Flexo Printing Failure Analysis and Exclusion

Print failures often occur in flexo printing. The so-called print failure refers to the sum of the phenomena that affect the normal production or print defects in the printing process.
The reasons for the occurrence of printing failures are more complex and reflect the various defects of the originals, printing plates, printing materials, printing equipment, printing processes, printing environments, and operators. Therefore, when a print fault is found, the cause of the fault may be analyzed based on the fault phenomena exhibited on the printed product, and then the factors may be analyzed and eliminated one by one. Finally, the main cause may be found out. Once you have identified the main cause of the failure, troubleshooting will be eliminated.
This article only specifically analyzes flexographic faults. The analysis method is to first understand the phenomenon of failure, through meticulous visual inspection or instrument testing, compare it with proofs, standards, etc., grasp the nature of the fault, and then list the various factors and links that cause the fault, and finally analyze and exclude each one. .
1. Imprinting is not allowed (1) Definition. In the process of chromatic printing, the error of the overlap of the prints exceeds the specified range.
(2) Form. There are three main forms of misregistration: vertical misregistration, horizontal misregistration, and vertical and horizontal misregistration.
Longitudinal overprinting is not allowed: Overprinting of crosshairs results in printing faults due to large errors in the sheet feed direction.
Transverse overprinting is inaccurate: Printing faults due to overprinting of crosshairs in the transverse direction (roller axial direction).
Vertical and horizontal misregistration at the same time: the above two forms of simultaneous printing failures.
(3) Causes and remedies 1 Vertical overprinting is not allowed:
• Uneven printing tension in each color group: adjust the printing tension.
· The printing pressure is uneven: Adjust the printing pressure of each unit.
• Larger errors occur in the printing diameter of the plate cylinders of each unit: The diameter of the plate cylinder, the thickness of the printing plate, and the double-sided tape, etc. should be carefully adjusted.
• The drying temperature of each unit is too high, too low or uneven.
2 Overprinting is not allowed:
·Axial tamping of the plate cylinder: detecting the axial clearance of each roller and controlling its axial turbulence.
· Roll axial oscillation: replace the roll.
• The drying temperature of each unit is too high or uneven.
3 Vertical and horizontal misprints are not allowed at the same time: When these two factors occur at the same time, they should be fully adjusted.
2. Imprinted (clear)
(1) The phenomenon of failure. The vagueness of the printed artwork is not clear.
(2) Causes and Remedies 1 Large area paste:
• The ink drying speed is too fast: The ink should be adjusted to reduce its drying speed.
• Insufficient printing pressure or inconsistent pressure on both sides of the roller: Reasonably adjust the printing pressure.
• The number of lines of the anilox roller is too high or there is severe wear, resulting in reduced ink transfer: the anilox roller should be replaced.
Inking pressure or printing pressure is too large: reasonable adjustment of ink pressure and printing pressure.
2 partial paste:
· The layout is uneven and the amount of ink applied is uneven: it should be trimmed or pad plated.
• Improper cleaning of the local anilox roller network reduces the volume of the local network pocket: inspection should be performed and thoroughly cleaned.
3. Overprint (1) Definition. It refers to the phenomenon that the graphic and text on the same printing plate produce a double offset and prints.
(2) Form. Overprinted directionality, ie, vertical overlap printing, horizontal overlapping printing, and oblique overlapping printing.
(3) Causes and remedies 1 Longitudinal overlap printing • Poor uniformity of printing pressure: The accuracy of the cylinder body, the consistency of the printing plate and the double-sided tape thickness are poor, and inspection and adjustment should be carried out.
· Poor sticker: Including double-sided adhesive tape does not meet the requirements.
· Machine vibration.
• The bearing clearance is too large.
• The backlash of the printing roller drive gear is too large.
• The drum has poor parallelism.
2 Horizontal overlap printing • The printing roller axial movement is too large.
· Machine vibration.
• I have poor levels when the machine is installed.
3 Inclination Overprinting The result of the simultaneous effect of various factors that cause vertical and horizontal overprinting. When this kind of failure occurs, it should be thoroughly checked and carefully adjusted.
4. The definition of ink bars (1). In the case of screen printing or field printing, a printing failure occurs on the print with strip marks parallel to the axial direction of the roll.
(2) Form. According to the reasons for producing ink bars, there are mainly three forms, namely gear-like ink bars, roller-type ink bars, and vibrational ink bars.
(3) Causes and remedies 1 Geared ink bars: The phenomenon of failure is shown in Figure 1. The main feature is that the spacing A of the ink bars is equal to the pitch of the roller or anilox roller transmission gears.
a) Relevant parts that cause ink bars: Rollers and anilox rollers.
b) Main defects of roller and anilox roller driving gears:

Figure 1 Gear wheel

· Wear of gears and bearings.
• The error of the drive gear is excessive.
• There are foreign objects on the gear teeth.
Poor lubrication.
2 roll roller. The phenomenon of failure is shown in Figure 2. Because the anilox roller itself is not high precision or poor installation, it is easy to produce roller-like ink bars.

Figure 2 roller roller

• The anilox roller geometry is too large.
• The anilox roll is poorly mounted and there is a large radial runout.
• Bad bearing installation.

Figure 3 Vibrating flash

3 Vibrational ink bars. The phenomenon of failure is shown in Figure 3. The ink stick caused by the vibration of the relevant part.
• Vibration caused by wear of the bearing.
• Vibration caused by poor plate adhesion.
· Printing pressure loses the vibration caused by the steady.
• Vibration caused by poor stability of the drive motor.
Vibrational ink bars are more complex and are mostly related to the performance of the machine and the maintenance and maintenance of the equipment. Once this kind of failure occurs, it should be fully tested and checked to find out the root cause of the failure, otherwise it is difficult to eliminate it.
5. Uneven density (1) definition. Refers to the failure of a certain part of the printed product concentration or the density of each print.
(2) Form. There are three main forms:
a) uneven density on one side of the printed product;
b) Gradual variation in the concentration of printed matter during printing results in uneven density;
c) Inconsistent density occurs after printing several sheets.
(3) Causes and remedies 1 The density of one side of the printed product is uneven. Part of the density is uneven:
• There is a large difference in the thickness of the plate or the thickness of the double-sided tape.
• The back of the plate or the surface of the plate cylinder body is partially uneven or has foreign objects.
The unevenness of the density in the left and right direction of the printed product:
• The degree of parallelism between the plate cylinder and the impression cylinder or the plate cylinder and the anilox roll is poor, causing the balance of transverse pressure between the rollers to be lost.
• Scraper edge and anilox roll surface tangent defect.
· The blade edge has defects.
• The thickness of the printing plate and the double-sided tape has a large error in the transverse direction of the roller.
Therefore, the above links should be tested and adjusted.
2 Gradual changes in the density of prints during printing result in uneven density:
·Ink performance changes?
• The main lubrication area is poorly oiled.
3 After printing several sheets, the density is not uniform, or the density unevenness occurs when the printing is recommenced after printing is stopped:
· Overprinting is not allowed or caused by ink sticks.
· The performance of the ink in the ink fountain has undergone major changes, such as the reduction of the solvent and the deposition of the pigment.
6. The color is dim and is not clear In flexo printing, the dullness of prints and unclear breakdowns often occur. The main reasons are as follows:
• Insufficient ink in the ink, or toner deposition after standing: The colorant should be added to the ink. If ink is deposited in the ink fountain, the stirring of the ink in the ink fountain can be enhanced.
• Too much solvent in the ink, excessive dilution: should stop adding thinner, or increase the original ink adjustment.
• Too much resin in the ink: The ink formulation should be adjusted and the production adjusted.
7. The ink is too dark. The ink is not as deep as possible. If the ink is too dark, the color reproduction will be affected.
The reason why the ink is too dark is as follows:
• Ink content is too high: The pigment content in the oil should be reduced appropriately.
• Insufficient ink transfer: The number of anilox roller lines is too low, or the ink pressure is too high. Adjust the ink pressure and increase the number of lines of the anilox roller.
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