Seeing ink streaks and ghosting in flexo printing? Here's the fix!

Flexographic printing is a direct printing method where the printing plate has elastic, raised images. Double-sided tape is used to fix the printing plate onto a plate roller, and the printing plate is supplied with ink by a metal or ceramic anilox roller. There are various printing equipment capable of flexo production, such as the most common unit-type flexography machines, which have many design routes, such as upper and lower paper flows; There are many types of inks capable of flexo printing, such as water-based inks and UV inks; There are countless products capable of flexo printing. With such a complex and varied factor, various issues inevitably arise during flexographic printing. Today, let's talk about common issues like ink streaks and ghosting in flexographic production, as well as their solutions.

01

Printed ink bar

Printed ink bars commonly have stripes perpendicular to the printing direction and evenly spaced with gear pitch dimensions. The main causes of ink rods are gears and mechanical vibrations, such as the level of mechanical precision, whether the gear design is reasonable, and whether the gear assembly is appropriate-all of which can lead to ink rods. If ink bar issues occur during printing, you can try the following methods:

Adjust the gear

If the gears move back and forth during transmission, it can cause ink bars to form on the printed product and wear the printing plate. Since the helix angle is determined by the main drive gear, this parameter of the plate roller gear must be the same. However, different manufacturers have different methods for eliminating axial force effects. The common approach is to chamfer the top corner of the gear at 45° to remove the edge, but a well-known manufacturer uses oblique grinding from the tooth root to the tooth tip on both sides of the gear, with the grinding height reaching one-third or one-half of the tooth height at a height of 1/3 or 1/2; Leave a gap for axial force to prevent axial force during gear operation from causing gear vibration.

Stagger the printing plates for layout

The purpose of staggered typesetting is to reduce roller bounce caused by printing plate typesetting issues. Practice has proven that relying solely on the two side grating strips is difficult to support plate roller runout caused by unreasonable layouts. We found that in severe cases, plate roller runout can cause anilox roller vibration, which in turn leads to abnormal scraper wear, especially noticeable when printing long sheets at high speeds.

Use softer double-sided tape as the pad for cushioning

Using softer double-sided tape as a backing for cushioning is a very practical solution, but when using this method, it is necessary to consider the pinhole issues caused by medium-density or low-density double-sided tape during on-site printing.

02

Print ghosts

The characteristic of printed ghosting is that when printing each product with a repeating perimeter, localized ink tones appear lighter at fixed positions. Ghosting is not caused by color registration; ghosting issues can also occur during monochrome printing. Ghosts often occur in the live versions, and because it's hard to explain their origins, foreign peers call them Ghosting, while domestic peers refer to them as Ghosting. In short, the cause of ghosting is actually the marks left by the printing plate on the anilox roller.

Common methods to solve ghosting problems generally include the following:

The repeat perimeter of the printing plate ≥ the perimeter of the anilox roller. Obviously, when the anilox roller diameter is small-meaning the roller spins faster and the ink replenishment frequency is high-ghosting does not occur. Therefore, the product perimeter should be specifically selected based on the perimeter of the anilox roller of the flexo printing machine, which must be considered during the finalized design process.

On the pattern of imposition products, the color blocks should be placed in the gaps of repeated patterns according to the difference between the circumference of the anilox roller and the plate roller perimeter, rather than overlapping with the original color block parts. Since most of our products use imposition printing, only a few can truly achieve repeated perimeter ≥ anilox roller perimeter per unit. Therefore, when many products have a repeat perimeter less than the perimeter of the anilox roller, we must check the difference between the perimeter of the anilox roller and the plate roller to check for large color block patterns at this position. If there is a gap here, the product will not have ghosting; if there is a large color block pattern here, the probability of ghosting is very high.

Use slow-drying agents. When you truly encounter this unavoidable design issue, using slow-drying agents can be a very practical solution. The main purpose of adding slow-drying agents is to improve the leveling properties of the ink on the anilox roller, making it dry as slowly as possible. When printing with UV ink equipment, the chance of ghosting is relatively low. This is because, without a UV curing device, UV ink dries very slowly. Water-based inks and alcohol-solvent inks can both add slow-drying agents, which is more helpful for ghosting treatment.

Ghosting often appears during the field printing process, which is related to many operators preferring to increase pressure during field printing. Therefore, another way to solve ghosting is to reduce the pressure of inking. Sometimes, when printing speed is slow, ghosting is noticed, and by increasing the speed, ghosting gradually disappears-this is actually the reason stress is reduced.