First, the expansion and contraction of paper
Usually in a printing house, the thread of paper is determined based on the direction of the paper fiber and the axis of the cylinder.
Any printed paper whose fiber arrangement direction is parallel to the cylinder axis is called longitudinal strand paper, and paper whose fiber arrangement direction is perpendicular to the cylinder axis is called horizontal strand paper.
Longitudinal paper should be used for printing in offset printing. This is because: from the perspective of paper feeding, the longitudinal strand paper is relatively stiff in the direction of the (axial) direction, and it is not easy to tear in the radial direction, which is easy to blow loose and convey. After the fiber is completely wetted, its diameter may increase by 30%, while its length increases by 1-2%. Therefore, horizontal thread paper stretches more and more (axially) than longitudinal thread paper, which directly affects the accuracy of overprinting. The longitudinal thread paper stretches more up and down (radially), but the overprinting can be made accurate by adjusting the liner or other methods.
[Case]
A factory uses a single-color offset printing machine to print a batch of three-color and four-color jobs. The paper used is a single sheet of offset paper, and the cutting method is a T-shaped cutting method. When the first color was printed, when overprinting the second color, it was found that irregular overprinting on the left and right sides was inaccurate.
[case study]:
This phenomenon often occurs, and offset press operators mistakenly believe that the paper is of poor quality. The actual reason is that the upper process uses a T-shaped cutting method, and the horizontal and vertical strands of paper are mixed to make the paper stretch. Inconsistent, so it leads to inaccurate left and right overprint and irregularity.
[Expert Tips]:
All the cut single sheets of paper must be clipped with warning strips. When printing, separate the vertical and horizontal lines of paper without mixing.
Paper is a variable material. The fibers that make up the paper have a hygroscopic effect. The air contains a certain amount of moisture. Therefore, as long as the paper is in contact with the air, it will continuously balance with the air humidity. When the air humidity in the surrounding environment is higher than the humidity contained in the paper, the paper will absorb the moisture in the air and expand and elongate. On the contrary, when the air humidity in the surrounding environment is lower than the humidity contained in the paper itself, the paper will be released The moisture shrinks and shortens to achieve a balance with the ambient humidity. The so-called tight edges and lotus edges of the paper are all related to the moisture content of the paper. In order to prevent the inaccurate image overprint caused by the expansion and contraction of the paper, the longitudinal thread paper (called the horizontal thread paper in the paper mill) is used as much as possible to control the moisture content of the layout. If the paper quality is not good, the requirements for overprinting are higher, and the multi-color overprinting of the monochrome offset press is used, as far as possible, first press the clear water again.
Second, the paper corner
The flick angle refers to the phenomenon of inaccurate fit on one side of the trailing part of the printed paper. In recent years, most corner swing failures have occurred in multicolor offset printing. The main reasons are:
(1) The front gauge is not on a parallel straight line, which causes the paper to reach the front gauge with less paper on one side.
(2) The thickness of the paper varies greatly
(3) The pressure of each color group is increasing
(4) The blanket of rubber blanket is uneven
(5) There are differences in the tightness of the two sides of the impression roller gripper
(6) The individual tooth pads on both sides of the teeth of the impression cylinder are smooth or too low
(7) Due to the large blank area on both sides of the back of the printing plate, the amount of wetting fluid in this area is large, which causes the paper to attach more wetting fluid, causing the trailing portion to expand and contract, forming a flick angle.
(8) The amount of liquid used in the blank areas on both sides of the printing plate is different, and one side will also form a flick angle.
Similar failures have occurred in monochrome offset presses.
[Case]
A factory uses a four-color folio offset press to print atlases, using 80g /? Offset paper. It is often found that some red and blue sleeves are not on the trailing edge, especially some special symbols, such as some city symbols are blue traps red. This phenomenon mostly occurs in the second half of the printed web.
[case study]:
The above example is an obvious flicker phenomenon. The main reason for this phenomenon is that the printing pressure is not decreasing enough, so the printing pressure should be re-measured and adjusted. In addition, it is also related to the tightness of the paper and the tightness of the grippers.
[Case]:
In the process of debugging the CP2000-CD102 machine, a factory carried out color printing tests after passing the inspection of various indicators, and added portraits of people in different positions on the black, cyan, magenta, and yellow 50% flat screens. The test results show that the coated paper and paperboard above 128g /? Are normal, and when the 80g /? Coated paper is tested, the phenomenon of "swing angle" occurs at the lower right corner of the printed matter, and the overprint of the corner gauge is not accurate.
[case study]:
This kind of "swing angle" failure is generally not related to the pressure of the various color groups of the feeding plate and the blanket lining. It may be related to the gripping teeth, but when checking the gripping teeth, the gripping force is normal, and then observe the double-diameter impression cylinder gripping teeth. When gripping the paper, the gripper does not grip the paper within a length of about 4mm in the lower right corner. The paper deforms slightly when pressed, resulting in a "swing angle".
Approach:
Reset the position of the paper side pull gauge, adjust all related parts, and then turn on the trial printing to eliminate the fault.
[Expert Tips]:
Whether it is a multi-color offset printing machine or a monochrome offset printing machine, do not recklessly modify some key parts, pay more attention to the position and scope of the gripper. For harder paper, such failures are not easy to occur. [next]
3. Paper fluff and powder
The fluffing of paper is mainly caused by two reasons. The first is the quality of the paper, and the second is the printing conditions. The quality of the paper refers to the surface strength of the paper; the surface strength of the paper is a physical quantity that measures the binding force between the fibers, fillers, and rubbers on the surface of the paper. The ability to tear. Paper with high surface strength has less hair loss during printing. Therefore, in practical applications, the surface strength of the paper is also called the fluff resistance of the paper. It reflects the ability of paper to resist fluff during printing.
The printing conditions include many contents, such as the supply amount of the dampening liquid, the viscosity of the ink, the viscosity, the printing speed and so on.
â‘´ Paper fluff and ink pile
The fluff of paper is generally divided into dry fluff and wet fluff.
In a monochrome offset press, the fluff of the paper is dry fluff, which occurs when the binding force between the fibers or pigments is less than the adhesive force of the ink. Wet hair loss occurs under this condition with the participation of water.
In the process of multi-color machine printing, in addition to the first color ink of the first color group cylinder, the inks of the other color groups must be printed on the surface of the printed ink or white paper, and this white paper surface is wetted. Liquid wet. After wetting the paper, its surface strength will inevitably decrease. Obviously, if the last color is printed on the white paper, the surface strength of the white paper is the weakest.
Fibers, paint particles and paper deposited on the blankets from the paper will cause the blanket to pile up ink. Multi-color offset presses have different levels of ink stacking depending on the printed graphics. But generally speaking, it is not easy to happen on the blanket roller of the first color group, and it often happens in the second color group or later, which is the main reason for the pile of ink on the paper. The viscosity caused by the residual viscosity increases and wetting caused by the wetting of the wetting fluid. Therefore, the closer to the rear roller, the easier the ink accumulation occurs.
When the pile of ink begins to form, the image is first roughened. This is due to the flaking fibers and paint particles peeling off with the ink and forming a pile of ink on the blanket. As a result, the ink transfer of the convex portion is poor, which causes the printing plate flower to get stuck.
(2) Handling methods for paper lint and ink pile
The most fundamental way to reduce paper fluff is to choose high surface strength paper for printing. However, in practice, this kind of paper is not necessarily used, so from a process point of view, first of all, the wetting fluid should be controlled in the minimum range, and secondly, the printing speed is reduced to reduce the viscosity of the ink in the printing process and buffer the paper shed . For more serious lint-free paper, in order to ensure the quality, press the water once before printing or print the white oil or wash the blanket multiple times before printing fine products.
[Case]:
A printing factory used four-color offset printing to print a batch of "Body Art Appreciation" albums. The printing machine is an imported four-color offset printing machine, and the paper used is 128g /? Double-sided coated paper. When the operator completed the preparation work and less than 500 sheets were officially printed, it was found that the original smooth and delicate human skin had many spots of different sizes, and at this time, the machine was immediately stopped. The captain suspected that the color sequence was not appropriate, and then changed the color sequence before printing, but spots still appeared in different positions.
[case study]:
After illuminating the paper, it was found that the dustiness of the paper was too high, and then due to the formation of the unique wet powder dropping phenomenon after printing with the four-color machine, the above failures occurred.
Treatment method: replace the paper.
[Expert Tips]:
Similar to this example, there are quite a lot of actual work. Some papers are not suitable for printing high-end color jobs due to paper quality problems. Never make up.
[Case]:
A factory J2108 machine used 128g /? Double-sided coated paper to print color jobs, and there was no abnormality during printing. Later, it was changed to 80g /? Single-sided coated paper to print the same graphics and text. There was ink accumulation on the surface of the blanket, which was later caused by insufficient blanketing of the blanket cylinder.
[Expert Tips]:
Insufficient blanket blanketing is often the main factor for ink accumulation. This is because the ink cannot be transferred well due to insufficient lining, resulting in thicker accumulation on the blanket cylinder to form a pile of ink. Therefore, the thickness of the blanket must be increased accordingly. It can be seen that the ink pile failure is not necessarily caused by poor paper quality.
Four, paper static electricity
The static electricity of paper affects paper feeding and printing. One is that the paper has static electricity before it is used on the machine, and the other is that the static electricity is not obvious before the paper is printed. The static electricity suddenly increases after imprinting, making the paper uneven. Water is involved in the offset printing process, but it is not uncommon to increase static electricity after printing.
Common methods for eliminating static electricity include: using a humidifier to increase the relative humidity of the room where the paper is hanging and the workshop to increase the moisture content of the paper; place the paper next to the machine in advance to adapt to the temperature and humidity of the printing workshop. If the paper feeder is acceptable, but the printed product is uneven at the delivery location, at this time, it can be effective to increase the moisture on the layout slightly. In addition, a static eliminator or antistatic agent can also be used to eliminate the phenomenon of static electricity on the paper.
[Case]:
A factory used offset paper to print a batch of colored jobs. During the paper feeding process, it was either double or skewed, and it could not be printed normally. It was judged that the paper had static electricity, but the factory had no equipment for hanging the paper. The ground was sprinkled with some water, and all the paper was shaken and piled up. The paper was reloaded the next day, and the paper was fed normally. The above situation no longer happened.
[Expert Tips]:
The static electricity of paper has a great relationship with the temperature and humidity of the printing environment, so attention must be paid! [next]
Five, paper bow wrinkle
In the printing process, paper bowing (pleating) is one of the most common failures, and the economic loss is relatively large. The reasons for such failures are mainly two aspects: One is caused by the paper itself (mostly in non-coatings) Paper), the second is the improper use of the machine adjustment, according to its shape to further analyze the cause and processing method.
1. Small radiating pleats
The reasons for this type of pleats:
(1) The "lotus edge" produced after the paper absorbs moisture (the paper itself has uneven moisture content or the workshop relative humidity is high, and the paper moisture content is low).
(2) The ink on the back of the printed piece on both sides is "stacked cylinder" (the accumulation of ink on the surface of the impression cylinder is too thick, causing a change in local pressure).
Approach:
Hang the paper again; if it is printed on both sides, be sure to wash the impression cylinder frequently.
2. Thin straight pleats that do not come out (the pleats are in the middle of the paper)
The main cause of such pleats is the deformation (tight edges) around the paper.
Approach:
Hang the paper again. If some printing factories do not have the equipment to hang the paper, you can put the uncut printing paper to be printed and shake the wind one week in advance, put it in the printing workshop to adapt to the temperature and humidity around the machine, and then cut it when printing on board.
3. Large crooked pleats
The reasons for this type of pleats:
(1) The corners of Shuida paper are raised, and the paper teeth are not good.
(2) The grip of the paper teeth on one side of the paper is too light.
(3) The paper corner floats up.
(4) The position of the side is not correct, and the corner of the paper touches the gap of the gripper.
Approach:
After the paper is knocked back, it is printed on the machine; it needs to be readjusted for the loose gripper, and the paper corner floats mainly because the gripper cannot hold the paper.
[Case]:
A factory J2105 machine printed middle school textbooks, color inserts. It was printed on 128g /? Coated paper, and shortly after the official printing, it was found that occasionally folds appeared on the outside. After observing the position on the outside, after careful observation, it was determined that the paper was pulled by the side rule and the paper was pulled. The gaps collide, causing pleats to appear.
Approach:
The side gauge is properly displaced to avoid the gap.
[Expert Tips]:
Similar to this phenomenon, the captain should deal with it in advance, and can not wait for the problem to appear and then deal with it.
4. Circular arc pleats
The main reason for this kind of pleats is: the middle part of the gripper is too loose, which causes the phenomenon that the gripper cannot hold the paper, and it is pulled out during printing to produce folds.
Approach:
Correct the grip of paper teeth. If the pad is too smooth, you can replace it with a new one.
[Case]:
A factory used a four-color offset printing machine to print the advertising brochure of an enterprise, and found that circular folds (not every one) appeared in the place where the portraits were printed in the second half.
[case study]:
According to the conventional treatment method, it is necessary to check the gripping force of the gripper, but in this case, the gripper gripping force is good. Later, it was found that the black density of the folds is large, especially the density of the red ink is too large, resulting in here If the peeling force is increased and the gripper is slightly unsuitable, the above failure will occur.
Approach:
Re-make the plate, put the portrait of the person on the mouth or the first half of the paper, and the fault disappears.
[Expert Tips]:
General portraits or products with strict requirements for fitting, such as portraits or smaller pictures, are generally best placed in the first half of the mouth or printing paper during imposition. In addition, those with high ink density should not be printed in the second half.
5. Large radiating pleats
The reasons for this type of pleats:
(1) Two or more gripper teeth of the impression cylinder are too loose in one place, and cannot even hold the paper.
(2) The unevenness of the delivery tooth pads causes the edge of the gripper paper not to be a straight line and becomes a wavy curve. When the impression roller grips the paper and presses the paper, the length of the paper is shortened. Pleat.
Treatment method: Check the gripping force of the gripper of the impression cylinder.
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