The essence of the formation of "print ghosts" (Part 2)

Performance characteristics of ghosts

(1) Uncommon. In printing, the appearance of ghosts is uncommon. No matter what kind of graphic structure of the printed matter, there is a contradiction between the return of ink and the residual ink. There are some factors that suppress ghost production. What are these factors? We can deduce from the trajectory of the reflow of ink.

First determine how much residual ink flows back. The thickness of the ink layer of the offset printing product is 2μm. Based on the calculation of the ideal ink transfer rate of 50%, the thickness of the ink layer on the blanket is 4μm, the printing plate is 8μm, and the plate roller is 16μm. The pressure difference caused by the corresponding pressure on each area of ​​the form roller is only 16μm, and the form roller is the source of the residual ink reflow, and its reflow ink layer is also the thickest. Therefore, in the entire ink path system, the reflow causes deformation The maximum change is 16 μm. Compare the thickness of the ink layer with the ink end. The simple calculation of the SM ink path shows that the thickness of the ink layer of the ink leveling roller connected to the ink transfer roller is at least 512 μm. The comparison of the two ink amounts shows that the ink amount at the ink end is much larger than the ink transfer end. The ability to deform the channel is far stronger than the ability to passively change the ink transfer end to deform the channel deformation. This is because the ink layer at the ink transfer end is too thin to suppress ghost formation.

The second factor is Mo Lu's ability to self-regulate the amount of ink. The ink path has a certain ink storage coefficient, and has a certain self-regulating ability to the amount of ink in the ink path. When the ink flow back from the ink transfer end causes the ink path to accumulate too much ink, at the ink supply end of the ink, the ink The road is no longer complete

Accept the ink from the ink transfer roller. This is the self-regulation of Mo Lu to suppress ghost formation.

Furthermore, the axial movement of the ink fountain roller. When the residual ink of the plate roller flows back, the ink channeling roller will evenly distribute the residual ink axially to make the residual ink layer uniform, so that the residual ink difference in each area is reduced, the pressure difference is reduced and the channel is reversely changed. The degree is reduced. This is because the axial movement of the ink fountain roller reduces the degree of channel deformation and suppresses ghosting. Furthermore, the inequality of the ink rollers of the ink path. The inequality of each ink roller is originally to make the stress points of each ink roller not overlap, and the formation of ghosts is less easy to form due to the suppression of various factors. It is due to the accumulation of ink to deform the channel, and The deformation of the channel requires more pressure, and because the ink layer of the plate roller is too thin, the pressure is not enough, so there must be an overlap of pressure points to accumulate the pressure. This is because the inequality of the ink roller reduces the accumulated pressure required to form ghosts and suppress ghosts. The inhibitory effects of these factors make ghosting uncommon.

(2) Suddenly appear and disappear suddenly, with short duration. Ghosts appear suddenly because the pressure accumulation required for the deformation of the reverse control channel of the ink path reaches a certain level, and the deformation of the reverse control channel changes the amount of ink to produce ghosts, but does not change the previous printing conditions and the accumulation of pressure The fixed value is not foreseen by us, so suddenly. The sudden disappearance of ghosts is because there are many factors in the ink path that inhibit the formation of ghosts. Once a ghost is formed, each inhibition in the ink path self-adjusts in the ink path, and the adjustment process is not What we know, so disappearing makes us feel abrupt. The short duration is because the control in the ink path consists of active control deformation and reverse control deformation. Two kinds of control exist at the same time and compete at the same time, and active control is much stronger than reverse control. After a long-term accumulation of power, once the power accumulates to a fixed value to obtain control, it is to release the power, and after the power is released, it must be handed over to active control. So the duration of ghost formation is short. The key to ghost formation-the cumulative frequency of force is due to the small amount of residual ink reflow, which causes a small change in pressure difference, so the key to ghost formation is the accumulation of small pressure, which is also not easy because of the ink roller Unequal, the movement of the ink-blowing roller makes the accumulated position of each pressure transmission will not overlap. If roller A and roller B have different diameters, the diameters of the two are X, Y, the two are squeezed, there is a D force point on A, the first D point is transmitted to the B roller D point, and the second D point is transmitted again At point D, the two rollers A and B rotate X circle and Y circle respectively. It can be seen that the overlapping of pressure points is more difficult, but because the ink layer is an area rather than a dot, and the amount of movement of the ink roller is also small, it is difficult to overlap the pressure of the ink layer, but the pressure area of ​​the ink layer intersects It is easier. The intersection of the ink layers makes it easier to accumulate the force in the intersecting area. This also determines the unfixed morphology of ghost formation, because we are not clear about the inequality of the ink roller diameter and the ink roller. The influence of the dynamics, the intersecting shape of the force area. The cumulative number of forces. Due to the development of modern printers at high speeds, the speed of rotation per unit time is very fast, so the accumulation of force in the unit time is also very large. If the diameter of the ink roller of a machine is X, the diameter of the plate cylinder is 6X, and the printing speed of the printing machine is 10,000 / hour, the ink roller is 60,000 rpm. The increase in speed increases the number of intersections of the force zone per unit time, and increases the cumulative number of forces per unit time. The cumulative force of frequency refers to the cumulative force of the force area and the number of intersections per unit time. Due to the different graphic structure of each printed product, the frequency accumulation of its accumulation force is different, and due to the difference in printing speed, the accumulation frequency in unit time is different. Also, due to the different deformation forms of the ink roller rubber, the frequency of accumulation of the frequency accumulation force is also different. It can be said that the formation of ghosts requires the accumulation of the accumulation force at the same frequency, but the actual situation is that it is not easy to maintain the same frequency in the ink path, so the difference in frequency is also a factor that makes ghosts uncommon and has a short duration. Now the devices for eliminating ghosts of various printer manufacturers are mainly designed from the perspective of changing the cumulative frequency of the ink path. For example, the device for eliminating ghosts in Beiren's printing presses can change the cumulative force frequency of the ink path by changing the movement of the ink-blowing roller. The device for eliminating ghosts in Roland printing presses can change the axial position of the plate roller To change the frequency of the cumulative force of the ink path, compared to the two, changing the axial position of the plate roller changes the frequency source of occurrence, and changing the size of the ink channel roller is to change the frequency transmission process, so the conclusion is drawn It is more effective to suppress the formation of ghosts by changing the axial position of the plate roller.

A little thought about the Roland press's ghost elimination device. The author has not operated such a machine, but knows that the Roland machine has such a device, so he made the following guess based on his own knowledge:

1. The method of changing the position of the plate roller. The way of changing the axial position of the form roller is absolutely not the same as the way of the axial movement of the ink roller, it is impossible to change the axial position continuously during the printing process, because

(1) If the axial position is continuously changed during printing, violent friction will occur between the ink-blowing roller and the printing plate, which will reduce the printing durability of the printing plate and increase the abrasion of the ink roller.

(2) Continuously changing the axial position can change the cumulative force, but since the cumulative force needs to be accumulated to a certain degree, as long as the cumulative force changes the frequency before reaching the critical value, so the continuous change depends on the plate roller The axial position is not necessary. It can be deduced from this that in this device, the axial position of the form roller is changed discontinuously.

2. The position changes where the printing machine runs. The change of the axial position of the form roller should be the movement of the form roller to the gap of the plate cylinder, because if the axial position of the form roller is changed in the printing section, the dots will be deformed, resulting in ink sticks, Printing problems such as ghosting.

3. Change the axial position of several plate rollers. There are generally four form rollers, because the change of the axial position of the form roller must be discontinuous, and it must be changed at the gap of the plate cylinder. Therefore, from the difficulty of achieving motion control, only the first Rooted by plate roller

It is easier to change the axial position. At the same time, the ink path is regarded as a whole. Changing one of the form rollers achieves the overall frequency of the ink path. Therefore, it is concluded that this device has only the first form roller The axial position has changed. Exclusion of ghost ghosts In the printing process, the most difficult to avoid and exclude is mechanical ghost ghosts, and the most common type of mechanical ghost ghosts is lack of ink ghost ghosts. It is more likely to occur when printing with transparent ink, but rarely occurs when using opaque ink for proofing. In cooperation with the plate-making and printing process, some measures can be taken to reduce ink-out ghosts. Therefore, attention should be paid to avoid designs that are prone to ink-out ghosts, such as large borders and large fields. If such a design must be adopted, the following measures can be taken:

(L) Use pictures as much as possible in the design. For screen printing that does not involve the field, there will generally be no ghosting.

(2) Change the design. Reduce the density of large borders or large fields, balance the density difference between the image on the web page and the part of the field, and try to balance the ink volume of the entire page to reduce the appearance of ghosts. For example, if two pictures are placed on the right side of the printed sheet, ghosts are likely to occur during printing; if the design is changed so that the two pictures are placed separately, one is placed on each side, and the amount of ink used to balance the entire version can avoid ghosts .

(3) When typesetting, design the layout reasonably. Appropriate layout arrangement can ensure that the ink roller has sufficient time for inking to avoid ghosts. For example, if a design is to place a large picture on the ground, when imposing, if the printing press is large enough, it can be placed after two pictures on a white background.

(4) Use the color bar in the cutting part other than the graphic part to transfer the excess ink on the ink roller. These color bars are called ink transfer bars.

(5) Adjust the printing machine to increase the amount of ink supplied to the ink fountain of the ink-deficient ghost. Moreover, some printing machines are equipped with inclined ink rollers, which can effectively solve the problem of ink shortage ghosts, that is, use different parts of the ink roller to print different parts of a large area on the ground, so that the ink roller will not be short of ink.

(6) Rotate the design layout by 90 °. Take the example of printing large bold characters on the ground. If the ink roller first inked the large bold characters, and then inked the solid, printing one by one, it is easy to cause ink shortage ghosts. But if the printed sheet is turned 90 °, so that part of the ink roller inked the font and the other part is inked on the ground, it can effectively avoid the lack of ink ghosts.

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