Offset "bar" analysis
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In the practice of lithography, "bars" are print quality problems that often occur and are difficult to completely solve. The production of "bars" involves various factors such as the design of the printing machine, the materials and processing technology of the mechanical parts, the operation of the staff, the selection of printing materials, and the maintenance of mechanical equipment. The main reason for the occurrence of "bars" in printing is the vibration generated by the printing cylinder of the printing press and the inking, the water conveyance mechanism, or the paper web connection, causing the relative position of the print to slip. Therefore, our existing lithographic equipment and technology cannot eliminate the "bar", and only through the improvement of technology and operation technology to minimize and reduce the "bar". This article explores the generation and solution of "bars", and hopes to play a role in attracting jade.
1 "bar" concept and classification
1) The concept of "bar"
On the lithographic print, a strip-like imprint, which is parallel to the direction of the axis of the roll, is caused by irregular deformation of the dot, which is also called "bar" and "strip".
2) Classification of "bars"
The ink color of the "bar" can be divided into two types: the ink bar and the water bar.
Ink bar: In the printing area of the printed matter, the dot is irregularly enlarged, and the ink layer is thickened, and there is a distinct dark band-like imprint (also called "color bar") than the ink color printed at the same time.
Water bar: In the printing area of the printed matter, the dot is irregularly reduced, and the ink layer is thinned, and there is a distinct light-colored strip print (also called "white bar") than the ink color printed at the same time.
According to the reason of the "bar", it can be divided into gear bars, vibration bars, roller bars and other bars.
Gear bars: The precision of the transmission gears of the rollers, inking parts and water delivery parts of the offset printing machine is insufficient, resulting in tooth surface wear or tooth damage, and chattering occurs in the meshing transmission, resulting in a relatively slippery printed image on the drum. Move to the "bar". It is characterized in that the pitch of the bars in the printed sheets is equal to the pitch of the gears, and the dense layout is shaped like a "washing board".
Vibrating bar: When the offset printing machine is working, it will suddenly vibrate at a certain moment of constant speed operation. This vibration destroys the drum to rotate at a constant speed, causing a slight sway between the printing rolls and creating a “bar”. Such as the printing cylinder working surface and neutral shift, roller clutch, the opening and closing of the drum bite, the motion inertia of the cam linkage mechanism or the bearing wear of each transmission component, etc., will cause shock and vibration, resulting in relative slippage of printed graphics And appeared.
"Bar". The feature is that the "bar" appears in the local position of the printed sheet and the position is fixed, and the position of the "bar" is related to the periodic and regular vibration of the mechanical movement.
Roller bar: Roller bar is because the offset printing machine's uniform ink, uniform water system design is unreasonable or rubber roller processing precision, improper matching precision, causing slippage between water, ink roller and plate cylinder and roller during operation Or, due to improper pressure regulation, the contact position is recessed, which interferes with the normal transfer of ink, thereby forming a "bar". The utility model is characterized in that the number of bars f is slightly more, the bar band is wider, the bar width is equal to the surface circumference of the ink roller, the position is not fixed, and the ink color before and after the printing product is different.
Other bars: Bars produced by factors such as blankets, liners, plates, paper, inks, dampening fluids, and operating processes involved in the printing process. Because of its different characteristics, this article will not elaborate on this.
2 reasons and solutions for the "bar"
1) The backlash of the roller meshing gear is too large
1 The backlash caused by the center distance adjustment is too large. The drum gear of the offset printing machine is basically meshed by helical gears. The gear exchange coefficient of the gear is large, and the meshing side clearance space is small, so as to reduce the impact in the transmission and the chattering during printing, and maintain the smoothness and accuracy of printing. In actual work, the backlash of the drum gear mesh is changed and adjusted according to the printing of paper of different thickness. The thicker the paper, the larger the center distance adjustment is, the smaller the exchange coefficient of the meshing surface of the drum gear is, and the side gap becomes larger. The greater the difference in the linear speed of the printed roll surface, the greater the relative slip f produced on the printed surface and the more pronounced the bar. Solution: Under the premise of ensuring proper printing pressure, the center distance between the three rolling elements should be minimized, and the meshing backlash should be properly pressed to reduce the degree of "bar".
2 The side gap caused by gear wear is too large. After a period of work, the drum gear is worn due to load or foreign matter in the tooth gap, resulting in excessive gear meshing backlash, the gear involute cannot be rolled at a constant speed according to the meshing line, or the foreign object "top tooth" makes the gear in operation. The chattering occurs, causing the friction between the rollers to increase to cause slippage, and the printed image portion produces an ink ribbon equal to the pitch of the gear at the moment of the sliding, that is, the gear bar. Solution: Strengthen the cleaning and maintenance work of mechanical equipment, clean and remove the debris between the gears, and maintain a good lubrication state; according to the gear wear condition, recalculate the adjustment center distance, properly press the backlash, and replace the new one immediately. gear.
2) Bearing wear
1 Rolling bearing wear. After the offset printing machine is used for a long time, especially the roller without the roller, the impact shock vibration is large, the bearing is easy to wear, and the gap between the journal and the bearing is increased and the sleeve is loose, and when the roller is pressed and pressed, The mouth vibrates due to the deviation, and the sliding friction creates a "bar". Especially when the load is not normal or the lubrication is not good, the shaft head and the bushing of the drum are intensified, and the "bar" is most likely to be generated near the mouth. Solution: The mouth part of the rubber inner liner can be cut into a trapezoidal shape to reduce the impact force when the pressure is combined. When the roller bearing wears seriously, the bearing should be updated in time.
2 Ink roller bearing wear. Water, ink roller bearings, bearing housings, bearing locks, debris or wear can also cause water and ink rollers to undergo significant axial turbulence and radial runout during motion, resulting in "bars", especially When the bushing or transmission gear of the ink roller wears, the vibration generated by the meshing of the roller gear affects the normal water leveling, ink leveling and transmission of the roller, which causes the occurrence of “bars”.
3) Ink roller wear
After the ink roller is used for a period of time, the ink roller is worn due to cleaning of dust, paper, ink, and other debris, or improper installation, uneven pressure adjustment, etc., the surface of the ink roller is uneven or the axis is not straight. In severe cases, the two ends of the ink roller are thin or thin at the middle. When the offset printing machine is working, it will cause the ink roller to jump, causing the local friction to increase and causing intermittent strip-shaped "bars". The cloth sleeve of the water roller is incomplete due to the suture interface, or the inner diameter is not suitable, or the cloth sleeve is loose and wrinkled after a long time of use, or the new water roller cloth sleeve is seriously depilated to form a pile pile, which may cause a normal strip. The occurrence of the bar.
4) Improper pressure adjustment
1 The pressure between the rollers is not properly adjusted. The roller of the offset printing machine completes the printing by relative extrusion. During the rolling process of the roller, the corresponding two rollers are repeatedly turned from the working surface (graphic contact surface) to the neutral position, and then the neutral position is transferred to the working surface twice. The conversion causes the load to mutate and produces an impact. This impact causes instantaneous pressure changes and relative slippage of the printed image contact surface, resulting in "bars". If the pressure between the rollers is too large, the resulting bars are noticeable. Solution: Adjust the printing pressure to a suitable or air cushion blanket.
2 Ink roller and plate cylinder pressure is not good. Although the layout of the four inking rollers of the offset printing machine is thick and thin in the front and the back, different diameters are used to avoid the overlap of the “bars”, but the four inking rollers are respectively in contact with the printing plates, and each of the two inking rollers simultaneously and one The root roller is in contact with the roller. When the first inking roller is in contact with the plate cylinder neutral position (the mouth position), the impact force is generated to cause the inking roller to be lifted up, and then falls, and a slip occurs between the inking roller and the inking roller, affecting the first inking. The amount of ink transferred from the roller to the layout changes, thereby producing a "bar", which also affects the uniformity of the ink film of the second toner roller; when the second toner roller is in contact with the neutral of the plate cylinder, the ink is also inked. The roller undergoes the process of being lowered and lowered. At this time, the first inking roller is in the printing plate, and the falling inking roller impacts the first inking roller to generate a "bar". In the same way, the third and fourth inking rollers will also produce "bars" on the plates, so that the entire layout appears as a light and heavy "bar" group. Solution: Always check the diameter and axis tolerance accuracy of each ink roller to ensure the normal working state of the ink roller. Accurately adjust the pressure of each roller and plate in the order of the specification, generally can be adjusted to the width of the indentation of 5mm, 5mm, 4mm, 3mm, and ensure that the axial movement of the inking roller is less than 0.3mm.
3 The pressure of the roller and the ink roller are not good. The ink rollers of the inking part of the offset printing machine basically adopt a soft-hard matching mode. When the pressure between the rollers is adjusted too much, the between the hard roller (the roller) and the soft roller (the ink roller and the roller) occurs seriously. Extrusion, the soft roller under the action of the pressure, the contact surface is sunken and the micro-drum on both sides affects the ink transfer to produce a "bar", or the ink roller reversing time does not match the rolling operation, the 窜 roller is 窜The amount of ink on the ink roller is affected to produce a "bar". Solution: Install the working positions of each roller in the order of specification, and accurately adjust the pressure of each roller and the squeezing roller. The width of the embossing roller is about 3-5mm. Each reversing of the squeezing roller should be completed within the neutral time of the roller.
4 Water roller pressure is not properly adjusted. When the pressure of the water roller and the plate cylinder is too large, the impact force generated by the plate cylinder during the switching between the working surface and the neutral gear causes the water roller to vibrate, and the water supply is uneven and a "bar" is generated. Or because the friction between the water stick and the plate cylinder is intensified, the residual ink layer of the dot on the printing plate is destroyed, so that the base of the dot is destroyed, and the lack of ink causes a "bar". Solution: When adjusting the water roller and plate cylinder pressure, use a 0.15-0.2mm feeler gauge to insert the test pressure between them, so that the frictional resistance of the hand is felt, but it can still be smoothly extracted. The water roll pressure above should be greater than the water roll pressure below, and the pressure at both ends should be the same.
5) Improper roller lining
In actual work, the blanket cylinder is an elastomer with a slight error when subjected to pressure. The drum with thick liner is not in pure rolling state, but has a certain range of sliding, which is easy to vibrate and produce “bars”; when the lining is too large, the compressible rubber cylinder is under pressure and the surface of the blanket is generated. Forward or backward convex hull, excessive "friction bar" plate appears "bar"; when the replaced blanket is not stretched or used to produce slack with a long blanket, under the printing pressure, the blanket is on the roller Slippage on the surface can also cause "bars" to appear. Sometimes, because the part of the mouth is too shallow and partially padded, the impact force of the drum neutral period becomes large, and "bars" are also caused. The lining of the drum is generally soft, medium and hard. The soft lining is generally not easy to appear as a bar, but it is less used because of its elasticity, and the printing effect is clear, but the lining is flexible. Small, "bars" are prone to appear on offset presses with low precision.
Solution: The lining of the drum must be determined by precise calculations to ensure the thickness of the lining under the premise of ensuring the minimum print pressure. In the premise of ensuring clear imprinting and small deformation of the dot, the medium-hard lining should be used as much as possible. It is best to use an air cushion blanket.
6) Transmission mechanism caused by vibration
The offset roller of the offset printing machine generally relies on the strong spring to make the gripper close the bite to print the paper. The roller at the end of the biting shaft rotates with the drum. When the cam opening plate fixed on the impact wall panel is reached, the pressing spring will be pressed. The teeth are opened, and when the roller is separated from the open plate, the force acting on the spring is lost, and the teeth are closed. During the exercise, the teeth need to be opened and closed twice to complete the transfer from the transfer paper to the completion of the delivery of the delivery. Due to the large impact during opening and closing, the stability of the operation of the impression cylinder is affected, resulting in slippage between the impression cylinders, and "bars" appear in serious cases. Similarly, the drum clutch pressure and the cam drive mechanism work also cause vibration to produce a "bar".