Study on coating glazing technology for printing surface

4. Online coating options

(1) Comparison of coating methods

Any printing unit can be coated with oil-based varnish and UV curing varnish using ordinary offset printing plates. Waterborne varnish and UV curing varnish can also be applied with the ALCOLOR dampening device.

However, the above two methods have two common disadvantages: that one printing unit is occupied by the coating operation, so it cannot be used for normal printing; the coating amount is relatively small, and the oil-based varnish is applied as an example, 2g/ M2 (wet film) has been considered a very high coating amount. In order to control the drying problem, the coating amount should generally not exceed 1 ~ 1.5g/m2.

(2) Comparison of drying methods

Most of the oil-based varnish is dried by chemical (oxidation) methods, and a small part is physically dried, ie the mineral oil therein is absorbed by the printing paper. Oxidation allows the oil-based varnish coated on the surface of the print to form a hard polymer film consisting of resin and oil. The oxidative drying process generally takes several hours to complete. If some solid materials are added to the oil-based varnish and heated and blown during the drying process, the formation of the polymer film can be accelerated and the drying speed can be increased.

UV-cured varnish can dry immediately under UV radiation. In the radiation curing process, virtually all coating materials are undergoing polymerization rather than drying as we generally understand.

The method of drying the waterborne varnish is completely different. It is completely dried by physical methods. The drying reaction is in part dependent on the absorption of moisture by the printing material and subsequent gradual drying, but most of the drying process takes place. It is completed before the printed sheets reach the delivery pile. 50% to 70% of the moisture contained in the varnish is dried by direct evaporation.

The use of a drying aid helps to immediately evaporate a significant portion of the water contained in the aqueous coating, thereby greatly reducing the dimensional instability of the aqueous coating.

Heating can also promote evaporation of water in the waterborne varnish coating. The water absorption capacity of air at 100°C is about 35 times higher than that of air at 20°C (the former absorbs 0.6 liters of water per cubic meter of air and the latter absorbs 0.017 liters of water per cubic meter of air). Heating with infrared radiation has proven to be a very effective drying method.

In addition, moist air containing more moisture must often be replaced with dry air with a strong water absorption capacity in order to remove as much moisture as possible. Replacement of moist air with dry air can be achieved by means of air knives, air blasts and air extractions in order to minimize the risk of the sheets being stuck to each other in the delivery pile.

Assume that the coating quantity of the waterborne varnish is 8g/m2 (wet film coating quantity), the area of ​​the printed sheet is 70mm x 100mm, and the printing speed is 10,000 pieces per hour, then there will be more than 30 liters of water per hour. Printed on sheets. In this case, it is obviously very important to choose the appropriate drying method.

General printing machines can be equipped with a series of different drying devices, such as: infrared drying devices, hot air drying devices, cold air drying devices, UV curing devices.

The infrared radiation emitted by the dual quartz tubes strikes the surface of the printed sheet 97%. The short-wave infrared radiation has a stronger penetrating power, while the medium-wave infrared radiation mainly radiates on the surface of the coating. The blowers located between the radiant lamps can act as cooling and keep the quartz lamps clean.

UV curing varnish provides the best gloss and best protection for quality prints. High-energy ultraviolet radiation with a wavelength of 250-400 nm enables the entire coating on the surface of the sheet to harden immediately.

In general, UV-cured varnish is applied to prints that use UV-curable inks. The curing process can be divided into intermediate curing (curing of special UV inks) and final curing. We must essentially distinguish between these two cures. The intermediate cure uses only one UV lamp and the output power is typically 160W/cm. The final curing generally uses two UV lamps, the output power is 240W/cm, or three 160W/cm UV lamps are used.
(to be continued)

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