MIM Process

The MIM process has four steps: compounding metal powder with a thermoplastic binder into feedstock, injection molding the feedstock into a steel mold, removing the binder, and sintering at 1,200-1,400 C to above 97% density. Each step controls a different failure mode, which is why all four run under one roof.

The four steps, and what each one decides

Feedstock compounding. Fine metal powder (typically 5-20 µm) is mixed with a thermoplastic binder until homogeneous, then pelletised. Uniformity here decides whether every cavity shrinks the same amount.
Injection molding. The feedstock is injected into a steel mold at high pressure, producing a green part up to 20% larger than the finished component. Tool design decides where the material welds and where it traps air.
Debinding. The binder is removed by catalytic, thermal or solvent methods, leaving 2-3% residual binder to hold the brown part together. Debinding rate decides whether the part cracks before sintering.
Sintering. At 1,200-1,400 C the remaining binder burns off and the part shrinks isotropically to above 97% density. Furnace profile decides density, carbon content and dimensional drift.

After sintering, only the operations the drawing actually needs are added: hardening, CNC touch-up, plating, tumbling or assembly.

Process variables we control

Step	Key variable	What goes wrong if it drifts
Feedstock	Powder size distribution, binder ratio, mixing time	Uneven shrinkage, cavity-to-cavity variation, weld lines
Molding	Injection pressure, melt temperature, mold temperature, gate position	Short shots, voids, internal stress that shows up after sintering
Debinding	Temperature ramp, atmosphere, time	Cracking, blistering, residual carbon
Sintering	Peak temperature, hold time, atmosphere, cooling rate	Low density, distortion, wrong carbon level, dimensional drift
Finishing	Machining allowance, heat-treat cycle, coating thickness	Tolerance stack failures, hardness out of spec

What the process can and cannot do

MIM suits small, complex parts in volume: 0.1-150 g typical, walls from 0.3 mm, as-sintered tolerance ±0.3%, and 10,000+ pieces per year for the economics to work. It runs stainless steels, low-alloy steels, tool steels, titanium, tungsten heavy alloys, cemented carbide, soft magnetic alloys and ceramics.

It is the wrong process for very large parts, thick sections that cannot be sintered evenly, single-digit quantities, or features that need tolerances tighter than ±0.1% without secondary machining. In those cases CNC, investment casting or PM is the better answer, and we say so at quotation.

Process comparison in detail: MIM vs CNC vs casting. Design limits: MIM design guide.

How the process is controlled in serial production

Feedstock lots are traceable and checked for binder ratio before use.
Sintering profiles are logged per batch; density is measured per lot, not per year.
Critical dimensions run under SPC with a dimensional report on first articles.
Any change to powder supplier, binder system or furnace profile goes through change control before it ships.
ISO 9001 system, with material certificates supplied per lot.

Capacity is 50+ million pieces per year, including 10+ million gears.

From process to price

Four process decisions set the piece price: cavity count (tooling versus cycle time), part weight, tolerance class (as-sintered versus machined) and finishing steps. A part specified at ±0.3% as-sintered with no secondary machining is often 40-60% cheaper than the same part machined from bar stock at volume.

Send a drawing or STEP file and we return a DFM review with an indicative piece price — contact us.

Frequently asked questions

What are the four steps of the MIM process?

Feedstock compounding, injection molding, debinding and sintering. Secondary machining, heat treatment and plating follow only where the drawing requires them.

Why is sintering done at 1,200-1,400 C?

That range is where the powder particles diffusion-bond into a dense solid. Below it, density and strength stay low; above it, grain growth and distortion increase.

What is a green part and a brown part?

The green part is the molded part before debinding, up to 20% larger than final size. The brown part is what remains after debinding, held together by 2-3% residual binder before sintering.

How much does a part shrink during sintering?

Shrinkage is isotropic and is compensated in the mold design, typically in the range of 15-20% linear for common stainless grades. The exact value is measured on first articles.

Which step causes most defects?

In practice, molding and sintering. Molding sets internal stress and weld lines; sintering decides density and dimensional drift. Both are controlled by logged parameters, not by operator judgement.

Can MIM replace several machined parts with one component?

Often yes. Features that would need separate setups, or a part assembled from three pieces, can come out of one mold. That is usually where the largest cost reduction sits.