Thermal transfer printing device

A thermal transfer printing device includes a thermal printing head of the corner-edge type having heating elements disposed on one edge and forming a printing bar. The edge faces a backing roller, is parallel to the axis thereof, and around which an ink ribbon is led directly away from a letter or envelope to be printed. A shell which is provided on the underside of the thermal printing head faces the backing roller. The shell extends immediately up to the printing bar and has a bottom surface with a region adjacent the printing bar at approximately the same height as the printing bar. The thermal printing head including the shell and the backing roller, are adjustable relative to one another. Good ink particle detachment and a relatively low contact force are provided. Disadvantages with regard to soft and sensitive mail can be avoided and the service life of the thermal printing head is prolonged due to a lower contact force. A good print quality is achieved, irrespective of the finish of the letters or envelopes and at a high transport speed of the latter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to a thermal transfer printing device for a franking 
and/or addressing machine. 
In the thermal transfer printing process, a printing medium is usually led 
between a thermal printing head and a backing roller disposed opposite the 
latter, on the side of the backing roller. An ink ribbon is led through on 
the side of the thermal printing head. It is necessary for the printing 
head to rest on the ink ribbon with an appropriately high force and for 
the ribbon to in turn rest on the printing medium, in order to transfer 
molten ink particles from the ink ribbon to the printing medium. 
The ink may be bonded to the ink ribbon in a wax layer, or recently in a 
layer of polyimide as well. 
In the case of an office printer which works on the thermal transfer 
printing principle, operations are carried out with constant printing 
relationships, such as the same contact force for the thermal printing 
head, single sheets of a prescribed paper grade, in particular paper 
thickness, and constant paper guidance. 
Contrary to the office printer, in the case of franking and/or addressing 
machines, the finish of the letters, in particular their thickness and 
stiffness, frequently changes because of different fillings and the paper 
quality of the envelopes. 
The printing relationships become particularly critical in the case of 
processing air mail, since the envelopes used therefor are often very soft 
and sensitive. Although more careful operation would be possible by 
reducing the contact pressure, the print quality would then in turn be 
correspondingly impaired. On the contrary, in the case of so-called soft 
letters, the contact pressure must even be increased with respect to 
normal letters in order to achieve the same print quality. However, higher 
contact forces in turn result in greater wear on the thermal printing head 
and, accordingly, to a correspondingly shortened service life, and may 
additionally lead to damage to the letters. 
Published European Patent Application 0 787 592 A1 discloses a thermal 
printer in which the letters are led along while lying flat along the 
broad side of a thermal printing head. The heating elements are disposed 
in the center of the plate-like printing head, in a row transverse to the 
transport direction of the letter or envelope. The letter, together with 
the ink ribbon, is pressed against the heating elements through the use of 
a spring-mounted roll. The broad side is significantly larger in 
comparison with the region having the heating elements, so that the ink 
ribbon and letter, following the printing operation, are guided in such a 
way that they rest on one another for a relatively long time. 
Due to the contact over a large area, that method of guiding the letters 
certainly takes care of the paper, but the ink particles are more 
difficult to detach from the ink ribbon. Compensation is possible only 
through the use of a longer thermal action time, which in turn 
necessitates a lower transport speed. In addition, because of the greater 
contact area, a higher contact force is required, as a result of which 
once more the thermal printing head wears more rapidly. 
The conditions are similar in another franking machine known from Published 
European Patent Application 0 724 234 A2, in which the row of heating 
elements is displaced more to one end of the plate-like thermal printing 
head. However, the joint path of the ink ribbon and the letter, following 
the thermal action, is still too long, since the ink ribbon is only led 
away from the letter by a deflection roller disposed downstream. 
Furthermore, a thermal transfer printing device is known from European 
Patent 0 434 340 B1, in which the heating elements are fitted to one edge 
of a thermal printing head, and the latter only presses in the direction 
of a backing roller with that edge. The ink ribbon is pivoted away from 
the letter over a roll disposed downstream of the edge. In that case too, 
the residence time of the ink ribbon on the letter is still too long. 
Added to that is the fact that the edge of the thermal printing head, 
which is provided with a small radius of curvature, loads the letter 
mechanically so severely that, in the case of envelopes made of thin paper 
and having a multilayer soft filling, it is possible for creases to arise, 
extending through to damage. In addition, that results in a 
correspondingly poorer print quality. 
Finally, a thermal printer is further known from Published European Patent 
Application 0 329 478 A1, in which the heating elements are fitted 
directly to a narrow front edge of a flat thermal printing head, and the 
ink ribbon is led away from the printing medium directly downstream of the 
heating element printing bar. Favorable conditions are provided in that 
way in relation to the detachment of ink particles, but any use for 
multilayer printing media, such as soft letters, is associated with the 
risk of creasing. 
That thermal printing head represents the so-called "corner-edge type", 
while the two devices described at the beginning correspond to the 
so-called "flat type". 
SUMMARY OF THE INVENTION 
The purpose of the invention is to improve functional properties and to 
widen the field of use. It is accordingly an object of the invention to 
provide a thermal transfer printing device, which overcomes the 
hereinafore-mentioned disadvantages of the heretofore-known devices of 
this general type, which achieves a good print quality irrespective of 
finish, quality or condition of letters or envelopes and at a high 
transport speed of the latter and which keeps wear on the thermal printing 
head as low as possible. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, a thermal transfer printing device for 
franking and/or addressing machines, comprising a backing roller having an 
axis; a thermal printing head of the corner-edge type having an underside 
facing the backing roller, an edge for leading an ink ribbon around the 
edge directly away from a letter or envelope to be printed, the edge 
disposed closest to the backing roller, facing the backing roller and 
parallel to the axis of the backing roller, and heating elements disposed 
on the edge and forming a printing bar disposed at a given height; a shell 
disposed on the underside of the thermal printing head and extending 
directly up to the printing bar, the shell having a bottom surface with a 
region adjacent the printing bar at approximately the given height; and 
the backing roller and the thermal printing head with the shell adjustable 
relative to one another. 
In accordance with another feature of the invention, the shell has an edge 
rounded off towards the thermal printing head in a region adjacent the 
printing bar, the shell is broader than the thermal printing head, and the 
shell has rounded corners in an extension of the edge of the thermal 
printing head. 
In accordance with a further feature of the invention, the edge of the 
thermal printing head has a given length, and the backing roller is longer 
than the printing bar but shorter than the given length. 
In accordance with an added feature of the invention, the shell is 
detachably fastened to the thermal printing head. 
In accordance with an additional feature of the invention, the shell is 
formed of a non-rusting metal, the shell has a region adjacent the 
printing bar, the shell has an insulation layer supporting the shell on 
the thermal printing head in the region, and the bottom surface of the 
shell is smoothed in the region. 
In accordance with yet another feature of the invention, the shell is 
formed of a plastic with a low coefficient of friction, such as an acetal 
copolymer with TEFLON. 
In accordance with a concomitant feature of the invention, the shell is 
formed of a metal, such as aluminum, with a TEFLON coating. 
The combination of a commercially available thermal printing head of the 
corner-edge type with the shell shaped according to the invention makes it 
possible, contrary to expectations, to use such a thermal printing head 
and to benefit from its advantages, namely good ink particle detachment 
and lower contact force, for franking and/or addressing machines. 
As a result of placing the shell on the underside of the thermal printing 
head, and extending it as far as immediately by the printing bar, careful 
pre-smoothing of the letters is achieved without the contact force between 
the thermal printing head and the backing roller having to be increased 
for a good print quality. Due to the lower contact force, the thermal 
printing head itself is also taken care of, and as a result its service 
life is prolonged. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
thermal transfer printing device, it is nevertheless not intended to be 
limited to the details shown, since various modifications and structural 
changes may be made therein without departing from the spirit of the 
invention and within the scope and range of equivalents of the claims.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now in detail to the figures of the drawings, which are somewhat 
diagrammatic for the purpose of simplification and ease of understanding, 
and first, particularly, to FIG. 1 thereof, there is seen a letter or 
envelope A which is disposed between an ink ribbon 4 and a backing or 
counter roller 3. A thermal printing head 1 and a shell 2 located on the 
underside of the thermal printing head, rest on the ink ribbon 4. The 
thermal printing head 1 is of the corner-edge type and makes contact with 
the ink ribbon 4 only at its edge 12. A printing bar 11 made of individual 
heating elements 111 is fitted to the edge 12, as is seen in FIGS. 2A and 
2B. The shell 2 has a bottom surface 21 which makes contact with the ink 
ribbon 4 only in a region that is located in the vicinity of the edge 12 
and the printing bar 11. The shell 2 is extended at two outer edges as far 
as a location just in front of the edge 12, and is provided with rounded 
corners 212. 
The shell 2 and the thermal printing head 1 are constructed to be broader 
than the ink ribbon 4, at least in that region which makes contact with 
the ink ribbon 4. Due to the rounded edges of the shell 2, the letter or 
envelope A is carefully smoothed. 
The shell 2 expediently is formed of a non-rusting metal and the bottom 
surface 21 is smoothed in a sliding region of the ink ribbon 4 and of the 
letter or envelope A. It is also possible to use a plastic, an acetal 
copolymer or a TEFLON-coated metal. 
The ink ribbon 4 is guided around a deflection roller 5 upstream of the 
shell 2, in such a way that a lower edge of the deflection roller 5, the 
bottom surface 21 and the printing bar 11 lie in one plane, as is also 
seen in FIG. 3. The ink ribbon 4 is stored in a non-illustrated 
conventional ink ribbon cassette having an unwinding spool for fresh ink 
ribbon and a winding spool for used ink ribbon. The ink ribbon 4 passes 
from the unwinding spool around the deflection roller 5, along the bottom 
surface 21, to the edge 12 having the printing bar 11, and around the 
latter to the winding spool in accordance with the direction of the arrow. 
The backing roller 3 is located axially parallel under the edge 12. The 
backing roller 3 and the edge 12 are adjustable relative to one another 
with a contact force corresponding to a required pressure for the action 
of heat on the ink ribbon 4, in order to achieve reliable transfer of ink 
particles to the letter or envelope. The deflection roller 5 and the 
backing roller 3 are rotated in accordance with the direction of the 
arrows as shown. In the case of the backing roller 3, a resilient covering 
31 with a good coefficient of friction is applied to a solid base body 32, 
which in turn surrounds an axle 33. 
According to FIG. 2A, the shell 2 is fitted to a lower surface 13 of the 
thermal printing head 1 so as to have largely the same contour. 
The heating elements 111 of the printing bar 11 are fitted equidistantly to 
the edge 11, as can easily be seen in the enlarged view of FIG. 2B. The 
ink layer on the ink ribbon 4 is partially detached by using the heating 
elements 111, depending on the activation pattern, and then the ink 
particles are transferred to the letter or envelope A through the use of 
adhesion. 
FIG. 3 shows how the shell 2 is fastened through the use of a single screw 
connection 23 to the thermal printing head 1. FIG. 3A shows that the 
bottom surface 21 of the shell 2 has a rounded edge 211 which is supported 
on the underside 13 of the thermal printing head 1, close to the edge 12, 
through an insulation layer 22. The heating elements 111 surround the edge 
12 in the form of the printing bar 11.