dark ong>anong>d pale part of the bar code. A contrast closest to 1 is desired. The unused error is a value between 0 ong>anong>d 100. It represents the amount of error correction that was needed to read the code. An unused error of 100 meong>anong>s that no error correction was needed ong>anong>d thereong>forong>e the code was not damaged at all. An unused error of more thong>anong> 75 is considered satisfactory. PAINTING ONLY After the painting process, it became clear that the cell size was very importong>anong>t ong>forong> the readability of the Data Matrix. Of the four dimensions that were tested, only two resulted in 2D codes that were readable. Those with cell sizes of 0.5 mm ong>anong>d 0.75 mm were not readable regardless of the parameters used. This meong>anong>s that cell sizes of 0.75 mm ong>anong>d below is not suitable ong>forong> this application. The influence of the ong>markingong> parameters on the resistong>anong>ce to painting of the codes was also investigated. Two different ong>markingong> speeds ong>anong>d three different line-spacings were studied. Each combination gave enough contrast to result in good readability. We did not see a significong>anong>t difference between results, so the fastest parameters should be selected. In figure 10a ong>anong>d 10b, we cong>anong> see a picture of the code after painting, with ong>anong>d without a white background, respectively. a) b) Figure 10- Pictures of data matrices after E-coating with a white background (a) ong>anong>d without a white background (b), with cell size of 1 mm The parameters presented in table 5 were determined to be the best ong>forong> resistong>anong>ce to E-coating. This was done without a white background. In addition to being the fastest set of parameters, it also showed a good visual aspect (figure 10b), a good contrast ong>anong>d was readable from six different ong>anong>gles. The unused error value of these marks is 100, so no error correction was necessary to read the code. As these parameters are stong>anong>dard parameters ong>forong> ong>markingong> on aluminum, it was also possible to read the code beong>forong>e the process of e-coat painting. Laser ong>markingong> is then ong>anong> effective solution ong>forong> identification of parts in the die cast industry that have e-coating process in their production line. Table 5- Best parameters ong>forong> resistong>anong>ce to paint. The average contrast ong>anong>d unused error come from six tests with the barcode reader at different ong>anong>gles. They were all readable ong>anong>d the unused error valued varied between 83 ong>anong>d 100. Marking time 2 s Cell size 1.25 mm Code size 20 mm Marking speed 600 mm/s Linespacing 0.175 mm Best contrast/unused error 0.753/100 Average contrast/unused error 0.733/69 Figure 11 shows pictures of the same barcode taken from different ong>anong>gles. In both cases the contrast is high, but ong>forong> the second situation the barcode is white while the background is black. This does not influence the readability of the code, since unlike the 1D code, only a difference in gray level is required without giving importong>anong>ce to which element is which color. This shows that the ong>anong>gle at which the camera is located with respect to the illumination has a great impact on the observed result.
SHOT BLAST ONLY Figure 11- Data Matrix seen from two different ong>anong>gles with cell size of 1 mm. Shot blast is ong>anong> abrasive treatment that chong>anong>ges the roughness of the surface. To resist these kinds of treatment, much deeper codes must be marked. A “deep ong>markingong>” technique, described below, is thereong>forong>e used. The individual cell size is ong>anong> importong>anong>t factor influencing the readability of the 2D code after shot blasting. The marks done with 0.4 mm cells were not readable after the shot blast treatment while the 0.6 mm ong>anong>d 0.8 mm show very good results. Figure 12 presents the evolution of the contrast with respect to the number of etching passes ong>forong> the 0.6 mm ong>anong>d 0.8 mm cell sizes. We saw a higher drop of contrast ong>forong> the 0.8 mm thong>anong> ong>forong> the 0.6 mm cell size code. We think this is due to the fact that the bottom of the etched cases, where the black ong>markingong> is located, is more exposed to the shot in the process. The optimal cell size seems to be 0.6 mm since at 0.4 mm they are not readable ong>anong>d at 0.8 mm contrast is lost. Note that the average contrast beong>forong>e the shot blast process was 0.61, so the contrast drop due to the shot blast process is only in the order of 10% or less. The cycle time is dependent on the depth of ong>markingong>. As we cong>anong> see in figure 12, it is not necessary to mark deeply. In fact, there is a depth beyond which the ong>markingong> is protected. It was observed that the fastest mark, with three ong>laserong> passes, was not deep enough. It was completely erased ong>anong>d thereong>forong>e not readable. After 6 passes, these marks became readable, but a substong>anong>tial amount of error correction was needed. With 9 passes good results were achieved, both in contrast ong>anong>d unused correction error. With 12 passes, the contrast reaches a maximum which is very close to the average contrast of 0.61 measured beong>forong>e the shot blast process. For the 6 different reading ong>anong>gles, the contrast varies between 0.525 ong>anong>d 0.616 while the unused error is between 10 ong>anong>d 90. With these parameters, it takes 37 seconds to mark a 12.3mm Data Matrix with the 100W ong>laserong>. Such a code has a data capacity of 60 numeric characters or 43 alphong>anong>umeric characters. Figure 12- Best contrast of 6 readings after shot blast as a function of the number of passes ong>forong> cell size of 0.6 mm ong>anong>d 0.8 mm. The technique of deep ong>markingong> cong>anong> produce 2D codes resistong>anong>t to shot blasting but requires a relatively high cycle time. Some applications may not allow such a long cycle time ong>anong>d part ong>markingong> should never be the bottleneck in the process.