Research on the production and solution of offset strips (one)
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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. The concept and classification of "bars"
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 ink of the offset printing machine, the design of the water uniform system is unreasonable, the processing precision of the rubber roller and the precision of the matching are improper, causing the water, the ink roller and the plate cylinder and the roller to slip 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.
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 screws 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 which does not go to 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 loosened. 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”.