Analysis of debugging principle of offset printing unit

1. The impression cylinder is the benchmark of the printing unit

The functions and adjustments of the components of the printing unit have been described separately, so what kind of relationship should they be put together? If the coordination is not accurate, how should it be adjusted? The following takes the basic structure of a printing unit as an example for detailed analysis, as shown in Figure 3.32.

Because the impression cylinder plays the role of paper transfer, its position change directly affects the accuracy of overprinting. Once the mutual cooperation of the paper transfer routes is determined, the position of the impression cylinder is completely determined.

From the perspective of image and text transfer, the image and text on the printing plate are transferred to the impression cylinder through the rubber cylinder. Since the graphics on the printing plate cannot always be kept in one position, it is usually necessary to calibrate with a plate-drawing mechanism or a plate-adjusting device, and it is not possible to adapt to the requirements of the plate cylinder by changing the position of the impression cylinder.

From the point of view of the contact of the rollers, to transfer images, there should be line contact or uniform surface contact between the rollers, that is, the axes of the rollers should be in line with each other. Since the position of the impression cylinder has been determined, the axis of the blanket cylinder can only be adjusted relative to the impression cylinder, and the axis of the plate cylinder can only be adjusted relative to the blanket cylinder.

From the perspective of clutch pressure, the position of the rubber roller is determined by the interaction of clutch pressure and pressure regulation. Once the pressure is different, the position of the rubber cylinder is different, but the impression cylinder cannot change with the position of the rubber cylinder.

Click here to see all news pictures Figure 3.22

In summary, in the printing unit, the position of the impression cylinder is not adjustable, which is the absolute reference of the printing unit. The rubber cylinder can be adjusted relative to the impression cylinder, but not to the plate cylinder. Therefore, the rubber roller is the first relative reference. Similarly, the plate cylinder can be adjusted relative to the rubber cylinder, but not to the ink mechanism, so the plate cylinder is the second relative reference.
From the perspective of pressure adjustment, when the pressure between the rubber cylinder and the impression cylinder is found to be inappropriate, it should be adapted by changing the position of the rubber cylinder; when the pressure between the plate cylinder and the rubber cylinder is found to be inappropriate, it should be Adjust the ink mechanism to meet the requirements.

From the perspective of time and displacement coordination, when it is found that the cooperation between the rubber cylinder and the impression cylinder is inappropriate, it should be adapted by changing the gear position of the rubber cylinder; when the cooperation between the plate cylinder and the rubber cylinder is found to be inappropriate, It should be adapted by changing the position of the plate cylinder gear. When it is found that the cooperation of the ink mechanism (such as the cross motion of the ink roller) and the plate cylinder is inappropriate, the position of the ink mechanism should be changed to meet the requirements.

The mutual adjustment between the various components mentioned above is carried out under the condition that each component's own working state is completely normal. Do not pay attention to this point when adjusting, that is, carefully check each component, such as the unevenness of the blanket. In particular, it should be noted that it is absolutely irreversible, that is, to reverse the benchmark.

Whenever the printing unit is adjusted, it is necessary to ensure the parallelism of the rotary axis, which is an important condition for its stable operation, so the pressure on the contact surface should be as uniform as possible. [next]

2. Axial serial motion of the rotary shaft

During the printing process, due to the presence of axial forces, the cylinder may have a series of axial movements. Once the axial crosstalk will cause dot slip or ghosting. The size of the axial series of the drum is generally controlled by the lock nuts at both ends. From the perspective of the installation and processing of thrust bearings, it is impossible to be completely uniform, and a certain clearance is required to allow them to operate relatively; another one may cause the bearing to be stuck due to the heating or compression of the rotating shaft. From an aspect perspective, a certain amount of axial series movement must be left during commissioning. The magnitude of this series momentum is different for different manufacturers. Generally less than 0.03mm. Without affecting the rotation of the bearing, the smaller the series momentum, the better. The usual adjustment method is: after the locking female is completely locked, it can be reversed by half a turn or a third of a turn. A more accurate method can be detected by a dial indicator.

3. Radial runout of the rotary shaft

The radial runout of the rotary shaft is mainly caused by errors in the machining and installation of the machine. The radial runout of the drum is a major indicator of equipment inspection before leaving the factory. Once it is found that the radial runout of the drum is too large, it is necessary to find out whether it is not processed or the installation is defective. The diameter jump of the cylinder affects the stable operation of the machine, and also causes the instability of the printing pressure, causing a series of printing failures. Therefore, the radial runout of the drum must be reduced as much as possible.

Click here to see all news pictures Figure 3.23 4. Commissioning of multi-unit printing unit

How should the printing unit of two or more units be adjusted together? As above, the key is the choice of benchmark. As shown in Figure 3.23, the paper enters from the right and passes through the first unit to reach the second unit. From the perspective of paper conveyance, the first group of impression cylinders must first transfer paper accurately and stably to ensure the normal operation of the second group of impression cylinders. If the second group of paper transfer is not good, the first group of paper transfer is no problem, obviously can not adjust the first group of impression cylinder to meet the requirements of the second group of paper transfer. If there is a problem with the transfer of the impression cylinder of the first group, it makes no sense to adjust the impression cylinder of the second group. Therefore, the second group of impression cylinders can only be adjusted relative to the first group of impression cylinders, and so on.

It can also be seen from the above figure that the first group of impression cylinders reaches the second group of impression cylinders after handing over the paper to the transfer cylinder. According to the transmission chain, the transfer cylinder is the first relative reference, The second group of impression cylinders is the second relative reference, and so on. According to this relationship, during commissioning, the rollers on the paper transfer path are first adjusted according to the reference sequence, and then each printing unit is adjusted. The adjustment of the printing unit is performed according to the method described above. [next]

5. Paper transfer roller

The role of the paper transfer roller is mainly to transfer paper between color groups. For single-sided multi-color printing, there must be an odd number of paper transfer cylinders in the middle, so that the printing surface can always be guaranteed to face up on the impression cylinder, as shown in Figure 3.24.

Click here to see all news pictures Figure 3.24

The number of paper transfer cylinders is selected according to the size of the printing format and the size of the operating space. For four start-ups, a large-diameter paper transfer roller can be used between the two units, because of its small size, it can be operated on both sides at the same time. If three rollers are placed in the middle, it takes up a lot of space and has no greater practical significance, but affects the accuracy of overprinting. For the three paper transfer rollers that can be used in the middle of the startup, because the folio is relatively large, it is not convenient to operate on both sides, and it is necessary to operate in the middle of the two groups.

At present, there are also full-size rollers used in the paper path for starting up, as shown in Figure 3.24b.

Click here to see all news pictures Figure 3.24b

In this way, the middle transfer roller becomes one. The moment the impression cylinder delivers the paper to the transfer cylinder, the printing of large paper may not be completed, so that the transfer cylinder actually plays the role of printing. Therefore, the requirements for the teeth of this cylinder are exactly the same as those of the impression cylinder. Another requirement for this cylinder is to prevent the back side of the paper from being dirty, because the printing surface is sticking to the surface of the cylinder, but the best solution should be solved by design, that is, after printing on the impression cylinder, it is passed to the transfer paper roller. [next]

6. Paper motion analysis in the printing unit

The movement of the paper on the impression cylinder has been analyzed in detail. The movement of the paper on the other cylinders is exactly the same as the impression cylinder, that is, the paper must always be in a controlled state (retention of positioning) in the printing unit. The key to the movement of paper in the drum is the movement in the process of mutual exchange, as shown in Figure 3.25 and Figure 3.26.

Click here to see all news pictures Figure 3.25

Click here to see all news pictures Figure 3.26

(1) The first stage of handover At this time, the first group of teeth gritted the paper, while the second group of teeth gritted just opened.

(2) The second stage of handover At this time, the first group of teeth is still biting the paper, and the second group of teeth is also biting the paper.

(3) At the third stage of the exchange, the first group of teeth was opened, and the second group of teeth continued to bite the paper, so the handover of the paper was completed.

The handover of the paper should be carried out under the premise that the dental pads are parallel, so the gap between the two parallel sets of dental pads has an important effect on the paper transfer. In terms of printing requirements, the front opening of the paper should always be kept in a horizontal plane, but this is difficult to achieve during the handover process, as shown in Figure 3.27.

Click here to see all news pictures Figure 3.27

It can be seen from the figure that there is only one kind of suitable paper, that is, the thickness of the paper is equal to the distance between the two sets of dental pads. When the thickness of the paper is too different, the wave shape at the front of the paper becomes more prominent, so each machine has its applicable range of paper thickness. To check the working condition of the teeth, you can use the thinnest paper printed on the ground given by its technical conditions to observe.

In terms of printing requirements, the paper should always be kept in close contact with the cylinder, that is, surrounded on the surface of the cylinder. However, due to the different thickness of the paper, the thinner the paper, the better the flexibility, the smaller the tendency of tangential orientation, and the easier it is to adhere to the surface of the drum; the thicker the paper, the worse the flexibility, the stronger the tendency of tangential orientation, The less likely it is to adhere to the surface of the cylinder, the more likely it is to collide with other contact surfaces and make the surface of the printed matter dirty.

Therefore, the thicker the paper, the larger the diameter of the roller, but the increase in the diameter of the roller is limited, and it is impossible to make it like a platform at any time. In order to still prevent the paper from colliding with other parts, an air suction device is installed inside the drum to suck the tail of the paper, and a blower device is installed outside the drum to press the paper against the surface of the drum.

7. Open and close tooth cam

When the teeth of the drum are exchanged, they must be opened and closed by a special mechanism. This special mechanism is to open and close the tooth cam. There are two types of open and close tooth cams: one is to close teeth at high points and open teeth at low points; the other is to open teeth at low points and close teeth at high points. To close the teeth at the high point is to force the teeth to be closed by the cam, which is stable and reliable, and the teeth are opened at the low point by the restoring force of the spring. Closed teeth at a low point is to close the teeth at a low point with a spring. When the machine is low speed and the actual ground is not large, this way of opening and closing teeth is OK. When the speed is high, the solid printing surface is increased, and the spring may cause instability due to its own inertia. At this time, the spring has the smallest force, which is easy to cause the paper to slide. When opening teeth, it is forced to open by the cam. From a mechanical point of view, the high-point closed teeth make full use of the deformation of the spring, while the low-point closed teeth do not realize the maximum potential of the spring. However, the high-point closed-cam must be full-cycle, while the low-point cam need not be. Therefore, when closing teeth with high points, the importance should be considered. Generally, the closed teeth of the impression cylinder should be closed with high points, and the intermediate transfer roller can be closed with low points.

8. Analysis of drum requirements

The cylinder is the most used part of the printing unit, and it is inseparable from the quality of the printing, so its requirements are more demanding.

(1) Utilization rate of the drum The utilization rate of the drum is the ratio of the arc length of the non-notched part of the drum to the circumference of the drum (between 0-1). The larger the ratio, the higher the utilization rate of the drum. Therefore, the large format of the small cylinder must be the main direction of the development of the rotary offset press.

(2) Diameter of the cylinder The smaller the diameter of the cylinder, the better, and the smaller the diameter, the easier it is to ensure the processing accuracy. However, due to the limitation of the printing width, it cannot be reduced indefinitely.

(3) The length of the cylinder The shorter the cylinder, the better the rigidity and the more uniform the pressure. Therefore, the products printed on four machines should print better on the same machine under the same conditions.

(4) Balance of the roller The balance of the roller directly affects the stability of the operation, so it is best to perform a dynamic balance test on the roller.

(5) Processing accuracy of the drum surface The higher the machining accuracy of the drum surface, the higher the quality of the printed matter, so the drum surface generally has to be ground.

(6) The surface material of the cylinder has many acidic substances in the printing environment, which can easily rust the surface of the cylinder, so the surface must be resistant to corrosion. In addition, the material on the surface of the cylinder must also have high strength and hardness, so as to ensure the quality of the print and resist fatigue impact.

(7) The weight of the drum The greater the weight of the drum, the greater the rotational inertia and the more difficult it is to brake. Therefore, this point must be fully taken into account when designing a new drum. Under the premise of strength guarantee, the lighter the weight, the better.

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