The balance of water and ink in lithographic printing is the most important relationship. If ink-ink balance is not controlled properly, it will cause various failures and seriously affect the quality of printed products. Therefore, we need to control the ink-and-wash balance as one of the key tasks of daily “three levels†work. To ensure the printing of high quality prints.
Incorrect balance of ink and ink
1. Small ink water
After the ink is diluted with a varnish, the thickness of the ink is thinned, the flowability is increased, and the ink performance is changed. (1) Increased ink polarity. The increase in the content of the varnish increases the polarity of the ink and decreases the sense of fat. It is easy to form greasy or dirty on the blank part of the printing plate or dots, and the ink on the plate is out of balance. (2) Decrease in ink resistance to water. As the water resistance of the ink is reduced, most of the water used for printing on the plate is absorbed by the ink, causing insufficient water in the plate to cause ink and ink imbalance. (3) Ink cohesion decreases. The ink flowability increases, the cohesion force decreases, and the dots increase under the effect of the roller pressure, the dots dot the bar, and the stencils merge.
2. Ink thick water
Generally, the original ink has a large viscosity and small fluidity, and the amount of printing ink is reduced. However, this can easily cause the following problems: (1) The polarity of the ink is small. The original ink has small polarity and is more suitable for process requirements. It is not suitable to blindly dilute, but it must be kept small in polarity and it is not easy to flow and spread. (2) The ink has high water resistance. The original ink has high water resistance, high tension, and reduced emulsification level. The water supplied to the layout is not easily absorbed by the ink, and the amount of water used is sufficient to wet the plate and balance it. (3) Ink cohesion. After the printing plate is inked, the graphic ink is affected by the cohesive force and the leveling property is small, the dot is not easy to increase, and the blank portion can resist the erosion of the ink and keep balance with less water.
3. Ink water small
Graphic structure is different, and its ink load is also different. Under normal temperature conditions, the amount of ink on the layout graphic is large and the fluidity becomes large. Therefore, the amount of water must be increased accordingly during printing. However, under the influence of mechanical force, part of the water emulsified layout, resulting in insufficient water to wet the layout and cause ink and water imbalance.
4. Small ink water
In the printing process, the high-profile part of the layout, the use of ink is small, thin ink layer, then the amount of water must be reduced accordingly, otherwise it will cause a large imbalance of ink.
Ink emulsification analysis
Under the pressure of a certain mechanical force in the printing, water is dispersed in the ink in fine droplets, showing an emulsion, so that the ink is emulsified. At this time, the amount of water can not be too large, otherwise it will significantly reduce the quality of the printed matter and give the process Bring a series of difficulties and even make printing impossible.
1. Quality problems caused by ink emulsification
(1) Dot distortion, emulsified ink coloring power and saturation decrease, color fades, and increased ink consumption will cause network consolidation and paste, unclear level. (2) Inconsistent printing ink color, resulting in changes in the density of the field value and color error. (3) The color balance was destroyed. Since the emulsified ink contains many fine droplets, the gray scale increases, the dots are juxtaposed, the color shift occurs after overlapping, and the ink is dull and dull. (4) The printing plate is prone to appear the flower version, lose the edition, floats the dirty, pastes the edition and so on. (5) The drying speed of the ink layer is slowed down and it is easy to make the back of the printed product sticky.
2. Factors that cause ink emulsification
Many factors in the printing affect the ink balance and emulsify the ink.
(1) Ink properties. 1 ink viscosity. This is the main factor for emulsification. The viscosity is large and the ink resistance is large. Adding No. 0 varnish to the ink can reduce emulsification. The ink has low viscosity, increased fluidity, reduced water resistance, and easy emulsification, but it is not allowed. Add any No. 6 varnish to reduce the viscosity. 2 pigment properties. Pigments are hydrophilic and hydrophobic, and their emulsification mainly depends on the wettability of pigments by oil and water. If carbon black is hydrophobic, black ink is not easy to emulsify, but blue. Light red and other ink easy to emulsify. 3 ink acid value. The ink is polar and is often measured as the acid value of the binder. The higher the acid number, the thicker the fountain solution is needed and the easier it is to emulsify.
(2) The paint and the glue in the printing paper are water-soluble, and they will be emulsified when they reach the ink fountain.
(3) The gum arabic in the fountain solution emulsifies the ink at the oil-water interface to form a stable emulsion.
(4) The layout water consumption is too large and the emulsification is caused by the pressure during printing.
(5) Dilute agents and desiccants should not be used excessively, and they should be avoided or reduced as much as possible. Otherwise, ink emulsification may easily occur.
(6) Increased speed of the press will accelerate ink emulsification.
(7) The degree of emulsification increases with increasing humidity and decreases with decreasing humidity.
How to identify the size of the layout moisture
1. Influencing factors of printing plate water volume
There are many factors that affect the moisture content of the printing plate, but the general principle is that the amount of water used must be based on the amount of ink contained in the plate graphic.
(1) Distribution of layout area. Large picture area, large amount of ink, large amount of water, and vice versa, small; dense graphic distribution, large amount of ink, large water consumption, and vice versa; It is necessary to conform to the law of "great water, big water, small ink and small water".
(2) Paper properties. When the surface strength, tightness, smoothness, and sizing degree of the paper are changed, the amount of water supplied on the layout should be controlled and adjusted according to the changes in these properties. Under normal circumstances, the surface strength of the paper is high and the smoothness is high. If the degree is high, then water is small, and vice versa.
(3) The nature of the ink. Ink viscosity is large, fluidity is small, water consumption is small, otherwise large; emulsified value is large, water consumption is large, and vice versa is small; pigment dispersion is high (small particles), water consumption is small, otherwise large; resin type The ink has a large water resistance, is not easily emulsified, and has a small amount of water; the oil type ink has a low water resistance, is easily emulsified, and has a large amount of water.
(4) workshop temperature and humidity. The workshop temperature is high, the water evaporates quickly, and the water consumption is large. On the contrary, it is small; the humidity in the workshop is large, the moisture is not easy to evaporate, and the water consumption is small, and vice versa.
(5) The speed of the printing press is inversely proportional to the page layout water consumption.
(6) The printing plate is good, the sand spot is good, and more water is stored, so the water consumption is larger. Therefore, the quality of the printing plate is slightly larger than that of the old printing plate.
2. How to identify the size of layout water
At present, there is no instrument to measure the size of water in the layout, generally based on practical experience to observe and determine the amount of water in the layout, specific methods:
(1) Visual inspection. After observing the brightness of the layout, after the surface of the plate is ground to sand, the surface of the plate is not bright, but the water film is formed after being wetted by water, and the brightness will be bright. Generally, the greater the brightness of the plate surface, the stronger the brightness. When observing the bright reflection of water in the layout, it should maintain an appropriate angle with the printing plate, so that the incident angle of light is equal to the reflection angle. At the same time, attention must be paid to the following issues: (1) The ratio of the size of the layout image and the blank area, the intensity of the light source, and the observation position Affect the brightness of the layout. (2) The water reflectance of the new printing plate is small, but with the increase of the printing quantity, the reflection of the water film will gradually increase. (3) The surface of the general coarse sand printing plate can be lead gray, and the surface of the fine sand printing plate can be of medium brightness.
(2) There are fine water droplets in the ink fountain. The surface of the ink roller indicates that the amount of water is too large.
(3) When ink is added to the ink roller, the ink is not easily added due to the presence of water on the ink roller. When the ink roller is applied with ink, slippage occurs, indicating that the amount of water is too large.
(4) When the water consumption of the layout is large, the softness and weakness of the printed sheets after the printed sheets are printed, and the jitter sound is very small. When the paper is delivered, the curling of the four sides is not neat.
(5) There are wavy water traces in the mouth of the blot, and the ink is colored. This proves that the amount of water is too large.
(6) When the amount of water in the layout is too large, the difference between the ink and the ink before and after the interruption of the printing is large, and a large amount of ink is not uniform. After printing is interrupted for a period of time, water remains on the layout.
(7) When the amount of water in the layout is too large, there is a trace of water on the tip of the blanket, and when it is severe, there is a small drop of water.
To sum up, we only strictly follow the above process requirements when printing, strictly control the amount of water. To achieve a good ink balance, you can print high quality prints.
Water balance conditions
After understanding various influencing factors, we summed up the conditions for ink and water balance.
(1) The grain on the surface of the printing plate is uniform and dense, and it must have a certain mechanical strength, so that the graphic part and the blank part have a firm adsorption force, that is, the graphic part is kept lipophilic. The hydrophilicity of the blank part is stable.
(2) Strictly control the temperature of the workshop. humidity. Because temperature changes can cause ink viscosity, fluidity changes. An increase in temperature will accelerate the evaporation of water and cause the ink and water to become unbalanced. It is best to control the workshop temperature at 22-25°C and the humidity at 55%-65%.
(3) Use of ink and paper. Different inks. Paper has different requirements for ink-ink balance, so it is necessary to grasp their nature changes under the same conditions.
(4) Diluents and desiccants will change some properties of the ink, so it is necessary to control its amount.
(5) The pH of the fountain solution is one of the main factors affecting the ink-water balance. Therefore, it is necessary to maintain the pH value stable and not to have a wide range of fluctuations.
(6) When printing, the roller pressure, ink roller, and water roller pressure must be accurately adjusted. At the same time, normal speed and constant speed are maintained during operation, and the machine can quickly and easily degrade the ink and ink.