Create white underlays for printing on transparent foil using "Create spot color plate based on ink amount"


When printing on transparent substrates – such as transparent foil for shrink sleeves – it can become essential to print a white primer onto the substrate before printing the actual print content. If all of the surface of thee substrate is treated with a white primer or white underlay, it would not make any sense to use a transparent substrate to begin with.  

Thus it is desirable to print a white primer only where some print content is printed afterwards. In addition, the degree to which the ink amount of the print content increases, less – if any at all – white primer might be needed.  As in most printing architectures, a white primer is relatively expensive, it would be nice to be able to limit the amount of white primer used as much as possible. Page areas printed with 200% of ink or more  do not tend to gain anything anymore from printing white below them beforehand (of course this will depend a lot on the  opacity of the inks used).

The fixup "Create spot color plate based on ink amount" makes it possible to create a spot color plate where at each position on the page its tint value is derived from the ink amount of the actual print content at that position – using a simple curve file that turns the input value of ink amount into a tint value of the newly generated spot color plate.

The tutorial described in the section below illustrates how the "Create spot color plate based on ink amount" may be used. The tutorial is divided into two parts: first, a simple use case is illustrated. Second, a more complex scenario is explained where the "Create spot color plate based on ink amount" is combined with using the "Create shape" feature to flexibly address the distinction between white and transparent parts of a page.

Create white underlay for packaging label printed on transparent foil (Part 1)

Example file "Peppery Pumpkin Yoghurt"

The sample label used in this tutorial is meant for printing on transparent foil, to be used as a glued on lid for yoghurt cups. The goal is to only print a white underlay in areas, where actual print content is present, and to only print as much of the white primer as actually needed – more on light areas of the print content, less in darker areas with a higher ink amount. To keep this tutorial simple we are ignoring different opacities of different inks.

The brownish red areas and the small green part have an ink coverage of just above 200%, whereas the orange areas sit at just above 100%. The remainder of the lid lingers mostly between 25% and 50%. Our assumption is that once ink coverage rreaches 200%, no white underlay is needed. Very light areas up to around 50% should be printed on 100% white primer, the range from 50% to 200% should go from 100% to 0% primer.

Create the "Create white underlays for printing on transparent foil" fixup

Create a new fixup, by choosing "Create spot color plate based on ink amount" as the fixup type (entering 'plate' in the search field will make locating it very easy), giving it a suitable name, and going through the configuration options, setting them to the values as illustrated below:


Regardless how the appearance of the spot color to be created is defined, the new spot color plate  will be created on top of all existing page content, and will be set to overprint, so that it does not disturb the existing page content.

To make the spot color plate's content as smooth as possible, it is recommended to set the "Interpolate key for spot plate image" to "On".

Setting up the ink amount to spot mapping curve

In order for this Fixup to work, a mapping curve is necessary that looks at the ink amount values across the page (at the configured resolution – this example uses 96 ppi; for crisper results value up to 300ppi may make sense) and uses them to look up the tint value to use for the spot color plate.

Click on the '+' or 'duplicate' button next to the "Ink to spot mapping curve" drop down:

This will take you to a mapping curve window. The easiest way to creating your own is to duplicate an existing one (like done above) (make sure to give a meaningful file name). Curve files can be found under preferences:

Users/User/Library/Preferences/callas\ software/callas\ pdfToolbox\ 13/Repositories/Custom/20210921_100038_0001/InkToSpotCurves/WhiteUnderlaySampleCurve.crv

In the example shown below the following has been done:

  • Put the name of this curve file – as it should be displayed in the "Ink to spot mapping curve" drop down list – after "DisplayName [tab] 0  [tab]", the example uses "Ink amount to spot - 100%  from 0% to 50% ink, then down to 0% at 200% ink"
  • below the line with "INPUT [tab] [OUTPUT]" put three lines with the following content:
    • 0.0 [tab] 1.0
      • → 0 % ink amount in the page will result in a 100% tint value on the spot color plate
    • 0.5 [tab] 1.0
      • → 50 % ink amount in the page will result in a 100% tint value on the spot color plate; in addition,
        • all values between 0% and 50% ink amount will also result in a 100% tint value on the spot color plate
    • 2.0 [tab] 0.0
      • → 200 % ink amount in the page will result in a 0% tint value on the spot color plate; in addition
        • all values between 50% and 200% ink amount will  result in a tint value on the spot color plate going from 100% down to 0% (via linear interpolation)
        • all values above 200% ink amount will also result in a 0% tint value on the spot color plate

Once the new curve file has ben created, go back to pdfToolbox, save the fixup as configured so far and then reopen the fixup editing dialog – this is necessary to update the "Ink to spot mapping curve" drop down list. Now select the ink to spot mapping curve.

Testing the newly created "Create white underlays for printing on transparent foil" fixup

Instead of closing the fixup edit dialog and running the fixup on the example file – or any other file – it is recommended t use the "Test" mode as then you can stay in the fixup editing context and are in a position to easily adjust the configuration without having to close and reopen the dialog and saving out modified files to your hard disk again and again.

The result is displayed to the left – here the new spot color plate shows up in light Cyan colored overlay.

Additional control step: execute the fixup and inspect the new spot color plate using the "Individual separations" view

To be able to control the newly created spot color plate on its own, execute the fixup on the example file – or any other file – and inspect the result in the "Individual separations" view:

The "White underlay" spot color appears as a somehow inverted page image, with the darkest parts in areas where the page content is relatively light, and vice versa.

Create white underlay for packaging label printed on transparent foil (Part 2)

The approach explained in Part 1 runs into problems, if

  • the print content contains white areas (e.g. white 'background' for inverted type) and transparent areas (e.g. content outside of the actual label, where nothing is to be printed at all)
  • the desire is not to put any white underlay in areas  where there is no print content at all but at the same  time there are areas inside the print content where a white background is necessary; when printing on transparent foil (as opposed to more less white paper or more or less white paper like  substrates) there is no pre-existing background. Instead depending on whatever the transparent foil will sit on will become the background, often in a not suitable manner.

In order to be able to distinguish between transparent areas and white areas, we ned to combine the fixup as illustrated above with a something that suppresses the "White underlay" in transparent areas of the page. This is achieved by created a shape reflecting the transparent areas of the page and applying a fill color – set to overprint – of 0% "White underlay". This will knock out the "White underlay" spot color (in transparent areas of the page) but will disturb any other content. The steps below illustrate how to set up the Shape based fixup.

As a further refinement, the shape generation can be configured such that the"White underlay" bleeds into the transparent areas by a few millimeters to compensate for registration issues or to make the print content stand out more on the transparent foil.

As this additional shape generation fixup needs to be applied right after creating the spot color plate for the "White underlay", it is best not use it on its own but instead to create a simple Process Plan that applies both fixups one after the other. The Process Plan is attached below.

Also, a slightly different version of the "Peppery Pumpkin Yoghurt" label is attached below – it has white background in some parts of the print content, while the page is transparent outside of the intended print content.

Create a new fixup for creating a Shape reflecting transparent areas of the page content

Make sure to exclude the already created "White underlay" spot color when determining transparent areas

Configure how to fill the shape

Fill the shape with 0% of "White underlay" spot color

Bring it all together in a Process Plan

Testing the new Process Plan using the Test feature

Using the Process Plan on the label variant with white areas in the print content and a transparent area around the print content creates a "White underlay" with a tint value of 100% in white areas, but with 0% "White underlay" outside the intended print content. In addition, 3mm of "bleed" are created for the "White underlay" spot color.