A device manufacturer in the Netherlands sent us a drawing set last autumn for a reusable stainless instrument tray insert. The drawing carried a UDI requirement, a DataMatrix symbol position, and a note that the code must grade B or better. It said nothing about which process step the mark belonged to, and nothing about what the surface would look like when the mark was made. The first article passed dimensional inspection and failed the code grade, because the mark had been placed on a bead-blasted area. Twelve weeks went into a fix that was one line on the drawing.
UDI direct part marking is where regulatory requirements, marking physics and machining tolerances meet, and the failure mode is almost never the laser. It is the absence of a process position and a surface specification. This page is about what a machined part drawing has to say so that the mark works the first time.
What UDI Direct Part Marking Actually Requires
The rule is narrower than most people assume, and knowing its shape saves a lot of unnecessary marking.
In the United States the requirement sits in 21 CFR 801.45. A device that must carry a UDI on its label must also bear the UDI permanently marked on the device itself when it is intended to be used more than once and to be reprocessed before each use. The regulation is deliberately technology neutral. Section 801.45(c) asks only for automatic identification and data capture, or any alternative technology, that will provide the UDI on demand, and it names no symbology. The issuing agency requirement sits elsewhere, in 21 CFR part 830 subpart C: the UDI itself has to be built to the rules of an FDA accredited issuing agency, and FDA has accredited three of them, GS1, HIBCC and ICCBBA.
There are four exceptions, and they are written into the same section:
- Any type of direct marking would interfere with the safety or effectiveness of the device.
- The device cannot be directly marked because it is not technologically feasible.
- The device is a single-use device that is subjected to additional processing and manufacturing for the purpose of an additional single use.
- The device has been previously marked under paragraph (a) of the section.
There is one more requirement in the same section that is easy to miss: a labeler relying on any of these exceptions has to document the basis for that decision in the design and development records.
The direct marking requirement phased in by device class: Class III devices from 24 September 2016, life supporting and life sustaining devices from 24 September 2017, Class II devices from 24 September 2018, and Class I and unclassified devices from 24 September 2022. One thing catches people out even now: FDA states that implantable devices are not required to be directly marked, because they are not reprocessed before each use.
The practical reading for a machinist is this. The parts that need direct marking are the reusable ones that get reprocessed, so surgical instruments, trays, cases, reusable handpieces and similar hardware. The parts that do not are the single-use components that never leave the sterile barrier. If a customer cannot tell you which of the two they are making, that is the first question to settle, because it changes the drawing.
FDA does not mandate GS1 DataMatrix specifically, but GS1 is one of the accredited issuing agencies and GS1 DataMatrix is the carrier the industry has settled on in practice, because it holds the identifier in a small area and survives repeated sterilization. A QR code that only redirects to a web page does not meet the on demand requirement of paragraph (c)(2), and is not the carrier to put on a drawing. The full text of 21 CFR 801.45 is worth reading once at source rather than through a summary, and the FDA recognised consensus standards database covers the marking and sterilization standards a supplier may cite.
Where the Code Goes on the Part, and How Much Room It Needs
Code placement is a machining decision that gets made too late, usually by whoever is standing at the marking station.
A GS1 DataMatrix carrying a GTIN plus a serial number is not small. GS1's own number for exactly this case is 2.5 by 2.5 mm: the GS1 DataMatrix guideline sets a minimum X-dimension of 0.100 mm for laser direct part marking, and cites a target useable area of 2.5 by 2.5 mm carrying a GTIN plus a serial number, with 0.200 mm as the target module size. Our shop rule is to reserve 3 by 3 mm, because at that size a typical 20 to 24 module symbol lands at roughly 0.13 to 0.15 mm per module, clear of the GS1 floor and with margin left for curvature and reflection. Below 2.5 by 2.5 mm we will tell you the grade is at risk before we quote, not after. The quiet zone around the code matters as much as the code: leave a clear margin of at least one module width on all four sides and the grade will improve without changing anything else.
Four placement rules cover most failures we see:
Put the code on a machined or ground surface, not a blasted one. Bead blasting raises a matte texture that scatters the reader's light and drops the grade, sometimes by a full letter. If the part must be blasted, mark it before the blast and accept a lower grade, or mask the code area during blasting.
Avoid the functional faces. A code on a sealing face, a bearing seat or a cutting edge is a defect waiting to be written. The code belongs on a non-functional surface, and the drawing should say which one.
Allow for the curvature. On a cylindrical part the code has to wrap, which distorts the modules at the edges. A flat pad, a milled recess or a slightly flattened band is cheaper than a better camera.
Keep it away from edges and chamfers. A mark that runs over a radius reads inconsistently and, on an instrument that gets polished during refurbishment, wears away first.
The Mark Belongs Before Passivation, Not After
This is the same argument that applies to instrument marking generally, and it is worth repeating because UDI marking makes it a regulatory exposure rather than a cosmetic one.
A laser mark on stainless or titanium locally removes or re-forms the passive oxide layer. That is true of the dark or engraved marks used for a machine readable code, and less true of annealed or cold marks, which are chosen precisely because they disturb the surface less. On a reusable instrument that will be steam sterilized hundreds of times, that area is where corrosion starts. If the mark is made after passivation, nothing restores the layer. If the mark is made before passivation, the acid bath treats the mark with the rest of the part.
So the UDI mark sits in the route before the passivation or anodize step, not at the end of the line. That single sequencing decision does more for mark durability through reprocessing than any choice of laser parameters, and it costs nothing. It does require the marking decision to be made at the process planning stage rather than when the parts are already finished and sitting in a crate.
On anodized aluminum the same principle applies but the geometry reverses. Anodizing is a conversion coating, not a paint film: roughly half the coating thickness grows outward above the original surface and half consumes the aluminium beneath it, so the part changes size as well as colour. Type II coatings run about 8 to 20 um and Type III hardcoat about 25 to 75 um, and either way the coating ends up on top of any mark made before it, which is where a shallow laser mark loses contrast. The practical routes are to mark after anodizing and accept that the mark is a surface feature, or to mask the code area before anodizing and mark into bare metal afterwards. Either way it is a decision that belongs on the drawing, not on the shop floor. Our anodizing guide covers film growth and masking, the process side of the marking itself is in our laser marking service guide, and the general marking trade-offs are in our part marking methods comparison.
Grading: What B or Better Actually Means
A code either reads or it does not, and a grade is what turns that into something you can specify.
This page covers the UDI specific side of marking: which devices the rule reaches, where the code sits in the process route, and what a drawing has to reserve. The wider comparison of marking methods, their depth and their durability, belongs to our part marking methods comparison, and the identification of a part back to a machining lot is its own subject in our CNC part traceability marking guide. This page assumes the code has to survive regulatory scrutiny, not just a shop floor.
Grading for directly marked parts is done under ISO/IEC TR 29158, the direct part mark quality guideline, which incorporates the earlier AIM DPM-1-2006. It scores a symbol on several axes including cell contrast, cell modulation, fixed pattern damage and axial non-uniformity, and returns one of five letter grades: A, B, C, D or F. There is no E, and F is a fail. The grade for the symbol as a whole is the lowest of the individual parameter grades, which is why one weak parameter sinks an otherwise good mark. Grade B is the level we see asked for most often on medical device drawings, and grade C is regularly accepted on small parts. On the materials side, the same roughness and residue limits that decide whether a mark reads are the ones set for medical device machining cleanliness, and the supplier qualification framework behind them is set out in our ISO 13485 CNC machining guide. What matters for a machining drawing is that the grade depends on the surface, not just the mark, so specifying "grade B" without specifying the surface it will be marked on is only half a specification.
The variables a machine shop can control, in the order that matters:
| Variable | Effect on grade | What we do about it |
|---|---|---|
| Surface texture at the mark | Blasted or as-cast surfaces scatter light and lose a grade or more | Mark on machined, ground or milled surfaces, or mask the area |
| Module size | Below roughly 0.1 mm the reader struggles on curved parts | Reserve 3 by 3 mm minimum, 2.5 by 2.5 mm floor |
| Contrast | Laser parameters and material both matter | Parameter set developed on the actual grade and finish |
| Curvature and angle | Wrapped symbols distort at the edges | Flat pad or recess where the geometry allows |
| Post-mark processing | Blasting, polishing and coating all degrade a mark | Sequence the mark before the destructive steps |
| Cleaning and passivation | Restores the passive layer over the mark | Mark before passivation, never after |
Using UDI as a Traceability Spine
Once the code is on the part, it is worth connecting it to everything else, because the cost of doing so is close to zero and the value in a complaint investigation is high.
A serialized DataMatrix that maps back to a machining lot gives you a route from a part in a hospital back to the material certificate, the machine, the inspection record and the operator. That is a different thing from a UDI, which identifies the device model; the serial number is what identifies the unit. We build both into the same mark when the drawing calls for it, and the records that sit behind it are described in our CNC part traceability marking guide and our first article inspection guide.
For a medical program the traceability chain is not optional in either direction. ISO 13485 clause 7.5.9, and specifically 7.5.9.2 for implantable devices, requires traceability records to include the components and materials used and the conditions of the work environment where those could cause the device not to meet its specified safety and performance requirements. It also requires the organization to require its distributors or suppliers of distribution services to keep distribution records available for inspection, and to keep the name and address of the consignee of each shipping package. A serialized mark is the practical way to make that chain work at part level rather than at lot level.
What to Put on the Drawing
Five lines, and they are the difference between a first article that passes and a twelve-week loop.
- The UDI requirement: which rule applies, and whether direct part marking is required or excepted.
- The carrier and content: symbology, and whether the variable data is a serial number, a lot, a date code or a combination.
- The location: a nominated face, with a minimum clear area of 3 by 3 mm.
- The surface: machined or ground, not blasted, and whether the area must be masked through any surface treatment.
- The process position: before passivation or anodize, and what happens to the mark if the part is reworked.
Add the required grade and the person who will verify it, and the specification is complete. A mark that meets a grade on a drawing is a contract term. A mark that "should be readable" is a conversation after the fact. The measurement side of verifying it, and how a mark is checked against the record it points to, are covered in our CNC inspection methods guide. AAMI is the other reference worth having, since its technical information reports cover the sterilization and reprocessing practice that a directly marked device has to survive.
We build the marking step into the process route on any medical part that carries variable data, and we will tell you at the quote stage if the surface you have nominated will not hold the grade you have asked for. Send the drawing and the UDI requirement and you will have a quotation within 24 hours, with the mark position and the process step written into it rather than left to the shop floor. Start a conversation with our engineering team.
FAQ: common questions on UDI marking
Which devices need UDI direct part marking?
Under 21 CFR 801.45, a device that needs a UDI on its label and is intended to be used more than once and reprocessed before each use must also carry the UDI permanently marked on the device. Single-use devices are generally not required to be directly marked, and there are further exceptions where marking would affect safety or effectiveness or is not technologically feasible.
Does the UDI have to be a DataMatrix?
FDA does not mandate a specific symbology. The rule requires automatic identification and data capture using a method that meets the requirements of an FDA accredited issuing agency, and GS1 DataMatrix is the carrier the industry uses most because it packs the identifier into a small area and survives repeated sterilization.
Can laser marking damage the part?
The mark itself is shallow and does not affect structural performance on a properly developed parameter set. What it does affect is the passive oxide layer on stainless and titanium, which is why the mark belongs before passivation in the process route rather than at the end of the line.
How much room does the code need on the drawing?
Reserve a minimum clear square of 3 by 3 mm for a serialized GS1 DataMatrix, with 2.5 by 2.5 mm as the absolute floor, plus a quiet zone of at least one module width around it. Put it on a machined or ground surface rather than a blasted one, and keep it off sealing faces and cutting edges.