This article is part of the FLEXO FLAW FIXING series

Even in the most intricate design patterns, package printing frequently necessitates a large amount of solid-color imagery. That dark background behind a product image? A strong image.ย 

That big, single-color logo on the side of a cardboard box? Yes, it is essentially a solid image. You will encounter mottled print when printing these unique color schemes. 

Among the 12 most common defects we see, it’s one of six that are frequently attributed to ink transfer issues. And, as with many of these flaws, there can be additional causes of the mottled image in flexo printing due to a variety of factors:

  • Ink viscosity
  • Print sleeve/cylinder/plate maintenance
  • Irregular substrate surface
  • Pressure
  • Ink drying speed
  • Surface tension
  • Anilox condition

We will work through these problems to gain a better understanding of frequently occurring flexo printing defects such as mottling, and return to producing consistent, high-quality prints.

Uncovering the Mystery of Mottled Print in flexo printing: Identifying Symptoms and Investigating Causes

Upon inspecting your freshly printed substrate, you may notice that the areas with solid colors have uneven print density and slight to moderate variations in shade or color. Mottled print defects in flexo printing may also be present, which can give a bumpy appearance to the image similar to an orange peel. The primary cause of these issues is often related to ink transfer, so it’s important to investigate that possibility first.

Troubleshooting Ink Transfer Issues: Causes and Solutions

The successful outcome of flexographic printing largely depends on ink transfer, which refers to the quantity of ink that is moved through the different components of the printing process and ultimately onto the substrate. Three primary factors control ink transfer in flexography:

  • Anilox
  • Image carrier (cylinder, sleeve, or plate)
  • Substrate

For a high-quality print, the ink must completely dry on the substrate. But, before that can happen, the appropriate amount of ink must be transferred from the anilox to the image carrier. The amount of ink transferred to the printing form by the anilox is determined by its cell volume and line count. 

In essence, fewer cells and less volume mean less space in the anilox for the ink to be picked up and transferred to the printing form or image carrier. Because there is less ink, the coverage on the substrate will be less uniform. Too much ink on the substrate can cause streaking and running.

Choose an anilox with the best cell volume/cell count for the job at hand. If the ink transfer is insufficient, contemplate using an anilox with a higher cell volume or lower cell count. Consider a lower cell volume/higher cell count anilox if too much ink is transferring.

Check the cleanliness and physical condition of the anilox roller as part of routine maintenance and clean or replace it as needed. Anilox cells frequently become clogged with dried ink or other contaminants, reducing the volume the anilox can carry. If it is dirty, it must be thoroughly cleaned.

How Surface Tension Can Make or Break Your Printing Process

A substrate with a lower surface tension than your ink could result in a mottled appearance. The ability of the substrate to pull ink from the image carrier is referred to as surface tension. Adequate ink transfer is facilitated by a proper surface tension chain (ink, image carrier, and substrate).

The Art and Science of Ink Viscosity: Why Finding the Right Flow Matters

Managing ink viscosity can be a challenge. Consider viscosity in terms of how long it takes a liquid to pour out of a cup. It has a low viscosity if it pours like water. 

It has a high viscosity if it has the consistency of ketchup. Your ink’s viscosity must be optimal for the printing job. You can change the physical properties of your ink by following the instructions provided by your ink supplier.

Mastering the Art of Printing: The Impact of Pressure on Print Quality

More flexo printing defects are caused by pressure than by any other variable in the press room. The majority of cases involve skewed images caused by excessive pressure, excessive dot gain, or pushing ink beyond intended printed areas.

In the case of a mottled image, however, insufficient pressure between the print cylinder and the substrate results in insufficient ink transfer. As a result, the solid color will be lighter than intended, uneven, or incomplete.

Reviving Your Print: Addressing Issues with Damaged or Dirty Substrate

Full wet-out is nearly impossible to achieve on a damaged or dirty substrate surface. If your substrate is damaged or polluted, ink transfer will almost certainly be affected. As a solution, you must identify and address the source of the damage or contamination.

Mastering Flexo Printing: A Guide to Fixing Mottled print and Other Common Defects in flexo printing

Orange Are you glad we talked about how to fix a mottled image? All kidding aside, this flexo printing flaw may appear to be an orange, but it’s anything but. If not addressed promptly, it can cost you time and substrate.

While it is most likely caused by a problem with ink transfer, viscosity, pressure, or substrate irregularity, it could also be caused by chemicals used to treat the plate or substrate. It could be caused by the speed with which the ink dries.