Coating glazing process analysis II

When it comes to coating the amount of coating, first of all, the difference between the amount of dry film coating and the amount of wet film coating should be differentiated. The ratio of the dry film coating amount to the wet film coating amount is approximately equivalent to the proportion of the solid material in the coating material in the total material. This ratio is on average 75% when using oil-based varnish, and about 40% when using water-based varnish. For example, if the wet film coating amount of the water-based varnish is 10 g/m 2 , the dry film coating amount is about 4 g/m 2 . Waterborne varnish usually has no light and light. The actual use depends on the needs of the packaging design. Water-based gloss coating is generally 6 ~ 8g/m2 (wet film). The thickness of the coating can be expressed in micrometers, and the above ratio can also be used for the ratio of the thickness of the dry film to the wet film. There is no reliable method for measuring the wet film coating thickness of a print on a machine. In the long version, the consumption of coating oil can be calculated based on the total coated area on the printed sheet. The smoother the paper surface, the better the gloss of the gloss coating. The thickness of the waterborne varnish film is twice as thick as that of the oil-based varnish. The same thickness of waterborne varnish film can show higher gloss.

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, which is composed 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 of waterborne varnish is 8g/m2 (wet film coating), the coverage area of ​​the printed sheet is 70mm x 100mm, and the printing speed is 10,000 sheets per hour. More than 30 liters of water per hour will be applied to the printed sheets. on. In this case, it is obviously very important to choose the appropriate drying method. The general printing machine can be equipped with a series of different drying devices. The short-wave infrared radiation has stronger penetrability, and the medium-wave infrared radiation mainly radiates on the coating surface. The blowers located between the radiation classes 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 mm enables the entire coating on the printed surface to harden immediately.

In general, UV-cured varnish is applied to prints using UV-curable inks, and the process can be divided into intermediate curing (curing transmission of 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.

(to be continued)