Causes and Treatment of Ink Film Drying in Sheet-fed Offset Printing

In the printing process, whether the ink can be fixed on the surface of the substrate in a timely manner, and whether it can be quickly dried after the printing is completed, directly affects the quality of the printed matter and the production efficiency. If the ink is not dried in time, it will cause the back of the printed product to become dirty, stick back, and cause reverse overprinting or color mixing failure when entering the next group of printing, affecting the post-press production period. Therefore, we hope that the ink will be quickly fixed and dried after it is transferred to the surface of the substrate. Next, we will analyze the drying problem of the ink that occurs in the sheet-fed offset printing press (hereinafter referred to as offset printing).

To solve this problem, we must first understand these issues:

Dry forms of ink The dry forms of ink used in offset printing are mainly of the following three types:

Penetration drying (mainly relying on the penetration of low-viscosity components in the ink during printing, leaving high-viscosity components <resin and pigment> on the surface of the paper to form a film)

Oxidative polymerization drying (reacts with oxygen in the air, is a three-dimensional spatial distribution of dry vegetable oil molecules into a network-like macromolecular structure.)

UV drying (the photosensitive material is added to the ink binder to decompose activated atoms or atomic groups under the irradiation of ultraviolet rays of a certain wavelength, and undergo chain reaction with the ink component containing unsaturated bonds, crosslinking the molecules and curing the ink into a film ).
<In fact, there are many ways to dry the ink, such as volatile drying, infrared drying, microwave drying, hot air drying, but it is rarely used in current sheet-fed offset printing presses.

The composition of the ink The composition of the ink is mainly composed of pigments, binders, fillers and auxiliary agents, and the main component that determines the drying method is the binder. The ingredients of the binder are different, and the drying mechanism is also different.
At present, there are two main inks used in sheet-fed printing: ordinary resin ink and quick-drying glossy ink.
The main components of the binder in the ordinary resin ink are resin, mineral oil and a small amount of dry vegetable oil, which mainly rely on the penetration of the mineral oil component in the binder to make the resin binder on the surface of the substrate continuously increase the viscosity and achieve Dry, in which the dry vegetable oil is further dried by oxidative polymerization with oxygen in the air. This also determines the application range of this ink is mainly for the printing of the surface of the substrate with good absorption.
The content of dry vegetable oil in the quick-drying gloss ink is obviously higher, and the content of mineral oil is lower than that of ordinary resin ink. The drying of the ink is mainly achieved by the two steps of mineral oil penetration and oxidative conjugation of dry oil. , The dry oil component contained in its auxiliary agent can promote the oxidation process of the dry oil in the ink (principle: adsorption of phospholipid antioxidants in the ink, and promote free radicals, free radicals with high activity Continue polymerization). Mainly used for substrates with smooth surface and poor absorption (such as: exquisite picture book, printing on copperplate paper of calendar).

The transfer process of ink from the printing machine to the surface of the substrate is firstly the transfer of ink from the ink fountain to the surface of the blanket through the ink leveling system and the printing plate.


High-viscosity inks are subject to continuous splitting, shearing, and friction on the printing press. The viscosity decreases and the fluidity increases (this is due to the thixotropy * of the resin colloid in the ink.)


<The so-called thixotropy is the property that the colloid exhibits reduced viscosity and increased fluidity under the action of external force. When the external force is removed, the viscosity and fluidity of the colloid are restored.


Next, the ink is transferred from the blanket to the surface of the substrate.


The ink adsorbs to the surface of the substrate under the pressure, and at the same time, the low-viscosity phase (mineral oil <ink oil> and a part of vegetable oil) in the ink begins to penetrate into the paper. Due to the loss of the low-viscosity phase, the viscosity of the ink begins to increase and flow The property is reduced, the ink changes from liquid to semi-solid, and can no longer flow and transfer, this is the stage of ink fixation.


Then the substrate leaves the embossing area and enters the next color printing (multi-color machine).


Due to the disappearance of pressure, the surface of the ink and the substrate is relatively still, and the viscosity of the ink is further improved (the thixotropy of the ink). The strength of the semi-solid ink film formed at this time is to ensure the good fixation of the ink on the paper, and to ensure that the ink film will not be damaged during the next overprinting and the stacking of the finished product, and will not cause the back surface to be stained.


The last is the static process after the substrate is printed on the printing machine. In this process, the remaining mineral oil in the ink film on the substrate continues to penetrate into the paper (we refer to this penetration as free penetration), and the viscosity of the ink continues to increase; at the same time, the dry vegetable oil in the ink reacts with the oxygen in the air Form a huge network of molecular structures. This process is a thorough stage of ink.


So far, the ink printed on the surface of the substrate is completed. After understanding this process and its mechanism, we will find that it is very simple to analyze the drying problem of ink, because almost all the problems of slow drying of ink in printing are caused by the intervention of a certain step in this process. . Let's do some specific analysis on some factors that affect the drying of ink.

1. The influence of ink components and auxiliary agents on the drying of ink film Dilution agent: The main components of the diluting agent are aluminum hydroxide and connecting material. Aluminum hydroxide has strong water absorption and dissociates in water, which promotes the emulsification of ink and slow drying. It also adsorbs the drier, making it ineffective.

White oil: White oil itself is a white transparent emulsifier, and its texture is easy to emulsify the ink *.

Debonding agent: Its main component is a wax-like substance (such as aluminum stearate) with a small specific gravity, which will float on the surface of the ink film, hinder the contact between the ink film and oxygen, and affect the oxidative drying of the ink.

Desiccant: Excessive use will make the ink ink thicker and the ink emulsified *, making the print difficult to dry. (It will also cause the printing plate to be dirty and pasted. The dots are enlarged and deformed, and the hairs and thorns are difficult to dry.)


<* Water droplets dispersed in the emulsified ink will affect the drying speed of the ink, and will form an oil-in-water emulsified ink, thus affecting the drying of the blot. 〉


Table 1 provides the amount of dry oil with different ink colors for reference only.


Table I

Amount of white dry oil in color% Amount of red dry oil% BK 5 ~ 10 2 ~ 5 M 5 2 ~ 5 C 3 ~ 4 1 ~ 3 Y 3 ~ 4 1 ~ 3

Ink properties: Organic inks dry slowly, thin inks dry slower than thick ones, and those with more oil content are slower than those with less oil content. The metal ions in the ink components can promote the oxidative drying of the ink, and their ability to promote drying is in order:


Co Mn Pb Ce Cr Fe Zn Ca


(For example, the chrome yellow pigment is a mixture of lead sulfate and lead chromate, and the iron blue pigment is potassium ferrocyanide. Because they all contain a large amount of metal ions that can promote the oxidation and drying of the ink connecting material, they are used as pigment inks. The drying speed is relatively fast. Carbon black does not have a drying effect, but will adsorb the desiccant to delay drying. Therefore, the black ink using carbon black as a pigment has a longer drying time. Tungsten and molybdenum salt pigments will also delay the drying of the ink.)


Most organic pigments have the effect of delaying the drying of the ink, inhibiting the oxidative polymerization of the binder, and acting as an antioxidant.


(For example, the gold red pigment contains naphthol groups, and there are multiple aniline groups in the light blue chemical composition, so they dry relatively slowly. In production, we often use the light blue to add black ink to increase color, we must At the same time, consider the factors of drying. In addition, the lightness of the light blue is too heavy and it is not recommended to use it alone.)


The proportion of drying oil used in modern offset printing tends to be smaller, the characteristics of resin solidification are more obvious, and the influence of pigment on the drying speed of ink is relatively reduced.


Through the above analysis, we will find that the good ink used in printing is the key to ensure the timely drying of the ink film on the surface of the substrate. At the same time, we must pay attention to control the amount of ink auxiliary agent in printing. When the problem of slow ink drying occurs, you can increase the amount of drying oil in an appropriate amount, but do not exceed the amount in Table 1 otherwise it will delay the drying of the ink.

2. The nature and structure of the substrate 1. If the printed paper becomes acidic, it will dry slowly. (For inks with oxidized conjunctiva, if the PH value drops from 5.6 to 2.5, the drying time of the ink is extended four times!)

2. The ink on the surface of the paper with smooth surface, dense structure, poor permeability and low sizing degree is slow to dry. See Table 1 for dosage.


This is because the mineral oil in the ink cannot penetrate into the interior of the paper, which directly hinders the penetration and drying process of the ink.


Conventional prints (coated paper, offset paper, newsprint, book paper, gravure paper, etc.) can be used for general infiltration drying, oxidative conjunctival drying, and a combination of both in printing. For printing on non-coated paper with good permeability, for the sake of cost, use ordinary offset resin ink mainly based on penetration drying. Printing on coated paper requires the use of bright, quick-drying ink. Never use ordinary offset resin ink on the coated paper, otherwise the printed matter will be difficult to dry.


For substrates with poor surface permeability, a desiccant can be appropriately added to the ink during the printing process.


<If the surface absorption is very poor (such as PVC, gold and silver cards, glass cards), try to use UV, IR, microwave and other drying methods. 〉

3. The moisture content of the paper is too large: First, it affects the speed of the penetration of the binder in the ink, and then the evaporation of the water in the paper affects the drying of the ink.

3. Process in printing production 1. The pH value of the wetting fluid: If the PH value is low, it will replace with the metal salt in the dry oil, thus inhibiting the drying of the ink. The pH of the dampening plate should be strictly controlled during printing (between 4.0 and 5.6).


Table 2 shows the effect of different pH values ​​of fountain solution at normal temperature on the drying time of ink:


Table II

Paper

Drying time

PH = 7.0

PH = 3.8

PH = 3.6

PH = 3.0

PH = 2.0

Offset paper front

12

16

17

22.5

70

Bond paper front

15

twenty one

twenty two

32

84

2. Moisture on the plate surface: During the printing process, if the moisture on the plate surface is too large, not only will the paper absorb too much water and deform, but the water droplets dispersed in the ink will affect the drying speed of the ink, and will also form an oil-in-water emulsified ink, which will affect Blot drying.

3. Excessive stacking of printed sheets: the area of ​​printed sheets is large, the surface of the paper is smooth, the stack of paper is high, and air is difficult to enter.

4. The printing speed is too fast: the ink can not be dried in time after it is transferred to the substrate, and then it enters the next color printing or piles up on the delivery table. Practice has proved that if the printing speed exceeds 13,000 in the printing of a single sheet of paper, ordinary domestic inks are prone to problems such as backside sticking, color mixing, and sticking back.


<This is also related to the quality of the ink itself. If the thixotropy of the ink itself is good, it can be printed at a very high speed. Sometimes the printing speed can be opened to 14,000 and there will be no return during the printing process. Sticky and mixed colors. The author uses two inks for comparison in the factory, one is butterfly ink from Japan, and the other is crystal ink from Tianjin. As a result, it was found that the drying speed of Nissan Butterfly Ink was significantly stronger than that of Crystal Ink. >

5. The influence of the temperature and humidity of the environment: the internal energy of the molecule increases and the speed of movement increases as the temperature rises, prompting the dry vegetable oil to generate more peroxides, and accelerating the oxidative polymerization reaction. Thermal reaction, the heat released will promote drying. The decrease in temperature is the opposite.


The following gives a set of experimental data on the influence of the temperature and humidity of the environment and the pH value of the paper on the drying time of the ink:


Table 3

Production environment conditions

Effect of different temperature and humidity conditions on the drying of oil type and oxidative polymerization drying ink

PH value of offset paper

(RH) 65%, 18 ℃

(RH) 75%, 20 ℃

6.9

6.1

12.4

5.9

6.6

14.1

5.5

6.7

23.3

5.4

7.0

30.1

4.9

7.3

38.0

4.7

7.6

60.0

4.4

7.6

80.0

Through the above table, we will find that the environmental temperature, humidity, and paper PH value together affect the drying of the ink on the paper surface. Therefore, it is more important to ensure the constant temperature and humidity of the printing environment in actual printing.


<The environment temperature and humidity of the workshop should be controlled at 20 ± 2 ℃, and the relative humidity should be 50 ~ 65 %>


Finally, we must note that the drying capacity of manganese and lead is not enough when the relative humidity reaches 70%, and a cobalt catalyst must be used.

Fourth, the ink film is too thick The ink film is too thick will reduce the contact between the middle part of the ink layer and the air, especially after the film surface of the ink layer will prevent the oxygen in the air from entering, and the middle part dries slowly.
Therefore, we must strictly prevent the following situations in printing:
(1) The imprint caused by other reasons is not strong, and the thickness of the imprint is increased by mistake.

(2) The ink color is light due to excessive moisture on the layout, but the ink color is increased by increasing the ink supply.

(3) Too much emphasis on the bright and thick imprint. Improperly increase the ink supply.

(4) The coloring power of the printing ink is too low but the thickness of the ink layer should be increased to remedy.
(When preparing light-colored spot ink, pay attention to the slightly higher color saturation, so that the ink film on the substrate can be made thinner during printing, which is conducive to drying. In addition, the more ink types used when preparing ink Less, the less emulsified the ink when printing, it is also conducive to drying)
The thickness of the ink layer on the surface of the substrate and the distribution of graphics.
Large areas of evenly distributed graphics and text are also consumed uniformly at this place, and the emulsification of the ink is consistent, but the graphic areas on both sides are directly in contact with the excess water in the blank part, making the ink emulsification on both sides more important than the middle. More severe, drying also becomes slow. In the same way, when the area of ​​the graphic on the lithography is smaller, especially the small area of ​​the solid block or the small area of ​​the dark tone screen, the emulsification value of the ink will also be large, and the problem of the ink layer not drying is more likely to occur. In addition to an appropriate increase in the amount of desiccant, it is also necessary to strictly control the amount of water in the layout.

Some of the solutions to the problem of non-dry printing in printing are actually in modern multi-color offset printing, even if good printing conditions and reasonable processes are used, it is difficult to achieve the ink in the printing process or after the printing is completed The front ink can be dried in time, so we often use the following three methods in production to accelerate the drying of the ink film or prevent the back of the printed matter from being smudged.

Powder spraying: Powder spraying is a relatively common method in printing. It is a more effective method for backside smearing caused by the slow drying of the ink film. When the ink layer is thick, the amount of powder spraying can be appropriately increased, especially the substrate When there is a small area of ​​dark tone net or solid ink stick on the surface, more attention should be paid to appropriately increase the amount of powder sprayed. In general, the amount of powder sprayed should be as small as possible in the effective range, otherwise it will affect the ink color of the printed matter, especially in the products required by bronzing and laminating, the amount of powder sprayed should be controlled.

Drying rack: In the paper collecting part of the printing machine, a layered paper collecting method is adopted, that is, every 5 to 20? (The thickness of the separated paper layer can be adjusted according to the density of the substrate and the thickness of the surface ink layer. The density of the substrate is large. When the layer thickness is large, the separated layer height should be appropriately reduced) layered paper collection to prevent the destruction of the ink film that has not been dried under the weight of the paper, or the back felt is dirty.

Shaking and drying: Prints with poor surface absorption and thick ink film generally dry slowly. Due to the stack of paper and the low oxygen contact in the air hinders the process of oxidative drying, the paper is gently shaken at intervals to make it breathable. Increasing the contact between ink and air can effectively accelerate the drying of ink.

Finally, let ’s review the key mentioned in this article to control the slow drying of the ink film on the surface of the printed matter, as well as some process methods for the slow drying of the printed matter:


1. Select the appropriate ink, 2. Use additives and auxiliary agents as little as possible in the printing process, 3. Use dry oil in an appropriate amount, 4. Control the amount of plate surface water and the PH value of the fountain solution in printing, 5. Ensure the environment Constant temperature and humidity are very important. 6. Control the thickness of the ink film of the image and text. 7. Spray powder, rack, shake and dry.

So far we have analyzed some of the reasons why the ink film on the printed matter dries slowly and some solutions when the ink dries slowly, hoping to help everyone.

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