The stable and accurate paper feeding is an important condition to ensure the printing quality. Due to the frequent changes of the substrate, the structure and motion of the paper feeding part are more complicated, and there are many reasons for the failure to make an accurate judgment immediately, which often makes it difficult for the operator to deal with. Paper feed faults usually include: empty sheets, double sheets, multiple sheets of paper skewed, and the paper feeds quickly and slowly. The occurrence of faults, except for paper, most of the faults are related to the adjustment of the separation mechanism of the paper.

1. Blank

This type of fault is the most common type of fault most easily discovered by operators. The causes of the fault are:

(1) The unevenness of the paper makes the stack too low.
(2) The paper feed nozzle is too low.
(3) The separation nozzle is too high.
(4) The back of the printed color is stuck.
(5) There is a failure in the automatic lifting of the paper stack.
(6) The paper blocking brush or paper pressure steel sheet is too low or reaches too much into the paper stack.
(7) The amount of blowing and suction is too small.

Through the analysis of the cause of the blank sheet failure, it can be seen that the height adjustment of each nozzle and the amount of air volume are the key. In addition, the stacking operation level is also very important. At the same time, the flatness of the stack should be adjusted through the wooden wedge.

[Expert Tips]
On the one hand, the wooden wedge is used to adjust the flatness of the paper pile, and on the other hand, it can be used to adjust the height of the paper pile and control the rise of the paper pile, so the operator must use the wooden wedge well.

2. Double or multiple sheets

This kind of fault is a fault that causes the operator to feel "fear" during the paper feeding process. Because it is not only harmful, but even extremely destructive to the machine, thousands of cases of "smooth car" accidents are basically caused by double or multiple sheets of paper. The reason for this failure :
(1) The edge of the paper curls up and falls, the paper is cut in different sizes, and the stack of paper is uneven.
(2) The paper has static electricity.
(3) The rise of the stack is too large or unstable.
(4) Improper adjustment of the components of the separation mechanism, mainly including: the paper blocking brush is too high; the paper blocking brush is too low or the amount of penetration is too small; the separation of the paper is not enough to blow; the separation of the separation nozzle is not high or low or the rubber ring is not selected ; The pressure paper blowing nozzle (if the pressure paper is not blown, it is called the pressure paper foot) too little paper.

From the point of view of the above failure reason and adjustment mechanism, the substrate is the main contradiction. The mechanism of separation mechanism is different with the substrate and the adjustment method is different. If the paper has strong static electricity, no matter how to adjust the mechanism of the separation mechanism, it is useless. Only hanging the paper to adjust the humidity or increase the air humidity. Generally speaking, when printing thin paper, there is a higher probability of double or multiple sheets.

Print thinner paper, especially paper under 60g / ?. Due to its poor strength and light weight, when the loose paper mouthpiece blows the loose paper edge, the paper edge is fluffy and wavy. When the separation nozzle sucks up the first piece of paper, the fluffy paper edge is difficult to return to a flat free state. When the presser foot is pressed down, it is easy to insert under the second or multiple sheets of paper, resulting in double or multiple sheets.

In view of the above situation, the following adjustments are made to the relevant machines of the separation mechanism:

(1) Appropriately reduce the air volume of the loose paper blowing nozzle to ensure that the loose paper blowing nozzle only blows a few sheets of paper steadily at a time.
(2) The height of the paper separation nozzle is not appropriate from the paper surface, otherwise when it moves down the paper suction, it will squeeze the air between the upper and lower two papers and make the two papers suck together.
(3) The presser foot should be pressed more often without touching the edge of the paper. If the presser foot is equipped with a blower, the amount of blower can be increased appropriately.
(4) Press the tablet, brush down as far as possible without affecting the nozzle, and extend it. Be sure to adjust the double-sheet controller to avoid the "smoky car" accident.

[Expert Tips]

When printing coated paper and offset paper, the adjustment method of paper feeding is different. The surface smoothness of coated paper and offset paper is different. The surface of the coated paper is smooth, the friction between the loose paper nozzle and the surface of the paper is small, and the paper can be blown up during printing (for the CD102 type machine, the side blowing is blown up, the air volume of the presser foot and the front blowing is increased), shrink The distance between the paper suction nozzle and the paper, even the paper suction nozzle can keep in contact with the paper at all times, and there will be no skewed or double sheets. The surface of the offset paper is rough. Before the paper suction nozzle and the paper are not sucked, if the distance is too small, a large friction force will be generated and skewed, and even the paper below will be affected to produce double sheets. Therefore, it is necessary to increase the distance between the paper suction nozzle and the paper before it is not sucked, and the suction amount is large and uniform. [next]

[Case 3-1]
Due to the failure of the double-sheet detector, the SZ206 type paper feeder of a certain factory entered the rubber-press roller of the whiteboard paper, causing a "smooth car" failure. The operator took many measures and failed to make the machine rotate. Finally, use the injection needle to inject water to make the paper fibers expand and become soft, and then use the hacksaw blade to pull out the whiteboard paper.

3. Paper skew

The parts on the paper conveyor basically follow the principle of symmetry. For example, brushes, tablets, blocks, blow nozzles, suction nozzles, paper receiving wheels, etc., are distributed in pairs and are symmetrical in front, rear, and left and right positions, and have the same contact pressure. Obviously, the blowing and suction volume should also be uniform and symmetrical, which requires the paper surface to be flat, the front opening and the side edges to be flat, and there is no resistance between the papers, but in actual production, this ideal state is often difficult to achieve, one is because The electrostatic attraction between the paper, the ink adhesion, and the uneven local resistance caused by the curvature of the paper surface make the paper transmission easily skewed. The second is that the left and right sides of the paper have different bending conditions. The third is that the front stopper is not in a straight line. Fourth, there are many parts of the separation mechanism, and the action is complicated. It is inevitable that the jitter will be unstable during the process of separating and handing the paper. Fifth, there is a deviation between the two sides of the delivery roller adjustment. These factors can easily cause paper skew and are difficult to overcome. Therefore, some machines, such as CD102, do not have an adjustment knob above the separation mechanism, which can correct paper skew.

[Case 3-2]
A factory has an imported folio multi-color offset printing machine. When the printing is positive or large folio, the paper feed is normal, but the paper is skewed when every four open jobs are changed.

[case study]

This situation often occurs in many printing plants, which is due to the small opening of the paper and the inadequate adjustment of the paper transport mechanism. The most critical is the uneven distribution of force on the paper surface of the paper delivery wheel (two) and the double-sheet detector control wheel.

4. The paper moves quickly or slowly or jitters

The causes of such failures are not only the broken pins connecting the feeder to the host, the wear of gears or other components, and the static electricity of the paper, but the improper adjustment of the delivery roller is also the main factor that causes the paper to move quickly and slowly. The drop time of the wheel is 5mm across the contact point between the paper pick-up roller and the paper pick-up roller, and the paper pick-up nozzle is ready to put the paper. The paper pick-up roller should press the paper in time and the hand-over should be Completed within a short period of time, but not out of control. If the drop of the delivery roller presses the paper prematurely and the delivery nozzle is not released, a "paper snatch" is formed. When the delivery nozzle releases the paper, it will cause the paper to be unstable or shake. Conversely, if the delivery roller falls too late, the paper is not controlled in time when the delivery nozzle releases the paper, and the paper will have a significant pause during transportation.

There must be proper contact pressure between the paper delivery roller and the paper delivery shaft. If the pressure is too low and the friction is insufficient, the paper between the paper delivery roller and the paper delivery shaft will slide relatively, especially when the paper is static or When the paper nozzle has more suction, the paper feeding speed will obviously change. The so-called residual suction phenomenon, that is, the paper separation nozzle still sucks the paper when the paper feeding nozzle advances the paper. The paper suction nozzle sucks the paper and pulls it back on the way back.

[Case 3-3]
During the printing production process of a factory, the paper conveying speed sometimes changes. The main reason is that the first few sheets of paper are slower when the air pump is turned on after the paper machine is turned on, while the last one is faster after the air pump is turned off.

[case study]:
This phenomenon is obviously due to the effect of suction. When there is suction, the paper is slow down, and when there is no suction, the paper is fast. It means that there is excess suction in the paper suction nozzle after the air pump is started. After the air pump is turned off, there is no suction. There is no excess suction in the wind.

Treatment method: clean the gas path

5. Poor gas path

Poor gas path is an important reason for the paper to move quickly or slowly or jitter. If the suction pipe is blocked, it will inevitably lead to a decrease in the amount of suction and shorten the suction time, which will cause the standard movement and working time of the paper separation nozzle and the paper delivery nozzle to change. The distance and speed of the paper sent will change. The function of the mouth replenishment port is to balance the internal and external air pressure, so that the suction nozzle can completely relax the paper. Because the suction pipe is blocked and the air supply path is not smooth, the suction nozzle will inevitably cause the phenomenon of residual suction, which will cause the paper conveying speed to slow down or stop.

[Related Links]

The surface of the coated paper is smooth, and generally does not produce powder or hair loss, so the side air inlet of the paper suction nozzle is not easy to block and can keep the suction smooth. Compared with coated paper, offset paper is easy to lose powder and fluff, so it is easy to appear unstable when printing offset paper. [next]

In order to facilitate the operator to identify and eliminate the fault as soon as possible, modern sheet-fed offset presses are often equipped with an "information display" at the operation position of the paper feeder. As shown in Figure 3-20

Figure 3-20 Offset press machine information display

Figure 3-21 Machine speed and paper monitoring display

Figure 3-22 Paper tilt display

(1) Number of machine prints (2) Error message (3) The paper arrives early or too late (4) The paper arrives late (5) The paper is skewed (6) The paper is wrong (7) The drawing rule is wrong (8) Double sheets [next]

If the paper transfer or paper feed fails, there is a display (#) under the corresponding mark as shown in Figure 3-22. This indicates that the paper is tilted or the paper is fed incorrectly. During paper feeding or production operation, the paper correction control system confirms the position of the skewed paper that appears at the front gauge through the sensor, which is displayed by the line marking (Figure 3-22). In addition, if the paper automatically reaches the limit position of the control, it is displayed with the symbol? ← (Figure 3-23). This display has no effect on the offset press control system.

Tip: For the information display of the offset press, only the paper feed monitoring is involved, and other fault monitoring will be explained later.

Figure 3-23 Paper arrival control display.

[Related Links]

For the main faults of the web offset printing machine in paper feeding, one is that the tension is not constant, and the other is that paper breaks often occur during the printing process.

When printing on roll paper, the paper tape must have a certain tension, because only the paper tape can be controlled to move its tension. In order to stabilize the printing process, the tension of the tape must be kept constant and the size is appropriate. Too small tension will cause process failures such as wrinkling and overprinting. Excessive tension will cause paper deformation or breakage and increase the machine The load. Paper tape with unstable tension will produce jitter, causing problems such as inaccurate overprinting and ghosting. The tension of modern web offset presses can be reflected on the computer interface.

In order to make the paper belt tension constant, a brake must be set on the paper roll, and the control system performs the necessary control on the paper roll tension controller. The tension of the paper belt is determined by the speed difference between the paper feed roller, printing cylinder, paper take-up roller and paper take-up roller.

The main reasons for paper breaks during web printing are:

(1) The tension system does not match the tensile capacity of the paper tape used.
(2) There is a problem with the quality of the paper roll.
(3) There are foreign objects on the blanket.
(4) The rubber cloth was not scrubbed after cleaning the rubber roller before starting.
(5) Improper operation, the general operating procedure should be "press pressure first, then water".

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