EP0500334A2

EP0500334A2 - Dot-matrix thermal recording device

Info

Publication number: EP0500334A2
Application number: EP92301345A
Authority: EP
Inventors: Takanori C/O Riso Kagaku Corp. Hasegawa; Koichi c/o Riso Kagaku Corp. Okusawa
Original assignee: Riso Kagaku Corp
Current assignee: Riso Kagaku Corp
Priority date: 1991-02-21
Filing date: 1992-02-19
Publication date: 1992-08-26
Also published as: JPH04265759A; EP0500334A3; JP3043443B2

Abstract

Provided is a device which can form favorable perforated images corresponding to the resolution of the thermal head, reproduce faithful printed images for all kinds of original picture images, avoid ink transfer, and adapt itself to different environmental conditions, and is suitable for use with different thermal recording materials such as thermal recording paper, OHP TP sheets, and thermal stencil master plates.

In the device of the present invention, a thermal head 4 consisting of a plurality of heat emitting elements 5 arranged in a single row in the primary scanning direction is directly contacted to the recording surface of a thermal recording material such as thermal recording paper, and the thermal recording material 1 is moved relative to the thermal head 4 in the secondary scanning direction which is perpendicular to the direction of the row of the heat emitting elements so that picture images may be formed with a dot matrix by selectively heating the thermal heat emitting elements, the ratios of the length of each heat emitting element 5 in the primary and secondary scanning directions to the pitches of the primary and secondary scanning are set 30 to 70% and 60 to 95%, respectively.

Description

TECHNICAL FIELD

The present invention relates to a thermal recording device for forming an image with a dot matrix by applying a thermal head to a heat-sensitive recording material such as heat-sensitive printing paper, thermal transfer ribbon, and thermal stencil master plates made by laminating a thermo-plastic film over a porous support.

BACKGROUND OF THE INVENTION

The thermal recording device which forms images with a dot matrix by using a thermal head is conventionally known, and such a thermal recording device forms images by applying a thermal head consisting of a plurality of heat emitting elements onto thermal recording paper, an OHP coloring TP sheet, an OHP frosted TP sheet, recording paper in conjunction with the use of thermal transfer ribbon, or a recording surface of heat sensitive recording material such as a thermal stencil master plate, and by selectively heating the heat emitting elements. Such thermal recording devices are widely used as facsimiles, printers for ticket dispensers, handheld copiers, OHP transparency making devices, and thermal master plate making devices.
In facsimiles, the feed speed of the thermal recording paper in the longitudinal direction or in the secondary scanning direction is determined by a unified standard, and the size of each heat emitting element is determined according to the feed speed in the secondary scanning direction. Further, the aspect ratio of each heat emitting element is determined to be a/b = 1/2 by a communication standard where a and b are the lengths of each heat emitting element in the primary and secondary scanning directions, respectively, the primary direction corresponding to the lateral direction of the paper or the direction of the row of the heat emitting elements.
Therefore, in the high resolution mode (fine mode) of the facsimile standard in which P_A = P_B where the dot pitch in the primary scanning direction is P_A and the dot pitch in the secondary scanning direction is P_B, b > P_A = P_B holds, and there will be some overlapping in the heat emitting regions of the heat emitting elements for each small distance along the secondary scanning direction.
If a thermal stencil master plate is processed or made by forming stencil images on a thermo-plastic film of a thermal stencil master plate with a thermal head for a facsimile of the above described kind in a mode equivalent to the high resolution mode of the facsimile standard, continuous openings will be formed in the thermo- plastic film of the thermal stencil master plate along the secondary scanning direction due to the above-mentioned overlapping. This causes not only the thickening and blurring of the lines of printed character and line images but also excessive deposition of ink onto the printing paper in solid areas of the picture images which could in turn cause conspicuous smearing of the reverse surface of the printing paper by ink transfer in continuous printing.
To overcome this problem, it has been proposed to make a thermal stencil master plate with a thermal head using heat emitting elements each of which is shorter in length along the secondary scanning direction than the pitch of the secondary scanning, and to ensure formation of unaffected parts between the perforations along the secondary scanning direction as disclosed in Japanese patent laid open publication No. 2-67133.
According to this proposal, since the perforations formed in the thermo-plastic resin film of the thermal stencil master plate are formed so as to be independent from each other in both primary and secondary scanning directions, it is possible to faithfully reproduce character images by printing, and to control excessive deposition of ink and reduce ink transfer from one sheet to another.
However, images formed by perforation of a film of a thermal stencil master plate are inferior in quality as compared to those formed by using thermal coloring type media such as thermal recording paper in terms of reproducibility (resolution) as compared to the original images, in particular the evenness of fine lines and small characters, legibility of small outlined characters, the sharpness of fine black and white patterns such as halftone screen images, and digitally reproduced photographic gradations.
Further, in high temperature environment, the perforation of the thermo-plastic resin film of the thermal stencil master plate due to melting tends to be excessive, and, combined with the lowering of the viscosity of the ink, the thickening and blurring of lines of character images become more pronounced, as compared to the original images, than in normal or low temperature environment. Additionally, the smearing or the ink transfer of the printing paper tends to be more pronounced due to increase in the amount of ink deposition, and the acceptable temperature range becomes narrower.
Thus, it has not heretofore been possible to provide a thermal recording device which can achieve the picture quality equivalent or comparable to those of the picture images produced by the coloring of thermal recording paper in the picture images produced by using the thermal stencil master plate, and achieving a desired uniformity in picture quality even when thermal recording materials having different recording properties are used.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of the present invention is to eliminate such problems and to provide a thermal stencil master plate making device which can form favorable stencil images for a given resolution of the thermal head, reproduce faithful printed images for all kinds of original picture images, prevent ink transfer, and adapt itself to various environmental conditions.
A second object of the present invention is to provide a thermal recording device which is suitable for use with a wide range of thermal recording materials having different recording properties, such as thermal recording paper, OHP TP sheets, and thermal stencil master plates.
These and other objects of the present invention can be accomplished by providing a thermal recording device for forming an image with a dot matrix by applying a thermal head consisting of a plurality of heat emitting elements arranged in a single row at a first pitch along a primary scanning direction onto a surface of a thermal recording material and moving the thermal recording material relative to the thermal head in a secondary scanning direction perpendicular to the primary scanning direction, and selectively heating the heat emitting elements for each successive movement of the thermal recording material at a second pitch in the secondary scanning direction, wherein: a ratio of a length of each of the heat emitting elements of the thermal head in the primary scanning direction to the first pitch is 30 to 70%, and a ratio of a length of each of the heat emitting elements of the thermal head in the secondary scanning direction to the second pitch is 60 to 95%.
In the thermal recording device of the present invention, since the size of each of the heat emitting elements of the thermal head is determined such that:

length in the primary scanning direction ---> 30 to 70% of the pitch of the primary scanning
length in the secondary scanning direction ---> 60 to 95% of the pitch of the secondary scanning

The primary reason which makes the quality of the picture images formed by thermal stencil master plate printing less favorable to that by thermal recording paper printing is found in the fact that the shape of the perforated dots in the film of the thermal stencil master plate are not so uniform as the colored dots of the thermal recording paper and, even though they may form independent dots, for instance, when three consecutive heat emitting elements along the secondary scanning direction are heated at the same time to form an image by perforation, the heat emitting elements are affected by the adjacent ones and the behavior of the melting and shrinking of the part of the perforated thermo-plastic resin film which directly contacts the heat emitting elements depend on the way the film is supported by the porous support fibers. In particular, when there is no support fibers under the thermoplastic resin film upon which the heat emitting elements are pressed, the melting and shrinking of the film tends to be excessive. If such an area not supported by fibers extends over a number of heat emitting elements and is heated by several of the heat emitting elements at the same time, the dots may excessively expand or clog adjacent ones by expansion with the result that the adjacent dots are affected and the sizes of the perforated dots become uneven.
Further, in the process of preparing a thermal stencil master plate in high temperature environment, the thermal effect from adjacent heat emitting elements becomes so pronounced that the thickening and blurring of fine lines tends to be significant, the quality of picture images become even more inferior to those of the thermal recording paper, and the excessive deposition of printing ink onto the printing paper through the expanded dots increases the possibility of ink transfer or the smearing of the reverse surface of the printing paper.
On the other hand, according to the thermal recording device of the present invention, since the length of each heat emitting element of the thermal head in the primary scanning direction is 30 to 70% of the pitch of the primary scanning and the length in the secondary scanning direction is 60 to 95% of the pitch of the secondary scanning to the end of avoiding the deterioration of the quality of the picture images due to the unevenness of the shape of the perforated dots, each of the dots would not be affected by the heating of the dots adjacent thereto along the primary scanning direction, and stable perforation may be achieved on the thermo- plastic resin film of the thermal stencil master plate so that the evenness of the perforated dots can be improved, and the quality of the printed images becomes comparable to that of the thermal recording paper. Further, in carrying out the process of plate making in high temperature environment, perforations may be formed in a stable fashion to an extent which has not heretofore been attainable, and the quality of picture images may be improved with the added advantage of eliminating ink transfer.
Since each of the perforated dots is independent from each other, and the shape of the dots is highly uniform, the part remaining between the perforated dots of the thermo-plastic resin film of the thermal stencil master plate is made uniform, and the strength of the film is improved so that the number of sheets of paper that can be printed with the same master plate may be increased.
The thermal recording device of the present invention offers a significant advantage over the method of making a recorded article with a number of steps such as the method involving the steps of processing a thermal stencil master plate and making printed materials, and the method of processing printing paper by using such thermal recording media as thermal transfer ribbon, and can be used in conjunction with the method of making recorded materials with a single step by using such materials as thermal recording paper and OHP coloring TP sheets. According to the thermal recording device of the present invention, the printed records (printed characters) are formed by independent dots, and the density of the printed characters (colored images) may become slightly less dark due to the reduction in the area of each printed (colored) dot. But, it is not significant, and the reproducibility and legibility of small characters and images actually improve.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with reference to the appended drawings, in which:

Figure 1 is a schematic side view of an embodiment of the thermal recording device of the present invention;
Figure 2 is a schematic plan view of the thermal head used in the thermal recording device of the first embodiment;
Figure 3 is a microscopic photograph of a part of the thermal stencil master plate of the first embodiment obtained with a scanning electron microscope at a magnification factor of 100;
Figure 4 is an enlarged view of a part of Figure 3 at a magnification factor of 1000;
Figure 5 is a microscopic photograph of a part of the thermal stencil master plate of the fourth embodiment obtained with a scanning electron microscope at a magnification factor of 10;
Figure 6 is an enlarged view of a part of Figure 5 at a magnification factor of 100;
Figure 7 is a microscopic photograph of a part of the thermal stencil master plate of the first example for comparison obtained with a scanning electron microscope at a magnification factor of 10; and
Figure 8 is an enlarged view of a part of Figure 7 at a magnification factor of 100.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows the general structure of an example of thermal recording device according to the present invention. In the illustrated thermal recording device, thermal recording material 1 is held between a pair of conveyer rollers 2, and is conveyed in the direction indicated by the arrow A (secondary scanning direction) until it is placed between a platen roller 3 and a thermal head 4. Then, heat emitting elements 5 provided in the thermal head 4 are directed contacted to a recording surface (surface 1a a in the drawing) of the thermal recording material 1, and recorded images are formed on the recording surface 1a a of the recording material 1 by selectively heating the heat emitting elements 5.
The recording surface 1a of the thermal recording material 1 corresponds to the surface carrying the coloring layer of thermal printing paper or coloring type TP sheet, or the thermo-plastic resin film of a thermal stencil master plate, or the base film of thermal transfer ribbon.
Figure 2 is a schematic plan view of the thermal head 4. As shown in this drawing, the heat emitting elements 5 of this thermal head 4, each having a rectangular shape, are arranged in a single row, at a pitch of Pa, along a primary scanning direction which is perpendicular to the secondary scanning direction given as a direction in which the thermal stencil master plate is conveyed or as the direction of the relative movement. The two ends along the secondary scanning direction of each of the heat emitting elements 5 are connected to electrodes 6, respectively, so that electric power may be supplied individually to each of the heat emitting elements 5.
The dot pitch of the dot matrix of the images formed on the recording surface 1a of the thermal recording material 1 in the primary scanning direction is determined by the pitch Pa of the heat emitting elements 5 in the primary scanning direction, and the dot pitch of the matrix in the secondary scanning direction is determined by the heat emitting response property of the heat emitting elements 5 of the thermal head 4 in relation with the moving speed of the thermal recording material in the secondary scanning direction. In the thermal recording device of the present invention, various parameters are so selected that the dot pitch of the dot matrix of the images formed by the heat from the heat emitting elements 5 of the thermal head 4 in the secondary scanning direction is made to be equal to the dot pitch in the primary scanning direction.
If the lengths of each of the heat emitting elements 5 in the primary and secondary scanning directions are a and b, respectively, the thermal recording device of the present embodiment is characterized by the size of each of the heat emitting elements 5 being as follows:

0.30 Pa a 0.70 Pa,
0.60 Pa b 0.95 Pa, and
Pa = Pb.

Thus, as mentioned earlier, the dot pitch (secondary scanning pitch Pb) of the dot matrix of the images formed by the heat from the heat emitting elements 5 in the secondary scanning direction is equal to the dot pitch in the primary scanning direction which is equal to the pitch (primary scanning pitch) Pa of the heat emitting elements 5 in the primary scanning direction.
Therefore, when the lengths of each of the heat emitting elements 5 in the primary and secondary scanning directions are short as compared to the corresponding dot pitches, the region of heat generation of each of the heat emitting elements will not be affected by the heat from the adjacent heat emitting elements 5, and the recorded traces or, in the case of thermal recording paper, the colored dots, the perforated dots in the case of the thermal stencil master plate, and the frosted dots in the case of the OHP frost type TP sheet will be independent from each other both in the primary and secondary directions, leaving gaps of unrecorded regions between the recorded dots. The size of these dots depends on the size of the heat emitting elements, the sensitivity of the thermal recording material or the medium, the coloring properties in the case of the thermal recording paper, the perforation property of the thermo-plastic resin film in the case of the thermal stencil master plate, and the melting and transferring properties of the ink sheet onto the printing paper in the case of the transfer ribbon.
The gaps between the recorded dots are particularly useful for such thermal recording materials as thermal stencil master plate and thermal transfer ribbon which can rely on the seeping of ink, and the plate making or the printing by the device of the present invention can produce optimum gaps in the recording material.
Meanwhile, in the case of the thermal recording paper and the OHP coloring type TP sheets, the expansion of the colored parts corresponding to the seeping of ink cannot be expected as much as in the case of the thermal stencil master plate, but when characters (records) are printed by the device of the present invention, solid areas will be slightly light in gradation as compared to the characters (records) printed by the conventional thermal head (although the density of each colored dot may have reached a saturated density level, the gaps extending between the dots reduce the area of each dot in the high density regions). However, it is not visually discernible, and actually achieves some improvement in the reproducibility and legibility of small character images.
When the used device is such that the ratio of the length of each of the heat emitting elements 5 in the primary scanning direction to the scanning pitch in the primary scanning direction does not satisfy the conditions defined for the device of the present invention, in particular when the ratio is greater than that of the device of the present invention, the perforated dots are connected in both the secondary and primary scanning directions particularly under a high temperature condition, and unfavorable results such as the thickening and blurring of the lines of images and the ink transfer from one sheet of the printing paper to another tend to occur. If the ratio related to the dot pitch is smaller than that of the device of the present invention, the distance between adjacent perforated dots becomes excessive, and the thinning of picture images and lowering of gradation level in solid areas tend to occur.
Now, embodiments of the present invention and examples for comparison are now described in the following. The results of evaluating the embodiments and the examples for comparison are summarized in Tables 1 and 2.

(Embodiment 1)

A thin film type thermal head of a 400 dots/inch (abbreviated as DPI hereinafter) resolution with the following specifications was mounted on a digital stencil master plate making and printing device (sold by Riso Kagaku Kogyo KK under the tradename of Risograph RC115D), and a thermal stencil master plate (tradename: RC Master 55) was processed by using an original containing character images and solid images. The ink used in this embodiment had a spread meter reading of one minute value of 33, and the printing device was the same as above (the same thing applies to the subsequent embodiments). The processing of the thermal recording paper (tradename: Riso thermal paper sheet type C-197) and OHP TP sheet (tradename: Riso TP film T-113) was also carried out with the single copy mode of the aforementioned device. The ambient temperature was 23°C.

Length of each heat emitting element in the primary scanning direction ---> a = 25 µm
Length of each heat emitting element in the second scanning direction ---> b = 60 µm
Dot pitch (primary and secondary scanning directions) ---> Pa = Pb = 63.5 µm
Heat emitting energy ---> 68.8 - 50.0 pJ/dot

The thermal stencil master plate was fabricated by laminating a polyester film (2 µm in thickness) and a porous support (9.5 g/m², manila hemp thin paper) with a bonding agent, and applying a release agent on the surface of the film facing the thermal head.
The thermal recording paper consisted of base paper carrying a layer of heat sensitive coloring agent with a density of 57 g/m2.
The OHP TP film consisted of a polyester film (50 f..lm in thickness) provided with a layer of a coloring agent.
As indicated by the microscopic photographs in Figures 3 and 4, the perforated dots which formed solid parts of the picture image were independent from each other, and formed a uniform dot matrix so that the unprocessed gaps between the adjacent dots may extend in both the primary and secondary scanning directions uniformly in the manner of a grid.
When the quality of the character image on the thermal recording paper and the picture image formed on the OHP TP sheet were evaluated by using a microscope, it was found that the unprocessed gaps existed between colored dots, but they were visually indiscernible enough for the solid parts to be regarded as such. In regards to character images consisting of fine lines, they were also faithfully reproduced from the original. The projected images of the processed TP sheet were also quite satisfactory.
When prints were made by using such a processed thermal stencil master plate, the parts corresponding to the unprocessed gaps between perforated dots observed in the master plate were filled by the seeping of the ink, and the printed solid parts were quite favorable. In regards to character images also, printed images faithful to the original were obtained without involving any thinning, thickening or blurring. In particular, favorable reproduction of minute character images was achieved. The images were comparable to those obtained by using thermal recording paper.
A prescribed number of prints were made by operating the aforementioned recording device at the rate of 60 to 130 sheets per minute, and the reverse surface of each of the printed sheets piled into a stack showed substantially no smearing by ink or no ink transfer.
The durability of the master plate was found to be favorable.

(Embodiment 2)

The same operation as that of the first embodiment was carried out at the ambient temperature of 10°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly smaller than those of the first embodiment, but a required picture quality was ensured in each case without creating any problem.

(Embodiment 3)

The same operation as that of the first embodiment was carried out at the ambient temperature of 35°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly larger than those of the first embodiment, but a required picture quality was ensured in each case without creating any problem.

(Embodiment 4)

The recordability of the thermal recording materials (thermal stencil master plate, thermal recording paper and OHP TP sheet) was investigated by using a thin film type thermal head of 400 DPI which was set up as described above and the same device and original as the first embodiment. The ambient temperature was 23°C.

Length of each heat emitting element in the primary scanning direction ---> a = 35 µm
Length of each heat emitting element in the second scanning direction ---> b = 60 µm
Dot pitch (primary and secondary scanning directions) ---> Pa = Pb = 63.5 µm
Heat emitting energy ---> 75.0 - 55.0 pJ/dot

When a part of the thermal stencil master plate obtained in this embodiment was observed with a scanning electron microscope, the condition of the plate in the solid regions was found to be favorable as shown in the microscopic photographs of Figures 5 and 6. In other words, the perforated dots forming the solid areas were independent from each other, and formed a uniform dot matrix by defining unprocessed gaps between consecutive dots in both the primary and secondary directions in the manner of a grid.
When the quality of the character images on the thermal recording paper and the picture image formed on the OHP TP sheet was evaluated by using a microscope, it was found that the unprocessed gaps existed between colored dots in the solid regions, but they were visually indiscernible enough for the solid parts to be regarded as such in regards to both solid images and character images.
When prints were made by using a processed thermal stencil master plate, the parts corresponding to the unprocessed gaps between perforated dots observed in the master plate were filled by the seeping of the ink, and the printed solid parts were quite favorable. In regards to character images also, printed images faithful to the original were obtained without involving any thinning, thickening or blurring. In particular, favorable reproduction of minute character images was achieved. The images were comparable to those obtained by using thermal recording paper.
A prescribed number of prints were made by operating the aforementioned recording device at the rate of 60 to 130 sheets per minute, and the reverse surface of each of the printed sheets piled into a stack showed substantially no smearing by ink.
The durability of the master plate was found to be satisfactory.

(Embodiment 5)

The same operation as that of the fourth embodiment was carried out at the ambient temperature of 10°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly smaller than those of the fourth embodiment, but a required picture quality was ensured in each case without creating any problem.

(Embodiment 6)

The same operation as that of the fourth embodiment was carried out at the ambient temperature of 35°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly larger than those of the fourth embodiment, but a required picture quality was ensured in each case without creating any problem.

(Embodiment 7)

Length of each heat emitting element in the primary scanning direction ---> a = 44 f..lm
Length of each heat emitting element in the second scanning direction ---> b = 60 µm
Dot pitch (primary and secondary scanning directions) ---> Pa = Pb = 63.5 µm
Heat emitting energy ---> 81.5 - 60.0 pJ/dot

In this case, the perforated dots forming the solid areas were independent from each other, and formed a uniform dot matrix by defining unprocessed gaps between consecutive dots in both the primary and secondary directions in the manner of a grid.
When the quality of the character images on the thermal recording paper and the picture image formed on the OHP TP sheet was evaluated by using a microscope, it was found that the unprocessed gaps existed between colored dots in the solid regions, but they were visually indiscernible enough for the solid parts to be regarded as such in regards to both solid images and character images.
When prints were made by using a processed thermal stencil master plate, the parts corresponding to the unprocessed gaps between perforated dots observed in the master plate were filled by the seeping of the ink, and the print quality of the solid parts was quite favorable. In regards to character images also, printed images faithful to the original were obtained without involving any thinning, thickening or blurring. There was no smearing of the reverse surface of the printing paper.

(Embodiment 8)

The same operation as that of the seventh embodiment was carried out at the ambient temperature of 10°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly smaller than those of the seventh embodiment, but a required picture quality was ensured in each case without creating any problem.

(Embodiment 9)

The same operation as that of the seventh embodiment was carried out at the ambient temperature of 35°C.
The perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet had a tendency to be slightly larger than those of the seventh embodiment, but a required picture quality was ensured in each case without creating any problem.

(Example 1 for Comparison)

The recordability of the thermal recording materials (thermal stencil master plate, thermal recording paper and OHP TP sheet) was investigated by using a thin film type thermal head of 400 DPI which was set up as specified below and the same device and original as the first embodiment for the purpose of comparing it to those of the embodiments 1 through 9. The ambient temperature was 23°C.

Length of each heat emitting element in the primary scanning direction ---> a = 53 f..lm
Length of each heat emitting element in the second scanning direction ---> b = 60 µm
Dot pitch (primary and secondary scanning directions) ---> Pa = Pb = 63.5 µm
Heat emitting energy ---> 87.5 - 65.0 pJ/dot

As shown in the microscopic photographs of Figures 7 and 8 taken with a scanning electron microscope and showing a solid picture image formed in a thermal stencil master plate, the perforated dots forming the solid areas were expanded in the primary or secondary scanning direction, and are merged with the adjacent ones, demonstrating the thermal influences from adjacent heat emitting elements. Therefore, the unprocessed gaps between consecutive dots were extremely small in some areas as compared to the above described embodiments, and the perforated dot matrix forming the solid regions was found to be inferior in terms of uniformity as compared with the above described embodiments.
When prints were made by using a processed thermal stencil master plate, the character images involved substantial thickening and blurring, and the solid areas contained a substantial amount of imprints of the fibrous support. This was caused by the parts of the film corresponding to those dots which were thermally affected by adjacent heat emitting elements and excessively melted, and the fluidized film which entangled with the fibers of the porous support and formed resolidified film or lumps. Further, the perforated dots became uneven in size, and the height of the ink deposited on the printing paper became uneven thereby causing unevenness in the density of the picture image.
There was a substantial amount of ink transfer because the expansion and blurring of the perforated dots became excessive, and the amount of ink deposition was accordingly great, thereby slowing the process of drying the printing ink.
As for the printing durability, the unprocessed gaps between the perforated dots are less than those of the embodiments, and the mechanical strength of the film was diminished, thus producing generally less favorable results than those of the above mentioned embodiments.
As for the coloring performances of the thermal recording paper and the OHP TP sheet, the colored dots forming solid regions were continuous, and a sufficient density was obtained. However, small character images involved thickening and blurring of lines, and legibility was diminished as compared to the above described embodiments.

(Example 2 for Comparison)

The same operation as the first example for comparison was carried out at the ambient temperature of 10°C.
The extent to which the perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet expanded and merged with the adjacent ones was eased as compared to the first example, and the thickening and merging of the lines in the character images was reduced. However, the sensitivity of the perforation and coloring was reduced as compared to that of the first example, and generation of unperforated dots and reduction in the area of each colored dot caused whitening or reduction in the density of solid areas.

(Example 3 for Comparison)

The same operation as the first example for comparison was carried out at the ambient temperature of 35°C.
The extent to which the perforated dots of the thermal stencil master plate and the colored dots of the thermal recording paper and the OHP TP sheet expanded and merged with the adjacent ones became even worse as compared to the first example, and the thickening and merging of the lines in the character images was more pronounced, resulting in a poor picture quality. In particular, the perforated dots forming solid images became more random in terms of size, shape and arrangement. It was presumably because each of the perforated dots was affected by the heat from adjacent heat emitting elements. The condition of perforation did not reflect the resolution of the thermal head (400 DPI) at all, and the prints produced by the processed master plate involved excessive ink transfer.

(Example 4 for Comparison)

The recordability of the thermal recording materials (thermal stencil master plate, thermal recording paper and OHP TP sheet) was investigated by using a thin film type thermal head of 400 DPI which was set up as specified below and the same device and original as the first embodiment. The ambient temperature was 23°C.

Length of each heat emitting element in the primary scanning direction ---> a = 44 f..lm
Length of each heat emitting element in the second scanning direction ---> b = 85 µm
Dot pitch (primary and secondary scanning directions) ---> Pa = Pb = 63.5 µm
Heat emitting energy ---> 100.0 - 75.0 pJ/dot

In regards to the coloring and recordability of the thermal recording paper or the OHP TP sheet, the density of the coloring in the solid areas was sufficiently high, and a microscopic observation revealed some continuity in the colored dots. The picture images were generally favorable except for some thickening and merging of the lines of small characters.
However, the perforations in the thermal stencil master plate were continuous in both the primary and secondary scanning directions, and the picture images contained more imprints of the fibrous support than the first example for comparison with the added disadvantages of more severe ink transfer and increased ink consumption.

(Example 5 for Comparison)

The same operation as the fourth example for comparison was carried out at the ambient temperature of 10°C.
The perforations of the thermal stencil master plate were continuous in both the primary and secondary scanning directions in some areas, but there were also areas where perforations were not produced (due to insufficient sensitivity). The prints contained excessive unevenness in density.
The character images of the OHP TP film involved thinning (breaks in fine lines) due to the insufficiency in sensitivity.

(Example 6 for Comparison)

The same operation as the fourth example for comparison was carried out at the ambient temperature of 35°C.
A majority of the perforated dots of the thermal stencil master plate were continuous in both the primary and secondary scanning directions, and the printability was extremely poor with severe thickening of character images and ink transfer.
In regards to the thermal recording paper and the OHP TP sheet, the density of solid areas was favorably high, but excessive merging and thickening of the lines of the character images prevented reproduction of clear images.
The results of evaluating the above described embodiments and examples for comparison are given in Tables 1 and 2.
In Tables 1 and 2, "00" denotes "very good", "O" denotes good, " "denotes fair, "X" denotes poor, and "XX denotes "very poor". The criteria for each item of evaluation are as given in the following:

1. Evaluation of the thermal stencil master plate
- 1) Condition of the perforation
  - 00 - The perforation dots are independent from each other and define a uniform dot matrix.
  - O - The arrangement of the perforation dots is uneven, but are independent from each other.
  - - The perforation dots are partly continuous.
  - X - A substantial part of the perforation dots are continuous.
  - XX - Expansion and merging of the perforation dots are severe.
- 2) Condition of the prints
  - 00 - The uniformity of solid areas and the reproducibility of character images are both favorable.
  - 0 - The quality is generally acceptable, but the lines of character images are partly thickened.
  - _o- Thinning or merging of the lines of character images can be seen.
  - X - Thickening of images is conspicuous.
  - XX - Thickening of images is severe, and the images are blurred as a whole.
- 3) Ink transfer
  - 00 - There is almost no ink transfer.
  - 0 - There is a slight ink transfer.
  - - The solid areas give rise to ink transfer.
  - X - There is a significant ink transfer.
  - XX - There is a severe ink transfer.
- 4) Plate durability
  - 00 - More than 5,000 prints.
  - O - About 5,000 prints.
  - - About 4,000 prints.
  - X - Less than 4,000 prints.
- 5) Ink consumption (The number of prints of B4 paper with an image ratio of 20% that can be made with 1,000 cc of printing ink)
  - 00 - More than 10,000 prints.
  - O - More than 9,000 prints.
  - - More than 8,000 prints.
  - X - More than 7,000 prints
  - XX - Less than 7,000 prints.
2. Evaluation of the thermal printing paper
- 1) Coloring of solid areas
  - 00 - Particularly favorable with a sufficient density.
  - 0 - Solid areas are in a favorable condition.
- 2) Reproducibility of character images
  - 00 - Legibility of even the small characters is favorable.
  - 0 - There are some merging of lines in parts of the small characters
  - □ - There are thinning or merging of lines, and the images lack evenness.
  - X - There are conspicuous merging and thickening of the lines of the character images.
3. Evaluation of the OHP TP sheet

The same as the thermal recording paper.
Since the ratios of the lengths of each heat emitting element in the primary and secondary scanning directions are 30 to 70% and 60 to 95%, respectively, to the dot pitches in the corresponding directions in the thermal plate making device of the present invention, faithful reproduction is possible for all kinds of images including small character images and solid images, and one can obtain other advantages such as a favorable ink transfer prohibiting property, a high plate durability, a favorable print capability with controlled ink consumption, and an expanded environmental adaptability which can cover a wide temperature range.
Further, the thermal recording device is suitable for use with thermal recording paper and OHP TP sheets, and is particularly advantageous in reproducing minute character images.
Although the present invention has been described in terms of preferred embodiments thereof, it is obvious to a person skilled in the art that various alterations and modifications are possible without departing from the scope of the present invention which is set forth in the appended claims.

Claims

1. A thermal recording device for forming an image with a dot matrix by applying a thermal head consisting of a plurality of heat emitting elements arranged in a single row at a first pitch along a primary scanning direction onto a surface of a thermal recording material and moving said thermal recording material relative to said thermal head in a secondary scanning direction perpendicular to said primary scanning direction, and selectively heating said heat emitting elements for each successive movement of said thermal recording material at a second pitch in said secondary scanning direction, wherein:

a ratio of a length of each of said heat emitting elements of said thermal head in said primary scanning direction to said first pitch is 30 to 70%, and a ratio of a length of each of said heat emitting elements of said thermal head in said secondary scanning direction to said second pitch is 60 to 95%.

2. A thermal recording device according to claim 1, wherein said first pitch is substantially equal to said second pitch.

3. A thermal recording device according to claim 1, wherein said thermal recording material consists of a thermal stencil master plate.

4. A thermal recording device according to claim 1, wherein said thermal recording material consists of heat sensitive paper.

5. A thermal recording device according to claim 1, wherein said thermal recording material consists of thermal transfer ribbon.