artigo - solid freeform fabrication (sff) - spectrum 1999
TRANSCRIPT
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
1/10
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
2/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
3/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
4/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
5/10
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
TKUM
FFURUARY 1999
8
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
6/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
7/10
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
FEBRUARY
1999
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
8/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
9/10
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
-
7/26/2019 Artigo - Solid Freeform Fabrication (SFF) - Spectrum 1999
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