Short answer: about 0.5 mm high and about 1.5 mm wide, as raised dots on a flat face.
Why: fingers read braille by touch. Dots that are too low feel like a rough surface. Dots that are too tall or too sharp feel unpleasant, and they can snap off. A 0.4 mm nozzle makes a round dome of about 1.5 mm without trouble.
Rules of thumb: - Raised dots only. Sunk-in dots are hard to feel. - About 2.5 mm between the dots in one cell. - About 6 mm from one cell to the next. - Layer height from 0.12 to 0.2 mm, so the dome comes out smooth. - Print the label flat, face up. Avoid curves. - Put the printed word beside the dots, for sighted readers.
Always print one test label and ask a person who reads braille to check it. Sizes differ a little between braille standards, so a real reader is the best test.
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