U.S. patent application number 14/169566 was filed with the patent office on 2015-08-06 for apparatus for forming an image on a flexographic media.
The applicant listed for this patent is Richard R. Bielak. Invention is credited to Richard R. Bielak.
Application Number | 20150217558 14/169566 |
Document ID | / |
Family ID | 52347404 |
Filed Date | 2015-08-06 |
United States Patent
Application |
20150217558 |
Kind Code |
A1 |
Bielak; Richard R. |
August 6, 2015 |
APPARATUS FOR FORMING AN IMAGE ON A FLEXOGRAPHIC MEDIA
Abstract
An apparatus for forming an image on a flexographic plate
includes a laser for exposing a back of the flexographic plate to
form a floor; a computer for providing a screened image; wherein
the computer locates isolated dots on the screened image; wherein
the laser exposes a front of the flexographic plate to form the
image, isolated dots and scaffold dots adjacent to the isolated
dots; and wherein the scaffold dots do not extend to the floor.
Inventors: |
Bielak; Richard R.; (Port
Coquitlam, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bielak; Richard R. |
Port Coquitlam |
|
CA |
|
|
Family ID: |
52347404 |
Appl. No.: |
14/169566 |
Filed: |
January 31, 2014 |
Current U.S.
Class: |
355/18 |
Current CPC
Class: |
B41C 3/00 20130101; G03F
7/2022 20130101; G03F 7/3042 20130101; G03F 7/3035 20130101; B41C
1/1008 20130101 |
International
Class: |
B41C 3/00 20060101
B41C003/00 |
Claims
1. An apparatus for forming an image on a flexographic plate
comprising: a laser for exposing a back of said flexographic plate
to form a floor; a computer for providing a screened image; wherein
said computer locates isolated dots on said screened image; wherein
said laser exposes a front of said flexographic plate to form said
image, isolated dots and scaffold dots adjacent to said isolated
dots; and wherein said scaffold dots do not extend to said
floor.
2. The apparatus of claim 1 wherein a solvent is applied to wash
away unexposed material and said scaffold dots from said
flexographic plate.
3. The apparatus of claim 1 comprising: said laser images an
intermediate film with said screened image; a laminator laminates
said film to top of said flexographic plate; and said laser exposes
said flexographic plate through said film.
4. The apparatus of claim 1 further comprising: a dryer for drying
said flexographic plate to remove said solvent.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] Reference is made to commonly-assigned copending U.S. patent
application Ser. No. ______ (Attorney Docket No. K001696US01NAB),
filed herewith, entitled FORMING AN IMAGE ON A FLEXOGRAPHIC MEDIA,
by Bielak; the disclosure of which is incorporated herein.
FIELD OF THE INVENTION
[0002] The present invention relates to an apparatus for forming an
image on a flexographic plate.
BACKGROUND OF THE INVENTION
[0003] In graphic arts technology, a number of well-established
printing processes utilize image carriers with three-dimensional
(3D) representation of data, the most popular of them being
flexographic printing, which uses flexible relief plates or
sleeves. In a traditional flexographic prepress process with
chemical etching there is no possibility of fine control of relief
properties other than depth of relief.
[0004] Specifically, the shape of the cross-section profile
directly influences the quality of reproduction of small features
such as highlight elements and/or file linework details, process
tolerance to changes in pressure applied by plate and/or sleeve to
substrate and other vital characteristics.
[0005] Flexographic printing uses a flexible relief plate to print
on a wide variety of substrates including paper, cardboard,
plastic, and metal films. The Kodak Flexcel NX plate is one such
relief plate. The process used to produce an image on the plate
usually comprises the following steps: [0006] 1. Exposing the back
of the plate to UV light. [0007] 2. Exposing an intermediate film
to the desired image. [0008] 3. Laminating the film to the top of
the plate. [0009] 4. Exposing the plate through the film using UV
light. [0010] 5. Removing the film. [0011] 6. Using a solvent to
wash away the unexposed plate material. [0012] 7. Applying
additional exposure to harden the plate. [0013] 8. Drying the plate
to remove as much of the solvent as possible.
[0014] The back exposure is used to establish the floor of the
plate. The intensity of the exposure decreases as the illumination
penetrates the plate because of absorption in the plate material.
Once the intensity drops below a threshold value, there is
insufficient cross linking in the polymer comprising the plate and
the remaining under-exposed polymer can be washed away. This is
usually the top 0.5 mm of the plate. To form the relief, the front
of the plate is exposed, through an image layer, with enough
intensity that sufficient cross linking occurs all the way down to
the plate floor.
[0015] For every opening in the image layer, a cone of UV light
with an angle of about 40 degrees from the normal propagates
through the plate forming cone shaped relief dots. A cross-section
of a plate 500 is shown in FIG. 5. The following features are
depicted in the cross-section 500: a solid area 504; an isolated
dot 508; and an array of closely spaced dots created by a halftone
screen 512. The height of the plate relief is shown by numeral 516
and plate floor by numeral 520.
[0016] Isolated dots, such as isolated dot 508, can be problematic.
There may be insufficient exposure to solidly and anchor the dot to
the plate floor 520. Even if the dot forms properly, excess
printing pressure could cause the dot to deform during printing.
The dots in the middle of the halftone array 512 fair better since
they are supported to either side by nearby dots. However, dots at
the edge of the array 512 could suffer from some of the same
problems as the isolated dot 508. Dot deformation can cause a large
objectionable blot to form on the printing substrate. This is
called a scum dot in the industry. Ensuring good dot formation and
eliminating the possibility of scum dot formation is the object of
this invention.
[0017] Large dots can support themselves even in isolation. For the
Flexcel NX plate, a minimum dot size of 4.times.4 pixels is usually
sufficient to ensure proper dot formation in all cases. However, a
large minimum dot in a halftone makes it difficult to print light
grey tones. Bump curves or screening strategies are used to try to
mitigate this problem with mixed success.
[0018] The typical plate relief is 20 mils (0.5 mm). Reducing the
relief, improves the dots ability to stand on its own. The
disadvantage is that over a long print run, dirt can collect in the
wide areas of the floor and if sufficient dirt accumulates then
this dirt will transfer to the substrate.
[0019] A method that has been successfully used in laser ablation
mask plates (LAMS) is to deliberately expose dots 608 that are too
small to properly form in the areas surrounding dots 604 that need
additional support as is shown on a printing plate profile 600 in
FIG. 6. There is some risk the dots may print despite the lower
relief--ink may accumulate on the recessed dots over several print
cycles and then transfer to the next substrate all at once. Debris
accumulation may also be a problem.
[0020] Another method suggests small dots which are interspersed
with large dots. Halftone screen 700 as is shown in FIG. 7. As
halftone dots 704 become sparse, rather than remove a dot
completely a halftone dot is replaced with small printing dot 708.
This is not ideal but often the tonal value of the resulting
halftone screen is less than the sparse array because the remaining
dots have additional support.
SUMMARY OF THE INVENTION
[0021] Briefly, according to one aspect of the present invention an
apparatus for forming an image on a flexographic plate includes a
laser for exposing a back of the flexographic plate to form a
floor; a computer for providing a screened image; wherein the
computer locates isolated dots on the screened image; wherein the
laser exposes a front of the flexographic plate to form the image,
isolated dots and scaffold dots adjacent to the isolated dots; and
wherein the scaffold dots do not extend to the floor.
[0022] These and other objects, features, and advantages of the
present invention will become apparent to those skilled in the art
upon a reading of the following detailed description when taken in
conjunction with the drawings wherein there is shown and described
an illustrative embodiment of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 represents, in diagrammatic form, a prior art digital
front end driving an imaging device;
[0024] FIG. 2 represents, in diagrammatic form, a prior art laser
imaging head situated on the imaging carriage imaging on a plate
mounted on an imaging cylinder;
[0025] FIG. 3 shows a prior art halftone rendered image;
[0026] FIG. 4 shows a prior art rendered image on flexographic
plate;
[0027] FIG. 5 shows a prior art cross-section of an imaged printing
plate;
[0028] FIG. 6 shows prior art engraved dots that are too small to
properly form the areas surrounding larger dots;
[0029] FIG. 7 shows prior art engraved flexographic plate showing
small dots which are interspersed with large dots;
[0030] FIG. 8 shows exposure intensity as a function of distance
into the plate;
[0031] FIG. 9A shows a cross-section of an engraved isolated
printing dot surrounded by non-printing scaffold dots;
[0032] FIG. 9B shows a cross-section of the engraved isolated
printing dot after the surrounded by non-printing scaffold dots
have been washed out by a solvent;
[0033] FIG. 10 shows a two-dimensional top view of how a typical
halftone in the highlights might look like;
[0034] FIG. 11 shows scaffold dot placements required to ensure
that scaffold dots do not become attached to other imaged features
on the plate; and
[0035] FIG. 12 shows another restriction with the placement of
scaffold dots.
DETAILED DESCRIPTION OF THE INVENTION
[0036] In the following detailed description, numerous specific
details are set forth in order to provide a thorough understanding
of the disclosure. However, it will be understood by those skilled
in the art that the teachings of the present disclosure may be
practiced without these specific details. In other instances,
well-known methods, procedures, components and circuits have not
been described in detail so as not to obscure the teachings of the
present disclosure.
[0037] While the present invention is described in connection with
one of the embodiments, it will be understood that it is not
intended to limit the invention to this embodiment. On the
contrary, it is intended to cover alternatives, modifications, and
equivalents as covered by the appended claims.
[0038] FIG. 1 shows an imaging device 108. The imaging device is
driven by a digital front end (DFE) 104, which comprises a computer
or microprocessor, which receives printing jobs in a digital form
from desktop publishing (DTP) systems (not shown), and renders the
digital information for imaging. The rendered information and
imaging device control data are communicated between DFE 104 and
imaging device 108 over interface line 112.
[0039] FIG. 2 shows an imaging system 200. The imaging system 200
includes an imaging carriage 232 an imaging head 220, which
comprises a plurality of lasers and which is mounted, imaging head
220 are controlled by controller 228. The imaging head 220 is
configured to image on a substrate 208, the substrate can be a film
to be attached as a mask to a flexographic plate, or alternatively
a flexographic plate that will be directly imaged by imaging system
200. The substrate 208 is mounted on a rotating cylinder 204 for
exposure. The carriage 232 is adapted to move substantially in
parallel to cylinder 204 guided by an advancement screw 216. The
substrate 208 is imaged by imaging head 220 to form an imaged data
212 on substrate 208.
[0040] FIG. 3 shows a halftone rendered image 300. The rendered
image 300 was prepared by DFE 104, to be further imaged on
substrate 208. FIG. 4 shows rendered image 300 imaged by imaging
head 220 on substrate 208 forming an imaged substrate 400.
[0041] The ideal solution to the problem of small dots with
insufficient support is to raise the floor of the plate surrounding
the dot. Small dots with carefully controlled size and spacing are
used to modify the floor height. The method takes advantage of the
side affect of back exposure that the plate material above the
plate floor is partially exposed. Exposure above a threshold value
causes the plate material to solidify. Exposure is a linear
integrating process; therefore, front exposure 816 can be combined
with back exposure 804 to exceed the threshold 808. The exposure
intensity 820 as a function of distance into the plate is shown in
the FIG. 8.
[0042] The figure shows the ultra violet (UV) radiation intensity
as a function of distance through the plate thickness. This
intensity decays as it penetrates the plate material because of
absorbers added to the polymer mix. The intensity changes according
to Beer's Law:
I=I.sub.0e.sup..alpha.x
[0043] Plate material that is not exposed above a threshold value
is washed away when the plate is processed. The shaded area shows
the thickness that remains and where the plate floor 824 is.
However, the plate material between the plate floor and the front
surface of the plate is partially exposed. The exposure deficit
812, shown in FIG. 8, represents the additional exposure needed to
solidify the remainder of the plate. Note that near the plate
floor, very little additional energy is required to exceed the
threshold value. Therefore a front exposure that does not create a
dot can be used to raise the plate floor.
[0044] The printing plate cross-section 900 shown in FIG. 9A shows
and isolated printing dot 908 surrounded by non-printing scaffold
dots 916. The scaffold dots 916 are used to raise the floor 912 of
the plate surrounding the isolated dot. The cross-section 900
depicts a one-dimensional profile. Printing dots 908 and
non-printing scaffold dots 916 are formed on an imaged film mask
904 prior to UV exposure. FIG. 9B shows a cross-section of the
isolated printing dot 908 with raised floor 912 and plate floor 920
after the scaffold dots 916 have been removed by the solvent.
Alternatively substrate 208 is a printing plate which can be imaged
directly by imaging system 200 without a need to use intermediate
steps of imaging a mask on a film, laminating the film on the plate
and to apply UV exposure steps.
[0045] FIG. 10 shows a two-dimensional top view of how a typical
halftone 1000 in the highlights might look like. The intensity of
the scaffold dot beam decays with distance from the mask 904
because of absorption in the plate (Beer's Law) and because the
beam expands as it propagates through the plate (Inverse Square
Law). The intensity equation is:
I = I 0 - .alpha. x x 2 ##EQU00001##
[0046] The intensity drops rapidly and passes below the threshold
for plate solidification. If the dot is small enough (I.sub.0 is
small enough) then intensity will drop below threshold before
reaching the plate floor. The resulting conical plug (of the
scaffold dots) 916 of solid plate material will be washed away by
the solvent in the plate processing step.
[0047] As seen in the cross-section, the beam continues to the
floor of the plate and beyond. Near the floor, the additional
exposure needed to solidify the polymer is small and the floor is
raised (912). In addition, the beams from adjacent dots begin to
overlap and the added exposure further raises the floor. This
places an additional restriction on the scaffold dots--the density
of the dots cannot exceed a maximum value else the floor will rise
to meet the solidified conical plugs and the plugs will not be
washed away.
[0048] A further restriction on scaffold dot 916 placements is
required to ensure that scaffold dots do not become attached to
other imaged features on the plate. FIG. 11 illustrates the
problem. With the scaffold dot plug 1104 anchored to an adjoining
feature 1108, the dot will not be washed away by the solvent. The
solution is to maintain a minimum distance between scaffold dots
and other features.
[0049] There is one additional restriction with the placement of
scaffold dots 916 that is illustrated in FIG. 12. The figure shows
part of a large plate feature 1208. The front exposure to create
this feature propagates through the plate and is partially
scattered by changes in refractive index as the plate material
forms cross-links This scatter spreads out and raises the floor
near the edge of the feature 1208. The effect is proportional to
the size of the feature and decays with distance from the edge of
the feature. Small features 1204 do not raise the floor
significantly as there is insufficient energy in the refractive
scatter. For reference, the dotted line in the figure shows the
location of the floor had no scatter occurred 1212. FIG. 12 shows
how a scaffold dot 916 could become attached to this raised floor.
Detection of a raised floor and suppression of scaffold dots in the
area of that floor is a further aspect of this invention.
[0050] In summary the main features of this invention are: Use
small dots 916 (scaffold dots) that have insufficient exposure to
solidify all the way to the plate floor. Control the maximum
density scaffold dots to ensure that floor is not raised to meet
the bottom of scaffold dot, maintain spacing of the scaffold dots
far enough away from other features to ensure that they do not
connect and refrain from placing scaffold dots in regions where the
floor could be raised by back scatter.
[0051] While the invention has been described with respect to a
limited number of embodiments, these should not be construed as
limitations on the scope of the invention, but rather as
exemplifications of some of the preferred embodiments. Other
possible variations, modifications, and applications are also
within the scope of the invention. Accordingly, the scope of the
invention should not be limited by what has thus far been
described, but by the appended claims and their legal
equivalents.
PARTS LIST
[0052] 104 digital front end (DFE) [0053] 108 imaging device [0054]
112 interface line [0055] 200 imaging system [0056] 204 rotating
cylinder [0057] 208 substrate [0058] 212 imaged data on substrate
[0059] 216 screw [0060] 220 imaging head [0061] 228 controller
[0062] 232 carriage [0063] 300 rendered halftone image to be imaged
on substrate [0064] 400 rendered image imaged on substrate [0065]
500 flexographic printing plate profile [0066] 504 solid area
[0067] 508 isolated dot [0068] 512 array of closely spaced dots
created by a halftone screen [0069] 516 plate relief [0070] 520
plate floor [0071] 600 flexographic printing plate profile [0072]
604 printing dot [0073] 608 partially printed dots [0074] 700
halftone screen [0075] 704 halftone dots [0076] 708 supporting dots
[0077] 804 back of plate [0078] 808 threshold [0079] 812 exposure
deficit [0080] 816 front of plate [0081] 820 exposure intensity
[0082] 824 plate floor [0083] 900 printing plate cross-section
[0084] 904 exposure mask (TIL) [0085] 908 printing dot [0086] 912
raised floor [0087] 916 scaffold dot [0088] 920 plate floor [0089]
1000 highlights view in a typical halftone [0090] 1104 scaffold dot
plug [0091] 1108 plate adjoining feature [0092] 1204 small plate
features [0093] 1208 large plate feature [0094] 1212 plate floor
had no scatter occurred
* * * * *