Clean up your act! OK, so we know dirty print is not all your fault.

Among other common flexo printing defects, dirty print is particularly confounding. It can stem from contamination, ink consistency, plate/sleeve issues, pressroom atmosphere, and everything in between. Dirty print often appears as speckles in the ink that give the print a dirty appearance or instances of excessive dot gain in high-resolution areas.

Find Solutions to Your Flexo Printing Defects: What Causes Dirty Prints?

Are you experiencing printing quality issues? Let’s figure them out together. We’ll start with the most obvious cause, which happens to be this printing defect’s namesake: Dirt.

Dirt & Contaminants

Microscopic airborne particles and contaminants surround us everywhere we go. It’s true in our own living rooms, but it’s especially true in an industrialized press room. In any industrial setting, machinery in action promotes static electricity, which attracts floating dust, tiny paper particles, and other contaminants to positively or negatively charged objects in the vicinity.

The result? A thin layer of dust, likely invisible to the naked eye, accumulates on your flexographic printing press and its sleeves/cylinders/plates, anilox roll, doctor blade, ink chambers, and more. From there, the composition of the ink wherever it contacts these contaminants will be compromised.

How can you stop what you sometimes can’t see? Well, you can’t. But routine maintenance and cleaning and adequate press room ventilation should keep contaminants to a minimum.

Drying Ink

Along the same lines, dried ink in anilox cells or on printing sleeves, cylinders, and plates also can cause dirty prints. How often do you inspect your dryers? They may easily be one of the most consistently neglected press components.

If ink dries too quickly, incorrectly, or out of balance, it may linger on the anilox, doctor blade, and/or image carrier to create serious issues with ink transfer. These ink transfer problems on one end of the spectrum could include skip out (which occurs when too little ink is transferred) or a flexo printing defect like dirty print (when too much is transferred).

Pressroom conditions from temperature to humidity always affect how quickly and completely inks dry. The more hot and arid the atmosphere, the more quickly the ink’s water, alcohol, and solvent bases will evaporate. Industry experts believe it’s best to keep the press room around 72°F and 50% humidity to minimize climate as a potential cause for dirty printing.

Dive deeper into Ink transfer with the free ebook below:

Incorrect Ink Viscosity

Ink can be runny or have the consistency of maple syrup. If it’s thick and “syrupy,” it’s more viscous. This has a huge impact on how ink releases from the anilox roll to the image carrier and from there to the substrate.

The more viscous the ink is, the more easily it accumulates along with the press ink-transfer components. In addition to dirty printing, this can also cause flexo printing defects like bridging and filling in.

Improper Anilox

The anilox is a cylinder that transfers ink onto the image carrier. The amount of ink delivered is controlled by a pattern of small, precisely sized dimples called cells in the anilox surface.

Too much ink on the anilox can ultimately bring too much to the substrate.

You must choose an anilox with the proper cell volume and cell count:

• Cell volume: The total space available to hold ink in 1 sq. inch on its surface. Generally, greater volumes are used for brighter, bolder work, and smaller volumes for jobs requiring fine detail.
• Cell count: The number of cells per inch the anilox has when measured along the engraving angle, which is the arrangement of cells in relation to the axis of the roll.

Furthermore, industry experts also strongly suggest using an anilox roll with an LPI (lines per inch) at least 6 times the printing LPI. This avoids a phenomenon known as dot dipping, in which small dots on the printing sleeve or plate essentially collect excess ink.

The guide below offers a more comprehensive view of all types of printing problems:

How to Clean Up Your Dirty Prints

Flexo printing is a pressure-sensitive process with many variable parts and processes. That’s why, in any scenario, you should always look to optimize the pressure at the ink contact points along the press.

But in regard to dirty printing, many issues from ink viscosity and drying speed to anilox selection and ink contamination can be a root cause. Your inks could also have excess pigment, or even the mounting tape may be too thick or hard.

These issues are a lot to digest in one sitting. If you have any questions or would like a second opinion, we’re always here to offer troubleshooting solutions. Call (888) 545-2270 to speak with a Luminite technical support rep, or reach out using the link below:  

(Editors Note: This blog was originally published in March 2018 and was updated in September 2021 to reflect changes in the market and new information)