Cause analysis and elimination of bar in the most

2022-07-26
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Cause analysis and elimination of "bar" in offset printing press

"offset bar" (hereinafter referred to as "bar") is a common quality problem in offset printing. In the actual production, when we encounter the failure of the product bar, we all feel that it is difficult to solve. This is because the bar is often caused by a variety of factors, and the causes are complex. Therefore, many technicians and operators have been studying and discussing for a long time, how to quickly and accurately judge, find out the causes of the bar, and quickly eliminate it. Based on my years of experience in enterprise work practice, this paper makes a rough analysis of the causes of the bar on the sheet fed offset press, and discusses the key points of identifying, searching and determining the root causes of the bar, so as to discuss with colleagues

I. definition and classification of bar

1 Definition of bar:

the so-called bar refers to the ink streaks of different depths formed in the direction parallel to the axis of the drum when the dot or ink film thickness changes irregularly during printing. Due to the influence of some factors, there is a small slip between the rubber rollers, between the plate roller and the plate cylinder, and between the plate cylinder and the paper. In the process of ink transfer, this small slip changes the ink film thickness or the shape and size of points, forming a strip ink color change

2. Classification of bars:

(1) according to the appearance characteristics, bars can be divided into ink bars and water bars

ink bar: irregular expansion, deformation and elongation of dots, aggravation of ink color, and formation of an obvious deep stripe (also called ink bar) on the layout

water bar: the dot is irregularly reduced, and the ink color is diluted, forming an obvious shallow stripe (also known as white bar)

(2) according to the causes, the bar can be divided into four types: roller bar, gear bar, vibration bar and other bars

roller bar: due to the unreasonable design of the inking part, insufficient precision and improper adjustment of the rubber roller, the small slip between the rubber roller and the printing plate or the interference with the normal ink transfer will change the printing density and produce the rubber roller bar

gear lever: the transmission gears of each drum and ink uniform and wet part of offset press have various errors such as tooth shape, tooth space, tooth surface finish, etc., which produce vibration during meshing, which can not ensure smooth operation, resulting in gear lever

vibrating bar: during the operation of the offset press, there are many unbalanced factors, such as the drum gap, the impact of the opening and closing of the paper teeth, the impact vibration caused by the reciprocating motion and the inertia of the cam linkage mechanism, and the matching clearance of the transmission parts, resulting in the relative sliding of the transfer part due to vibration, resulting in the vibrating bar

other levers: as the offset printing process is a comprehensive and complex process combining mechanical action and physical and chemical reaction. For the potion, ink, printing plate, blanket, liner, paper, etc. involved in the printing process, the bars caused by improper treatment or selection of one factor are classified as other bars. Due to space limitations, this paper will not analyze this kind of bar

II. Analysis of the causes of the bar

during the production of the bar, there were sudden and gradual exposures. As far as offset press itself is concerned, there are many reasons for the bar. In this paper, it is analyzed from three aspects: improper operation adjustment, mechanical problems and design problems

1. Improper operation adjustment

(1) improper adjustment of ink roller pressure. The pressure between the inking roller and the plate is too high. When the inking roller contacts the neutral position of the plate cylinder, the inking roller touches the bite edge of the plate to produce an impact. The inking roller vibrates and jumps on the plate, and the linear speed changes instantaneously. The normal ink film is disturbed (as shown in Figure 1 (a)). The ink film of point a is disturbed and transferred to a 'on the printing plate to form an ink bar, which may reappear at a "(the arc length of a' a 'is the circumference of the ink roller)

also in Fig. 1 (b), the ink film at B may be disturbed and transferred to the plates B 'and B "to form a bar (the arc length of B' B" is equal to the circumference of the inking roller)

although the printing plate cylinder was advanced by about 70mm in the design to avoid the ink bar at the bite due to the rubber roller vibration, when the second ink roller jumped up, the ink roller was also pushed up. When the second ink roller jumped down, the ink roller also fell down. This jump of the ink roller also affected the first ink roller. At this time, the first ink roller was in the picture, As a result, a bar is generated (as shown in Figure 1 (c)). A sheet fed offset press generally has four ink rollers and two lower ink rollers (as shown in Figure 1 (d)), so more than one light and heavy bar is formed. In addition, since the two inking rollers ③ and ④ not only play the role of inking on the printing plate, but also play the role of ink collection, the existence of the above phenomenon is most likely to cause a lever, so it should be adjusted accurately

the pressure of inking roller in the same series is too high. Because the inking roller rotates by the friction between the printing plate cylinder and the inking roller, the pressure between the inking roller and the inking roller is too large, which makes the inking roller rely on the inking roller too much. The friction on both sides of the inking roller is different, and it may cause friction and sliding on the printing plate surface, resulting in a bar

in the same way, when the pressure between the water landing roll and the water cascade roll is too heavy, it will affect the uniform transmission of water film and cause white bars

(2) excessive drum lining. As the upper or lower pressure is too small, the drum lining is increased, which makes the drum lining excessive, and intensifies the micro speed difference and micro friction of the drum during rolling. From the microscopic point of view, due to the influence of various factors, the linear speed of the drum is not absolutely equal, but is a relative balance. To a certain extent, there is always a small speed difference. Excessive lining causes excessive friction of the blanket under the extrusion, resulting in intermittent wear and imprinting of the printing plate, thus creating a bar

(3) the pressure of rubber cylinder and impression cylinder is too high. The offset press is completed by three rollers, and there is a certain amount of extrusion force when the working surfaces of the rollers are opposite. When the drum operates to neutral, the load will change (as shown in Figure 2): when point B contacts and disengages from neutral, impact will occur at point a. Due to the small clearance in the roller bearing and the elastic deformation of the roller and the bearing, the roller will produce small displacement and cause vibration. The excessive pressure between the rubber roller and the embossing roller will aggravate this vibration. For the printing press without roller pillow, this vibration can not be eliminated, resulting in a bar

(4) the blanket is loose. When the rubber cloth is too loose, due to the rolling action, the rubber cloth slides back and forth on the surface of the drum, and this relative movement will produce one or several bars

(5) too much moisture in the layout. In the process of offset printing, too much moisture in the layout will affect the uniform transfer of the ink film, making the printing density too low and aggravating the appearance of the bar. Therefore, for offset printing, on the premise of ensuring that the layout is not dirty, the water supply should be controlled within the minimum range, and the water supply and ink supply should be in a relatively stable state. To control the balance of ink and water is an important step to improve the printing quality

2. Mechanical problems in recent years

(1) the drum gear is worn and the accuracy is reduced

the accuracy requirements of the offset press cylinder gear are very high, which is almost close to the aircraft aviation gear. In order to make the center distance slightly adjusted (to adapt to the printing of papers of different thicknesses within a certain range), the increase of tooth backlash shall be minimized as far as possible to ensure smooth operation. The offset press mostly adopts involute gear and 14.5 ° ~ 15 ° tooth profile pressure angle. Generally, the small periodic speed change between gears caused by gear transmission is not enough to show the bar. However, when the tooth surface of the gear is excessively worn or individual teeth are damaged, and there are foreign matters in the tooth groove, the gear cannot roll at a uniform speed according to the meshing line, resulting in vibration during operation, which will lead to inconsistent mutual speeds of the rollers, resulting in relative slip between the transfer surfaces, resulting in periodic gear bars. The feature of the gear bar is that the spacing between the bars t= π MS (MS is the end module of the gear), which is like a washboard densely distributed in the whole layout. If several teeth are damaged or there are foreign objects in the tooth groove, several bars at corresponding positions will be generated

(2) the shaft sleeve of inking roller is seriously worn or the gear of inking roller is excessively worn. When the clearance of the shaft sleeve is enlarged, the vibration generated by gear meshing will be transmitted to the inking roller, affecting the normal transmission of the ink film and causing the appearance of the bar. Due to wear or sundries on the fitting surface of the shaft head, the installation of the three inking rollers is not concentric, causing the jumping of the inking rollers to affect the transmission of the ink film, resulting in a bar

(3) the roller bearing is worn. Roller bearing wear can cause various levers, depending on the position of the bearing. When the load is abnormal, the lubrication is poor or sundries are immersed, resulting in bearing wear, improper bearing fit clearance, and the drum load changes, resulting in vibration and causing the bar

(4) ink roller bounce and end clearance. Aging, uneven outer circle of inking roller, bent mandrel, worn shaft seat lock and other factors will cause the ink roller to jump, resulting in the production of bar. The inking roller shaft seat is worn and the bearing step length error is too large, resulting in end play. The inking roller will produce excessive axial movement under the action of the inking roller, affecting the transfer of the ink film and causing the bar

(5) vibration and impact of transmission system load. The structure of the sheet fed offset press is complex, and there are quite a lot of impact and vibration parts in production, such as the clutch of the drum, the neutral entry of the drum into rolling, various reciprocating paper transfer mechanisms and cam driving mechanisms, the opening and closing tooth mechanism for paper transfer, the ink roller serial motion and the swing mechanism for water and ink transfer rollers, etc. Due to the vibration and impact of these mechanisms, the uniform and synchronous rolling between the rollers will be destroyed, resulting in instantaneous speed change. Jinan experimental machine factory Jinan new era Gold Testing Instrument Co., Ltd. has always adhered to the principle of production for so many years, making the rollers slide to form a bar. Especially when the main motor belt is loose, these shocks and vibrations make the offset press more unstable. There are defects in design and manufacturing, as well as improper adjustment and use

(6) excessive wear of transmission system parts. The speed 1 of various zero open oil return valves of the transmission system must be slow, such as the wear and slip of the transmission belt, the wear and deformation of various key connections, the wear and slip of the coupling, the wear and tear of the cam, gear and bearing parts, etc. the excessive wear of these parts will affect the balance of the offset press operation and the possibility and severity of the bar to varying degrees

3. Design problems

the appearance of a bar in an offset press depends not only on the structural form of the core component drum and its bearings, but also on whether the printing drum sleeps, but also on the structure and stiffness of the machine wallboard, frame and chassis, as well as the structure and reliability of water and ink transmission components. This article only describes the defects of the pillow and the lower ink conveying system

(1) there are two printing methods for the walking pillow and non walking pillow

sheet fed offset press: one is the printing method of roller shoulder iron contact, that is, the walking pillow printing method. The offset press without shoulder iron contact will cause drum vibration in high-speed operation due to the existence of drum neutral, reciprocating and intermittent motion mechanisms, as well as the increase of drum bearing clearance and gear side clearance during the printing process, which is easy to produce bars

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