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 photo curable layer. Diffusion of oxygen 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 dots and a dot shape that provide optimal print performance on various substrates, including corrugated board.

Claims

1. A flexographic relief image printing element comprising a plurality of dots in relief, and wherein at least a portion of 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 photopolymer layer; b) a shoulder angle of the dot is such that either (i) the overall shoulder angle of the dot is greater than 50 or (ii) .sub.1 is greater than 70 and .sub.2 is less than 45; and c) an edge sharpness of the dots is such that the ratio of r.sub.e:p is less than 5%.

2. The flexographic relief image printing element according to claim 1, wherein shoulder angle of at least a portion of the dots 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 at least a portion of the dots is such that overall shoulder angle is greater than about 70.

4. The flexographic relief image printing element according to claim 1, wherein the shoulder angle of at least a portion of the dots is such that .sub.1 is greater than 70 and .sub.2 is less than 45.

5. The flexographic relief image printing element according to claim 1, wherein the ratio of r.sub.e:p is less than 2% for at least a portion of the dots.

6. The flexographic relief image printing element according to claim 1 wherein a dot relief of the printing element is greater than about 9% of the overall plate relief.

7. The flexographic relief image printing element according to claim 6, wherein the dot relief of the printing element is greater than about 12% of the overall plate relief.

8. Relief dots created in a relief image printing element and forming a relief pattern, wherein said relief dots are created during a digital platemaking process, and wherein said 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, or (ii) .sub.1 is greater than 70 and .sub.2 is less than 45; (c) a depth of relief between the relief dots, measured as a percentage of the overall plate relief, is greater than about 9%; and (d) an edge sharpness of the relief dots, is such that the ratio of r.sub.e:p is less than 5%.

9. The relief dots according to claim 8, 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.i, is greater than the total thickness of the photopolymer layer.

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

11. The relief dots according to claim 10, wherein the shoulder angle of the relief dots is such that the overall shoulder angle is greater than about 70.

12. The relief dots according to claim 8, wherein the shoulder angle of the relief dots is such that .sub.1 is greater than 70 and .sub.2 is less than 45.

13. The relief dots according to claim 8, wherein the depth of relief between the relief dots is greater than about 12% of the overall plate relief.

14. The relief dots according to claim 8 wherein the edge sharpness of the relief dots is such that the ratio of r.sub.e:p is less than about 2 percent.

15. The relief dots according to claim 8, wherein said 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%.

16. The relief dots according to claim 8, wherein said 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) a shoulder angle of the relief dots is such that .sub.1 is greater than 70 and .sub.2 is less than 45; 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%.

17. A method of using a photocurable printing element comprising a laser ablatable mask layer disposed on at least one photocurable layer to manufacture a flexographic printing plate comprising a plurality of printing dots, 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) selectively applying a diffusion barrier over at least portions of the uncovered photocurable layer; (c) exposing the laser ablated printing blank to at least one source of actinic radiation through the in situ mask and the diffusion barrier to selectively cross link and cure portions of the photocurable layer, wherein the diffusion of oxygen into the at least portions of the photocurable layer is limited by the diffusion barrier; and wherein at least a portion of said plurality of dots comprise at least one characteristic selected from the group consisting of: i) 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 photopolymer layer; ii) a shoulder angle of the dot is such that either (i) the overall shoulder angle of the dot is greater than 50 or (ii) .sub.1 is greater than 70 and .sub.2 is less than 45; and iii) an edge sharpness of the dots is such that the ratio of r.sub.e:p is less than 5%.

18. The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.910.sup.9 m.sup.2/sec.

19. The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.910.sup.10 m.sup.2/sec.

20. The method according to claim 17 wherein the oxygen diffusion coefficient of the diffusion barrier is less than 6.910.sup.11 m.sup.2/sec.

21. The method according to claim 17 wherein the diffusion barrier is applied as a liquid and is selectively applied to portions of the printing element and then dried to a solid or semi-solid before exposure of the printing element to actinic radiation.

22. The method according to claim 21 wherein the liquid is applied by ink jet.

23. The method according to claim 17, wherein shoulder angle of at least a portion of the dots is such that the overall shoulder angle is greater than about 50.

24. The method according to claim 17, wherein the shoulder angle of at least a portion of the dots is such that overall shoulder angle is greater than about 70.

25. The method according to claim 17, wherein the shoulder angle of at least a portion of the dots is such that .sub.1 is greater than 70 and .sub.2 is less than 45.

26. The method according to claim 17, wherein the ratio of r.sub.e:p is less than 2% for at least a portion of the dots.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying figures, in which:

[0051] FIG. 1 depicts a printing element with a plurality of dots demonstrating the unique dot/shoulder structure achieved application of the diffusion barrier as compared to the dots of a printing element achieved without application of the diffusion barrier.

[0052] FIG. 2 depicts a schematic representation of four dot shape measurements related to the creation of an optimum dot for flexographic printing.

[0053] FIG. 3 depicts rounded edges on a 5% flexo dot wherein the entire dot surface is rounded.

[0054] FIG. 4 depicts a diagram of increasing contact patch size with impression on a dot with a non-planar top.

[0055] 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.

[0056] FIG. 6 depicts the measurement of the dot shoulder angle .

[0057] FIG. 7 depicts dot shoulder angles for 20% dots made by different imaging techniques along with their respective dot reliefs.

[0058] FIG. 8 depicts a dot with two shoulder angles.

[0059] FIG. 9 depicts examples of compound shoulder angle dots created by applying the diffusion barrier as compared to compound shoulder angle dots created by a direct write imaging process without application of the diffusion barrier.

[0060] FIG. 10 depicts the shoulder angles and relief depth between compound shoulder angle dots.

[0061] FIG. 11 depicts relief image definitions.

[0062] FIG. 12 depicts a range of dot relief levels with their respective dot shoulder angles.

[0063] FIG. 13 depicts rounded dot edges on a 20% dot made by standard digital imaging of a flexo plate.

[0064] FIG. 14 depicts well-defined dot edges on 20% dots.

[0065] 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.

[0066] 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.

[0067] FIG. 17 depicts chord measurement calculations for the top of a flexo dot in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0068] The inventors of the present invention have found that the shape and structure of a printing dot has a profound impact on the way it prints. Knowing this, one can manipulate the resultant shape of the printing dots to optimize printing by utilizing the printing methods described herein. This invention allows for selective manipulation of portions of the dots on a printing plate. 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 application of a diffusion barrier.

[0069] 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 in certain applications, as shown in FIG. 2. The geometric parameters that characterize the optimum flexographic printing dot, especially in digital flexo printing, include:

[0070] (1) planarity of the dot surface;

[0071] (2) shoulder angle of the dot;

[0072] (3) depth of relief between the dots; and

[0073] (4) sharpness of the edge at the point where the dot top transitions to the dot shoulder.

[0074] 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. Thus it is important to allow the flexibility of selectively manipulating portions of the dots on a printing plate and either leaving portions of the dots unmanipulated regarding oxygen diffusivity, or manipulated in a different manner or to a different extent as compared to other portions of dots on the same printing plate.

[0075] Firstly, the planarity of the dot surface 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 surface is preferred throughout the tonal range. Most preferred are planar dot surfaces, even on dots 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 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. However, in some cases it is preferred to have round top dots, or dots of varied flatness in other portions of the print field on a printing plate.

[0076] The planarity of the top of a dot can be measured as the radius of curvature across the top surface of the dot, r,, as shown in FIG. 15. Preferably, the top of the dot has a planarity where the radius of curvature of the dot top 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.

[0077] 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).

[0078] A second parameter is the angle of the dot shoulder, which was found to be a good predictor of print performance. The dot shoulder is defined as shown in FIG. 6 as the angle formed by the dot's top and side. At the extreme, a vertical column would have a 90 shoulder angle, but in practice most flexo dots have an angle that is considerably lower, often nearer 45 than 90.

[0079] 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 dots to have the largest shoulder angle possible in most instances.

[0080] 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.

[0081] There are two competing geometric constraints on shoulder angledot 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. Most dots should be formed in such a way as to have an angle that represents a compromise between these two needs.

[0082] 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 when viewed from the side as depicted in FIG. 8. The angle closest to the print surface, .sub.1, would have a large angle so as to minimize impression sensitivity, while the angle closer to the dot's base, .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.

[0083] 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.

[0084] 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 .sub.1 (the angle nearest the dot top) of >70 or more and .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 and a line representing the adjacent dot side wall as shown in FIG. 6. As used herein, .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 nearest the top of the dot as shown in FIG. 8. As used herein, .sub.2 means the angle formed by a horizontal line and a line representing the sidewall of the dot at a point nearest the base of the dot, as shown in FIG. 8.

[0085] A third parameter is plate relief, which is expressed as the distance between the floor of the plate and the top of a solid relief surface 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 in tone patches (i.e., the dot relief), which is a result of the close spacing of the dots in tonal areas. The low relief between dots in tonal areas means that the dots 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, causing dot bridging or dirty print.

[0086] The inventors have found that deeper dot relief 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 is to a certain extent linked to the dot's shoulder angle, as shown in FIG. 12 which demonstrates dot relief 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 made by standard analog and digital imaging processes (Samples 2 and 14, respectively) often have dot reliefs that are less than about 10% of the overall plate relief. In contrast, enhanced imaging processes can produce dot reliefs that are greater than about 9% (Sample 13) or more preferably, greater than about 13% of the plate relief (Sample 12).

[0087] 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.

[0088] 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.

[0089] Edge sharpness can be defined as the ratio of r.sub.e, the radius of curvature (at the intersection of the shoulder and the top of the dot) to p, the width of the dot's top 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 and its edge, where p is the distance across the dot top 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.

[0090] 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 1pi.

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%

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%

[0091] 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.

[0092] 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).

[0093] 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: [0094] (1) applying, preferably selectively, a barrier membrane on top of the flexo plate to cover at least portions of the uncovered photo curable layer. The membrane can most beneficially be applied, preferably selectively to selected portions of the printing element, 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 barrier membrane 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. [0095] (2) coating, preferably selectively, at least portions of the uncovered photopolymer layer with a liquid layer, and then preferably drying the coated liquid layer to a solid or semi-solid;

[0096] wherein the barrier membrane and/or liquid layer have a coefficient of oxygen diffusion of less than 6.910.sup.9 m.sup.2/sec, preferably less than 6.910.sup.10 m.sup.2/sec and most preferably less than 6.910.sup.11 m.sup.2/sec.

[0097] 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.

[0098] 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:

[0099] a) selectively laser ablating the laser ablatable mask layer to create an in situ mask and uncovering portions of the photocurable layer;

[0100] 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 a portion of the photocurable layer is limited during the exposing step by a method selected from at least one of: [0101] i) applying, preferably selectively, a barrier membrane onto at least portions of the photocurable layer before the exposure step; and [0102] ii) coating at least portions of the photocurable layer with a layer of liquid, and preferably drying the coated liquid layer to a solid or semi-solid, prior to the exposure step.

[0103] 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 in the condition it exists prior to exposure) should be less than 6.910.sup.9 m.sup.2/sec., preferably less than 6.910.sup.10 m.sup.2/sec. and most preferably less than 6.910.sup.11 m.sup.2/sec.

[0104] 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. Preferably the barrier membrane is coated on one side with adhesive so that it can easily be applied and adhered to the printing element.

[0105] 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.

[0106] 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%.

[0107] The barrier membrane needs to be sufficiently impermeable to oxygen diffusion so that it can effectively limit diffusion of oxygen into the photo curable 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.

[0108] In addition to limiting the diffusion of oxygen into the photo curable 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.).

[0109] 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.

[0110] The barrier membrane may be laminated to the surface of the printing plate using pressure and/or heat in a typical lamination process. Or it may be simply applied with pressure. It is preferable that the barrier membrane be capable of being selectively applied such that it can effectively be applied to portions of the printing element but not other portions thereof.

[0111] In another embodiment, the printing element may be selectively covered with a layer of liquid prior to the exposure step. The liquid here serves as a form of a barrier layer. 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 or be dried to a solid or semi-solid prior to exposure. The inventors herein have determined that a liquid layer from 1 m to 100 m 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 may also be an ink that is selectively applied to the surface using ink jet application technology or other equivalent selective application means. The liquid may be selectively applied using screening technology. The liquid should be spread upon at least portions of the surface of the printing element after the in situ mask is created but before the printing blank is exposed to actinic radiation. Preferably the liquid is dried to a solid or semi-solid prior to exposure of the printing element.

[0112] 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.

[0113] Finally, the relief image printing element is mounted on a printing cylinder of a printing press and printing is commenced.

[0114] 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 selectively manipulate and optimize certain geometric characteristics of the relief dots in the resulting relief image.