This article is part of the FLEXO FLAW FIXING series

What Is The Pinhole Effect?

Sometimes, the ink does not completely cover the print surface. Small holes that appear at random in print are known as pinholes, and this is the Pinhole Effect. The pinholing problem in flexo printing is generally due to the qualities of the anilox roll in conjunction with the plate and substrate.

The Main Causes of Pinholing problem in flexo printing.

Flexo uses anilox rollers to offer a controlled process for transferring ink to the plate surface in a “known” volume. Ink is applied to the plate’s character as a sequence of dots, with the high-speed rotating action of the press often transforming the dots into ink ridges.

These ink ridges are transferred from the plate to the substrate, frequently with extended spaces on the final substrate. The anilox cell allows for a regulated ink flow, and it also influences how the plate transmits ink to the substrate. The ink supplied to the anilox roller is not completely transferred. The conventional rule of thumb is:

  • 50% of the volume moved from the anilox roller to the plate
  • 50% from the reservoir to the substrate

Approximately only 25% of the original anilox volume is transferred to the substrate. However, numerous press and component parameters can affect performance.

Unfortunately, merely throwing more ink at the pin-holing does not address the underlying issue; more ink implies more ink and more energy to dry it. More ink also necessitates the use of a larger volume anilox roll.

However, these rolls frequently have a lower line per inch value, with more giant cells and a more significant gap between the centers of each ink dot, potentially resulting in more significant gaps between the ink ridges.

The Appearance of The Pinhole Effect

Some parts of your substrate may not print completely. This one has a peculiar, unfinished appearance. Tiny unprinted areas the size of a pinhole.

What Are The Flaws Of Pinholes In Flexo Printing?

Your pinhole effect might be mechanical or chemical.

  1. Improper Ink Transfer

Your inks aren’t entirely wet on the substrate. Obtaining a smooth coating from your anilox roll to your image carrier to your substrate requires optimal ink transfer.

Depending on the type, it may be drying too rapidly and failing to transfer onto the printing form. This effect might be due to the environment in your press room. Keep an eye out for the following variables:

  • Humidity
  • Temperature
  • Circulation of air

These factors can cause inks, mainly water-based and alcohol-based, to dry too rapidly or too slowly. Adjust the power of the dryers and use a solvent with a slower evaporation rate.

  1. High Ink Viscosity

Tackiness issues might arise with more viscous inks. Higher ink tack makes image transfer to the substrate more challenging. Adjust the ink viscosity with additions if required to increase fluidity without becoming runny.

  1. Dirty Anilox or Low Cell Volume

Your anilox may decrease cell volume due to unclean cells or wear over time. Less volume means less room for the ink to be taken up and transmitted to the image carrier on the anilox. As a result, the ink will not produce a homogeneous coating on the substrate.

During routine maintenance, inspect the anilox roller and replace it as needed. However, in most situations, the anilox roller is dusty from dried ink and has to be thoroughly cleaned using non-caustic chemicals and a non-abrasive brush. It is good to clean the imprint cylinder that regularly exerts pressure on the plate/sleeve cylinder.

  1. Irregular/Damaged Substrate Surface

Full wet out is difficult, if not impossible, on an uneven or damaged substrate surface. If your substrate is unclean, refusal of ink transfer occurs. As a workaround, you can modify the substrate’s surface treatment. A material modification may be a general solution in some instances.

How to Overcome Pinholing problem in flexo printing?

One method for eliminating the pin-holing effect is to lessen the distance between the anilox cells on the roll. This way can also improve ink flow. Another alternative, and the one we’ll look at here, is to adjust the surface of the plate. Traditionally, this method entailed generating microscopic cells on the plate’s surface that carried ink to transfer more ink to the print. 

However, for the reasons described above about the usage of larger ink lay-downs, this solution is only a partial workaround that frequently results in higher production costs rather than addressing the fundamental source of the problem.

Solutions To Prevent The Pinhole Effect In Flexo Printing

  1. Adjust the power of the dryers and use a solvent with a slower evaporation rate. Boost the printing speed.
  2. Change the viscosity of the ink. Increase the thickness of the ink film, Increase the printing form to the imprint of the substrate. 
  3. The substrate’s surface may be uneven, pitted, or filthy. Examine the metering system.
  4. Examine the anilox roller’s condition and replace it if necessary.
  5. The imprint cylinder has to be clean.
  6. Use softer printing paper or mounting tape. Examine the integrity of the substrate’s surface treatment.