The shape of the future?

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  • MIM focus

    In the space of a mere 30 years, metaland ceramic injection mouldingtechnology, better known by theiracronyms MIM and CIM, have

    come a long way, becoming the almost $1billion industry it is today and now pre-dicted to pass the $2 billion mark duringthis decade.

    There are hundreds of MIM firmsworldwide, all producing, or trying toproduce more or less the same products.These are mainly parts for watches, medical applications, orthodontics appliances, computer disk drives, andparts for the automotive industry.Understandably some are a little betterat it than others.

    While the globalisation of MIM willundoubtedly continue, it will inevitablylead to a plateau when supply of MIMparts overtakes demand. The ensuingbattle for market share will naturally heatup and - as some soothsayers predict -culminate in takeovers and a generalshakeout from which only the fittest willemerge.

    But who will be those 'fittest'? Will wesee a repeat of the pattern followed bySilicon Valley's semiconductor industry inthe 1960s when, drawn by the lure ofcheap labour, entire manufacturing opera-tions were transplanted to S. E. Asia andother countries? In that case we can fore-see a shift in the centre of gravity of theMIM industry towards Eastern Europe,China, India or even Africa.

    However, cheap labour, like a mirage,is a fleeting illusion and now increasinglyredundant with the availability of robot-ics and automation. As in evolution theo-ry, the survivors will be those who willadapt to the ever changing market condi-tions and advances in technology.

    The majority of today's commercialMIM applications are what we could callimprovements over conventionally man-ufactured products. MIM has successfullydisplaced many investment cast parts suchas orthodontic appliances, parts requiringextensive 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 exampleceramics or carbides.

    MIM's full potential however has yetto be exploited. MIM can do so muchbetter than merely produce more cost-effective versions of existing products. Itis these unprecedented and, in some cases,impossible applications - by today'sstandards - that constitute MIM's nextfrontier. And the gate is wide open to theintrepid explorer.

    Three obstacles

    But, like the great explorers of yore,to increase our chances of reaching thetreasures that lie hidden in strangenew worlds where no one has gonebefore, we would do well to first look atthe obstacles we will encounter on ourpath. Unlike the 12 labours of Herculesthere are only three, but they are ofstature. They are Fineness, Atmosphereand Gravity. Let's examine them one byone and see how we can control them,and perhaps even turn them into ourallies.

    Fines have always been the bane of PMand in fact of metallurgy in general, per-haps even of life on Earth, causing allkinds of problems from dust-laden desertwinds to explosions, volcanic ash, atmos-pheric haze, turbidity in liquids, pollu-tion, contamination, abrasion, asthma,allergies, cancers, silicosis, etc.

    Powders for PM are usually classifiedto remove the undesirable fines whichhave poor pressing characteristics. MIM'sgreatest contribution has been to givethose good-for-nothing fines the prop-erty of plasticity, thus allowing us toshape them into value-added products.

    Tomorrow's technology-based manu-factured goods will have to be madesmaller. We already use our cellularphones to surf the web, buy shares on thestock market and take photographs.Before long we'll also use them to trans-mit and receive video signals, monitor ourbody functions, or to instruct the homerobot to cook fettuccine alla carbonaraand select a nice wine to go with it. Allthis amounts to squeezing more thingswith greater functionality into the limitedspace of a handheld gadget, drivingdesigners of electronic products into so-called system-on-a-chip (SOC), chip-scale-package (CSP) and multilevel pack-age design - and undoubtedly up the wallas well.

    But miniaturisation can have a radicaldomino effect. Size reduction of integrat-ed systems implies that not just one butall components of the system be madesmaller, including the tools to fabricatethese. The situation is analogous to thatof the Swiss watch industry at the turnof the 18th Century when fabrication oftiny gear trains had to wait until micro-gear cutting machines were firstdesigned. Things may go fairly well until suddenly an impassable obstaclepops up.

    As integrated circuit technology forgesahead towards higher integration, there isa corresponding increase in the I/O(input/output) count. That is the numberof interconnections between the siliconchip and the external circuitry, whichrequires the bond pitch - the distancebetween contiguous interconnections - todecrease proportionally. This in turnrequires that wirebonding tools, theminuscule ceramic tubelets with orificesas small as 25 micrometres (m), mustalso be made smaller. State-of-the-artwirebonding tool manufacturing technol-ogy is unable to oblige. This problemconstitutes today's biggest bottleneck inUltra Large Scale Integration.

    In summary, conventional machiningtechnology is unable to produce presentday's microprecision and tomorrow'snanoscale 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, its potential problems andways to circumvent them...

  • MIM focus

    using MIM technology however is sensi-ble, cost-effective and, in some cases, theonly viable alternative. But attempting tofit a 10 m powder particle, a typical con-stituent in today's MIM powders, into a12 m concave mold cavity feature wouldbe like asking a blindfolded player toscore in basketball. Thus we have nochoice but to use sub-micrometre powdersor nanoparticulates as they are nowincreasingly called.

    While fineness is key to achievingmicro- and nano- design features,nanoparticulates bring along their ownproblems. Producing them is difficultenough and always costly. The real and asyet unsolved problem is their handling.Imagine, we are talking about particlesthe size of those in cigarette smoke, i.e.typically 0.01-1.0 m.

    As a powder's particle size goes down,its specific surface area and consequentlyits surface activity go up with a concomi-tant increase in the tendency to formstrongly bonded agglomerates resulting indifficulty to achieve a high volume load-ing (also called packing density) in thefeedstock. Nanoparticulates also displaylower sintering temperatures, faster sin-tering kinetics with associated increasedgrain growth.

    A particulate material's average parti-cle diameter allows us to estimate its spe-cific surface area, usually expressed inm2/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 is the density, in g/cm3, and d is the particle diameter, in micro-metres (m)

    Equation (1) shows us that, at con-stant density, the product of the specificsurface area times the average particlediameter is a constant. Thus, if we wereto comminute a powder consisting ofuniform spherical particles of diameter30 m into 0.03 m diameter nanos-pheres, the specific surface wouldincrease thousand fold.

    State-of-the-art MIM is unable to han-dle the high surface activity associatedwith nanoparticulate materials. When

    making a MIM feedstock, the polymericbinder has to stick to or wet the fillerpowder in order to get a high volume load-ing. The higher the filler's surface activitythe more difficult it becomes to wet it.

    Improved wettability

    A particulate material that doesn'twet will not disperse in the binder butinstead forms strong agglomerates as theaffinity between particle surfaces isstronger than between particle surfaceand binder molecules,

    MIM can overcome the surface activi-ty problems associated with nanopartic-

    ulate materials by the use of surfaceactive agents or surfactants. By coatingthe surface of the particulates with amolecular monolayer of a suitable sur-factant, the surface activity can be dras-tically reduced so that the polymericbinder will now wet the thus surfactant-coated particulates.

    The amount of surfactant needed isonly an infinitesimal fraction of the totalmass of the binder. Also, in most cases itis unnecessary to coat the entire surfacearea of the particulates.

    When as little as 25 per cent of theirsurface is coated, their wettability will

    metal-powder.net September 2003 MPR 23

    Figure 1: The Shuttles 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 doesnot carry additional benefits.

    If aliens from some distant planetwere to visit us, their first questionwould probably be: How can you guyssurvive in this poisonous gas? Our scien-tists tell us things haven't always beenthat bad and that there were the goodold days, at least for the anaerobic lifeforms from which we evolved, when therewas hardly any gaseous oxygen on ourplanet. But right now there's plenty of itand it burns - sometimes slowly, some-times fast - our cars, our ships, ourbridges, our forests, the Eiffel tower, andeven ourselves.

    Reactive powders

    Because of atmospheric oxygen, finemetal powders are pyrophoric, hencetheir use in fireworks and rockets. TheSpace Shuttle is put into orbit by burningabout 352,000 pounds of fine aluminiumpowder, all of it in less than two minutes.Most nanopowders are so reactive theyneed to be constantly kept under a blanket of inert gas. This makes fabrica-tion and processing of nanoparticulatematerials complicated and extremelycostly.

    MIM can overcome the pyrophoricityproblem associated with nanoparticulatematerials by coating the surface of thealready surfactant-coated particles with apolymeric binder that will effectivelyshield them from contact with atmospher-ic oxygen. Consequently, a properly pre-pared feedstock can be handled andmoulded without having to place theentire operation under inert gas (with

    moulding operators in scuba diving out-fits). As dewaxing and sintering are cus-tomarily performed in an oxygen-freeatmosphere, the issue of pyrophoricitybecomes immaterial once the green partshave been moulded. Clearly the use ofwater as a solvent to extract water-solublebinders from green nanostructuresbecomes questionable.

    With a few rare exceptions, most of usspend our entire lives forcibly stuck to theEarth's surface. We are so complacentabout living in a gravitational field thatwe hardly ever realize that, each time westep on our bathroom scale, we measureits effect on our body. MIM part produc-ers on the other hand, are constantlyreminded of gravity.

    During binder removal, whatever contri-bution the binder was making to the greenpart's tensile strength evanesces. At the out-set of sintering, interparticulate bond for-mation gradually builds up the tensilestrength again. Between these two events,the compact's tensile strength goes througha minimum that is often insufficient tocounter the gravitational pull. As a resultthe part will droop or sag. This is a majorproblem in MIM and at present there areonly partial solutions to alleviate it.

    The magnitude of gravitational sagdepends on a number of factors. Factorsrelated to the filler are its particle size,shape, surface morphology and density.Large dense particles are subjected to agreater force than small lightweight onesfollowing Newton's second law of motion.Smooth spherical particles will sag morethan spiky ones which tend to mechanical-ly interlock. Factors related to the greenpart are its geometry and density.

    A green part in the shape of a pyramidwill 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 theprocessing environment include the sinter-ing atmosphere, the rate of temperaturerise, the support on which the part isplaced, mechanical vibration transmittedfrom circulating fans and vacuum pumps,among others. Although the problem ofgravitational sag has not been entirelyovercome, many proprietary "tricks" existto mitigate its effects.

    So now that we are at least aware ofthe obstacles, let's boldly go and see whatlies ahead in MIM.

    Designer materials on demand

    In the early days of MIM the onlyfine metal powders available were car-bonyl iron and nickel. Almost anticipat-ed for MIM, these powders, with particlesizes in the 3-8 m range, were essential-ly spherical, relatively inexpensive andeasy to process as they could be sinteredto near full density at temperatures ofonly 1200C (2200F) in forming gas, anon-flammable mixture of 10 per centhydrogen in argon, thus obviating theneed for sophisticated sintering equipment.

    MIM's first large-scale commercialproduction of nickel-iron parts was sin-tered in cheap ceramic hobbyist kilnsplaced in a steel tank through whichforming gas was made to flow. Part buyersin need of stainless steel had to contentthemselves with nickel-iron alloys con-taining sufficient nickel to make themcorrosion resistant. As things stand, manycontemporary MIM part producers haveyet to emerge from MIM's nickel-ironage.

    With the erection of the world's largestinert gas atomiser in the late 1970s byAvesta (now Carpenter Powder Products),fine spherical prealloyed 316L stainlesssteel powder became routinely available,soon followed by Pfizer's (now AmetekSpecialty Metal Products) MIM 17-4PHstainless steel, developed for a militaryapplication.

    These days, although many powdersuppliers will - for a price - producealmost any alloy composition in gradessuitable 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 cant follow.

  • MIM focus

    tungsten carbide powders and Alcoa'ssuperground alumina remain the main-stay of today's MIM industry. So, after 30years, we have just half a dozen powdersthat are used to produce probably wellover 90 per cent of all contemporaryMIM applications.

    It is a common scenario for a partbuyer to approach a MIM firm in thehope that his machining-intensive appli-cation, say a brass watchcase, can be pro-duced more economically by MIM. Theclient's material specification calls forbrass only because this material is avail-able, cheap, and easy to machine. In a sit-uation like this the MIM part producerwill, almost invariably, suggest to pro-duce the watchcase in 316L. If, for someoutlandish reason - fear of material sub-stitution is a very common one - theclient would insist on brass, it would cer-tainly be possible to produce a MIMbrass feedstock but its cost would over-shadow that of 316L so that machiningthe watchcase from brass bar stockwould, in the end, remain the cheapermanufacturing route.

    This points to two significant problemsin MIM. One is the persistent limited

    availability of fine metal powders in a widerange of compositions and at prices thatwill 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. Theseare material compositions with specialproperties or combinations of properties,e.g. high temperature superconductivityand corrosion-resistance.

    Also for applications such as MEMS,new material compositions may have tobe designed to overcome the shortcom-ings of the materials we have been using for the past century. One of theseshortcomings is inhomogeneity at thesubmicrometre level. To visualise thisproblem, all we have to do to is look atmetallographic microstructures whereone grain of austenite in a steel may becontiguous to a chromium-rich carbideprecipitate.

    Finally, the development of new andadvanced products always requires so-called first article batches for productevaluation. It is therefore urgent thatMIM firms be in a position to procureeconomically and rapidly - within hoursor at most a few days - small quantities offiller materials, much like one can todaygo to a coffee bean shop, blend a mix ofdifferent coffees from all over the worldand grind it to whatever degree of fine-ness desired.

    The term nanotechnology hasbecome a buzzword in recent years. It isalso one that is increasingly misunder-stood. For many scientists, nanotechnolo-gy means the research aimed at eventuallybuilding structures from individual atomsand molecules. Often the more appropri-ate term molecular manufacturing ispreferred to avoid confusion with anotherdefinition of the term, namely any manu-facturing technology aimed at creatingnanostructures - physical features withdimensions in nanometres. Thus, underthis second definition nanoparticulatesare simply submicrometre particles. Inthis context we clearly refer to nanotech-nology in the sense of nanoscale, notmolecular manufacturing, and yet

    On the other hand, the term micro-electro-mechanical systems (MEMS), orMicrosystems 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 sphericalparticles of diameter d = 10 m and density d = 7.8 g/cm3 and align them neatly sothat they just touch each other, like a row of marbles or a string of pearls. Uponraising the temperature sufficiently the particles sinter to each other and the centreto centre distance between contiguous particles becomes smaller. If we continue toraise the temperature, and neglecting any frictional forces, - say we conduct thisexperiment in the cargo bay of the Space Shuttle - we will eventually end up with asingle stainless steel sphere of diameter D, given by

    (2)

    D is also the ultimate sintered dimension to which the length of our originalgreen string has now regressed. So far everything looks normal, right? But waittill 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, shrinkageis independent of particle size. In MIM, green part shrinkage is unaffected by thefiller'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 themicroscopic devices combining mechani-cal and electronic components nowincreasingly found in defense, medical,electronic, communication, and automo-tive applications.

    The importunate crux of the matter isthat we have to make things smaller.This is not the latest short-lived crazebut a pressing necessity for modern life.In his visionary lecture entitled There'sPlenty of Room at the Bottom present-ed at the California Institute ofTechnology in December 1959 - for manythe very foundation stone of nanotech-nology - Richard Feynman, the eccentric,irreverent, conga-playing Nobel laureatephysicist, alluded to the possibility, insurgery, to swallow a mechanical sur-geon who would go on an inspectiontour of the patient's innards and fixthings wherever needed.

    The idea, derided at the time by thescientific community, was quickly pickedup by Hollywood in The FantasticVoyage in which a team of shrunken sur-geons, including shapely medical assistantRaquel Welch, travel in a micro-submarineto the patient's brain to undo a blood clot.

    Forty years after Feynman's talk, Britishscientists developed a video pill that, whenswallowed by the patient, travels throughthe gastrointestinal tract taking and send-ing pictures, a modest yet important firststep 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 factthat today's nanosys-tems are made likesemiconductors.Hence the selectionof materials andfreedom of designare limited, mass-production is com-plicated and invest-ment and operatingcosts of wafer fabtype cleanrooms areprohibitive.

    But the mostpressing issue is theintegration of the

    molecular machinery of nanosystems intoMEMS, essential to the commercial devel-opment of nanotechnology applications.No matter whether it is a molecular ornanoscale manufactured device, it has tobe packaged into some kind of box sothat it is protected and can be implantedor integrated.

    In machining we work from the out-side towards the inside. WhenMichelangelo made his four- metre (14 ft)tall sculpture of David, he chipped awayat a block of marble for three years. ATudor Oyster watch case takes 162painstaking (Tudor's words, not theauthors') machining operations. Inmoulding we work from the inside to theoutside. We basically splash a muddy sub-stance against a solid wall, let it solidify alittle 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 samemould for many products. This is muchbetter than micromachining where it isdifficult to rigorously hold the samedimensions, especially to within submi-crometre tolerance limits. But a micro-mould can be made out of a very hardmaterial like tungsten carbide or cubicboron nitride, then kept at strictly con-stant temperature so that it will have avirtually constant shape. Then we canmass produce to our heart's content. Soall we need now is micromoulds. Whowill machine these? Even the best oftoday's best Swiss watchmaking shops arenot up to the challenge. Feynman, in hislandmark lecture, clearly foresaw theproblem:

    Why can't we drill holes, cut things,stamp things out, mould different shapesall at an infinitesimal level? What are thelimitations as to how small a thing has to be before you can no longermould it?

    He also envisaged a solution, suggest-ing the use of a lathe to machine the com-ponents of a smaller lathe and then usingthat smaller lathe to machine the compo-nents of a yet smaller lathe. But the realproblem, Feynman conceded was:

    it is something, in principle, thatcan be done; but in practice, it has notbeen done because we are too big.

    Nature has given us this wonderfulphenomenon called shrinkage which wehaven't fully exploited, and certainlynever commercially.

    Shrinkage upon sintering is a formi-dable tool for miniaturisation. To make amicromould we first fabricate a macromould as small as present daymicromachining techniques will allow us to do.

    Now we mould a green part in thismacromould and process it upon which itshrinks. Next, using the sintered part as acore insert in a mould cavity, we mould anew green part around it. This will giveus, after processing and shrinkage, aminiature replica of our original macro-mould that would have been impossibleto machine. We then repeat the wholesequence as many times as we want.After n iterations - an iteration being onecomplete miniaturisation from mouldinga green part; processing it; using it as acore; moulding a second green part andprocessing that one too - and assumingwe always use the same feedstock withshrinkage factor K (cavity dimensiondivided by sintered dimension), the origi-nal dimension Lo of our macromouldwill have become Ln with

    (4)

    From which we get

    (5)

    Equation (5) is our miniaturisationformula, giving us the shrinkage factoras a function of the size reduction rateand 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 originalmacromould. Of course we want to fabricate our micromould in the mini-mum number of iterations, but are notsure we can formulate a workable mould-ing feedstock with a large enough shrink-age factor. Let's see if we can get by witha single iteration, i.e. n = 1

    Eq. (5) with and n = 1 yields

    (6)

    While feedstocks with larger shrinkagefactors have been processed, it would bequite a challenge to work with it. Let's seewhat happens if we do two iterations, i.e.n = 2

    (7)

    This shrinkage factor is typical ofmoulding feedstocks in common use today.Thus, in just two iterations we can producea micromould that is half the size of amacromould which, itself, is at the very lim-its of what micromachining can produce.

    We have solved Feynman's dilemma ofhaving to fabricate smaller and smallerlathes for which, in any case, there wouldbe no operators. In his landmark paper,Feynman had thought of everything andsuggested training ants who would trainmites to operate the lathes. Now we canleave the ants and mites in peace as wedon't need the lathes anymore. We simplylet the phenomenon of sintering do the job.

    The final frontier

    The ideal location for futuristic MIM isthe near-Earth microgravity environment.In a microgravity environment parts obvi-ously would not sag during sintering. Norwould they distort as a result of theinevitable friction on their support duringshrinkage since in space there would be noneed for a support. This would remove thecurrent size constraints of MIM parts. Inspace, parts as big as a house would be noproblem. This holds important rewards forthe fabrication of large structural elementsof future space stations.

    All kinds of interesting scenarios canbe imagined. If a small meteor shoulddamage a space station, there would beno need to send somebody back to Earthto get spares. New beams, nuts and bolts,

    could be shaped from a ball of feedstocklike clay dough, then sintered.

    Yes, you may say, but you still have togo back to Earth to fetch your raw mate-rials. But that is not really true.

    Space around the Earth abounds withfine cosmic dust which is something of aheadache for spacecraft. Over 100tonnes of it, containing silicates, car-bonaceous material but also nickel, ironand 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 theparts from atmospheric oxygen wouldalso be superfluous so there would be noneed for a sintering furnace either. Wecould just heat the parts as they orbit,using a modern version of Archimedes'"burning mirror".

    A proposal to sinter MIM parts onboard 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: Price262.

    A book worth reading