The thermal transfer printer is an on-line printer to print on a web in packaging apparatus adapted to print either when the web is in motion or during a stop cycle. The printer is configured to interface with a computer having a RS 232C serial port to permit changing messages or product identifications using stored data, fonts and graphics. Thousands of product identifications and descriptions can be stored and called up in minutes. The inker incorporates a web tensioning device which permits feeding a middle portion of the web through a printing nip at a speed different from the speed of the remainder of the web.