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Minerals & Metals Review - January 2012 � 84

Metal injection moulding - An emerging

manufacturing technologyhe need of manufacturingtechnology for small,precise, complex shaped

parts in large volumes with shortlead times and at competitiveprice has triggered the MIMtechnology. MIM Technologyevolved from plastic injectionmolding & powder metallurgy, formanufacturing complex shapedprecise metal parts. It consists ofprocess steps like compounding,injection molding, debinding &sintering. The raw material in theform of fine metal powdertransforms into complex metalpart. Like plastic injectionmolding, MIM offers threedimensional shape capabilitiesbut with properties near that ofwrought metal. Improvedstrength, reduced costs anddesign freedoms are the benefitsof transforming an assemblyproduced by conventionalmanufacturing process to a onepiece MIM component.

The process involvescombining fine metal powderswith plastic binders which allowsthe resulting metal powder andplastic mix, referred as 'feedstock',to be injected as a liquid into ahollow mold using equipmentsimilar to standard plasticinjection molding machines. Afterthe part is cooled and de-molded,but before the plastic bindingmatrix is removed, the part isreferred to as a 'green part'. Thenext step is to remove the plasticbinders from between the metal

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particles with the application ofsolvents and/or thermal processes.The resulting slightly porous metalpart referred to as a 'brown part' issintered at temperatures greatenough to bind the surfaces of theparticles to one another, but notenough to melt the metal outright.Sintered parts normally attain thedensities of 96-98% of that oftheoretical density.

The MIM process is very similarto plastic injection molding andhigh-pressure die casting, and it canproduce much the same shapes and

configuration features. However, itis limited to relatively small, highlycomplex parts that otherwise wouldrequire extensive finish machiningor assembly operations if made byany other conventional metal formingprocess.

The advantages of the metalinjection molding process lie inits capability to producemechanical properties nearlyequivalent to wrought materials,while being a net-shape processtechnology with good dimensionaltolerance control. Metal injection

Metal injection Moulding (MIM) is an

emerging manufacturing technology

across the world for the small, complex, precise &

net shape metal parts. Some of the key

technological aspect, capabilities, application

aspects & the engineering attributes associated

with MIM are reviewed.

Sachin Malgave

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Minerals & Metals Review - January 2012 � 85

molded parts offer a nearlyunlimited shape and geometric-feature capability, with highproduction rates through the useof multi-cavity tooling.

The limitation MIM is subjectto is one of overall part size, withmost components generally notexceeding 200 g.

Process flowThe process flow for MIM is asfollows:Raw material: Fine metal powder& polymer binders,Compounding: Fine metalpowder is mixed with a polymerbinder. This mixture isgranulated to form "feedstock",Molding: Feedstock is heated toform viscous slurry and injectionmolded to form "green" part,

Debinding: Part of the binders areremoved through solvent de-binding to get "brown" part, andSintering: The brown part isdensified through sintering to get'finished' part.

MIM vs. other competingmethodsThe comparison of MIM withother methods is as follows:MIM vs conventional PM: MIMcan produce geometries thateliminate secondary operationsand offers superior density,corrosion performance, strength,ductility. It also combines two ormore PM components into one,reducing part count.MIM vs machining: The MIMdesigns save material & weightand provide cost savings throughbetter material utilization - spruesand runners can be reground andreused as feedstock with nocompromise to final properties.

Further, molding from a singletool eliminates multiple set-upoperations. Also difficult-to-machine materials can be moldedinto a net shape.MIM vs investment casting: MIMcan produce thinner wallsections, sharper cutting pointsand produce better surface finish.It is better for small-diameterblind and through holes. FurtherMIM greatly reducesrequirements for finishmachining and produces highvolumes of small components ata lower cost, faster lead time.

Viable MIM partA viable MIM part involves highshape complexity, intricate partsdifficult to machine, complexcross sections, high strength &wear performance, require toachieve "close" tolerances, need tolower cost, medium to largevolumes having annualproduction between 25,000 and10 million pieces.

MIM should be considered forthe parts which require discretemachining, multiple secondaryoperations, multi-piece assemblyconversions, unique materialcompositions, higher strength,etc. It can also be considered forthe parts that require complex

Fig 1: MIM Process Flow Fig 2: Production Quantity Vs Shape complexity

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Minerals & Metals Review - January 2012 � 86

geometry; part performanceinadequate in case of PM, die-casting or plastic; and currentprocess that cannot handle largevolumes.

Design guidelinesThe design guidelines for MIM areas follows: "Plastics mold design"principles apply, corner radii ofgreater than 0.3 mm, upto 2° drafton walls longer than 10 mm,minimum hole diameter 0.50 mm,etc. Among others it also includesmin wall thickness 0.50 mm, max5 mm (coring recommendedbeyond 5 mm), gradual sectionthickness transitions, uniformwall thickness recommended asfar as possible and stiffening ribs/coring normally adopted.

Here the general thumb rule is:

The author is working in R&D Group,Indo-US MIM Tec Pvt Ltd

on a 10 mm dimension, atolerance of +/-0.05 mm ispossible

Design advantagesThe advantages observed includesflexible chemistry, close control onweight; hexagonal, square, splined,blind and flat bottom holes even atangles to each other feasible; co-molding / sintering of two differentmaterials possible, etc. Furtherknurled features are feasible alongwith the external/internal threadsfeasible also logo / part number /raised letters / impressions or cavityID can be done.

Metallurgical advantagesMIM produces an annealedmicrostructure and is 96-99 %dense. It exhibits homogeneousmicrostructure / isotropic physicalproperties and is amenable for allthe secondary operations. Theparts can be fabricated out ofdifficult to melt alloys and thereis no interconnected porosity.

The minimum densityachievable is 96% as against 85%on PM Parts. The increaseddensity ensures tensile propertiesalmost double that of PM part

Finishing operationsIn general all operations done onwrought material can be appliedon MIM parts including heattreatment / case hardening, sizing/ calibration, conventionalmachining, grinding / honing,

Fig 3: Unit cost of manufacturing

tumbling / burnishing, electro-polishing / plating, welding, etc.

MIM LimitationsSome of the limitations include:- Weight Limit < 200 Grams :Ideal weight: 10 ~ 15 Grams,- Length Limit < 200 mm : Ideallength Limit < 60 mm- Wall thickness range: 0.5 mm -5mm (If > 5mm Coring advised)- Tolerances tighter than ISO2768(M)- Tolerances less than +/- 0.5%of base dimension @ 1.33 Cpkmay call for secondary operations- Typical number of dimension onthe 2-D drawing: 25- Low melting materials like brass,aluminum, magnesium

Wide applicationsThe major applications of theMIM include:Automotive: Safety systems, fuelinjection systems, interiorsystems, power train, electricalsystems;Consumer goods: Multi-utilityhand tools, power tools, mobilephone & computer peripherals,sporting goods;Industrial: Locks, electronicsensors, hydraulics, fastener &hardware, plumbing, oil & gas;Medical: Surgical instruments,orthopedics, orthodontics&dental, hearing aid device, etc.

ConclusionMetal injection Moulding (MIM)is a manufacturing technology isa cost effective alternative formanufacturing the small,complex, precise & net shapemetal parts required in mediumto high volume requirement. Itcan produce the metal parts withmechanical properties nearlyequivalent to wrought materialswith good dimensional tolerancecontrol and offer a nearlyunlimited shape and geometric-feature capability, with highproduction rates.

Fig 4 Metal Injection Molded (MIM) parts