Flexo title

This article commences a new mini-series targeting the issue of ink drop-off in flexo printing and how to handle this whilst operating within Adobe Illustrator using Phantasm CS Studio and Publisher. It's intended for all designers and operators of Illustrator in the packaging and flexo print industry as well as designers who may be required to produce packaging artwork.

If you want an introduction into flexo – or "flexographic" – printing, it's suggested you refer to this Wikipedia entry and the introduction video produced by the industry promotion group Flexo4All. Flexo printing is used widely in the packaging industry where printing is possible on many substrates including plastics.

In this introduction article, we'll cover the basics of ink drop-off and how to adjust an embedded image to either simulate the issue or compensate for it. Following articles will concentrate on the specific requirements to control drop-off levels within Illustrator on objects such as drop shadows and gradients, and attributes including transparency.

What is "ink drop-off"

The issue of ink drop-off, also referred to as "cut off levels" and "minimum dot size", is determined by the method in which flexo prints are produced. Rather than the printing plates for each ink separation commonly used in offset/lithographic printing, flexo printing typically relies on photopolymer printing plates which are, as their name implies, flexible.

It is thoroughly recommended that you read the explanation published by FlexoGlobal entitled Minimum Dot for Photopolymer Flexo Defined. This covers the mechanics of the situations as well as touching on solutions possible in Photoshop. These industry-accepted solutions form the basis of the methods due to be applied here, except with a more detailed overview of Illustrator artwork specifics.

An excellent example of the problems that can result from ink drop-off levels is shown above (image source: FlexoGlobal, reproduced with kind permission). This scan of artwork produced using the flexo print method clearly shows an unwanted and distracting black hard edge in the right-hand area of the sky. It is due to the fact that the smallest black separation ink dots required for the first few percent of black channel tints physically could not be reproduced.

Gradient 10% cut-off

To see what's happening more clearly, the above gradients – one grayscale and one process CMYK, each have a 10% cut-off level applied. This is higher than the levels experienced on flexo printing (typically between 1 and 3%), but does show how the eye quickly notices the abrupt end to a continuous change in tone. Each numbered cut-off stage is detailed as follows:

  1. Grayscale gradient first appears at 10% tint
  2. Yellow cut-off in process gradient
  3. Cyan cut-off in process gradient
  4. Magenta cut-off in process gradient
  5. Black cut-off in process gradient

Notice how the process gradient's inks all end at different stages when a uniform 10% cut-off is applied. This is because each of the CMYK inks start at different levels and all blend towards the left-hand edge where each ink dwindles to zero.

Showing ink drop-off in Illustrator

This uses exactly the same methods as would be employed in Photoshop, except that with the use of the Curves tool in Phantasm CS, the drop-off levels may be applied to both embedded vector (shapes, text, gradient fills, etc.) and raster (TIFF, JPEG, etc.) object types simultaneously.

As an example, the following image is placed and embedded into a CMYK Illustrator document:

Breakout example - original

The separations for this original image can be seen as follows:

Original image separated

First open the Curves Filter tool (we want to make a permanent change in this example) via Filter » Phantasm CS » Curves in Illustrator CS3 or below or Object » Filters » Phantasm CS » Curves in Illustrator CS4 or above:

Phantasm CS Curves window with cut-off

The Curves graph is available in two sizes; small and large. By default the graph is small and should be toggled to the larger size by clicking on the lower right button (1) to allow for greater accuracy when defining cut-off levels that will equate to the ink drop-off in flexo. Once enlarged, opt for the "pencil" curve editing mode (2). Then carefully draw a horizontal line (3) at the bottom left of the graph area ensuring that the line starts at an Input value of 0% and ends at 10% whilst maintaining a constant Output value of 0%.

Before applying this, it's possible to save this graph (or a more relevant cut-off value of 0-2% or 0-3%, etc.) by clicking on the Save… button. This will allow you to save the graph as a Photoshop-compatible .amp settings file. Alternatively, if you already have a series of cut-off settings originally created in Photoshop correct for a specific flexo setup, you could import and use these via the Load… button.

Upon applying this 10% cut-off level, the following result may be seen:

Breakout example image cut-off at 10%

The highlighted areas show the areas of maximum visual damage due to the ink drop-off. In the case of the right-hand highlight, it's the cyan separation which abruptly cuts out. Refer to the original separations, above, where it may be seen that the cyan channel hovers around the 10% tint level just where the problem occurs. The left-hand highlight shows all three CMY separations being cut-off in the shadow which is a common problem.

Whereas simulating this drop-off effect in Illustrator using Phantasm CS' tools doesn't cure the problem, it does, in combination with the array of color control tools on offer in Phantasm CS, allow a designer or operator to work around this issue. (Remember that the cut-off level is more likely to be a couple of percent and not 10% as shown previously.) For example, in the image above, it's possible to boost the cyan channel so that it doesn't near the cut-off level, although this may have to be compensated for with the other channels to ensure the end result isn't badly tinted. This is where the skill and experience of the designer or operator comes into play…

Compensating for the drop-off level

An accepted method of ensuring no point in the artwork goes below a cut-off level causing flexo printing ink drop-off, is to use the Curves tool to adjust the image so that the white point moves up to the cut-off level. By example, we'll return to the previous original image (before the cut-off curve was applied) and re-open the Phantasm CS Curves Filter window:

Curves flexo drop-off compensation

Note: if the old cut-off setting remains from the previous application of a Curve Filter, first revert to the curve editing more (1), hold down the Alt key and notice that the Cancel button temporarily becomes Reset. Click on this button whilst in the Reset state.

To make the compensation "curve", click on the bottom left curve node to select it (it changes from a hollow filled node to a solid fill) and enter a value of 10 in the Output box. It's important to note that the node was not dragged as Input/Output values are rounded up/down and manually dragging the node would be less precise meaning you could be applying a 9.6% cut-off instead of 10%. This matters more when a realistic cut-off level is nearer to 1%.

Applying this will result in a darker image (less noticeable if the image is compensated for only by a more realistic 2%). It's possible to verify the resultant adjusted image in Photoshop using Phantasm CS Studio and Publisher's Edit Embedded Image tool (Edit » Edit Image):

Checking compensated image levels in Photoshop

With the CMYK image in Photoshop, the Info panel (1) is opened, if not already so via Window » Info. The sample mode is set to 32-bit (0.0-1.0) by clicking on the tiny pipette menu pull-down icon (2) for added accuracy (the right-hand information (4) remains at 8-bit (0-255) for clarity).

By passing the pointer over the areas of concern (3), the Info panel confirms that no channel goes below a 10% tint (4). Therefore this image would no incur any ink drop-out on a flexo print press. Note that the left-hand shadow doesn't feature drop-out issues either.


Before we continue this mini series, it may be useful to refer to the following useful resources on flexo printing:


Kiwi fruit example image courtesy of Fotolia, supplied freely with Computer Arts issue 130.

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