Waterless lithography

Let's take a look at a variety of developments that promise to offer a competitive challenge to the dominance of web offset.

After more than two decades of trying, the practical development of waterless lithography is on the verge of creating a process revolution in both sheet-fed and web. The elimination of water in the lithographic process has, in effect, created a whole new printing process. For lack of a better name, we may call it "waterless lithography;" the concept is, in reality, a fundamentally new way to print.

When water is taken out of lithography, the basic chemistry of the process changes. Although still using the principle of offsetting an image onto a blanket for transfer to paper or other substrates, the characteristics of press operation and the resulting product change.

The elimination of the conventional dampening system and, therefore, all of the process variables associated with achieving ink-water balance is at the heart of the driography (waterless) process' potential advantage.

Along with eliminating water, alcohol and alcohol substitutes, the utilization of the environmentally friendly process results in quality improvements. "Cleaner" appearing resolution screens can be run - 200- to 300-line screens are common, with reports of screen rulings in excess of 600 lines. Although there are claimed economical advantages for driography, it is the upgraded quality that will propel the process into competition with conventional offset.

One of the principal reasons for enhanced quality in driographic printing is that printing is accomplished with non-emulsified inks due to the elimination of water. This results in a reduction in dot gain since one of its primary causes is ink emulsification. Reduced dot gain makes it possible to run finer screens without plugging.

Waterless heatset web printing yields dramatic enhancement of offset quality that approaches continuous-tone printing and offers a faster, environmentally friendlier alternative to conventional printing.

While driography takes the water out of the printing process, flexography and gravure are working to create process advantages over offset by putting water into their processes.

Flexography is an established process widely used for such products as paperback books, labels and packaging products.

Several years ago, water-based flexo was developed as an ecologically friendly upgrade. This new flexo technique has achieved significant, but nevertheless small, penetration into the worldwide newspaper industry. Unlike petroleum-based offset and letterpress inks, the new generation water-based flexo inks dry on the surface of porous newsprint, avoiding the wicking phenomenon that creates the fuzzy edges and graying of the sheet.

Flexo printed newspapers are characterized by sharp-edged printing with bold, vibrant colors, extraordinarily black blacks and a whiter-appearing paper. However, on an overall quality scale, the best of flexo generally is viewed as lower in quality than the best of offset - particularly for single-color halftones.

In newspapers, flexo's success generally has come when there has been a conversion from an obsolete letterpress to an updated printing process. Flexo can win in the conversion situation, in part because it offers lower newsprint costs. The nature of the new generation newspaper flexo process is such that virtually the first paper out of the press is commercially salable, creating little, if any, makeready waste.

Gravure and heatset web printers have eyed each other's markets for some time. Gravure, looking to make changes to compete more effectively at shorter run lengths, meets heatset web, attempting to be more competitive at longer run lengths.

Gravure advocates substantiate their technical advances with filmless cylinder making, automated press operation, advantages achievedthrough paper and ink used by the process, and their utilization of advanced material handling

systems.

Many gravure printing operations, particularly in Europe, achieve reduced costs and faster throughput by using modern materials handling techniques, such as automated conveyors andgathering devices to take the product away from the press and advanced handling systemto store and transport signatures.

The gravure industry also is improving productivity by making presses that are more automated, run faster and are wider.

Gravurepressmanufacturers are increasing and improving their automated control systems, while at the same time undertaking operational simplification programs with the ultimate goal of one press, one operator.

Strides forward also are being made in gravure weak spots such as energy utilization and pollution control. The explosive nature of the solvents used in the gravure process, the need to recapture and recycle thosesolvents and the requirement for expensive pollution control and containment equipment is, in today's sensitive environment, a severe limitation for this printing process.

Developments in computer technology, refinements in non-impact imaging and the development of new imaging technology all are accelerating to make the digital printing press a reality in an ever-expanding range of markets.

There are many technical contenders for the output mechanism in a digital press, including lasers, various forms of electrostatics, electronic beam imaging, ion deposition, ink-jet, and several that have not yet been announced.

There will be an increasing variety of choices for printers to make in selecting the process that is optimal for their specific market. But these choices, and the utilization of the best technology for customer requirements, are exactly what will keep print alive and well in a highly competitive world of information and communications.

Litho Platemaking

Most supply companies producing platemaking materials market pre-sensitised plates, and these are normally grouped under two headings - diazo compounds or polymer resin coatings. The basic difference is that diazo images are non-receptive to grease while the polymer group is naturally grease receptive.

Diazo coated plates are supplied with either negative or positive - working coatings. Due to the peculiarities of this coating, it is necessary to use plates that have a fine grain, but this has the advantage of allowing excellent tonal reproduction. When negative - working coatings are exposed they are rendered insoluble to the solutions used on the press; the non-exposed areas are then developed away. The image may subsequently be inked or lacquered and finally treated with a de-sensitising solution. However there is at least one type of plate that needs no lacquering after processing as the image lacquer is applied at the time of manufacture.

Positive - working coatings, unlike other photo-litho coatings which are hardened upon exposure to ultra-violet light, are rendered soluble in the alkaline developer solution, leaving an image of unexposed coating. After development, the image is lacquered or inked and finally de-sensitised. Any unwanted areas, such as film edges, may be removed with a correcting fluid before final de-sensitising.

Polymer-coated plates are negative-working and rely on the action of U-V light to polymerise the coating in the image areas. The unexposed coating is washed away with developer. Because of the grease receptive nature of polymer images, they do not need lacquering or inking if the plate is to go direct to the press.

Apart from diazo compounds and polymer resin coatings used in preparing presensitised plates, there are a number of other surface treatments used in platemaking; they are nearly as varied as the different plate processes used.

Main types of plate materials in current use are divided into five categories; direct image and electrostatic; chemical transfer; surface; deep etch; and bimetal.

Date: 2016-05-14; view: 404; Нарушение авторских прав