Discussion on the suitability of flexible letterpress and lithographic printing inks
We are a big printing company in Shenzhen China . We offer all book publications, hardcover book printing, papercover book printing, hardcover notebook, sprial book printing, saddle stiching book printing, booklet printing,packaging box, calendars, all kinds of PVC, product brochures, notes, Children's book, stickers, all kinds of special paper color printing products, game cardand so on.
For more information please visit
http://www.joyful-printing.com. ENG only
http://www.joyful-printing.net
http://www.joyful-printing.org
email: info@joyful-printing.net
When we study printability, there are many aspects to study from the nature of the ink, that is, to study the printability of the ink. Because China's printing industry and the ink industry belong to different departments, the relationship is relatively distant. Ink workers rarely explain the nature of the ink to the printing workers in detail, which makes many printing suitability problems not well studied. Now briefly introduce the composition of the ink, and then discuss the relationship between the properties of the ink and printing. For everyone to study some printability, the content can not be exhaustive, please forgive.
First, the composition of the ink
The pigment (coloring material), the filler, the binder and the auxiliary agent are mixed and ground to uniformly and finely distribute the pigment, the filler and the auxiliary agent in the binder, and become a colloidal dispersion system, and the appearance is a uniform and delicate fluid. Or paste, this is the ink.
Due to the different printing processes, inks are required to have different properties. If the gravure ink is very thin, the viscosity is also small, otherwise it cannot be filled in the recess of the plate. It can't be scraped off by the scraper. Most of the ink for printing is used for the letterpress wheel transfer machine. The printing speed is as fast as 10,000 marks/hour. The paper used is the news paper with better absorption, so the ink of the printing ink is only thicker than the gravure ink. And the osmotic drying is very good. Otherwise it will not meet the requirements of high speed printing and rapid drying. The letterpress rotary press of the printed book is slower, the paper used is firmer and the absorption is poor. Therefore, the ink of the printed book is thicker and thicker than the ink, and the osmotic drying is slightly slower, and some even have the property of drying a part of the oxidized aggregate film. Make the writings of the books and magazines can be printed more clearly, and it is not easy to wipe them off after drying. Color letterpress printing is mostly used for printing trademarks, packaging and other printings, and the printing speed is slower. The paper used is from letterpress paper, offset paper, coated paper to glass powder coated paper. The printed matter is required to have vivid colors, clear graphics, solid letterpress ink, or color lead ink, and other ink (including copper-zinc ink). The printed ink is much thicker and more sticky, otherwise the dot printing is not clear. Because the printing speed is slower and the paper absorbability is poor, the solid-state oxidative polymerization of the conjunctiva is dry. In addition, it must be bright and have a certain luster. As for printing plates, coated papers and the like, generally, offset inks with higher color density are used, and even the bright light fast-fixing offset inks are used for bright colors and excellent gloss. Offset printing uses the oil-water mutual exclusion principle to print, so the offset ink must have a certain water resistance, and should not be heavily emulsified on the printing machine and bleed in the syrup. Because the offset printing speed is now faster than the embossing, and the rubber transfer, the viscosity of the ink is not too high, so the thickness is thicker than the color lead ink, and the viscosity is small. The papers used are both offset paper and coated paper, so the oxidative polymerized conjunctiva is also dried. Some require that the back side is not dirty, so it is necessary to use fast fixing ink. If the gloss is required to be excellent, it is more necessary to use a bright light to fix the ink.
The nature of the ink is actually determined by its composition, such as ink color, tinting strength, light resistance, acid resistance, alkali resistance, soap resistance, heat resistance, resistance to various solvents and water resistance, mainly determined by the pigment. The properties such as drying property, fast fixing property, glossiness, abrasion resistance, and flex resistance of the ink are mainly determined by the binder. The properties of the viscosity, viscosity, fluidity, etc. of the ink are largely related to the binder, but also to the nature and amount of the pigments and fillers used. Therefore, ink workers should understand the printing process and the properties of the substrate before designing, in order to design and manufacture the ink that meets the requirements. The printing worker should also have a certain degree of understanding of the nature of the ink in order to use the ink reasonably.
Second, the relationship between various properties of ink and printing
1. Color tinting strength and transparency
The color of the ink, especially the color of the three primary inks, is often measured in terms of density, chromatic aberration, grayscale, and actual color efficiency. It is usually measured and calculated using a color density meter in the field or color code portion of the printed matter or proof. Got it. However, if the measurement is made from the quality of the ink, a certain amount of ink must be taken according to the international standard method, and the color block is printed on the specified coated paper with a certain pressure and printing area by the IGT adaptability tester, and then the color density meter is used. Make measurements and calculations. Otherwise these valid data are not standard and contrast.
For the three primary color inks, the color density should be within a certain range, usually 1.00 ± 0.05 for yellow, 1.45 ± 0.05 for magenta, and 1.55 ± 0.50 for green. However, due to the different requirements of the original color tone, it may be slightly different. When the density of the three primary colors is within the above range, gray balance can generally be achieved, and thus the calculated color difference, gray scale and actual color efficiency are also more practical. The standard color ink that conforms to the ideal of the three primary colors in the world does not exist. However, the chromatic aberration should be as small as possible, which is more conducive to the gray balance and the reproduction of the original color. The gray scale is small, that is, the brightness is high. When the color of the original is very bright, the three primary colors with large gray scale cannot reproduce the vivid original color. As for the picture or some of the originals whose shades are relatively dark, it is not necessary to require the gray of the three primary colors to be small when printing, and sometimes it is necessary to increase the gray scale or the background color or the spot color. The actual color efficiency refers to the purity of the main color, and the higher the color efficiency, the larger the range of the pure color with the other two three primary inks. In general, an ink having a small color shift and a small gray scale has a large actual color efficiency. To measure the color quality of the three primary color inks, generally, the color should be small, the gray scale is small, and the actual color efficiency is good.
The tinting strength of the ink is also known as the color concentration, the ink with a large tinting strength, the smaller the amount of ink used to achieve a certain color density of the printing, and the ink with a small tinting force requires a large amount of ink. It brings disadvantages to the printing process, such as dot enlargement, paste and even dirty. However, it should be noted that the gray color of the ink with large tinting strength is also large, which affects the vividness of the color. Therefore, the tinting strength of the three primary color inks should be equivalent. When the tinting strength of one of the colors is too large or too small, the gray balance and color reproduction will be affected.
The transparency of the ink is also very important. The transparency of the three primary inks must be above 90% in order to better represent the intermediate color and discoloration. For example, in the past, the transparency of yellow was not good, at around 85%. At that time, the general printing color sequence was mostly yellow, yellow, black and black. Yellow ink is the base, opaque is not good, so no problem occurs. Recently, most of them are black, green, yellow, yellow, and yellow ink. When the transparency is not enough, the yellow plate is heavier, and the thicker ink layer will make the whole picture have a faint color.
In the packaging and printing, the background color is often used to highlight and highlight the theme, usually red, ochre, orange, orange, light yellow, light blue, lake blue, grass green, light green, brown, brown, beige, snow blue, Light gray and other colors, which have an understanding of the color properties of the ink in order to get the desired color. The following points should be noted.
① The color matching of the ink is the color reduction effect. The more the color is mixed, the darker it is. Therefore, the color should be used as little as possible. First, use two colors to adjust. After adjusting to a fairly close color, add a third color. Minor adjustments to achieve the desired color. Because the background color is often light color, and finally add the rinse ink to dim, you can make a small amount of flushing first, and think that it is necessary to re-amplify, so as to avoid too much waste and waste, especially after diluting, there is still a certain gap, often Adding and splicing together, the amount of spelling too much will become more and more, and more than the required amount. The color ink supplied by the ink factory has many intermediate colors and discoloration in addition to the three primary colors. When adjusting the ink, it is necessary to adopt two kinds of close tones to mix and match, so as to achieve the requirement that the color tone is relatively clear and not too dark. Below is a color circle diagram, which arranges the names of the inks of each color. The colors of the two similar colors are more vivid, and the distance is far away. If you want to mix fresh grass green, you can use lemon yellow (light yellow) and green. When you use medium yellow and medium blue, the emerald green is very dark.
② When the color is dilute, the color of the ink that is transparent and dilute is darker, but it is not dark when printed on white paper. When you use white ink to dilute, the ink looks brighter, but with pink. It is required to have a light color without pink and is brighter. Some people often add a small amount of white ink when using a large amount of transparent and dilute ink. Although the color tone is relatively bright, the white ink has an ink with poor light resistance, which is more likely to fade. Disadvantages, must pay attention. Therefore, it is best not to add white ink when the paper with better whiteness is printed. For paper with poor whiteness, it is a different matter.
③ The flesh color is also often used to dilute the gold red ink, although the color tone is good, but the gold red is very easy to fade after diluting, so it is not appropriate. After the pink ink is diluted, it is also very easy to fade. Dilute with 05-06 red or 05-06 deep red, although the color is slightly darker, the fading situation is much better. It is considered that when the yellow tone is not enough, the orange ink can be slightly added, and the ink has good light fastness and is not easy to fade. Lightening blue lake blue should be diluted with sky blue ink, do not use medium blue or dark blue, because it is easy to fade. Green ink and Qinglian ink have two kinds of light resistance and light resistance. When light color, it is also good to use light, otherwise it is easy to fade. The yellow ink is now lightfaster than the sky blue ink and the green ink. Therefore, when adjusting the green color, it should be considered that the yellow color will fade first, and the printed matter of the display window should be paid attention to.
④ packaging printing sometimes uses aluminum foil paper or thin iron sheet, which often needs to consider the transparency of the ink in the background color or pattern printing. The effect of opaque yellow ink and transparent yellow ink on aluminum foil or iron sheet is completely different. A lot of candy-wrapped aluminum foil or tin box, first printed with a layer of transparent yellow ink, can make the metallic luster reflect through the yellow ink layer to become a gold color. The yellow ink with low transparency is very ineffective. The high-transparent pink or magenta ink, sky blue ink and green ink are used to make the aluminum foil paper or metal box show a very beautiful and bright color. Sometimes it is made with white ink, which is completely required to cover the metallic luster of aluminum foil or iron, and then printed like a pattern printed on white paper.
2. Fineness
The fineness of the ink is measured by a squeegee fineness meter, which indicates how large the diameter of the largest solid particles in the ink is. Generally, the fineness of the letterpress ink is 20 micrometers or less, and the offset ink is 15 micrometers or less. This data is actually the diameter of a solid surface coated with a thick layer of oil. The true solid is only half the diameter, ie the relief base is below 10 microns and the offset ink is below 7.5 microns. The so-called maximum particle counts the total number of particles of the total solid particles of the ink, which is actually very small, and the particles of most inks are much smaller. Even at 15 micrometers, the area of the printed dots is comparable to the area of the ink particles. A dot can accommodate a large number of ink particles. See Table 1 for details. The 250-line 1-point area listed in the table is also larger than the 15 micron ink particles. The area is 6 times larger, and the 4% point is 23 times larger. Moreover, most of the ink particles are much smaller than 15 microns, so the qualified inks are generally not affected by the fineness problem.
The problem was that the printing factory reflected the poor fineness of the ink and affected the quality of the printed matter. On the spot, except that the ink was thickened due to the accumulation of particles in the long-term storage, or improper storage and use, the surface of the ink was crusted into it. Most of the paper is powdered and detached into the ink to cause the ink to become thick.
3. Rheological properties of the ink
Offset or embossed inks are all sticky and thick pastes. They are physically called plastic fluids. They do not deform or flow like external fluids like water. They must be applied to a certain extent before they begin to deform. Or flow. As shown in Figure 2, when the external force is applied, the ink does not change, and the rheology starts when the force is applied to point A. The flow deformation at this time is a situation of near solid motion, called plug flow. That is, the relationship between the force and the rheology represented by the curve AC' in the figure. When the force is increased to C', the rheology of the ink flows in a laminar flow like the ideal fluid, and the rheological curve is straight. The force A that causes the ink to begin to flow is called the minimum yield value, and the force C that causes the ink to start laminar flow is called the maximum yield value. The linear extension above the C' point and the force axis are changed to point B, and the force at point B is called the ideal yield value. A preliminary understanding of the rheological properties of the plastic fluid of the ink can be studied in relation to the printing suitability requirements of the printing process.
① viscosity, yield value and thixotropy and its determination
The relief and offset inks are viscous, thick plastic fluids. To determine the internal resistance generated during flow, the so-called viscosity, first overcome the internal stress equivalent to the minimum yield value, so Ford Cup (paint cup) or falling ball Viscometers are not suitable. A typical rotary viscometer, a drop bar viscometer or a parallel plate viscometer is used to determine the viscosity and yield value, or a rheological curve is drawn to study its rheological position and thixotropy.
●Rotary viscometer
Commonly used are the Brukerfeld viscometer and the Amira viscometer, the latter being used in China. The structure and principle are that the ink is placed in a small cylinder which can be insulated, and a rotor is inserted into the ink. When the rotor rotates, the spring of the connecting rotor is deflected by the viscosity of the ink, and the pointer can be directly read. . By changing the cylinder and the rotor and changing the rotational speed by the gearbox, the viscosity of various inks from 0 to 1.000.000 centipoise can be measured. The structural principle of the Bruker Felt viscometer is basically the same as that of the Emily Viscometer, but it can change a lot of speeds step by step, from the lowest speed stepwise to the selected one of the highest speeds to determine the series of relief and offset inks. The data pair of shear rate and shear stress is depicted as an upwardly curved curve. A series of shear rate and shear stress data pairs are then reduced from the highest speed to the lowest speed to become a straight line. As shown in Figure 3. The size of the area enclosed by the middle of the two lines indicates the size of the contact. In fact, this is because the pigment particles in the ink attract each other and condense in the static state, so they are in a thick and condensed state. When subjected to mechanical agitation, the flocculation force is destroyed, and a thin and easy-flowing state is obtained. But after a period of inactivity, it will flocculate again. This is the touch.
●Dropstick viscometer (Rare viscometer)
The main structure is a horizontal short cylinder mounted on a vertical bracket, hollow through, inserting a long round bar in the empty cylinder, a certain gap between the round rod and the cylinder, adding a certain amount of ink to the gap, so that the round rod and the circle The middle of the cylinder is filled with ink, and weights of different weights are placed on the round bar. The time required for the round bar to fall freely by 10 cm was measured. Because of the resistance of the thin layer of ink when the round bar falls, and because of the different weights on the round bar, the different time required to indicate the shear rate of the ink, the flow curve of the ink can be drawn accordingly and calculated Theoretical yield value and plastic viscosity.
●Parallel plate viscometer
Between the two plates placed horizontally, a certain amount of ink is placed, and the upper plate has a predetermined weight, which can be horizontally freely dropped at a fixed height and pressed on the ink for the ink to be circular in two parallel The middle of the board is expanded. The diameter of the ink spread becomes larger as time increases, and the diameter of the ink spread is recorded at different times such as 10 seconds, 60 seconds, 100 seconds, ... 30 minutes, and the slope, intercept, and yield value of the ink rheology curve can be calculated.
② viscosity and its determination
When the ink is running on the ink roller of the printing press, it is torn by the ink roller which rotates in the opposite direction. The resistance generated at this time is called the viscosity of the ink, which is the comprehensive force after the cohesion and adhesion of the ink are balanced with each other. Performance. Unlike the viscosity of the above ink, there is no absolute unit. The viscous index is determined by an ink meter (ink viscous meter). Its main structure is three ink rollers and a connected weight and balance 抒. When a certain amount of ink runs between three ink rollers at a certain temperature, the ink layer is The resistance to tearing is transmitted to the balance bar, and the weight is read by balancing the weight to obtain a viscous reading. The use of viscosity to study the transfer of ink on a printing press is more practical than the viscosity, so it has been valued by the ink and printing industry in the world.
③ fluidity and its determination
0.1 ml of ink was subjected to a total weight of 250 g in two round glass sheets having a diameter of about 70 mm at a temperature of 25 ° C. After 15 minutes, the ink was flattened to expand into a circular diameter of millimeters. This is a comprehensive expression of the thickness, softness and hardness of the ink and the yield value.
④ relationship between various rheology of ink and printing adaptability
Because the relief and offset inks are a plastic fluid with complex rheology, the structure and performance of various presses have different requirements for inks, so the relationship between the rheology of these various inks and printability It is also interrelated and very complicated, and only a rough introduction can be made here.
Viscosity is not as obvious and strict as printing for flexographic and offset inks, as is the case with flexographic inks and gravure inks. However, there is a general rule that printing speed is slow and inks with higher viscosity can be used; printing speed is fast. The viscosity of the ink used is lower. For example, the transfer of ink on the wheel is much smaller than the viscosity of the printed ink. The offset ink used in the high-speed four-color offset press is less viscous than the offset ink used in a typical printing speed monochrome or two-color offset press. However, the viscosity of the ink becomes smaller as the temperature increases. After the high-speed offset printing machine is opened, the speed of the ink roller is high, the viscosity of the ink will gradually become smaller, so the viscosity is slightly higher. It does not matter whether the substrate has a viscosity requirement for the ink. The smooth and firm paper can be used higher. Viscosity inks, loosely structured papers use less viscous inks, otherwise it is easy to drop powder and remove hair.
Viscosity and viscosity are related, that is, the viscosity of a large ink is generally high. However, there are also irrelevant times, so some inks have similar viscosity but different viscosities. Some inks have similar viscosities and different viscosities. In this case, the ink should be selected based on the viscosity. For example, the color offset ink is more suitable in the country of 5-9. The applicable range of the color printing ink viscosity is 5-11. The rotary offset printing ink has a printing speed of 3-6 because the printing speed is much faster. The viscosity of the ink has a great relationship with the printing speed. The separation speed of the fast ink layer is also fast, and the viscosity of the ink is also large, which is the main reason for the particularly low viscosity of the rotary offset printing ink. The relationship between viscosity and ink transfer is relatively large. In the applicable range, the ink with high viscosity is better, and the ink layer printed on the paper is thicker, and the dots are more realistic. However, if the viscosity is too large, the ink roller may not be evenly spread or even the stacking roller or the pile rubber may be piled up. At this time, the dots of the printing are also hairy and unreal. When the viscosity is too small, the ink transfer is poor, and finally the ink layer printed on the paper is thin, and the dot is easily enlarged. The viscosity of the ink has a lot to do with the paper. For example, the same viscous ink is printed on the offset printing machine. The relatively solid imported coated paper can be printed very well. If the domestically produced coated paper is used, the powder will be dropped or even peeled. It is the paper that cannot withstand the stickiness of the ink layer when it is separated from the paper. Viscosity is related to temperature. When the temperature is high, the viscosity is lowered. The offset printing machine stops after the printing stops, and the paper is fluffed and peeled. It is because the car is cold and the viscosity of the ink is increased because of the pause. First print some paper with better paper, and the car will be normal after a while. On four- or more-color offset presses, the viscous relationship of the four-color inks is also important. In general, the viscosity of the latter ink cannot be greater than the previous color, otherwise the ink of the previous color on the print will be pulled down.
The yield value and fluidity are related to the "body bone" of the ink. The so-called "bone" ink is often a large yield value. The ink surface with a large yield value looks harder on the ink surface and is drawn on the ink roller. Shorter, the measured fluidity is smaller. In the appropriate range, the ink with a small yield value and a small fluidity is better to be printed on a embossing or offset printing machine. Since the drawing is short, the flying ink phenomenon is less, the ink body has a harder fluidity, and the dots on the printing material are smaller. thick. When it is too large and out of range, the ink transfer is poor, and the ink roller is not evenly smeared, and sometimes the ink is not inked.
The thixotropy ink, due to the flocculation of the pigment particles, can prevent the ink from flowing due to flocculation during a short period of time. This is often caused by the ink in the embossing and offset printing fountains, resulting in a lighter and weaker printing. The main reason. But the thixotropy ink, the printed dots are thicker, so it is not as small as possible. As for the scope of large or small, there is no data published in the general ink industry.
4. Fixation and dryness of the ink
After the resin ink is printed on the paper, the low-viscosity solvent and the dry vegetable oil can quickly penetrate into the pores on the surface of the paper, so that the synthetic resin and the dry vegetable oil-coated pigment remain on the surface of the paper and solidify. This state is called dry. solid. At this time, a small amount of pressure on the printed matter will not stick to the back, but the pressure will be too thick. The ink layer in the fixed state is further oxidized and polymerized to form a strong film, which is true drying. Generally, the resin ink has two stages of fixation and complete drying, and the fast fixing is called fast fixing ink. Fixation is actually measured in seconds, but due to the relationship between test methods and instruments, it is currently calculated in cents. Dryness is comparable to the time the print is completely dry, in hours.
Fast-fixing inks have a significant effect on coated or coated paper because the pores on the surface of the coating are very small and numerous. It can be in a fixed state by inhaling an appropriate amount of solvent in a very short time. Generally, the pores on the surface of the offset paper are coarse and small, and a certain amount of solvent cannot be inhaled in a short time to fix the ink, so the effect is not good. The fixing state of the ink is not really dry, so only a small amount of the printed matter can be stacked and cannot be moved by hand, because the excessive pressure is too large, or the partial pressure is excessively pressed by the hand, and it is only fixed. The ink layer will still be destroyed and the back side will be colored. The ink printed on the paper, whether fixed or not, acts on the unsaturated double bonds of the dry vegetable oil due to the oxygen in the air, first oxygen, then polymerization, gradually increasing the molecular weight and finally becoming a strong film, this is the real dry. Therefore, the conditions for drying the ink, one is the need for oxygen in the air, and the other is the heat or chemical accelerator that causes or promotes the oxidative polymerization, that is, the dry oil.
Third, the factors affecting the drying of oxidative polymerization ink
The mechanism of oxidative polymerization is more complicated. Due to the fact that the length cannot be detailed, the relevant points in the printing process and printing quality are discussed as follows:
1. The role of desiccant and the point of use
The desiccant generally used for inks is a salt of a metal such as lead, cobalt or manganese, and its function is to shorten the induction period of oxidative polymerization, accelerate the combination of oxygen and dry oil, and accelerate oxidation and polymerization. The lead salt promotes the oxidative polymerization to be slow, and the bottom and surface of the ink layer are almost simultaneously dried. As a support for oxygen, cobalt salt promotes the oxidation of dry oil. The iron ink is often used as an example. The ink layer on the iron often forms a film on the surface and the inside does not harden. Generally speaking, The efficiency of promoting drying is five or six times greater than that of lead salt. The role and efficiency of the manganese salt is between the two. The red dry oil on the market is a reddish brown poly-like fluid dominated by cobalt ruthenate and cobalt rosinate. White dry oil is a white paste containing three salts of lead, cobalt and manganese. In general, the drying effect of red dry oil is several times faster than that of white dry oil, but the surface is dry quickly and the internal drying is slow. Although the dry oil is less efficient, the drying time of the bottom surface is relatively close. The amount of red dry oil is about 1%-3% of the weight of the ink, and the amount of white dry oil is about 3%-15%. If too much dry oil is added, one drying speed does not increase equal to waste; the second dry oil is Polar substances, easy to cause ink emulsification, the disadvantage of paste on the offset printing.
2. Effect of humidity on drying
The water vapor in the air will adsorb on the surface of the paper and the ink layer, preventing the contact of oxygen and ink, hindering the oxidative polymerization of the ink, and slowing the drying. When the relative humidity is 85% and 65%, the drying speed of the ink layer of the printing will be doubled. When the humidity is as high as 95% or more, the difference is 2 to 3 times.
3. The effect of temperature
Heat is one of the conditions for initiating and promoting oxidative polymerization, and the oxidative polymerization of dry oil itself is also an exothermic reaction. Therefore, the temperature has a great influence on the drying of the ink. When the temperature is 25 ° C, the drying speed can be doubled compared with that at 15 ° C.
4. Effect of offset dampening solution
Moisture can delay the drying of the ink, and the acidic substance in the dampening solution can react with the metal salt in the desiccant to reduce the driering effect. Therefore, the pH value of the dampening solution is less than 5.5, and the drop is small. When it is below 4.5, the drying time will be doubled. When the pH value is below 3.5, and the water amount is large, the emulsification of the ink is serious, which may cause the ink to not dry for a long time.
5. Dryness and paper relationship
The surface structure of the paper is related to the fixation of the ink and has a great relationship with the drying of the conjunctiva. Paper that absorbs too much ink will also draw in a large amount of dry oil and synthetic resin from the ink. There is not enough dry oil and resin on the surface of the ink to form a strong film. Therefore, heavier touch or rubbing will be wiped off, which is called pulverization on printing. Due to the different manufacturing processes, the paper also has different PH values. The alkaline one is beneficial to the drying of the ink, and the acidic one is not conducive to the drying of the ink.
6. The relationship between printing process and ink drying
In the packaging and printing, the spot color is often printed first to highlight the theme pattern. In order to achieve the purpose of bright and bright background, the ink layer is often printed thick or printed twice, and the dry oil is also added more. Since the ink layer completely covers the paper and the drying is too fast, the surface of the ink layer is often made into a very smooth film layer, like glass, so that the second color is not printed, even if the printing is not firmly adhered, this phenomenon is called The ink film is crystallized or vitrified. The ink film is both crystallized and printed with a second color. In principle, there are two ways: one is to enhance the adhesion of the second color ink, and the other is to destroy the crystallized ink film. Alkyd resin, epoxy resin and polyamide resin are the materials with high adhesion. It can enhance the adhesion by adding ink, but these resins are not miscible with the ink, can not be added more, and at the same time add more and dilute The color of the ink, so the adhesion is still not big enough, the effect is not good. Adding an organic solvent to the ink to dissolve the crystallized ink film is a method tried by the printing worker, but the crystallized ink layer has become a three-dimensional network structure, and the smooth film layer is not easily dissolved, so the effect is not good. . Some add alkali or soap to the ink to enhance the polarity, making it easier to print, but at the time it seems to be printed, but it is not easy to wipe off after drying. Only a large area of the cover is printed with a layer of clear ink or zero varnish, and the effect is better when it is dried and the second color ink is printed. However, this method is time-consuming and labor-intensive, and the cost is increased. It is preferable to grasp the dryness of the ink in printing so as not to overprint the ink to print the second color.
It is also possible to add a TM-27 coupling agent to the ink system or to add a small amount of adhesion promoter to solve the adhesion fastness problem.