This article is part of the FLEXO FLAW FIXING series

What is Ink Mottling?

Flexographic ink mottling is a problem that plagues food packaging and other package printing that demands a lot of solid colour graphics. The ink does not wholly wet out on the substrate in certain circumstances.

Print density and shade/hue differences are visible in printed regions meant to be a solid colour. This mottling issue seems rough and has the appearance of an orange peel.

The most likely causes are variations in print density caused by poor ink transfer between the anilox, image carrier, and substrate. This defect usually starts with selecting an anilox with the appropriate cell count/cell volume and cleaning all ink-transferring components on your press.

Package printing requires many solid-colour images, even with the most complicated design patterns. You will encounter patchy print while printing these unique colour schemes.

Flaws of Ink Mottling in flexographic printing

Among the 12 most common flaws we see, one of six is ink transfer difficulties. Moreover, there can be other causes of the speckled image in flexo printing owing to several factors:

  • The viscosity of the ink
  • Print sleeve/cylinder/plate upkeep
  • Surface Pressure on an Irregular Substrate
  • Speed of drying ink
  • The surface tension
  • Anilox situation

The Appearance Of Mottled Ink

When your substrate comes off the press, the print density in the solid colour sections is noticeably inconsistent, with minor-to-moderate shade or hue variances. Mottled photos can have a rough appearance and the visual consistency of orange peel.

What are the Causes of Mottled Ink?

The most common suspicion is ink transfer. Look into it first.

  1. Issues With Ink Transfer

All successful flexographic printing relies on ink transfer. The amount of ink is transferred through the printing process’s components and eventually onto the substrate. Three factors essentially govern ink transfer in flexo:

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

For a high-quality print, the ink must completely dry. But, before that can happen, the appropriate amount of ink must be transmitted from the anilox to the image carrier.

As a result of cell volume and line count, the anilox determines how much ink transfers to the printing form. In essence, fewer cells and less volume mean less area in the anilox for the ink to be taken up and transmitted to the printing form or image carrier. 

Since there is less ink, the coverage on the substrate will be less uniform. Too much ink on the substrate can create streaks and run.

Choose an anilox with the best cell volume/cell count for the work at hand. Consider a greater cell volume/lower cell count anilox by transferring a little ink and a lower cell volume/higher cell count by moving too much ink.

Check the cleanliness and physical condition of the anilox roller as part of regular maintenance and clean or replace it as needed. Anilox cells frequently become clogged with dried ink or other pollutants, reducing the amount the anilox can transport. 

  1. The Tension Of The Surface

A substrate with a lower surface tension than your ink might result in a mottled look. The capacity of the substrate to draw ink from the image carrier refer to as surface tension. The facility of adequate ink transfer is by an appropriate surface tension chain (ink, image carrier, and substrate).

  1. Viscosity of Ink

Managing ink viscosity may be a challenge. Consider density in terms of how long it takes a liquid to pour out of a cup. It has a very low viscosity if it flows like water. It has a high density if it has the consistency of ketchup.

The viscosity of your ink must be optimal for the printing task. You can change the physical properties of your ink by following the instructions provided by your ink provider.

  1. Under Stress

More flexo printing faults are due to pressure than any other variable in the press room. Most occurrences include skewed pictures caused by high pressure, excessive dot gain, or pushing ink beyond intended printed regions.

However, insufficient pressure between the print cylinder and the substrate results in bad ink transfer in the event of a mottled picture. Therefore, the solid colour will be lighter than planned, irregular, or incomplete.

  1. The Substrate That Is Damaged Or Dirty

Full wet out is very hard to achieve on a damaged or unclean substrate surface. If your substrate is damaged or dirty, ink transfer will probably be affected. As a remedy, you must identify and treat the cause of the harm or contamination.

Solutions To Prevent Ink Mottling Problem In Flexographic Printing :

  1. Add new liquid or medium to adjust viscosity for increased visibility.
  2. Increase the imprint of the plate on the substrate. Make the ink viscous. Thick the ink film. Utilize a softer plate durometer.
  3. Thoroughly clean the container using a suitable cleaning solution. It is necessary to reprint the printing plate.
  4. It is essential to carefully remove any ink and foreign elements from the impression roller.
  5. Make use of softer or collapsible plates and cushion mounting tape. Examine the treatment levels on the substrate.
  6. Verify that the solvent balance is correct. Replace the ink system. Make contact with the ink and substrate suppliers.
  7. Replace old ink with new.
  8. Thoroughly clean the plate. Check that the ink filters are operational. Check with the provider about the plate’s level. Ensure that no harsh solvents are present on the plate surface.
  9. Inspect anilox roll for signs of wear, clogging, or excessive land regions. Use a higher-quality anilox roll.