Data Matrix codes for metal part traceability

Traceability starts with a mark that can be read by machine years after manufacture. On a metal part, the code must withstand oil, scratches, cleaning and repeated reading. Data Matrix is a common choice because it is compact and provides error correction.

Data Matrix ECC 200

Data Matrix ECC 200 is a two-dimensional matrix code made of dark and light modules. It has an orientation pattern (the letter L) on two sides, and alternating light and dark modules on the opposite sides to identify the grid and size.

The ECC 200 designation means the code uses Reed-Solomon error correction, so some damaged or dirty modules can be reconstructed. This reserve is not unlimited and does not replace a poor mark.

Cell size and scanner

The most common mistake in code preparation is to determine the size on screen, regardless of the scanner. Each module on the finished part must be large enough for the scanner’s optics to distinguish it from its neighbour; with small cells the edges blur and the scanner does not decode the code reliably.

Size must be matched to the reading distance, field of view and scanner resolution. A code read up close by an office scanner is not necessarily readable with a handheld scanner in production.

Quiet zone

An empty quiet zone must remain around the code. For Data Matrix it is at least one module width; without it the scanner cannot distinguish the code from its surroundings. On a ground or uneven surface, it is sensible to leave more.

The quiet zone must not contain other graphics, a stamp or the edge of the part; for a code near an edge, the distance must be verified.

Contrast and surface

The scanner needs a difference between dark and light modules. On metal this can be created by a dark thermal mark on a light surface or a light mark on a dark coating. A polished surface reflects light and can reduce contrast, while a ground surface scatters it more.

On curved surfaces the code is optically distorted; reading must be tested in the position in which the part will actually be.

GS1 element strings

In industrial traceability, GS1 element strings are often embedded in Data Matrix codes. The identifiers in brackets are intended for people; only the content is written into the code.

• (01) is the product GTIN, usually 14 digits.
• (10) is the batch or lot number.
• (21) is the serial number.

Where a separator is needed to delimit variable data, a special FNC1 character is inserted into the code, not an ordinary character. If this is omitted, the scanner combines the elements incorrectly. The content of the code must therefore be defined together with the scanner and the information system, not only at marking.

Quality verification

The quality of directly marked codes on parts can be assessed according to ISO/IEC 29158, which describes parameters and measurement for marks on parts. The assessment is not a subjective judgement of appearance: measurements are taken with a suitable measuring system by an agreed method.

If the customer requires a quality assessment, the method, verifier and acceptable values must be agreed before the series.

Before the code goes into production

Code preparation starts with the content and the scanner, not with the file. The data, scanner and reading distance must be defined, then the module size, quiet zone and placement on the part chosen.

The last step is a test on the real part with the scanner from production; only this shows whether the code is readable after cleaning.