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    PRINT FINISHING THROUGH VARNISHING IN OFFSET PRINTING

In printing, bookbinding and converting, more than in any other industry, the quality and efficiency of products depends on close attention to a large number of minor and major production details. To a large extent, the effectiveness of production control is in the hands of the individual operator, a fact which underlines the importance of trained staff in the graphic arts industries.
The quality level expected of a printed product has risen worldwide, and past exhibition has showed that this tendency holds on. The wide variety of varnish applications and the large number of application fields for the varnishes offered on the market today are still growing. Major improvements in printing inks have been achieved, and printers have a broader range of varnishes to choose from. Varnishing frequently used to be made in a separate run after printing where as today the printing press manufacturers offer additional varnishing units with which varnish can be applied online after printing. Varnished printed products are much better protected against abrasion, scratching, humidity and dirt. Thus the service life and the effectiveness for advertising can be increased.
The table shown below shows the basic problem of the varnishing processes. The main criteria for the useful applications are the drying process as well as easy handling. In the course of the last few years, water-based varnishes have turned out to be the most successful alternative.
The progress of the consumption of the water-based varnishes in Europe underlines this tendency. With oil-based varnishes and UV varnishes decreasing, the consumption of water-based varnishes has sharply risen. This is an increase of 250%. Due to the high technical expenditures and due to high costs, UV varnishes are mainly used for special application as package printing, label printing and printing on board.
   
The right drying method is the crucial factor
   
Print varnishes have chemical structure similar to that of natural offset inks. They mainly dry by oxidation, i.e. by absorbing oxygen. Part of the process also consists of physical drying when the varnish e.g. sets in the stock. The dry oxidized printing ink forms a hard, polymeric varnish film. The process of oxidative - chemical - drying takes several hours and can be enhanced by adding drying agents and by blasting heat or oxygen (blowing air). 
UV varnish, on the other hand, sets instantaneously as a consequence of UV radiation. UV radiation results in a polymerization of the varnish. High-energy UV radiation with wavelengths of 250 to 400 nm results in an instantaneous polymerization of the printing ink.
Water-based varnishes dries physically, i.e. mainly as the consequence of the evaporation of the 50-70% content in the varnish. Just as printing inks do, print varnishes also set in the stock followed by secondary drying. The use of drying agents is aim to achieve that that the highest share possible share of water evaporates immediately after the varnish has been applied. It is a known fact that heat enhances the evaporation of water. Air with a temperature of 100oC will absorb 35 times the amount of water that water with a temperature of 20oC will. Experience has shown that the drying of water-base varnish by means of measured doses of infrared drying combined with a consistent circulation of air is very favorable preventing excessive heat in the pile as well as secondary drying and blocking in the pile.
In the case of water-based varnish of 8g/sqm and a sheet size of 70x100 cm as well as a printing speed of 10,000 sph, roughly 30 litres of water are sprayed onto the paper per hour. Modern machines are equipped with specific units as e.g. IR radiation, air knife and drying tunnels. In addition most of the drying units on sheet fed offset machines can be adjusted to the use of UV inks, i.e. the drying units are equipped with IR and UV radiators.
  
Varnish for creative for creative designs 
   
The various fields of application for varnishes in offset printing allow the use of varnish in both gloss and matt. Partial varnishing - spot varnishing - makes it possible to draw the attention to specific areas and/or to cut out areas for further processing e.g. gluing. There is no limit to graphic designs with varnishes. Furthermore, varnishing increases the quality of a product since the varnish also protects the whole printed matter. For printing on heavy board, an additional coat of varnish is recommended.
The development of the varnishing techniques has not yet come to an end. Sure to your ink and varnish supplier will give you useful hints as to which type of varnish is suited for specific print job.

  Figure - 1

Type of varnish Structure Drying pros cons
Print varnish Similar to printing ink, but without pigments; portion of solids: 75% Chemically can be processed like printing ink; no difficulties with areas left blank; good protection against humidity sets slowly; thin layer of varnish; there is a danger of yellowing; powdering necessary; not odourless
Water-based varnish portion of solids: 40%; the rest is water Physically high gloss; good rub-resistance; fast setting; no powdering necessary; no yellowing; odourless more difficulties with areas left blank; light papers tend to distort
UV-Varnish 100% solids Radiation drying; chemically sparkling gloss; instantly dry; hard/resistant; no powder necessary; high rub-resistance high expenditure for technical operations; high varnish and energy cost; not odourless; inline-processing is difficult; conventional offset inks

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