Research on overprinting problems
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First, overprint and overprint error concept
The monochromatic printing plates of different screen angles obtained by separating the originals are sequentially overlapped and arranged according to the printing color sequence, and finally printed with the same level and color tone as the printing, which is called printing overprinting.
Overprinting accuracy is the most basic requirement for printing, but due to various subjective and objective factors, overprinting inevitably has certain deviations (generally referred to as "overprinting errors"). Therefore, what we often say "accurate overprinting" actually means that the error of overprinting is controlled within a certain range.
Second, the evaluation of overprint accuracy
The tolerance values for overprinting are generally based on the human eye's visual observation. Studies have shown that under normal observation conditions, the ability of the human eye to distinguish fine lines can reach 0.10 mm. Therefore, the tolerance allowed for overprinting is set to 0.10 mm, that is, 0.10 mm is used as a reference for overprinting accuracy (but at this time, the human eye can recognize extremely fine hues). Practice has confirmed that for wet-stacked monochrome overprinting, the tolerance of the overprinting tolerance is specified to be 0.10 mm; however, for wet-wet multi-color overprinting, the allowable overprinting error must be equal to or less than 0.03 mm. Under normal circumstances, the overprinting standard is usually set in the range of 0.05 to 0.10 mm, and for printing products with low quality requirements, the overprinting standard can even be relaxed to 0.15 to 0.20 mm.
The law of overprinting error is neither uniform nor chaotic, and it conforms to the normal distribution in mathematical statistics. Further research confirmed that the actual overprint error obtained in practice is 68.26% in the range of single standard error (1s), 95.45% in the double standard error (2s), and 99.73% in three standard errors ( Within 3s). This provides an important basis for the evaluation of overprint accuracy.
In order to determine the accuracy of the overprinting, the "positioning mark method" is usually used to detect the changes occurring during the overprinting, so that the printing operator can take timely measures to improve the accuracy of the overprinting. In the offset printing process, the "cross mark method" is generally used for positioning. This method is calibrated at the four corners and the middle portion of the printing plate at the time of plate making, and is cut off together with the paper edge after printing.
In multi-color printing, it is possible to check the overprinting of the top, bottom, left and right, and diagonal lines of the printed product from the misalignment or skewness of the line marks of various colors. These marking lines are commonly referred to as rule lines and are sometimes referred to simply as "regulation lines." The rule line is mainly divided into three types: cross line, angle line and cutting line. Since the line plays an important role in positioning, it must be very careful in making it. It must ensure that the size and geometric accuracy of the line itself are high, and the line is high. Be small, straight, and clear.
Generally, most of the overprinting occurs at the two corners of the tail of the paper, and the image of the cornice is less likely to be overprinted. Moreover, the side where the overprint is not allowed to be more serious must be the opposite side of the side gauge.
Third, the factors affecting overprinting
The following focuses on some of the main factors affecting the accuracy of overprinting during the printing process.
Machinery
The key to overprinting accuracy is to transfer the paper conveyed by the paper feeder to the impression cylinder accurately in the up and down direction (ie, the printing direction, along the radial direction of the drum) and the left and right direction (ie, the vertical direction of the printing direction, along the axial direction of the drum). on.
The overprint adjustment structure consisting of paper stoppers, swinging teeth, grippers, etc., only allows the paper to be fixed in the correct position for precise movement, and the movement of each component must be coordinated, without any mistakes, to register.
The overprinting error caused by mechanical factors is mainly due to improper adjustment and use of various parts of the machine, resulting in misalignment or mechanical wear between the components. Generally it mainly occurs in the following four processes:
(1) Paper feeding and conveying process of the paper feeder;
(2) The pre-defined process;
(3) side-pointing process;
(4) The transfer process of paper transfer (ie, the transfer of the front gauge to the transfer paper and the transfer of the transfer tooth and the roller jaw).
The paper is sent from the paper feeding table to the pre-specified position of the paper feeding until the end of the embossing. The paper is matched between the components, and the following principles must be met in the process:
(1) In the process of separation, transportation, imprinting, etc., the paper must always be strictly and correctly controlled by organic parts;
(2) The working cycle and trajectory action of each component of the paper feeder must be closely matched with each other accurately;
(3) In the paper feeding, the paper requires the spacing of the paper to be accurate;
(4) Maximize the paper positioning performance to avoid the change of positioning time caused by various factors;
(5) Correctly adjust the paper feeding time of the paper feeder to match the main drum cycle;
(6) When the front gauge, side gauge, and transfer paper are in paper positioning and transfer handover, the time must be accurate;
(7) Before the specified position, the paper should not be bitten, bitten or bitten, and the upper and lower limits should not exceed 3~5mm;
(8) When the transfer paper and the roller bite are transferred, the time must be accurate, and the lead or hysteresis must not occur;
(9) When the machine is handed over and controlled, it must be very stable and not subject to the influence of other external forces to change the accuracy of the work;
(10) The bite of the roller biting paper and the transfer paper should be uniform and larger than the pushing force of the roller on the paper;
(11) Each drum gear should be meshed at the pitch point of the pitch, and the distance between the two sides of the drum should be equal and parallel;
(12) The paper feeding table, the side gauge, the teeth and the drum cannot have axial movement;
(13) The distance between the platen roller, the brush wheel, the brush and the like, and the position of the paper and the pressure of the platen control paper, which are accurately positioned by the paper feeding table, must be strictly controlled and controlled.
The paper transfer relationship is quite critical. The content of the handover relationship includes two aspects: one is the position of the handover, that is, the relative position of the two handover parts when the paper is handed over, wherein one machine is used as a reference, and the position of the other mechanism is adjusted to be compared with the reference mechanism. Coordination, the adjustment between the various parts has a sequence. The second is the time of handover, that is, the time when the two transfer parts control the paper at the transfer position (near) at the transfer position, that is, the paper is controlled from the paper machine, and the paper machine starts to control the paper. After a period of time, the paper machine is stopped when the paper is released. There is no order requirement for handover time.
It is worth mentioning that in order to prevent the misregistration of the gripper teeth from being uneven, the following points should be achieved:
(1) The adjustment of the gripper should be carried out in the test print, and the grip force should not be changed during the imprinting process. For each color unit of the multi-color machine, the gripper force of the gripper on the impression cylinder should be consistent, and the gripper of the gripper must be flat to ensure the uniformity of the force.
(2) For the products of self-overprinting on the front and back sides, the crepe paper of the impression cylinder must be uniform.
(3) The support spring of the crepe paper shaft should have good elasticity, prevent the paper squeezing force from being insufficient, and the paper positioning is not accurate.
(4) Tighten the tight ring of the gripper to prevent the jaws from moving in the axial direction. Using a high-precision crepe paper shaft or bushing, the crepe paper is in a stable state during the creping process, and the crepe paper distance is always maintained at a constant value.
2. Plate
The commonly used printing plate material is a thin and light metal plate which greatly affects the overprinting accuracy due to the shape or the like.
(1) Tensile deformation
When the two ends of the printing plate are subjected to the pulling force, due to the ductility of the metal plate, irreversible radial elongation deformation will occur, and the graphic surface of the printing plate will inevitably elongate, and the elongation value will be generated by the graphic. Overprint error.
Under the action of the tensile force, the metal plate has an elongation value ΔL which is proportional to the tensile force F and the length L, and is inversely proportional to its cross-sectional area A and the elastic modulus E. Expressed by the formula:
△L=F·L(E·A)
From the above formula can be launched, and the practice proved that:
The tensile deformation ΔL of a printing plate is generally caused by operations such as loading and pulling.
The larger the tension F of the compact plate, the larger the deformation of the plate. Therefore, do not use excessive force when loading and pulling.
The greater the thickness of the 3 plates (i.e., the cross-sectional area A), the smaller the deformation of the plate. Therefore, from this point of view, the use of thicker materials for the printing plate can reduce overprinting errors.
The longer the length L of the 4 plates, the larger the stretching value, which is more serious on machines with larger printing areas.
The larger the elastic modulus E of the five plates, the smaller the deformation of the printing plate.
In addition, it is also easy to know in actual operation that the machine with a small wrap angle has a small stretching value and a uniform distribution of the stretching value, and can be pulled by the frictional force by pulling the vibration during the drawing. Plate; a machine with a large wrap angle, the plate is not easy to be pulled, and the axially moving parts must be prepared to make the plate axial displacement and eliminate the friction on the back side to drive the plate.
(2) Bending deformation
After the printing plate is mounted on the plate cylinder, it becomes a cylindrical shape from a regular sheet shape, and bending deformation is inevitably caused. Formulated as follows:
△L=L·b/(2R+b)
Where b is the thickness of the plate, R is the radius of the plate cylinder, and L is the original length of the graphic.
It can also be inferred from the above formula:
1 When the thickness of the printing plate is constant, the bending deformation value of the printing plate decreases as the radius of the printing plate increases. When the radius of the plate cylinder is constant, the bending deformation of the plate increases as the thickness of the plate increases.
2 The surface coating of the multi-layer metal plate should not be too thick, otherwise it will be excessively deformed due to the bending of the plate surface, causing the plating layer to partially crack, forming a dense moire pattern, causing the blank portion of the printing plate to be dirty. In addition, a flexographic printing plate such as a photosensitive resin plate has a thick plate base and a large deformation. Therefore, in the layout design, we must pay attention to the problem of deformation.
3 Multi-color printing presses should use the same thickness of the printing plate, so that each plate produces a bending deformation close to ensure accurate overprinting.
4 high-speed rotary printing press, the printing speed is high, the radius of the plate cylinder is small, and the surface utilization factor is large, so that the bending deformation caused by the printing plate is large. In order to control the bending deformation of the printing plate, a thinner printing plate is required.
(3) Method of adjusting the printing plate
It is particularly worth mentioning that, in general, we found that overprinting was not accurate when printing, and most of them were adjusted by stretching the plates. The greater the pulling force, the greater the tensile deformation of the plate and the uneven distribution of the tensile deformation (the force is greatest at the edge of the mouth and the edge of the blade). From the above analysis, it is known that this is unfavorable for overprinting.
In the operation of the printing press, the standard action of adjusting the printing plate should be: when the printing plate needs to move axially, the intimate friend first removes the circumferential tension of the printing plate, so that the pad can be rubbed against the printing plate. The force disappears or is reduced to a sufficient extent; when the plate needs to be moved circumferentially, the end of the plate must be removed from the tension at the other end, so that the plate can be loosened, and absolutely must not be too strong. Otherwise, the amount of tensile deformation of the plate is greatly increased. Therefore, when adjusting the position of the plate, the axial movement and circumferential movement of the plate must be coordinated. Do not hard pull hard.
In addition, if the printing plate is loose during printing, the position of the imprint will be jumped during the printing process, which will also affect the overprinting accuracy.
In addition, the platesetters of multicolor machines are made under the same plate making conditions to reduce or eliminate various disadvantages.
3. Paper
In the process of multi-color overprinting, the image overprinting error is often caused by the deformation of the paper. It is also a major factor affecting the accuracy of overprinting.
Paper deformation is summarized, with natural deformation and compression deformation. The former refers to the paper which, under the condition of no external force, undergoes water absorption or dehydration due to changes in its own water content, resulting in dimensional elongation or shortening deformation. This deformation is closely related to the temperature and humidity of the surrounding environment. The latter refers to the plastic deformation of the linear size and area caused by the misalignment of the fibers when the paper is subjected to the pressing force in the rolling of the drum.
Overprinting errors caused by the two deformations of paper are almost inevitable. However, effective control of the water content of the paper, reasonable adjustment of the ambient temperature and humidity and the pressure of the drum can ensure the linear size and area of the paper to be stable, reduce the deformation caused by the paper, and thus reduce the overprinting error.
Both theoretical and practical production have confirmed that during the overprinting process, the water content of the paper should not change by more than ±1%, otherwise the accuracy of overprinting will be affected. Therefore, the temperature of the workshop must be strict, and the general variation is not more than ±3 °C.
The silk thread of the paper also has a great influence on the overprint. In the printing, the paper is mostly used. Due to changes in temperature and humidity, a dried fiber, after complete wetting, may increase its diameter by 30%, while the length direction increases by only 1% to 2%. Therefore, the transverse thread is much larger than the expansion ratio of the longitudinal thread, which directly affects the accuracy of overprinting. The expansion and contraction of the longitudinal wire can be compensated by adjusting the lining and the like.
In addition, the amount of moisture in the printing process also affects the overprint accuracy. Generally, the moisture should not be too large, otherwise the paper will be stretched too much.
In addition, the cutting accuracy of the printed paper will also affect the overprinting error. The cutting precision of paper mainly includes two aspects of dimensional accuracy and verticality.
Good cutting accuracy generally requires:
(1) The verticality of ∠BCD is very important, and its right angle must be controlled within the allowable precision:
(2) The cutting specification of the paper must be AB=CD, and AD is parallel to BC, and CD is perpendicular to BC.
4. Pad
Changing the thickness of the liner causes misregistration of the print on the print and the size of the cylinder plate image, usually in the radial direction of the drum. There are two cases of this type of change in the size of the picture due to variations in the thickness of the roller pad:
1 When the radius of the plate cylinder is larger than the radius of the blanket cylinder, the size L of the plate image is smaller than the size L of the paper image. The registration error at this time is: △ L = L paper - L version.
2 When the radius of the plate cylinder is smaller than the radius of the blanket cylinder, the size L of the plate image is larger than the size L of the paper image. At this time, the registration error is: △ L = L version - L paper.
In order to change the error value of the image elongation or shortening, the error can be balanced by changing the thickness of the roller liner. In practice, in order to make the size of the plate graphic stretch in the radial direction, it is often used to reduce the thickness of the plate cylinder liner; or to reduce the thickness of the plate cylinder liner, and increase the blanket in equal amounts. The thickness of the roller pad is used to maintain the phase of the pad. Conversely, in order to shorten the size of the printing plate in a complicated labor direction, according to the same principle, increase the thickness of the plate cylinder liner; or increase the thickness of the plate cylinder liner while reducing the blanket cylinder by an equal amount. The thickness of the liner keeps the gasket up and down equal.
However, when increasing or decreasing the pad operation, it should be noted that the maximum allowable value of the roller pad is generally limited to ±0.10 mm.
5. Blanket
The deformation of the blanket will also cause the change of the size of the graphic to cause the overprinting error. It mainly comes from the pressure, the pulling force and the quality of the blanket itself.
The blanket is stretched on the drum, which not only produces a relative frictional force on the surface of the drum, but also has a close relationship with the offset error caused by the extrusion deformation and elongation of the blanket. According to the law of deformation of the blanket, its predation deformation mainly depends on two points: 1 the direction and size of the friction; 2 the elongation of the blanket itself. Under the same frictional force, if the elongation of the blanket itself is different, the degree of deformation must also be different. At the same time, it also determines the degree of tension of the blanket on the drum, that is, the tighter the tension, the smaller the elongation during the printing process, the smaller the dimensional change of the graphic, but the friction amount of the layout is increased; Of course. In addition, if the blanket (including the underlying liner) changes during the printing process, the imprint often changes, which also affects the registration accuracy.
Analysis of the reasons for the overprinting error caused by the above-mentioned blankets, which must be noted in daily production:
a. Try to use a blanket with less elongation to reduce deformation;
b. Keep the blanket tight, not too loose or too tight, and keep it constant;
c. The overprint error caused by the tension of the blanket caused by the dimensional change of the graphic shall not exceed 0.10mm;
d. For long periods of downtime, it is best to loosen the blanket that is tight on the drum.
Finally, it must be noted that among the above factors, factors such as machinery, paper and printing plates are the most important factors.
Fourth, measures and recommendations to improve the accuracy of printing overprinting
1. When designing and making plates, be sure to ensure the accuracy of overprinting. In particular, it is necessary to compensate for the amount of deformation of the plate in the layout design.
2. The operation of adjusting the plate must be standardized.
3. Properly control the printing pressure (preferably using the minimum pressure), increase or decrease the thickness of the lining according to the actual situation, and reduce the deformation of the printing plate, paper, blanket, and the like.
4. Strictly control the temperature and humidity of the printing environment, reduce the natural deformation of the paper, and ensure the cutting precision of the paper.
5. Adjust all the parts that affect the overprinting accuracy, such as the paper feeding mechanism, positioning mechanism and paper transfer mechanism of the printing press, and their transfer relationship to the optimal position.
6. Enhance the operator's sense of responsibility, do a thorough inspection of the printed products, find faults, and eliminate them in time.