The problem was not cost. It was consistency.
This customer manufactures reusable laparoscopic instruments. The jaw component grips tissue, so the internal serration profile determines how well the instrument holds without tearing. The drawing specified 14 serrations with a 0.35 mm pitch on a curved internal face, plus an undercut where the jaw pivots.
They were producing it on a 5-axis machine with an EDM operation for the serrations. It worked, but it worked slowly: 22 minutes of cycle time per part, and an 82% yield because the thin serration tips were prone to burring and occasional chipping during deburr. Roughly one part in six was scrapped or reworked, and the rework itself sometimes changed the serration profile enough to fail inspection.
They came to us asking whether MIM could hold the serration geometry, not whether it could save money. Cost was secondary.
Feasibility assessment
Serrations at 0.35 mm pitch are within MIM capability, but they demand attention. We ran the assessment across three questions:
- Can the tool be cut? Yes — the serration form was produced as an EDM insert in the mould, which is a far easier EDM job than doing it on every part.
- Will the feedstock fill it? The serration tips are 0.22 mm at the narrowest. That is fine for MIM feedstock provided the gate is positioned so the flow front reaches the serration face without a knit line. We ran mould flow simulation before cutting steel.
- Will the geometry survive sintering? This was the real risk. Fine features on a curved face can distort during the 15–20% linear shrink. We designed a sintering support fixture specific to this part and validated it across three trial batches.
Material and surface requirements
316L was fixed by the customer's regulatory submission, which suited us — it is our highest-volume MIM grade. The requirement that mattered more was surface condition: as a reusable instrument, the part is autoclaved hundreds of times and must not harbour contamination in surface porosity.
Sintered density came in at 7.82 g/cm³, or 98% of theoretical, with closed isolated porosity. We then electropolished to Ra 0.28 µm and passivated per ASTM A967. The customer's cleaning validation confirmed the surface performed equivalently to their machined parts.
Validation programme
Medical programs carry more validation than industrial ones, and we planned for it from the first quotation:
- First article with full CMM dimensional report against every drawing callout
- Metallographic sectioning to confirm porosity distribution and absence of sinter cracking at serration roots
- Tensile bars sintered alongside production lots, tested per ISO 6892
- PPAP Level 3 including control plan, PFMEA and MSA
- Three consecutive validation lots before production release
Total validation took 14 weeks from tool release. That is longer than an industrial program and we set that expectation at quotation stage rather than discovering it later.
Results
- Yield rose from 82% to 97%. The remaining 3% is dominated by handling damage in finishing, not process variation.
- Serration profile variation dropped sharply. Because the geometry comes from one mould insert rather than a per-part EDM operation, serration-to-serration and part-to-part consistency improved to the point the customer relaxed their inspection sampling.
- Unit cost fell 31% — a welcome side effect rather than the objective.
- Tooling paid back in seven months. Slower than a high-volume industrial part, but acceptable given the quality gain.
- Capacity ceiling removed. The customer is no longer limited by 5-axis machine hours when demand spikes.
The honest caveat
MIM did not make everything easier. The validation burden was real, the tooling cost more than a simple part because of the serration insert and the sintering fixture, and the first two trial batches showed distortion we had to engineer out. A buyer expecting a drop-in replacement in six weeks would have been disappointed.
What MIM did do was move the difficulty from the production line to the tool. Once the tool was right, every part was right — which is exactly the trade a medical device manufacturer wants to make.
Talk to the engineer who ran this program
If your part sits in the same territory — similar size, similar volume, similar frustration with the current process — the fastest route is a drawing and your current unit cost. We will come back with a process recommendation, a tooling estimate and a break-even volume, usually inside 48 hours.