Effect of Printing Process Operation on Gloss of Prints

The printing of the product must be squeezed by the surface of the plate cylinder of the offset printing machine and the surface of the blanket cylinder and the impression cylinder, which is the imprinting pressure of the offset printing. To transfer the inking graphics on the printing plate to the blanket cylinder and then to the paper, during the two transfer processes, the printing could not be completed without the printing pressure. It can be seen that the printing pressure of the lithographic printing machine mainly comes from the plate cylinder, the blanket cylinder, and Impression roller.

The printing pressure directly affects the degree of ink transfer, whether the ink layer transfer thickness of the imprint reaches the standard, and affects the gloss of the printed matter. Excessive pressure is transmitted to the printing plate, and the ink on the Internet becomes distorted, the dots are expanded, the thickness of the ink layer does not reach the standard, and the gloss of the printed matter is poor. Too light pressure, the ink layer floating on the surface of the paper is not true, the formation of rough, affect the gloss effect. After two or more colors are overprinted, especially when the field version is overprinted with the field version, if the fixing of the previous color is not good or the auxiliary agent is not properly added, the gloss of the printed matter may be affected.

The ideal printing pressure means that the minimum pressure is evenly used on the basis of a clear printing pressure range, a clear print, a thick ink layer, and a good tune and color reproducibility. The offset press must control the proper ideal pressure, accurately measure and adjust the center distance of the cylinder, accurately measure and calculate the thickness of the lining, correctly detect the unevenness between the printing plate and the blanket, effectively fill the unevenness, and make the roller surface contact evenly. , To achieve the ideal state, according to the smoothness of the paper, flexible control of printing pressure. The network expansion rate is controlled at about 15%, so that the printed products are both thick and open, and printed materials are also bright.

Effect of pH of fountain solution

The pH of the fountain solution is too high or too low to wet the plate. The pH of the fountain solution is too high, the plate is poor in hydrophilicity, and the blank part of the printing plate is easy to get dirty. If an ink mainly composed of a dry vegetable oil chain material is used, the ink and the dampening plate when the pH of the fountain solution is greater than a certain value The interfacial tension of water will drop, causing the ink to be heavily emulsified, affecting the gloss of prints. If the pH of the fountain solution is too low, the surface of the printing plate will be severely corroded, and the blank part of the printing plate will not be damaged by the grease layer and the appearance of trachoma will not form a strong hydrophilic layer. The dampening water with too low pH will also affect the ink transfer and imprint drying. In the worst case, it will further expand with corrosion, and the combination of graphic and metal matrix will be damaged, resulting in damage or loss of the mesh, and reducing the gloss of printed products.

In terms of printing suitability, the fountain solution needs to maintain a suitable acidity, and a pH of 4.5 to 5.5 is generally considered as the preferred pH value.

Effect of ink emulsification

Ink emulsification refers to the mixture of a fluid in another fluid that is immiscible with it, a mixture of very fine dispersions, both in the form of oil-in-water and water-in-oil, and the oil-in-water type in offset printing. It rarely occurs during the process, and the water-in-oil type is inevitable.

During the printing process, the ink moisture is emulsified and the ink layer is very thick. The ink molecules become oil-in-oil. As a result, after the imprint is dried, the color is dark, the gloss is reduced, the outline is blurred, and the color changes greatly. Sample color errors produce a series of other problems: slower drying of the print, dirty print on the back of the print, reduced ink viscosity, scumming of the printing plate, agglomeration of the pigment particles, and pattern and pasting.

In the printing process, adhere to the small ink thick water method, as much as possible to make the ink thick, small amount of water, because the thick ink viscosity, the ink cohesion increases, the corresponding increase in the adhesion between the ink molecules, thereby increasing the ink resistance Water performance, reduce the ratio of water droplets dispersed in the ink, reduce the ink emulsification value, ensure the printed product pure hue, deep saturation, bright color, thick ink layer, small dot expansion value, and clear, full circle real.

Effect of ink adjuvant

Various inks used in the printing process, some properties do not meet the printing requirements or due to changes in printing conditions, printing process and technical conditions, etc., the physical properties of the ink, chemical properties or printability can not be compared with the printing job Matching requires an ink adjuvant to adjust properly. Most of the auxiliary agents such as anti-sticking agent, detackifier, varnish, and desiccant have adverse effects on the gloss of printed products. The common No. 6 varnish is diluted in the ink. Reducing the viscosity of the ink and putting it in the ink increases the fluidity of the ink and reduces the dryness of the ink, but it is detrimental to the gloss of the print.

Offset printing inks have sufficient drying performance under normal temperature, but in the case of low temperature and humidity in winter or in the case of anti-adhesives, de-sticking agents, varnish, etc., a desiccant should be added. The proper use of desiccant in the process operation is beneficial to glossiness of the printed matter, because it takes a certain amount of time for the paper to absorb the binder, and the binder is condensed as quickly as possible during the process until the drying of the conjunctiva is the key to good gloss of the finished product. The ink preparation process should be strictly controlled. Addition and release of adjuvants, if considering the gloss effect, should be less or no adjuvant.

The thickness of the ink layer

The correct use of ink is an important part of ensuring the color of the printed matter. The ink of the printed color product is too thick and too thin, which is not conducive to uniform ink and cloth. The ink is too thick, the printing ink layer is thick, the ink fluidity is poor, transmission is not smooth, the screen dark tone regional outlets are blurred, the color darkens, the level is boring; the ink is too thin, the printing ink layer is too full, the printing ink is pale. Proper ink blending will make the printing ink layer moderate, the color close to the original, and the level will be clear, and the bright areas will reflect a bright level. The thickness of the ink layer printed on the paper is generally measured by photometry. The density value of the ink film printed on the paper is measured by a reflection density meter. The larger the density, the thicker the ink layer. Experience has shown that black ink 1.8 ~ 2.0, cyan ink 1.45 ~ 1.7, magenta ink 1.25 ~ 1.5, yellow ink 0.9 ~ 1.05, the printing effect is more ideal.

No reflection densitometer, mostly printed proofs have been signed to print proofs, see the ink on the ground, but also look at the dot area ink, the ink school to print the proof is basically the same, the thickness of the printing ink is slightly thicker. If you print with offset paper, the ink layer should be thicker.

Zhang Hong

Reprinted from: Packaging World