A thermal printing company requested our assistance with a mysterious problem—barcodes that intermittently grade well and poorly. They had a trusted verifier but their confidence was shaken. Even on the same label, grades were randomly good and then not good. Samples were requested to be sent to our barcode testing lab.

What we received was alarming: Code 128 barcodes encoding lots of data on a small label which required a very small bars. Challenging when everything is working perfectly. Very small bar width provides little tolerance—but these barcodes did not show any significant gain or loss on the 5 mil bar widths.

Reviewing the verification reports the customer provided, we noted that the ISO grading also reported a 5 mil aperture was used: a 5 mil aperture for 5 mil bars. The industry standard is an aperture of 80% of the smallest bar. That could lead to inconsistent grading, but they were seeing dramatic grade differences—B’s and F’s. We brought out the heavy weapons—the magnifier.  It was inspirational–and horrifying.

When examining problematic thermal and thermal transfer barcodes, we always look for evidence of burned out pixels: white streaks from top to bottom of the direction of travel. Easy to detect in ladder orientation, harder to detect in picket pence. Depending on factors such as gain, when a verifier uses an aperture the same size (or larger) than the X dimension, the scan may be unable to detect minute voids in bars. Depending on where those voids are located, a barcode may scan and grade well in one location, but scan poorly in a slightly higher or lower area of its height. This would account for intermittent grading.

Here is what we saw:

We saw a barcode pocked with spots of missing pigment in the bars. Spots of various sizes in various random locations.

A small spot in the middle of a wide bar may be undetectable to the verifier’s scanner, but a wider spot could make that wide bar appear to be two adjacent narrow bars. A spot on the edge of a wide bar would make it narrower. A spot on the edge of a narrow bar could sever it altogether, making it invisible to a scanner pass, altering the count of transitions from light to dark, disrupting the decode algorithm and causing a failing grade.

How could this happen in thermal transfer printing? Pocks or voids are caused by pigment not adhering to the label substrate. Dust, dirty or oily substrate can do this, but usually you would see this only at the beginning of a print run, where the first few feet of the label roll was exposed to artifacts or vapors in the local environment.

Pocks and voids in the entire print run are the result of mismatched ribbon and label. Synthetic labels behave differently than plain paper. Coated and uncoated label adds another variable. Ribbons can be wax, resin or a blend of both. Ribbons and labels must be matched to each other to perform properly. When they are not, pocks and voids occur.

A mismatch caused the intermittent grading that alerted the customer to the problem.

A visualization of the problem, from the verifier software, illustrates this. A successful complete pass is a solid green line. Intermittent breaks in the scan pass are shown in red.

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