Halftone printing method

Distortion, smearing, and/or slurring are reduced in printing halftone images using printing apparatus with cylindrical printing surfaces by forming the image to be printed as a plurality of toned lines substantially perpendicular to the ink transfer nips in the printing apparatus. The invention is particularly suited to printing halftone images on rough or irregular substrates and on substrates, such as the outer side surfaces of truncated conical containers, which are not of uniform conformity with the cylindrical printing surface.

BACKGROUND OF THE INVENTION 
This invention relates to printing methods, and more particularly to 
methods for printing halftone images on substrates having irregular 
surfaces and/or surfaces which are not of uniform conformity with the 
printing surface. This invention also relates to printing halftone images 
with apparatus in which high pressure is required for any reason at any 
ink transfer nip, and apparatus in which overinking is a problem. The 
invention has particular application to printing halftone images around 
truncated conical substrate surfaces such as the outer side surfaces of 
plastic containers. 
Printing presses with cylindrical printing members have been adapted for 
printing on the outer side surfaces of plastic containers (e.g., cups) 
which have the shape of a truncated cone. In one common arrangement the 
already formed container is mounted on a rotatable mandrel and held so 
that the outer side surface of the container is in line contact with the 
cylindrical surface of the printing member. The printing member rotates 
about its longitudinal axis, thereby rotating the container and 
transferring ink from the printing surface to the container at the line 
contact or nip between the surfaces. Because the printing surface is 
cylindrical and the container surface has a truncated conical shape, the 
container surface is not uniformly conformable to the printing surface. 
Typically, the upper portion of the container, which has the larger 
circumference, has a higher linear velocity than the adjacent printing 
surface. The lower portion of the container, which has the smaller 
circumference, has a lower linear velocity than the adjacent printing 
surface. Only at some intermediate portion of the container is the linear 
velocity of the container surface the same as the linear velocity of the 
adjacent printing surface. Accordingly, the container surface is generally 
overfed near the top of the container and underfed near the bottom of the 
container. This causes circumferential elongation of the portion of the 
image near the top of the container and circumferential foreshortening of 
the image near the bottom of the container. Only the intermediate portion 
of the image is printed without distortion. 
Not only are portions of the image distorted as described above, they are 
also frequently smeared or slurred. For example, the overfeeding of the 
top portion of the container surface tends particularly to slur the 
trailing edge of each feature of the image on that portion of the 
container. 
Many printing substrates have localized non-uniformities which interfere 
with image transfer to them. For example, the wall thickness of 
thermoformed or molded plastic containers typically varies considerably. 
To insure good ink transfer to the container surface despite these surface 
variations or irregularities, substantial pressure is required between the 
printing surface and the container. Similar high pressure is required for 
satisfactory ink transfer to many other possible substrate materials with 
irregular surfaces such as corrugated cardboard, high basis weight 
cardboard, wood, nonwoven fabrics, kraft paper, polyethylene coated paper, 
and textured or embossed substrates such as embossed plastic film. 
Wherever such high pressure is required for good ink transfer, increased 
smearing or slurring of the printed image is frequently experienced. 
Depending on the type of printing process involved, high pressure at ink 
transfer nips other than the nip at which the image is finally transferred 
to the substrate may also cause smearing or slurring of the printed image. 
In old or worn presses, high pressure may be required between the inking 
roller and the image cylinder to insure thorough inking of the image 
despite worn bearings, irregular surfaces, etc. If the image or plate 
cylinder is not used as the printing surface, the image must be 
transferred from the plate cylinder to a blanket cylinder which is then 
the printing surface. Again, high pressure may be required between the 
plate cylinder and the blanket cylinder for good image transfer to the 
blanket cylinder despite worn or irregular parts. High pressure at any of 
these ink transfer nips tends to cause slurring of the transferred image 
so that the final printed image is similarly slurred. 
Overinking, which may occur occasionally in any printing operation and 
which is particularly common in old or worn presses, is another frequent 
cause of image smearing or slurring. 
All of the foregoing problems are particularly aggravated in attempting to 
print small image details. Halftone images are made up entirely of small 
image elements and are therefore extremely difficult to print under the 
conditions described above. Image distortion of the kind encountered in 
printing or truncated conical surfaces such as plastic containers makes it 
very difficult to achieve uniform image density vertically on the finished 
container. The halftone image tends to be lighter or less dense than 
desired near the top of the finished container and darker or more dense 
than desired near the bottom of the container. Smearing or slurring of the 
image as a result of any or all of the above factors (i.e., non-uniform 
conformity of the substrate with the printing surface such as is 
experienced with conical containers, high pressure at any ink transfer 
nip, and/or overinking) also interferes with good halftone printing. The 
halftone dots are distorted by the slurring, thereby degrading the image. 
A small amount of distortion of each halftone dot has a large cumulative 
effect on the overall image. Intended levels of shading cannot be 
maintained and contrast may be lost. If the slurring is severe enough, the 
halftone dots may run together with the result that image details are 
completely lost. 
All of the foregoing problems become even more severe in printing 
multicolor halftone images in which several monochromatic halftone images 
must be superimposed in proper registration and with proper density to 
achieve the desired composite result. 
In view of the foregoing, it is an object of this invention to provide 
improved methods for printing halftone images on substrates having 
irregular surfaces and/or surfaces which are not of uniform conformity 
with the printing surface. 
It is a more particular object of this invention to provide improved 
methods for printing halftone images on the outer side surfaces of 
truncated conical thermoformed or molded plastic containers. 
It is another more particular object of this invention to provide improved 
methods for printing halftone images in any application in which high 
pressure is required at any ink or image transfer nip, or in which 
overinking is a frequent problem. 
SUMMARY OF THE INVENTION 
These and other objects of the invention are accomplished in accordance 
with the principles of the invention by forming the image as a plurality 
of parallel toned lines substantially perpendicular to the nip between the 
printing surface and the substrate surface. The master or plate is 
prepared using a line screen (rather than the usual halftone dot screen) 
with the lines oriented substantially parallel to the printing direction. 
The plate then has an image made up of a plurality of toned lines 
substantially parallel to the printing direction and therefore 
substantially perpendicular to all of the ink transfer nips in the 
printing apparatus. The printing apparatus is operated in the conventional 
way to print images which are also made up of toned lines substantially 
parallel to the printing direction and therefore substantially 
perpendicular to the nip between the printing surface and the substrate 
surface. 
Multicolor halftone images are formed by superimposing monochromatic 
partial images, each of which is formed as a plurality of parallel toned 
lines having a unique angular orientation as nearly perpendicular to the 
nip between the printing and substrate surfaces as is consistent with 
preventing moire in the printed image. Preferably, the lines forming the 
monochromatic partial image of the most important color are substantially 
perpendicular to the nip between the printing and substrate surfaces, and 
the lines forming other partial images deviate from perpendicular in 
inverse relation to their importance to the appearance of the final image. 
Halftone images printed in accordance with the principles of this invention 
on such surfaces as truncated conical containers are less degraded by the 
lack of conformity of the printing and substrate surfaces than 
conventional halftone dot images. Most of the slurring occurs along the 
toned lines and therefore has much less effect on the appearance of the 
image. The method of this invention also reduces the effect of smearing or 
slurring due to overinking or high pressure at any ink transfer nip. 
Again, most of the smearing or slurring occurs along the toned lines and 
therefore has less effect on the appearance of the image. 
Further features of the invention, its nature and various advantages will 
be more apparent from the accompanying drawing and the following detailed 
description of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIGS. 1 and 2, a typical arrangement for printing on the outer 
side surface 12 of truncated conical container 10 includes inking roller 
20, plate cylinder 30, and blanket cylinder 40. Each of elements 20, 30, 
and 40 rotates about its central axis in the direction indicated by the 
associated arrow. The central axes of all of these elements are parallel, 
and all have the same surface velocity. Container 10 is mounted on a 
mandrel (not shown) having a central axis of rotation which intersects the 
axis of rotation of blanket cylinder 40. Container 10 is typically a 
plastic material which has been formed by any conventional method. For 
example, container 10 may have been thermoformed by any of several 
processes such as vacuum forming, pressure forming, plug assist forming, 
matched tool forming or the like. Alternatively, container 10 may have 
been molded by such processes as blow molding or injection molding. 
Container 10 may also have been formed by a hybrid of the above processes 
such as in a Hayssen monaformer. 
Plate cylinder 30 has a master or plate 32 mounted on the periphery 
thereof. (The thickness of plate 32 is greatly exaggerated for purposes of 
illustration in FIGS. 1 and 2.) The image on plate 32 is inked by contact 
with inking roller 20. Inking roller 20 is inked in turn from an ink 
supply. In the simplified apparatus of FIGS. 1 and 2 inking roller 20 is 
inked from ink supply 16 maintained between a portion of the surface of 
roller 20 and doctor blade 18, although in actual practice inking roller 
20 is typically inked by a more sophisticated arrangement (e.g., an ink 
train including a plurality of rollers for forming a uniform film of ink 
or inking roller 20). The inked image on plate 32 is transferred by 
contact to one of blankets 42 on the periphery of blanket cylinder 40. 
(Again, the thickness of blankets 42 is greatly exaggerated in FIGS. 1 and 
2.) Finally, the image on one of blankets 42 is transferred by contact to 
the side surface 12 of container 10. When a complete image has been 
printed on container 10 (i.e., when one of blankets 42 has rotated past 
container 10 and container 10 has accordingly been driven through 
approximately one revolution), container 10 is moved away from contact 
with blanket cylinder 40 and another container is moved into its place in 
time to receive an image from the next successive blanket 42. While 
container 10 is in contact with blanket cylinder 40, it is driven about 
its axis by contact with cylinder 40. 
As is apparent from the foregoing, ink is transferred from inking roller 20 
to plate 32 at the nip between inking roller 20 and plate cylinder 30. 
Similarly, an inked image is transferred from plate 32 to successive 
blankets 42 at the nip between plate cylinder 30 and blanket cylinder 40. 
An inked image is also transferred from one of blankets 42 to container 
surface 12 at the nip between blanket cylinder 40 and container 10. 
Although a particular printing arrangement is shown for illustrative 
purposes in FIGS. 1 and 2, it will be understood that any other printing 
apparatus can be used in which an image is transferred from a printing 
surface to a substrate surface at a line contact (nip) between the 
surfaces. For example, blanket roller 40 could be omitted and the image 
printed directly on container 10 from plate cylinder 30. In that case, 
plate 32 would be the printing surface. 
Because container 10 has the shape of a truncated cone, the circumference 
of container 10 is less near the bottom 14 of the container than near the 
top of the container. Accordingly, the top portion of container surface 12 
has greater linear velocity than the bottom portion of that surface. This 
generally means that as blanket cylinder 40 drives container 10, the top 
portion of container surface 12 moves somewhat faster than the adjacent 
portion of blanket 42, the bottom portion of container surface 12 moves 
somewhat slower than the adjacent portion of blanket 42, and only an 
intermediate portion of container surface 12 moves at the same speed as 
the adjacent portion of blanket 42. This means that only the intermediate 
portion of the image will be printed on the container without distortion, 
slurring, or smearing. The distortion, slurring, or smearing of the 
remainder of the image can seriously degrade the appearance of the printed 
image, particularly a halftone dot image wherein the distortion, slurring, 
and/or smearing of each dot has a large cumulative effect on the overall 
appearance of the image. 
In accordance with the principles of this invention, halftone images are 
printed by means of toned line images rather than toned dot images, the 
toned lines being oriented substantially perpendicular to the nip between 
the blanket cylinder (or other printing surface) and the container surface 
(or other substrate surface) to greatly reduce the deleterious effects of 
the distortion, slurring, and smearing described above. This is 
accomplished in the printing arrangement shown in FIGS. 1 and 2 by forming 
a halftone image on plate 32 comprised of a plurality of parallel toned 
lines substantially perpendicular to the ink transfer nip between plate 
cylinder 30 and blanket cylinder 40. Because plate 32 is wrapped around 
the cylindrical surface of plate cylinder 30, it will be understood that 
the toned lines on plate 32 are said to be "perpendicular" or 
"substantially perpendicular" to the ink transfer nip between cylinders 30 
and 40 with reference to a line perpendicular to this ink transfer nip 
which has been wrapped around the cylindrical surface of plate cylinder 
30. This "wrapped" perpendicular line lies in a plane perpendicular to the 
ink transfer nip. "Perpendicular" and "substantially perpendicular" have 
the same meaning when applied to toned lines on any other cylindrical 
surface. In FIG. 2, plate 32 is shaped with lines perpendicular to the nip 
between cylinders 30 and 40, although the scale of FIG. 2 is too small to 
illustrate how these lines form an image. 
The parallel toned lines forming the image on plate 32 need not be exactly 
perpendicular to the nip between cylinders 30 and 40, but may deviate 
somewhat from perpendicular. Preferably, the angle between the toned lines 
and a perpendicular is no more than 30.degree., and more preferably no 
more than 15.degree.. These angles are measured on the cylindrical surface 
of plate cylinder 30 (or any other pertinent cylindrical surface). 
The toned lines are actually formed on plate 32 by any conventional 
technique. For example, if plate 32 is made photographically, the 
necessary toned line image can be made by using a line screen having the 
desired orientation of screen lines, rather than the usual dot screen. In 
all other respects, the photographic process of making plate 32 may be the 
same as when a dot screen is used. Suitable photographic techniques for 
making plate 32 are described in "The Contact Screen Story by Du Pont", E. 
I. Du Pont De Nemours & Company (Inc.), Photo Products Department, 
Wilmington, Delaware 19898, Publication A-80172, March 1972. Suitable 
straight line screens are described on page 41 of this publication, and an 
enlarged straight line screen is illustrated on that page. Typically, the 
toned line image has from about 55 to about 150 toned lines per inch 
measured perpendicular to the toned lines. 
Plate 32 is repeatedly inked in the usual manner and the inked image is 
transferred by contact to successive blankets 42 on blanket cylinder 40. 
Like the original plate image, the inked images on blankets 42 are 
comprised of parallel toned lines substantially perpendicular to the nip 
between plate cylinder 30 and blanket cylinder 40, and therefore also 
substantially perpendicular to the nip between blanket cylinder 40 and 
container surface 12. 
Finally, the inked image on one of blankets 42 is transferred by contact to 
container surface 12. Because the printed image is made up of lines 
substantially perpendicular to the nip between blanket cylinder 40 and 
container surface 12, substantially all of the distortion, slurring, 
and/or smearing which occurs is along the toned lines and therefore has 
much less effect on the appearance of the printed image than in comparable 
halftone dot images. 
FIGS. 3a-d illustrate in a very general and simplified manner why 
beneficial results are achieved in accordance with the principles of this 
invention. FIG. 3a shows a single row of greatly enlarged halftone dots to 
be printed on container surface 12 perpendicular to the ink transfer nips 
in the printing apparatus. It is assumed that this row of halftone dots is 
to be printed on a portion of container surface 12 in which smearing or 
slurring of the image is likely to occur. FIG. 3b shows how the row of 
halftone dots of FIG. 3a is actually printed on container surface 12. 
Instead of the nearly circular dots shown in FIG. 3a, each dot in FIG. 3b 
is slightly smeared or slurred, mostly in the direction of printing (i.e., 
perpendicular to the ink transfer nips). The slight smearing or slurring 
of each dot has a relatively large cumulative effect on the printed image. 
For example, if 10% is added to the area of each dot as a result of 
smearing or slurring, the printed image will be approximately 10% denser 
or darker than intended. Larger amounts of smearing or slurring have an 
even greater effect on the appearance of the printed image. 
FIG. 3c shows a single toned line to be printed on container surface 12 
perpendicular to the ink or image transfer nips under the same conditions 
as in FIGS. 3a and 3b. FIG. 3d shows how the toned line of FIG. 3c is 
actually printed on container surface 12. Again, the printed image is 
somewhat smeared or slurred in the direction of printing. However, only a 
relatively small area is added to the line as a result of this smearing or 
slurring. Much of the smeared or slurred ink remains within the intended 
area of the line and only a small amount is smeared beyond the intended 
end of the line. Accordingly, much less than 10% is added to the area of 
the line and the effect on the printed image is much less severe than with 
toned dots printed under similar conditions. 
Although the smearing or slurring described above is the result of the 
non-uniform conformity of the printing and substrate surfaces (i.e., the 
use of a cylindrical blanket to print on a truncated conical container), 
use of the method of this invention also reduces the effects on printed 
halftone images of smearing or slurring due to other factors such as 
overinking and/or high pressure at any of the ink transfer nips. 
Overinking causes conventional halftone dots to spread out in all 
directions, but especially in the direction of printing. If the overinking 
is substantial, the dots may spread out so that they meet and begin to 
fill in the intermediate areas. The result is loss of detail, tone, and 
contrast. Overinking may occur accidentally in any printing operation and 
is a frequent problem in old or worn presses in which the inking apparatus 
is difficult to adjust and control. 
High pressure at one or more ink transfer nips (e.g., the ink transfer nip 
between inking roller 20 and plate cylinder 30 in the apparatus of FIGS. 1 
and 2, or the image transfer nips between plate cylinder 30 and blanket 
cylinder 40 and between blanket cylinder 40 and container surface 12) 
affects conventional halftone images in much the same way that overinking 
does. As in the case of overinking, high pressure ink transfer causes the 
halftone dots to spread out, thereby altering the intended image density 
and possibly causing loss of detail, tone, and contrast. High pressure is 
typically required for good image transfer to irregular substrate surfaces 
such as the walls of thermoformed or molded plastic containers, corrugated 
cardboard, high basis weight cardboard, wood, non-woven fabrics, kraft 
paper, polyethylene coated paper, and textured or embossed substates such 
as embossed plastic film. As used herein, the term "high pressure" in this 
context means a "squeeze" of 0.006 inch or more (i.e., maximum total 
deformation of opposing surfaces at the ink or image transfer nip of 0.006 
inch or more), and especially a squeeze of 0.006 to 0.060 inch. Ordinary 
printing on regular substrate surfaces such as printing quality papers 
does not normally require such high pressures. A squeeze of 0.004 inch or 
less is generally sufficient for good printing on the usual grades of 
paper. However, high pressure as that term is defined above may even be 
required for satisfactory image transfer to ordinary paper if the press is 
old or worn. High pressure may also be required at ink transfer nips other 
than the final image transfer nip in old or worn presses. 
Use of a toned line image having lines substantially perpendicular to the 
ink transfer nips in accordance with the method of this invention (instead 
of a conventional halftone dot image) substantially reduces the 
deleterious effects of overinking and/or high pressure ink transfer. Most 
of the smearing of ink occurs in the direction of printing, i.e., along 
the toned lines, and therefore has relatively little effect on the 
appearance of the printed image. Also, for a given density, the lateral 
spacing between the boundaries of adjacent toned lines is somewhat greater 
than the lateral spacing between the boundaries of adjacent rows of toned 
dots. Accordingly, lateral spreading of ink due to overinking and/or high 
pressure is less likely to cause adjacent toned lines to run together than 
adjacent rows of toned dots. 
The principles of this invention are also applicable to printing multicolor 
halftone images made up of two or more partial monochromatic images. If 
the partial images do not overlap, each partial image is printed in the 
same way that a single monochromatic image is printed, i.e., the parallel 
toned lines forming each partial image are oriented substantially 
perpendicular to the ink transfer nips in the printing apparatus. If the 
partial images overlap to provide mixtures of the colors of the partial 
images, the toned lines of each partial image must have a unique angular 
orientation sufficiently different from the angular orientation of the 
lines of all other partial images to prevent moire and achieve uniform 
blending of the colors in the printed image. In general, the angular 
difference between the lines of each partial image must be at least 
30.degree. to prevent moire (although for colors with low visible contrast 
to the background, such as yellow on a white background, the angular 
difference may be substantially less than 30.degree. (e.g., 15.degree.) 
because the prevention of moire is less critical for such colors). On the 
other hand, the lines of all partial images are preferably as nearly 
perpendicular to the ink transfer nips in the printing apparatus as is 
consistent with preventing moire to reduce the adverse effects of smearing 
and slurring described above. Thus, the lines of all the partial images 
preferably deviate from perpendicular to the ink transfer nips by no more 
than 45.degree., more preferably by no more than 30.degree.. 
FIG. 4 illustrates apparatus for printing a three-color image on cut 
substrate sheets. Each of plate cylinders 130, 230, and 330 is provided 
with a plate for a respective one of three monochromatic partial images. 
Each of these plates is inked with ink of the appropriate color by a 
separate inking roller 120, 220, 320, each supplied with ink from an 
associated ink supply 116, 216, 316. The inked partial images are 
transferred in proper registration from plate cylinders 130, 230, and 330 
to form a composite image on blanket cylinder 140. This composite image is 
then printed on cut substrate sheet 110 which passes between blanket 
cylinder 140 and pressure cylinder 112 at the appropriate time to receive 
the image. 
Although in the apparatus shown in FIG. 4, the monochromatic partial images 
are superimposed on blanket cylinder 140 and the resulting composite 
transferred to substrate 110, it will be understood that three separate 
printing surfaces could be used to successively print the three partial 
images in proper registration on the substrate. Similarly, although the 
apparatus of FIG. 4 is capable of printing a three-color image, plate 
cylinders can be added or deleted to print images having more or less than 
three colors. The apparatus shown in FIG. 4 can alternatively be used to 
print multicolor halftone images on truncated conical containers by 
holding the container against blanket cylinder 140 as in the apparatus of 
FIGS. 1 and 2. 
Assuming that the multicolor image to be printed by apparatus of the type 
shown in FIG. 4 is one in which the monochromatic partial images are at 
least partially overlapping to provide mixtures of colors, FIG. 5 
illustrates how the lines forming each of the three partial images may be 
oriented in accordance with the principles of this invention to prevent 
moire in the printed image and achieve uniform blending of colors, while 
at the same time reducing the effects of slurring and smearing described 
above. In FIG. 5, line 50 is perpendicular to the ink transfer nips in the 
printing apparatus. The toned lines forming one monochromatic partial 
image are oriented parallel to line 50. The toned lines forming a second 
partial image are oriented parallel to line 52 which deviates from 
perpendicular line 50 by an angle A. Angle A is preferably in the range 
from 30.degree. to 45.degree., more preferably about 30.degree.. The toned 
lines forming the third partial image are oriented parallel to line 54 
which deviates from perpendicular line 50 (in the opposite angular 
direction from line 52) by an angle B. Like angle A, angle B is preferably 
in the range from 30.degree. to 45.degree., more preferably about 
30.degree.. 
FIG. 6 shows how the toned lines of four monochromatic partial images may 
be oriented in accordance with the invention. As in FIG. 5, the line 50 is 
perpendicular to the ink transfer nips. Line 62 deviates from 
perpendicular line 50 by an angle C. Angle C is preferably about 
15.degree.. The toned lines of a first partial image are oriented parallel 
to line 62. Line 64 deviates further from perpendicular line 50, forming 
an angle D with line 62. Angle D is preferably about 30.degree.. The sum 
of angles C and D is preferably no more than 45.degree.. The toned lines 
of a second partial image are oriented parallel to line 64. Lines 66 and 
68 deviate from perpendicular line 50 by angles which are preferably equal 
to but opposite from the angles of deviation of lines 62 and 64, 
respectively. The toned lines of third and fourth partial images are 
oriented parallel to lines 66 and 68, respectively. 
The foregoing examples are illustrative only, and it will be understood 
that any orientations within the ranges set forth above may be chosen for 
the toned lines of the monochromatic partial images forming a multicolor 
printed image. If one or more partial images are more important to the 
printed image than other partial images (e.g., if the appearance of the 
printed image is more seriously affected by smearing or slurring of one or 
more partial images than by smearing or slurring of other partial images), 
the partial images are preferably oriented so that the deviation from 
perpendicular is inversely related to the importance of the partial image. 
In the arrangement illustrated by FIG. 5, for example, the most important 
partial image would be formed by lines parallel to line 50, and the less 
important partial images would be formed by lines parallel to lines 52 and 
54. Similarly, in the arrangement shown in FIG. 6, the more important 
partial images would be formed by lines parallel to lines 62 and 66, and 
the less important partial images would be formed by lines parallel to 
lines 64 and 68. 
It will be understood that the foregoing is illustrative of the principles 
of this invention only, and that various modifications may be made by 
those skilled in the art without departing from the scope and spirit of 
the invention. For example, the method of the invention may be carried out 
on various types of printing devices as discussed above.