Rotary offset ink performance and quality control (below)

Second, the quality control of rotary offset printing ink

The quality control of offset printing inks is mainly achieved through the one-by-one detection of ink performance indicators.

Viscosity

Viscosity refers to the ability of the ink to resist molecular motion due to friction between internal molecules of the ink under external forces. Viscosity has a great influence on the print quality, and it controls the transferability of the ink, the reproducibility of the dots, and affects the gloss and drying speed of the ink. Viscosity is too large or too small to be used properly. The viscosity depends on the speed of the press and the tightness and looseness of the paper structure. The viscosity of web offset ink is Laray viscosity 50-80Pa·s/25C°.

At present, there are rotary viscometers, laray viscometers, and balance plate viscometers for measuring ink viscosity.

2. Viscosity

Viscosity is the size of the ink's ability to resist splitting forces between the ink films. Viscosity and viscosity are the manifestations of different forces. There is no definite rule to follow. However, in general, the greater the viscosity, the greater the viscosity.

The faster the printing machine rotates, the greater the viscosity exhibited by the ink. Therefore, the viscosity of the design ink must be determined according to the speed of the printing press, so as not to affect the quality of the printed matter. Otherwise, the phenomenon of pulling and flying ink will occur. . Under normal circumstances, high-speed ink stickiness is 3 to 4, medium-speed ink stickiness is 4.5 to 5.5, and low-speed ink stickiness is 6 to 7.

3. Liquidity

Fluidity is the ability to measure the outward flow of ink under non-external forces. The larger the expanded diameter, the better the ink flowability.

4. Color

The ink produced by the ink factory is required to have the same color for each type and batch. At present, many newspaper companies have purchased different types of printing presses, and the style of each newspaper is not the same. This requires the ink factory to carry out “special ink special-purpose special offers” according to the requirements of the newspapers. Although it is also a four-color ink, The hue and other characteristics of the four colors used to supply different newspapers are not the same. However, for a newspaper office, the required four-color ink should be relatively stable in terms of hue and other characteristics.

There are two main methods for detecting the color of inks: 1 Routine test methods, ie, scraping contrast methods. This method is based on the person's eyes and the sample under the standard light source for parallel observation and comparison. 2 Use the printability tester (IGT) to print the proofing method.

5. Dryness

Ink drying is too slow, or even dry, the newspaper will appear dirty, pressure dirty, sticky hands and other serious impact on the appearance of newspapers.

The drying method of newspaper printing is mainly osmotic drying, and the main body that affects printing ink drying is the binder.

The reasons that affect the ink penetration drying rate are:

1 The compatibility of resin and mineral oil. When the compatibility of the resin and the mineral oil is poor, the release of both rapidly proceeds and quickly penetrates into the paper; when the compatibility of the resin and the mineral oil is good, the release and penetration of the solvent on the surface of the paper becomes slow; Absorption; 3 ink viscosity; 4 ink emulsification level; 5 newspapers on the graphics section of the ink layer thickness.

At present, the measurement method for the dryness of the cold-set web offset printing ink is the pressure rubbing method, which causes the ink on the newsprint paper and the blank newsprint to rub under a certain pressure for a certain time, and when the blank newsprint is no longer dyed due to friction, the scratching is calculated. Friction time, which indicates the dry value of the ink.

6. Gloss

At present, domestic ink companies use reflectance gloss meter to measure the gloss of ink. The gloss meter is placed on the printed sample of the printability meter and the gloss value can be read directly.

7. Fineness

At present, the fineness of the ink used in the ink factory is determined by the scraper fineness meter method. The ink fineness is required to be less than 15 μm. The fineness of the ink in the ink has a great influence on the ink gloss, the finer the pigment is grinded, and the gloss of the ink. The higher the ink, the finer the ink will directly affect the print resistance of the PS plate. With the other printability constant, the finer the ink, the higher the printing plate strength of the PS plate.

8. Water resistance

Offset printing is accomplished using the principle of ink repulsion. Ink roller and water roller will make the ink and fountain solution contact each other under the printing pressure, causing the ink to emulsify, and the traditional Chinese medicine water is reversed during the printing process and is easily transferred. In the ink fountain, the fluidity of the ink is changed, and even the ink is dried poorly, the gloss is lowered, and the imprinting becomes lighter and lighter.

In actual production, the ink factory in order to solve the problem of ink emulsification, in the ink production process mainly takes the following approach: 1 the use of high viscosity and low viscosity resin binder; 2 increase the viscosity of the ink; 3 using ink balance agent; 4 Strictly control the water resistance of raw materials.

Source: "Printing Technology"