One: Quality Control of Printed Products in Offset Printing
Simply speaking, the printing process is the process of copying the original textual information on the substrate surface. In this regard, to evaluate the quality of printed matter, the first thing to look at is the faithfulness of the printed matter to the original. We know that due to the limitations of equipment, materials, printing processes, etc., it is impossible to completely faith in the manuscript. Therefore, the high-quality copying we have mentioned means that the printed matter is very close to the original. At the same time, because of the difference between print and manuscript, we not only require a small difference between print and manuscript, but we also put forward the stability of this difference, that is, the stability of copying. This stability is similar to printing. is very important.
In addition to the standards for quality evaluation of prints mentioned above, we have extended the surface quality standard (empirical quality standard) of prints from the aesthetic point of view of prints as follows:
a. The ink is bright and the depth of the screen is uniform;
b. The ink layer is thick and glossy;
c. The outlets are clean, clear, and free of burrs;
d. Conforms to the original and has a clear color tone;
e. Accurate printing;
f. Text is not missing;
g. The printed sheet has no wrinkles, no imprints, stains and fingerprints, and the product is neat;
h. The back is clean and free of dirt.
From the time when the manuscript is subjected to gradation compression, color correction, color separation, printing plate making until the end of printing, each step in the process will affect the reproduction quality of the printed matter. In the next article, we will focus on the quality control of prints in the printing process.
As we mentioned earlier, due to the limitations of various conditions in actual production, our print copy cannot be completely consistent with the original. Let's make a hypothesis. When all the materials, equipments, and processes that we use to participate in printing reach our desired ideal state, our printed matter will be very close to the original. Through this hypothesis we will easily think that what we need 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 better the quality of the prints we can get.
Below we discuss these conditions and processes separately.
First, the choice of printing supplies.
(1) Paper.
The nature of the paper will directly affect the quality of the reproduction of the printed matter. Therefore, in the design of the printing process, a trade-off must be made between the quality of the paper and the cost of printing (often, low cost and high quality cannot be obtained at the same time). The most ideal paper for print reproduction should have the following properties:
1 has absolute whiteness, that is, it is completely non-absorbable for various types of light. In this way, high image contrast can be printed. In addition, light can be prevented from entering the paper near the dot, and is absorbed by the dot from the inside of the paper, resulting in unnecessary optical dot gain.
In fact, paper with absolute whiteness does not exist at all, and none of the papers can completely absorb external light. In the printing process design engineering, what we need to do is to select as much whiteness as possible within the allowable cost range (except for products with special process requirements, of course) in order to meet the requirements for image reproduction in order to achieve richer level.
2 has a good gloss. Similar to the above, but the difference is that the gloss is not as great as possible, too much glossiness hinders the normal reading of printed matter.
3 smoothness. The smoothness of the paper refers to the roughness of the paper surface, and the ideal paper surface smoothness should be at least 500 seconds or more. If the smoothness of the paper during printing is too low, the high-profile dots will be easily lost, the low-key dots will be merged, and the real part will be empty.
• Also for the consideration of printing costs (sometimes due to the special effects of printed materials, such as textured paper), it is not possible to use high-quality, high-smoothness paper for each type of print. When encountering paper with low smoothness, the number of printing screens should be appropriately reduced during the process design, and the printing pressure should be increased during the printing process to make up for the surface smoothness of the paper.
4 has a neutral pH. Acidic paper can cause the ink to be unable to dry in the printing process. This can lead to over-emulsification of the ink in the printing process. As a result, a series of quality problems may occur in the printed product, such as blemishes of the printed material, blurring of the edges of the dots, and enlargement of dots. Deformation and other issues. When the pH value of the paper itself is alkaline, it reacts with a large number of pigments of the ink, which causes the printed product to fade after the printing is completed. This phenomenon is particularly evident in the printing of gold and silver inks.
· In the choice of paper, try to choose a paper that is near neutral, but if the paper has been selected, PH value has been determined, we must pass the pH of the dampening solution to the pH of the paper during the printing process. Make some degree of compensation. The general effect is also more obvious.
5 paper shape stability. The ideal paper should not be deformed, either under the influence of the printing pressure or under the dampening of the fountain solution. This ensures overprinting during the printing process and avoids a series of problems due to paper deformation.
• What we need to do is not to select paper that is not deformed at all, but to make trade-offs in the cost of the process and the highly stable paper. Such as the printing of newspapers, due to the low requirements and quality requirements are not very high, so when the choice of paper, stability is not very important, and newsprint can be accepted. For printing such as maps and other printed matter, we must choose papers with good stability to avoid overprinting problems caused by paper deformation and changes in the finished product size.
6 paper surface strength. The ideal paper requires no stripping and lint-free during the printing process, avoiding the wear of the printing plate due to this, and the reduction of image quality due to the adhesion on the blanket, printing plate, and ink roller.
· The paper we use in the printing process will generally fall off more or less during the printing process. In addition to selecting the paper before printing, we must judge the surface strength of the substrate and clean it in time during the printing process. Foreign material such as paper powder on the plate and blanket.
7 paper silk. The ideal paper is no silk direction, and the strength of the paper is large enough. However, as long as the cost is taken into consideration, we must not use paper with a lot of direction. In the printing, we must pay attention to the direction of the paper and the direction of the printer's paper, which can greatly improve the level of overprint.
(2) Ink.
Due to the nature of offset printing (need to print with the participation of water, the printing plate and the blank 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. Completely water-repellent offset inks cannot be printed on the machine. This is due to the fact that during the ink transfer process, water that has reached the ink path due to the high-speed shearing of the printing press will inevitably be encountered. When the oil and water are completely incapacitated, the normal ink transfer will be hindered.
· In printing, in order to ensure the ink's water resistance, we add additives (desiccant, remover, varnish, anti-fouling agent, etc.) to the ink to avoid reducing the cohesion of the ink, and in some cases According to the situation, a tackifier is added to the ink to restore the cohesion of the ink so as to restore the ink's water resistance. At the same time, the ink in the ink path and the ink fountain should be regularly cleaned to prevent the use of the residual ink on the machine and affect the water resistance.
2 sticky and elastic. Good adhesion is the premise to ensure that the ink can be properly transferred and transferred; and the ink's elasticity can ensure that the ink can quickly recover and level after transferring to the surface of the substrate. Good elasticity is the prerequisite for ensuring the smoothness of the surface of the printing dot.
• As mentioned above, except that good quality inks are selected, what we do in printing is to maintain the properties of inks. Many additives are added as mentioned above to minimize the ink properties due to human operations The change. Also pay attention to the control of the wetting fluid (discussed in detail in the process).
Thixotropic. The ink used in offset printing must have good thixotropy, that is, the fluidity increases under the action of the mechanical shear force when the ink passes out of the ink fountain, and the mechanical force when the ink is transferred from the surface of the blanket to the surface of the substrate. The disappearance of ink, the rapid decline in the flow of ink, began with the penetration of the binder in the ink on the surface of the substrate together to promote the initial curing of the ink. This process is very important for multi-color printing, can greatly affect the multi-color machine's overprint effect.
• Choosing a good thixotropic ink is very important for printing.
4 ink transparency. Offset inks require good transparency.
In fact, the transparency of printing inks in printing cannot be guaranteed, and the transparency of inks of different colors is not always the same. Therefore, in the printing process, the color sequence of the ink should be considered. A reasonable arrangement of the color sequence can effectively improve the color saturation of printed materials. degree.
5 ink particle size and coloring. Good printing inks require that the finer the particles of the ink pigment, the better the coloring properties. Select ink must not ignore these two points, the same is still the old saying, in the cost allowable conditions to select the finest particles of ink, high colorability of the ink, are conducive to print quality control.
6 ink stability. The stability of the ink includes the ability of the ink to resist light, heat, acid, alkali, and alcohol, and it requires that the ink does not change color and age under certain conditions, especially for inks with special uses (such as printing on billboards that need to be displayed in the sunlight, etc.) ), it is even more demanding that inks have this special resistance.
(3) Wetting fluid
The role of wetting fluid
The blank part of the printing plate forms a uniform water film to resist the infiltration of the graphic part of the ink to the blank part of the printing plate, preventing the dirty plate; forming a new hydrophilic layer, thereby maintaining the hydrophilicity of the blank part of the printing plate; Wet fluid to control the temperature of the plate surface. Because of the above effects of the wetting fluid, the requirements of the wetting fluid in the printing are: high surface activity (add surfactant or low surface energy substances in water); there is a certain degree of acidity (pH: 4.5- Between 6).
· The control of wetting fluid in the printing process is mainly considered from the pH value of the fountain solution. Here we briefly talk about the control of the PH value:
1 When the amount of desiccant in the ink is increased, although the drying speed is increased, the viscosity of the ink rises, the particles become thicker, and the adhesiveness to the blank part of the printing plate increases, and the dirty plate is prone to occur, so the amount of addition and discharge of the raw liquid is increased. The pH of the wetting fluid slightly decreased, which is beneficial for preventing the dirty plate.
2 If the non-coating paper with loose texture is used for printing, if the viscosity of the ink is large, the powder falling off is serious and the printing plate is dirty due to wear, it is necessary to properly reduce the pH of the wetting liquid. With advanced coated paper printing, the pH of the wetting fluid can be appropriately increased.
3 In the case of solid-printing, the pH of the wetting fluid can be lower. On the other hand, if dot printing is used, the pH of the wetting fluid can be higher.
4 When the temperature of the printing shop increases, the viscosity of the ink drops, and the fluidity increases. At the same time, the dry vegetable oil separates more free fatty acids and causes the layout to become dirty. Therefore, the pH of the wetting fluid should be lower.
(4) Blanket.
The ideal blanket should be completely non-absorbent, good ink, should have good elasticity, good compressibility, and can quickly restore the original shape after the pressure disappears, that is, all the elasticity in the printing performance Sensitive elasticity.
1 In printing, we want the blanket to deliver ink only to the surface of the substrate, and the moisture is that we do not want to transfer to the surface of the paper.