Method of improving print performance in flexographic printing plates

Abstract

A method of making a relief image printing element from a photosensitive printing blank is provided. A photosensitive printing blank with a laser ablatable layer disposed on at least one photocurable layer is ablated with a laser to create an in situ mask. The printing blank is then exposed to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer. Diffusion of air into the at least one photocurable layer is limited during the exposing step and preferably at least one of the type, power and incident angle of illumination of the at least one source of actinic radiation is altered during the exposure step. The resulting relief image comprises a plurality of dots and a dot shape of the plurality of dots that provide optimal print performance on various substrates, including corrugated board.

Claims

1. A flexographic relief image printing element comprising at least one photopolymer layer on a backing layer, wherein the at least one photopolymer layer comprises a cured floor layer therein and wherein the cured floor layer establishes an overall depth of plate relief; wherein a plurality of dots in relief are created in the at least one photopolymer layer by selectively exposing the at least one photopolymer layer to actinic radiation to selectively crosslink portions of the at least one photopolymer layer and separating and removing the uncrosslinked portions of the at least one photopolymer layer, and wherein said plurality of dots comprise at least one characteristic selected from the group consisting of: a) a planarity of a top surface of the dot is such that the radius of curvature of the top surface of the dot, r.sub.t, is greater than the total thickness of the at least one photopolymer layer; b) a shoulder angle of the dot is such that the overall shoulder angle of the dot is greater than 50°; and c) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r.sub.e, to the width of the top of the dot, p, is less than 5%; and wherein a dot relief of the flexographic relief image printing element is greater than about 9% of the overall plate relief.

2. The flexographic relief image printing element according to claim 1, wherein shoulder angle of the dot is such that the overall shoulder angle is greater than about 50°.

3. The flexographic relief image printing element according to claim 2, wherein the shoulder angle of the dot is such that overall shoulder angle is greater than about 70°.

4. The flexographic relief image printing element according to claim 1, wherein the ratio of r.sub.e:p is less than 2%.

5. The flexographic relief image printing element according to claim 1, wherein the dot relief of the printing element is greater than about 12% of the overall plate relief.

6. A plurality of relief dots created in a relief image printing element, wherein the relief image printing element comprises, in order, a laser ablatable mask layer, at least one photopolymer layer, and a backing layer, wherein a plurality of dots in relief are created in the at least one photopolymer layer during a digital platemaking process, wherein the at least one photopolymer layer comprises a cured floor layer therein and the cured floor layer establishes the overall depth of plate relief; the digital platemaking process including the steps of (i) laser ablating a laser ablatable mask layer to create an in situ negative; (ii) selectively exposing the at least one photopolymer layer to actinic radiation through the in situ negative to selectively crosslink portions of the at least one photopolymer layer and (iii) separating and removing uncrosslinked portions of the at least one photopolymer layer, and wherein said plurality of relief dots comprise at least one geometric characteristic selected from the group consisting of: (a) a planarity of a top surface of the relief dots, measured as the radius of curvature of the top surface of the dot, r.sub.t, is greater than the total thickness of the photopolymer layer; (b) a shoulder angle of the relief dots is such that (i) the overall shoulder angle is greater than 50°; and (c) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r.sub.e, to the width of the top of the dot, p, is less than 5%; and wherein a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%.

7. The plurality of relief dots according to claim 6, wherein said planarity of the top surface of the relief dots is such that the radius of curvature of the top surface of the dot, r.sub.t, is greater than the total thickness of the photopolymer layer.

8. The plurality of relief dots according to claim 6, wherein said shoulder angle of the relief dots is such that the overall shoulder angle is greater than 50°.

9. The plurality of relief dots according to claim 8, wherein the shoulder angle of the relief dots is such that the overall shoulder angle is greater than about 70°.

10. The plurality of relief dots according to claim 6, wherein the depth of relief between the relief dots is greater than about 12% of the overall plate relief.

11. The plurality of relief dots according to claim 6 wherein the edge sharpness of the relief dots is such that the ratio of r.sub.e:p is less than about 2 percent.

12. The plurality of relief dots according to claim 6, wherein said plurality of relief dots comprise the following geometric characteristics: a) a planar top surface, such that the radius of curvature of the top surface to the dot, r.sub.t, is greater than the total thickness of the photopolymer layer; b) an overall shoulder angle of the relief dots is greater than 50°; c) a depth of relief between dots, measured as a percentage of the overall plate relief, is greater than about 9%; and d) a ratio of r.sub.e:p is less than about 5%.

13. The flexographic relief image printing element according to claim 1, wherein the top surface of each of the plurality of relief dots comprises a defined topographic pattern.

14. The flexographic relief image printing element according to claim 1, wherein an average surface roughness of the relief image printing element is less than about 100 nm.

15. The flexographic relief image printing element according to claim 13, wherein an average surface roughness of the relief image printing element is between about 700 and about 800 nm.

16. A plurality of relief dots created in a relief image printing element, wherein the relief image printing element comprises, in order, a laser ablatable mask layer, at least one photopolymer layer and a backing layer, and wherein the at least one photopolymer layer comprises a cured floor layer therein and the cured floor layer establishes the overall depth of plate relief; and wherein the relief image printing element is created by (i) laser ablating the laser ablatable mask layer to create an in situ negative; (ii) laminating an oxygen barrier membrane to a top of the laser ablatable mask layer; (iii) selectively exposing the at least one photopolymer layer to actinic radiation through the in situ negative and the oxygen barrier membrane to selectively crosslink portions of the at least one photopolymer layer; and (iv) developing the relief image printing element to separate and remove uncrosslinked portions of the at least one photopolymer layer, and wherein said plurality of relief dots comprise at least one geometric characteristic selected from the group consisting of: (a) a planarity of a top surface of the relief dots, measured as the radius of curvature of the top surface of the dot, r.sub.t, is greater than the total thickness of the photopolymer layer; (b) a shoulder angle of the relief dots is such that the overall shoulder angle is greater than 50°; and (c) an edge sharpness of the dots is such that the ratio of the radius of curvature at the intersection of the shoulder and the top surface of the dot, r.sub.e, to the width of the top of the dot, p, is less than 5%; and wherein a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying figures, in which:

(2) FIG. 1 depicts a printing element with a plurality of dots demonstrating the unique dot/shoulder structure of the invention as compared to the dots of a printing element exposed without the benefit of this invention.

(3) FIG. 2 depicts a schematic representation of four dot shape measurements related to the creation of an optimum dot for flexographic printing.

(4) FIG. 3 depicts rounded edges on a 5% flexo dot wherein the entire dot surface is rounded.

(5) FIG. 4 depicts a diagram of increasing contact patch size with impression on a dot with a non-planar top.

(6) FIG. 5 depicts a mathematical representation of the increase in contact patch size of a non-planar dot with increasing impression. The top line depicts the rate of increase without the effect of bulk compression, while the bottom line includes a correction factor for the bulk compression.

(7) FIG. 6 depicts the measurement of the dot shoulder angle θ.

(8) FIG. 7 depicts dot shoulder angles for 20% dots made by different imaging techniques along with their respective dot reliefs.

(9) FIG. 8 depicts a dot with two shoulder angles.

(10) FIG. 9 depicts examples of compound shoulder angle dots created by a method in accordance with the present invention as compared to compound shoulder angle dots created by a direct write imaging process.

(11) FIG. 10 depicts the shoulder angles and relief depth between compound shoulder angle dots.

(12) FIG. 11 depicts relief image definitions.

(13) FIG. 12 depicts a range of dot relief levels with their respective dot shoulder angles.

(14) FIG. 13 depicts rounded dot edges on a 20% dot made by standard digital imaging of a flexo plate.

(15) FIG. 14 depicts well-defined dot edges on 20% dots.

(16) FIG. 15 describes a means of characterizing the planarity of a dot's printing surface where p is the distance across the dot top, and r.sub.t is the radius of curvature across the surface of the dot.

(17) FIG. 16 depicts a flexo dot and its edge, where p is the distance across the dot top. This is used in the characterization of edge sharpness, r.sub.e:p, where r.sub.e is the radius of curvature at the intersection of the shoulder and the top of the dot.

(18) FIG. 17 depicts chord measurement calculations for the top of a flexo dot in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(19) The inventors of the present invention have found that the shape and structure of a printing dot 1 has a profound impact on the way it prints. Knowing this, one can manipulate the resultant shape of the printing dots 1 to optimize printing by utilizing the printing methods described herein. FIG. 1 depicts a printing element with a plurality of dots 1 demonstrating the unique dot/shoulder structure of the invention as compared to the dots of a printing element exposed without the benefit of this invention.

(20) More particularly, the inventors of the present invention have found that a particular set of geometric characteristics define a flexo dot shape that yields superior printing performance, as shown in FIG. 2. The geometric parameters that characterize the optimum flexographic printing dot 1, especially in digital flexo printing, include: (1) planarity of the dot top 2; (2) shoulder angle 4 of the dot; (3) depth of relief 6 between the dots 1; and (4) sharpness of the edge 8 at the point where the dot top 2 transitions to the dot shoulder 4.

(21) However the dot shape shown in FIG. 2 is not necessarily the most optimum dot shape, depending on the substrate being printed, among other factors.

(22) Firstly, the planarity of the dot top 2 was found to be a contributing factor to printing performance. Flexo plates imaged by typical digital imaging processes tend to create dots with rounded tops, as seen, for example, in FIG. 3, in which a 5% dot is shown. This well-known phenomenon is caused by oxygen inhibition of photopolymerization and tends to affect smaller dots more than larger ones as described in more detail above. A planar dot 2 is preferred throughout the tonal range. Most preferred are planar dot top 2, even on dots 1 in the highlight range (i.e., 0-10% tonal). This is illustrated in FIG. 4, which shows a diagram of increasing contact patch size with several impression levels on a printing dot 1 having a non-planar top. Furthermore, FIG. 5 shows a mathematical representation of the increase in contact patch size of non-planar dot with increasing impression. The top line shows the rate of increase without the effect of bulk compression and the bottom line includes a correction factor for bulk compression.

(23) The planarity of the top of a dot can be measured as the radius of curvature across the top surface 2 of the dot 1, r.sub.t, as shown in FIG. 15. Preferably, the top of the dot 2 has a planarity where the radius of curvature of the dot top 2 is greater than the thickness of the photopolymer layer, more preferably twice the thickness of the photopolymer layer, and most preferably more than three times the total thickness of the photopolymer layer.

(24) Thus, it can be seen that the rounded dot surface is not ideal from a printing perspective because the size of the contact patch between the print surface and the dot varies exponentially with impression force. In contrast, a planar dot surface should have the same contact patch size within a reasonable range of impression and is thus preferred, especially for dots in the highlight range (0-10% tone).

(25) A second parameter is the angle of the dot shoulder 4, which was found to be a good predictor of print performance. The dot shoulder 4 is defined as shown in FIG. 6 as the angle θ formed by the dot's top 2 and side 4. At the extreme, a vertical column would have a 90° shoulder angle, but in practice most flexo dots 1 have an angle that is considerably lower, often nearer 45° than 90°.

(26) The shoulder angle can vary depending on the size of the dots as well. Small dots, for example in the 1-15% range, may have large shoulder angles, while larger dots, for example greater than about 15% dots may exhibit smaller shoulder angles. It is desirable for all dots to have the largest shoulder angle possible.

(27) FIG. 7 depicts dot shoulder angles for 20% dots made by different imaging techniques. In flexo plates made by analog imaging processes, dot shoulder angles are often close to 45° as seen in Sample 2 of FIG. 7. Digital imaging processes for flexo plates increase this angle, especially for smaller dots, into the more preferred range of greater than about 50°, but this angle is not conferred on larger dots as seen in Sample 14 of FIG. 7 and comes with the undesirable side effect of rounded dot tops or edges. In contrast, through the use of the imaging technology process described herein, dot shoulder angles of digital flexo plates can be improved to greater than about 50°, even for large dots such as the 20% dot shown in Sample 13 of FIG. 7 which depicts dots that were produced in accordance with the process described herein.

(28) There are two competing geometric constraints on shoulder angle—dot stability and impression sensitivity. A large shoulder angle minimizes impression sensitivity and gives the widest operating window on press, but at the expense of dot stability and durability. In contrast, a lower shoulder angle improves dot stability but makes the dot more sensitive to impression on press. In practice today, most dots are formed in such a way as to have an angle that represents a compromise between these two needs.

(29) An ideal dot would eliminate the need for compromise between these two requirements by separating the sections of the dots which perform the two functions (print impression and dot reinforcement) and giving each dot a shoulder angle that is especially suited for its purpose. Such a dot would have two angles 10 and 12 when viewed from the side as depicted in FIG. 8. The angle 10 closest to the print surface, θ.sub.1, would have a large angle so as to minimize impression sensitivity, while the angle 12 closer to the dot's base, θ.sub.2, would be smaller so as to confer the greatest physical reinforcement of the dot structure and the greatest stability. However, dot shapes of this type are not easily obtained by conventional analog or digital flexographic photopolymers and imaging techniques, because the dot shape is to a large extent determined by the imaging technique used.

(30) Imaging techniques such as the process described herein have been able to create such compound shoulder angle dots as shown in FIG. 9. The two figures on the left of FIG. 9 depict dots produced in the process of the present invention while the figure on the right depicts dots produced in a direct write imaging process. The compound shoulder angle dots of the present invention have very high shoulder angles nearest the dot top (the print surface) but are structurally sound due to the broad base and the much lower shoulder angle near the dot's base, where it attaches to the “floor” of the plate as seen in FIG. 10. This compound shoulder angle dot has been shown not only to print very well at optimum impression levels, but also exhibits extraordinary resistance to print gain at higher impression levels.

(31) A dot shoulder angle of >50° is preferred throughout the tonal range. A dot shoulder angle of >70° or more is preferred. Most preferred is a dot having a “compound angle” shoulder with θ.sub.1 (the angle nearest the dot top) of >70° or more and θ.sub.2 (the angle nearest the dot floor attachment) of 45° or less. As used herein, dot shoulder angle means the angle formed by the intersection of a horizontal line tangential to the top of the dot 2 and a line representing the adjacent dot side wall 4 as shown in FIG. 6. As used herein, θ.sub.1 means the angle formed by the intersection of a horizontal line tangential to the top of the dot and a line representing the portion of the adjacent shoulder wall 10 nearest the top of the dot as shown in FIG. 8. As used herein, θ.sub.2 means the angle formed by a horizontal line and a line representing the sidewall of the dot 12 at a point nearest the base of the dot, as shown in FIG. 8.

(32) A third parameter is plate relief, which is expressed as the distance between the floor of the plate 20 and the top of a solid relief surface 2 as shown in FIG. 11. For example, a 0.125 inch thick plate is typically made so as to have an 0.040 inch relief. However, the plate relief is typically much larger than the relief between dots 6 in tone patches (i.e., the “dot relief”), which is a result of the close spacing of the dots 1 in tonal areas. The low relief between dots 6 in tonal areas means that the dots 1 are structurally well-supported, but can cause problems during printing as ink builds up on the plate and eventually fills in the areas between dots 6, causing dot bridging or dirty print.

(33) The inventors have found that deeper dot relief 6 can reduce this problem significantly, leading to longer print runs with less operator interference, a capability that is often called “cleaner printing.” The dot relief 6 is to a certain extent linked to the dot's shoulder angle, as shown in FIG. 12 which demonstrates dot relief 6 changes with dot shoulder angle. The four samples are taken from plates having a 0.125 inch total thickness and an 0.040 inch thick plate relief. As seen in FIG. 12, dots 1 made by standard analog and digital imaging processes (Samples 2 and 14, respectively) often have dot reliefs 6 that are less than about 10% of the overall plate relief, in contrast, enhanced imaging processes can produce dot reliefs 6 that are greater than about 9% (Sample 13) or more preferably, greater than about 13% of the plate relief (Sample 12).

(34) A fourth characteristic that distinguishes an optimum dot for flexo printing is the presence of a well-defined boundary between the planar dot top and the shoulder. Due to the effect of oxygen inhibition, dots made using standard digital flexo photopolymer imaging processes tend to exhibit rounded dot edges. For dots above about 20%, the center of the dot remains planar, but the edges show a profoundly rounded profile as seen in FIG. 13, which shows rounded dot edges on a 20% dot made by digital imaging of the flexographic plate.

(35) It is generally preferred that the dot edges be sharp and defined, as shown in FIG. 14. These well-defined dot edges better separate the “printing” portion from the “support” portion of the dot, allowing for a more consistent contact area between the dot and the substrate during printing.

(36) Edge sharpness 8 can be defined as the ratio of r.sub.e, the radius of curvature (at the intersection of the shoulder 4 and the top of the dot 2) to p, the width of the dot's top 2 or printing surface, as shown in FIG. 16. For a truly round-tipped dot, it is difficult to define the exact printing surface because there is not really an edge in the commonly understood sense, and the ratio of r.sub.e:p can approach 50%. In contrast, a sharp-edged dot would have a very small value of r.sub.e, and r.sub.e:p would approach zero. In practice, an r.sub.e:p of less than 5% is preferred, with an r.sub.e:p of less than 2% being most preferred. FIG. 16 depicts a flexo dot 1 and its edge 8, where p is the distance across the dot top 2 and demonstrates the characterization of edge sharpness, r.sub.e:p, where r.sub.e is the radius of curvature at the intersection of the shoulder and the top of the dot 2.

(37) Finally, FIG. 17 depicts yet another means of measuring planarity of a flexo dot. (AB) is the diameter of the top of the dot, (EF) is the radius of a circle with chord (AB) and (CD) is the segment height of a circle with radium EF transected by chord (AB). Table 1 depicts data for various dot % at 150 lines per inch (LPI) and Table 2 depicts data for various dot % at 85 lpi.

(38) TABLE-US-00001 TABLE 1 Chord measurement calculations (mils) at 150 lpi EF Dot % AB CD CD/AB 67 1 0.75 0.001049 0.1% 67 2 1 0.001866 0.2% 67 7 2 0.007463 0.4% 67 17 3 0.016793 0.6% 67 45 5 0.046658 0.9% 67 84 7 0.091488 1.3% 67 95 8 0.119510 1.5%

(39) TABLE-US-00002 TABLE 2 Chord measurement calculations (mils) at 85 lpi EF Dot % AB CD CD/AB 250 1 1.33 0.000884 0.1% 250 2 1.88 0.001767 0.1% 250 7 3.51 0.006160 0.2% 250 15 5.14 0.013210 0.3% 250 45 8.91 0.039700 0.4% 250 85 12.42 0.077140 0.6% 250 95 14.09 0.099284 0.7%

(40) Furthermore, in order to reduce print fluting when printing on corrugated board substrates and to produce the preferred dot structure described herein, the inventors of the present invention have found that it is necessary to (1) remove air from the exposure step; and preferably (2) alter the type, power and incident angle of illumination.

(41) The use of these methods together yields a dot shape that is highly resistant to print fluting and shows exceptional impression latitude on press (i.e., resistance to print gain changes when more pressure is applied to the plate during printing).

(42) The inventors herein have discovered that a key factor in beneficially changing the shape of printing dots formed on a printing element for optimal relief printing is removing or limiting diffusion of air into the photocurable layer during exposure to actinic radiation. The inventors have found that diffusion of air into the photocurable layer can be limited by:

(43) (1) laminating a barrier membrane on top of the flexo plate to cover the in situ mask and any uncovered portions of photocurable layer. The membrane can most beneficially be applied after the laser ablation used to create the in situ mask, but before exposure to actinic radiation. The inventors of the present invention have also found that this sheet can be used to impart a defined texture to the print surface of the plate, which is an additional capability and benefit of this method.
(2) coating the in situ mask and any uncovered photopolymer layer with a liquid layer, preferably an oil; wherein the barrier membrane and/or liquid layer have a coefficient of oxygen diffusion of less than 6.9×10.sup.−9 m.sup.2/sec, preferably less than 6.9×10.sup.−10 m.sup.2/sec and most preferably less than 6.9×10.sup.−11 m.sup.2/sec.

(44) Altering the type, power and incident angle of illumination can also be useful in this regard and can be accomplished by multiple methods. For example, altering the type, power and incident angle of illumination can be accomplished by using a collimating grid above the plate during the exposure step. The use of a collimating grid for analog plates is described with respect to analog printing plates in U.S. Pat. No. 6,245,487 to Randall, the subject matter of which is herein incorporated by reference in its entirety. In the alternative, the use of a point light, or other semi-coherent light source can be used. These light sources are capable of altering the spectrum, energy concentration, and incident angle to varying degrees, depending on the light source and exposure unit design. Examples of these point light sources include Olec Corporation's OVAC exposure unit and Cortron Corporation's eXact exposure unit. Finally, a fully coherent (e.g., laser) light source can be used for exposure. Examples of the laser light sources include U.V. laser diodes used in devices such as the Luescher Xpose imager and the Heidelberg Prosetter imager. Other light sources that can alter the type, power and incident angle of illumination can also be used in the practice of the invention.

(45) In another embodiment, the present invention relates generally to a method of making a relief image printing element from a photosensitive printing blank, said photosensitive printing blank comprising a laser ablatable mask layer disposed on at least one photocurable layer, the method comprising the steps of: a) selectively laser ablating the laser ablatable mask layer to create an in situ mask and uncovering portions of the photocurable layer; b) exposing the laser ablated printing blank to at least one source of actinic radiation through the in situ mask to selectively cross link and cure portions of the photocurable layer, wherein the diffusion of air into the at least one photocurable layer is limited during the exposing step by a method selected from at least one of: i) laminating a barrier membrane to the in situ mask and any uncovered portions of the photocurable layer before the exposure step; and ii) coating the in situ mask and any uncovered portions of the photocurable layer with a layer of liquid, preferably an oil, prior to the exposure step.

(46) A wide range of materials can serve as the barrier membrane layer. Three qualities that the inventors have identified in producing effective barrier layers include optical transparency, low thickness and oxygen transport inhibition. Oxygen transport inhibition is measure in terms of a low oxygen diffusion coefficient. As noted, the oxygen diffusion coefficient of the membrane (or the liquid layer) should be less than 6.9×10.sup.−9 m.sup.2/sec., preferably less than 6.9×10.sup.−10 m.sup.2/sec. and most preferably less than 6.9×10.sup.−11 m.sup.2/sec.

(47) Examples of materials which are suitable for use as the barrier membrane layer of the present invention include those materials that are conventionally used as a release layer in flexographic printing elements, such as polyamides, polyvinyl alcohol, hydroxyalkyl cellulose, polyvinyl pyrrolidinone, copolymers of ethylene and vinyl acetate, amphoteric interpolymers, cellulose acetate butyrate, alkyl cellulose, butryal, cyclic rubbers, and combinations of one or more of the foregoing. In addition, films such as polypropylene, polyethylene, polyvinyl chloride, polyester and similar clear films can also serve well as barrier films. In one preferred embodiment, the barrier membrane layer comprises a polypropylene film or a polyethylene terephthalate film. One particularly preferred barrier membrane is a Fuji® Final Proof receiver sheet membrane available from Fuji Films.

(48) The barrier membrane should be as thin as possible, consistent with the structural needs for handling of the film and the film/photopolymer plate combination. Barrier membrane thicknesses between about 1 and 100 microns are preferred, with thickness of between about 1 and about 20 microns being most preferred.

(49) The barrier membrane needs to have a sufficient optical transparency so that the membrane will not detrimentally absorb or deflect the actinic radiation used to expose the photosensitive printing blank. As such it is preferable that the barrier membrane have an optical transparency of at least 50%, most preferably at least 75%.

(50) The barrier membrane needs to be sufficiently impermeable to oxygen diffusion so that it can effectively limit diffusion of oxygen into the photocurable layer during exposure to actinic radiation. The inventors herein have determined that the barrier membrane materials noted above in the thicknesses noted above will substantially limit the diffusion of oxygen into the photocurable layer when used as described herein.

(51) In addition to limiting the diffusion of oxygen into the photocurable layer, the barrier membrane can be used to impart or impress a desired texture to the printing surfaces of the printing element or to control the surface roughness of the printing surfaces of the printing element to a desired level. In one embodiment of the present invention, the barrier membrane comprises a matte finish and the texture of the matte finish may be transferred to the plate surface to provide a desired surface roughness on the surface of the printing plate. For example, in one embodiment, the matte finish provides an average surface roughness that is between about 700 and about 800 nm. In this instance the barrier membrane comprises a polypropylene film with a cured photopolymer layer thereon and the cured photopolymer layer has a defined topographic pattern defined thereon. The texture or roughness of the barrier membrane surface will be impressed into the surface of the photopolymer (photocurable) layer during the lamination step. In general, surface roughness in this regard can be measured using a Veeco Optical Profilometer, model Wyko NT 3300 (Veeco Instruments, Plainville, N.Y.).

(52) In another embodiment of the present invention, the barrier membrane comprises a smooth nanotechnology film with a roughness of less than 100 nm. In this embodiment, the average surface roughness of the printing plate can be controlled to less than about 100 nm.

(53) The barrier layer may be laminated to the surface of the printing plate using pressure and/or heat in a typical lamination process.

(54) In another embodiment, the printing plate may be covered with a layer of liquid, preferably a layer of oil, prior to the exposure step, and the oil may be either clear or tinted. The liquid or oil here serves as another form of a barrier membrane. As with the solid barrier membrane, it is important that the liquid used be optically transparent to the actinic radiation used to expose the photocurable layer. The optical transparency of the liquid layer is preferably at least 50%, most preferably at least 75%. The liquid layer must also be capable of substantially inhibiting the diffusion of oxygen into the photocurable layer with an oxygen coefficient of diffusion as noted above. The liquid must also be viscous enough to remain in place during processing. The inventors herein have determined that a liquid layer from 1 μm to 100 μm in thickness comprising any of the following oils, by way of example and not limitation, will meet the foregoing criteria: paraffinic or naphthenic hydro-carbon oils, silicone oils and vegetable based oils. The liquid should be spread upon the surface of the printing element after the in situ mask is created but before the printing blank is exposed to actinic radiation.

(55) After the photosensitive printing blank is exposed to actinic radiation as described herein, the printing blank is developed to reveal the relief image therein. Development may be accomplished by various methods, including water development, solvent development and thermal development, by way of example and not limitation.

(56) Finally, the relief image printing element is mounted on a printing cylinder of a printing press and printing is commenced.

(57) Thus, it can be seen that the method of making the relief image printing element described herein produces a relief image printing element having a relief pattern comprising relief dots to be printed that are configured for optimal print performance. In addition, through the platemaking process described herein, it is possible to manipulate and, optimize certain geometric characteristics of the relief dots in the resulting relief image.