In lithographic offset printing, the occurrence of tearing in the printing process of paper can be said to be a relatively common failure. It is possible to promptly determine the cause of the failure and to handle it in a timely manner, which is undoubtedly very useful for the work.
In fact, the tearing of paper is not a very mysterious failure. Intentive masters generally do not immediately make adjustments to the equipment or blindly replace parts. Instead, consider carefully, and under what circumstances the tearing of the paper occurs. In general, the most common cases occur in three situations: after the equipment has just been overhauled (commissioned); after a long-term fixed print and live is suddenly replaced; and during the printing process, there is a continuous increase in tearing. Why should we analyze these situations? Because in different situations, the cause of the problem is obviously different, this analysis can help us find the root cause of the fault quickly and accurately. Then deal with the fault in a targeted manner.
For the convenience of discussion, the author divides the common tear into several situations:
1. Paper is not flat during feeding.
It is easy to appear in the printing process of relatively thin paper, and the tearing time is longer, and it is often accompanied by overprinting.
For relatively thin paper, be sure to pay attention when adjusting the machine, because it is easy for such problems to occur in feeders, paper feeders, and in the subsequent paper transfer.
The most common reason for this type of problem is that the roller pressure on the paperboard is too high, which causes the paper to be rolled out of the arch during the process of travel. This will inevitably cause problems with tearing of the paper during subsequent printing and transfer.
For the relatively thin paper, in the adjustment process, the author suggests that Feida's blowing and suction should be properly reduced until it can reach normal paper feeding. Excessive air volume will cause paper deformation.
The pressure of the pressure roller on the paperboard should be adjusted lightly and shifted slightly outwards (outer eight characters) in order to level out the uneven paper.
2. Inconsistent paper feeding and positioning
Such failures are often extreme. For example, the entire sheet of paper is gargled with teeth, or is half-rowed. It often appears to be very serious, and the offset of register lines is quite serious.
This type of failure is often due to the fact that feeder time is relatively fast relative to the take-up roller or that Fly is transferred to the front gauge relatively quickly (commonly referred to as paper speed). When the paper with the relative position ahead reaches the positioning position, since the limit that can be compensated for by the positioning has been exceeded, the mouthpiece will be bent and deformed until it is bitten by the delivery tooth. At this time, the mouth of the paper is not straight, so it cannot be avoided in the course of the subsequent transfer.
The treatment of such failures is only necessary if the relative angles of the paper are adjusted correctly.
3. The front position is too far behind (the paper is too large)
The whole or half of the mouth tears (the spacing is the spacing of the teeth) and the degree of tear varies depending on the size of the crepe.
If the current gauge is positioned too far behind, it will directly lead to an increase in the length of the feed paper and all the following gritted papers. At this time, the paper webs may not be able to be smoothly transferred during the mutual transfer process (when the next row of gritters enters the take paper In the position, the open teeth cannot avoid the oversized mouth, or the teeth after passing out the paper can't open enough space to open the paper) and cause the mouth to tear.
In fact, the processing method is very simple, that is, the position of the front gauge can be changed accordingly.
However, such a simple failure often leads to a great trouble for the operator due to a mistake in judgment. The author has seen an example of the same fault, and the operator immediately after the discovery is due to the abnormal transfer of the clenched teeth caused by the result of adjusting the bite force of the teeth over and over again, adjusting the transfer position.
When a problem arises, judgment is often more important than handling. Of course, prevention is more important than judgment. This example is due to the operator's abnormal regulation of the former rule (exceeding the range of adjustment allowed by the former regulation). (No one will adjust, the former rule will not normally move its position, of course, except for very special circumstances).
In addition, the author proposes that there is no electric adjustment front and rear position of the machine, under normal circumstances try not to use the way to change the position of the front regulation to adjust the plate registration. In particular, pay attention to old-fashioned equipment (the scale is not obvious or lost due to multiple repairs), or shift production.
4. The edges of the paper are just barely biting
The position of the tearing mouth is only about 2-3 cm on the edge, and the length of the tearing mouth is not large. The overprinting of the entire sheet has no effect, but local overprinting problems often occur near the tearing mouth. Generally not Zhang Zhang, the frequency of appearance is irregularly distributed.
Such problems usually occur after the conversion of India and Japan, especially when the paper format has changed. The root cause of the problem is that the edge of the paper should have at least one bite (feeder) crouching into the embossing area, so that it can always be transferred in the control. When the edge of the paper corner is not gnawing, this paper corner can easily be broken during delivery.
Appropriate skewing of the side gauges ensures that the edge of the paper is as controlled as possible. At the same time adjust the axial position of the printing plate.
Sometimes, we must also pay attention to the fact that the edges of the paper creases are upturned or squashed. This problem is also prone to problems. The solution is to turn the paper off (no paper can be lost, or paper with a problem at the end of the paper. Flatten the paper by tapping paper or soft paper) $ page break $
5. Tooth pads are too close or too far in the transfer position
When the dental pad is very far, the length of the tearing mouth is very short. Usually the fault occurs when the paper is just replaced with a paper that is relatively â€œsensitiveâ€ to the machine (such as relatively crisp or thin paper). When the occlusal pad is near, the tearing mouth sometimes has only the width of the occlusal pad, and the length is quite stable, and the tearing mouth is very smooth.
When the distance between the occlusal pads is relatively long, if the paper with better flexibility is printed, the problem of tearing does not occur, but when the paper with poor mechanical strength is encountered, the problem is manifested. Two rows of teeth in the transfer position, to the two rows of hooks, at the same time bite the paper easily tear the paper. A tooth pad that is too close is like a set of files, and the paper is easily cut under such a file. This situation often occurs when a long pad is suddenly replaced with a new one.
The general model can use a 0.2 mm thick steel bar (about 1 cm wide and long for the width of the row of teeth) to assist in adjusting the distance of the pad when in the transfer position. Generally in the transfer position, two rows of teeth can straighten the steel bar (the tooth pad is preferably a slight distance, that is, the steel bar is slightly curved). Then re-adjust the bite force.
6. Clenching teeth open too small
This is the most common cause of tearing in paper during printing. It is characterized by the fact that the tearing paper has a short mouth and is often turned upside down on the side of the dental plaque (we often use this phenomenon as an important basis for judging whether it is an odd-numbered row or an even-numbered row).
When the size of the clenched teeth does not meet the requirements (too small, generally too large, there is no problem), when entering the crossover line, you can not avoid the paper at the same time enter the crossover line, open the corresponding position of the paper to kick, often this At the same time, the action caused the paper torn and folded over.
There are two reasons that can cause such a low molarity.
One is that the whole row of bite teeth is becoming smaller. In this case, we must first consider whether the open tooth bearing is damaged. (High-point open teeth machine), general open spit block may not be damaged.
Another situation is that the individual teeth are too open. This is most likely due to the baldness of the small top thread on the tooth. It may also be due to long-term wear between the braces and the shaft. Caused. In the actual work, the above two situations often occur, sometimes due to the normal wear and tear of the machine, but more often due to artificially increased bite force resulting in excessive force between parts, resulting in abnormal wear. of. Therefore, the author would like to remind the operator again that it is very important to properly regulate and maintain the equipment. Otherwise, it will cause trouble for himself.
The number of handovers for a sheetfed offset press is very high, especially for four-color offset presses. The number of rows of teeth is sometimes more than a headache. If the trials in one row and one row are too troublesome, it means that the work is also costly. Not necessarily obvious. Therefore, it is very important to quickly determine the clenched teeth.
In the long-term work, the author explored some tips for judging problems with teething.
1 If it is an elemental problem, then the frequency of occurrence must be proportional to the number of rows in the chain. (If it is a machine with 11 rows of chains, then the gap between paper and paper with a tear must be a multiple of 11 or 11). Conversely, if the interval between tears is proportional to the number of rows in the chain, then we can almost certainly have problems with one or more rows of teeth.
If it is determined that it is a problem with the teeth, it will simply cost more than a dozen sheets of paper (because the problem of the teeth will not affect the overprint, so it will not affect other non-tearing prints), in one of the rows. Mark the eye-catching mark (such as white ink), visual inspection during the printing process, will quickly find the problem of the teeth.
2 If it is the problem of roller teething, then tearing will not be so regular, or Zhang Zhang has. However, we have found that tearing due to insufficient height of the bite opening is generally accompanied by the phenomenon of the nest edge. According to the direction of the nest edge, we can further narrow the scope of the fault. According to experience, the direction of the nest is always in the direction of the dental chip.
3 If it is a very regular one, or every other two, then we must first consider the double roller.
4 This kind of tearing due to insufficient opening of the teeth is often accompanied by a very inconspicuous phenomenon of partial overprinting. If the color order is blackish yellow, black and yellow are registered separately, and there is a problem of overprinting between them. Then we can further lock the faulty teeth on the roller between the blue and the green.
Based on the entire judgment process above, in most cases, we can quickly locate the faulty tooth based on judgment.
When the judgment is over, the processing is the simplest process, as long as the fault teeth found, the corresponding whole row can be readjusted (pay attention to the adjustment of the whole row, otherwise the tear is solved, due to the change of tooth force. It is likely that the overall pressure will be exerted on other individual teeth, which will lead to new problems in the near future.)
7. Long handover time
Generally this kind of situation is difficult to appear in the normal working process, but after overhauling or debugging, it sometimes appears. Its tearing feature is that there are tears but generally there will be no nesting (because the paper is broken of). But sometimes the shape of the tear is not easy to distinguish from the above. When judging, the final consideration is based on the possibility of occurrence.
When the two rows of teeth in the transfer process, to a common bite of paper movement, this is for the smooth transfer of paper, and then the two rollers will be separated in their respective circumferential direction, if the paper bite teeth in the specified corner If you haven't opened your teeth, you will see a paper grab. Tear the paper.
Adjust the open teeth of the corresponding teeth
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