Postpress Technology 1000 Questions (34-43)

34. What is a UV coating process?

According to the needs of the products for coating, localized coating is required on the areas that need to be highlighted, such as trademarks and packaged printing products. Local glazing products have a great contrast between light and dark parts, unique style, and can produce unique artistic effects. In addition, some packaging boxes can use partial uV coating to set aside the life, which creates the conditions for the use of ordinary adhesives.

35. What printing plate is used for the local glazing process?

The partial glazing version uses a rubber relief or photosensitive resin plate, preferably a screen plate.

36. What are the types of coating methods for uV Varnish?

When the paper is coated with uV varnish, it can be printed first, and the coating oil can be coated in an off-line manner after finishing. This processing speed is generally 30 to 60 m/min. There are mainly the following ways of printing glazing

1 offset blanket blanket coating, processing speed 7000 "-9000 / h.
2 The last unit of the gravure rotary press is coated with a processing speed of 100 to 20 Om/min.
3 Flexo printing press final unit and special coating machine, processing speed 100-20Om/min.

37. Use uV coating materials should pay attention to what the problem?

1 light source. The uV irradiation light source generally uses a high pressure mercury lamp or a metal halide lamp. High-pressure mercury lamp output power is generally 80-120W / cm, in order to ensure the curing speed of uV coatings <0.5s.

2 Paper. uV coatings are not suitable for the processing of easily permeable paper. Because the low-molecular materials in the uV coating can easily penetrate into the paper, it causes the paper to become dark or even permeable. In this case, it is possible to cure completely only if both positive and negative sides of the paper are irradiated with ultraviolet light. In order to prevent the infiltration phenomenon, the underlayer may be first coated with a material other than the uV paint to avoid infiltration when the uV paint is applied.

3 Ink. Need to pay attention to the combination of uV paint and ink, if necessary, apply a primer.

4 Labor protection. uV Varnish is irritating to the skin, and if it does not evaporate, it adheres to the skin and clothes. If it is in the hot weather in summer, it may enter the skin with sweat to cause inflammation, so if the skin is stained with uV Oil should be washed with soapy water and replaced as soon as possible. In addition, it is necessary to pay attention to the temperature control of the uV paint, and the pasteability and hot stamping properties of the uV paint after processing.

38. What are the main factors that affect the quality of glazing and the solutions?

1 Effect of paper properties on polish quality. The effect of paper on coating quality is mainly reflected in the effect of paper smoothness on coating quality. High-smoothness papers have a significant effect upon polishing, while papers or paperboards with low smoothness have a poor effect after polishing, since the varnish is almost completely absorbed by rough paper or paperboard. In order to solve this problem, you can use a casein base or two times before the application of light.

2 Influence of temperature on coating quality. The glazing temperature is between 18~20°C, which can achieve the best results. In winter, the varnish is easily solidified, and the surface film of the varnish product is uneven. In order to solve this problem, the coating oil needs to be stored in a warm place. If the temperature of the coating oil is too low, it must be properly diluted with a solvent.

8 The effect of printing ink on the quality of coating. Printing inks used for printing products after printing must be solvent-resistant and heat-resistant, otherwise the printed matter will be discolored or wrinkled. The solution is to pay attention to the following points when selecting printing inks: First, use alcohol-resistant, ester-based solvents, and acid-base inks. The second is the use of long-term non-color and good gloss ink. The third is to select inks that have good adhesion to paper.

4 The influence of the quality of printed products on the quality of glazing. The printing time is too long, the ink area on the bottom of the print is too large, the dry oil is excessively added, and the glass crystal film is formed on the ink blotting surface, which often causes the glazing and printing to fail, or to produce phenomena such as “face” and “dot”. The solution is: Adding 5% lactic acid to the varnish usually can be used after stirring. The modified varnish is applied to the printed matter so that the vitrified film on the surface of the printed matter is damaged, and the varnish is evenly applied to the surface of the printed matter to form a glossy film.

39. Why do some prints turn yellow after they polish?

Coatings for glazing should be colorless and transparent to ensure that the original color effects of prints are not changed.

The effect of different glazing methods on color printing results is shown in Table 4.

40. Why does the phenomenon of dirty printing on the surface of the printing products appear?

Dirty plate phenomenon is that there are small particles of impurities appear on the surface of the glossy print. The main reason for this is that the coating material contains impurities that should be filtered before use. In addition, the glazing machine paint bucket and rubber roller should also be cleaned before glazing to prevent dirt from mixing.

41. Why do some stripes or other marks appear on the printed surface after polishing?

The stencil failure of the coating products was mainly caused by the uneven pressure between the squeegee and the impression roller. The pressure should be adjusted to eliminate streaks.

42. Why does the phenomenon of change in the color of the glazing print appear?

Fast on-line speed, general drying device can only reach the surface coating curing, not yet dry curing coating solvent dilution solvent can not be excited or volatilized, only under the surface of dry curing, activities along the coating A hole in the middle and bottom of the coating, an ink printing layer and a paper coating a paper, are gradually absorbed by the paper. Once the solvent and diluent have been absorbed, the molecules that have penetrated into the printing layer of the ink dissolve into the ink printing layer and act on the polymer, which causes the viscosity of the binder to decrease and gradually dissolve. The pigment particles lose their protection and become free, which changes the smoothness of the ink layer. The structure reduces the adhesion of the ink pigment on the surface of the paper, and the color of the printed product changes, appearing dull and dull.

43. Why does the phenomenon of “gold and silver documents disappear?” appear after glazing?

The metal particles in the printed gold, silver, or hot stamped aluminum are much larger than the other particles in the ink, and have poor compatibility with the binder. After the binder or diluent decomposes the binder, The pigment is more easily released from the binder and forms a free metal powder that adheres to the surface of the print. When the glazing coating is too thin or the curing is incomplete, free metal particles or other larger pigment particles are easily rubbed off. Causes the loss of printed color or changes in hue. The free metal end chemically reacts with the acid or alkali in the coating and dampening solution paper and loses its brilliant luster. The “gold and silver graphic is missing”.