Below, we will explore some common issues encountered in flexographic printing of film labels and their solutions：
1. Poor Adhesion
If the PE or PP film has not undergone surface treatment, or if the treatment is ineffective, resulting in low surface tension, ink adhesion to the film surface will be poor. In such cases, it is necessary to choose a film material with suitable surface tension or re-electrode the material surface to improve it.
Over-diluting the ink during printing, damaging the adhesive, or using the wrong thinner can also result in poor ink adhesion, making the ink less resistant to rubbing and friction. In such cases, the correct thinner should be selected.
Additionally, poor adhesion of the ink to the film can lead to ink peeling. In such cases, it is advisable to switch to a different ink type or consult with the ink manufacturer for a solution.
2. Pile-up Issues
During film printing, ink build-up can occur if the ink contains impurities, is too viscous, or dries too quickly. Each situation requires specific analysis. If the ink contains impurities, it should be filtered or replaced with fresh ink. If the ink is too viscous, a suitable amount of thinner can be added to adjust the viscosity. If the ink dries too quickly, a slow-drying thinner can be added to reduce the drying speed.
3. Color Difference
Changes in ink viscosity, uneven printing pressure, and clogging or wear of the anilox roller can all lead to color variations during film flexographic printing, resulting in color discrepancies. To address this, operators should continuously add ink while stirring to ensure the ink is fully dissolved in the solvent and maintains the ideal viscosity. If possible, a mechanical circulation pump can be used for more effective stirring. Additionally, after prolonged use, thinner should be added to reduce ink concentration, approximately every 30 minutes. After adding thinner 2-3 times, some original ink should be replenished and stirred evenly to prevent the concentration from dropping further.
4. Color Mixing
Color mixing occurs when the color of subsequent ink is contaminated by the previous ink or when the color diffuses and migrates. The causes of color mixing include:
(1) The previous ink dries too slowly, or the subsequent ink dries too quickly. In such cases, appropriate solvents should be used (ensuring the previous ink is fully dried), or the temperature of the heating system in each section should be adjusted.
(2) The ink viscosity is too high; it can be reduced to an appropriate level.
(3) The plasticizer in the substrate film can also contribute to color mixing. It's important to understand the types of plasticizers used in the film and any necessary precautions. If unsuitable, the type of film should be changed.
(4) Improper pigments or dyes used in the ink can cause migration. In such cases, inks made with dyes as raw materials should be avoided whenever possible.
5. Adhesion Issues During Printed Material Rewinding
Causes of adhesion issues during printed material rewinding often include slow ink drying, excessive residual solvent in the printed material, excessive rewinding tension, and high temperature and humidity during rewinding. If the ink drying speed is too slow, resulting in high residual solvent content, the drying speed must be adjusted. This can be done by adding a fast-drying thinner or increasing the drying temperature to reduce residual solvent. If the rewinding tension is too high, it should be reduced appropriately. Additionally, if the ink drying temperature during rewinding is too high, it can cause the film surface temperature to rise excessively. The accumulated heat may not dissipate quickly enough during rewinding, or the cooling process might be too short, leading to insufficient film cooling. Therefore, the drying temperature should not be set too high, and the cooling time should be extended if needed.
6. Issues with Frayed Edges and Flashes
In film printing, frayed edges or irregular flashes around the printed image may occur due to improper pressure between the inking roller and the printing plate, often caused by excessive pressure. In this case, the pressure should be adjusted accordingly. If the ink has dried on the printing plate or anilox roller, a slow-drying thinner should be added to the ink, or the ink trough should be covered to reduce solvent evaporation. If the ink is too thick, its viscosity should be adjusted based on the printed material. Static electricity can also cause burrs and flashes at the edges of the printed image. To prevent this, an antistatic device should be installed, or an appropriate amount of antistatic agent should be added to the ink.
7. Issues with Scalpel Lines and Ink Streaks
Foreign objects, damage to the printing plate, excessive or worn doctor blade pressure, and impurities in the ink can lead to scalpel lines or ink streaks after printing. If these issues occur, the printing plate should be inspected and cleaned, and replaced if necessary; the doctor blade pressure should be adjusted, or a new doctor blade should be installed; and the ink should be filtered to remove impurities.
8. Smudges and Traces
Smudges or trackles occur when ink is transferred to the reverse side of the printed material. Poor ink-drying performance, residual solvent in the print, excessive rewinding tension, and the use of double-sided treated film materials can all cause smudging issues. To address these problems, use faster-drying inks or faster-evaporating thinners to adjust the ink viscosity to an appropriate level. Check the dryer temperature and airflow between different ink colors, and adjust the drying temperature or use faster-evaporating thinners to minimize residual solvent in the print. Reduce the rewinding tension as needed. When printing on one side, use single-sided treated film to avoid unnecessary waste and printing defects. If smudging still occurs after these adjustments, apply anti-smudging powder to prevent ink transfer.
9. Ghosting Patterns
Unwanted dots or patterns may appear in the printed design, or solid areas may exhibit wavy or perforated patterns. The possible causes include:
(1) The ink is too thin and has lost its viscosity. To address this, the ink viscosity should be adjusted and controlled according to the specific requirements of the printed material.
(2) The anilox roller is worn, or the screen line angle is incorrect. In this case, the anilox roller should be replaced promptly.
(3) The ratio of the anilox roller's screen line count to the printing plate's screen line count is incorrect. In this case, the anilox roller should be replaced with one that has a suitable screen line count.
10. Pinholes and Intermittent Spots in Printing
Uneven film surfaces or poor filler fineness in the film can lead to pinholes or intermittent spots in the printed pattern. These defects occur when the ink fails to form a continuous film, often due to dirt on the impression cylinder, wear on the anilox roller, or ink drying too quickly. Solutions include replacing the film, increasing the ink film thickness, or adjusting the printing pressure. Additionally, clean the impression cylinder, inspect the anilox roller and printing plate cylinder, and replace them if they are excessively worn. Using a slow-drying thinner to adjust the ink viscosity or increasing the printing speed can also help resolve the issue.
11. Ink color too light
The causes of ink color being too light include:
(1) The ink is too thin and has insufficient viscosity. To address this, add an appropriate amount of original ink or binder to adjust the viscosity and enhance the ink's coloring power.
(2) The ink has dried on the anilox roller or printing plate, preventing it from transferring to the substrate. To resolve this, add an appropriate amount of slow-drying thinner to reduce the ink's drying speed.
(3) Water or other immiscible substances have contaminated the ink, impairing its performance. To correct this, replace the ink or add original ink or a small amount of ester solvent to restore the ink's performance before resuming printing.
(4) The film surface has poor ink adhesion. Check whether the ink type is compatible with the film, or if the film's surface tension meets the printing requirements.
(5) The pressure between the anilox roller, printing plate cylinder, and impression cylinder is improperly set.
(6) The anilox roller line count is too high, leading to insufficient ink transfer.
