Bidirectional Hot Melt Ink Jet Printing

Technical Field

This invention relates to bidirectional hot melt ink jet printing systems and, more particularly, to a new and improved hot melt ink jet printing system which provides improved image quality.

Background Art

In conventional ink jet printing systems, a re- ciprocating ink jet head having an array cf ink jet orifices projects corresponding arrays of ink drops onto a substrate and, for bidirectional printing, the ink drops are applied during each pass of the ink jet head during its reciprocating motion in opposite di- rections adjacent to the record medium. When a bidi¬ rectional ink jet printing system is arranged to pro¬ vide hot melt ink printing, certain problems may arise which result in degradation of the quality of the image produced by the printing. For example, as de- 'scribed in the copending Application Serial No.

07/359,736, banding may result from variations caused by bidirectional printing and this can be made less apparent by interlaced bidirectional printing. On the other hand, where two adjacent hot melt ink lines are printed in succession during opposite successive mo¬ tions of a printhead, the first line may not be fully solidified before the second line is printed. If the successive adjacent lines are laid down in two differ¬ ent colors, the inks may coalesce along the border between the lines, producing an undesired line of the composite color.

Even if there is no undesirable mixing of colors, the heat transfer resulting from application of the first line increases the temperature of the adjacent

substrate region. As a result, if the line applied during the next scan of the ink jet head is immedi¬ ately adjacent to the preceding line, the increased substrate temperature in the region where the second line is applied may cause the ink in that line tc spread to a greater extent, producing a different image quality.

Furthermore, when two primary colors are over¬ printed to produce a composite color, the first color often spreads on the substrate to a greater extent than the second color spreads on the first color, so that each drop of the second color ink is surrounded by a marginal region in which only the first color has been printed. This causes the hue of the composite color to be shifted toward that of the first color printed. When different adjacent regions made up of the same composite color are printed by application of the primary colors in reverse order, which may occur during bidirectional color printing, the hues of the composite color in the adjacent regions will be shifted toward that of the primary color first ap¬ plied, so that regions in which the composite color is applied in opposite directions will show color band¬ ing. In addition, the second ink to be applied in the same region will not usually flow into the substrate, which may result in lower adhesion, leading to chip¬ ping or flaking of the second ink from the image.

Disclosure of Invention

Accordingly, it is an object of the present in- vention to provide a new and improved hot melt ink jet printing system which overcomes the above-mentioned disadvantages of the prior art.

Another object of the invention is to provide a new and improved printing system in which variations in the hue of a composite color resulting from bidi¬ rectional printing are avoided.

A further object of the invention is to provide a system for producing a composite color image in which a subsequent primary color applied to a first primary color is not subject to chipping, flaking or other adhesion problems.

These and other objects are attained according to one aspect of the invention by providing a hot melt color ink jet printing system in which a second pri¬ mary color ink is printed over a first primary color ink before the first primary color ink has solidified. In this way, mixing of the two primary color inks can be effected so that a composite color of the same hue is provided throughout the region in which both pri¬ mary color inks are applied and also assuring good adhesion of the second ink to be applied.

For this purpose, an ink jet head is provided having a plurality of orifices from which inks of different colors are printed, in which the orifices are arranged to apply different primary color inks to the same location on a record medium in closely spaced succession, so that drops of a first primary color ink applied to the region of the medium are still at least partially molten when superimposed drops of another color ink are applied to the same region. Thus, dif- ferential spreading of the two inks on the substrate and problems resulting from inadequate adhesion of the second ink are eliminated.

The time delay between the application of the first and second ink drops at the same location on the substrate may vary depending upon the difference be¬ tween the melting point and the application tempera¬ ture of the inks and temperature of the substrate. In general, however, the time delay should be less than about 100 milliseconds and, preferably, less than about 50 milliseconds in order to assure that the first ink drop to be applied is still sufficiently fluid to permit mixing when the last drop is applied. One form of ink jet apparatus in accordance with this

aspect of the invention has a reciprocating ink jet head arranged to scan a substrate at a rate of at least 10 cm/sec. and, preferably, at least about 100 cm/sec, in which the ink jet orifices supplying dif- ferent colors of ink are aligned in the direction of scanning motion of the head and are spaced by no more than about 10 cm. , desirably by no more than about 5 cm, and preferably by no more than about 2 cm.

According to another aspect of the invention, the lines of ink drops produced by the aligned orifices during one scan of the ink jet head have a spacing greater than the line spacing in the image to be pro¬ duced, and the image lines located between the lines produced during one scan of the ink jet head are formed during one or more subsequent scans of the ink jet head. In this way, color banding resulting from hue variations is rendered less detectable. Prefer¬ ably, each line produced during a scan of the ink jet head is spaced from a line produced during the previ- ous scan by at least one line width to reduce thermal interaction between the lines.

Brief Description of Drawings

Further objects and advantages of the invention will be apparent from a reading of the following de- scription in conjunction with the accompanying draw¬ ings in which:

Fig. 1 is a schematic diagrammatic view illus¬ trating the operation of a typical prior art color ink jet system; Fig. 2 is a graphical representation illustrating the threshold for visual detectability of image band¬ ing as a function of contrast and spatial period;

Fig. 3 is a schematic diagrammatic view illus¬ trating the arrangement of ink jet orifices in a rep- resentative ink jet head in accordance with the pres¬ ent invention; and

Fig. 4 is a schematic diagrammatic view illus¬ trating the arrangement of ink jet orifices in another form of ink jet head in accordance with the invention.

Best Mode for Carrying Out the Invention In the schematic illustration of Fig. 1 showing the operation of a typical prior art color ink jet system, an ink jet head 10 has an array 11 of orifices from which ink is ejected drop by drop onto a sub¬ strate 12 during successive reciprocating motions of the head 10 in the direction indicated by the arrow 13 so as to produce a pattern 14 on the substrate. To produce a color image, the array 11 of orifices is divided into four orifice groups 15, 16, 17 and 18, each having, for example, six orifices, to apply four different color inks, such as yellow, magenta, cyan and black, respectively, to the substrate 12. For bidirectional color printing, i.e., printing during motion of the head in both directions, the substrate 12 may be advanced in the direction of the arrow 19 after each scan of the head 10 by a distance 20 which is equal to the length of each of the orifice groups 15-18 in the direction of motion 19 of the substrate.

In order to provide high-resolution printing, the spacing in the direction of motion of the substrate 19 between the adjacent orifices in the array 11 should be as small as possible and, to provide an ink jet head having a smaller effective spacing, the array of orifices is usually tilted at an angle with respect to the direction of reciprocating motion 13 of the head, as illustrated in Fig. 1. In addition, closer spacing of the lines produced by the ink jet head may be ob¬ tained by interlaced scanning, in which the substrate 12 is advanced by a distance corresponding to an odd multiple of half the spacing of the orifices in the direction of the arrow 19 after each scan of the head, as described in the above-mentioned copending applica¬ tion. In the schematic illustration of the head 10

shown in Fig. I, only 24 orifices in the array 11 are shown, but it will be understood that the array may be extended so that a larger number of orifices, such as 48, 72 or 96, is provided. In the type of operation in which the substrate 12 is advanced in the direction 19 by a distance 20 between successive transverse motions of the ink jet head 10, the ink applied to the substrate by the groups of orifices 15, 16, 17 and 18 will appear in four adjacent bands 21, 22, 23 and 24, respectively, during the first scan of the head and, during the next scan, each group of orifices will apply the corre¬ sponding color ink to the next adjacent band in the group so that after four successive scans all four colors of ink have been applied at the appropriate locations in the band 24 so as to produce the desired composite colors. Similarly, successive scans of the head 14 adjacent to each of the other bands in the image 13 produces composite colors in those regions. In such color printing systems, certain problems arise. For example, printing of one color in a region immediately adjacent to a region printed with another color before the first printed color has solidified may cause the two colors to mix and produce an unde- sired line of the composite color between the regions. Thus, for example, if magenta ink from the orifices 17 in the head 10 is applied concurrently with or immedi¬ ately after cyan ink from the orifices 16 is applied and before that ink has solidified, the magenta and cyan inks along the line between the bands 22 and 23 in the image 13 may flow together and coalesce to produce an undesired blue line 25 between them. More¬ over, even if the adjacent constituent color inks do not mix to produce an undesired color line in this way, the thermal transfer between the region in which one line is printed to the region in which an adjacent line is to be printed at the same time or immediately thereafter causes the ink in the second line to spread

to a different extent, resulting in image irregular¬ ities.

In many cases it is desirable to have two or more groups of orifices providing the same color ink in an array of orifices in an ink jet head. Thus, for exam¬ ple, the array 11 may be extended by adding two or more sets of orifice groups similar to the orifice groups 15-18. In such cases, it is possible to pro¬ duce a given composite color by printing the constitu- ent colors in either order, i.e _. ., blue may be produced by printing first magenta and then cyan or first cyan and then magenta. Because the second color to be applied usually does not spread as much as the first color applied, however, some of the ink in each drop of the first color to be applied will not be covered completely by a corresponding drop of the second color to be applied, causing the hue of the composite color to be shifted toward that of the first ink color. Thus, the hue of a blue color produced by printing cyan before magenta will be shifted slightly toward the blue, and the hue of a blue produced by printing magenta before cyan will be shifted slightly toward magenta. Consequently, when two adjacent bands of a composite color are produced by printing the constitu- ent colors in the reverse order, a detectable banding may result, as described in the above-mentioned co- pending application.

In accordance with the invention, a bidirectional color hot melt ink jet system is arranged to overprint one or more additional colors at the same region on the substrate before the first printed color has solidified, thereby assuring mixing of the constituent color inks and avoiding differences in the spreading of drops of successively printed colors which cause variations in hue. For this purpose, an ink jet head is provided in which each of the orifices which ap¬ plies a primary color ink to the substrate is aligned in the direction of motion of the ink jet head with

orifices for applying the other primary color inks and closely spaced therefrom so that, during each pass of the ink jet head adjacent to the substrate, drops of all three primary colors may be applied in succession within a limited time period to the same region of the substrate.

Since hot melt ink substantially solidifies on a substrate within about 100 milliseconds after applica¬ tion and usually within about 50 milliseconds, the spacing of the orifices and the rate of motion of the ink jet head are selected to assure that all drops of constituent ink required to produce a composite color are applied to the same region of the substrate in substantially less than 100 milliseconds, desirably less than 50 milliseconds, and, preferably less than about 25 milliseconds. Thus, with a head 10 scanning the substrate 12 at a rate of 100 cm/sec, the spacing between the orifices supplying the first and last drops of ink at the same region of the substrate to form a composite color should be spaced by no more than 10 cm. , desirably no more than 5 cm. , and prefer¬ ably no more than about 2.5 cm. For slower scan rates such as 20 cm/sec, correspondingly smaller maximum orifice spacings such as 2 cm., desirably 1 cm. and, preferably, 0.5 cm. should be provided. At a scan rate of 10 cm/sec. the maximum spacing should be no more than 1 cm. and preferably 0.5 cm.

As previously mentioned, bidirectional ink jet printing may cause variations in density or hue which produce visually detectable color banding. For exam¬ ple, as described above, the hue of colors produced by overprinting a combination of primary colors may vary depending upon the order in which the primary colors are applied to the substrate. Fig. 2 illustrates on a logarithmic scale the effect of variations in contrast between adjacent bands of differing spatial period, with percent contrast represented on the ordinate scale, and variations in the spatial period repre-

sented in inches on the abscissa scale, on the visual detectability of such bands in an ink jet image. The line 26 in Fig. 2 represents the limit of visual de¬ tectability when the banding is a square wave function of intensity and the line 27 in Fig. 2 represents the limit when the banding is a sinusoidal function of intensity. As shown by both the lines 26 and 27, where the contrast between adjacent bands is 10%, banding is detectable for all spatial periods greater than about 0.01 inch (0.25mm), whereas for 1% contrast between adjacent bands, banding is detectable for sinusoidal variations in density only if the spatial period is between about 0.02 and about 0.5 inch (0.5mm and 1.25mm) and for square wave variations of density at all spatial periods above 0.02 inch (0.5mm).

Fig. 3 illustrates the arrangement of orifices in one form of ink jet head arranged to produce both black and multicolor printing in accordance with the invention. In this case, a head 33 has an array 34 consisting of 16 orifices arranged to project black ink and three adjacent arrays 35, 36 and 37 of eight orifices each arranged to project yellow, magenta and cyan inks, respectively, these orifices being aligned with the alternate orifices in the array 34 and have a total spacing, for example of 1 cm. When the head 33 is used for bidirectional color printing, all of the colored ink orifices in the arrays 35, 36 and 37 and the eight black ink jet orifices in the array 34 which are aligned in the scanning direction 38 with the colored ink orifices are used, and the substrate is advanced by an odd multiple of half the distance be¬ tween adjacent color ink orifices between successive passes (80 lines per cm.), for example, so that inter¬ laced lines are provided. Thus, with image line spac- ings of 200 lines per inch (80 lines per cm) for exam¬ ple, any differences in intensity or hue resulting from the reversal of the order of laying down of col¬ ored ink drops has a spatial period which is less than

the lower limit for visual detection of the banding effect as shown in Fig. 2.

Moreover, because each color ink orifice in the arrays 35, 36 and 37 is aligned with other color ink orifices in the direction of motion of the head 33 and those orifices are closely spaced on the head, with a scanning speed of, for example, 20 cm. per second, the successive drops of different color inks applied to the same region of the substrate to produce a compos- ite color image will be applied within about 50 milli¬ seconds, i.e., before the first drop has solidified. Consequently, the inks will be mixed and hue varia¬ tions resulting from application of the inks in re¬ verse order will be avoided. When the head 33 is used to print black images, the entire array of black orifices 34 may be utilized. In this case, the substrate is advanced by a distance equal to the length of the array 34 plus the distance between adjacent orifices in that array between suc- cessive passes of the head, thereby producing images at twice the speed of color printing. If edge ragged- ness results from position errors between passes, only alternate orifices in the array 34 are used, and the ^• lines produced by successive passes of the ink jet head are interlaced in the manner described in the copending Application Serial No. 07/359,736, filed May 31, 1989.

Fig. 4 illustrates another arrangement of ori¬ fices in an ink jet head to provide prompt application of drops of the constituent colors of a composite color in accordance with the invention. In this case, the array of orifices contains three times as many orifices for black ink as for each of the primary colors, yellow, magenta and cyan, thereby permitting images consisting solely of black ink to be printed at three times the speed of images containing color.

In the enlarged schematic illustration of Fig. 4, an ink jet head 40 has twelve black orifices 41-52

arranged in adjacent pairs, and three sets of four orifices each 53-56, 57-60 and 61-64 for each of the primary colors, cyan, magenta and yellow, respec¬ tively, are arranged in adjacent pairs. As indicated by the four parallel lines 65, 66, 67 and 68, which extend parallel to the direction of reciprocating motion of the head, represented by the arrow 69, each of the primary color orifices is aligned with one of each of the other primary color orifices and one black orifice in the direction of motion of the head. Thus, the black orifice 42, the cyan orifice 53, the magenta orifice 57 and the yellow orifice 61 are aligned along the line 65 in the direction of motion of the head 40, and the cyan orifice 54, the magenta orifice 58, the black orifice 45 and the yellow orifice 62 are aligned along the line 66. In addition, two black orifices 43 and 44 are uniformly spaced between the lines 65 and 66 so that, when an image consisting only of black ink is printed and no color mixing is involved, those orifices, along with the black orifices on the lines 65-68 and the other black ink orifices which do not lie on those lines can be used to print images at a rate three times as fast as the color printing rate. With the orifices for projecting the same color ink arranged in adjacent pairs, the advantages described in the Hoisington et al. Patent No. 4,835,554 can be provided.

For color printing, the spacing of the lines 65, 66, 67 and 68 preferably does not correspond to the spacing of adjacent lines in the final image printed on the substrate. Instead, the scanning is arranged so that the spacing of the lines in the final image printed on the substrate is a fraction, such as one- sixth or one-eighth, of the spacing of the lines shown in Fig. 4 by causing the substrate to be advanced by a distance which is smaller than that spacing between successive scans of the printhead in the manner de¬ scribed hereinafter. In addition, although only 24

orifices are shown in the schematic illustration of Fig. 4, the orifice array may be extended to include a larger number of orifices such as 48, 72 or 96 ori¬ fices. For convenience of manufacture, the minimum spac¬ ing in the scanning direction between adjacent ori¬ fices is preferably no less than about 0.5 mm. Thus, the first and last orifices disposed on each of the lines 65, 66, 67 and 68 may be spaced by about 5 mm. so that, with a scanning rate as low as 10 cm/sec, the time between application of the first and last drops of ink at the same location is less than about 50 milliseconds. At higher scanning rates, such as 100 cm/sec, the time between the first and last drop application is correspondingly lower, such as 5 milli¬ seconds.

Consequently, with this arrangement, as with that of Fig. 3, the last drop of a constituent color of a hot melt ink required to produce a composite color is applied at the same region before the first drop has solidified, thereby causing the different colors to be mixed. This prevents hue variations in the composite color which could result from spreading of a first constituent color ink drop to a greater extent than a subsequent constituent color ink drop. Therefore, the hue of the composite color will not be changed as a result of a different order of application of the constituent drops resulting from application of the constituent colors in the opposite order during motion of the printing head in opposite directions in a bidi¬ rectional printing system.

In order to avoid thermal interaction between the ink in one line of an image and the ink in a subse¬ quently printed line which can cause different ink spreading and possible mixing of constituent colors to produce an undesired composite color line, the ink jet printing system of the invention is preferably ar¬ ranged so as to assure that no line of a color image

is printed within two image lines of an image line printed during the previous scan of the head. For this purpose, the lines 65-68 of Fig. 4 containing the primary color ink orifices have a spacing of at least four image lines, such as six image lines, i.e _. ., a spacing which requires five scans of the head 40 to produce the lines intervening between the image lines printed by the orifices on the lines 65-68 during one scan. For color printing with this arrangement, only the orifices located on the lines 65-68 are used and, in the illustrated embodiment, only four lines are printed during each scan. With this arrangement, advancing the substrate by successive sequences of image line increments of 2, 2, 3, 2, 2 and 13 produces a complete color image. If the image line spacing is 150 per inch (6 per mm.), any bidirectional banding effect which might result, for example, from varia¬ tions in the time of flight of drops applied to the substrate during motion of the head in opposite direc¬ tions, is below the visual threshold described above with respect to Fig. 2. If a larger number of ori¬ fices such as 48 or 96 is provided, with a correspond¬ ing increase in the number of color lines printed during each scan, the last substrate advance in the foregoing sequence is correspondingly increased, i.e.., to 37 or 85 lines rather than 13 lines. When no color is required in the image, all of the black orifices are used and, since those orifices are spaced by two image lines in the illustrated embodiment, the sub¬ strate may be advanced in the manner described in the above-mentioned copending application to avoid edge raggedness, for example, one image line after one pass and 23 images lines after the next pass. With image lines spaced at 150 per inch (6 per mm. ) , the average interlace frequency provided by this arrangement is 100 per inch (4 per mm.). As shown in Fig. 2, any banding produced by this arrangement is

not detectable unless the contrast between adjacent bands exceeds about 10%.

If an array having more than 24 orifices is pro¬ vided, such as a 96-orifice array, it is possible to make the successive substrate advance distances more uniform by adding appropriate multiples of 6 to each of the increments in the sequence specified above. The most uniform sequence of substrate advance dis¬ tances in this case would be 14, 20, 15, 14, 14, 19. Since more uniform substrate advance distances allow for greater substrate advance accuracy, this arrange¬ ment may be desirable in many cases. In addition, any density variations which might be periodic at 96 lines with a 96-orifice array are distributed among six different regions of the image, causing the variations to occur at a higher spatial frequency.

If the aligned arrays of orifices on the lines 65-68 were spaced by only two or three image lines rather than six image lines, undesired thermal inter- action might occur as described above. While such thermal interaction may be avoided if the lines of orifices are spaced by four image lines, this arrange¬ ment requires two successive substrate advances of two image lines, followed by a skip to avoid overprinting of the lines printed during the first scan. Moreover, with image lines spaced at 150 per inch (6 per mm.), the interlace frequency at four-image-line spacing of the lines is only 75 per inch (3 per mm.) which could result in detectable banding if the contrast between adjacent bands is 3% as shown in Fig. 2.

On the other hand, a spacing of eight image lines for the lines of the orifices 65-68 provides the same advantages as the above-described arrangement having a spacing of six image lines. In addition, with a sub- strate advance sequence of seven advances of three image lines each followed by an eleven-line advance, a complete image can be formed. With a line spacing of 150 per inch (6 per mm.), the interlace frequency in

the image is 150 per inch (6 per mm.), eliminating any detectable banding regardless of contrast as shown in Fig. 2. If the number of orifices in the array is greater than 24, the sequence of substrate advances should consist of seven three-line advances followed by an advance of, for example, 25 lines for a 48- orifice array printing eight lines per scan or 73 lines for a 96-orifice array printing sixteen lines per scan. Moreover, by adding appropriate multiples of eight to each of the first seven substrate advance increments, more uniform substrate advance distances may be obtained in a manner similar to that described above with respect to the embodiment having a spacing of six image lines. Although the invention has been described herein with reference to specific embodiments, many modifica¬ tions and variations therein will readily occur to those skilled in the art. For example, the black and color orifice arrays 34 and 35-37 in the head 33 shown in Fig. 3 could be oriented at different angles to the direction of motion of the printhead, in a manner similar to that described in connection with Fig. 2 of the Fischbeck Patent No. 4,864,328, but with the dif¬ ferent color orifices in the arrays and some of the black orifices aligned in the direction of motion of the printhead. Accordingly, all such variations and modifications are included within the intended scope of the invention.