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