1.The source file wasn't created in the ideal quality:
Print the sample label to test if your printer is working well. You can download the sample label from USB Driver:

OR website: Sample 4 x 6 Shipping Label.

2.High print speed or density:
Try reducing your print speed (try 3 in/sec) and your print density (try 3 as well) to see if you obtain a better result.
For Mac:
-> Select "Printer Features" in "Preview"

-> You can change the printing speed and the Darkness of your printing
(Larger number means higher Darkness)

For Windows:
-> Go to "Control panel" -> "Devices and Printers"
(Right click on "This PC", choose "Properties", then click on "Control panel"-> "Devices and Printers")

-> Right click on "Label Printer" -> choose "Printer Preferences"

-> Go to "Option"

-> You can change the printing speed and the Darkness of your printing

(Larger number means higher Darkness)

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