Today's prints are not only beautifully printed, but also look beautiful. In addition to fine levels and realistic color image printing, some additional surface treatments are also important measures to improve the quality of printed materials. Printed metal inks, embossing, embossing, embossing of electro-aluminum foils, surface composite clear plastic films and coating varnish are some of the additional surface treatment options available.
Of course, these additional surface treatments must be as economical as possible. Nowadays, many printing companies have realized that the application of varnish to the surface of printing products is a very good surface finishing method. It can not only be completed at the same time with the printing operation, but also has a very low cost. Manroland, Heidelberg and other offset presses can be used for in-line coating.
First, coating glazing process
1. The role of coating oil
1Decorative function: Apply a layer of varnish to the surface of the printed product to give the printed material a matt, glossy or high gloss surface.
(2) Protective function: The coating film applied on the surface of the printed ink layer has a protective effect on the printing ink layer, especially during the transportation of printed products.
Any print can be coated with varnishes. The only question is: Is it appropriate and economical to coat varnishes in some way under certain conditions?
2. The main types of polish oil
The three most commonly used coatings for in-line coating are oil-based varnish, UV-curable varnish, and waterborne varnish.
In general, the cost of applying an oil-based varnish is equivalent to printing another color of ink. The cost of applying the same dry film thickness of waterborne varnish is generally the same as that of the oil-based varnish, but the varnish of the waterborne varnish usually needs to be relatively thick, so the cost of the varnish used is correspondingly higher. In terms of coating effect, coating UV-curing varnish is the best online coating glazing process, but it is also the most expensive on-line coating glazing method.
In any case, on-line coating of glazing oil is always much more economical than compound BOPP film, off-line coating of varnishes or calendering and other additional post-treatment methods. Therefore, on-line coating of glazing oil (especially The use of waterborne varnish has been increasingly used.
(1) Oil-based varnish
The composition of the oil-based varnish is basically the same as that of the offset printing ink, but does not contain a pigment, and contains about 75% of the solid content, and is dried by a chemical method.
The advantages of coated oil-based varnish are:
1 The processing method is the same as the general ink printing;
2 easy to local finish;
3 coating has good moisture resistance.
The disadvantages of applying an oil-based varnish are:
1 Oil based varnish drying slowly;
2 coating is relatively thin;
3 coating tends to yellow;
4 It is necessary to spray powder after coating in order to prevent the printing material from sticking;
5 It may be necessary to use a frame delivery device.
(2) Waterborne Varnish
The waterborne varnish contains approximately 40% solids, the remaining 60% being water and physically dried.
The advantages of coating waterborne varnish are:
1 coating has excellent gloss;
2 coating excellent wear resistance;
3 Waterborne Varnish Dry quickly after coating;
4 No need to spray powder or only a small amount of powder after coating waterborne varnish;
5 print stacking stability;
6 coating will not turn yellow;
7 Waterborne varnish has no bad odor and is rarely limited in application areas.
The disadvantages of coating waterborne varnish are:
1 When using lightweight paper, the printed sheet may curl due to water absorption;
2 It is difficult to perform partial finishing (coating with a metering roller).
(3) UV curing varnish
UV-cured varnish contains approximately 99% solids, which is “dried†by radiation curing. Actually, the entire coating is polymerized into a solid film under ultraviolet irradiation. The curing effect depends on the ultraviolet wavelength emitted by the UV lamp. And radiant power.
The advantages of coating UV curing varnish are:
1 gloss coating can produce a very high gloss;
2 coating is strong and durable;
3 As long as the UV lamp can meet the requirements of the application, the UV curing varnish applied to the printing product can be instantly dried;
4 No need to spray powder after coating, no print adhesion will occur;
5 coating has high wear resistance.
The disadvantages of coating UV curing varnish are:
1 Machinery and equipment are expensive;
2 UV curing varnish has high cost and high energy consumption cost;
3 is not completely odorless;
4 It is difficult to directly apply UV curable varnish directly on the printed products with ordinary offset printing inks. It must occupy two coating glazing units, one before and one after.
3. Coating amount of varnish
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 . Generally, waterborne gloss oil has no light and light, and the actual use depends on the requirements 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-run operation, the consumption of coating oil can be estimated 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.
(to be continued)