Actually the thermal printers doesn’t need the ink, the thermal printer use the thermal lines to print it, it just need a thermal paper, it will be printing very clear. Thermal printer work principle would help you understand why don’t need ink, but can print the characters on it.

The thermal printer working principle is that heating thermal paper to form text by thermal printer head.

When someone click the “Print” button, the printing task will be sent to printer by communication interface, such as usb, WIFI, Bluetooth etc. After the printer get the “printing task”, it will be submit to cpu of the printer, the CPU convert the text to binary send it to thermal film of the printer head, “0” is not heating to black, “1” is heating to black spot. All of the text consist of black spots.