Rubber and plastics stereotypes

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The demand for flexible duplicates which are capable of printing illustrated magazines has been largely superseded by the use of photopolymer materials which can produce plastics plates direct from the negative, but for long runs of text and line work such as bookwork and commercial stationery, rubber and plastics duplicates continue to be used in large quantities.

For the flexographic printing of packaging materials, rubber stereos continue to be used in increasing quantities and further growth in the use of rubber is taking place in wallpaper and textile printing. A comparatively recent development in rubber platemaking is the introduction of grades suitable for printing 120-line process colour onto plastics foils. These provide an attractive alternative to gravure in shorter runs where the cost of gravure cylinders is difficult to justify.

Taking as a basis the amount of platemaking materials being used, the present position of rubber and plastics stereotyping can be summarised as indicating a rapid increase in the use of rubber and a continuing use of thermoplastic materials by larger book printing units.

The materials used for the production of rubber and plastics stereotypes are supplied in the form of powders, granules, pellets and sheets of various thicknesses and although referred to as simply a ‘rubber' or a 'plastics', these materials are in fact compounds of many different substances mixed together to produce grades in which the true rubber or plastics content may be less than 75 per cent of the bulk. Further, this broad classification into rubber or plastics material is confused by the existence of materials which consist of blends of rubber and plastics. A more satisfactory classification therefore is one based upon the behaviour of the material under platemaking conditions since all of the materials in use are either heat setting or heat softening.

Heat-setting materials are those which, under the influence of heat and pressure, can initially be made to flow and take up a new shape and with continued heating undergo a chemical change permanently setting the material into the new shape. The basic platemaking operations consist of heating the mass until it reaches a stiffly flowing or malleable condition and after pressing it into the new shape holding it under heat and pressure for a 'curing' or 'vulcanising' period. Those materials which set into a rigid condition are the 'thermosetting plastics' and those which set into a resilient mass are those normally classified as 'rubbers'. It should be noted that, because the curing takes place as a result of heat, no chilling press is required and also that, because the curing reaction in irreversible, neither thermosetting plastics or rubber materials can be reclaimed for use in platemaking.

Heat softening materials also can be made to flow and take up a new shape when subjected to heat and pressure but, because there is no curing reaction, they remain soft until cooled. Materials with these characteristics are the 'thermoplastic plastics', and platemaking equipment will include both a heating and a chilling press. Thermoplastic materials are available in rigid grades which produce plates superficially similar to those produced in thermosetting plastics and in grades having flexible properties resembling rubber. An important feature of thermoplastics is their low heat resistance. This makes it impossible to include them in work which is to be hot moulded, but it also implies that the material can be re-used for platemaking.

Thermosetting platemaking materials are supplied as fibre boards and moulding powders based upon the polymer phenol-formaldehyde. In the cured condition these materials will withstand temperatures up to 250°C and are therefore particularly suitable for production of the matrices from which all types of rubber and plastics plates can be made. These same phenolic materials, hot pressed into one of these matrices, are used to produce rigid thermosetting plates. Such plates however are only suitable for flat-bed printing or for use as embossing dies.

Rubbers are either natural, synthetic or plasticised, the latter consisting of blends of synthetic rubber and a plastics such as PVC. All are available in sheets of various thicknesses and in a range of different hardnesses, but the plasticised rubbers are usually supplied in granular form. Stereo rubbers based upon natural rubber are considered to produce plates of the highest resiliency and longest printing life, but are attacked by the solvents used in letterpress printing inks as well as some of those used in flexographic inks. Oil and solvent resistance is obtained by using one of the synthetic rubbers, but it is important to choose the correct type for the particular inks being used. Solvent attack can cause the rubber to swell and lead to variations in printing impression or, in extreme cases, complete disintegration of the plate.

An important feature of rubber plates is their long printing life, many million copies being commonplace. The plasticised rubbers, although not capable of such long runs have the ability to print finer screens than is possible with unblended rubbers of comparable hardnesses, and have found applications in the production of frozen food wrappers.

Thermoplastic platemaking materials are available in the form of powders, granules, pellets and sheets of hardnesses suitable for all classes of flat-bed and rotary letterpress printing. Most materials are based upon PVC and are resistant to all normal inks and solvents. Flexible grades have printing characteristics similar to those of rubber, but printing life is only of the order of 100, 000 impressions. For rotary printing, plates made in these grades can be curved by the application of gentle heat. Thermoplastic platemaking offers potentially faster production than rubber and it is possible to reclaim offcuts and reject plates but the plates suffer from the disadvantages of a more limited printing life and the necessity to use both a heating and a chilling cycle. Laminated plates of many different types are in current use and may consist of two or more layers of different grades of the same material or layers of dissimilar but compatible materials such as PVC and synthetic rubber. The most usual reason for lamination is to give a flexible plate with a surface hard enough to print fine detail, but occasionally the objective is lower material costs.

An attractive feature of rubber and plastics platemaking is the comparatively small amount of equipment required.

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.

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