First, what is the rejection angle?
Swing angle, also known as loose tail, refers to the extension of the two sides of the printed sheet to the outside of each extension, forming a trapezoidal shape, resulting in inaccurate overprint or graphic size distortion. This is a relatively common printing fault in offset printing, especially in large format offset printing. In the printing process once the prints appear to shake the problem is always a headache.
Second, what are the characteristics of the rejection angle in print?
1. Obvious when printing large format prints;
2. Severe when printing thin paper;
3. The problem of throwing angle during the printing process is always aggravated from the direction of the paper in the printing direction to the dragging part of the paper.
4. The problem of rejection in multi-color printing presses starts from one color and gradually decreases. Lead to overprinting.
Third, what are the factors related to the emergence of rejection?
It is mainly the deformation of the paper, which is caused by the combination of the paper's water-absorption deformation and the paper's tensile-compression deformation.
Followed by the structure of the printing press, and the adjustment during use.
How is the paper deformed during the printing process? Why is the deformation of the dragging part more serious than that of the mouthwash?
1. We know that in offset printing, it is necessary to use water to complete the entire printing process. During the imprinting process, water is transferred to the surface of the paper through the printing plate blanket (in fact, we only hope that water can prevent ink from being blank on the printing plate during printing. Partial adhesion and expansion are sufficient, but this transfer to the surface of the paper is something we cannot avoid anyway.)
During the printing process, the paper is dragged by the teeth of the impression cylinder, enters into the embossing area, begins to receive the imprinting, and also starts to contact the moisture transferred from the printing plate to the blanket, since the main component of the paper is containing a large amount of hydrophilic The group's cellulose, and the paper surface is rich in pores, water can instantly adsorb on the surface of the paper and spread around, while the pressure of the roller into the interior of the paper. However, this is a process in which paper is subjected to high pressure under high pressure. Therefore, the initial contact with the paper does not completely penetrate the paper, but accumulates at the end of the paper in the opposite direction of the roller and the paper under pressure. In simple terms, the more moisture that accumulates on the surface of the paper, the more water will penetrate into the paper. Similarly, the moisture that has penetrated the paper in the mouth of the paper will not be bound to the fiber immediately. A part of the water will also accumulate in the tail part under the pressure, which will cause the paper to absorb moisture. From the beginning of the paper to the end of the paper, there must be an increasing trend. From this we have come to the conclusion that the amount of water absorbed by the paper is higher than that of the paper.
Tests show that the fiber in the paper will expand rapidly after absorbing moisture, and it will deform and expand by 20%-30% after absorbing sufficient moisture. It is easy to understand how much water is absorbed at the end of the paper, and of course the deformation will be relatively large.
2. The pressure deformation is the deformation of the straight line size and area caused by the paper being squeezed in the drum and the fibers being misaligned. In the printing process, due to the action of the roller, the paper extends from the mouth to the outside and becomes closer to the two sides of the trailing edge. The overprint error is greater.
In addition to being subjected to positive pressure on the surface of the drum, the paper is also subjected to a pulling force opposite to the running direction along the tangential line of the drum. This negative tension causes the paper to undergo tensile deformation and the paper is compressed due to the positive pressure on the paper. As a result of the accumulation of paper on the tail of the paper, the result is that the deformation at the end of the paper will, of course, be much larger than at the mouth.
We obtained a conclusion from the above analysis: due to the water absorption deformation of the paper roll, and due to the deformation caused by the force between the rolls, no matter which kind of deformation will tend to increase at the tail of the paper, when such deformation occurs When the quantity reaches a certain value, we will see the rejection angle that we do not want to see.
Fifth, in the printing of multicolor machines, we will find that the first color is the most serious when the corners are thrown. That is to say, the first color is the largest, and then the color is smaller and then the color is smaller. Of course, it will not overprint accurately. Why is this?
We know that the fibers in the paper will swell and distort after absorbing water. When the relatively dry paper is printed in the first color group, the amount of water absorbed is relatively large, so the moisture content of the paper will increase greatly. When the second color is printed, because the moisture content of the paper has been increased a lot, the amount of water that the printing unit can provide does not change significantly. Although the moisture content of the paper will increase further, the relative increase There has been no increase in the relative amount of time when printing in the first color. That is to say, when the paper is printed in the second color, the amount of dimensional change is smaller than the first color. Of course, the relative amount of deformation of the third color of the paper will gradually become smaller.
In the process of imprinting paper, the force deformation is similar to the process of water absorption deformation. Paper deformation can be divided into elastic deformation and plastic deformation. When the first color is printed, the paper will be deformed. At this time, the plastic deformation cannot be recovered, and the elastic deformation of the paper will not be restored enough due to the high speed printing. In this case, the ink can only be printed in the second color. It is transferred to the already deformed paper, and another plastic deformation occurs on the paper. However, the amount of deformation at this time has been reduced due to the deformation in the first color. In the same way, the third color and the subsequent color group are the same, that is, the amount of paper deformation is gradually reduced.
Sixth, by the printing press itself or machine use adjustment process, caused by the problem of what?
The manufacture of the machine itself, the structure, and the adjustments to the operation of the printing press can also cause rollover failures. The PZ4880-01A printing machine originally produced in China has a much more pronounced rejection angle. The reason for this is that sinking of the pad, uneven bite force, and inconsistency can easily lead to a rejection angle during the printing process. During the imprinting process, the paper is subjected to the pulling force of the rolling force. If there is not enough force to maintain the stability of the paper, the paper will easily generate a relative slip on the impression cylinder during the imprinting, and the slippage will occur. It will also be accumulated and enlarged at the end of the paper, resulting in rejection of the corner.
Seventh, for the paper deformation and machine adjustment, what way to reduce the existing rollover failure?
First of all, when we select the paper, we must select the corresponding paper according to the requirements of the print job. If printing requires precise overlay, then we must choose paper with low pulp content, high surface sizing, and high mechanical strength when selecting paper. Secondly, before the printing, the paper should be subjected to humidity conditioning (the conditional unit is best to use a forced moisture adjustment method, which can ensure that the paper has a relatively high moisture content), so that the water content of the paper is stable, and the stability is improved during the printing process.
For the reasons of the machine itself, the rejection of corner rejection, we can adjust from the following aspects.
1. The teeth of the first color group are normally bitten by the teeth, and the teeth of the two ends of the impression cylinders of the 2-4 groups are slightly smaller.
2. Put 2-4 sets of rubber blanket liners (about 0.05mm or so can be reduced). Doing so can bring some convergence to the paper that has already been thrown, but this requires the operator to have a very rich printing experience to master the scale.
3. In addition, the pad at both ends of the impression cylinder can be increased, so that the higher portion can be firstly brought into contact with the pressure, so that the two corners of the paper can be converged in phase to prevent diffusion.
8. What solutions have been used in the design of modern printing presses to solve the problem of rejection?
China's modern printing press has been able to solve the problem of rejection in the printing process, such as:
1. The use of high-strength alloy steel in the overall structure of the cavities ensures that the paper is smooth and avoids ghosting and flicking.
2. The post-card is segmented to solve the phenomenon of “sweeping angle†in printing.
3. A plurality of support screws are used on the support rod of the impression pad of the impression cylinder to prevent the failure of the corner drop caused by sinking of the pad.
Several foreign presses also have different solutions. Such as:
1. In the process of machine design, the form of the impression cylinder is directly occlusal.
2. The teeth of the roller teether are designed to be flexible and have a pre-bending action before biting paper, which can be adjusted as needed, so that the rejection of the corner rejection can be avoided.