Talk at MEDICA TECH FORUM, 16 November 2026
From simulation to series production: how we shrank a coupling from 6 to 4 mm in diameter, with the same performance and without an export licence.
Outer diameter, one third smaller
of the prototype's pull-out torque: 1.12 mNm
NdFeB without heavy rare earths instead of SmCo
SUMMARY
A medtech manufacturer wants to take a blood pump on a catheter into series production. The prototype measures 6 mm and runs on SmCo magnets, which need an export licence from China. The series design measures 4 mm and uses NdFeB without heavy rare earths: it transmits 1.12 mNm, 91 % of the prototype, and the signal at the Hall sensor deviates by only 3.8 %.
BOMATEC overmolds the outer magnet into the impeller, qualifies the process and secures a supply without an export licence.
Representative case, combined from two BOMATEC projects. Dimensions and figures come from a magnetic field simulation with assumed geometry, not from customer data. The measured value for the model check is assumed as well.
STARTING POINT
The coupling transmits the torque from the motor in its sealed titanium housing to the impeller in the blood. No shaft passes through the wall, so there is no shaft seal in the blood path.
A Hall sensor in the motor flange, 3 mm in front of the magnet, reads speed and blockage from the rotating field. If its signal stays the same, the evaluation and validation of the electronics stay unchanged. That is why the field at the sensor counts as much as the torque.
Outer diameter, instead of 6 mm for the prototype
Pull-out torque within ±20 % of the prototype, field at the Hall sensor within ±5 %
NdFeB without heavy rare earths (dysprosium, terbium, gadolinium, holmium), the magnet withstands the process heat
WHY IT IS DIFFICULT
Smaller, just as strong and licence-free: each of these requirements makes the others harder. That is why they have to be solved together.
Magnet cross-section if every dimension shrinks with the diameter from 6 to 4 mm. It takes more remanence than SmCo delivers: hence NdFeB.
Flux density that a magnetized NdFeB magnet loses during overmolding. Its coercivity drops two to three times as fast with temperature as that of SmCo.
The usual remedy restores the coercivity and brings the export licence back.
THE PATH
01 · ENGINEERING
Before we change anything, we rebuild the 6 mm prototype as a magnetic model: magnets, titanium wall, impeller hub and Hall sensor. The model gives a pull-out torque of 1.23 mNm and 37.4 mT at the sensor. Then we check it against a measurement on the real part; in this representative case the measured value is assumed.
The flushing sleeve sits in the field. That is why we specify it as non-magnetic: 316L instead of a magnetic steel, because its walls are too thin for PEEK. We check its influence with FEM.
Model check: 37.4 mT simulated, 38.6 mT measured (assumed value). Deviation −3.0 %.
02 · ENGINEERING
We vary the inner magnet, ring thickness, length, grade, number of poles and segments. Two targets apply at the same time: pull-out torque 0.98 to 1.47 mNm and field at the sensor within ±5 %.
Simply scaled down, 0.33 mNm remain. Four segments or four poles add torque, but cost 41 % or 91 % of the sensor signal. Both targets come back mainly through a thicker outer ring: only the 316L flushing sleeve, 0.15 mm thin, covers it (prototype: 0.5 mm wall). On top of that come 7.5 instead of 5 mm length for the sensor signal and BMN-52SH, the strongest grade free of heavy rare earths.
Result: 1.12 mNm and −3.8 % at the sensor, the best of 156 candidates.
03 · OWN PRODUCTION
We overmold the outer magnet directly into the impeller, at our plant in Malaysia. Magnet, flushing sleeve and impeller are joined and sealed in the same shot: no adhesive, no separate seal in the blood path. The 316L flushing sleeve forms the outer wall over the Parylene-coated magnet, the plastic closes the end faces.
The flushing sleeve goes into the mold as an insert; its channels are laser-cut after overmolding. Grade and volume make up for the flux loss of about 10 % from overmolding.
04 · QUALITY
New magnet material, new process: IQ, OQ and PQ run again. We document our process step, as a full PPAP on request, and support you up to the FMEA.
05 · LOGISTICS
Our plant in Malaysia injection molds, overmolds, presses and assembles, outside China. Sintered NdFeB magnets without heavy rare earths need no export licence; on request we source them from partners outside China.
A coupling, a sensor magnet, an actuator: wherever the magnet turned out to be harder than expected. Patrick Muschak, Sales Engineer Medical & Life Sciences, looks forward to hearing from you.
RESULT
Same performance at two thirds of the diameter and without an export licence. The price is length: 7.5 instead of 5 mm, which the catheter can accommodate.
Pull-out torque, within the ±20 % band
Field at the Hall sensor, target ±5 %
NdFeB without heavy rare earths
WHAT YOU CAN TAKE AWAY
FAQ
Samarium has been on China's control list since April 2025, so SmCo from China needs an export licence. The authority has up to 45 working days to decide, it often takes longer, and the rules change with politics. We have no source for SmCo outside China.
These heavy rare earths make NdFeB more temperature-resistant, but are themselves subject to China's export controls. We would have swapped one licence for the next.
At 4 mm, magnet volume is missing. A thicker ring brings most of it back, possible because only the 0.15 mm thin flushing sleeve covers it. The sensor signal also needs length: 7.5 instead of 5 mm. More than 1.5 times the prototype length was not allowed.
Because the outer ring is too small for that at 4 mm. It is magnetized beforehand and loses about 10 %; grade and volume make up for it.
The approach, yes. Whether a licence-free material is enough, we check for each magnet individually.
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