Match the driver to the stock
Thermal labels are not A4 pages. Set the exact stock size in both the designer and the printer driver. A 4 by 6 inch label is 101.6 by 152.4 millimeters; a 2 by 1 inch label is 50.8 by 25.4 millimeters. Use a one-label-per-page PDF for thermal stock and print at 100%.
Calibrate feed and darkness
Follow the printer manufacturer’s instructions for detecting label gaps or black marks. Choose a suitable darkness and speed for the material. Excess heat can thicken bars and close narrow spaces; insufficient heat can leave gaps. Cleaning the print head may resolve streaks that are not present in the PDF.
Respect printer resolution
Common thermal printers use 203 or 300 dots per inch. Choose bar widths that map cleanly to device dots and avoid resizing a bitmap repeatedly. Keep adequate quiet zones. Validate the final print with the intended scanner; desktop image appearance is not a print-quality certification.