Tiny Medical Devices - Designing for Success (part 2)


Picture this – your medical device is designed and you’re ready to validate the tiniest component – the technology piece that enables your intellectual property. You dive in head first, knowing your part has +/- 5 micron tolerances on a plastic part. You start like any engineer would start – with a solid stack-up tolerance plan and <20% gage R&R – you should be fine, right?

Consider this general micro molding rule of thumb – BEFORE you even design the injection mold. If you work backwards with the numbers, you’ll have ~0.0004” total tolerance

Total   10 microns = (0.000397”)

Tooling = 20% of 10 microns = 0.00008” (wow, really? This is difficult to do but SO critical to hitting the final Cpk.

Cpk of 1.33 or better on this tolerance means you need ~20% of the tolerance left (and be tightly controlled) to hit your CPK.  The error breakdown generally speaking is:

Tooling 20%
Gage R&R 20%
Molding process 20%
Material lot to lot variation 10% - melt flow, venting
In the bank- 10% for some unforeseen couple of microns

Starting with the tooling (it’s almost always about the tooling), anything we can do to reduce the 20% of steel tolerance in a critical tolerance will increase the numerical dance of success as explained above. Achieving steel tolerances of +/- 0.0002” is fairly straight forward in the precision mold-making industry, however achieving 20% of this is not straight-forward and requires some good research to make sure you have one that can. Selecting the right tooling supplier is the #1 absolute critical contributor to overall micro molding product success.

It’s equally important to understand the gage R&R error in the still measurement as well as the end plastic component. Creating the steel may require EDM, milling, wire EDM, or grinding. Validating steel or pieces of steel that make up a critical feature requires that the metrology equipment is capable of one more resolution higher than the tolerance. For example 20% of 0.000397” = < 0.00008” which requires 0.00001” or better in measurement capability in both steel and plastic. If the tooling supplier doesn’t have this capability, there’s no way to quantify the very start of the process to creating a micro molded part. If the tooling supplier has this capability, you are off to a great start in achieving the final goal of CPK 1.33 or better – a necessary requirement for many medical and drug delivery devices.

Stay tuned for our next installment in this VITAL (Validation+Inspection+Tooling+Automation+Learning)  series of Small Talk and on our Blog when we will tackle this critical next step of the micro molding process – Inspection and Gage R&R.