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Dots all over the white areas of the label
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Health report
An area of the label that should be plain white comes out
peppered with small dots. Nothing in the design is there — no text, no
graphic, no colour. The cause is on one of two sides, and the fastest way to
know which is to look at the file before touching the printer.
Render the file first
Open the ZPL in the ZPL preview. If the
dots appear in the rendered image, the printer is innocent: the file carries
them. The usual culprit is a bitmap (^GF) converted from a logo
or a scan whose background was not pure white — light grey, JPEG noise, a
paper texture. Conversion to one bit per pixel turns every grey into a
scatter of black dots, and the printer reproduces them faithfully. The fix is
in the source image: clean the background to white, or convert again with a
threshold that drops the light tones — the
image to ZPL converter lets you set that
threshold and see the result before it reaches the printer.
If the preview is clean, it is the printer path
A clean render with dots on paper means the dots are added between the
file and the media. Three mechanisms cover almost all of it. Darkness
too high for the media: the heat meant for black elements bleeds
into neighbouring dots and direct-thermal paper starts to grey; lower the
darkness in steps (^MD in the format, or the printer's own
setting) until the white is white and the barcode still scans.
Contamination on the printhead: adhesive and dust from the
liner heat up under the elements and mark the paper; clean the head with the
manufacturer's recommended method and material, never with anything abrasive.
Worn or damaged elements: individual heater dots that stay
on or leak heat leave marks at the same horizontal positions on every label —
that is a printhead replacement, not a setting.
Telling wear from dirt from darkness
Print a few labels and compare. Dots at identical positions every
time point to the head — dirt or wear; if cleaning removes them it was dirt.
Dots scattered differently on every label, denser where the black
elements are, point to darkness or media. Dots that appeared with a new media
roll and nowhere else point to the media.
Keep the fix on the right side
Cleaning a printhead does nothing for a noisy bitmap, and re-converting an
image does nothing for a scorched background. One render decides which side
you are on, and it costs no labels.
