Quality control of printed matter in offset printing
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The printing process is the process of copying the original graphic information on the surface of the substrate. In this regard, to evaluate the quality of printed matter, the first thing to look at is the loyalty of the printed matter to the original. We know that due to the limitations of equipment, materials, printing processes, etc., it is impossible to make the printed matter completely faithful to the original. Therefore, the high-quality reproduction we have said is that the printed matter is very close to the original. At the same time, due to the difference between the printed matter and the original, we not only require the difference between the printed matter and the original, but we also propose the stability of the difference, that is, the stability of the copy. This stability is the same for printing. is very important.
In addition to the above-mentioned criteria for quality evaluation of printed matter, from the aesthetic point of view of printed matter, we have made the following extensions to the surface quality standard (empirical quality standard) of printed matter:
a. The ink color is bright, and the depth of the picture is uniform;
b. The ink layer is thick and shiny;
c. The outlet is smooth, clear and free of burrs;
d. It conforms to the original and has a clear color tone;
e. Overprinting is accurate;
f. The text does not lack pen breaks;
g. The appearance of the printed sheet is free from wrinkles, imprints, dirt and fingerprints, and the product is clean and tidy;
h. The back is clean and free of dirt.
From the beginning of the tone compression, color correction, color separation, printing plate to the end of printing, every part of the process will affect the quality of the print. In the following article, we will focus on the quality control of printed matter during the printing process.
As we mentioned earlier, due to various conditions in actual production, our print copy cannot be exactly the same as the original. Make a hypothesis that when all the materials, equipment, and processes we use to print are as ideal as we want, then our prints will be very close to the manuscript. With this assumption we will easily think that what we have to do in printing is to narrow the gap between the conditions and processes that we can control and change, and the ideal state. The smaller the gap, the higher the quality of the prints we can get.
Below we discuss these conditions and processes separately.
First, the choice of printing consumables.
(1) Paper.
The nature of the paper will directly affect the quality of the print, so in the design of the printing process, it is necessary to make a trade-off between the quality of the paper and the cost of printing (often low cost and high quality can not be obtained at the same time). The most ideal paper for printing reproduction should have the following properties:
1 has absolute whiteness, that is, it does not absorb at all for all kinds of light. In this way, a high degree of image contrast can be printed, and in addition, light can be prevented from entering the paper near the dot, and absorbed from the inside of the paper by the dot, resulting in an unnecessary increase in the optical dot.
· Paper with absolute whiteness does not exist at all. No paper can absorb light from the outside world. In the printing process design project, what we need to do is to choose paper with high whiteness (except for products with special process requirements) within the allowable cost range to meet the requirements of image reproduction, in order to achieve more abundant. level.
2 has a good gloss. Similar to the above, but the difference is that the gloss is not as large as possible, too much gloss is an obstacle to normal reading.
3 smoothness. The smoothness of the paper refers to the roughness of the paper surface. The ideal smoothness of the paper surface should be at least 500 seconds. If the smoothness of the paper is too low during printing, it will easily cause high-profile dots to be lost, low-key dots to be level, and the solid part to be empty.
• Also for printing cost considerations (sometimes for print special effects, such as textured paper), it is not possible to print on high quality, high smoothness paper for each type of print. When encountering paper with low smoothness, the number of printing screen lines should be appropriately reduced during the process design, and the printing pressure should be appropriately increased during the printing process to compensate for the surface smoothness of the paper.
4 has a neutral pH value. Acidic paper can cause the ink to not dry in time during the printing process, which will lead to excessive emulsification of the ink during the printing process, which will cause a series of quality problems in the printed matter, such as smudging of the printed matter, blurring of the edges of the dots, and increase of the dots. Problems such as deformation. When the pH of the paper itself is alkaline, it reacts with a wide variety of ink pigments, which causes the printed matter to fade after printing, which is especially noticeable in the printing of gold and silver ink.
· In the choice of paper, try to choose paper that is close to neutral. However, if the paper has been selected and the pH has been determined, the pH of the paper should be passed through the acidity and alkalinity of the dampening solution during the printing process. Make a certain degree of compensation. The general effect is also obvious.
5 paper shape stability. The ideal paper should be undeformed, either under the pressure of the printing or under the wetting of the fountain solution. This ensures overprinting during the printing process and avoids a series of problems caused by paper deformation.
· What we have to do is not to choose paper that is not deformed at all, but to make trade-offs between cost and high-stability paper in the process design process. For example, in the printing of newspapers, because the requirements are low and the quality requirements are not very high, then when choosing paper, the stability is not very important, and the news paper can be accepted. For printing such as maps, we must choose paper with good shape stability, avoid overprinting problems caused by paper deformation, and changes in the size of the finished product.
6 Surface strength of the paper. The ideal paper requires no flakes during the printing process to avoid the resulting wear on the printing plate and the deterioration of the image quality due to adhesion on the blanket, printing plate and ink roller.
·The paper we use in the printing process, generally more or less, will lose powder during the printing process. In addition to selecting the paper before printing, we must judge the surface strength of the substrate during the printing process and clean it in time. Foreign matter such as paper powder on the plate and blanket.
7 paper silk. The ideal paper is not in the direction of the wire, and the strength of the paper is large enough. But as long as the cost is taken into account, we can't help but use paper with multiple directions. In printing, we must pay attention to the uniform direction of the paper and the direction of the paper feed of the printing machine, which can greatly improve the level of overprinting.
(2) Ink.
Due to the nature of offset printing (requires printing with the participation of water, the graphic and blank on the printing plate are almost on the same plane), the offset ink must have good water resistance, good adhesion, elasticity, and very good touch. transsexual.
1 water resistance. For smooth printing, it is necessary to select inks with good water resistance, but unlike other conditions, the water resistance of offset inks is not as strong as possible, and the completely water-repellent offset inks cannot be printed on the machine. It is because during the ink transfer process, the water that reaches the ink path due to the high-speed shear of the printing press is bound to be encountered. When the oil and water are completely unacceptable, the normal transfer of the ink is hindered.
·In the printing, in order to ensure the water resistance of the ink, additives (desiccant, debonding agent, varnish, anti-smudge agent, etc.) are added to the ink to avoid reducing the cohesive force of the ink. A tackifier is added to the ink according to the situation to restore the cohesive force of the ink to achieve the purpose of restoring the water resistance of the ink. At the same time, it is necessary to regularly clean the ink in the ink path and the ink fountain to prevent the residual ink on the machine from being used repeatedly, which affects the water resistance.
2 viscosity and elasticity. Good adhesion is the premise to ensure the normal transfer and transfer of the ink; and the elasticity of the ink can ensure that the ink recovers and leveles quickly after being transferred to the surface of the substrate. Good elasticity is the prerequisite for ensuring the smooth surface of the printing dot.
· As mentioned above, in addition to selecting good quality inks, what we do in printing is to maintain the properties of the ink. Many additives are as little as mentioned above, to avoid the ink properties caused by human operation. The change. Also pay attention to the control of the wetting fluid (discussed in the process).
3 thixotropy. The ink used in offset printing must have good thixotropy, that is, the fluidity increases under the action of mechanical shearing force when the ink is ejected from the ink fountain, and the mechanical force is transmitted when the ink is transferred from the surface of the blanket to the surface of the substrate. The disappearance of the ink, the fluidity of the ink rapidly decreases, and together with the penetration of the binder in the ink on the surface of the substrate together contributes to the initial curing of the ink. This process is very important for multi-color printing and can greatly affect the overprinting effect of multi-color machines.
• Choosing a good thixotropic ink is very important for printing.
4 ink transparency. Offset inks require good transparency.
·The transparency of printing ink in actual printing is not guaranteed, and the transparency of inks of different colors is often not the same. Therefore, in the printing process, the color sequence of ink should be considered, and the arrangement of color sequence can effectively improve the color saturation of printed matter. degree.
5 The particle size and colorability of the ink. A good printing ink requires that the finer the particles of the ink pigment, the better, and the higher the colorability, the better. The choice of ink must not ignore these two points, it is also an old saying, the choice of the finest particles of ink under the conditions of cost, the ink with high coloration, is conducive to quality control in printing.
6 ink stability. The stability of the ink includes the ability of the ink to resist light, heat, acid, alkali and alcohol, and requires the ink to not change color and age under certain conditions, especially for inks with special purposes (such as printing on billboards that need to be displayed in the sun, etc.) ), the ink is required to have this special resistance.
(3) Wetting fluid
The role of dampening fluid
The blank portion of the printing plate forms a uniform water film to resist the infiltration of the ink in the graphic portion to the blank portion of the printing plate to prevent the dirty plate; forming a new hydrophilic layer to maintain the hydrophilicity of the blank portion of the printing plate; Wet to control the temperature of the surface of the plate. Because the wetting fluid has the above effects, the requirements for the wetting fluid in printing are: high surface activity (addition of surfactant or low surface energy in water); certain acidity (pH: 4.5- Between 6).
· The control of the dampening solution during the printing process is mainly considered from the pH value of the dampening solution. Below we briefly talk about the control of the PH value:
1 When the amount of the desiccant in the ink is increased, although the drying speed is increased, the viscosity of the ink is increased, the particles are coarsened, the adhesion to the blank portion of the printing plate is increased, and the dirty plate is liable to occur, so that the amount of the raw liquid is increased and the amount of the original solution is increased. The pH of the dampening solution is slightly reduced, which is good for preventing dirty plates.
2 Use non-coated paper with loose texture. If the viscosity of the ink is large, the powder is severely damaged, and the printing plate is dirty due to abrasion. Therefore, it is necessary to appropriately lower the pH of the dampening solution. The pH of the dampening solution can be appropriately increased by printing on advanced coated paper.
3 Using solid plate printing, the pH of the dampening solution can be lower. On the contrary, if the screen printing is used, the pH of the dampening solution can be higher.
4 When the temperature of the printing workshop rises, the viscosity of the ink decreases, the fluidity increases, and the dry vegetable oil separates more free fatty acids, causing the surface to become dirty, so the pH of the dampening solution should be lower.
(4) Blanket.
The most ideal blanket should be completely non-absorbent, good in ink, should have good elasticity, have good compressibility, and can quickly restore the original state after the pressure disappears, that is, all the elasticity is expressed in printing. Sensitive.
1 In printing, we want the blanket to transfer ink only to the surface of the substrate, and the moisture is not desired to be transferred to the surface of the paper, otherwise it will cause the surface strength of the paper to be reduced or even deformed. Therefore, we hope to get a blanket of ink-repellent ink.
· In fact, it is impossible to obtain a blanket that is completely non-absorbent and has excellent ink-repellent properties. In addition, in the printing process, the hydrophilic substances in the paper, the hydrophilic substances such as fibers, and the pigment in the ink are on the blanket. Adsorption causes the blanket's ink-receiving ability to decrease during the printing process, while the hydrophilic ability is improved. Therefore, in addition to selecting a blanket with good surface properties before printing, it is necessary to pay attention to the cleaning of the surface of the blanket during printing to maintain good ink-repellent properties of the blanket.
2 Since the blanket is periodically subjected to compression and elastic recovery during printing, this cycle is quite short on current high-speed printing equipment. If the elasticity of the blanket exhibits hysteresis, it cannot be immediately after the pressure disappears. Restore, then the compression range of the blanket will be affected during the compression of the next cycle.
• The hysteresis of the blanket is unavoidable, and even irreversible plastic deformation occurs, which are factors that affect normal embossing during printing. Before printing, we have to choose the appropriate usable blanket, and after the blanket has been imprinted for a while, we have to check the printing pressure (for example, it can be detected by the method of pressing the streak). And advocate the use of two blankets. Give the blanket a recovery time and reduce the aging cycle of the blanket.
3 The ideal blanket should be compressible, ie the volume reduction occurs only in the compression zone, and the volume of the non-compression zone does not change. This can reduce the ratio of the linear velocity of the surface of the drum during the printing process within a considerable range, thereby reducing the deformation of the dots due to the speed difference.
·The blankets used in our daily production are generally incompressible, that is, in the vicinity of the embossed area, the front and convex hulls of the blanket appear, which makes the difference in the line speed of the embossed area more change. Therefore, it has a great influence on the correct transfer of the outlets. Relatively speaking, the compressibility of the air-cushion blanket is quite good. For high-quality prints, the use of air-cushion blankets can achieve good printing results.
Second, printing equipment
(1) Transmission stability of printing equipment
The overall stability of the printing equipment will be directly related to the stability of the image reproduction process. The stability of the printing equipment mentioned here includes the transmission precision of the printing machine, the dynamic balance of the printing transmission parts, the uniformity of the transmission speed, and the various components. The coordination accuracy between the two is stable. These problems will directly affect the quality of the transfer of the print reproduction process, such as the stability of the dot, the stability of the ink usage, and the stability of the pressure. These will directly affect the quality of the printed matter.
· In actual printing, we must consider the accuracy and stability of the equipment itself for different printing products. If the stability of the equipment itself is not up to standard, it is absolutely impossible to print good prints. This also proposes that in daily production, while ensuring the output, do not neglect the maintenance and maintenance of the equipment itself. A stable operation of the equipment is a prerequisite for printing good prints.
(2) The positioning of the paper in the printing machine is accurate and stable.
After the paper enters the machine from Feida, it is continuously transmitted in the positioning and transfer mechanism of the machine. The precise operation of each step will be related to the accurate overprinting of the substrate.
· Firstly, Feida, including the various components on Feida, must ensure the stability of Feida's paper separation and paper transfer at the same time, and also pay attention to the maintenance of the air pump, and pay attention to replacement and repair of the parts with serious wear.
·There are then various parts on the paperboard, including the platen roller, brush, double-plate controller, paper conveyor belt, etc., to ensure the symmetry of the components on both sides of the paperboard, while ensuring reasonable adjustment of working position and tightness. Sex. Avoid skewing the paper.
· Front gauge, side gauge, swinging teeth. Adjust in strict accordance with the angular position of the equipment to avoid misalignment and handover errors.
·The drum teeth (including the impression cylinder teeth, the paper roller to bite the teeth), as above, pay attention to the accuracy of the adjustment.
All the institutions mentioned above are directly related to the overprinting relationship of printed materials. It is necessary to reasonably adjust and ensure the absolute precision of these institutions to make the printed products have more accurate overprinting.
(3) Imprinting mechanism. The ideal working state of the imprinting mechanism is that the ideal printing pressure is used, and the pressure is stable during the operation without change. (See the process control for pressure regulation principle)
(4) Water (ink) road. The requirement of the water (ink) road is to make the water (ink) spread evenly, the pressure between the water (ink) rolls is uniform, and the pressure between the plates should be uniform and reasonable, and the specific requirements are determined according to the requirements of different machines.
Third, the control of the printing process
With the ideal materials and good printing equipment, we can print good printed products, but we can't necessarily print good prints. The next important point is the need for proper printing process control.
(1) Printing plate. Printing is a very important part of the printing and copying process. Let's take a look at the factors that need to be controlled during the printing process:
1) Determine the correct exposure time;
2) Determine the correct pumping time;
3) Dust removal work;
4) Step range: 2%-98%;
5) Determine the position of the plate of the plate;
6) Quality control strip.
· The problems we should pay attention to in the printing work are:
Pay attention to the quality problem of the PS version
PS background is not good.
Improper development time.
3. After development, there is a "blue bottom" that causes the print to get dirty.
4. It is not easy to apply ink, and it is not easy to apply ink when printing on the PS plate.
5. Partial "flower version", paste version.
6. The printing process is interrupted.
7. Partial drop.
8. Partially lost.
9. PS surface roughness is not good
10. The PS version has poor ink resistance and alkali resistance.
11. Photosensitive ratio is not appropriate.
12. The photosensitive film is too thin.
The above problems are all we should pay attention to in the printing work. Any occurrence of any one of them will lead to the decline of the quality of the printed matter, or even invalidation.
(2) Ink balance control in printing
1 water quantity control. In fact, we don't need water at all in the final print. Even we always complain that water has caused various defects in our products (such as the low gloss of the ink on the printed surface due to the presence of water). Therefore, we are always looking for a lithographic printing that does not require water at all. Therefore, waterless offset printing has appeared. Although it has various advantages that are unmatched by water offset printing, only one defect is that the cost is too high, which is enough to make it impossible. Better promotion. Therefore, we have to start with water printing. Because of the limitations of plates and inks, we still can't leave water for traditional water printing equipment. What we can do in printing is to not ink on the blanks as much as possible. Under the premise, reduce the water consumption, this can reduce the ink emulsification, make the edge of the dot clearer (the oil and water boundary is clear), and secondly, the ink can be smoothly transferred to the surface of the substrate, and the third can avoid reducing the amount of water. A large reduction in the surface properties of the substrate and the water absorption deformation of the substrate.
· It is a basic principle that we use water in offset printing to ensure that the outlets are clear and the blanks are not dirty. Some people say that there is water printing. If you control the water, you will master the printing. It makes sense to produce better printing quality.
(1) How to judge the excess amount of water
The color of the ink on the printed matter is light, even if the amount of ink is increased, the ink color is not deepened in time;
The amount of ink accumulated on the ink roller increases, and the ink particles become thicker;
The printed sheet absorbs excessive moisture, the water volume on both sides of the front and back is uneven, and the paper is curled;
The surface of the ink film retains too much moisture, and the ink added to the ink fountain is not easily stirred evenly, and the roller has a slip phenomenon;
There is moisture sticking at the tip of the blanket, and water drops are dropped at both ends of the drum;
Before and after the interruption of printing, there is a big gap between the darkness of the printed ink color;
After the shutdown, the layout is not dry.
(2) Ink quantity control. The amount of ink we control in printing is mainly based on the print. This process is mainly carried out through the vision of the human eye, but experienced operators can often do very well.
First of all, we must determine a reasonable color sequence according to the process characteristics of the product. Different color sequences have different effects on print quality.
Then, the sample after the test is compared with the print. There are generally two ways to compare this. One is the visual method, which compares the printed sheets and the proofs to identify the color depth of each part (if it is The monochrome machine is not sure about the yellow color of the first print, and can be determined by the red or blue color filter. One is the hand touch method, which is to press the graphic on the printed sheet just by hand, and feel it if the hand is raised. Sticky, which means that the amount of ink is more suitable. However, these two methods require very experienced operators to use them, and they are subjective and unstable.
Modern and relatively advanced printing systems have successively introduced relatively perfect ink color control devices. Such as HEIDELBERG's CPC system; ROLAND's CCI system; KOMORI's PQC system provides intelligent ink color control functions, replacing the artificial judgment of ink color with the intelligent judgment of the computer. These systems offer the possibility of scientific and data-based printing of ink.
(3) Pressure control during printing
The presence of printing pressure can lead to many problems such as dot gain, but since printing pressure is a prerequisite for ink transfer during the printing process, it is impossible to completely avoid it. Therefore, as with the water volume control mentioned above, a general principle is also proposed for pressure control, that is, under the premise that the ink can be reasonably transferred, we advocate the use of the minimum printing pressure. This has many advantages, such as: minimizing the spread of the dots under pressure; minimizing the amount of deformation of the blanket under pressure, thereby reducing the line speed difference on the surface of the drum and reducing the speed due to this speed. Defects caused by poor dot; reduce the wear on the printing plate, reduce the number of times the plate is replaced during printing, enhance the stability of printing quality; improve the service life of the blanket, and at the same time make the pressure during the printing process more stable……
(4) Overprint control of printed matter
The overprinting of printed matter is usually an important factor in determining the quality of printed matter. If the problem of overprinting is not guaranteed, then there is no higher quality problem.
The control of the overprinting accuracy is precisely the precise adjustment of the printing equipment mentioned in the "equipment" (provided that the printing equipment is available), as well as the following factors:
1. Condition the substrate so that its own humidity is equal to or slightly higher than the ambient humidity of the plant.
2. Keep the workshop environment constant and constant humidity.
3. Minimize water usage during the printing process.
4. Ensure that the thickness of the plate liner of each color group is consistent during the printing process. Due to the calendering problem of multi-color printing and the participation of water in the printing process, it is now more advisable that the thickness of the printing plate liner of the post-color printing is slightly smaller. The front color pad thickness is 0.03 mm.
In fact, there are still many factors that can improve the accuracy of print overprinting, which are not listed here.
Fourth, other aspects of control
In addition to some of the above mentioned, operators in the printing work should also pay attention to the following issues:
(1) Read the ticket carefully and strictly follow the requirements of the ticket (such as the specifications of the printed product, the type of paper, and other special requirements) to avoid quality accidents caused by failure to follow the requirements of the ticket.
(2) Adhere to the operation of “three operations”. The so-called three diligence is in the printing process, diligently stir the ink in the ink fountain, diligently look at the amount of water in the plate, and sample the control sample.
(3) Strengthen the storage of semi-finished products. The requirement for the storage of semi-finished products is to meet the overprinting requirements of the under-color or back-printing, and to avoid the defects such as paper deformation, crystallization of the imprint, lack of imprinting, and staining of the back surface due to poor storage. At the same time, in order to ensure the quality of semi-finished products, we must make the following points:
1 The piles of paper are piled neatly. If they are not uniform, a large amount of paper edges will be exposed to the air. This will easily cause deformation due to changes in humidity. In addition, paper edges will be easily damaged and folded due to uneven paper edges during stacking. Phenomenon, these conditions will affect the progress of the next process, and there is no benefit to the quality of printing.
2 Since the paper around the stack and the surface of the paper are exposed to the air, it is easy to deform the size. Therefore, in the stacking process, 20 pieces of waste pieces should be placed on the stack to protect the printed surface. In addition, if it is stacked for a long time, it is necessary to wrap the paper stack with kraft paper or plastic film.
3 Stacking height of the stack. It should be decided according to the specific situation. Generally, there is no large-scale solid printing. The paper stack can be piled up higher. For large solid areas or high surface smoothness, the stacking height should be reduced appropriately. For printed products with a large amount of ink (especially when printing on coated paper or white cards), the drying rack should be considered. This is good for drying the ink while avoiding smudging on the back.
4 When encountering a print with a very large amount of ink, when the stack is piled up, the pile should be loosened at regular intervals. Auxiliary drying.
The factors that can affect the quality of printing during the printing process are not only the factors mentioned above. There are many factors in the actual printing that cause the printing quality to have such problems or the like, so we need to do it in our printing work. We carefully discover and deal with it. In addition, in the work, we should promote the standardization and standardization of management and operation, record the changing factors in the work, and explore the experience for data management, so as to achieve stable printing quality.