The printing form is the object with which text in sentence letters , images and line elements are transferred to the printing medium - for example paper - by means of color . The shape of the printing surface and the material of the printing form are on the used printing method depending on: When printing , the printing form is off letters or clichés of different types of materials assembled in gravure etched or engraved metal cylinders are used, while in the offset printing is printed by means of a chemically treated metal plate . In screen printing , the printing form is a screen, the non-printing parts of which are color-impermeable. When flexographic printing is engraved directly with a rubber cylinder or a digital fine photopolymer -Druckplatte which is clamped to a cylinder, is employed.
However, there are also printing processes without a printing form, which belong to the so-called NIP processes .
In the relief printing process, the printing elements are on a higher level than the non-printing areas. Letterpress and flexographic printing essentially belong to letterpress printing. The letterpress uses printing plates made of hard material and highly viscous printing ink, while flexographic printing uses printing plates made of soft material and relatively liquid printing ink.
Book printing, invented by Johannes Gutenberg around 1450, is the oldest printing method in Europe and for almost five centuries was the only usable method for the mass production of printed matter. The printing form initially consisted of innumerable individually arranged types or letters, which were combined with bars , book decoration elements and woodcuts to contain text and graphics.
The stereotype is a further development of the letterpress form . It is a metal duplicate and is created by molding using a die into which a metal alloy is poured. In this way, instead of the printing form consisting of individual letters, a plate is obtained from one piece. Curved stereo plates were used in rotary printing presses for newspaper printing until the 1970s .
With the invention of the autotype it was possible to print rasterized black and white and color images in letterpress printing. A wax impression was made from these screened zinc clichés , which received a thin galvanic copper coating. The so-called galvano was filled with typometal and you received a detailed image of the original cliché. This method was preferred for quality prints.
Since the 1970s, book printing has gradually been almost completely replaced by the more economical offset printing .
In flexo soft printing plates and thin liquid inks are used. This makes it an ideal printing process for non-porous materials such as aluminum foil and polyethylene . The printing forms consist either of a UV- sensitive photopolymer plate or of rubber. The photopolymer printing form is exposed to UV light through a negative mask and the printing elements are developed in a washout process. The production of the printing forms from rubber material is done by means of laser engraving . After their production, the printing plates are stretched or glued onto the printing cylinder of the flexographic printing machine.
In gravure printing, printing forms are used in which the printing areas are recessed while the non-printing elements are at a constant level. The entire printing form is flooded with ink before printing and the excess ink is then removed with a squeegee . The transfer of the color to the paper takes place under high pressure. Gravure printing includes rotogravure as a commercial application and a number of artistic techniques such as etching , aquatint , copperplate engraving, and steel engraving .
In rotogravure printing , the printing form consists of a steel cylinder of considerable weight on which there is a thin surface layer of electroplated copper. The printed image is chemically etched or electronically engraved into this thin copper layer in the form of tiny cells. Finally, the copper layer is also chrome-plated in order to achieve better durability for longer runs. After printing, the thin copper coating is peeled off to reuse the steel cylinder.
Depending on the size and depth of the cells, a distinction is made in gravure printing between the conventional variable depth and the modern variable depth and area processes. In the conventional method, the cells are the same size, but different depths. To create the cells, a positive film with the print image is copied onto evenly rasterized pigment paper . The pigment paper has a coating of gelatin and color pigments , which is made photosensitive with potassium dichromate . After exposure and development, a gelatin relief is created that is transferred to the printing cylinder. After the paper has been peeled off, the gelatin relief remains on the metal and is deeply etched in a multi-stage etching process. However, conventional gravure printing has lost a lot of its importance because the production of printing plates cannot be standardized. For this reason, the electronically generated gravure form with variable depth and area has established itself in practice.
Engraving with a diamond graver has gradually been replaced by faster and more precise laser engraving. This can also optimize the cell shape, so that frequency-modulated printing grids and edges without a sawtooth effect are possible.
In the case of planographic printing, the printing and non-printing areas lie in one plane. The principle here is based on the chemical contrast between fat and water. While the printing areas are fat-friendly, the non-printing areas are moistened with a film of water and repel the fat-rich printing ink. For lithographic printing count of lithography , the offset printing and the light pressure .
In the 19th century, lithographic printing was the only printing process that allowed larger editions of colored printed matter. It was based on an invention by Alois Senefelder from 1798. A limestone slate that was quarried in Solnhofen in Bavaria was used as the printing form . The drawing to be printed was placed on the stone by the lithographer with ink or chalk and transferred to paper in the lithographic press. However, the drawing on the stone could not be printed without preparation. The lithographer and the lithographer called this chemical process etching . The properties of the grease-friendly, printing parts should be reinforced and the non-printing parts of the stone should remain grease-repellent and water-absorbent. The etch consisted of a mixture of nitric acid , gum arabic and water and optimized the compressive properties of the stone. In addition to specialist knowledge, a lot of experience is required for this activity. This is why artists today have their lithographs treated by an experienced lithographer on a commission basis in order not to endanger the result of their work.
Until the 1930s, lithographic printing was a very frequently used printing technique for a wide variety of printed matter, but was then gradually replaced by offset printing and is now only used in the artistic field. For today's mass production of printed matter, lithography is unsuitable because it is uneconomical compared to other modern printing techniques.
In the early 20th century, the principle of lithography was further developed. A process was discovered in which a thin metal plate could first be printed onto a rubber surface and then onto paper. As the soft rubber blanket adapts to the surface structure of the printing material, rough papers can also be printed. The flexible metal plates were initially made of zinc , later multi-metal and aluminum plates .
The transfer of the print image onto the plate takes place from reversed positive films onto a thin, light-sensitive layer with which the plate was previously coated. The exposure under the copier lamp decomposes the coating and the now soluble, exposed areas are washed out during development. The right-sided print image is created on the plate, which has to be optimized for printing in a similar way to lithographic printing.
The conventional copying process has now largely been replaced by electronic computer-to-plate technology (CtP). In this case, the printing plate is imaged directly with computer data in an imagesetter, without going through a film. The printing plate is developed online, i.e. directly in the imagesetter or in a separate developing machine. The latest development since 2008 is called Computer to Press or Direct Imaging , in which the printing plates are imaged directly in the printing machine. This eliminates the need to manually clamp and set up the printing plate. The advantages of this process are the shortening of the set-up times for the printing press and the automation of all associated operations. However, this process has not yet established itself in practice because the printing press is not producing when the printing plate is being imaged and modern printing presses have optimized the printing plate change so that a new job can be started in a few minutes.
Collotype is used today exclusively for artistic print products with very small editions. The support of the printing form is a 10 mm thick, matt-etched glass plate with a light-sensitive emulsion layer made of chromate gelatine. A photographic halftone negative, usually a color extract from the original, is copied onto the plate under UV light . During the exposure process, the chromate salts contained in the gelatine change the water solubility of the gelatine and create a gelatine relief . This process is known as the tanning of the gelatine and a so-called wrinkle grain is created instead of a grid as in other printing processes. After copying, the plate is immersed in 5 ° C to 10 ° C cold water, which washes out the chromates so that further exposure is prevented.
In the case of color reproductions, a printing form per printing color must be created for each color separation . For facsimile prints of works of art you need up to 20 plates, which when printed on top of each other achieve an unmatched fidelity to the original.
The best-known screen printing process is screen printing or serigraphy , in which the printing ink is pressed through a fine-meshed fabric onto the material to be printed with a wiper-like tool, the rubber squeegee .
The printing form for screen printing consists of a frame that is covered with a fabric made of metal or plastic. The stencil on the fabric contains the printed image. For simple, flat printed matter, manually created templates are used and attached to the underside of the screen. A light-sensitive photopolymer coating is used exclusively for raster work and multi-color prints. After the screen has been coated and dried, a correct-sided positive master copy of the print image is exposed onto the screen under UV light. The UV light hardens all transparent areas, but the image areas themselves remain soluble and are removed with water in a development process. Defects can be removed with a screen filler.
Screen printing is mainly used in advertising and lettering, textile and ceramic printing and for industrial applications.
Electronic printing process
All conventional printing processes use a static printing form that transfers the same print image with each printing process. The production of these printing forms and the set-up times in the printing machine cause high costs, which make long running times of the machine and high print runs necessary in order to work economically. For very small editions of printed matter, electronic printing processes that work without a printing forme and are electronically loaded directly from the computer are ideal today. The electronic printing processes include electrophotographic printing , thermal transfer printing and inkjet printing , which are also summarized under the name digital printing or NIP processes . Best-known applications are books on demand and photo books , but personalized printed matter such as invitations, wedding announcements, thank you notes and business cards are also enjoying increasing popularity.
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