Lithographic Plate Composition
A conventional lithographic plate consists of two distinct surface areas. The image areas are oleophilic, meaning that the surface will readily attract oily ink. Nonimage areas are hydrophilic, or water attracting. Because oil-based ink and water tend to repel each other, these two areas remain separate on the plate, even though water and ink are distributed over the entire plate by the dampening and inking systems.
Plate base. Today, most lithographic plates have a thin metal base. Aluminum is , by far, the most common metal used, although plates can also be made of stainless steel, mild steel, or brass. Aluminum has the advantage of being relatively light, flexible ( for wrapping around the cylinder), and hard enough to withstand reasonable surface compression forces.
. Gauge tolerance. It is critical that sheet metal used in the manufacture of plates meets strict gauge tolerances and is free of surface defects. Plates up to 22 x 34 in. (559 x 864 mm) should not vary mor than 0.001 in. (0.025 mm) in thickness. For example, a 0.012in. (0.305 mm) plate should not be more than 0.0125 in. (0.318 mm) nor less than 0.0115 in. (0.292 mm) thick in any area. In this example, the tolerance is expressed as +/-0.0005 in. (+/-0.013 mm). An excessive variation in tolerance will create pressure variations in different areas of the plate, resulting in uneven tints and solids.
. Plate gauge. The plate gauge varies depending upon the cylinder undercut and press size. standard thicknesses range from 0.0055 in. (0.14 mm) to 0.020 in. (0.51 mm), while sizes of the plates may be as large as 59 x78 in. (1499 x1981 mm). Generally, a large-diameter press cylinder requires thicker plates to minimize stretching and plate cracking. Plate thickness should be matched to the cylinder undercut so that minimal packing is required. Several layers of packing are more likely to creep and compress than a single sheet.
Plate graining. Before a metal can be used as a base for a lithographic plate, its surface must be properly prepared. The surface of a sheet of aluminum is very smooth. When water is coated on the smooth surface it will have a tendency to bead. Because a lithographic plate must accept a film of water without beading, the surface of the plate is given a grain. This is accomplished either by roughening the surface mechanically, or treating it chemically or electrolytically. This mechanical or chemical graining process significantly improves press latitude, reduces drawdown time in the vacuum frame, and helps to eliminate halation. A grained, anodiged aluminum plate at 2 different magnification is shown in Figure 2-30.
Most plates in the United states are grained on a machine in which a continuous web of aluminum is passed under a series of rotating nylin brushes and grained with a mixture of abrasives and water -- a process referred to as slurry brush graining. Introducing a uniform form grain produces a surface that is dark in color. This brush grain is very fine and is satisfactory for presensitized and wipe-on plates. Using a chemical etch after brush graining produces a much lighter, cleaner, and slightly rougher grain.
As an option to mechanical brush graining, several methods of cleaning and slightly roughening plates chemically are currently in commercial use. They are used primarily for treating relatively smooth, short-run plates prior to coating in the manufacture of presensitized plates. These plates are usually double-sided plates for use on small presses. Most bimetal aluminum plates are also chemically grained, but they are made much rougher than the presensitized chemically grained plates. Some premium long-run plates are electrochemically grained to produce a uniform, relatively rough grain. As an exception to grained plates, stainless-steel bimetal plates are smooth and without grain. This is because stainless steel is naturally hydrophilic.
Rougher-grained plates have several advantages. They provide better latitude for ink/water balance on press, faster drawdown in a vacuum frame, less trouble with dirt and hickeys, better durability on press, and less tendency for dot slur. However, these plates are not able to hold fine highlight dots as well as smooth-grained plates.
Silicating. In addition to roughening the surface, chemical treatments are also needed for some processes, especially wipe-on and negative-working diazo presensitized plates. Diazo compounds, which are ink-receptive when exposed, can react with untreated metals. Therefore, the aluminum is usually treated in a hot sodium silicate solution to create a barrier layer that prevents a reaction between the diazo and the aluminum. This treatment also desensitizes the plate so that it will be more water-receptive, as well as helping to make the plate surface more receptive to bonding with the diazo.
When positive-working diazo presensitized plates are made, surface treatments may not be necessary; fine graining and /or cleaning usually precede the application of positive-working diazos.
Anodizing. Most high-quality plates are anodized after graining. Aluminum anodizing is a process by which a thin, uniform layer of extremely hard aluminum oxide is produced electrolytically on the grained aluminum. This anodic layer has many extremely small pores, similar to a honeycomb. The anodic layer must be sealed before the photosensitive coating is applied. Usually, hot solutions of sodium silicate are used to treat the anodized layer, making it highly water-receptive. This process also prepares the plate surface to receive the light-sensitive coating. Furthermore, the anodic layer is hard, abrasion-resistant, and highly durable.
Diazo sensitizers. Both water and solvent-soluble diazos are commonly sued as a sensitizing agent for plates. Plsitive-acting plates are coated with diazo oxides or quinone diazides. Exposure to UV light converts negative-acting diazos directly to insoluble resins that have good ink receptivity and durability for printing. Positive-acting diazos decompose upon exposure to light and become soluble in the developer while the unexposed diazo remains on the plate, forming the image areas.
Photopolymer sensitizers. A thin polymer coating makes an excellent oleophilic surface. A number of different reactive polymers can be sensitized with a suitable photoinitiator for use as a plate coating. On exposure to light, the exposed parts of the coating become insoluble in the same solvents that dissolve the unexposed portions of the coating. Thermal plates often employ photopolymer coatings that harden when exposed to light. The resultant images are tough, and the plates generally withstand long runs. Most photopolymer coatings are aqueous (water-developable).
Silver-halide sensitizers. Silver-halide coatings have been used to sensitize graphic arts films for decades. Manufacturers are now using silver-halide as a sensitizer for plates. The light-sensitive siver-halides are suspended in an emulsion layer, which lies on top of what is called the nuclei layer. A barrier separates these two layers. T he laser in the platesetter exposes the nonimage areas of the plate, reducing the silver-halides. During the processing, the unexposed silver-halides diffuse through the barrier layer into the nuclei layer, where it is reduced to molecular silver, which is oleophilic in nature. A second processing step removes the exposed silver and corresponding nuclei layer from the plate surface.
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