1. Field of Invention
This invention relates to a thermal printer having a thermal print head which effects printing operation through heat transfer.
2. Discussion of Prior Art
In a commonly known thermal printer which prints on sheets of ordinary paper, i.e. non-heat sensitive paper, printing operation is achieved by a thermal print head acting through a thermal ribbon (accommodated in a cassette) having a thermally transferable ink layer. In such thermal printer, the thermal head is moved for printing along a paper supporting platen while it is held in pressed contact, via the thermal ribbon, with the printing surface of the paper placed on the platen. Consequently, relative movement between the paper and the thermal ribbon causes friction between the ink layer of the ribbon and the printing surface of the paper sheet, thus resulting in undesired placement of ink on the printing surface of the paper. For this reason, thermal printing is effected without such relative movement of the thermal ribbon with the paper.
Accordingly, bidirectional printing is easy when the thermal head prints directly on a sheet of thermosensitive paper without the use of any thermal ribbon. However, on the other hand, it has been found that bidirectional printing on a sheet of ordinary paper using a thermal head and a thermal ribbon is difficult because of problems involving construction of the thermal ribbon supporting mechanism and of the thermal ribbon feeding mechanism. Consequently, printing operation using a thermal ribbon has been limited to printing in a single fixed direction, and hence, increase in printing speed is almost impossible.