Direct Part marking with a bar code symbol has had increasing drive in recent times as the need for traceability of parts history (manufacturer, materials used, etc) for years after they have been put into service, has become necessary. Marks made with Laser etching, chemical etching and dot peening are a few of the methods adopted so far.
Data Matrix bar code can be printed with virtually any size of element as long as the rest of the criteria for the bar code is followed accurately. AIM (Automatic Identification Manufacturers) has established a standard for Data Matrix bar codes and we will assume the use of the ECC-200 (Error Correction Code methodology) standard for this document, although it is really not a restriction. The air transportation industry has adopted their “ATA SPEC2000”, which is one of the first industry attempts to address the quality aspects of direct part marking.
Quality has been the biggest issue in the use of the marks. If the mark had been printed with a ribbon onto a paper label, there are many software products that can create the barcode with very accurate dimensions. The problem with etching onto the part directly causes not only the material but also the finish of the material to become part of the “quality” of the imprinting. Based on the metallurgical or chemical composition of the part to be identified, the best method of directly marking it varies. Some materials react very well with a low energy laser beam (like softer metals and some plastics) while other materials require significant care (like directly etching glass due to the “crazing” or micro-fractures that can occur under the instantaneous high heat of a laser).
The actual accuracy of the etched mark depends on the correct choices of power, frequency and software as it relates to the material being etched when using lasers, and similar considerations when using chemical etching or dot peening. It is easy to under-etch or over-etch the bar code if quality is not monitored closely. This over or under etching causes the individual elements to be outside of the parameters outlined by the AIM and other specifications. Some decoding schemes do not consider a dot peened mark to be a “legal” formation of the code due to the “L-pattern” (which is used as a finder pattern for data matrix identification) having breaks between each dot instead of being a continuous, solid line.
A data matrix mark is either a square symbol or a rectangular format being twice as wide as it is high, but in both cases it is formed by two sides being a “solid L-pattern”, with alternating timing marks on the other two sides. Marks are made with light and dark elements (reversed images are acceptable) but the ratio of dark and light elements should be 50% (with a minimum of 35% to maximum of 65% per the AIM standard). Please consult the AIM standard for more details.