Dirty phenomenon and prevention on the back of printing
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When printing the dark part of a color product and printing a large area on the spot with a spot color, the back side is dirty due to the thicker ink layer, which is extremely prone to failure, ink viscosity, pH, thickness of the ink layer, and roughness and penetration of the paper surface. If there is a problem with sex, it will cause the back to be dirty.
In offset printing, the ink of the previous sheet is not dried, and the latter sheet is superimposed thereon, and the ink of the previous sheet may be partially transferred to the back of the latter sheet, which is called back smear. The dirty back will not only make the ink on the back of the printed matter, but also cause the front image to be spotted, which will seriously affect the quality of the product and even cause the whole batch to be scrapped. Especially when printing the dark-tone part of a color product and printing a large-area field with a spot color, since the ink layer is thick and the back side is dirty, it is extremely easy to malfunction, so care must be taken.
In essence, the smear on the back of the printed product is caused by the adhesion of the ink on the surface of the previous printed ink film to the back of the latter printed sheet. This adhesion is the intermolecular between the ink on the surface of the previous printed ink film and the subsequent printed sheet. Secondary force fb. The surface ink is also affected by the intermolecular secondary force fa of the front side of the previous sheet and the cohesive force fc of the ink itself. Only when fb≥fa+fc, the back side smear occurs.
Since the intermolecular secondary force will decrease sharply with increasing distance, fa is mainly related to the thickness of the ink film, which will drastically decrease as the thickness of the ink layer increases. Fb is related to the distance between the surface layer of the ink film and the latter sheet. When the thickness of the ink layer is determined, fb is only related to the distance between two adjacent sheets, and sharply decreases as the distance increases. Fc depends on the nature of the ink and the drying of the ink. Because the cohesive force rises sharply as the ink dries and solidifies. Therefore, the fundamental solution to the back smudging is to reduce the thickness of the ink layer, increase the distance between the sheets, improve the performance of the ink, and speed up the drying of the ink on the paper. From the above, it is discussed how to prevent the backside of the printed matter from being smeared in the printing.
First, the performance of the ink
Different types and colors of inks have different binders, pigments and proportions, drying methods, and drying speeds vary greatly. In addition, the ink with large thixotropy will decrease in viscosity during the printing process due to the shearing action between the ink rollers, and the viscosity will rise rapidly after the transfer is completed. Since viscosity is an external manifestation of cohesion, an increase in viscosity indicates an increase in cohesive force, which is conducive to preventing smudging on the back of printed matter. Therefore, when printing color prints without background color removal and large-area on-site, inks with fast drying and high thixotropy should be used as much as possible.
In order to adjust the printability of the ink, it is sometimes necessary to add a detackifier to reduce the viscosity of the ink. The viscosity is reduced, that is, the cohesive force is reduced, and the back surface is easily dirty. Therefore, it is necessary to strictly control the amount of the debonding agent when printing the coating paper which is easy to produce the back surface, and control the viscosity of the ink.
Appropriate addition of drier can significantly accelerate the drying of the ink, but the drier is also an emulsifier, which will promote the emulsification of the ink. Excessive addition of the drier will delay the drying of the ink.
Second, the ph value and dosage of dampening solution
When the printing is prone to large areas of the back surface, the thickness of the ink layer tends to cause the ink to expand toward the blank portion. At this time, lowering the pH of the fountain solution is advantageous for suppressing the expansion of the ink. However, if the pH is too small, the fountain solution will react with the drier to delay the drying of the ink. Therefore, when printing such products, the pH of the fountain solution needs to be determined according to the layout and color of the layout.
If the printed ink layer is thicker in the spot color, and the middle contains small lines, or prints a layered picture based on dark tones, the dampening solution ph is lower because it is easy to paste.
When printing red ink, the pH of the dampening solution should be lower; when printing other colors of ink, the pH should be slightly higher.
The more severe the ink emulsification, the more moisture is mixed into the ink layer, and the ink film is less likely to dry. The use of alcohol dampening solution can reduce the amount of dampening solution, reduce emulsification, and facilitate ink drying.
To use a common dampening solution, first pay attention to the proportion of each component in the formula, and then maintain the ink balance with as little water as possible. At the same time, controlling the distribution of moisture on the plate is also important to prevent back smudging. For example, a printing machine has an axial width of 1020 mm, a printed paper width of 700 mm, and a large area of graphics is almost full of sheets. In this case, since the ink layer is thick, the water supply amount is large, and the printing machine is uniformly supplied with water in the axial direction, neither the ink and the water are emulsified or the ink is applied to the left and right ends of the printing plate and the area other than the paper. The blank portion of the paper absorbs moisture, and as a result, excess moisture breaks into the left and right ends of the paper, resulting in excessive emulsification of the printed image and lightening of the ink. In order to maintain the uniform color of the printing plate and increase the amount of ink, the water at the left and right ends of the paper has a large ink and is not easy to dry. Therefore, when printing a large area of the ground, the back side of the dirt is most likely to appear on the left and right sides of the paper. It is reasonable to use a water pipe blowing device or a paper strip on the water roller to reduce the amount of water supplied to both ends of the plate. If the number of prints is not large, you can also consider cutting the paper slightly larger, so that the blank portion at both ends of the paper absorbs some of the moisture.
Third, the nature of the paper
The roughness and permeability of the paper surface is an important factor affecting the back smudging. Paper with good permeability, the binder can penetrate into the paper quickly and more, and accelerate the solidification of the ink. Paper with a rough surface facilitates the infiltration of air from the gap between the papers, while the surface of the paper has a large amount of air in the capillary, which is beneficial to the oxidative polymerization of the ink. Therefore, back smudging mostly occurs when a thicker ink layer is printed on coated paper. The printing of aluminum foil paper with an offset printing machine is particularly prone to back smearing, because the surface of the aluminum foil paper has no penetration ability at all, and even if only a relatively thin ink layer is printed, it is necessary to use dusting to prevent the back surface from being dirty.
When using wet-drying overprinting, if the previous color is already a high number of dots or solids, when the latter color is overprinted, since the original paper surface has been covered by the previous ink film, the permeability of the latter ink will be greatly reduced. Small, easy to cause sputum on the back.
The difference in acidity and alkalinity of the paper also affects the drying speed of the ink. The ink printed on the acid paper dries slower than the ink printed on the alkaline paper.
Fourth, the thickness of the ink layer
The back smear occurs mostly in areas where the printed ink layer is thick.
In the process of ink drying, for the resin-based ink commonly used in sheet-fed printing, the drying is firstly accomplished by rapid penetration of high-boiling kerosene in the binder and rapid thickening of the remaining binder. The oxidative polymerization is slow. The process is carried out to complete the complete curing of the ink.
For a certain type of paper, there is a limit to the penetration of the binder under a certain printing pressure. When the ink layer is thick, the amount of penetration of the binder in the surface ink is greatly reduced after the bottom material is infiltrated into the paper, and the ink on the surface layer cannot be quickly thickened and solidified by the ink to improve cohesion. Therefore, for a certain type of paper, within a certain thickness of the ink layer, a suitable printing process is adopted, and the back surface is not dirty. Once the thickness of the ink layer exceeds a certain limit, the backside smear will appear rapidly, and the severity of the back smear is sensitive to changes in the thickness of the ink layer. Therefore, when printing a color image based on dark tone, it is preferable to use a method of removing the under color to minimize the thickness of the ink layer. When printing a large area of the ground, although the thicker the ink layer, the thicker the ink, the thickness of the ink layer must be strictly controlled within a certain range. If the color of the ink to be formulated is too light, it is easy to cause the back of the printed matter to be dirty by increasing the thickness of the ink layer to add a dark color.
When the ink layer is thin, the intermolecular force fa of the printed sheet and the surface layer of the ink film is large. When the ink layer is thickened, the intermolecular force is rapidly reduced, which is also a factor causing backside smudging.
Fifth, printing pressure and printing speed
The penetration of ink on paper is divided into two types: pressure penetration and free penetration. Increasing the printing pressure can promote the pressure penetration of the ink; reducing the printing speed, can increase the imprinting time, and can also promote the pressure penetration of the ink. At the same time, after the printing speed is lowered, the previous sheet has a longer time for the free penetration of the capillary before the next sheet is stacked on the previous sheet, so when printing a large area without a dot, the printing pressure is appropriately increased. It can promote the ink color to be thicker and more uniform, and accelerate the drying speed. If the number of prints is small, the printing speed can be lowered to prevent the back surface from being dirty.
Sixth, environmental temperature and humidity
As the ambient temperature rises and the ink drying increases, so the winter and summer should be different in terms of adding a drier.
As the ambient humidity rises, the ink drying speed becomes slower, and the back surface is more likely to be dirty in a very humid environment.
Seven, the distance between paper
The smoother the paper, the higher the paper, the higher the density of the paper. Under the action of the gravity of the paper, the smaller the paper spacing at the bottom of the paper stack, the intermolecular secondary bonding between the ink film surface and the adjacent printed sheets on the printed sheet. The larger the force fb, the more likely the back is dirty. At the same time, the smaller the paper spacing, the less easily the air penetrates from the gap between the papers, which is detrimental to the oxidative polymerization of the ink. Therefore, when printing a thicker ink layer on a coated paper, it is not appropriate to stack the paper too high. Conditional printers can use a dedicated drying rack that is separated by layers to reduce the height of the crepe paper.
Another method commonly used in the printing industry to prevent back smear is dusting. After the special powder is sprayed on the printed sheet, the support between the sheets is increased by the support of the powder, and a good anti-back smear effect can be obtained. However, the use of dusting also brings many drawbacks.
Firstly, dusting pollutes the environment and endangers the health of the printing workers. Therefore, the powder spraying is only used when other measures are taken to avoid the back surface smearing, and the spraying of the powder should be adjusted according to the size of the printed sheet which is prone to back smudging. Area, try to reduce the amount of dusting. Secondly, dusting affects the quality of printed matter. If the printed sheets after dusting are printed with other colored inks, the printing quality of the printed inks will be seriously affected. Therefore, the color to be dusted is usually placed in the final print. If the printed sheet after dusting needs to be coated, the granulated powder will appear more obvious after the film is coated. Especially when the powder is sprayed on the black background, it is generally required to be embossed by the printing machine under the condition that the ink roller and the water roller are pressed off, and the blanket is sprayed on the black background by frequently scrubbing the blanket. The powder is removed.
Eight, other
When printing thick cardboard, the distance between the symmetrical paper organizers on the delivery table should be slightly larger than the width of the paper. Otherwise, when the paper cutter is tilted inward, because the cardboard is hard, the two ends are pressed easily. The lower arch gives a larger force to the next printed ink layer, causing the back to be dirty. Before the printing, if the paperboard has been hygroscopically deformed, it is also easy to cause uneven distribution of the contact pressure on the paper surface due to the difference in the deformation of the paperboard, and partial back smearing occurs.
If there is static electricity of opposite polarity between the papers, the paper will be close together due to the suction of positive and negative charges, resulting in dirty back.
After the printed paper is initially fixed, about half an hour or so, it should be moved once, and it is best to shake it to facilitate air circulation and accelerate the oxidative polymerization reaction, which is beneficial to prevent the back of the printed matter from being dirty.