Thermal and thermal transfer printers use a printhead made up of individual dots that heat up to melt ribbon onto a label (thermal transfer) or darken some heat sensitive media (direct thermal). In either case you’ll need to determine the best printhead resolution for your application.
Printhead resolutions are specified in dots per inch, (dpi) and are nominally 200, 300, 400, or 600 dpi. The individual dot size for these are 5 mil (.005 inch), 3.3 mil, 2.5 mil, and 1.6 mil. Of course, a printer can’t print anything smaller than its dot size. Printheads are usually 4 inches wide, but you can also get them in 2, 6, and 8 inch widths.
Which resolution should you use? That depends on what you are printing.
If you’re printing shipping labels for UPS or Fed Ex and all you need is a legible address and bar codes that the shipping companies can scan, 200 dpi is the choice. Replacement printheads are cheaper and print speeds are higher at 200 dpi.
A few years ago the only way to print more data into a bar code in a small space was to print narrower bars. Not many applications used narrow bars (called the X dimension of a code) smaller than 5 mils. Today you would use a 2D code like Datamatrix to fit more data in a small space, making printhead resolution less important when printing small labels.
The real reason to use a higher resolution printhead is when you need to print a logo or small text; marketing people are very fussy about logos and the look of their finished goods labels. Here’s the same logo printed at 200 and 400 dpi. You’ll see that the 200 dpi suffers from jagged edges when printing curves or angled lines.
The effect of resolution is a lot more apparent when printing small images and text. Labels in the medical industry often require small images and warnings or instructions in multiple languages. It’s common to prepare these in Photoshop and save them as a bit map image to avoid the complexity of printing multiple languages on the thermal printer. Here are two examples, again at 200 and 400 dpi:
This image is blown up slightly, but the 200 dpi printing is unacceptable, the 400 is legible.