(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
Simply speaking, 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 how faithful the printed matter is to the original. We know that due to the limitations of equipment, materials, printing technology and other aspects, it is simply impossible to make prints faithful to the originals. Therefore, the high-quality reproduction we have mentioned means that the prints are very close to the originals. At the same time, due to the difference between the printed matter and the original, we not only require that the difference between the printed matter and the original is small, we also proposed the stability of this difference, that is, the stability of replication, this stability is also the same for printing is very important.
In addition to the above-mentioned evaluation standards for print quality, from the perspective of the aesthetics of the printed matter, we have made the following extensions to the surface quality standards (experience quality standards) of the 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 dots are smooth, clear and free of burrs;
d. Conform to the original, with clear tone levels;
e. Overprint is accurate;
f. There is no shortage of pens and strokes;
g. The printed sheet has no wrinkles, no marks, dirt and fingerprints, and the product is neat and tidy;
h. The back is clean and free of dirt.
From tone compression, color correction, color separation, and printing plate making to the end of printing to the end of printing, every link in the process will affect the copy quality of the printed matter. In the next article, we will focus on the quality control of the printed matter during the printing process.
We mentioned earlier that due to the limitations of various conditions in actual production, our printing and copying cannot be exactly the same as the original. Make a hypothesis, when all the materials, equipment, and processes we use to participate in printing have reached the ideal state we want, then our prints will be very close to the original. Through 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 this gap, the higher the quality of the prints we can get.
Below we discuss these conditions and processes separately.
1. Selection of printing consumables.
(1) Paper.
The nature of the paper will directly affect the copy quality of the printed matter, so when designing the printing process, a trade-off must be made between the quality of the paper and the printing cost (often low cost and high quality cannot be obtained at the same time). The most ideal paper for printing and copying should have the following properties:
â‘ It has absolute whiteness, that is, it is completely absorptive to all kinds of light. In this way, a high image tone contrast can be printed. In addition, unnecessary light dots can be prevented from being absorbed by the dots from the inside of the paper due to light entering the paper near the dots.
· In fact, paper with absolute whiteness does not exist at all, and no paper can completely absorb external light. In the printing process design project, all we have 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 a richer level.
â‘¡ has good gloss. Similar to the above, but the difference is that the gloss is not as large as possible. Too large a gloss is an obstacle to normal reading of printed matter.
â‘¢Smoothness. The smoothness of the paper refers to the roughness of the surface of the paper. The ideal smoothness of the surface of the paper should be at least 500 seconds. When the smoothness of the paper is too low in printing, high-profile dots will be easily lost, low-profile dots will be graded, and the field will be partially empty.
· Also due to printing cost considerations (sometimes due to the special effects of the printed matter, such as textured paper), it is impossible to use high quality and smooth paper for every type of printed matter. When encountering paper with low smoothness, the number of screens for printing should be appropriately reduced during the process design, and the printing pressure should be appropriately increased during the printing process to compensate for the smoothness of the surface of the paper.
â‘£ Has a neutral PH value. Acidic paper can cause the ink to fail to dry in time during the printing process, which can cause excessive emulsification of the ink during the printing process, which can cause a series of quality problems in the printed matter, such as smudging of the printed matter, blurred dot edges, and increased dot Deformation and other issues. When the PH value of the paper itself is alkaline, it will react with many kinds of ink pigments, which will cause the printed matter to fade after printing, which is especially obvious in the printing of gold and silver ink.
· In the selection of paper, try to choose a paper that is close to neutral, but if the paper has been selected and the PH value has been determined, the pH value of the paper should be passed through the acidity and alkalinity of the wetting fluid during the printing process. Compensate to a certain degree. The general effect is also obvious.
⑤ Paper form stability. The ideal paper should not be deformed, either under the action of printing pressure or under the dampening of the fountain solution. This can ensure overprinting during the printing process and avoid a series of problems caused by the deformation of the paper.
· What we have to do is not choose paper that is not deformed at all, but try to make a balance between cost and paper with high form stability in the process design process. For example, in the printing of newspapers, due to the low cost and low quality requirements, when selecting paper, the shape stability is not very important, and newsprint is acceptable. For the printing of printed materials such as maps, we must choose papers with good form stability to avoid overprinting problems caused by paper deformation and changes in finished product size.
â‘¥ The surface strength of the paper. The ideal paper requires no dusting and fluffing during the printing process, to avoid the resulting abrasion of the printing plate, and the image quality degradation due to adhesion on the blanket, printing plate, and ink roller.
· The paper we use in the printing process, generally more or less will be powdered and depilated 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 matters such as paper dust on the printing plate and blanket.
⑦The thread of paper. The ideal paper does not have a silk direction, and the paper is strong enough. However, as long as the cost is taken into consideration, we cannot but use paper with repeated orientation. In printing, we must pay attention to the same direction of the paper and the printing paper, 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 graphics and the blank on the plate are almost on the same plane), the offset ink must have good water resistance, good viscosity, elasticity, and very good touch. transsexual.
â‘ Water resistance. For smooth printing, it is necessary to choose inks with good water resistance, but unlike other conditions, the water resistance of offset printing inks is not as strong as possible. Completely water-repellent offset inks cannot be printed on the machine. This is because during the ink transfer process, the water reaching the ink path due to the high-speed shearing of the printing press is bound to be encountered. When the oil and water are completely intolerable, it will hinder the normal transfer of ink.
· In printing, in order to ensure the water resistance of the ink, the additives added to the ink (drying agent, debonding agent, ink adjustment oil, anti-smudge agent, etc.), so as not to reduce the cohesion of the ink, sometimes in some cases Add tackifier to the ink according to the situation to restore the cohesion of the ink to achieve the purpose of restoring the water resistance of the ink. At the same time, the ink in the ink path and the ink fountain should be cleaned regularly, so as to avoid the residual ink on the machine from being used repeatedly and affecting the water resistance.
â‘¡ Sticky and elastic. Good viscosity is the prerequisite to ensure the normal transmission and transfer of ink; the elasticity of the ink can ensure that the ink recovers quickly and leveled after being transferred to the surface of the substrate. Good elasticity is the premise of ensuring the surface of the printed dots.
In addition to choosing good quality inks, as we mentioned above, what we do in printing is to maintain the properties of the ink. Many additives are as mentioned above. Try to add as few as possible to avoid the ink properties caused by human operations. The change. At the same time pay attention to the control of the wetting fluid (will be discussed in detail in the process).
â‘¢ Thixotropy. The ink used in offset printing must have good thixotropy, that is, when the ink is transferred from the ink fountain, the fluidity increases under the action of mechanical shearing force. When the ink is transferred from the blanket surface to the substrate surface, due to the mechanical force The disappearance of the ink quickly reduces the fluidity of the ink, and together with the penetration of the binder in the ink on the surface of the substrate together contribute to the initial curing of the ink. This process is very important for multi-color printing, which can greatly affect the overprint effect of multi-color machines.
· It is very important to choose ink with good thixotropy.
â‘£ Transparency of ink. Offset printing inks require good transparency.
· In fact, the transparency of printing ink cannot be guaranteed in printing, and the transparency of inks of different colors is often not the same, so in the printing process, the color sequence of the ink should be considered, and a reasonable arrangement of color sequence can effectively improve the color saturation of the printed matter. degree.
⑤ The particle size and colorability of the ink. A good printing ink requires the finer particles of the ink pigments, the better, and the higher the colorability, the better. The selection of inks must not ignore these two points. It is also an old saying. Choosing the ink with the finest particles and the ink with high coloring under the conditions of cost allows for quality control in printing.
â‘¥Ink stability. The stability of the ink includes the ability of the ink to resist light, heat, acid, alkali, alcohol, and requires that the ink does not change color and aging 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 wetting fluid
A uniform water film is formed on the blank part of the printing plate to resist the infiltration of the ink of the graphic part into the blank part of the printing plate to prevent the dirty version; a new hydrophilic layer is formed to maintain the hydrophilicity of the blank part of the printing plate; Wet liquid to control the temperature of the printing plate surface. Because the wetting fluid has the above effect, the requirements of the wetting fluid in printing are: high surface activity (added surfactant or low surface energy substance in water); certain acidity (pH value: 4.5- 6).
· The control of the wetting liquid in the printing process is mainly considered from the PH value of the fountain solution. Let us briefly talk about the control of the PH value:
â‘ When the amount of desiccant in the ink is increased, although the drying speed is accelerated, the viscosity of the ink increases, the particles become thicker, the adhesion to the blank part of the printing plate increases, and dirty plates are prone to occur, so the amount of the original solution is increased to increase The pH value of the wetting liquid drops slightly, which is beneficial to prevent dirty plates.
â‘¡Use loose-texture non-coated paper for printing. If the viscosity of the ink is large, the powder will fall off and the hair will be serious, and the printing plate will become dirty due to abrasion. Therefore, it is necessary to appropriately reduce the pH value of the wetting liquid. Using high-quality coated paper for printing, the pH of the wetting fluid can be increased appropriately.
â‘¢ Using field printing, the pH value of the wetting liquid can be lower. On the contrary, if the dot printing plate is used for printing, the pH value of the wetting liquid can be higher.
â‘£When the temperature of the printing workshop rises, the viscosity of the ink decreases and the fluidity increases. At the same time, the dry vegetable oil separates more free fatty acids, causing the plate to become dirty, so the pH value of the wetting liquid should be lower.
(4) Blanket.
The ideal blanket should be completely non-absorbent, good in ink absorption, should have good elasticity, have good compressibility, and can quickly return to its original state after the pressure disappears, that is, all the elasticity is shown in printing Sensitive elasticity.
â‘ In printing, we hope that the blanket only transfers ink to the surface of the substrate, and moisture is not what we want to transfer to the surface of the paper, otherwise it will cause the surface strength of the paper to decrease and even deform. Therefore, we hope to get a water-repellent and ink-friendly blanket.
· In fact, it is impossible to get a blanket that is completely non-absorbent and has good ink affinity. Adding hydrophilic materials such as rubber and fibers in the paper and pigments in the ink on the blanket during the printing process Adsorption causes the ink-absorbing ability of the blanket to decrease during the printing process, while increasing the hydrophilic ability. Therefore, in addition to choosing a blanket with good surface properties before printing, pay attention to cleaning the surface of the blanket frequently during printing to maintain the good ink-repellent and hydrophobic properties of the blanket
â‘¡ Due to the periodic compression and elastic recovery of the blanket during printing, in the current high-speed printing equipment, this cycle is quite short. If the elasticity of the blanket shows a lag, that is, it cannot be immediately after the pressure disappears Restore, then the compression range of the blanket will be affected during the compression process in the next cycle.
· The hysteresis of the blanket cannot be avoided, and even irreversible plastic deformation will occur. These are factors that affect the normal embossing in printing. Before printing, we must choose the appropriate available blanket, and after the blanket is pressed for a period of time, we must check the printing pressure (for example, it can be detected by the method of embossing). And advocate the use of two blankets in turn. Give the blanket a time to recover and reduce the aging cycle of the blanket.
â‘¢ The ideal blanket should be compressible, that is, the volume reduction occurs only in the compression area, and the volume in the non-compression area does not change. This can reduce the ratio of the linear speed of the cylinder surface in 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 imprinting area, the front convex hull and the rear convex hull appear in the blanket, which makes the difference in the linear velocity of the imprinting area have a greater change , Which has a great influence on the correct transfer of outlets. Relatively speaking, the compressibility of the air-cushion blanket is quite good. For high-quality prints, using the air-cushion blanket can get a good printing effect.
2. 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 copy process. The stability of the printing equipment mentioned here includes the transmission accuracy of the printing machine, the dynamic balance of the printing transmission parts, the uniformity of the transmission speed, and various components The accuracy of the coordination between the stable. These problems will be directly related to the quality of the transfer of the print copy process, such as the stability of the dot, the stability of the amount of ink, and the stability of the pressure. These will directly affect the quality of printed matter.
In actual printing, we must consider the accuracy and stability of the equipment itself for different printed products. If the stability of the equipment itself does not meet the requirements, it is absolutely impossible to print good prints. This also puts forward that in the daily production, while ensuring the output, don't neglect the maintenance and maintenance of the equipment itself. Having a stable operation of equipment is a prerequisite for printing good prints.
(2) The positioning of the paper in the printing process is accurate and stable.
After the paper enters the machine from Feida, it is continuously transferred in the positioning and handover mechanism of the machine. Each step of precise operation will be related to the accurate registration of the substrate.
· First, Feida, which includes the various components on Feida, to ensure the stability of Feida paper separation and paper transfer at all times, but also pay attention to the maintenance of the air pump, and pay attention to the replacement and repair of the parts with serious wear.
· Then the various components on the paperboard, including the pressure roller, brush, double-sheet controller, paper conveyor belt, etc., on the premise of ensuring a reasonable adjustment of the working position and tightness, more attention should be paid to the symmetry of the components on both sides of the paperboard Sex. Avoid skewed paper.
· Front, side and swing teeth. Adjust in strict accordance with the angular position of the equipment to avoid inaccurate positioning and handover errors.
· Crushing the teeth of the rollers (including impression rollers and paper rollers), as above, pay attention to the accuracy of adjustment.
All the organizations mentioned above are directly related to the overprint relationship of the printed matter. It is necessary to make reasonable adjustments and ensure the absolute precise operation of these organizations in order to make the printed products have more accurate overprint.
(3) Imprint mechanism. The ideal working state of the embossing mechanism is that the ideal printing pressure is adopted, and the pressure is stable and unchanged during the operation. (See the process control for the principle of pressure regulation)
(4) Water (ink) road. The requirement of the water (ink) path is to make the water (ink) spread evenly, the pressure between the water (ink) rollers should be uniform, and the pressure between the printing plates should be uniform and reasonable, the specific requirements should be determined according to the requirements of different machines.
3. Control of printing process
With ideal materials and good printing equipment, it is possible to print good printed products, but by no means can we print good printed products. The next most important point is the need for a reasonable printing process control.
(1) Exposure. 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 in the printing process:
1) Determine the correct exposure time; 2) Determine the correct pumping time; 3) Dust removal work; 4) Tone range: 2% -98%; 5) Determine the gripping position of the printing plate; 6) Exposure Quality control strips.
· The issues we should pay attention to in the printing work are:
Pay attention to the quality of the PS version
1. The background color of the PS version is not good. 2. Improper development time. 3. After development, a "blue background" appears, causing the printing to become dirty. 4. It is not easy to add ink, and it is not easy to add ink when printing on PS. 5. Partial "flower version", paste version. 6. The printing process was interrupted. 7. Partially lost words. 8. Partly off-page. 9. The surface roughness of the PS version is not good 10. PS version has poor ink affinity and alkali resistance. 11. Improper ratio of photosensitive ratio. 12. The photosensitive film is too thin.
The above problems are what we should pay attention to in the printing work. The appearance of any one of them will lead to a decline in the quality of the printing, or even void.
(2) Water and ink balance control in printing
①Water volume control. In fact, we don't need water at all in the final printed matter. Even we always complain that water has caused various defects in our products (such as the low gloss of the ink on the surface of the printed matter due to the presence of water). So we are always looking for a lithographic printing that does not require water at all, so waterless offset printing appears. Although it has various advantages that water offset printing cannot match, but one defect is that the cost is too high, which is enough to make it impossible. Better promotion. So we have to start with water printing, because of the limitation of plate material and ink, we still ca n’t do without water for the traditional water printing equipment, all we can do in printing is not to ink the blank part as much as possible. Under the premise of reducing the amount of water used, this can reduce the emulsification of the ink and make the edges of the dots clearer (the boundaries between oil and water are clear). Second, the ink can be smoothly transferred to the surface of the substrate. The large decrease in the surface properties of the substrate and the deformation of the substrate's water absorption.
· To ensure that the dots are clear and the blank parts are not dirty, it is a basic principle for us to use water in offset printing as much as possible. Some people say that there is water printing, and control the water to master the printing. It really makes sense to produce better printing quality.
§ Attached (1) How to judge the excessive amount of water
The ink color on the printed matter is light, even if the amount of ink is increased, the ink color will not deepen 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 amount of water on the front and back sides is uneven, and the paper is curled;
There is too much water remaining on the surface of the ink film, the ink added to the ink fountain is not easy to stir evenly, and the transfer roller has a slip phenomenon;
There is sticky flow of water at the trailing end of the blanket, and drops of water are dropped at both ends of the roller;
Before and after the printing is interrupted, there is a large gap between the shades of the printed products;
The layout will not dry after shutdown.
(2) Ink volume control. Our control of the amount of ink in printing is mainly based on the printed samples. This process is mainly carried out through the vision of the human eye, but experienced operators can often do well.
First of all, we must determine a reasonable color sequence according to the process characteristics of the product. Different color sequences have different degrees of impact on print quality.
Then is to compare the printed sample with the printed sample. This comparison generally has two ways. One is the visual method, which is to compare the printed sample with the printed sample to identify the color shade of each part (if Monochrome machine is not sure about the yellow color printed first, you can use the red or blue filter); One is the feel method, that is, just print the picture on the printed sheet and press it by hand. Sticky means that the amount of ink is more appropriate. However, these two methods require very experienced operators to be able to use, and the subjectivity is relatively strong, the quality is not stable.
Modern and more advanced printing systems have successively launched relatively perfect ink color control devices. Such as HEIDELBERG's CPC system; ROLAND's CCI system; KOMORI's PQC system, etc. provide intelligent ink color control function, replacing the artificial judgment of ink color with the computer's intelligent judgment. These systems provide the possibility of scientific and digital printing ink color.
(3) Pressure control during printing
The existence of printing pressure can cause many problems such as dot increase, but because printing pressure is a prerequisite for ink transfer during the printing process, it is impossible to completely avoid it. Therefore, like 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 transferred reasonably, 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 deformation of the blanket under pressure, thereby reducing the linear speed difference on the surface of the drum and reducing the speed due to this speed. The dot deformation caused by the difference; reduces the wear of the printing plate, reduces the number of printing plate replacement during the printing process, enhances the stability of the printing quality; improves the service life of the blanket, and at the same time makes the pressure during the printing process more stable……
(4) Overprint control of printed matter
Overprinting of printed matter is usually an important factor in determining the quality of printing. If the overprinting problem cannot be guaranteed, then there is no question of higher quality.
The control of overprinting accuracy comes from the precise adjustment of the printing equipment we mentioned in "Equipment" (provided that the printing equipment is available), as well as the following factors:
1. Humidify the substrate to make its own humidity equal to or slightly higher than the ambient humidity of the workshop.
2. Maintain constant temperature and humidity in the workshop environment.
3. Reduce the water consumption as much as possible during the printing process.
4. Ensure that the thickness of the printing plate liner of each color group is the same during the printing process. Due to the calendering problem of multicolor printing and the participation of water in the printing process, it is now more advocated that the thickness of the printing plate liner for post-color printing is slightly less than The thickness of the front color pad is 0.03 mm.
In fact, there are many factors that can improve the accuracy of printing overprint, which are not listed here.
4. Other control aspects
In addition to some of the aspects mentioned above, operators in printing work should also pay attention to the following issues:
(1) Read the work ticket carefully, and perform the operation strictly in accordance with the requirements of the work ticket (such as print specifications, paper types, and other special requirements) to avoid quality accidents caused by failure to follow the work ticket requirements.
(2) Adhere to the "three-industry" operation. The so-called Sanqin refers to the process of mixing ink in the ink fountain, looking at the amount of water in the plate, and sampling the control samples during printing.
(3) Strengthen the storage of semi-finished products. The storage requirements for semi-finished products are to meet the overprinting requirements for color printing or back printing, to avoid paper deformation, blot crystallization, dry blots, dirty back, etc. due to poor storage. In order to ensure the quality of semi-finished products, the following points must be made:
â‘ Paper stacking is neatly stacked, if not, it will cause a lot of paper edges to be exposed in the air, so it is easy to be deformed due to changes in humidity, and due to uneven paper edges, the paper edges are likely to be damaged or folded Phenomenon, these conditions will affect the next process, not good for printing quality.
â‘¡ Because the paper around and on the surface of the paper stack is exposed to the air, it is easy to deform the size. Therefore, about 20 pieces of waste pieces should be placed on the surface of the paper stack during the stacking process to protect the printed products on the surface. In addition, if it is stacked for a long time, it is necessary to cover the paper stack with kraft paper or plastic film.
â‘¢ The stacking height of the paper stack. It should be decided according to the specific situation. Generally, there are no large pieces of solid prints. The stack of paper can be properly stacked. For large solid areas or high smoothness of the paper surface, the stack height should be appropriately reduced. Prints with a large amount of ink (especially when printing on coated paper or white cards), should consider drying racks. Doing so is conducive to the drying of the ink, and at the same time avoids the back rubbing.
â‘£ When encountering printed products with a large amount of ink, shake the paper stack at regular intervals when stacking the paper stack. Assist drying.
In the printing process, the factors that can affect the printing quality are not only the factors mentioned above. In actual printing, there are many factors of one kind or another that will cause one or other problems in the printing quality, so we also need to use We carefully discovered and dealt with. In addition, in the work, we should promote the standardization and standardization of management and operation, make a record of the changing factors in the work, and find out the experience to facilitate the data management, so as to achieve the stability of printing quality.Ningbo XISXI E-commerce Co., Ltd , https://www.xisxipet.com