Printing ink viscosity and printing adaptability (2)

The meaning and conditions of stickiness

The general definition of the viscosity of a packaged printing ink is the adhesion of a substance. It can also be explained as follows: at a given speed, the printing ink separates two flat surfaces of a certain unit area (they are glued together by liquids) The force. In the past, people mistakenly thought that a printing ink with a large viscosity had a large adhesion force. In fact, viscosity and viscosity are related, but it refers only to the individual viscosity of the printing ink which has a large viscosity and is occasionally large, or an extremely unstable ink viscosity under certain conditions.

For many years, ink manufacturers in China have misled some packaging and printing operators. After many years of repeated practice, it was found that: Some printing inks have large differences in viscosity, but their fastness of printing adhesion is very similar, and they cannot With high viscosity printing inks, the stickiness is also big enough to justify. The author believes that: As a technical indicator for packaging printing inks, inks should be selected based on their stickiness. This is not to say that the viscosity is not taken into account, because the size of the adhesive not only determines the quality of the gloss, adhesion, hiding power, color density, etc., but also determines the amount of printing. For this purpose, I have made detailed exchanges and descriptions on quantity and quality issues with Wang Yong, manager of Shandong Yongfei Group, and Jin Yazhang, manager of Jiangsu Nanjing, and Dibang Wang, director of Zhejiang Hangzhou.

The author has compared the inks used by Yongfei Group, for example, using the inks printed by Nanjing Ink Factory:

(1) a coat of -4 cups of ink net weight 91.7g;

(2) The ink viscosity index is 200 seconds (actually 204 seconds)/25°C. Coated -4 cups;

(3) 100 seconds/25°C after adding 1:1 thinner;

(4) The viscosity is 57 seconds/25°C when added to 1:2 dilute material;

(5) 38 seconds/25°C·coating -4 cups when added to 1:3 thinner material;

(6) 23 seconds/25°C·coating -4 cups when diluted to 1:4.

While testing the yellow ink of a printing factory in southern Jiangsu:

(1) a coat of -4 cups of ink net weight 94.3g;

(2) The ink viscosity index is 67 seconds/25°C · Coated -4 cups;

(3) 54 seconds/25°C after adding 1:1 thinner;

(4) The viscosity is 42 seconds/25°C when added to 1:2 dilute material;

(5) 31 seconds/25°C·coating -4 cups when added to 1:3 thinner material;

(6) 22 seconds/25°C·coating -4 cups when diluted to 1:4.

The authors performed the same comparison under the same conditions. As a result, the latter was disadvantageous in terms of viscosity index (and when it was added to 1:4 thinner, it was indeed close to the former), and after the printing was finished, its printing The number of out of the former is more than 7% of the former.

Viscosity representation does not mean that the viscosity is large after printing. Facts have proved that for the packaging and printing companies not only require the ink provider, but also have a certain range of viscosity, not to judge the usual ink consistency or concentration as the viscosity. The viscosity we refer to refers only to the dynamic viscosity. The ink film left on the substrate is not a solvent, but a resin, a pigment, and a filler. In a word, seeking quality can only confirm quality, while quantity can effectively control printing production costs.

Of course, in the discussion of stickiness, we should pay more attention to the mutual stickiness between ink colors. Generally speaking, the stickiness of the latter color ink cannot be greater than the previous color, otherwise it will pull down the previous color ink on the printing (often called Dyeing and other malfunctions). In order to prevent the above failures, the packaging and printing operator adds as much as possible the true solvent to the prior printing ink system when adding the ink thinner, and it is advisable to add more diluent to the printing ink system later.

The general theory of stickiness

The viscous theory of packaging printing inks is different from ink's viscosity after people's long-term practice. In theory, the viscosity of the printing ink is not only the nature of the viscosity of the ink used, but also the elastic breakage characteristics of the solid.

Through the meaning of stickiness and the conditions for identification, we have seen that stickiness is a complex situation. It is related to elasticity, adhesion properties, surface tension and other factors. It is also related to the adaptability of packaging printing. We must stick below the critical point for the viscous coefficient and the viscosity of the ink to plastic, paper, and metal inks. Otherwise, the phenomenon of stencil or paper pull, wool meshing, etc. will occur at this speed. If the printing conditions allow, then the higher the better the viscosity of the ink, so that the printed matter can be clear and complete network points, printing ink can be added more dilute material or varnish to increase the number of printed products.

We know that if the overprinting property of the ink is not good, the imprinting will not be even, and even the ink film that was printed first can be pulled down. Therefore, the ink should have good overprintability, and each ink ink film must have anti-separation (sticky) effect.

Because the viscosity and viscosity are different: viscosity is the internal resistance of the molecule, and the viscosity is related to the separation ratio of the ink film. In general, the viscosity and viscosity are proportional to the theory is that the greater the viscosity of the ink, The stickiness is also greater, but not all.

Because the viscosity of the ink is related to the separation ratio of the substrate film. Therefore, the ink viscosity meter can be used to determine the viscosity of the ink at different speeds so that each ink has a good overprint. For example, when the ink tip is too long, the formed filaments may be drawn back, so that the prints may have spots, and the printing dots are not as complete as the shorter inks - so-called "stands". For engraved gravure and photosensitive flexographic inks, long inks do not easily wipe the plate.

For any fluid, when it is studied mechanically, it generally involves both viscous and elastic properties.

In general, the viscosity and viscosity of an ink are directly proportional. This is a very general and highly irresponsible concept. The viscosity of the ink depends on the split (off) relationship of the ink film. Therefore, at a certain speed, it is very important to determine the viscosity of an ink because the faster the speed, the greater the separation force. It can be seen that the printing speed of the packaging printing press is closely related to the printing performance of the ink. An ink may be very smooth at a low speed and may fail at a high speed. For example, pulling paper (peeling), beard, pinhole, shrinking, spotting, flying ink, drawing less ink (poor overprint performance), picking dirty and so on.

In the past, the stickiness test method generally used the so-called finger sensing method, ie, sticking some ink on the index finger, then closing the thumb, and then separating the two fingers, judging the ink according to the force felt by the two fingers separated. viscosity. This traditional method requires a lot of experience, but it cannot be expressed clearly by numbers. Despite the long-term practice of inventing a viscous ink stickiness meter, the end of the era of stickiness cannot be expressed digitally. However, it can only be used as a relative sticky digital expression, but it cannot be explained by the standard mechanical unit system. It still needs experience and data to accurately respond to the problems that appear in the packaging printing process at any time.

The Effect of Sticky Value on Package Printing

Over the years, during the printing process, people often print inks with lightening or lightening agents due to the increase or decrease of the PH value, or due to various additional inks or diluting agents, or thinner materials. There is no ink (it is often called off or not inked) when the printing speed is increased. Use the balance of solubility in the principle of “two-dimensional ink making” or add cross-linking and coupling agents to increase the value of tackiness and improve gloss and adhesion, in addition to the gelation, recoiling, and ink roller or printing plate web ( Lines) In the cavity, there is often an increase in viscosity, which affects the normal printing production.

Although China has developed a viscous inspection principle and an ink stickiness value-added inspection method (see QB564-33), it often accuses ink producers or packaging and printing units of the inferiority of their products rather than seriously studying and analyzing the causes and hazards of stickiness. The author believes that only by communicating or proposing a more reasonable printing production process, can we make up for the defects of packaging and printing products caused by ink. Only by grasping the viscosity and viscosity of the printing ink can we effectively improve the printing capacity in reducing printing ink costs. Improve the printing quality rate.

Source: "Soft Packaging Business"