Analysis and troubleshooting of the hottest flexo

  • Detail

Analysis and troubleshooting of flexographic printing

printing failures often occur in flexographic printing. The so-called printing failure refers to the sum of the phenomena that the parties using this standard in the printing process should explore the possibility of using the latest version of the following standards, which may affect the normal production or cause printing defects

the causes of printing failures are relatively responsible, which are the comprehensive reflection of various defects such as originals, printing plates, printing materials, printing equipment, printing technology, printing environment and operators. Therefore, when a printing fault is found, the possible causes of the fault should be analyzed according to the fault phenomenon shown on the printed matter, and then the factors should be analyzed and eliminated one by one. Finally, the main causes can be found out. Once the main cause of the fault is found, the troubleshooting will be easily solved

this paper only analyzes the flexographic printing fault in detail. The analysis method is to first find out the phenomenon of the fault, compare it with the samples and standards through careful visual inspection or instrument detection, grasp the essence of the fault, then list various factors and links that produce the fault, and finally analyze and eliminate them one by one

don't complain first, overprint is not allowed

(1) definition. In the process of chromatic printing, the error of imprint overlap exceeds the specified range

(2) form. There are three main forms of overprinting, namely, longitudinal overprinting, horizontal overprinting and longitudinal and horizontal overprinting at the same time

longitudinal overprint inaccuracy: printing failure caused by large error of overprint cross line along the paper feeding direction of the substrate

horizontal overprint inaccuracy: printing failure caused by large error of overprint cross line along the horizontal direction (drum axis)

inaccurate overprinting in both longitudinal and transverse directions: printing failure caused by the above two forms at the same time

(3) causes and troubleshooting

1. Inaccurate longitudinal overprinting:

· uneven printing tension of each color group: adjust the printing tension

· uneven printing pressure: adjust the printing pressure of each unit

· there is a large error in the printing diameter of the plate cylinder of each unit: including the diameter of the plate cylinder, the thickness of the plate and double-sided tape, etc., which should be carefully adjusted

· the drying temperature of each unit is too high, too low or uneven

2. Inaccurate horizontal overprint:

· axial wear of plate cylinder: detect the axial clearance of each cylinder and control its axial wear

· roll axial swing: change the roll

· the drying temperature of each unit is too high or uneven

3. Inaccurate overprinting of both vertical and horizontal: when the above two factors occur at the same time, it should be adjusted comprehensively

II. Unclear impression (paste version)

(1) fault phenomenon. The printed pictures and texts are not clear and fuzzy

(2) causes and troubleshooting

1. Large area paste:

· ink drying speed is too fast: adjust the ink to reduce its drying speed

· insufficient printing pressure or inconsistent pressure at both ends of the drum: reasonably adjust the printing pressure

· the number of lines of the corrugated roller is too high or there is serious wear, resulting in a reduction in ink transfer: the corrugated roller should be replaced

· excessive inking pressure or printing pressure: adjust inking pressure and printing pressure reasonably

2. Partial paste:

· partial uneven layout and uneven inking: it should be trimmed or padded

· improper cleaning of local holes of the wrinkle roll reduces the volume of local holes: it should be checked and thoroughly cleaned

III. overlapping printing

(1) definition. It refers to the phenomenon that pictures and texts on the same printing plate are printed with double and triple offsets

(2) form. Overlapping printing directionality, i.e. longitudinal overlapping printing, horizontal overlapping printing and inclined overlapping printing

(3) reasons and troubleshooting

1. Longitudinal overlapping printing

· poor uniformity of printing pressure: the accuracy of the cylinder body and the consistency of printing and double-sided tape thickness are poor, which should be checked and adjusted

· poor plate pasting: including the adaptability of double-sided tape does not meet the requirements

· machine vibration

· the side clearance of the transmission gear of the printing cylinder is too large

· poor parallelism of the drum

2. Horizontal overlapping printing

· the axial wear of the printing cylinder is too large

· machine vibration

· poor leveling during machine installation

3. Oblique overlapping printing

the result of the simultaneous action of various factors causing longitudinal and transverse overlapping printing. In case of such failure, comprehensive inspection and careful adjustment shall be carried out

IV. ink bar

(1) definition. A printing fault in which a strip impression parallel to the axial direction of the drum appears on the printed matter in the eye or field printing

(2) form. According to the different reasons for producing ink bars, there are three main forms, namely, tooth ink bars, roller ink bars and vibrating ink bars

(3) causes and troubleshooting

1. Gear ink bar: the phenomenon of the fault is shown in Figure 1. Its main feature is that the distance a between the ink bars is equal to the pitch of the driving gear of the drum or corrugated roller

a) the relevant parts causing the ink bar: roller and corrugation roller

b) main defects of driving gear of drum and corrugation roller:

· wear of gear and bearing

· the error of driving gear is too large

· there is foreign matter on the gear tooth surface

· poor lubrication

2. Roller ink bar. The phenomenon of the fault is shown in Figure 2. Due to the low accuracy of the corrugated roller itself or poor installation, it is easy to produce roller ink bars

· the geometric error of the corrugated roller is too large

· the corrugated roller is poorly installed and has large radial runout

· poor bearing installation

3. Vibrating ink bar. The phenomenon of the fault is shown in Figure 3. Ink bar caused by vibration of relevant parts

· vibration caused by bearing wear

· vibration caused by poor printing and pasting

· vibration caused by the loss of stability of printing pressure

· vibration caused by poor stability of drive motor

the vibrating ink bar is relatively complex, which is mostly related to the performance of the machine and the repair and maintenance of the equipment. Once such a fault occurs, it should be comprehensively tested and checked to find out the root cause of the fault, otherwise it is difficult to eliminate it

v. uneven concentration

(1) definition. It refers to the failure caused by the inconsistency of the concentration of a part of the print or the concentration of each print

(2) form. There are three main forms:

a) uneven concentration on one side of the print

b) the print concentration changes gradually during printing, resulting in uneven concentration

c) uneven concentration after printing several sheets

(3) reasons and troubleshooting

1. Uneven concentration on one side of the print

uneven concentration in some parts:

· there is a large difference in the thickness of the printing plate or the thickness of the double-sided adhesive

· local unevenness or foreign matters on the back of the printing plate or the surface of the printing plate cylinder

uneven concentration of prints in the left and right directions:

· the parallelism between the plate cylinder and the embossing cylinder or between the plate cylinder and the embossing roller is poor, causing the lateral pressure between the cylinders to lose balance

· the tangent between the blade edge of the scraper and the surface of the roller is poor

· the improvement of the portability of the blade is also essential for the defect of the mouth

· there is a large error between the thickness of the printing plate and the double-sided tape in the transverse direction of the drum

therefore, the above links should be tested and adjusted

2. The density of the printed matter changes gradually during printing, resulting in uneven concentration:

· ink performance changes

· poor oil supply to main lubricating parts

3. Uneven concentration after printing several sheets, or uneven concentration when printing again after stopping printing:

· overprint is inaccurate or ink bars are generated

· the performance of the ink in the ink bucket changes greatly, such as the reduction of solvents, the deposition of pigments, etc

VI. the color is dim and not bright

in flexo printing, there are often problems of dim and not bright colors of prints. The main reasons are as follows:

· insufficient pigment in the ink, or pigment deposition after standing: pigment should be added to the ink. If the pigment in the ink bucket is deposited, the mixing of the ink in the ink bucket can be strengthened

· there is too much solvent in the ink, and the dilution is excessive: stop adding dilution section, or increase the original ink adjustment

· there is too much resin in the ink: the ink formula should be adjusted and ball milled again

VII. The ink color of the print is too deep

the darker the ink color of the print, the better. If the printing ink is too dark, it will affect the reproducibility of color

the main causes and elimination methods of too deep ink in printing are as follows:

· the content of pigment in the ink is too high: the pigment content in the oil volume should be appropriately reduced

· excessive ink transfer: the number of lines of the corrugated roller is too low, or the inking pressure is too large, so the inking pressure should be adjusted reasonably to increase the number of lines of the corrugated roller

VIII. Pulverization

(1) definition. The pulverization mentioned here refers to the printing failure when using gold powder for ink printing, that is, after the printing is dry, once the printing surface is rubbed, the pigment in the ink layer will fall off. Generally speaking, this failure is called pulverization, or gold ink floating phenomenon

(2) reasons and troubleshooting

· the connecting material in the ink has too strong permeability to the substrate (Zhang Zhang), so that the pigment cannot be fully maintained on the surface of the substrate, forming an ink layer with certain friction resistance. Therefore, paper with weak permeability should be used

· insufficient resin content in ink: resin slurry should be supplemented appropriately

· the mesh number of gold powder is too low, and its particles are too coarse: use gold powder with higher fineness

IX. crystallization

(1) definition. Crystallization, also known as decolorization, refers to the phenomenon that the ink layer falls off the surface of the substrate after the printing is fully dried

(2) reasons and troubleshooting

there are two cases of decolorization: one is the crystallization of non absorbable substrate materials, such as plastic films; The second is the crystallization phenomenon when overprinting the absorbent substrate. When the first color printing is dry, the second color is overprinted. At this time, the ink layer of the second color is scraped off by scraping with the tip of your finger

1. Main causes and elimination of crystallization in plastic film printing

· the adhesion of film to ink is too poor, and corona treatment is not carried out: corona treatment should be added

· insufficient corona treatment: strengthen the intensity of corona treatment

· poor ink adhesion: replace with new ink

2. The main causes and elimination methods of crystallization during overprinting

add luster to the 65th anniversary of the establishment of diplomatic relations between China and Mongolia and the year of friendly exchange between China and Mongolia. If the interval between two-color printing is too long, too many driers are added to the ink, or smooth paper with low ink absorption such as manila paper is used for printing, crystallization is easy to occur. Therefore, the time between two-color printing should be reasonably controlled, and the content of driers in ink and the smoothness of paper should be strictly controlled. If the decolorization is not obvious, gel agent can be added to the ink to achieve a certain effect

X. penetration and through printing

the penetration and through printing of printing occur almost at the same time, which is sometimes difficult to distinguish in practical work. Since the causes of the two are similar, they can be considered together

(1) penetration

① definition. Penetration refers to the phenomenon that the brown oil in the ink penetrates into the back of the printing surface, as shown in Figure 4

② reasons and troubleshooting

· the permeability of paper is too strong: the permeability of paper should be reduced

· the viscosity of ink is too low: appropriately increase the viscosity of ink

(2) through printing

1. Definition. Transparent printing is also called perspective. The pictures and texts on the printed matter are made of paper

Copyright © 2011 JIN SHI