1. Printing failure caused by paper
(1) Unified quality factors
1. Uneven coating and sizing on paper
The paper is composed of fibers, and the paper surface cannot be completely uniform. Therefore, it is necessary to coat a layer of inorganic mineral pigment and binder coating on the surface of the paper to improve the surface quality of the paper. The finer the pigment, the smoother and denser the coated paper surface; otherwise, the rougher the paper surface.
In addition, in order to reduce the de-powdering and fluffing of paper, a surface sizing process is used in papermaking, that is, a layer of adhesive is applied on the surface of the paper to improve the strength and gloss of the paper surface. Due to the fast printing speed of modern printing presses, the ink and the paper surface are instantly separated, and the ink has a large pulling force on the paper surface, which requires the application of the paper surface to be uniform and consistent.
2. Poor paper flatness
The flatness of the paper is not good, the specific performance is:
â‘ Bubbles form bubbles on the surface of the paper. This is because when the paper passes through the thermal dryer, the rate of moisture escape on the surface of the paper is lower than the speed generated by the steam inside the paper. The internal steam generates pressure inside the paper.
â‘¡There are holes and pulp on the paper
This is formed in the screening and purification process of paper. The occurrence of holes is formed after the paper is formed, and impurities are peeled off from the paper surface. Large holes will cause downtime or waste products during printing; large and hard blocks will damage the printing plate and rubber during printing, which will affect the printing process.
In order to avoid the above phenomena, the main measures taken are:
â‘ The paper dryer has good ventilation equipment, so that the water vapor evaporated from the paper can be quickly discharged outside the house.
â‘¡Secondary drying accelerates water removal.
â‘¢ The operation is carefully screened to purify the pulp.
(2) The moisture content of paper
1. Reason analysis
1. During the process from paper mill to printing plant until it becomes a printed product, due to changes in the surrounding temperature and humidity during transportation, storage, use, etc., and the paper fiber molecular structure contains hydroxyl groups to form hydrogen bonds, which will be hydrophilic when wet Swelling occurs; dehydration shrinks when drying. The water absorption and dehydration speed around the paper is faster than the center of the paper. In this way, the moisture content of the center and the periphery of the paper is uneven, so that the paper is pressed and ruffled. If it is used for printing, it will lead to inaccurate overprinting; The finished product is wrinkled, causing waste. The specific performance is:
â‘ Tight edge phenomenon The paper is in a dry environment, and the dehumidification phenomenon occurs, that is, the dehumidification speed of the paper is less than the center of the paper, so that the paper is stretched around the tight edge phenomenon.
â‘¡ Lotus leaf entering phenomenon, the paper is in a humid air environment, and the phenomenon of moisture absorption occurs, that is, the surrounding moisture absorption and elongation speed is faster than the center of the paper, and the paper forms a lotus leaf edge.
2. Exclusion method
â‘ When the paper has tight edges, spray more water around the paper pile or the machine to reabsorb moisture around the paper, balance the water content of the paper, and restore the paper to a flat level.
Dry the paper in an environment where the relative humidity is 6-8% higher than that in the printing shop.
â‘¡When the paper has a lotus leaf edge, you can knock the paper by hand to knock out the crease with a width of about 1cm and a length of about 10cm, so as to increase the stiffness of the paper and facilitate the paper transport;
In severe cases, the paper must be hung again.
(3) Static factors of paper
1. Causes and hazards
During the printing process, the paper on the paper stack is separated and transported forward under the friction of the paper conveyor belt and the platen roller. There is always friction between paper and paper, and between paper and machine. Friction generates electric charge. When the charge is accumulated, the paper is charged. When the paper has the same polarity charge, the paper will repel each other, resulting in inaccurate overprinting and paper collection when printing; when the paper has a different charge, it will cause empty sheets, double sheets, multiple sheets during paper feeding , Overprinting and other defects; the surface of the paper with static electricity is also easy to absorb some paper wool and dust, resulting in print images or spots and other failures, so it is important to eliminate paper static electricity during printing.
2. Exclusion method
â‘ Eliminate with a static eliminator
There are several types of static eliminators used in printing machines, such as inductive static eliminators, high-frequency high-voltage static eliminators, power frequency high-voltage static eliminators, etc., which should be placed near the drum during installation.
â‘¡Improve the relative humidity of the printing workshop
The generation of static electricity is related to the relative humidity of the operating environment. Generally, when the relative humidity of the workshop is lower than 40%, static problems are easy to occur. Therefore, when printing, sprinkle some water around the paper stack and the machine, or use an air humidifier to adjust and improve the printing workshop Relative humidity to avoid static electricity.
â‘¢Moisture-conditioning the paper
When the moisture content of the paper is low, the paper is easily charged with static electricity. When the paper has serious static electricity, it is necessary to hang the paper or put the paper in a relatively humid environment for a period of time, but pay attention to prevent the occurrence of ruffles on the paper.
â‘£ Reduce the speed of the printing press
When the static electricity is serious, reduce the friction between paper and paper, and paper and machine by reducing the speed of the printing press, thereby reducing the generation of static charge.
2. Paper feeding failure caused by improper adjustment of printing machine
During printing, the paper passes through the feeder and paper conveying table, and then passes the pre-regulations and regulations, and finally is rolled by the rubber cylinder and the impression cylinder to complete the printing. In these links, improper adjustment of a certain part of the machine will affect the operation of the paper and cause printing failures, so the following machine parts must be adjusted.
1. Suction nozzle, blow nozzle, paper pressing rod
Due to the uneven blowing size of the paper blowing nozzle or the improper suction position of the suction nozzle, the paper cannot be separated and form double sheets or multiple sheets; the suction nozzle sucks too much, causing double or multiple sheets of paper, too small to form empty Zhang.
Remedy: Adjust the size of the blowing and inspiratory volume through the two air valves on the air pump until it is suitable; adjust the position of the suction nozzle to be suitable.
2. Elm paper belt and pressure roller
The inconsistent tension of the feeding belt leads to the inconsistent rotation speed of each feeding belt, which causes the paper to skew; when the position of the paper pressing wheel is improper or the rotation is not flexible, the paper is skewed, or the paper cannot stretch naturally on the paper table , Making the paper arch uneven, and after rolling, the printed product is wrinkled.
Remedy: By adjusting the axle of the conveyor belt under the conveyor table, make each conveyor belt on the cardboard be tight and tight; check whether the axle is worn and make the pressure uniform, and add lubricating oil regularly.
3. Side gauge pull
The paper is pulled to the side gauge baffle through the front and regular rules to complete the positioning. When the tension of the side gauge is too large, the paper arches under the resistance of the side gauge baffle plate, and after printing, the printed product forms a rounded arc-shaped fold.
Remedy: adjust the side gauge to a normal state.
4. Impression roller teeth row
When printing, when the impression cylinder's gripping force is inconsistent, the paper bite will form an arch. After imprinting, the printed product forms an arc-shaped fold at the bite; or the corner of the paper is just half bitten by the impression roller's teeth, which causes excessive force on the edge of the paper, and a fold is formed at the bite of the paper after printing.
Remedy: Check whether the teeth of the teeth and the pads are worn. If they are worn, they need to be replaced in time; adjust the force of the teeth to make them uniform; check whether the position of the teeth of the teeth is appropriate.
The relationship between printing quality and paper is very large. As a printing job, you must fully understand this, continue to accumulate experience, and take corresponding measures to improve the quality of printing and reduce the rate of waste caused by paper factors.
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