(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
Printing is a process that transfers ink to the surface of the substrate. Offset printing is of course not an exception. The entire printing process is an ink transfer process. First, the ink in the ink fountain is transferred to the surface of the printing plate by the ink roller under certain pressure and friction, and then the surface of the printing plate The ink of the ink transfers to the surface of the blanket under a certain pressure, and finally the ink on the surface of the blanket passes to the surface of the paper (this paper mainly discusses this paper) under the influence of a certain pressure. We will find that the whole process is carried out under the precondition of pressure. It is not difficult to imagine that without pressure in it, the ink cannot complete this series of transmission. This directly shows how important the role of pressure in printing is. It can be said that being able to master the reasonable adjustment of pressure is a prerequisite for good printing.
There are generally several types of pressure adjustments that are more common in our work: ink pressure adjustment, water pressure adjustment, and pressure adjustment between rollers. This article will discuss the adjustment principle of the pressure between the cylinders (which can also be directly called the printing pressure) and the printing problems caused by the pressure.
To adjust the printing pressure, I believe that the following steps should be followed:
1. Data preparation. There are three main sources of data:
1. Actual measurement. The actual measurement is mainly for the printing plate and blanket. Use a micrometer in the measurement. It should be noted that during the measurement, several points should be measured on different parts of the printing plate or the blanket at the same time, and then the average value is taken; in addition, considering the deformation of the blanket after stretching on the machine, in The thickness of the blanket measured under the machine should be slightly larger than the calculated thickness, and it is generally controlled to be: 0.1mm larger for ordinary blankets and 0.15mm larger for air cushion blankets, so that when the liner and blanket are loaded into the roller, it is just The correct thickness.
2. The data marked on the random description of the printing machine. The offset presses are marked on the random description. There are two main types of data on printing pressure: one is the absolute size of the cylinder and the roller pillow, including the diameter of the plate cylinder, blanket cylinder, impression cylinder, and three cylinder roller pillow The diameter is also marked with the roller pillow clearance of the machine when printing a specific paper (many machines are marked with 0.1? As the standard), and the total thickness of the printing plate and the blanket and its pad; there is also Directly give the amount of cylinder diameter reduction (the relative height of the surface of the roller pillow and the surface of the roller body), the excess of the roller (the amount of lining higher than the surface of the roller pillow), and the gap of the roller pillow; the concept of facilitating the operation of the printing press operator has become a The trend of machine design, of course, is also the same in the use of equipment. In the given data, it has become more and more a fool. For the previous calculation of printing pressure and cylinder gap, it has become less and less used. Arrived;
3. The printing pressure is determined by the setting of the printing process and the condition of the machine. At the same time, there is a more ideal setting for the roller lining (we will focus on the discussion in this article). This step relies mainly on the combination of calculation and practical experience.
2. Measure and adjust the center distance of the cylinder according to the reference value of the roller gap given on the printing machine instruction manual. This step can be done automatically in modern printing machines. However, this step is often very cumbersome for the older domestic-made printers, such as J2108 and J2205. First, the pad on the surface of the roller should be removed (otherwise it will affect the measurement results), then the roller pillow is cleaned, and finally the method of pressing the lead wire (in the closed state, the fine lead wire is placed into the gap between the roller pillows, and then the printing machine is jogged) Flatten it) First measure and then adjust; then measure and adjust again, repeat this process until the roller pillow clearance reaches the standard value. The reason for such a complicated process to adjust the printing pressure is that the data of the roller pillow clearance of the old-fashioned domestic printing machine are often displayed on the outside of the machine by mechanical means (such as the screw dial), especially for machines with a long service life, from The data read on the dial is often untrustworthy.
The center distance of the roller pillow is not fixed. When the machine is old and the roller gear wears seriously, sometimes the "gear ink bar" due to the abnormal gear meshing occurs. At this time, it should be appropriately reduced according to the needs. Roller pillow clearance (the adjustment process of roller pillow clearance is actually the adjustment of the center distance of the roller).
3. According to the data given in the instructions (in the more traditional printing machine instructions, the total thickness of the printing plate and the blanket and their respective linings are given), the measurement of the printing plate and the blanket to be used is Calculate the thickness of the liner, and properly line the printing plate and blanket.
Thickness of printing plate package = total thickness of printing plate and liner—thickness of printing plate
Blanket blanket thickness = total thickness of blanket and blanket-blanket thickness
4. After the lining is completed, jog the printing machine to check whether there is any mechanical improperness, and then jog the carriage and rub the surface of the printing plate or the blanket with your hand to check whether there are uneven surfaces. If there are any, it may be due to The liner is covered with foreign objects or caused by wrinkling. Need to be eliminated immediately.
The above four steps are a relatively complete pressure adjustment process. In fact, only the old-fashioned domestic printing presses must follow the above steps. On modern more advanced printing presses, this process has become quite simple. Enter the printing paper thickness and the desired printing pressure (usually the amount of compression of the blanket cylinder), and the remaining computer can be automatically completed.
The procedure mentioned above is to adjust the machine according to the instructions. Among them, whether it is the roller gap or the pad thickness, the printing process is generally set when the thickness of the substrate is the assumed value. So when the thickness of the substrate changes, some simple calculations are used:
Pressure of plate cylinder and blanket cylinder (compression amount) = (plate cylinder body diameter + blanket cylinder body diameter) / 2 + plate thickness + plate pad thickness + blanket thickness + blanket pad thickness— [(Diameter roller roller pillow diameter + blanket roller roller pillow diameter) / 2 + roller pillow gap]
The pressure (compression amount) between the blanket cylinder and the impression cylinder = (the diameter of the blanket cylinder body + the diameter of the impression cylinder body) / 2 + the thickness of the blanket + the thickness of the blanket pad + the thickness of the substrate — [(blanket blanket roll Pillow diameter + embossing roller rolling pillow diameter) / 2 + Roller pillow clearance]
Pressure (compression amount) of plate cylinder and blanket cylinder = thickness of plate and pad + thickness of blanket and pad—diameter reduction of plate cylinder + diameter reduction of blanket cylinder + plate cylinder and blanket Roller pillow clearance
The pressure (compression amount) between the blanket cylinder and the impression cylinder = the thickness of the blanket and its pad + the thickness of the substrate-the reduction of the blanket cylinder diameter + the reduction of the impression cylinder + the roller pillow of the blanket cylinder and the impression cylinder gap
In general, when we set the printing pressure, we mainly set it according to the instructions of the printing equipment. The printing pressure values ​​given in the operating instructions we can generally see are given on the assumption that the thickness of the printed paper is 0.1 mm. However, in the setting of printing pressure, the standard given by the equipment will often have some changes due to the actual situation.
First, let's take a look at the factors that affect the adjustment of printing pressure.
(1) Smoothness of paper
There are many types of offset printing products, including newspapers and periodicals, packaging and decoration products, and the surface conditions of printing paper are also very different, so the printing pressure required in printing is also different. Table 13-1 lists the data of the paper smoothness and printing pressure. On a sheet-fed offset press, the line pressure between the plate cylinder and the blanket cylinder is fixed at 5? / ?, printing at 4000 rpm. Speed, printing with paper of different smoothness, measured the linear pressure of the impression cylinder and blanket cylinder when obtaining the best quality prints.As can be seen from the table, the printing pressure required by newsprint with a rough surface is almost three times that of ultra-gloss coated paper with extremely smooth surface. When printing with low-smoothness paper, the printing pressure must be increased appropriately, otherwise it will appear The defects of printed matter outlets are false, and the handwriting is broken.
(2) Printing speed
Many production examples can prove that when the printing speed increases significantly, the printing pressure also increases. Because when the printing speed is increased, the contact time of the corresponding particles between the printing surfaces is also reduced. This requires a greater pressure to reduce the degree of incomplete contact between the printing surfaces, so that the ink can be quickly Transfer from the ink supply surface to the ink receiving surface, so that the ink can complete the necessary transfer in a short time.
It is also a rotary machine. The printing pressure of modern high-speed machines must be higher than that of old machines. In addition to the short stamping time, the high-speed machine has a larger cylinder utilization factor and a smaller radius. The compression width is also relatively reduced, which also affects the transfer of ink.
(3) Product quality requirements
Different products have different copy requirements. In addition to using different raw materials and printing plates, you can also adapt to the printing pressure. "Field" graphics can (and allow) make the pressure slightly higher to meet the requirements of strong imprint. The network graphic should emphasize the "ideal pressure" to avoid the loss of hierarchy due to the expansion of the network. High network cable products have higher requirements on the quality of replication. In particular, the printing pressure should be controlled to achieve the minimum value. When printing such products, in addition to using coated paper with a smooth surface as much as possible, you should also pay attention to the choice of plate quality and correct Strict control of water and ink balance on the surface of the printing plate.
(4) The elasticity of the blanket and its pad
The deformation of the blanket and its backing materials (backing paper, cushions, and also felt) is closely related to the printing pressure, and it should be used with elasticity, and the elasticity is not easy to lose after a large amount of printing Paper and felt to meet the basic requirements of ideal printing pressure. On the contrary, in order to reduce costs, if you choose a domestically produced rubber blanket of poor quality, you must increase the "absolute value of the minimum printing pressure." If the imported air cushion blanket printing pressure is generally set to 0.08 ~ 0.10mm, if the domestic blanket is used at the same time as Mao Ni as a liner, the general printing pressure is set to 0.2 ~ 0.25mm.
After the blanket is used for a certain time (number of prints), it will lose or reduce its original elasticity more or less quickly (this is determined by their quality). At this time, it is necessary to increase the pressure to solve the good transfer of ink The problem, which means that the new blankets and pad materials can achieve less printing pressure.
(5) Number of prints
In the printing process, with the increase in the number of printed products, the printing pressure has been reduced, because the blanket and liner will lose or reduce the original after a period of use, that is, after printing a certain amount of printed products Elasticity, thus reducing printing pressure. According to the experiment: for soft liners, the printing pressure is reduced by 20-30%, while for rigid liners, the pressure drop is not more than 10-15%. As the printing pressure decreases, the ink transfer rate will decrease. Therefore, in order to obtain stable prints, the pressure can be adjusted slightly higher before printing, so that the printing pressure after printing a certain amount is still sufficient to ensure quality.
This decrease in printing pressure is most noticeable after the liner is renewed. Therefore, after renewing the liner, it is necessary to trial run at the working speed for half an hour to an hour to stabilize the liner and blanket deformation, and then adjust the printing pressure, so that you can obtain excellent quality prints with basically the same printing pressure.
Two ways to change the printing method:
The offset printing machine mainly adjusts the printing pressure by changing the compression amount of the blanket, and this compression amount is mainly derived from the fact that the center distance of the cylinder after printing is smaller than the sum of the radius of the cylinder surface. For example, the pressure between the plate and the impression cylinder comes from the fact that the center distance between the two cylinders is less than the sum of the free surface radius of the plate cylinder and the free surface radius of the blanket.
There are two main ways to adjust the printing pressure. One is to change the printing pressure by changing the pad under the printing plate or blanket; the other is to change the center distance of the cylinder. The author believes that the adjustment of printing pressure should depend on the situation, and different pressure adjustment methods are used in different situations.
When the thickness of the substrate changes, different adjustment methods are adopted, as follows (take the thickness of the substrate as an example):
1. Method one: Reduce the liner under the blanket to increase the amount of substrate. At the same time, in order to ensure the pressure between the plate cylinder and blanket cylinder, the thickness of the liner in the plate cylinder is reduced by an equal amount.
2. Method 2: Directly increase the center distance between the impression cylinder and the blanket cylinder. The increase should be the amount of thickening of the substrate.
In older models such as domestic 01, 03, 08 and other machines, because the pressure regulator of the machine is not reliable, generally use method one to adjust when the paper thickness changes, unless there is a large change in the thickness of the substrate At this time (it must be adjusted by adjusting the center distance of the drum, the specific method will be described later), try not to change the center distance of the roller that has been accurately adjusted, this is mainly due to labor intensity and production efficiency. At the same time, we should also pay attention to the method of adjustment:
A Due to the change in the thickness of the plate cylinder liner, it is necessary to adjust the plate water roller and plate ink roller at the same time;
B It should be noted that after the diameter of the plate cylinder changes, the imprint on the substrate changes (because the cylinder diameter changes, and the tangent diameter of the cylinder does not change. When the plate pad is thickened, the imprint on the substrate is It will be shorter; the same is true when thinning is the opposite), and it is strictly prohibited to change the printing pressure in this way during a batch of printing.
In addition, the need to change the pad thickness of the drum are:
1) When the rubber cloth has been deformed for a long time, the pad of the rubber cloth needs to be properly thickened (this thickening generally needs to be tested a little at a time to achieve the best effect).
2) During the overprinting, circumferential overprinting is inaccurate. When such overprinting is overprinted along the size of the circumferential center. {Considering the problem of overprinting in multicolor printing, when setting the printing pressure, we often have to reduce the total thickness of the third and fourth groups of printing plates and their liners by 0.05mm (compression) compared to the first group. }
On more advanced printing machines, changing the center distance of the cylinder is as simple as turning the handle or pressing a button, and the pressure display device is generally more reliable. Therefore, it is generally recommended to directly use method two to complete the adjustment. The benefits of this are many:
1) The adjustment is simple and convenient, that is, when the thickness of the substrate changes, or when the pressure of the impression cylinder and the blanket cylinder needs to be adjusted during the printing process, we only need to change the center distance of the two cylinders. There is no need to increase or decrease the liner of the plate cylinder and blanket cylinder simultaneously. Adjusting the center distance of the cylinder on the current printing machine is a relatively simple job, and it can be operated remotely even on the console (now many printing machines, as long as the operator enters the printing paper thickness into the control system, the system will automatically adjust The default value of the pressure is adjusted automatically.)
2) It can effectively avoid the speed difference of the surface of the cylinder during printing due to the change in the circumference of the cylinder due to the use of pads. This speed difference often leads to the deformation of the dots in the printing, and even related faults such as overprinting and ghosting.
3) Due to the use of changing the liner in the printing plate to change the printing pressure, it will cause overprinting problems (when the surface diameter of the plate cylinder increases, the graphics and text that are finally transferred to the substrate will shorten in the circumferential direction).
The adjustment of the center distance of the drum is also applicable to the following situations:
A When printing paper with rough texture and uneven thickness, in order to increase the printing pressure of the blanket and impression cylinder, the center distance between the blanket cylinder and impression cylinder must be reduced to meet the needs of printing.
B. When the machine is used for a long time, the roller gear or roller bearing is seriously worn, and the backlash of the gear is too large to produce "ink bars" (strips) or other problems, it must be solved by appropriately reducing the roller spacing.
Inspection of printing pressure:
(1) Gradually add paper to evaluate the full version method:
1. Reduce the padding in the blanket by 0.4 mm (more than normal).
2. Wash the printing plate and fill it evenly with ink (yellow ink is recommended).
3. Close the pressure and transfer the full ink on the printing plate to the blanket.
4. Then observe whether the ink distribution on the blanket is even.
The above method is a relatively rough detection method, it can only detect whether the blanket is flat, and whether the printing pressure is within a reasonable range (this detection can only limit a relatively large range), and because the operation is more cumbersome, so in Not much is used in machine operation now. The method introduced in (2) (3) is used more often, and its work efficiency and effect are better.
(2) Use signal bar combined with full version network cable detection:
It is basically the same as the method of (1), except that the ink transferred to the blanket is the network line and the signal strip on the printing plate, and then the size of the printing pressure can be determined according to the detection result. Usually this detection is very accurate, especially 50 % Of the dots are very sensitive to printing pressure. When the printing pressure is a little abnormal, it can be clearly displayed.
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