Thermal Line Printer With Staggered Head Segments

The present invention relates to a thermal line printer of the type having a line head composed of a plurality of linear thermal head segments aligned in the widthwise direction of a recording medium sheet perpendicular to the feeding direction or the lengthwise direction of the recording medium sheet so as to cover the entire span or width of the recording medium sheet.

One type of conventional divided line head is composed of a plurality of linear thermal head segments aligned in a single row and coupled to each other at opposed ends of adjacent segments. Each linear segment has a given length sufficient to cover the span of A4 or B4 size recording paper and is formed with a plurality of heating elements arranged linearly on the segment at a given pitch. These linear head segments are connected in series to each other to constitute the divided line head which can cover the entire span of larger size recording paper such as A1 size and A0 size, and which has a higher yield rate than that of a monolithic line head of comparative length.

However, this type of conventional divided line head has the drawback that the pitch of heating elements is made irregular at the junction or connecting portion of adjacent segments to thereby degradate the quality of printed image pattern.

Another type of conventional divided line head is disclosed in U.S. Pat. No. 4,660,052. This conventional head is composed of a plurality of linear thermal head segments aligned in a pair of parallel rows in staggered relation and in partially overlapping relation at end portions of the linear segments between the parallel rows so as to completely cover the entire width or span of recording paper. In operation, the first row of linear segments is activated to effect a part of single line printing, and then the second row of linear segments is shifted in the lengthwise direction of the recording paper relative thereto through an interval corresponding to the distance between the parallel rows and is activated to effect the remaining part of the single line printing to thereby complete the single line printing.

FIG. 1 is a side view of the conventional staggered thermal printing head device. As shown in the figure, the first row of head segments 1a and 1c are opposed to a corresponding first platen 2A, and the second row of head segments 1b and 1d are opposed to a corresponding second platen 2b. The major surfaces of the planar head segments 1a, 1b, 1c and 1d in contact with a recording paper 3 are aligned in parallel to a common plane tangential to the pair of platens 2a and 2b. An edge portion of the head segments may block a feeding pass of the recording paper transferred along the common tangential plane between the head segments and platen due to slight difference between the levels of the head segments and due to error in set angles of the head segments relative to the common tangential plane.

FIG. 2 is a partial side view of a thermal printing head segment. The head segment 1a is formed at is edge portion with electrically resistive heating elements 6 as well as common electrodes 4 for supplying driving current to the heating elements and a protective film layer 5, these of which are raised upwardly relative to the heating elements 6. The raised or convex portion tends to protrude into the feeding pass of the recording paper to thereby disturb the linearity of pass between the pair of the parallel platens 2a and 2b. Therefore, a dot line part printed by the downstream or succeeding thermal head segments 1a and 1c is not aligned or matched with the remaining dot line part printed by the upstream or preceding thermal head segments 1b and 1d. As a result, the conventional staggered printing head device cannot print a perfect dot line because of disturbance or distortion of recording paper pass.

In view of the above noted drawback in the prior art, an object of the present invention is to arrange the head segments so as to eliminate disturbance or distortion of the straight feeding pass of the recording paper. The planar head segments are slanted at their raised portions relative to the feeding pass to escape from the feeding pass. By slanting the head segments, the raised or edge portion thereof can be positioned away from the pass of recording paper so as to eliminate distortion of the recording paper pass, which would otherwise be caused due to relative variation of the surface level of the head segments and relative variation of the set position and angle, thereby avoiding vertical dislocation of the printed image and mechanical damage on the recording paper.

FIG. 1 is a side view of the conventional line thermal head device;

FIG. 2 is a partial side view of the conventional head segment;

FIG. 3A is a schematic plan view of one embodiment of the line thermal head device according to the present invention;

FIG. 3B is a side view of the FIG. 3A embodiment; and

FIG. 4 is a side view of another embodiment of the line thermal head device according to the present invention.

FIG. 3A is a plan view of one embodiment and FIG. 3B is a partial side view of the same embodiment. As shown in the figures, the line thermal head device is comprised of a pair of platen rollers 2a and 2b disposed in parallel to each other. A plurality of planar head segments 1a, 1b, 1c and 1d are disposed in contact with the platen rollers in staggered relation such that the first pair of head segments 1a and 1c are arranged in a row along the first platen roller 2a and the second pair of head segments 1b and 1d are arranged in a row along the second platen roller 2b. By such arrangement, the plurality of head segments 1a through 1d can be operatively coupled in series to each other in the widthwise direction of a recording paper so as to cover substantially the entire span of the recording paper which is longer than that of each individual head segment. The first pair of thermal head segments 1 a and 1c have heating elements, at their edge portion, aligned linearly along the platen roller 2a in contact therewith, and the second pair of head segments 1b and 1d have heating elements, at their edge portion, aligned linearly along the second platen roller 2b in contact therewith.

Further as shown in FIG. 3B, the major surface of respective head segments 1a through 1d is slanted or inclined relative to a common plane A tangential to the parallel platen rollers 2a and 2b in a printing zone such that the edge or peripheral portion of the respective head segments is disposed away from the surface of the recording medium 3. Namely, the major surface of the respective head segments and the common plane A intersect with each other at an angle .alpha. such that the first pair of head segments 1a and 1c and the second pair of head segments 1b and 1d accommodate therebetween the platen rollers. When the slanting angle .alpha. is set less than 2.degree., the setting error of the head segments and the level difference between the head segments cannot be effectively absorbed so that the raised portion formed at the edge area of respective head segments may protrude into the pass of the recording paper 3. On the other hand, when the slanting angle .alpha. is set more than 8.degree., the lengthwise distance of the recording paper feeding pass is increased between the parallel rows of head segments, thereby decreasing the absolute accuracy of the lengthwise distance therebetween to cause vertical image dislocation. Moreover, since the winding angle of the recording paper 3 is increased relative to the platen rollers 2a and 2b, the variation of diameter of each individual platen roller and the nip of the platen roller (indentation of rubber platen surface produced by pressing of the thermal head) may cause a serious affect to generate dislocation of printed image. Thus, as a result of experiment, the slanting angle .alpha. of the respective head segments should be set in the optimum range from 2.degree. to 8.degree..

FIG. 4 shows another embodiment of the line thermal head device for line-sequentially printing image dot lines in the widthwise direction of a recording paper while feeding the recording paper in the lengthwise direction thereof. The head device is comprised of a single platen roller 2, and a plurality of head segments 1a through 1d arranged in staggered relation relative to each other in a manner similar to the FIG. 3A embodiment such that the recording paper 3 is interposed in contact between the platen roller 2 and the head segments 1a through 1d. The major surface of respective head segments is slanted relative to a plane tangential to the platen roller and defining a level of a feeding pass in the printing zone such that the edge portions of respective head segments opposed between a pair of rows of staggered arrangements are inclined away from the surface of recording paper 3.

As described above, in this embodiment, the thermal head segments are disposed in slanted relation to the tangential plane so as to eliminate the affect of level variation of head segments and error in the setting angle thereof to thereby reduce the image dislocation recorded by the plurality of thermal head segments arranged in staggered relation.