Thermal printer using a thermally transferable ink sheet

DETAILED DESCRIPTION It is therefore an object of the present invention to provide a thermal transfer sheet capable of continuous tone rendition.
The thermal transfer sheet of the invention comprises a heat resistive base, a thermally transferable ink layer on one side of the base, and auxiliary particles distributed in the layer so that they partially emerge from the surface of the layer to present an irregular surface.
The transferable ink layer comprises a mixture of a coloring agent and a hot melt binder, the mixture being transferable to a writing surface in response to application of heat to the other side of the base.
For a wide range of applications, the particles can be made to possess desired physical properties in relation to the ink material.
In one aspect of the invention, the particles are capable of acting as a conductor of heat to adjacent ink portions to cause them to melt earlier than other portions of ink, providing a passage for other portions of melted ink to permeate therethrough to the writing surface.
In another aspect of the invention, the particles are transferable with the melted ink to the writing surface to serve as an ink carrier and a spacer between the transfer sheet and the writing surface.
In a further aspect, the particles are rendered untransferable to the writing surface but capable of acting as a heat conductor.
In a still further aspect, the particles are rendered fusable when the particles are heated to a temperature higher than the temperature at which the binder becomes transferable.
The presence of the auxiliary particles in the thermally transferable ink layer enables ink dots to be modulated in quantity as a function of the amount of heat applied to each elemental area.