MIM Additive Manufacturing

Overview of MIM Additive Manufacturing

Metal injection molding (MIM) is an additive manufacturing process that combines plastic injection molding and powder metallurgy. MIM allows complex metal parts to be mass-produced to net shape with high precision and repeatability.

MIM has key advantages over other metal 3D printing processes:

• High volume production – Up to thousands of parts can be produced in each batch. This makes MIM suitable for end-use production applications.
• Low cost per part – The molding method brings economies of scale. Part cost decreases significantly with higher volumes.
• Wide range of metals – Stainless steel, tool steel, titanium alloys and other metals can be used.
• Excellent mechanical properties – Near full density and uniform composition is achieved.
• Complex geometries – Intricate shapes, interior features and thin walls are possible.
• Multiple post-processing options – Machining, etching, plating and other finishes can be applied.
• Established process – MIM has been in use since the 1970s. Standards and material databases exist.

MIM is ideal for small, complex metal parts needed in high volumes at lower costs. It bridges the gap between prototype 3D printing and high-volume manufacturing.

MIM Process Overview

The metal injection molding process has four main steps:

1. Feedstock preparation – Metal powder is mixed with a binder material to create a homogeneous feedstock. This mixture is pelletized for use in the injection molder.
2. Injection molding – The feedstock is melted and injected into a mold tool to form the desired “green” shape. Standard injection molding equipment is used.
3. Debinding – The binder material is extracted from the molded component through chemical, thermal or catalytic means. This leaves a “brown” part.
4. Sintering – The debound parts are sintered to fuse the metal powder into a dense end-use part. The parts shrink during sintering.

Secondary operations like machining, joining, plating and etching can further enhance the components. The basic MIM process flow is shown below:

Table 1: Overview of MIM Additive Manufacturing Process

Step	Description
Feedstock Preparation	Mixing of metal powder and binder into a pelletized feedstock
Injection Molding	Molding feedstock into desired green shape
Debinding	Removing the binder to leave a brown part
Sintering	Fusing of metal powder into final part via heat

Applications of MIM Parts

MIM is suitable for manufacturing small, complex, net shape metal components in medium to high volumes. Typical MIM applications include:

Table 2: Applications of MIM Additive Manufacturing

Industry	Example Components
Automotive	Fuel injector nozzles, spur gears, turbocharger parts
Aerospace	Turbine blades, impellers, initiators
Medical	Orthodontic brackets, scalpel handles, forceps
Consumer	Watch components, grills, decorative parts
Industrial	Knife blades, locking mechanisms, valves
Firearms	Triggers, hammers, safeties, slides

MIM enables consolidation of parts, weight reduction, better performance and lower manufacturing costs across many industries. The mix of geometrical freedom and productivity make MIM suitable for end-use production.

Compared to CNC machining, MIM allows more complex shapes to be mass produced. Minimizing assembly steps through part integration is made feasible with MIM.

Advantages of MIM Additive Manufacturing

MIM has unique benefits that make it a popular manufacturing choice:

Table 3: Advantages of MIM Additive Manufacturing

Benefit	Description
High Volume Production	Up to millions of parts can be produced per year via MIM
Low Cost per Part	Reduction in cost with higher production volumes
Design Flexibility	Complex geometries and microfeatures are possible
Range of Materials	Most alloy powders like stainless steel, tool steel and titanium can be used
Good Mechanical Properties	Near full density and uniform composition
Variety of Finishes	Machining, etching, plating and other finishes can be applied
Established Process	Standards, databases, years of experience available

The combination of design freedom, material capabilities and cost effectiveness gives MIM advantages over other processes like metal 3D printing, investment casting or machining.

Parts can be engineered with thinner walls, undercuts, hollow interiors and other complex elements. Consolidating multiple components into one MIM part is also feasible.

Limitations of MIM Additive Manufacturing

While having many benefits, MIM does have some constraints:

Table 4: Limitations of MIM Additive Manufacturing

Limitation	Description
Part Size	Typically limited to smaller components of up to 70 cubic inches
Low Ductility Materials	Some ductile alloys like aluminum are not readily MIM’able
Upfront Costs	Significant tooling investment needed for molds
Low Mix Production	Best suited for medium to high volumes of the same part
Post Processing	Additional machining or finishing may be needed

MIM has geometry limits since it involves molding. Thermoset plastics allow larger MIM parts but have lower strength.

Not all metal alloys can be easily formulated into MIM feedstocks. Ductile materials in particular present challenges.

Hard tool steel molds must be fabricated for each new part design. This adds time and cost.

Frequent design changes are less suitable for MIM due to the fixed mold tools. Other 3D printing methods allow easier iteration.

Depending on final dimensional and surface requirements, post molding operations may be needed.

MIM Design Considerations

MIM allows geometrical freedoms but parts must be designed with the process limitations in mind:

Table 5: MIM Design Considerations

Parameter	Guideline
Wall Thickness	Minimum 0.3mm, maximum 5mm. Uniform thickness is ideal
Tolerances	±0.5% is typical but depends on geometry
Surface Finish	As-sintered is around Ra 10-15 microns
Draft Angles	>1° draft angles required to aid demolding
Shape Complexity	Consolidating parts or optimizing topology is feasible
Features	Fine details like 0.1mm holes and slots are possible
Textures	Complex in-mold textures can be incorporated
Inserts	Incorporating other inserts in the mold is possible
Weight Reduction	Hollowing, reducing mass through lattices and topology optimization

The molding process imposes certain design rules. But MIM can still produce geometries unachievable by other methods.

MIM Material Options

A wide range of alloys, including highly demanded steels and titanium, are available for MIM:

Table 6: MIM Material Options

Material	Applications
Stainless Steel	Medical, marine, consumer products
Tool Steel	Cutting tools, molds, wear parts
Low Alloy Steel	Automotive, machinery components
Titanium Alloys	Aerospace, medical implants
Nickel Alloys	Aerospace turbines, marine hardware
Tungsten Heavy Alloys	Radiation shielding, vibration damping

High strength, corrosion resistant stainless steel such as 17-4PH and 304L are commonly used. Precipitation hardening grades allow further strength enhancement.

Tool steels like H13 are ideal for forming, stamping and injection molds needing good hardness, toughness and thermal stability.

Titanium alloys for bio-compatibility, nickel alloys for heat resistance and tungsten alloys for density are readily MIM’able.

New materials like MIM copper and aluminum alloys are also under development.

MIM Design Software Options

To assist with MIM design requirements, CAD and topology optimization software options exist:

Table 7: MIM Design Software Options

Software	Description
SolidWorks	Popular CAD with moldflow analysis plugins
Autodesk Moldflow	Dedicated injection mold simulation
nTopology	Topology optimization and design for AM software
Materialise 3-matic	Tool for designing lattices and lightweight structures
Netfab	Software for optimizing 3D meshes for MIM

Solidworks is commonly used for CAD design. The moldflow simulation can check manufacturability.

Specialized programs like Moldflow add advanced analysis and process modeling capabilities.

Topology optimization software like nTopology allows organic shapes tailored for AM and MIM. Lightweighting and consolidation are enabled.

Software like 3-matic helps design variable density lattices and generate support structures.

MIM Process Parameters

MIM involves optimizing feedstocks, molding, debinding and sintering. Typical parameters are:

Table 8: MIM Process Parameters

Step	Typical Range
Powder Size	5 – 25 microns
Binder	30 – 50% of feedstock volume
Solids Loading	55 – 70% in feedstock
Mold Temperature	150 – 185°C
Injection Pressure	60 – 110 MPa
Molding Cooling Rate	20 – 50°C/s quenching
Debinding Method	Solvent, thermal, catalytic
Debinding Time	Days to hours
Sintering Temperature	50 – 80% of melting point
Sintering Time	Hours to days
Shrinkage	13 – 17% linear shrink

The parameters depend on material, part geometry, production rate and properties needed.

Fine powders and high solid loadings aid resolution. Faster cooling and mold temperatures create better green strength. Lower debinding times and higher sintering temperatures increase production rates.

MIM Post Processing Options

As sintered MIM parts may require additional processing:

Table 9: MIM Post Processing Options

Process	Purpose
Heat Treatment	Change microstructure for enhanced properties
Plating	Apply decorative coatings like gold or chrome
Passivation	Create protective oxide layer on steels
Welding	Join MIM parts to each other or other components
Laser Marking	Permanent marks for logos, text or ID codes
Machining and Drilling	Higher precision dimensions or custom features
Vibratory Finishing	Smoothen surface and round sharp edges

Post molding steps can refine appearance, properties and interfaces to other components. Plating, anodizing and painting are popular finishes.

Connecting MIM parts via welding, brazing or gluing may be required for some assemblies. Additional machining can create precision mating surfaces.

MIM Equipment Suppliers

Established injection molding companies offer MIM equipment and services:

Table 10: MIM Equipment Suppliers

Company	Equipment
ARBURG	Electric and hydraulic injection molding machines
Milacron	Complete integrated MIM lines
Toshiba	Hybrid and electric molding machines
Netstal	High precision injection molding
Nissei	Vertical and horizontal injection molders
Sodick	High speed, high frequency molding

Specialized MIM services are also available from:

• PIM International
• MPP
• MIMITAL
• CN Innovations

These full service providers offer feedstock formulation, analysis, tooling, molding, debinding and sintering.

Cost Considerations for MIM Additive Manufacturing

MIM has relatively high startup costs but low per part costs at production volumes:

Table 11: MIM Cost Considerations

Cost Factor	Typical Range
Mold Tool	$10,000 – $100,000+ depending on complexity, material and size
Small Batch Setup	Under $10,000
Incremental Part Cost	$0.5 – $5 metal cost per piece
Production Rate	5,000 – 500,000 parts per year per tool
Finishing	$0.1 – $2 per part depending on process
Batch Size Breakeven	1,000 – 10,000+ parts vs other processes

Milling an MIM mold tool from tool steel can take weeks and cost over $100,000 for large components. Smaller less complex molds can be under $10,000.

Once the mold is made, ongoing MIM process costs are very economical for medium to high production volumes. MIM can produce up to one million parts annually from a single mold tool.

Choosing Between MIM and Other AM Processes

MIM is positioned between 3D printing and high volume processes:

Table 12: Comparing MIM and Other Metal AM Processes

Factor	MIM	Binder Jet 3D Printing	DMLS	Die Casting
Capital Cost	High for tooling	Medium	High	Very High
Per Part Cost	Lowest above 10k parts	Low at low volumes	Medium	Lower at very high volumes
Materials	Wide range of alloys	Limited range	Limited range	Aluminum and zinc alloys
Resolution	Medium ~0.1 – 0.3mm	Medium ~0.3 – 0.5mm	Highest ~0.05mm	Lower ~0.5mm
Production Speed	High	Medium	Slow	Very High
Post Processing	Medium	High	Medium	Low
Mechanical Properties	Good	Variable	Best	Good
Design Constraints	Some geometrical constraints	Few constraints	Some overhang constraints	High level of constraints

MIM provides the best economics for complex geometries in alloy materials needed at productions of 10,000+. Lower cost mass production processes become favorable at much higher volumes.

Conclusion

MIM is an attractive metal additive manufacturing process enabling complex geometries to be mass produced directly in a range of engineering alloys.

It combines the versatility of AM with productivity approaching conventional manufacturing. This powerful fusion leads to lower part costs, consolidation of assemblies, better performance and lightweight construction.

While needing some initial mold investment, MIM delivers valuable economies of scale. It is establishing itself as a complementary technique bridging the gap between prototype 3D printing and high-volume manufacturing.

Continued materials development and software integration will expand MIM applications across medical, aerospace, automotive, industrial and consumer sectors.

MIM Additive Manufacturing – FAQ

Q: How does MIM compare to die casting?

A: MIM can produce more complex, higher precision geometries than die casting but has lower production rates and volumes. Die casting is faster and cheaper for simpler shapes needed in the millions.

Q: What size parts can be made with MIM?

A: MIM parts typically range from 0.5 grams to 70 grams in weight. Larger components up to 250 grams are possible with equipment to handle higher pressures and tooling sizes.

Q: What determines the cost of a MIM mold tool?

A: Mold material, complexity, size, surface finishes, and turnaround time affect mold fabrication costs. Simple tool steel molds can be under $10k while large production hardened steel molds can exceed $100k.

Q: Does MIM require any post processing?

A: Some applications need additional heat treatment, machining, or surface finishing. But many components can be used as-sintered. Post processing depends on final dimensional and appearance requirements.

Q: How many parts can a MIM mold produce?

A: MIM production rates typically range from 5,000 to 500,000 parts annually per tool. With proper maintenance, millions of shots are possible over years of service life.

Q: What are common MIM design errors to avoid?

A: Insufficient draft angles, severe undcuts, thick to thin wall transitions and placing fine details on opposite sides of a core can all cause molding issues. Consulting with experienced designers is recommended.

Q: Can multiple materials be combined in MIM?

A: Yes, MIM enables multi-material parts by using powder mixtures or multiple feedstocks. Insert molding with other alloys or hard materials is also possible for composite structures.

Q: How is surface finish of MIM parts?

A: The as-sintered finish is around 10-15 microns roughness. This is suitable for many applications. Additional tumbling or polishing can further smooth surfaces if needed.

Q: How long does the MIM process take?

A: Lead times range from 6-12 weeks typically. Mold fabrication takes the most time if required. Once tools are made, batch production is quite fast for small components.

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