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MIM focus
In the space of a mere 30 years, metal
and ceramic injection moulding
technology, better known by their
acronyms MIM and CIM, have
come a long way, becoming the almost $1
billion industry it is today and now pre-
dicted to pass the $2 billion mark during
this decade.
There are hundreds of MIM firms
worldwide, all producing, or trying to
produce more or less the same products.
These are mainly parts for watches,
medical applications, orthodontics
appliances, computer disk drives, and
parts for the automotive industry.
Understandably some are a little better
at it than others.
While the globalisation of MIM will
undoubtedly continue, it will inevitably
lead to a plateau when supply of MIM
parts overtakes demand. The ensuing
battle for market share will naturally heat
up and - as some soothsayers predict -
culminate in takeovers and a general
shakeout from which only the fittest will
emerge.
But who will be those 'fittest'? Will we
see a repeat of the pattern followed by
Silicon Valley's semiconductor industry in
the 1960s when, drawn by the lure of
cheap labour, entire manufacturing opera-
tions were transplanted to S. E. Asia and
other countries? In that case we can fore-
see a shift in the centre of gravity of the
MIM industry towards Eastern Europe,
China, India or even Africa.
However, cheap labour, like a mirage,
is a fleeting illusion and now increasingly
redundant with the availability of robot-
ics and automation. As in evolution theo-
ry, the survivors will be those who will
adapt to the ever changing market condi-
tions and advances in technology.
The majority of today's commercial
MIM applications are what we could call
“improvements” over conventionally man-
ufactured products. MIM has successfully
displaced many investment cast parts such
as orthodontic appliances, parts requiring
extensive or complex machining opera-
tions like watch and disk drive compo-
nents, or parts where the material is sim-
ply too hard to machine, for example
ceramics or carbides.
MIM's full potential however has yet
to be exploited. MIM can do so much
better than merely produce more cost-
effective versions of existing products. It
is these unprecedented and, in some cases,
“impossible” applications - by today's
standards - that constitute MIM's next
frontier. And the gate is wide open to the
intrepid explorer.
Three obstacles
But, like the great explorers of yore,
to increase our chances of reaching the
“treasures” that lie hidden in strange
new worlds where no one has gone
before, we would do well to first look at
the obstacles we will encounter on our
path. Unlike the 12 labours of Hercules
there are only three, but they are of
stature. They are Fineness, Atmosphere
and Gravity. Let's examine them one by
one and see how we can control them,
and perhaps even turn them into our
allies.
Fines have always been the bane of PM
and in fact of metallurgy in general, per-
haps even of life on Earth, causing all
kinds of problems from dust-laden desert
winds to explosions, volcanic ash, atmos-
pheric haze, turbidity in liquids, pollu-
tion, contamination, abrasion, asthma,
allergies, cancers, silicosis, etc.
Powders for PM are usually classified
to remove the undesirable fines which
have poor pressing characteristics. MIM's
greatest contribution has been to give
those “good-for-nothing” fines the prop-
erty of plasticity, thus allowing us to
shape them into value-added products.
Tomorrow's technology-based manu-
factured goods will have to be made
smaller. We already use our cellular
phones to surf the web, buy shares on the
stock market and take photographs.
Before long we'll also use them to trans-
mit and receive video signals, monitor our
body functions, or to instruct the home
robot to cook fettuccine alla carbonara
and select a nice wine to go with it. All
this amounts to squeezing more things
with greater functionality into the limited
space of a handheld gadget, driving
designers of electronic products into so-
called “system-on-a-chip” (SOC), “chip-
scale-package” (CSP) and multilevel pack-
age design - and undoubtedly up the wall
as well.
But miniaturisation can have a radical
domino effect. Size reduction of integrat-
ed systems implies that not just one but
all components of the system be made
smaller, including the tools to fabricate
these. The situation is analogous to that
of the Swiss watch industry at the turn
of the 18th Century when fabrication of
tiny gear trains had to wait until micro-
gear cutting machines were first
designed. Things may go fairly well
until suddenly an impassable obstacle
pops up.
As integrated circuit technology forges
ahead towards higher integration, there is
a corresponding increase in the I/O
(input/output) count. That is the number
of interconnections between the silicon
chip and the external circuitry, which
requires the bond pitch - the distance
between contiguous interconnections - to
decrease proportionally. This in turn
requires that wirebonding tools, the
minuscule ceramic tubelets with orifices
as small as 25 micrometres (µm), must
also be made smaller. State-of-the-art
wirebonding tool manufacturing technol-
ogy is unable to oblige. This problem
constitutes today's biggest bottleneck in
Ultra Large Scale Integration.
In summary, conventional machining
technology is unable to produce present
day's microprecision and tomorrow's
nanoscale parts. Moulding these items
22 MPR September 2003 0026-0657/03 ©2003 Elsevier Ltd. All rights reserved.
The shape ofthe future?MIM experts R L Billiet and H T Nguyen pay tribute to the remarkable progress the industryhas made in 30 years while glancing forward tothe next frontier, it’s potential problems andways to circumvent them...
MIM focus
using MIM technology however is sensi-
ble, cost-effective and, in some cases, the
only viable alternative. But attempting to
fit a 10 µm powder particle, a typical con-
stituent in today's MIM powders, into a
12 µm concave mold cavity feature would
be like asking a blindfolded player to
score in basketball. Thus we have no
choice but to use sub-micrometre powders
or nanoparticulates as they are now
increasingly called.
While fineness is key to achieving
micro- and nano- design features,
nanoparticulates bring along their own
problems. Producing them is difficult
enough and always costly. The real and as
yet unsolved problem is their handling.
Imagine, we are talking about particles
the size of those in cigarette smoke, i.e.
typically 0.01-1.0 µm.
As a powder's particle size goes down,
its specific surface area and consequently
its surface activity go up with a concomi-
tant increase in the tendency to form
strongly bonded agglomerates resulting in
difficulty to achieve a high volume load-
ing (also called packing density) in the
feedstock. Nanoparticulates also display
lower sintering temperatures, faster sin-
tering kinetics with associated increased
grain growth.
A particulate material's average parti-
cle diameter allows us to estimate its spe-
cific surface area, usually expressed in
m2/g. For a population of uniform spheri-
cal particles we can use the formula
(1)
where
A is the specific surface area, in m2/g
δ is the density, in g/cm3, and
d is the particle diameter, in micro-
metres (µm)
Equation (1) shows us that, at con-
stant density, the product of the specific
surface area times the average particle
diameter is a constant. Thus, if we were
to comminute a powder consisting of
uniform spherical particles of diameter
30 µm into 0.03 µm diameter nanos-
pheres, the specific surface would
increase thousand fold.
State-of-the-art MIM is unable to han-
dle the high surface activity associated
with nanoparticulate materials. When
making a MIM feedstock, the polymeric
binder has to stick to or “wet” the filler
powder in order to get a high volume load-
ing. The higher the filler's surface activity
the more difficult it becomes to wet it.
Improved wettability
A particulate material that doesn't
wet will not disperse in the binder but
instead forms strong agglomerates as the
affinity between particle surfaces is
stronger than between particle surface
and binder molecules,
MIM can overcome the surface activi-
ty problems associated with nanopartic-
ulate materials by the use of surface
active agents or surfactants. By coating
the surface of the particulates with a
molecular monolayer of a suitable sur-
factant, the surface activity can be dras-
tically reduced so that the polymeric
binder will now wet the thus surfactant-
coated particulates.
The amount of surfactant needed is
only an infinitesimal fraction of the total
mass of the binder. Also, in most cases it
is unnecessary to coat the entire surface
area of the particulates.
When as little as 25 per cent of their
surface is coated, their wettability will
metal-powder.net September 2003 MPR 23
Figure 1: The Shuttle’s takeoff is assisted by more than 350 000 pounds of fine aluminiumpowder, all burned in less than two minutes. Photograph: Courtesy NASA.
MIM focus
already have greatly improved while coat-
ing more than 50 per cent habitually does
not carry additional benefits.
If aliens from some distant planet
were to visit us, their first question
would probably be: How can you guys
survive in this poisonous gas? Our scien-
tists tell us things haven't always been
that bad and that there were the good
old days, at least for the anaerobic life
forms from which we evolved, when there
was hardly any gaseous oxygen on our
planet. But right now there's plenty of it
and it burns - sometimes slowly, some-
times fast - our cars, our ships, our
bridges, our forests, the Eiffel tower, and
even ourselves.
Reactive powders
Because of atmospheric oxygen, fine
metal powders are pyrophoric, hence
their use in fireworks and rockets. The
Space Shuttle is put into orbit by burning
about 352,000 pounds of fine aluminium
powder, all of it in less than two minutes.
Most nanopowders are so reactive they
need to be constantly kept under a
blanket of inert gas. This makes fabrica-
tion and processing of nanoparticulate
materials complicated and extremely
costly.
MIM can overcome the pyrophoricity
problem associated with nanoparticulate
materials by coating the surface of the
already surfactant-coated particles with a
polymeric binder that will effectively
shield them from contact with atmospher-
ic oxygen. Consequently, a properly pre-
pared feedstock can be handled and
moulded without having to place the
entire operation under inert gas (with
moulding operators in scuba diving out-
fits). As dewaxing and sintering are cus-
tomarily performed in an oxygen-free
atmosphere, the issue of pyrophoricity
becomes immaterial once the green parts
have been moulded. Clearly the use of
water as a solvent to extract water-soluble
binders from green nanostructures
becomes questionable.
With a few rare exceptions, most of us
spend our entire lives forcibly stuck to the
Earth's surface. We are so complacent
about living in a gravitational field that
we hardly ever realize that, each time we
step on our bathroom scale, we measure
its effect on our body. MIM part produc-
ers on the other hand, are constantly
reminded of gravity.
During binder removal, whatever contri-
bution the binder was making to the green
part's tensile strength evanesces. At the out-
set of sintering, interparticulate bond for-
mation gradually builds up the tensile
strength again. Between these two events,
the compact's tensile strength goes through
a minimum that is often insufficient to
counter the gravitational pull. As a result
the part will droop or sag. This is a major
problem in MIM and at present there are
only partial solutions to alleviate it.
The magnitude of gravitational sag
depends on a number of factors. Factors
related to the filler are its particle size,
shape, surface morphology and density.
Large dense particles are subjected to a
greater force than small lightweight ones
following Newton's second law of motion.
Smooth spherical particles will sag more
than spiky ones which tend to mechanical-
ly interlock. Factors related to the green
part are its geometry and density.
A green part in the shape of a pyramid
will be more resistant to sagging than
a part with a long cantilever feature.
A highly loaded part, i.e. moulded from
a feedstock with high packing density,
will deform less. Factors related to the
processing environment include the sinter-
ing atmosphere, the rate of temperature
rise, the support on which the part is
placed, mechanical vibration transmitted
from circulating fans and vacuum pumps,
among others. Although the problem of
gravitational sag has not been entirely
overcome, many proprietary "tricks" exist
to mitigate its effects.
So now that we are at least aware of
the obstacles, let's boldly go and see what
lies ahead in MIM.
Designer materials on demand
In the early days of MIM the only
fine metal powders available were car-
bonyl iron and nickel. Almost anticipat-
ed for MIM, these powders, with particle
sizes in the 3-8 µm range, were essential-
ly spherical, relatively inexpensive and
easy to process as they could be sintered
to near full density at temperatures of
only 1200ºC (2200ºF) in forming gas, a
non-flammable mixture of 10 per cent
hydrogen in argon, thus obviating the
need for sophisticated sintering
equipment.
MIM's first large-scale commercial
production of nickel-iron parts was sin-
tered in cheap ceramic hobbyist kilns
placed in a steel tank through which
forming gas was made to flow. Part buyers
in need of stainless steel had to content
themselves with nickel-iron alloys con-
taining sufficient nickel to make them
corrosion resistant. As things stand, many
contemporary MIM part producers have
yet to emerge from MIM's “nickel-iron
age”.
With the erection of the world's largest
inert gas atomiser in the late 1970s by
Avesta (now Carpenter Powder Products),
fine spherical prealloyed 316L stainless
steel powder became routinely available,
soon followed by Pfizer's (now Ametek
Specialty Metal Products) MIM 17-4PH
stainless steel, developed for a military
application.
These days, although many powder
suppliers will - for a price - produce
almost any alloy composition in grades
suitable for MIM, the iron and nickel car-
bonyls, 316L and 17-4PH, together with
24 MPR September 2003 metal-powder.net
A
DE
CB
Figure 2. Getting smaller! Miniaturisation is an area where MIM can go, but others can’t follow.
MIM focus
tungsten carbide powders and Alcoa's
superground alumina remain the main-
stay of today's MIM industry. So, after 30
years, we have just half a dozen powders
that are used to produce probably well
over 90 per cent of all contemporary
MIM applications.
It is a common scenario for a part
buyer to approach a MIM firm in the
hope that his machining-intensive appli-
cation, say a brass watchcase, can be pro-
duced more economically by MIM. The
client's material specification calls for
brass only because this material is avail-
able, cheap, and easy to machine. In a sit-
uation like this the MIM part producer
will, almost invariably, suggest to pro-
duce the watchcase in 316L. If, for some
outlandish reason - fear of material sub-
stitution is a very common one - the
client would insist on brass, it would cer-
tainly be possible to produce a MIM
brass feedstock but its cost would over-
shadow that of 316L so that machining
the watchcase from brass bar stock
would, in the end, remain the cheaper
manufacturing route.
This points to two significant problems
in MIM. One is the persistent limited
availability of fine metal powders in a wide
range of compositions and at prices that
will allow MIM to compete with alterna-
tive forming techniques.
MEMS and nanotechnology
The second problem is that the bur-
geoning micro-electrical mechanical sys-
tems (MEMS) and nanotechnology indus-
tries are generating a demand for applica-
tion-specific “designer” materials. These
are material compositions with special
properties or combinations of properties,
e.g. high temperature superconductivity
and corrosion-resistance.
Also for applications such as MEMS,
new material compositions may have to
be designed to overcome the shortcom-
ings of the materials we have been
using for the past century. One of these
shortcomings is inhomogeneity at the
submicrometre level. To visualise this
problem, all we have to do to is look at
metallographic microstructures where
one grain of austenite in a steel may be
contiguous to a chromium-rich carbide
precipitate.
Finally, the development of new and
advanced products always requires so-
called “first article” batches for product
evaluation. It is therefore urgent that
MIM firms be in a position to procure
economically and rapidly - within hours
or at most a few days - small quantities of
filler materials, much like one can today
go to a coffee bean shop, blend a mix of
different coffees from all over the world
and grind it to whatever degree of fine-
ness desired.
The term “nanotechnology” has
become a buzzword in recent years. It is
also one that is increasingly misunder-
stood. For many scientists, nanotechnolo-
gy means the research aimed at eventually
building structures from individual atoms
and molecules. Often the more appropri-
ate term “molecular manufacturing” is
preferred to avoid confusion with another
definition of the term, namely any manu-
facturing technology aimed at creating
nanostructures - physical features with
dimensions in nanometres. Thus, under
this second definition nanoparticulates
are simply submicrometre particles. In
this context we clearly refer to nanotech-
nology in the sense of nanoscale, not
molecular manufacturing, and yet…
On the other hand, the term micro-
electro-mechanical systems (MEMS), or
Microsystems as they are called in
metal-powder.net September 2003 MPR 25
Figure 3. Folding wing mirrors on cars are a common example of a MEMS system application.
Let's perform an imaginary experiment by taking 1 g of stainless steel spherical
particles of diameter d = 10 µm and density d = 7.8 g/cm3 and align them neatly so
that they just touch each other, like a row of marbles or a string of pearls. Upon
raising the temperature sufficiently the particles sinter to each other and the centre
to centre distance between contiguous particles becomes smaller. If we continue to
raise the temperature, and neglecting any frictional forces, - say we conduct this
experiment in the cargo bay of the Space Shuttle - we will eventually end up with a
single stainless steel sphere of diameter D, given by
(2)
D is also the ultimate sintered dimension to which the length of our original
“green” string has now regressed. So far everything looks normal, right? But wait
till you see the length L of our “green” string, i.e. the number of particles times d,
thus
(3)
This hypothetical diversion is just meant to illustrate the amazing dimension-
reducing potential afforded by the sintering phenomenon. As can be seen, shrinkage
is independent of particle size. In MIM, green part shrinkage is unaffected by the
filler's particle size as it only depends on the feedstock's volume loading.
The suprise of sintering
MIM focus
26 MPR September 2003 metal-powder.net
Europe where the acronym MST
(Microsystems Technology) is also used,
is much easier to define as it refers to the
microscopic devices combining mechani-
cal and electronic components now
increasingly found in defense, medical,
electronic, communication, and automo-
tive applications.
The importunate crux of the matter is
that we have to make things smaller.
This is not the latest short-lived craze
but a pressing necessity for modern life.
In his visionary lecture entitled “There's
Plenty of Room at the Bottom” present-
ed at the California Institute of
Technology in December 1959 - for many
the very foundation stone of nanotech-
nology - Richard Feynman, the eccentric,
irreverent, conga-playing Nobel laureate
physicist, alluded to the possibility, in
surgery, to swallow a mechanical “sur-
geon” who would go on an inspection
tour of the patient's innards and fix
things wherever needed.
The idea, derided at the time by the
scientific community, was quickly picked
up by Hollywood in “The Fantastic
Voyage” in which a team of shrunken sur-
geons, including shapely medical assistant
Raquel Welch, travel in a micro-submarine
to the patient's brain to undo a blood clot.
Forty years after Feynman's talk, British
scientists developed a video pill that, when
swallowed by the patient, travels through
the gastrointestinal tract taking and send-
ing pictures, a modest yet important first
step towards Feynman's surgeon.
While the number of potential appli-
cations may be staggering, many practi-
cal, technological and economic chal-
lenges strew the path to commercialisa-
tion of nanostructures. Most of these
stem from the fact
that today's nanosys-
tems are made like
semiconductors.
Hence the selection
of materials and
freedom of design
are limited, mass-
production is com-
plicated and invest-
ment and operating
costs of wafer fab
type cleanrooms are
prohibitive.
But the most
pressing issue is the
integration of the
molecular machinery of nanosystems into
MEMS, essential to the commercial devel-
opment of nanotechnology applications.
No matter whether it is a molecular or
nanoscale manufactured device, it has to
be packaged into some kind of box so
that it is protected and can be implanted
or integrated.
In machining we work from the out-
side towards the inside. When
Michelangelo made his four- metre (14 ft)
tall sculpture of David, he chipped away
at a block of marble for three years. A
Tudor Oyster™ watch case takes 162
painstaking (Tudor's words, not the
authors') machining operations. In
moulding we work from the inside to the
outside. We basically splash a muddy sub-
stance against a solid wall, let it solidify a
little so that we can peel it off and bingo,
we have a perfect replica of the wall's
surface.
‘We are too big’
Moulding allows us to use the same
mould for many products. This is much
better than micromachining where it is
difficult to rigorously hold the same
dimensions, especially to within submi-
crometre tolerance limits. But a micro-
mould can be made out of a very hard
material like tungsten carbide or cubic
boron nitride, then kept at strictly con-
stant temperature so that it will have a
virtually constant shape. Then we can
mass produce to our heart's content. So
all we need now is micromoulds. Who
will machine these? Even the best of
today's best Swiss watchmaking shops are
not up to the challenge. Feynman, in his
landmark lecture, clearly foresaw the
problem:
“Why can't we drill holes, cut things,
stamp things out, mould different shapes
all at an infinitesimal level? What are the
limitations as to how small a thing
has to be before you can no longer
mould it?”
He also envisaged a solution, suggest-
ing the use of a lathe to machine the com-
ponents of a smaller lathe and then using
that smaller lathe to machine the compo-
nents of a yet smaller lathe. But the real
problem, Feynman conceded was:
“… it is something, in principle, that
can be done; but in practice, it has not
been done because we are too big.”
Nature has given us this wonderful
phenomenon called shrinkage which we
haven't fully exploited, and certainly
never commercially.
Shrinkage upon sintering is a formi-
dable tool for miniaturisation. To make a
micromould we first fabricate a
macromould as small as present day
micromachining techniques will allow
us to do.
Now we mould a green part in this
macromould and process it upon which it
shrinks. Next, using the sintered part as a
core insert in a mould cavity, we mould a
new green part around it. This will give
us, after processing and shrinkage, a
miniature replica of our original macro-
mould that would have been impossible
to machine. We then repeat the whole
sequence as many times as we want.
After n iterations - an iteration being one
complete miniaturisation from moulding
a green part; processing it; using it as a
core; moulding a second green part and
processing that one too - and assuming
we always use the same feedstock with
shrinkage factor K (cavity dimension
divided by sintered dimension), the origi-
nal dimension Lo of our macromould
will have become Ln with
(4)
From which we get
(5)
Equation (5) is our miniaturisation
formula, giving us the shrinkage factor
as a function of the size reduction rate
and the number of iterations. Let's try
Figure 4. This tiny cog, moulded by microMIM techniques, is just0.85mm in diameter. Photograph: Courtesy FraunhoferGesellschaft.
MIM focus
it out. Suppose we want to make a micro-
mould half the size of our original
macromould. Of course we want to
fabricate our micromould in the mini-
mum number of iterations, but are not
sure we can formulate a workable mould-
ing feedstock with a large enough shrink-
age factor. Let's see if we can get by with
a single iteration, i.e. n = 1
Eq. (5) with and n = 1 yields
(6)
While feedstocks with larger shrinkage
factors have been processed, it would be
quite a challenge to work with it. Let's see
what happens if we do two iterations, i.e.
n = 2
(7)
This shrinkage factor is typical of
moulding feedstocks in common use today.
Thus, in just two iterations we can produce
a micromould that is half the size of a
macromould which, itself, is at the very lim-
its of what micromachining can produce.
We have solved Feynman's dilemma of
having to fabricate smaller and smaller
lathes for which, in any case, there would
be no operators. In his landmark paper,
Feynman had thought of everything and
suggested training ants who would train
mites to operate the lathes. Now we can
leave the ants and mites in peace as we
don't need the lathes anymore. We simply
let the phenomenon of sintering do the job.
The final frontier
The ideal location for futuristic MIM is
the near-Earth microgravity environment.
In a microgravity environment parts obvi-
ously would not sag during sintering. Nor
would they distort as a result of the
inevitable friction on their support during
shrinkage since in space there would be no
need for a support. This would remove the
current size constraints of MIM parts. In
space, parts as big as a house would be no
problem. This holds important rewards for
the fabrication of large structural elements
of future space stations.
All kinds of interesting scenarios can
be imagined. If a small meteor should
damage a space station, there would be
no need to send somebody back to Earth
to get spares. New beams, nuts and bolts,
could be shaped from a ball of feedstock
like clay dough, then sintered.
Yes, you may say, but you still have to
go back to Earth to fetch your raw mate-
rials. But that is not really true.
Space around the Earth abounds with
fine cosmic dust which is something of a
headache for spacecraft. Over 100
tonnes of it, containing silicates, car-
bonaceous material but also nickel, iron
and magnesium fall on Earth daily.
Space is also full of organic molecules.
Recently scientists discovered vinyl alco-
hol in space dust.
Finally, the necessity to shield the
parts from atmospheric oxygen would
also be superfluous so there would be no
need for a sintering furnace either. We
could just heat the parts as they orbit,
using a modern version of Archimedes'
"burning mirror".
A proposal to sinter MIM parts on
board the Space Shuttle has been
submitted to a major NASA
subcontractor.
metal-powder.net September 2003 MPR 27
This article will formthe basis of onechapter of a forth-coming book by RomBilliet and HanhNguyen on MIM andCIM. Entitled A prac-tical guide to Metaland CeramicMoulding, it will bepublished by Elseviernext Spring: Price�262.
A book worth reading