Rubber and plastics stereotypes

The demand for flexible duplicates which are capable of printing illustrated magazines has been largely superseded by the use of photopolymer materials which can produce plastics plates direct from the negative, but for long runs of text and line work such as bookwork and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