Coldset Web Digital Plate Press Performance

Scope of the Document
This document provides a discussion of the factors that influence performance of digitally-imaged presensitized metal (aluminum) plates on coldset web offset presses.

The discussion is divided into two parts:

• factors that influence the lithographic performance of plates, and
• factors that influence the reproductive performance of plates.

For the purposes of this discussion, the lithographic performance of plates is defined as those characteristics of the plates having to do with ink and water, and the ability to achieve an acceptable balance between the two, as well as image durability and chemical resistance. The reproductive characteristics of plates are defined as those characteristics of plates having to do with resolution, register, and image fit. Though there are clearly links between the lithographic performance of a plate and its reproductive performance, for the purposes of clarity in this discussion, we will separate the two.

Lithographic Performance
In general terms, those factors that influence the lithographic performance of digitally-imaged, presensitized metal plates on coldset web presses are not much different from the factors that influence the performance of analog-imaged plates on the same presses. Principally, there are two factors to consider in this respect:

1. the ink-receptivity of image areas, and
2. the water-receptivity of non-image areas.

Sounds simple enough, right? Maybe not. It turns out that there is considerable scientific debate in the printing industry about the phenomena that permit the lithographic process to function. Some claim the phenomena are exclusively chemical: specifically, surface energy differences. Others claim principles of physics govern lithography: notably, the dynamics of surface interface and fluid dynamics. As is usually the case in such debates, there is truth in both arguments. In fact, lithography functions as a result of surface energy, surface interface, and fluid dynamics principles. That is, lithography works due to a mix of the scientific principles of chemistry and physics. Thus, plates with superior lithographic performance are plates with images and surfaces whose chemical and physical properties act to reinforce the lithographic phenomena.

Consider the three types of digital plates: silver-halide (AgX), photopolymer, and thermal. How do the properties of the image and surface of each respond with respect to the lithographic process?

Plate Surface
The task of the plate surface is two-fold:

1. to provide a support for the plate coating and a surface that grips the plate coating, and

2. to provide a surface that is receptive to the fountain solution, which in turn, prevents ink from adhering to the non-image areas of the plate.

All the plates under consideration have grained and anodized surfaces. This grained structure of the surface controls the plates' water-receptivity while the anodic layer prevents premature wear of the grained surface. All plates have somewhat different grain structures and each grain structure has slightly different water take-up characteristics. In practice, differences in plate grain structure will influence the take-up and spreading of fountain solution across non-image areas of a plate. Consequently, it may be necessary to adjust fountain solution dosage to obtain optimum results with different plates. However, neither the plate nor the fountain solution alone is the only factor to consider when optimizing non-imaging conditions for a given plate. The condition and settings of dampening system components will significantly influence a given plate's ability to take and hold the minimum fountain solution needed to produce a clean sheet in non-image areas.

Note also that mechanical wear of the plate surface in non-image areas can degrade the ability of the plate surface to hold a water film. Excessive or premature surface wear can be caused by blanket or plate overpacking and hard rollers. The uncoated groundwood sheets used in most coldset web printing can also contribute significantly to wear of a plate's non-image surface through ink or paper piling on the blanket.

The most notable differences in lithographic characteristics of digital plates have to do with the type of coatings applied to the grained and anodized surface. There are three types of plate coatings: silver, photopolymer and thermal polymer. Following is a discussion of the difference in lithographic characteristics of each.

Plate Image - Digital Silver Plates
Digital silver plates use film-like, silver-halide sensitive emulsions as the imaging and printing coating. These coatings depend mostly on surface energy to differentiate between ink and water receptive areas, and the limited ink-receptivity of the silver image may require the use of specialized fountain solutions. Silver plates can also be susceptible to poor performance in repeated start-restart environments—again, due to marginal ink-receptivity on the part of the silver image itself. Finally, silver is soft and can be more susceptible to scratching and wear on press than other coatings and there are no silver coatings that can be baked to improve durability or chemical resistance. In summary, silver coatings depend entirely on surface energy differences for their lithographic performance. There are no physical characteristics of the coatings that act to enhance a silver plate's lithographic properties. Thus, lithographic latitude can often be narrow with silver plates.

Plate Image - Photopolymer Digital Plates
Digital photopolymer plates use high-speed, visible-light sensitive polymer resin emulsions as their imaging and printing coating. Unlike silver coatings, polymer coatings have both surface energy and physical characteristics (images are slightly raised above the plate surface) that act together to differentiate between ink and water receptive areas. Polymer coatings are also more ink-receptive than silver coatings. This combination of attributes provides good start/restart performance and better litho latitude than with silver plates. In addition, many photopolymer coatings may be baked to improve their durability and chemical resistance. In coldset web applications with aggressive sheets, this may provide a significant press advantage.

Plate Image - Thermal Polymer Digital Plates
Thermal polymer digital plates use heat-sensitive polymer resin emulsions as their imaging and printing coating. Like the resin coatings on photopolymer plates, these thermally-sensitive emulsions have both surface energy and physical properties (images slightly raised above the plate surface) that act together to differentiate ink and water receptive areas. Thus, like digital photopolymer plates, thermal polymer plates have good litho latitude, good start/restart performance, and good ink receptivity. In addition, many photopolymer coatings may be baked to improve their durability and chemical resistance. In coldset web applications with aggressive sheets, this may provide a significant press advantage.

Reproductive Performance
The reproductive characteristics of plates are defined as those characteristics of plates having to do with resolution, register, and image fit.

All digitally-imaged metal plates provide image registration and image fit superior to analog-imaged plates. The press advantages of improved register and fit are straightforward: faster makeready with less waste. In terms of the type of digital plate used (silver, photopolymer, thermal polymer), improvements in register and fit are similar, and are significant when compared to all analog-imaged plates.

However, with respect to resolution, there are differences between digital plates types.

Digital Silver Plates - Resolution
Like film, digital silver plates are capable of very high resolution: typically 2% to 98% at 175 LPI. Silver digital plates are also capable of holding 20 micron stochastic images. For most coldset web applications, this resolution will likely prove more than adequate.

Digital Photopolymer Plates - Resolution
Digital photopolymer plates have several characteristics that effectively limit their resolution. The first is light scattering in the clear overcoat layer. As it happens, when one sensitizes photopolymers in the visible portion of the spectrum, they become unstable and will loose their photosensitivity if imaged areas of the coating come in contact with oxygen in the atmosphere. Thus, all photopolymer visible light plates are manufactured with a clear PVA (polyvinyl alcohol) overcoat layer, and this overcoat layer is an essential component of visible light photopolymer technology. During imaging, the clear overcoat scatters light, and thus limits the resolution of most digital photopolymer plates to 2% to 97% at 150 LPI; 25 micron or larger stochastic. (Fuji claims 2-98% @ 200lpi as do we for our PPV plates!) Though this resolution may prove adequate for most coldset web presswork, a second, resolution-limiting characteristic of digital photopolymer plates may prove problematic for some jobs.

Additionally, images on digital photopolymer plates have poorly-defined, sloped edges, and these thin image edges are the first to experience wear as a run progresses. It is the differential wear of image edges that accounts for into-the-run dot sharpening often seen with digital photopolymer plates. (Depending on the plate in use, one can expect the image on a digital photopolymer plate to sharpen from 3% to as much as 8% throughout a run.) The result is that one may experience color or tone shifts as a run progresses. Depending on the quality requirements of the presswork, this may prove problematic.

Performance of photopolymer plates can also be affected by heat and relative humidity. One of the surprising points of these plates is they change very quickly even in a controlled environment. Since they are negative working, they get slower with additional exposure to heat and humidity causing the plate to sharpen.

Thermal Polymer Plates - Resolution
Thermal polymer plates provide the highest resolution of all digital plate types: 1% to 99% at 300 LPI; 10 micron stochastic. This capability is likely in excess of that required for most coldset web applications.

An additional press advantage of thermal polymer images is the extremely sharp definition of image edges. This accounts for the fact that on press, thermal plates do not experience differential wear in image areas and thus, tone reproduction is stable throughout a run.

Conclusions
With respect to the lithographic performance of presensitized, metal digital plates, all the plate types discussed will have non-imaging performance that is similar, because all the plates under consideration have grained and anodized surfaces. It is likely that any notable differences in the non-imaging characteristics of these plates are attributable—not to the plate itself - but to various press conditions: roller conditions and settings, blanket grind and condition, settings and conditions of dampening system components, and fountain solution type and dosing. Conversely, it is certainly true that there are slight differences in the grained and anodized surfaces provided by various plate manufacturers, and it is likely that these surfaces will respond with slight differences in various press environments.

There are significant differences in the lithographic performance of the coatings used for presensitized, metal digital plates. Of the three coating types, silver, photopolymer and thermal polymer, silver coatings-though functional - have the poorest lithographic performance. Compared to polymer coatings, silver has poor ink receptivity, and silver coatings depend entirely on surface energy to differentiate image and non-image areas. Silver coatings are soft and cannot be baked to improve their durability or chemical resistance.

Polymer coatings used for both photopolymer and thermal polymer plates have excellent ink receptivity. This is because polymer coatings have a high degree of natural ink receptivity as well as surface energy and physical characteristics that act together to enhance the lithographic phenomena. The durability of polymer coatings is also better than that of silver, and many polymer coatings may be baked to improve both durability and press chemical resistance.

Notable differences also obtain with respect to the reproductive performance of digital plates. Silver plates and thermal polymer plates have similar reproductive characteristics, both with resolution capabilities likely in excess of the requirements for most coldset web applications. On the other hand, digital photopolymer plates have resolution limitations that are a result of the chemistry of photopolymer emulsions as well as differential dot wear on press. Whether these limitations prove problematic will depend on the resolution requirements of the presswork.

Finally, we note that all metal digital plates have register and image fit superior to analog imaged plates. These advantages can deliver significant benefits on press in terms of faster makeready with less waste.