization will make

a

3-D

m o c k u p t o j u d g e t h e l o o k of t h e

intended sh ape from all possible angles and

to ensure that potential weaknesses in pre-

sentation from a particular side have not

been overlooked The creation of th e final

physical artifact demand s a f irm com mit-

ment in time and energy, and the dedicated

artist wants to make sure that it is expended

toward a promising goal

For more th an a d ecade , a r t i s t B rent

C o l l i n s

of

C o w e r ,

M O ,

has been c rea t -

i n g a b s t r a c t g e o m e t r i c a l w o o d s c u l p -

t u r e s M a n y a r e h i g h l y o r g a n i z e d a s -

sembl ies

of

s a d d le s h a p e s t h a t s e e m t o

f lo w i n t o o n e a n o t h e r a n d t e r m in a t e i n

r im s t h a t f o r m i n t r i g u i n g p a t te r n s o r

c o m p l i c a t e d k n o t t e d s p a c e c u r v es T h e

i n t er n a l s a d d l e s h a p e s o f t e n a r e c l o s e

to minim al surfaces-the kind formed

b y s o a p f i l m s s p a n n i n g a c u r v e d w i r e

f rame-even tho ug h Col l ins heard of

tha t c once p t severa l years a f te r h is f i rs t

scu lp tu re of th is typ e

To

cons t ruc t th is compl ica ted shape ,

Col l ins f i r s t dev ises a ske le ton of t h e

basic to po logy us ing po lyv iny lch lo r ide

4 2 I E E E SIE(.TRUhl FEKRL I ARY I W J


10/10

p i p e s e c t i o n s a n d e m b r o i d e r y h o o p s .

Th is he covers wi th wire mesh ing and

b e e s w a x , w h i c h i n t u r n h e m o l d s

smooth ly to the mos t des i rab le shape .

The prototyping can take several weeks,

and often several attempts have to be

made before a shape is found worth exe-

cuting in wood-a proce ss tha t will take

another few months.

In 1995 , a f te r many concep tua l ex -

changes over the phone, author Sequin

started to develop

a

custom program to

generate shapes that hew ed to the particu-

lar flavor of Collins' recent artwork. The

idea was to greatly extend this family of

s h a p e s t o

a

leve l o f complex i ty tha t

Collins' prototyping process simply could

not handle. The parameterized sculpture-

generator program was fine-tuned for pro-

ducing and displaying virtual prototypes

for this special class of geometric sculp-

tures. The shape s are displayed fast enough

for enjoyable interactive e xploration, but

realistically enough that the viewer can

properly judge the qua lity of the resulting

geometry . F rom a menu , the des igner

selects the basic shape (tower o r ring) and

then adjusts a variety of parameters inter-

actively to obtain an artistically interesting

and p leas ing conf igura t ion . With th is

medium it is easy to explore

a

vast sp ace of

possible shapes, to concentrate on promis-

ing topologies, and then to fine-tune a par-

ticul ar shap e to perfection-all in a mat ter

of minutes Figure 7 shows a n example

generated by the program.

The author's program can also output

t h e b o u n d a r y r e p r e s e n t a t i o n of t h e s e

shapes in

STL

format, the standard data

e x c h a n g e f o r m a t f o r s o l i d f r e e f o r m

machines . The STL f i les output by the

sculpture generator program were trans-

m i t t e d o v e r t h e I n t e r n e t t o M e t a l c a s t

Engineering, Plynetics, Malmberg Engin-

ee r ing, Z -C orp . ,

3D

Systems, Stratasys,

and The University of Texas at Austin.

Small models ranging from

75

to 300 mm

in diameter were fabricated with stere-

o l i thography , se lec tive la ser s in te r ing ,

fused deposition modeling, or

3-D

print-

ing. With turnaround times as short as a

few days, a robust

3-D

artifact was

ob-

tained for inspection and to be shown to

friends and visitors.

In some instances, the solid freeform

model served as more than

a n

early de-

monstration

of

a future sculpture or as the

final proof of concept before Collins com-

mitted to months of toil. It also demon-

strated that a com plex structure was physi-

cally realizable when even an inspection

on the computer left some doubt as to its

freedom from self-intersections.

A

f inal word

Evidently, a variety of uses have al-

ready emerged for solid freeform fabrica-

tion. Many examples of improved time-

to-market and lower-cost product devel-

opment have been reported.

And

users are

finding ways to use the method for more

than just gene rating models. For instance,

the industry is developing th e technolog y

for rapid tool making, in which either pa t-

t e r n s f o r m a k i n g m o l d s , o r t h e m o l d s

themselves, are fabricated by layer-based

m a n u f a c t u r i n g . R e s e a r c h e r s a r e a l s o

advanc ing the techno logy to the po in t

where functional parts can be fabricated

rapidly with these technologies.

While solid freeform fabrication will

p r o b a b l y n e v e r r e p l a c e c o n v e n t i o n a l

manufac tu r ing fo r mass p roduc t ion , i t

can easily be imagined

in

use at remote

sites where spare parts are needed but

are difficult to stockpile . O n e day soo n,

solid freeform workstations may be i n -

stalled on aircraft carriers, the space sta-

tion, or even a Mars base.

Also worth noting is that the technol-

ogy can manufacture products in materi-

a ls o r shapes tha t o the rwise canno t be

manufactured. For instance, since most

forms of solid freeform fabrication are

additive processes, it is possible to con-

t ro l the mate r ia l compos i t ion a t every

point in a part. Consequently, function-

a l l y g r a d i e n t m a t e r i a l s , i n w h i c h t h e

material properties vary spatially, are on

the hor iz on , a l lowing t ru ly op t imized

structures to be fabricated.

It is worth noting, too, that

a

growing

number of researchers and industry prac-

titioners have spotted similarities between

solid freeform fabrication and the

VLSI

industry. Th e interest here is in standard-

izing the data exchange formats and in

developing and codifying design rules to

allow engineers to develop designs and

ship them off t o fabrication houses, confi-

dent that they will work as planned. [See

"NIST's

support of rapid prototyping stan-

dards," pp. 38-39]. Ma ny analogies can

be d rawn be tween even ts tha t a l lowed

VLSl manufacture to develop alon g these

l i n e s , a n d w h a t i s n e e d e d f o r s im i l a r

development for the newer technology.

Through this effort, the

SFF

industry may

one day do fo r mechan ica l des ign and

manufacture what the VLSl industry has

succeeded in doing for electronics.

Spect rum edi tor :

Cadi

Kaplan

C R A W F O R D K E A M A N OLII) FREEFORM FAHRICATION A NE W A1ANUFACTURINC 1 AKAI )ICM

4 3

Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999

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Transcript of Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999