Brief analysis of the reasons for the inaccurate overprinting of multi-color rotary machines
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The development of book printing has become a reality today, and “multi-color printing is becoming more efficient”. The multi-color rotary machine is an important equipment for book printing, with obvious economic benefits, high printing speed and folding linkage, which provides reliable technical support for the efficient production of books and periodicals. However, due to the constraints of many factors, the quality of multi-color rotary machines has many problems. Among them, the problem of multi-color overprinting is very prominent. In order to improve the quality of the product, it is necessary to analyze the reasons for the inaccurate multi-color printing overprinting, which can lay a good foundation for solving the problem.
First, the printing elements related to multi-color overprinting
There are many printing factors related to overprinting, which generally include: the accuracy of the printing machine, the number and color sequence of the printing color group, the printing plate quality, the transfer pressure, the printing pressure, the characteristics of the blanket, the characteristics of the paper, the working state of the printing machine. and many more. Among these factors, the precision of the printing press is the key to determining the quality of the product. Maintaining the working accuracy of the printing press is the central task of the maintenance of the printing equipment and the basis of accurate overprinting. The number of printing color groups and the effect of color sequence on overprinting are easier to understand. The more color groups, the more times the overprinting is, and the more chances of overprinting problems. The color sequence is wrong or unreasonable, and the chance of paper deformation increases a lot in the wet-press overprint printing mode. Plate quality is the basis of multi-color printing, and the consequences of problems with plate making can directly lead to the inability to print. The transfer pressure and printing pressure determine whether the size of the ink on the plate is consistent with the plate when it is transferred to the substrate, and it is also the key to accurate product overprinting. These factors are controllable, and the characteristics of the paper, such as the characteristics of the blanket, the characteristics of the paper, and the working state of the printing machine are varied. These changing factors influence and restrict the entire printing process, and therefore, the overprinting of the product. Quality plays a direct role.
Second, the analysis of the reasons for the inaccurate multi-color overprinting
In general, the overprints we see can be divided into the following two categories: the overprints with different pattern sizes are not allowed to be overprinted with the same size. Among them, the same size of the overprint is not allowed to be divided into three types: axial overprinting is not allowed, circumferential overprinting is not allowed, and the pattern is skewed. In the rotary machine without the diagonal pull plate device, the solution to the skew can only be solved by reinstalling. Under normal circumstances, the circumferential and axial plate overprinting can not be solved by mechanical means. The reason for the inaccurate overprinting of different pattern sizes is the focus of our analysis. The change in pattern size between color groups that appears in actual printing is often referred to as a scale-up phenomenon. It is a frequent quality problem, and there are many factors contributing to this quality problem. Here are a few of them.
1. Analysis of pattern changes on the blanket cylinder
The pattern on the blanket cylinder comes from the plate cylinder. Under normal circumstances, the plate can ensure the uniformity of the size of each plate in the manufacturing process. Then, the change factor of the plate pattern to the blanket cylinder will become the pattern of the plate. The key to transferring right or not. This change is manifested in the axial and circumferential directions of the drum:
The transfer of the pattern cylinder to the blanket cylinder is based on the transfer pressure for contact transfer. In this process, the corresponding positions of the respective circumferences of the rollers are constrained by the transmission gears, regardless of the diameter of the plate cylinder and the diameter of the blanket cylinder. There is a difference that the arcs of the drum gears are always equal. The arc length corresponding to the same arc is related to the radius of the drum. The larger the radius, the longer the arc length, and the smaller the radius, the shorter the arc length. The radius of the plate cylinder (including the plate) is essentially a fixed value on modern rotary machines. The length of the pattern transferred from the plate cylinder to the blanket cylinder is related to the radius of the blanket cylinder in the circumferential direction of the drum. The diameter of the blanket cylinder is large, the pattern is long, and the small diameter pattern is short. Therefore, it is very important to maintain the uniformity of the diameter of the blanket cylinder on the multi-color rotary machine, which ensures the dimensional stability of the various patterns in the circumferential direction. At the same time, this uniformity plays a more important role in maintaining the linear speed of the blanket cylinder.
In the process of pattern transfer, what is the influence of the axial dimension change of the printing plate? The non-parallel axis of the roller has little effect on the axial transfer of the pattern. The simple calculation from the following can prove:
A2=b2+c2
A-b=c2/(a+b)
a and b represent the plate length and the blanket cylinder print length, respectively. c represents the deviation value of the drum axis that is not parallel. (a-b) indicates the amount of change in the plate cylinder and the blanket cylinder. On the rotary machine, the values of a and b are usually close to 1000mm, and the value of c is no more than 0.1mm.
A-b=c2/(a+b)=0.12/(1000+1000)=0.000005mm
This result shows that when the non-parallelism of the cylinder axis exceeds 0.1 mm, the pattern transfer error is only 0.000005 mm, and this variation is completely negligible. Therefore, it can be said that the axial length of the printing plate pattern is constant when it is transferred to the rubber.
It can be seen from the above analysis that the non-parallel axis of the drum has little effect on the pattern transmission, and it is important to keep the diameters of the blanket cylinders of each color group consistent. This is the basic requirement for ensuring the same circumferential dimension of the blanket cylinder, and the line speed of the blanket cylinder is kept consistent. The basis for ensuring the equal size of the pattern during printing plays an important role in multi-color overprinting.
2. Analysis of the use and change of tension during printing
The purpose of using tension on a multi-color rotary machine is mainly to keep the movement of the paper strip smooth, and to overcome the printing difficulties caused by the non-circularity and unevenness of the web. Therefore, in the case of stable paper texture, the use of tension is to ensure printing. One of the key factors of quality. The paper tension exists in the same form as other elastic objects, but the paper is non-rigid and resistant to tension. When stretched by an external force, the paper will be deformed, that is, narrowed and thinned while being stretched. When the web is subjected to tension, the tension is generated at a certain elongation. The larger the external force, the larger the elongation deformation. The deformation increases with the increase of the tensile force, and the deformation increases. The amount is decreasing, and when the increase tends to zero, the tape is broken. On the printing press, the first stretch of the tape is produced by the braking force provided by the paper machine tensioning system between the paper feed mechanism and the first printing unit. This braking force causes the paper to be stretched, deformed, and strained. This tension will enter the individual printing units as the tape passes. Between the printing units, if the circumferences of the blanket cylinders are equal, the line speeds are the same, and there is no speed difference between the two printing units, the tension will remain the same until the printing of each unit is completed. If the linear velocity of the second printing unit is faster than the first unit, the tape is further stretched and narrowed between the first and second units to meet the speed requirement of the second unit, and the tension is increased. On the contrary, the width is shortened and the tension is reduced. Since the tension of the paper feeder is set, the first deformation of the paper tape is stable, and the speed difference between the printing units after entering the printing unit, the tension is constantly changing, and the paper tape becomes larger as the tension becomes larger. Narrow or widen as the tension decreases. Therefore, the tension of the paper feeder should meet the requirements of tensioning the paper to stabilize the tension. Trying to set the tension value can produce a relatively stable deformation of the paper, minimizing the deformation of the paper between the printing units. That is, to minimize the variation of the width of the paper, to avoid affecting the overprint.
3. The effect of printing pressure on overprinting
On the printing machine with the contact type roller, the contact of the roller ensures the stability of the center distance of the roller on the one hand, ensures the smooth running of the printing machine, and overcomes the impact and jumping caused by the opening of the blanket cylinder in high-speed printing. On the other hand, this makes the printing pressure subject to certain restrictions during adjustment. It is prominent that when adjusting the printing pressure, the pressure can not be increased by adjusting the center distance, and only the pad can be used to obtain a larger blanket. The amount of deformation gets the way to increase the pressure. When the printing pressure is too large, the pad is inevitably enlarged, and the amount of deformation of the blanket is also increased. The larger deformation amount produces a larger printing pressure, which reduces the working stability of the blanket cylinder. When the thickness of the paper changes greatly, the rubber cylinder will inevitably jump at high speed, which causes the printing pressure to be unstable, which affects the printing. The quality is stable. The adjustment of the pad size causes a change in the radius of the blanket cylinder, which causes the linear speed of the blanket cylinder to change, resulting in a change in speed between the printing units and a constant deformation of the tape. Therefore, in order to ensure the stability of the product quality, the minimum pressure can be used to the maximum extent under the premise that the pattern can be clearly reproduced, and the increase of the printing pressure caused by the increase of the liner and the change of the linear speed of the drum can be prevented, and the mechanical movement can be ensured smoothly. It is important to reduce the change in print width caused by paper width variation and drum runout.
4. Effect of paper characteristics on overprinting
For paper, the quality of printing has a very important effect on the quality of printed products. The impact on the quality of overprinting is as follows:
First, the thickness of the paper can cause a change in printing pressure. The thicker the paper, the greater the printing pressure, and the more the linear speed of the blanket can be changed. This change accelerates the change in paper tension and causes a narrow change in paper bandwidth. The same amount of paper, the thickness is large, the texture is loose, the tensile strength is small, and it is more easily deformed. Second, the difference in the uniformity of the paper is manifested by the inconsistency in the thickness of the cross section. Under the action of printing tension, the elongation of the paper section is different everywhere, and the thickness of the paper changes differently to maintain the same length. When printing, the paper strips fluctuate and the tapes are elastic on both sides. This change affects the overprinting, and in serious cases, the paper breaks. Third, the paper with good tensile strength has a small amount of deformation and strong recovery ability under the action of external force. Such a paper, when subjected to a large tension, has less elongation and recovers faster, and the amount of axial deformation is relatively small. This type of paper is usually longer in fiber and denser in texture. When the paper with weaker tensile strength is subjected to a large tension, the elongation is more, and the recovery is slower, and the axial deformation is larger, which often brings the deformation state to the next printing unit during printing, causing quality problems. . Therefore, selecting a paper with good tensile strength is very advantageous for printing.
Third, understand the essence of the problem and take effective measures to overcome the problem of overprinting
In the usual work, when we deal with the problem of overprinting, we should treat the cause of the deformation of the paper and the effect of this deformation on the axial overprint as a central issue, and summarize the above analysis:
First, we see that the size of the ink to be printed on the blanket is the same. If the tape is stretched and narrowed again before entering the second unit, the printed ink of the first unit will be narrowed with the tape, then The ink on the second unit blanket cylinder is inevitably wider than the ink of the first unit when the transfer is superimposed on the paper strip, and an axial expansion phenomenon occurs.
Second, in terms of printing press pressure adjustment, the printing pressure value that can avoid the rubber roller beating should be adopted, and the minimum printing pressure is used under the premise of ensuring the quality of the printing. This can avoid the quality problem caused by the rubber roller beating, and can also stabilize the working properties of the blanket and the cushion material, prolong the service life, and the printing pressure data remains unchanged. To achieve the purpose of controlling line speed and stabilizing product quality.
Third, in order to reduce the tensile deformation of the paper between the printing units during the printing process, it is necessary to use an appropriate tension. The tension value set when feeding the paper feeder should be such that the paper is in a stable deformation state before printing, and the deformation between the printing units is effectively avoided. As long as the tension control is in place, even if the paper changes between the units, only a small re-deformation will occur, and the overprint error value will be controlled within the range allowed by the product quality to ensure the normal operation of the printing operation.
Fourth, the nature of the paper itself determines the extent to which the paper is deformed by tension. Different papers have different tensile strengths and densities. This is the state of the machine equipment that is often seen. It is just the replacement of the paper, and the overprinting takes place. The reason for the significant change. Therefore, grasping data indicators such as paper thickness, uniformity, and tensile strength can guide the adjustment of the printing equipment to suit the characteristics of the printed paper. In order to better select and use paper, it is important to complete the printing of qualified products.
It can be seen from the above analysis that reasonable control of tension, proper use of blankets and liners, and understanding of the characteristics of the paper used are very important for solving the quality problem of overprinting. In addition, there are many factors that affect overprinting, such as ambient temperature and humidity, the nature of the fountain solution, the nature of the ink, and other characteristics of the paper. Therefore, when analyzing the reasons for inaccurate overprinting, comprehensively consider the influence of each element on the inaccuracy of overprinting, and eliminate them one by one, and finally find the main problem. Usually pay attention to accumulate various relevant data to provide a basis for analyzing problems, so as to be able to analyze and solve multi-color overprinting problems well.