Analysis of drying problem of sheetfed offset ink
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Whether the ink can be fixed on the surface of the substrate in time during the printing process 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 print to become dirty, sticky, and cause reverse overprinting or color mixing when entering the next set of printing, affecting the post-press production period. So we want the ink to quickly fix and dry after it is transferred to the surface of the substrate. Below we will analyze the drying problem of inks appearing in sheetfed offset presses (hereinafter referred to as offset printing).
To solve this problem, we must first understand these issues:
Dry form of ink
There are three main types of inks used in offset printing:
Permeation drying (mainly relies on the penetration of low-viscosity components in the ink in printing, leaving the high-viscosity component <resin and pigment> on the surface of the paper to form a film)
Oxidative polymerization drying (reacting 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 binder of the ink, and the activated atom or atom group is decomposed under the irradiation of ultraviolet rays of a certain wavelength, and the ink component containing the unsaturated bond is subjected to a chain reaction to crosslink the molecule, and the ink is solidified 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 there is still less use in the current sheetfed offset press.
Ink composition
The composition of the ink is mainly composed of a pigment, a binder, a filler, and an auxiliary agent, and the main component that determines the drying method is a binder. The composition of the binder is different, and the mechanism of drying is also different.
In the current sheetfed printing, we mainly use two kinds of inks: ordinary resin ink and fast drying bright ink.
The main components of the binder in common resin inks are resin, mineral oil and a small amount of dry vegetable oil, which mainly rely on the penetration of mineral oil components in the binder, so that the resin binder on the surface of the substrate continuously increases the viscosity. Dry solid, wherein the dry vegetable oil is further oxidatively polymerized with oxygen in the air to further dry. This also determines that the application of this ink is mainly for the printing of the surface of an absorbent substrate.
The content of dry vegetable oil in the fast-drying bright 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 realized by two steps of mineral oil penetration and oxidative conjunctiva of dry oil. The dry oil component contained in the auxiliary agent can promote the oxidation process of the dry oil in the ink (Principle: adsorbing the phospholipid antioxidant in the ink, and promoting the free radical, the highly active free radical The polymerization continues to occur). It is mainly used for substrates with poor surface absorption and poor absorption (such as: exquisite albums, printing of calendar paper).
Transfer process of ink from the press to the surface of the substrate
The first is that the ink is transferred from the ink fountain through the ink distribution system and the printing plate to the surface of the blanket.
High-viscosity inks are reduced in viscosity and fluidity due to constant splitting, shearing, and rubbing on the press (this is due to the thixotropy of the resin colloid in the ink*).
<The so-called thixotropic property is the property that the colloid exhibits a viscosity-lowering fluidity under the action of an external force, and the viscosity and fluidity of the colloid are restored when the external force is removed.
The ink then moves from the blanket to the surface of the substrate.
The ink is adsorbed to the surface of the substrate under the action of pressure, and the low viscosity phase (mineral oil <ink oil> and a part of vegetable oil) in the ink starts to penetrate into the inside of the paper, and the viscosity of the ink starts to rise due to the loss of the low viscosity phase, and the flow The property is reduced, the ink changes from a liquid state to a semi-solid state, and can no longer flow and transfer, which is the fixing phase of the ink.
The substrate then exits the embossed area and begins to enter the next color printing (multicolor machine).
Due to the disappearance of pressure, the ink and the surface of the substrate are relatively stationary, and the viscosity of the ink is further improved (thixotropy of the ink). The strength of the semi-solid ink film formed at this time is to ensure that the good fixation of the ink on the paper is a prerequisite for ensuring that the ink film is not damaged during the next overprinting and stacking, and that no back smudging occurs.
Finally, the stationary process of the substrate after printing on the printing press. During this process, the mineral oil remaining in the ink film of the substrate continues to penetrate into the paper (we call this penetration a free permeation), 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 the thorough phase of the ink.
The ink printed on the surface of the substrate has been completed. After understanding the process and its mechanism, we will find that it is very simple to analyze the drying problem of the ink, because almost all the problems of slow drying of the ink in printing are caused by the intervention of a certain step in the process. . Let's take a look at some of the factors that affect ink drying.
First, the effect of ink composition and adjuvant on the drying of the ink film
Dilution agent: The main component of the thinner is aluminum hydroxide and binder. The aluminum hydroxide has strong water absorption, dissociates in contact with water, promotes ink emulsification and slow drying. The drier is also adsorbed and rendered ineffective.
White oil: White oil itself is a white transparent emulsion, and the texture is easy to make the ink emulsified*.
Detachment agent: Its main component is a waxy substance (such as aluminum stearate) which has a small specific gravity and floats on the surface of the ink film, hinders the contact between the ink film and oxygen, and affects the oxidative drying of the ink.
Desiccant: Excessive use will make the ink of the ink thicker and the ink emulsified*, and the print is difficult to dry. (It will also cause the plate to be dirty and paste. The dots are enlarged and deformed, the hair is thick, and the barbed marks are difficult to dry.)
<*The water droplets dispersed in the emulsion ink will affect the drying speed of the ink, and will form an oil-in-water emulsion ink, which will affect the drying of the ink. 〉
Table 1 provides the amount of dry oil of different ink colors, for reference only.
Color Dosage of white dry oil % of dry oil
BK 5～10 2～5
M 5 2~5
C 3～4 1～3
Y 3～4 1～3
The nature of the ink: organic ink drying is slow, thin ink is slower than thicker drying, and the oil content is much slower than the oil content. The metal ions in the ink composition can promote the oxidative drying of the ink, and their ability to promote drying is:
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 iron ferrocyanide. Since they contain a large amount of metal ions which can promote the oxidative drying of the ink binder, the inks used as pigments thereof The drying speed is relatively fast. The carbon black does not have a drying effect, but the desiccant is adsorbed to delay drying. Therefore, the black ink made of carbon black as a pigment has a relatively long drying time. The tungsten molybdenum salt pigment also delays the drying of the ink.
Most organic pigments delay the drying of the ink, inhibit the oxidative polymerization of the binder, and act as an antioxidant.
(For example, the reddish pigment contains a naphthol group, and the chemical composition of the light blue has a plurality of aniline groups, so their drying is relatively slow. In production, we often use the blue light to add black ink, we must At the same time, the factors of drying are considered. In addition, the oiliness of the blue light is too heavy and is not recommended to be used alone.)
The proportion of dry oil used in modern offset printing tends to be small, the characteristics of solidification of the resin are relatively obvious, and the influence of the pigment on the drying speed of the ink is relatively reduced.
Through the above analysis, we will find that the selection of good ink in printing is the key to ensuring that the ink film on the surface of the substrate is dried in time. Also pay attention to controlling the amount of ink adjuvant used in printing. When the problem of slow drying of the ink occurs, the amount of dry oil can be increased in an appropriate amount, but it should not exceed the amount of Table 1 or it will delay the drying of the ink.
Second, the nature and structure of the substrate
1. If the printed paper is acidic, it will dry slowly. (For oxidized conjunctiva inks, if the pH is reduced from 5.6 to 2.5, the drying time of the ink is extended by four times!)
2. The surface of the paper with smooth surface, compact structure, poor permeability and small sizing degree is dry slowly. The dosage is shown in Table 1.
This is because the mineral oil in the ink cannot penetrate into the inside of the paper, which directly hinders the infiltration drying process of the ink.
Conventional printed matter (coated paper, offset paper, newsprint, book paper, gravure paper, etc.) can be generally dried by permeation drying, oxidized conjunctiva drying, and dried in combination. Printing on uncoated paper having good permeability is cost-effective, and a general offset resin ink mainly based on osmosis drying is used. When printing on coated paper, it is necessary to use bright and dry ink. Do not use ordinary offset resin ink on coated paper, otherwise the printed product will be difficult to dry.
A substrate having poor surface permeability can appropriately add a desiccant to the ink during the printing process.
<If the surface is poorly absorbed (such as PVC, gold and silver cards, glass cards), try to use UV, IR, microwave and other drying methods. 〉
3. The water content of the paper is too large: first, it affects the rate of penetration of the binder in the ink, and then the evaporation of water in the paper affects the drying of the ink.
Third, the process of printing production
1. The pH value of the dampening solution: When the pH value is low, it will undergo a displacement reaction with the metal salt in the dry oil, thereby suppressing the drying of the ink. The pH of the dampening in printing should be strictly controlled (between 4.0 and 5.6).
Table 2 gives the effect of a different pH value of the dampening solution at room temperature on the drying time of the ink:
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 70
Securities paper front 15 21 22 32 84
2. Layout moisture: In the printing process, if the moisture of the layout is too large, not only will the paper absorb too much moisture, but the water droplets dispersed in the ink will affect the drying speed of the ink, and will form an oil-in-water emulsion ink, thus affecting The blot was dried.
3. Excessive stacking of printed sheets: The area of the printed sheets is large, the surface of the paper is smooth, the paper is high, and the air is difficult to enter.
4. The printing speed is too fast: after the ink is transferred to the substrate, it can not be dried in time, and then enters the next color printing or piles up on the delivery table. Practice has proved that in the printing of sheetfed paper, if the printing speed exceeds 13,000, ordinary domestic inks are prone to defects such as sticky, mixed colors and sticky 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. Stick and mix colors. The author used two inks in the factory for comparison. One is Japanese butterfly ink, and the other is Tianjin crystal ink. It was found that the dry solidification speed of Nissan butterfly ink was significantly stronger than that of crystal ink. >
5. The influence of temperature and humidity on the environment: the temperature increases, the internal energy of the molecule increases, the speed of movement accelerates, and the dry vegetable oil produces more peroxide, accelerates the oxidative polymerization, and this oxidative polymerization is put The heat is reacted and the heat released promotes drying. The opposite is true for temperature reduction.
The experimental data of the influence of temperature and humidity of a group of environments on the drying time of the ink when the pH value of the paper changes is given below:
Table 3
Production environmental conditions Effect of different temperature and humidity conditions on oil type and drying of oxidative polymerization drying ink
Offset paper pH (RH) 65%, 18°C (RH) 75%, 20°C
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 ambient 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 constant humidity of the printing environment in actual printing.
<The ambient temperature and humidity of the workshop is preferably controlled at 20±2°C, relative humidity is 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
Too thick the ink film will reduce the contact between the middle part of the ink layer and the air. In particular, the film on the surface of the ink layer will hinder the entry of oxygen into the air, and the middle part will dry slowly.
Therefore, we must strictly prevent the following situations in printing:
(1) The imprint due to other reasons is not strong, and the thickness of the imprint is increased erroneously.
(2) The ink color caused by excessive moisture in the layout is light, but the ink color is increased by increasing the amount of ink supply.
(3) Excessive emphasis on the vividness and thickness of the imprint. Inappropriately increase the amount of ink supplied.
(4) The tinting strength of the printing ink is too low, but the thickness of the ink layer is increased to remedy.
(When dispensing a light-colored spot color ink, pay attention to the saturation of the color is slightly higher, so that the ink-ink film of the substrate can be made thinner during printing, which is good for drying. In addition, the type of ink used in formulating the ink is more Less, the less emulsification of the ink during printing, it is also conducive to drying)
The thickness of the ink layer on the surface of the substrate and the distribution of the image.
The large-area evenly distributed image is evenly consumed by the dampening solution, and the emulsification of the ink is uniform, but the image areas on both sides are directly contacted with the excess moisture of the blank portion, so that the ink on both sides is more emulsified than the middle. More serious, the drying also becomes slower. Similarly, when the area of the graphic on the lithographic plate is smaller, especially when a small area of a solid block or a small area of a dark-adjusted net, the emulsification value of the ink is also large, and the problem that the ink layer does not dry is more likely to occur. In addition to increasing the amount of desiccant in an appropriate amount, it is necessary to strictly control the amount of water in the layout.
Some solutions to the problem of printing in printing
In fact, in modern multi-color offset printing, even with good printing conditions and reasonable process, it is difficult to make the ink dry in the printing process or before the printing is completed, so we are producing The following three methods are often used to accelerate the drying of the ink film or to prevent the back of the printed matter from being dirty.
Dusting: dusting is a common method used in printing. It is a more effective means for backside smudging caused by slow drying of the ink film. When the ink layer is thick, the amount of dusting can be appropriately increased, especially the substrate. When there is a small area of dark-adjusted mesh or solid ink on the surface, it is necessary to pay attention to the appropriate increase in the amount of dust. Generally, the amount of powder to be sprayed should be as small as possible within the effective range, otherwise it will affect the ink color of the printed matter, especially in the products required for bronzing and film coating.
Drying rack: the method of layered delivery is adopted in the delivery part of the printing machine, that is, every 5~20cm (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, and the density of the substrate is large. When the layer thickness is large, the separated layer height is appropriately reduced.) The layered paper is conveyed to prevent the ink film which has not been dried from being damaged under the own weight of the paper, or the back surface is dirty.
Shake and dry: prints with poor surface absorption and thick ink film are generally dry slowly. Since the stacking of paper and the contact of oxygen in the air less hinder 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 the ink.
Finally, let us review the key points in this paper to control the slow drying of the ink film on the surface of printed matter, as well as some techniques for printing inks that are slow-drying:
1. Select the appropriate ink, 2. Use as little additive and auxiliary as possible during the printing process, 3. Use the appropriate amount of dry oil, 4. Control the pH of the plate and the pH of the dampening solution in the printing, 5. Ensure the environment Constant temperature and constant humidity are very important. 6. Control the thickness of the ink film of the graphic, 7. Spray, dry, shake and dry.
So far we have analyzed some of the reasons for the slow drying of the ink film on the printed matter and some solutions when the ink is drying slowly. I hope I can help you.