4. Selection of online coating
(1) Comparison of coating methods
Any printing unit can be coated with oil-based varnish and UV-cured varnish with ordinary offset plates. Water-based varnish and UV-curable varnish can also be applied by means of ALCOLOR dampening device.
However, the above two methods have two common shortcomings, namely: a printing unit is occupied by the coating operation, so it can not be used for normal printing; the coating amount is relatively small, taking oil-based varnish as an example, 2g / m2 (wet film) is considered to be a very high coating amount. In order to control the drying problem, the coating amount should not generally exceed 1 to 1.5 g / m2.
(2) Comparison of drying methods
Most oil-based varnishes are dried by chemical (oxidation) methods, and a small part is dried by physical methods, that is, the mineral oil is absorbed by the printed paper. Oxidation causes the oil-based varnish coated on the surface of the printed matter 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 heating and blowing are performed during the drying process, the formation of the polymer film can be accelerated and the drying speed can be improved.
UV curing varnish can be dried immediately under UV radiation. In the radiation curing process, in fact, all the coating materials are undergoing polymerization, rather than drying as we usually understand.
The drying method of water-based varnish is completely different. It is completely dried by physical methods. The drying reaction is to some extent indirectly through the absorption of moisture by the printing material and subsequent gradual drying, but most of the drying process This is done before the printed sheet reaches the stack. The 50 to 70% of the moisture contained in the varnish is dried by direct volatilization.
The use of drying aids helps to evaporate a considerable part of the water contained in the water-based glazing oil immediately, thereby greatly reducing the dimensional instability of the water-based glazing prints.
Heating can also promote the evaporation of water in the aqueous varnish coating. The water absorption capacity of 100 ℃ air is about 35 times higher than that of 20 ℃ air (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, it is often necessary to replace the humid air with more moisture with dry air with strong water absorption capacity in order to remove as much moisture as possible. The replacement of moist air with dry air can be achieved by means of air knives, air blowing and air extraction, thereby minimizing the risk of printed sheets sticking to each other in the stack.
Assuming that the coating amount of water-based varnish is 8g / m2 (wet film coating amount), the printing sheet coating area is 70mm × 100mm, and the printing speed is 10,000 sheets per hour, then more than 30 liters of water will be coated per hour On the printed sheet. In this case, it is obviously very important to choose a suitable drying method.
General printing machines can be equipped with a series of different drying devices, such as: infrared drying device, hot air drying device, cold air drying device, ultraviolet curing device, etc.
97% of the infrared radiation radiated by the double quartz tube is directed to the surface of the printed sheet, the short-wave infrared radiation has a stronger penetration ability, and the medium-wave infrared radiation is mainly radiated on the coating surface. The blower located between the radiation lamps can play a cooling role and keep the quartz lamp tube dust-free.
UV-curable varnish can provide the best gloss and best protection for high-quality prints. High-energy ultraviolet radiation with a wavelength of 250-400nm can harden the entire coating on the surface of the printed sheet immediately.
Generally speaking, printed matter using UV-curable ink is coated with UV-cured varnish. The curing process can be divided into intermediate curing (for curing special UV ink) and final curing. We must essentially distinguish these two types of curing. The intermediate curing only uses 1 UV lamp, and its output power is generally 160W / cm. The final curing generally uses 2 UV lamps with an output power of 240W / cm, or uses 3 160W / cm UV lamps.
Second, glazing version and glazing blanket
You can use ordinary blankets or special blankets to apply the varnish, and you can choose photosensitive resin plates for fine local glazing.
1. Coat with varnish
Many domestic printing plants still use ordinary blankets for the entire coating. If only the material cost is considered, this method is of course the most economical. But to do so, the blanket must be cleaned frequently, because ink tends to accumulate on the blanket in a relatively short period of time, causing multiple shutdowns.
From the perspective of the transmission of glazing oil, the use of rubber blankets specifically for glazing has better performance and is not likely to cause ink accumulation.
Both types of blankets can be cut to the size of the printed sheet in different ways. One of the cutting methods is to cut the standard format paper according to the size of the blanket, and then cut the blanket based on the standard format paper. Another method of cutting is to cut off the excess on the blanket. This is necessary because it prevents the varnish from being transferred to the impression cylinder during varnishing, which can cause the printer to become dirty.
With the above-mentioned blanket cutting method, a simple blanket for local glazing can also be cut out (the glazing part is a regular geometric figure).
When calculating the cost of the glazing version and the blanket, the downtime and miscutting caused by manual cutting of the blanket should be considered at the same time.
Using rubber blanket for glazing coating, its service life is much shorter than that of photosensitive resin plate.
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