(This article is an original article from Bisheng. It is forbidden to reprint, and reprinting must be corrected)
Sheet-fed offset presses use a variety of paper delivery methods, but in summary, they can be divided into three basic forms: direct paper delivery, indirect paper delivery, and transcendental paper delivery.
In recent years, the speed of printing presses has continuously increased, and the speed of sheet-fed offset printing presses has reached 16,000 sheets per hour or higher. Although the indirect transfer paper that is widely used has the characteristics of relatively smooth transfer and can meet the requirements of high-speed printing, this type of transfer method is also a mechanical reciprocating motion. It still transfers the positioned paper to the impression cylinder twice With a gripper, the faster the printing speed, the greater the inertia, the greater the impact force, and the more serious damage to the machine, it will inevitably produce errors and affect the accuracy of overprinting, thus hindering the further increase of printing speed, and the printing speed is increased to a certain level. After the degree, due to the limitations of manufacturing accuracy and objective reasons such as assembly error and vibration, it will inevitably produce errors and also affect the accuracy of overprinting.
For this reason, currently on some high-speed printing machines, the final positioning position of the paper is placed on the impression cylinder, that is, the final registration rule is set on the impression cylinder instead of on the cardboard. After the paper passes through the pre-regulation and side regulation on the paperboard, the speed gradually increases from zero to a speed of about 25% above the linear speed of the drum surface, and the paper is firmly pushed to the front regulation on the drum for positioning, so This method of paper delivery is called transcendental paper delivery.
The use of overtaking paper can shorten the positioning time of the paper at the rule as much as possible. The reciprocating paper acceleration mechanism accelerates the paper to exceed the surface speed of the roller surface, and finally locates on the roller, and then the roller grips print. In this way, there is no problem of destroying the paper positioning accuracy during the transfer process. This paper delivery method overcomes the shortcomings of direct and indirect paper delivery and is beneficial to increase the printing speed. It has the following advantages:
â‘ Compared with the indirect paper delivery method, a pair of paper delivery gripper mechanisms are reduced, the structure is compact, and the probability of paper delivery errors is reduced.
â‘¡The paper is positioned twice, and the final positioning is set on the impression cylinder, which reduces the number of intermediate transfers, and the positioning is accurate, especially suitable for the needs of high-speed printing.
â‘¢ The paper speed is greater than the linear speed of the impression cylinder (with a certain amount of overrun), which is conducive to full final positioning.
â‘£The final positioning plate can be easily adjusted, which can partially solve the problem of inaccurate overprint caused by paper deformation.
â‘¤ The impact, vibration and noise during the operation of the mechanism are small, and the paper transfer stability is good.
â‘¥ Convenient and fast operation, reduce labor intensity.
However, it must be pointed out that the transcendental transfer can only be applied to smaller format papers, that is, only to smaller printing presses; and for larger format papers or used on large printing presses, the accuracy will drop a lot.
Transcendence paper transfer is a new advanced paper transfer method. In recent years, such devices have developed very rapidly, and there are many types, such as friction roller type, vacuum belt type, suction wheel type, etc., but their basic principles are similar, and there is no qualitative difference. In the following, the composition and working principle of the surpassing paper delivery mechanism will be explained with the more common and advanced suction wheel surpassing paper delivery method as the focus.
The suction wheel surpassing paper delivery mechanism is mainly composed of two parts: one is the power transmission and variable speed movement mechanism; the other is the vacuum opening time control and negative pressure adjustment mechanism.
(1) Variable speed (intermittent) motion analysis of the suction wheel
The suction wheel is a key execution component in the transcendental paper delivery mechanism, and it is very important to analyze its working principle and movement characteristics. The variable-speed intermittent movement of the suction wheel is usually realized by using a multi-head conjugate disc-shaped indexing cam mechanism as a transmission mechanism. Therefore, it is particularly important to first analyze the motion characteristics of the conjugate disc-shaped indexing cam mechanism.
â‘ Conjugate disc-shaped indexing cam mechanism
The overrunning paper delivery mechanism has two cams with the same profile on the power input shaft arranged symmetrically, but they are installed in reverse phase according to a certain phase angle to form a pair of parallel conjugate disc-shaped indexing cams. When the conjugate disc-shaped indexing cam is continuously rotated, the follower turntable and the suction wheel of the driving mechanism transfer the paper to the feed roller according to a certain transmission ratio. The conjugate disc-shaped indexing cam always keeps in contact with the follower, forming a geometrical closure (as shown in Figure 1).
The conjugate disc-shaped indexing cam mechanism has the advantages of simple design and analysis process, high indexing accuracy, small impact vibration, suitable for high-speed operation, etc. It is very ideal to use it as the transmission mechanism of the surpassing paper transfer mechanism.
â‘¡Motion characteristics and contour curve of conjugate disc-shaped indexing cam mechanism
Relevant research shows that the motion characteristics of the conjugate disc-shaped indexing cam mechanism are universally improved trapezoidal acceleration, and the changes of acceleration and speed with time are shown in Figure 2.
The relationship between the contour curve of the conjugate disc-shaped indexing cam mechanism used on the printing machine's overfeed mechanism and the angle (time) is shown in Figure 3.
Figure 3 Conjugate disc indexing cam profile curve analysis diagram
â‘¢ Variable speed movement of the suction wheel
According to the movement characteristics and contour curve of the conjugate disc-shaped indexing cam mechanism of the driving mechanism, the movement form of the suction wheel of the follower can be divided into the following four stages:
a. Start-up phase (accelerated motion):
At this stage, the suction wheel sucks the stationary paper on the paper feed board. The suction wheel starts to accelerate until the speed exceeds the surface linear speed of the paper feed roller, so that the bite edge of the paper reaches the front stop board in time for a second positioning .
b. Handover phase (constant speed movement):
At this stage, the paper feed roller grips the paper and hands the paper to the platen roller, so that the paper leaves the pre-regulation completely and quickly.
c. Inert phase (deceleration movement):
At this time, the paper leaves the suction wheel completely, and the part without suction holes on the surface of the suction wheel is finished. It decelerates from high-speed operation and transitions to a static state, so that the starting part of the suction hole on the surface of the suction wheel is at the surface of the paper inlet .
d. Static phase: In this phase, the suction wheel is about to complete the entire motion cycle, waiting for the beginning of the next paper delivery cycle.
(2) Working principle of suction control mechanism
The negative pressure provided by the air pump of the mechanism controls the size of the vacuum volume through the regulating valve, and the set negative pressure enters the main air pipe of the mechanism. There is a ventilation valve on the wind pipe leading to each suction wheel. When the valve is fully opened, the negative pressure is led to each suction wheel wind pipe. When the piston rod is in the open position, the negative pressure will enter the air cavity, and the suction wheel can rely on the air pressure to attract the paper, and under the action of the speed change mechanism, it can achieve the overtaking paper delivery; when the piston rod is in the closed position, the negative If the pressure cannot enter the vacuum chamber, the air chamber communicates with the atmosphere, and the suction wheel loses the function of delivering paper; when the paper detection device finds that the conveyed paper has some problems, such as double sheets, empty sheets, skew, etc., the machine automatically The control mechanism will immediately produce a series of chain reactions, one of which is very important is to stop the air supply in the air cavity, so that the suction wheel cannot affect the paper stopped on the incoming paperboard.
Adjustment of suction-wheel over-paper delivery mechanism
(1) An important feature of the transcendental paper delivery mechanism-paper arch
Because the paper is stationary when the pre-regulation is pre-positioned, and the roller is in the running state, the speed of the paper delivery must be slightly greater than the linear speed of the surface of the impression cylinder (usually around 25%) in order to push the paper tightly and accurately The final positioning plate of the impression cylinder. Therefore, the first half of the paper will produce a degree of protrusion, usually called an arch (shown in Figure 4).
It is generally believed that the arch of the paper reflects the surpassing degree of the paper transfer, and also apparently shows that the running speed of the paper is higher than the linear speed of the paper feed roller, so it is one of the important characteristics of the paper transfer. If there is no such arch, the paper delivery speed is only equal to or less than the surface linear speed of the feed roller, which means that the paper has just been barely sent to the positioning plate of the feed roller or not sent to the front regulation at all. If the above phenomenon occurs, it will inevitably lead to the destruction of the paper's accurate positioning and overprint accuracy, and the printing quality will also be affected.
The amount of paper is different, and the amount of arching is also different, but it is generally in the range of 1-4mm. The general rule is that the greater the stiffness of the paper, the smaller the amount of arching.
(2) Adjustment method:
â‘ Adjustment of paper arch
Paper arch adjustment is very convenient. The adjustment device is generally located outside the protective cover of the machine transmission surface (ie, side II), and consists of an adjustment handle and a lock tension arch handle. First, the paper is delivered by the suction roller to the feed roller, and the machine is stopped at 128.5 Â° (the front edge of the paper is close to the front rule of the transfer roller, the teeth are not closed), then, check the amount of paper arch, if If it is too large or too small, you can loosen the locking handle and turn the value dial on the end of the handwheel to adjust. After adjustment, you must tighten the locking handle, and then let the paper run a few times, check the crown at 128.5 Â° of the machine .
â‘¡Adjusting the air volume of the suction wheel
a. First turn the machine to the 100 o ~ 110 o position (at this time, the air valve of the suction wheel is still open, and the suction hole is sucking the paper).
b. Put a piece of paper of common weight and type on each suction wheel to cover the suction wheel completely.
c. Turn on the air pump to turn on the negative pressure, and turn the adjustment handle to make the vacuum gauge show a vacuum of 0.5pa, the outline of the suction hole can be seen on the paper strip, and there is a resistance when pulling the paper strip .
d. There is an air valve on the ventilation duct of each suction wheel. As long as the valve is turned 900, the wind source of the suction wheel can be closed, and the suction wheel can be discarded.
In short, when the printing conditions change, especially when the nature of the paper changes, the adjustment of the suction wheel-type over-paper delivery mechanism is relatively simple, and no mechanical adjustment is required, as long as the paper arch and the suction wheel air volume are adjusted. The corresponding simple adjustment is enough.
Friction pressure roller type, vacuum belt type surpassing paper delivery mechanism
There is no essential difference in the working principle and mechanical mechanism of friction pressure roller type, vacuum belt-type over-paper delivery mechanism and suction wheel-type over-paper delivery mechanism. The main difference lies in the speed change mechanism used. The following will briefly introduce these two types of surpassing paper transfer mechanism, in order to compare and supplement with the suction wheel type surpassing paper transfer mechanism.
1. Friction pressure roller type surpassing paper delivery mechanism
The friction pressure roller-type surpassing paper delivery mechanism is mainly composed of upper paper delivery rollers, lower paper delivery rollers, paper blocking levers and other mechanical parts. The swing of the upper paper delivery roller and paper stopper and the rotation of the lower paper delivery roller are respectively driven by two cams on the shaft of the impression cylinder through corresponding mechanisms.
The basic working process is: after the paper reaches the front gauge and is pre-positioned, the side gauge falls to complete the initial positioning, and then the upper paper feed roller is dropped to press the paper. After that, the lower paper feed roller starts to rotate, the front gauge starts to let the paper forward, and the paper advances in a certain gap under the guidance of the paper stopper. The speed of the drum surface reaches the positioning plate of the drum to complete the second positioning. The platen is gripped by the platen of the impression cylinder to prepare for printing, and at this time, the paper delivery roller and the paper blocking lever are lifted to let the paper.
There are many adjustment items of the friction pressure roller type beyond the paper delivery mechanism, such as the pressure of the upper paper delivery roller, the rotation arc length of the lower paper delivery roller, the gap between the paper stopper and the paper feeding board, etc. In addition, it also has a very obvious defect: due to the use of upper and lower friction transmission, it is easy to smear the local graphics of the first print during the second print. Therefore, this type of overpass paper mechanism must be greatly improved.
2. Vacuum belt-type surpassing paper delivery mechanism
The vacuum belt-type surpassing paper delivery mechanism is composed of pre-positioning plate, suction nozzle, porous tape and other mechanical parts.
The basic working process is:
(1) After the paper reaches the pre-regulation for pre-positioning, the side gauge falls for positioning, and the suction stops at this time;
(2) The suction nozzle starts to suck, and the paper is attracted to the tape. While the paper is swayed down on the front plate, the tape rotates, the piston in the gas distribution wheel opens, and the air path creates a vacuum under the action of the vacuum pump. The vacuum belt and the air distribution port suck the paper, and rely on the suction to generate friction between the paper and the tape to accelerate the paper; at this time, the front rule falls to let the paper.
(3) The paper reaches the positioning plate of the roller at a speed greater than the surface of the roller, and the second positioning is completed.
(4) The impression cylinder grips the paper and transfers it. At this time, the suction is converted into a blower, and an air cushion is formed between the paper conveying table and the paper to facilitate the rapid transfer of the paper. The tape starts to reverse, and the pre-positioning plate enters the positioning position again, preparing for the next sheet of paper for positioning.
The adjustment of the vacuum belt-type surpassing paper delivery mechanism is also relatively simple: there is no need to adjust in the mechanical aspect, generally only need to adjust the vacuum degree and the blowing pressure. Therefore, it is more convenient to use than the friction-and-roll-over paper delivery mechanism. And because the device is installed under the paperboard, and the paper is renewed through suction paper, it also solves the problem of smudging.
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