Printing pressure adjustment principles and common problems (1)
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Printing is the process of transferring ink to the surface of a substrate. The offset printing process is a process of ink transfer. The first is the ink in the ink fountain. Under a certain pressure and friction, the ink is transferred to the surface of the printing plate by the ink roller, and then the ink on the surface of the printing plate is under a certain pressure. The surface of the blanket is transferred. Finally, the ink on the surface of the blanket is transferred to the surface of the paper under a certain pressure (discussed mainly on paper). We will find that the whole process is carried out under the premise of pressure. It is not difficult to imagine that there is no pressure in it, and the ink cannot complete the series. This directly shows how important the role of pressure in printing is. It can be said that mastering the reasonable adjustment of pressure is a prerequisite for printing.
There are several types of pressure adjustments that we have in common at work: ink pressure regulation, water pressure adjustment, and pressure adjustment between rollers. This article will discuss some of the principles governing the pressure between the rollers (also directly referred to as printing pressure) and the printing problems caused by pressure.
For the adjustment of printing pressure, the author believes that the following steps should be followed:
First, the data is prepared. There are three main aspects to data sources:
1. Actual measurement. The actual measurement is mainly the measurement of the printing plate and the blanket. Use a micrometer when measuring. When measuring, you should pay attention to measuring several points at different parts of the plate or blanket at the same time, and then taking the average value. In addition, considering the deformation of the blanket after stretching on the machine, The thickness of the blanket measured under the machine should be slightly larger than the calculated thickness. Generally, it is controlled by: 0.1mm for the ordinary blanket and 0.15mm for the cushion for the cushion, so that when the liner and the blanket are tightened into the drum, it is just The correct thickness.
2. Data marked on the random description of the press. The offset printing machine is marked on the random description. There are two main types of printing pressure data: one is to mark the absolute size of the roller and the roller, including the plate cylinder, the blanket cylinder, the diameter of the impression cylinder, and the three-roller roller. Diameter, also marked with the roller gap of the machine when printing specific paper (many machine marked with 0.1mm standard), in addition to the total thickness of the plate and blanket and its liner; Directly giving the reduction of the diameter of the drum (the relative height of the surface of the roller and the surface of the drum body), the excessive amount of the drum (the amount of the lining is higher than the surface of the roller), and the gap between the rollers; the concept of facilitating the operation of the operator of the printing machine has become a The trend of machine design, of course, is also the same in the instructions for the use of equipment. It has become more and more foolish in the given data. It has become less and less used for the calculation of printing pressure and roller gap. Arrived;
3. Determine the printing pressure by setting the printing process and the condition of the machine, and at the same time have a more ideal setting for the roller lining (we will have a concentrated discussion in this article). This step relies primarily on the combination of computation and practical experience.
Second, Measure and adjust the center distance of the roller according to the reference value of the roller gap given on the instructions of the printing machine. This step of modern printing presses can generally be done automatically. However, for older models of domestically produced printers, such as J2108 and J2205, this step is often very cumbersome. First, the liner on the surface of the drum should be removed (otherwise it will affect the measurement results), then the roller should be cleaned, and finally the method of pressing the lead wire (in the state of pressing, the fine lead wire is placed in the gap of the roller, and then the printing machine is clicked. Flatten it, measure it first, then adjust it; then measure and adjust again, repeat the process until the pillow gap reaches the standard value. The reason why such a complicated process to adjust the printing pressure is that the data of the roller gap of the old-fashioned domestic printing machine is often displayed mechanically on the outside of the machine (such as a screw dial), especially for machines with a long service life. The data read on the dial is often untrustworthy.
The center distance of the roller is not constant. When the machine is old and the roller gear is worn out, sometimes the “gear ink bar” is caused by the gear meshing abnormality. In this case, it should be appropriately reduced according to the needs. Roller clearance (the adjustment process for the roller gap is actually the adjustment of the center distance of the roller).
Third, According to the data given in the instructions for use (in the comparison of the traditional printing machine instructions, the total thickness of the printing plate and the blanket and their respective linings are given) for the measurement of the printing plate and blanket to be used. Calculate the thickness of the liner and properly coat the plate and blanket.
Plate lining thickness = total thickness of plate and pad - plate thickness
Blanket lining thickness = total thickness of blanket and lining - blanket thickness
Forth, After the lining is completed, click the printing machine to check whether there is any mechanical discomfort. Then, click the long car and rub the surface of the printing plate or the surface of the blanket to check whether there is any uneven surface. If there is any possibility, it may be due to The bag is lined with foreign matter or wrap wrap. Need to be excluded immediately.
The above four steps are a relatively complete pressure regulation process. In fact, only the old-fashioned domestic printing presses must follow the above steps. This process has become quite simple on modern and more advanced printing presses, and often only to the control window. Enter the thickness of the printed paper and the printing pressure you want to achieve (generally the amount of compression of the blanket cylinder), and the rest of the computer can be automated.
The procedure described above is an adjustment of the machine according to the instructions for use, in which the roll gap or the thickness of the pad is such that when the thickness of the substrate is assumed, the printing process is generally set. So some simple calculations are used when the thickness of the substrate changes:
Pressure of the plate cylinder and blanket cylinder (compression amount) = (plate cylinder diameter + blanket cylinder diameter) / 2+ plate thickness + plate liner thickness + blanket thickness + blanket pad thickness - [(plate cylinder roller diameter + blanket roller pillow diameter) / 2+ roller gap]
The pressure between the blanket cylinder and the impression cylinder (compression amount) = (rubber cylinder body diameter + impression cylinder body diameter) / 2 + blanket thickness + blanket liner thickness + substrate thickness - [(rubber cylinder roller Pillow diameter + impression roller diameter) / 2) roller gap]
Pressure of the plate cylinder and blanket cylinder (compression amount) = plate and pad thickness + blanket and pad thickness - plate cylinder reduction + blanket cylinder reduction + plate cylinder and blanket Roller gap of the drum
Pressure between the blanket cylinder and the impression cylinder (compression amount) = blanket and its liner thickness + substrate thickness - blanket cylinder reduction + impression cylinder reduction + blanket cylinder and impression cylinder gap
In general, when setting the printing pressure, we mainly set it according to the instructions for use of the printing equipment. The printing pressure value given in the instructions for use, which we generally can see, is given assuming that the thickness of the printing paper is 0.1 mm. However, in the setting of printing pressure, the standard given by the equipment tends to have some changes due to the actual situation.
First let's take a look at the factors that affect the printing pressure adjustment.
(1) Smoothness of paper
There are many types of offset printing products, including newspapers and magazines, as well as packaging and decoration products. The surface condition of printing papers is also very different, so the printing pressure required for printing varies. The paper smoothness and printing pressure data listed in Table 13-1 are fixed on a sheetfed offset press with a line pressure between the plate cylinder and the blanket cylinder of 5 kg/cm and printing at 4000 rpm. Speed, printed on paper of different smoothness, measured line pressure of the impression cylinder and blanket cylinder when the best quality print is obtained.
No. Paper type Smoothness (seconds) Line pressure (kg/cm)
1 super glossy coated paper 2472 4.4
2 coated paper 518 7.8
3 offset paper 51 11.1
4 Newsprint 17 12.2
As can be seen from the table, the printing pressure of rough surface newsprint is almost three times that of ultra-gloss coated paper with extremely smooth surface. If printing with low smoothness paper, the printing pressure must be properly increased, otherwise it will appear. The print outlets are imaginary, and the defects of the writing are broken.
(2) Printing speed
Many production examples can demonstrate that printing pressure increases as printing speed increases significantly. Since the printing time is increased, the time at which the corresponding dots between the printing surfaces are contacted is also reduced, which requires a large pressure to reduce the degree of incomplete contact between the printing surfaces, so that the ink can be faster. Transfer from the ink supply surface to the ink receiving surface allows the ink to complete the necessary transfer in a short time.
The same is the rotary machine, the high-speed machine of modern times must have higher printing pressure than the old one, because the high-speed machine has a larger utilization factor and a smaller radius than the imprinting time, so the embossing The compression width is also relatively reduced, which also affects the transfer of ink.
(3) Quality requirements of products
Different products have different replication requirements. In addition to using different raw materials and plates, you can also adapt to the printing pressure. The "field" graphic can (and is allowed to) make the pressure a little larger to meet the requirements of blotting. The network cable graphic should emphasize the "ideal pressure" and avoid the expansion of the network and the loss level. High-wire products have higher requirements for copy quality, especially to control the printing pressure to minimize the value. When printing such products, in addition to using smooth coated paper as much as possible, attention should also be paid to the selection and quality of the printing plate. Strict control of the ink balance on the surface of the plate.
(4) The elasticity of the blanket and its liner
The deformation of the blanket and its cushioning material (lining paper, cushion, and felt) is closely related to the printing pressure. It should be made of elastic fabric, which 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 other hand, if the domestically produced quality blanket is used in order to reduce the cost, it is necessary to increase the "absolute value of the minimum printing pressure". If the printing pressure of the imported air cushion blanket is generally set to 0.08~0.10mm, if the domestic blanket is used as the cushion at the same time, the general printing pressure is set to 0.2~0.25mm.
After the blanket is used for a certain period of time (printed number), it will lose or reduce the original elasticity quickly or slowly (this is determined by their quality). At this time, it is necessary to increase the pressure to solve the good transfer of the ink. The problem is that the new blanket and padding material enables a lower printing pressure.
(5) The number of printed materials
In the printing process, as the number of printed materials increases, the printing pressure decreases. This is because the blankets and pads lose or reduce the original after a period of use, that is, after printing a certain amount of printed matter. Elasticity, thus reducing printing pressure. According to the experiment, the printing pressure for the soft pad is reduced by 20-30%, and for the hard pad, it is not more than 10-15%. The ink transfer rate is lowered due to a drop in printing pressure. Therefore, in order to obtain a stable quality print, the pressure can be adjusted slightly higher before printing, so that the printing pressure after printing a certain amount is still sufficient to ensure the quality.
This reduction in printing pressure is most pronounced after the liner is renewed. Therefore, after updating the liner, it is necessary to test run at a working speed for half an hour to one hour to stabilize the liner and the blanket, and then adjust the printing pressure, so that a high-quality print can be obtained with substantially constant printing pressure.
Change the two channels of printing:
The offset printing machine mainly adjusts the printing pressure by changing the compression amount of the blanket, and the compression amount is mainly derived from the sum of the center distance of the drum after the pressing in the printing is smaller than the radius of the surface of the drum. For example, the pressure between the plate and the impression cylinder is derived from the fact that the center-to-center distance between the two rollers 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 adjust by changing the center distance of the roller. The author believes that the adjustment of printing pressure should be based on the situation, using different pressure regulation methods in different situations.
When the thickness of the substrate changes, different adjustment methods are used, as follows (indicated by the increase in thickness of the substrate):
1. Method 1: The pad under the blanket is reduced by the amount of the substrate, and in order to ensure the pressure between the plate cylinder and the blanket cylinder, the thickness of the pad 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, and the amount of increase should be the amount of thickness of the substrate.
In older models such as domestic 01, 03, 08 and other machines, since the pressure regulating device of the machine is not reliable, it is generally used to adjust the paper thickness when the paper thickness changes, unless the thickness of the substrate changes greatly. At this time (this must be adjusted by adjusting the center distance of the drum, the specific method is followed by a specific introduction), try not to change the center distance of the drum that has been precisely adjusted, which is mainly considered for labor intensity and production efficiency. At the same time, using Method 1 to adjust, we also pay attention to:
A Since the thickness of the plate cylinder liner is changed, it is necessary to adjust the plate water roller and the plate roller at the same time;
B It is necessary to pay attention to the change of the imprint on the substrate after the change of the diameter of the plate cylinder (due to the change of the cylinder diameter, the tangent diameter of the drum does not change, when the plate liner is thickened, the imprint on the substrate is It will become shorter; the opposite is true when thinning), and it is strictly forbidden to change the printing pressure in this way during the printing process of a batch of printing.
In addition, the need to change the thickness of the pad of the drum is:
1) When the blanket is deformed for a long time, it is necessary to properly thicken the pad of the rubber brist (this kind of thickening is generally required to add a little test at a time to achieve the best effect).
2) The circumferential overprinting is not allowed when overprinting, when the overprinting is not printed on the circumferential core size. {Considering the overprinting problem in multicolor printing, we often reduce the total thickness of the third and fourth sets of plates and their liners by 0.05 mm (compression amount) from the first group when setting the printing pressure. }
In the more advanced printing machines, since the change of the center distance of the drum is as simple as turning the handle or pressing the button, and the pressure display device is generally reliable, it is generally recommended to directly use the second method to complete the adjustment. There are many benefits to doing this:
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 is adjusted due to the process in the printing process, we only need to change the center distance of the two rollers. It is not necessary to simultaneously increase or decrease the pad of the plate cylinder and the blanket cylinder. Adjusting the center-to-center distance of the drum on the current press is a relatively simple task, even on the console, it can be operated remotely (now many presses, as long as the operator enters the printed paper thickness into the control system, the system automatically adjusts The default value of the pressure is output and automatically adjusted).
2) It can effectively avoid the speed difference of the surface of the roller during printing due to the use of the pad to change the circumference of the roller. This speed difference often leads to deformation of the dot in printing, and even related problems such as overprinting and ghosting.
3) Due to the use of the padding method in the replacement plate to change the printing pressure, the overprinting problem may occur (when the surface diameter of the plate cylinder is increased, the image which is finally transferred to the substrate is circumferentially shortened).
The adjustment of the center distance of the drum applies to the following situations:
A In order to increase the printing pressure of the blanket and the impression cylinder when printing rough and uneven paper, the center distance between the blanket cylinder and the impression cylinder must be reduced to suit the printing needs.
B Since the machine has been used for a long time, the roller gear or the roller bearing is seriously worn, and if the gear side gap is too large to cause a “ink bar” (bar head) or other problems, it must be solved by appropriately reducing the roller pitch.
Inspection of printing pressure:
(1) Gradually add paper to the full version of the law:
1. Place the pad in the blanket less than 0.4mm (normal).
2. Wash the plate and evenly apply the full version of the ink (yellow ink is recommended).
3. Press and transfer the full version of the ink on the plate to the blanket.
4. Then observe if the ink distribution on the blanket is uniform.
The above method is a relatively rough detection method. It can only detect whether the blanket is flat, whether the printing pressure is within a reasonable range (this kind of detection can only limit a relatively large range), and because it is cumbersome to operate, Not much is used in today's machine operations. The methods described in (2)(3) are often used more often, and work efficiency and effectiveness are better.
(2) Use the signal strip to combine the full version of the network cable detection:
The method is the same as (1), except that the ink transferred to the blanket is the wire and signal strip on the plate, and then the printing pressure can be determined according to the detection result. Usually, the detection is very accurate, especially 50. % of the dots are very sensitive to printing pressure, and can be clearly expressed when the printing pressure is not normal.
(3) Streaking marks
The pressure is checked by the width of the roller contact, which includes detecting the parallelism between the rollers and the pressure of the printing cylinder. The process is similar to the above two, in which the printing plate is first filled with ink, and then the machine is placed in the middle position between the printing plate cylinder and the blanket cylinder, and the pressure is manually pressed, and finally the streaks on the blanket are checked to meet the normal requirements. The same process is repeated between the blanket and the impression cylinder (when there is a substrate) to detect the pressure between the blanket cylinder and the impression cylinder. The table below gives a set of data for a more common model (the drum gear index circle diameter is between 270-300 and the impression cylinder is doubled):
Pad Type Compression (mm) Indentation Width (mm)
Plate - Eraser Eraser- Embossing Plate - Eraser Eraser- Embossing
Soft lining 0.2～0.3 0.2～0.4 7～8 8～9
Hard lining 0.08 or less 0.08 3～4 4～5
Since the embossing width and the diameter of the drum and the hardness of the blanket are greatly affected, it is for reference only.