MIM – Metal Injection Moulding

What is MIM

MIM (Metal Injection Moulding) combines the design freedom of plastic injection moulding with the mechanical properties of metals. The result is small, complex-shaped components with a density of over 95% of the theoretical value, dimensional tolerances of ±0.3% of the nominal dimension and a surface roughness of Ra 2.0 μm – straight from the manufacturing process, without the need for costly finishing machining.

MIM technology is the economically sound choice where CNC machining reaches its geometric limits, capacity constraints or rising unit costs, and where conventional casting fails to meet the required tolerances or surface finish.

The process begins with the feedstock – a homogeneous mixture of fine metal powder (particles smaller than 20 μm) and a polymer binder. This mixture is injected into a mould in the same way as plastic. The result is a so-called ‘green part’, which has a compensated geometry.

The binder is then removed chemically or thermally (debinding), producing what is known as a ‘brown part’, which undergoes sintering in a furnace at a temperature of 1200–1400 °C in a protective atmosphere. During sintering, the metal particles fuse together by diffusion to form a compact whole – shrinkage of 15–25 % occurs and the material’s density exceeds 95 % of the theoretical value.

The result: a metal component with properties comparable to those of machined steel, produced in runs of thousands at a fraction of the cost of machining.

Metal powder with spherical particle morphology up to 20 μm is mixed with a polymer binder system – a primary binder (paraffin waxes, polyoxymethylene, etc., to ensure fluidity) and a secondary binder (polypropylene or polyethylene to maintain shape during depolymerisation). The binder system accounts for approximately 40% of the mixture’s volume. The homogenised mixture is extruded and subsequently granulated, producing the feedstock for the MIM process

We develop our own feedstock – the composition is selected according to the specific application and the customer’s material requirements.

The feedstock is plasticised and injected under pressure into a precision metal mould. The machines are similar in design to plastic injection moulding machines, but adapted to cope with the abrasive nature of the metal powder. The moulds are deliberately oversized by 15–25 per cent to compensate for shrinkage during sintering.

The resulting green part has precise, compensated geometry – internal/external threads, undercuts, blind holes, thin walls from 0.4 mm, ribs and markings. All in a single injection.

The binder accounts for 30–40 % of the volume and must be removed without altering the dimensions of the part. We use three methods depending on the composition of the binder:

• Catalytic debinding – depolymerisation in a nitric acid atmosphere; proceeds from the surface at a constant rate, minimising internal stress
• Solvent debinding – selective removal of the wax component using a chemical bath; opens the pores for subsequent heat treatment
• Thermal debinding – controlled evaporation of the residual binder at a precisely controlled rate of temperature rise

The result is a brown part – a porous, brittle part that retains the shape and size of the green part.

The brown compact is sintered in a controlled-atmosphere furnace (nitrogen, hydrogen, argon or vacuum) at 1200–1400 °C. At 80–90 % of the metal’s melting point, atomic diffusion takes place: metal particles coalesce and pores are sealed.

Isotropic shrinkage to the final dimensions takes place. Material density: ≥ 95% of the theoretical value. Dimensional tolerance in the modern process: ±0.3% of the nominal dimension.

After sintering, the component has mechanical properties comparable to those of forged material – this is the key difference compared to piston pressing, which achieves only 85–90 per cent of the density.

Most MIM parts emerge from the sintering process in their final form – a surface roughness of Ra 2.0 μm and dimensional accuracy of ±0.3 % generally meet customer requirements without the need for further processing.

Where there are specific requirements for higher precision, higher strength or other surface properties, we offer:

• Machining of precision functional surfaces
• Thermal and chemical-thermal treatments – heat treatment, annealing, nitriding, carburising, carbonitriding
• Surface treatments – passivation, electroplating, PVD, DLC coatings, controlled oxidation
• Tumbling and sandblasting to achieve a specified surface roughness or meet aesthetic requirements

Each batch undergoes dimensional inspection, metallographic analysis of the structure, and strength and hardness tests. For parts of a higher criticality, we carry out non-destructive testing to detect internal porosity.

The entire process chain is managed in accordance with the ISO 9001 standard. The company’s quality policy is primarily focused on the customer and their individual needs. Every component is traceable from the powder batch right through to dispatch.

Design freedom that CNC cannot offer. Internal threads, blind holes, undercuts, thin walls from 0.4 mm – all in a single injection moulding cycle, without the need for re-clamping or the assembly of individual components. MIM allows multiple functions to be integrated into a single part.

Density and strength comparable to those of forged material. Density ≥ 95 % of the theoretical value. Unlike piston pressing (85–90 %), MIM achieves properties that can be used in structural calculations without the need for correction factors for porosity.

Dimensional accuracy of ±0.3% across the entire series. Once the mould has been fine-tuned, MIM produces hundreds of thousands of identical parts with minimal variation. This forms the basis for reliable assembly and predictable performance of the part in its application.

Lower unit cost for medium and large production runs. The material efficiency of MIM technology exceeds 95 per cent. Compared with CNC machining, where a significant proportion of the material ends up as swarf, MIM produces only minimal waste. In the CNC machining of expensive alloys, up to 80 per cent of the material is lost as swarf. For complex parts in runs of approximately 1,000–2,000 pieces, MIM becomes more cost-effective than machining – and the advantage increases further as production volumes rise.

Surface finish Ra 2.0 μm without further machining. MIM parts generally do not require further surface finishing. Precision casting typically achieves a surface finish of Ra 3.2 μm and requires finishing. MIM eliminates this step.

Compatibility with HIP. For applications with the most demanding fatigue strength requirements, we offer post-HIP treatment using our own equipment. The density approaches 100% of the theoretical value. For nickel superalloys (e.g. Inconel 718), the fatigue life following HIP is increased by a factor of 4–5 compared with standard heat treatment.

CNC requires no initial investment and offers high precision for simple shapes. However, the cost per unit rises linearly with the complexity of the part – every recess, internal cavity or, for example, rib increases the number of set-ups and machining time.
With MIM, the cost of the mould is a fixed initial expense. Once the mould is in place, the unit cost depends on the weight rather than the complexity of the geometry.

Break-even point: for a complex part with internal features, approximately 1,000–2,000 pieces per batch. For a simple shape, up to 12,500 pieces. Above this volume, MIM is cheaper – and the advantage increases as the volume rises.

For parts weighing up to 250 g where precision and surface finish are required, MIM is consistently superior. Above this weight, the cost of fine MIM powders becomes uncompetitive compared to casting materials.

Piston pressing achieves a density of 85–90 per cent and does not allow for parts with undercuts or parts with varying wall thicknesses along a single axis. MIM does not have these geometric limitations and achieves a density that is 7–12 percentage points higher. For structurally stressed parts with complex geometries, powder metallurgy pressing is not a comparable alternative.

Variable valve timing system components, locking mechanisms, connectors, control elements. Production runs of tens of thousands of units; IATF 16949 compliance required.

Trigger mechanisms, safety catches, stops, latches. Precise geometry and repeatability are essential here, not merely an advantage.

Surgical instruments for laparoscopy, orthopaedic fasteners, dental components. Biocompatible alloys (316L, Ti-6Al-4V), sterilisable surfaces, disposable multi-purpose instruments.

Shielding covers, connector housings, sensor housings. Dimensional accuracy and the ability to miniaturise to under 1 gram are key parameters.

Gear segments, sliding components, pump parts, valve components. Tool steels and low-alloy steels, heat treatment tailored to the application.

Micro-components with tolerances in the micrometre range, with a surface requiring no further machining.

• Retaining clips, latches, pins
• Precision gear segments and planetary gears
• Sensor housings and connector bodies
• Hinge and connecting elements
• Surgical clips, forceps, instruments for minimally invasive surgery
• Safety and trigger mechanisms
• Functional microcomponents for mechanical assemblies weighing from 0.1 g