Analysis of the most popular offset overprint faul

2022-08-13
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Analysis of offset overprint fault

overprint accuracy is an important aspect of measuring product quality. However, in the work, the overprint accuracy is often reduced due to various reasons, which seriously affects the quality of prints. Especially in offset printing, there are many reasons for overprint failure, mainly including printing materials, printing technology and equipment. This paper attempts to make a more complete analysis and Discussion on the formation and elimination of overprint faults from the aspects of influencing factors of overprint accuracy, analysis methods and control of each link

I. factors affecting the overprint inaccuracy of offset press

color plate making is a process of color decomposition, while color printing is a process of color combination, which is called overprint. The minimum quality requirement of color printing is to overprint accurately and make the printed matter faithful to the original. During the printing process, due to various reasons, it is easy to cause overprint error of products to exceed the standard, resulting in the scrap of printed products. The following is an analysis of several aspects involved in overprint error:

1. The influence of printing plate on overprint accuracy

lithography plates are now mostly thin and light aluminum. In printing, the deformation or volume change of the plate will affect the overprint accuracy

the deformation of the printing plate is roughly caused by two aspects, the first is tensile deformation, and the second is baking deformation. Stretch deformation refers to the misregistering caused by the artificial extension of the metal plate in the circumferential direction of the drum due to the improper force of the operator in loading and pulling the plate during the proofreading process. Baking deformation refers to the change of the linear size of the printing plate caused by improper baking when baking to improve the printing resistance of the dried PS plate. When the temperature in the oven is too high or uneven, the plate base becomes soft, affecting the dimensional stability of the plate

2. Influence of cylinder lining on overprint accuracy

only when the radius after lining is equal, can the linear speed of the cylinder of printing plate, rubber and embossing be consistent, and the transfer can be carried out effectively under a certain pressure

when the lining thickness is too large, the radius of the drum will increase correspondingly. Under the same angular speed, the linear speed of the drum surface will speed up, and there is a speed difference between the rollers, which will not only cause the wear of the layout text, but also cause the "shrinkage" of the layout text; The roller lining is too thin, the radius is reduced correspondingly, and the linear speed becomes slow. The dots will be "stretched" during the transfer process, which will cause the text and text to "zoom in", resulting in inaccurate overprint

3. Influence of paper on overprint accuracy

in lithography, paper is an important factor affecting overprint accuracy, especially offset paper. Due to the influence of paper, overprint accuracy is difficult to control

the influence of paper on overprint accuracy mainly comes from the deformation of paper. The deformation of paper is various, but it can be divided into two types, namely, natural deformation and pressure deformation

natural deformation is the characteristic of paper due to its fiber's water absorption and expansion, dehydration and contraction under the action of no external force. The water content of paper changes with the change of temperature and humidity. When the water content in the air is greater than that of paper, the paper absorbs water, and the paper edge is elongated and wavy, commonly known as "lotus leaf edge"; If the moisture content of the air is less than that of the paper, the edge of the paper will release moisture and there will be a "tight edge" phenomenon until the two reach equilibrium. Experiments show that the moisture content of paper will change by about 1% for every 10% change in relative humidity. Figure 1 shows the relationship between relative humidity and paper stretching. In the offset printing process, when the second color is set after the first color is printed, the overprint inaccuracy caused by the expansion of the paper is very obvious, especially those papers with loose structure, light sizing and strong water absorption

pressure deformation is the deformation of linear size and area caused by the extrusion of paper in the drum and the dislocation of fibers. In the printing process, due to the extrusion of the drum, the paper extends outward in a sector from its mouth. The closer it is to both sides of the trailing tip, the greater the overprint error. This phenomenon is often referred to as "angle swing"

in addition, the paper cutting size changes greatly, and there will also be inaccurate overprint when it is seriously skewed or folded

4. Influence of the amount of fountain solution used on the overprint accuracy

excessive roller solution during printing will increase the elongation and deformation of the paper, which is easy to cause various printing faults. When the water content of the printing plate is too large, the transferred paper will stretch because the fiber absorbs excessive water. During chromatic printing, the paper will be irregularly deformed, resulting in inaccurate overprinting. In addition, when the paper is normally printed with the advent of Fieldbus under a certain tension, if the moisture suddenly increases, the tension will change, and the regular line will also be offset, resulting in inaccurate overprint

in the offset printing process, whether the water ink balance is appropriate not only affects the accuracy of overprint, but also has a very close relationship with the normal transfer of imprint, the depth of ink and the dry state of printed matter. Therefore, whether we can correctly grasp and control the ink balance is the key to ensure the stability of printing product quality

5. The influence of viscosity on overprint accuracy after the implementation of ink Document No. 65

when the ink viscosity is too large, the peeling tension of the paper on the blanket is increased due to the effect of adhesion. When the paper is peeled off from the drum, it is subject to the action of adhesion and stripping tension. If the tooth holding force cannot overcome the stripping force, the paper will twitch and small displacement will occur. This phenomenon is manifested as inaccurate overprint on monochrome machines and ghosting (phantom) on multicolor machines. The other is that the paper is peeled off from the surface of the blanket under the action of the teeth of the imprinting cylinder and the traction force, so that the trailing part of the paper is stretched, and this phenomenon is easier to occur under the following conditions: (1) the ink viscosity is high; (2) The paper trailing tip is printed with the field; (3) Use thin paper printing; (4) Too much fountain solution is used. For the wet press wet printing method of multi-color offset press, when the degree of elongation at the trailing tip of each paper is inconsistent, there is a ghosting problem

6. Influence of printing color sequence on overprint accuracy

at this stage, four-color offset printing machines are mostly used, and the four-color offset printing machine adopts the wet press wet printing method. High speed increases the separation speed, drum curvature and inertia force between paper and drum, which is prone to problems such as paper stripping or roughening, color mixing or color deviation, dirty back, ghosting and so on. Therefore, the four-color offset press wet stack wet printing has strict requirements on the printing process arrangement. In addition to the requirements that the printing ink layer should be thin, the less the page moisture is, the better, the ink and paper printability is good, and the printing pressure is reasonable, the chromatic sequence must also be reasonably arranged

from the perspective of overprint accuracy, the general color arrangement sequence is: (1) overprint is strictly printed first, and the overprint with low requirements is printed later; (2) The primary color comes first and the secondary color comes second; (3) The bottom color comes first, and the pressing color comes last; (4) Several adjacent color overprint should be one color after another, not with two hues at the same time; (5) The line and point are first, and the field is last. On the basis of the above color arrangement, the two-color or multi-color offset printing machine should also pay attention to the light color in front, the dark color in the back, the secondary graphics and texts in front, the main graphics and texts in the back, the ones with less ink consumption in front, the ones with more ink consumption in the back, etc

in addition, it should be noted that the number of overprinting should not be too many, otherwise the possibility of error will be greater due to multiple overprinting

7. mechanical influence on overprint

the printing paper must be effectively controlled by the machine from beginning to end, and the relative position must be stable in the process of transmission and handover. Only in this way can we meet the requirements of overprint accuracy, otherwise the overprint accuracy will be greatly affected. There are mainly the following situations that have the greatest impact: first, the accuracy of the printing equipment itself is low and the error is large, that is, the mechanical wear is large due to the long-term use or improper maintenance of the printing equipment, resulting in the mismatch of various parts of the equipment and inaccurate overprint. This is mainly reflected in the problem of paper positioning, that is, regular parts; Second, during the transfer and handover of paper, the relative position of paper changes due to unstable handover, resulting in inaccurate overprint

II. Control of overprint accuracy

in view of the factors affecting overprint accuracy, corresponding control methods are adopted to achieve the purpose of overprint accuracy

1. to avoid the size change of the printing plate after baking. When baking the printing plate, the temperature in the oven must be uniform so that the plate is evenly heated. The temperature should not be too high and the time should not be too long. For short version products requiring high accuracy, it is best not to bake the version

standard operation should be carried out during version calibration. The position of the plate clamp should be centered, the printing plate should be aligned, and the plate clamp screws should be fastened. When pulling the plate, the jacking screw should be loose. As long as the force is moderate, the deformation of the printing plate can be generally avoided

2. When the sheet fed offset printing machine is used for chromatic printing, the internal image is often incorrectly overprinted vertically. Its characteristic is that the vertical and horizontal crosshairs are aligned, while the internal drawing of the bite or trailing tip cannot be aligned longitudinally, which affects the product quality

in addition to factors such as paper expansion and contraction and the weight of gestures when pulling the plate, another important factor is the inaccurate pad lining of the printing plate

in some printing plants, the operator does not have a dial card measurement when loading the plate, but depends on seeing, touching and experience to determine the number of printing plate liners. Although some machines are equipped with a dial card, they are not used. They are padded "almost", which artificially causes the problem that the diameter of the printing plate cylinder is sometimes large in this color and sometimes small in that color, resulting in inaccurate overprinting of the front and rear internal drawings. Especially for products with small white hollow words on the bite and trailing tip, it is difficult to register the hollow words on both sides

some offset machine operators do not know how much the plate pad pad will change the size of the text. Although some know that there is a change, they do not know how to change it? How much? The following is an explanation:

the cylinder diameter of the offset press is equal to the pitch diameter of the cylinder gear. For example, the cylinder diameter of J2108 machine is:

d pressure =d section =300mm

the arc length of the cylinder in one cycle of operation is:

d pressure · π = 300 × 3. =942.47778mm

the arc length per degree of operation is:

942.47778 ÷ 360=2.61799mm

if the printing plate is padded with 0.2mm, the cylinder diameter increases by 0.4mm, the cylinder diameter reaches 300.4mm, the cylinder circumference increases to 943.73441mm, and the arc length per degree increases to 2.62148mm. Assuming that the original length of the printing plate is 520mm, that is, the arc length is 520mm, and it is padded according to the printing plate standard, that is, when the printing plate cylinder diameter is 300mm, The degree of rotation of the plate cylinder during imprinting is:

520 ÷ 2.61779=198.64 °

when the plate is padded with 0.2mm and the diameter of the plate cylinder is 300.4mm, the degree of rotation of the plate cylinder is:

520 ÷ 2.62148=198.36 °

0.28 ° less than the original, and the arc length is:

2.62148 × 0.28=0.73mm

that is, because the printing plate is padded with 0.2mm more, in a certain rotation angle, the surface linear speed of the printing plate cylinder is faster than the surface linear speed of the rubber cylinder, and the specific length is 0.73mm, so the image and text size transferred on the rubber blanket is 0.73mm smaller than the original

on the contrary, if the printing plate is less padded by 0.2mm, the final image and text size will be 0.73mm larger than the original

therefore, it is unreliable to determine the number of printing plate liners by eyes, hand models and experience. Measure the thickness of each printing plate with a dial card, and then use appropriate backing paper for backing according to the standard backing data. Once the inner drawing is overprinted longitudinally due to the negligence of padding during production

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