Fault analysis of inaccurate overprint of the hott

Analysis of overprint inaccuracy fault (Part 2)

3. Overprint inaccuracy fault generated in the process of paper transfer

reason 1: the paper transfer tooth is too high at the handover position of the paper, which may lead to the uneven size of its mouth and the occurrence of overprint inaccuracy fault

solution: the correct position is the thickness of 3 thin papers or the thickness of thick papers plus 0.2mm

reason 2: the holding force of the paper delivery teeth and the drum teeth is too small, which is easy to cause problems in the transmission process of the paper, resulting in inaccurate overprint

solution: on the machines with closed teeth of the boss and high points controlled by the cam, it may be caused by the wear of the open teeth roller. Readjust the gripping force. The gripping force should be based on the ability to pull out the gripping paper tightly, but do not blindly increase the gripping force of the gripping teeth. Otherwise, it will cause serious wear of relevant parts

reason 3: the running track of the up swinging paper feeding tooth is unstable

solution: the running track of the up swing paper transfer tooth is an inverted water drop curve, which should be a smooth curve. However, if sawtooth type is found during inspection, there will be inaccurate overprint due to unstable paper transfer. You can tie a pencil to the side of the paper delivery tooth row to draw this track. Generally, in this case, it is recommended to overhaul the machine, because once the cam of the machine has this kind of fault, it proves that many parts of the machine have been seriously worn

reason 4: there is also a movable horizontal 10 head (10 beam) on the top of the tooth pad, which is seriously worn, causing the paper to slide backward in the process of transmission or imprinting

solution: it is recommended to replace it, and try to replace it in a whole row, so as to prevent the uneven force of its teeth, and it is still unable to accurately overprint

reason 5: the height of the tooth pad of the drum's teeth or paper delivery teeth is uneven

solution: adjust each row of dental pads to the same horizontal height

reason 6: the distance between the teeth of the drum and the tooth pad of the paper delivery tooth is not appropriate (the relative position of the tooth pad between the rollers of the multi-color machine will also be inappropriate). It is easy to cause local overprint inaccurate fault

solution: the height of the paper delivery tooth pad is adjusted according to the teeth of the first impression drum or the first paper transfer drum, while the multicolor machine starts from the teeth of the first paper transfer drum and adjusts backward in turn according to the paper feeding direction. The thickness of 3 thin papers or thick papers plus 0.2mm. When adjusting, the available length is the width of the machine. The elastic steel bar with a width of about 10mm and a thickness of about 0.2mm is held by the paper delivery teeth after the front gauge is positioned, and the paper delivery teeth are loosened after inching to the handover position, and then the height of the paper delivery tooth pad is adjusted according to the bending of the steel bar

reason 7: the gripping force of the impression cylinder is uneven

solution: it may be caused by wear or improper adjustment of the machine. Sometimes it is caused by the uneven force of the big tooth springs on both sides of the toothing shaft. The wear of many parts in the paper transfer process may cause overprint problems. During the operation, we must pay attention to lubrication, maintenance and timely repair

4. Inaccurate overprint caused by poor operation of the drum

cause 1: axial movement of the three drums during operation

solution: some may be caused by the loosening of the jacking screws at the shaft end, and some may be caused by the wear of the thrust bearings on both sides of the axial direction

cause 2: the axial and circumferential directions of the plate cylinder move during operation

solution: in addition to the above reasons, there are usually plate adjustment mechanisms on both sides of the plate cylinder of modern printing machines, which may sometimes be caused by circuit problems of the plate adjustment mechanism

printing process reasons

1. printing plate

reason 1: improper plate drawing methods pose great challenges to building a low-carbon life and a green society in China, resulting in the deformation of the printing plate

solution: it is recommended to remove the printing plate tension before pulling the plate. Hard pulling and hard top are resolutely prohibited

reason 2: the thickness or liner of the printing plate is different between the color groups

solution: take the 270mm drum measurement data result to accurately print 600mm pictures and texts as an example, the imprint will be shortened by 0.25mm every time the plate thickness or pad thickness increases by 0.1mm. Sometimes operators will also use this feature of the printing plate to change the length of the imprint

2. amount of water

reason: too much water causes paper deformation

solution: no matter from which point of view (printing drying, overprint, ink, etc.), it should be advocated to minimize the amount of water within a reasonable range

3. paper strand direction

it is generally advocated to use longitudinal (and the paper feeding direction of the printing machine) strands, and print with paper whose paper feeding direction is perpendicular to the wide edge of the paper

experiments show that when a fiber is filled with water, its diameter will increase by 25-30%, while its length will only increase by about 1%. When using this kind of paper for printing, the deformation of the paper caused by the backward tension and the reduction of the strength of the paper after water absorption can be avoided to a great extent

4. Temperature and humidity of the workshop

when the ambient temperature changes by ± 5 ℃, the water content of the paper changes by 0.15%. In multicolor printing, when the water content of the paper changes by 0.1%, overprint will be affected. Generally, the ambient temperature of the workshop should not change more than 3 ℃

when the environmental humidity changes by 10% and the relative humidity is below 80%, the water content of the paper changes by 1%; When the relative humidity of the environment exceeds 80%, the water content of the paper changes from 1.5% to 2.0%. Therefore, the environmental humidity should be strictly controlled below 80%

enterprises with conditions should better ensure that the workshop temperature is constant at 20 ℃ and the humidity is maintained at 55% - 65%

no, in fact, this kind of deformation is very small. Whether it is machine failure or improper process application, it can only be mentioned on the premise of correct registration of the printing plate. Above, the author briefly discusses some reasons for the inaccurate overprint of printed matter from the aspects of plate making, materials, printing, etc. in actual production, other problems may be encountered, which requires specific analysis of specific problems