Medical technology is the largest application field for metal injection moulding. Market analyses attribute roughly 29 percent of the MIM market to the medical and dental segment; about 41 percent of global MIM demand originates in medical device manufacturing. The reason is obvious: surgical instruments are small, geometrically demanding, needed in medium to high volumes — and made from materials MIM handles well.
What differs in this environment is not the manufacturing. It is everything around it: evidence, traceability, cleanliness, and one technical detail that weighs more heavily in MIM than in any other process — residual porosity.
The regulatory frame, briefly
As a component supplier you are not a medical device manufacturer. Responsibility under the EU Medical Device Regulation rests with whoever places the product on the market. Your task is to supply the evidence they need for their technical documentation.
| Standard or regulation | What it governs | Whom it concerns |
|---|---|---|
| ISO 13485:2016 | quality management system for medical devices | manufacturing plant and supply chain |
| EU MDR 2017/745 | market access, conformity, surveillance | device manufacturer |
| ISO 10993 | biological evaluation, biocompatibility | material and surface |
| AMS 2700 / ASTM A967 | passivation of stainless steels | process step at the supplier |
| UDI | unique device identification, traceability | batch control down to the part |
On timing: the MDR transition periods were extended by Regulation (EU) 2023/607. For Class III devices and implantable Class IIb devices they run until 31 December 2027, for other Class IIb, Class IIa and sterile Class I devices until 31 December 2028. This does not concern you directly as a component supplier — but it explains why your customers are currently working systematically through their supplier documentation.
Residual porosity — the MIM-specific point
This is the section for whose sake this article exists.
As sintered, MIM parts typically reach 95 to 98 percent of theoretical density. The remaining 2 to 5 percent are pores. In most applications that is irrelevant. In medical technology it is the central point, for three reasons.
Open versus closed porosity
What matters is not the quantity of pores but whether they are open to the surface. Closed pores inside the material affect strength marginally and nothing else. Open pores at the surface form crevices — and crevices are a problem in medical technology:
- Cleaning validation: residues from machining, handling or use can lodge in them. Cleaning effectiveness has to be demonstrated, not asserted.
- Passivation effectiveness: the passive layer forms on the geometric surface. If that surface is greatly enlarged by open pores and poorly accessible, passivation becomes uneven.
- Crevice corrosion: in narrow, poorly flushed crevices a different local environment can establish itself. That is exactly where pitting begins.
From about 97 percent density onwards, porosity largely closes — open pores become isolated, closed pores. This is not a sharp boundary, but it is the practically decisive order of magnitude.
| Density | Porosity character | Suitability |
|---|---|---|
| 94 – 96 % | predominantly open | uncritical parts without body contact |
| 96 – 98 % | mixed, predominantly closed | standard for instruments |
| 98 – 99.5 % | closed | demanding applications, prolonged contact |
| above 99.5 % (HIP) | practically pore-free | implants, permanent residence |
Residual binder and carbon
The second MIM-specific point. Before sintering, all binder must leave the part. If a remnant stays in the core it burns during sintering and leaves carbon behind. With austenitic steels such as 316L that is critical: carbon forms carbides with chromium at the grain boundaries, and where chromium is bound, the passive layer lacks it. Corrosion resistance falls without the batch chemical analysis showing anything unusual.
Carbon content after sintering therefore belongs in the specification — for 316L usually ≤ 0.030 percent. Why the material number alone does not carry here is set out in detail in K.10.
Materials in medical technology
| Material | Typical application | Points to note |
|---|---|---|
| 316L | instruments, clamps, endoscopy | density, carbon content, passivation |
| 17-4 PH | spring and load-bearing elements | heat treatment, lower corrosion resistance |
| Ti-6Al-4V | implant-adjacent, dental | limit oxygen pickup during sintering |
| CP titanium | osseointegration | high feedstock price |
| CoCrMo | joint components | demanding sintering control |
With titanium the oxygen content is the critical value. Titanium picks up oxygen during sintering, which reduces ductility. For medical applications it must be specified. The governing document is ASTM F2885, the standard for MIM-produced Ti-6Al-4V in surgical implants; the limits it sets for oxygen, nitrogen, carbon and hydrogen belong on the drawing unchanged. For orientation: in wrought material this very value separates Grade 5 at 0.20 percent from Grade 23 (ELI) at 0.13 percent oxygen. More on this in K.18.
Passivation — different in MIM than in wrought material
Passivation produces a chromium-oxide-rich protective layer. The process is governed by AMS 2700 and ASTM A967 and is standard in instrument manufacture.
With MIM parts, however, the effective surface is larger than the geometric one — because of residual porosity. In practice this means:
- Longer treatment time than for wrought material of the same geometry.
- More thorough rinsing, because acid residues can remain in pores. Un-rinsed acid is more dangerous than no passivation.
- Drying with sufficient dwell time, so that no moisture remains in pores.
- Testing to ASTM A967, for example copper sulphate or humidity test — on the MIM part itself, not on a reference coupon of wrought material.
How this runs in a real project is shown by our endoscopy clamp case study: 316L, 3.8 grams, 240,000 parts a year, passivation to AMS 2700.
Sterilisability
MIM parts in the materials named are sterilisable by all common methods. What matters is less the individual cycle than the number of repetitions.
| Method | Conditions | Assessment for MIM |
|---|---|---|
| Steam (autoclave) | 121 – 134 °C, saturated | uncritical with good passivation |
| Ethylene oxide | 37 – 55 °C | uncritical, observe residual gas release |
| Gamma irradiation | 25 – 40 kGy | metallurgically uncritical |
| Plasma / H2O2 | low temperature | uncritical |
The repeated steam cycle is the real test: with reusable instruments, hundreds of cycles accumulate. That is exactly where it shows whether density and passivation are right — or whether discolouration and pitting appear after fifty reprocessing runs.
What belongs in the specification
A drawing that states only dimensions and a material number is incomplete for a medical MIM part. Add:
- Minimum density as a percentage of theoretical density, with measuring method and inspection frequency
- Carbon content after sintering, not of the batch
- Oxygen content for titanium materials
- Passivation with standard reference and required testing
- Surface finish as an Ra value, stating whether as-sintered or reworked
- Cleanliness requirement, such as particle load or endotoxin limit, where relevant
- Traceability down to the feedstock batch
- Permitted sterilisation methods and expected cycle count
Our approach
Our manufacturing network is certified to ISO 13485; we supply the consolidated certificate package per project. The practical advantage of the one-supplier model is greater in medical technology than anywhere else: you carry one supplier in your supplier management, audit one, and receive one documentation package — even when primary and secondary source produce in different plants. Anyone who has taken two suppliers in parallel through a notified body review knows the difference.
Further reading
- MIM-316L is not 1.4404 — why density and sintering control decide corrosion resistance.
- PPAP / PSW for MIM parts — the evidence trail that also carries in medical technology.
- Endoscopy clamp case study — 316L, AMS 2700, 90,000 parts a year via MIM.Experts.