When the "K" in CMYK is not K

[Rabari]

When exporting a pdf for press printing, I can't get any graphical elements to export as K only, even though I define them for instance as 10 % K with the CMYK sliders on the Colour palette.

They turn up on every printing plate (C, M, Y, K). Even text defined as black is output to all plates. This is problematic, as simple b/w pages could be output as expensive four colour jobs, with unnecessary registration problems etc.

I've used a "Press (Print Ready)!" document setup, and a document Colour format which corresponds with the output I desire (Grey/8). I output the pdf to a pdf/x-4 format. I've tried creating my own colours in the Swatches palette, but I just can't figure this one out. 

 

Example:

1. Below are some elements, all defined as percentages of K. Selected is a 15 % K rectangle

 

2. The exported pdf opened in Acrobat, which shows the elements appearing on all plates. When deselecting Process Black, as in the screenshot, nothing should be visible, as all elements should be shades of  "pure" black)

[MickRose]

I think the Grey/8 Colour Format might be the problem. I seem to remember I had a similar proble a while back. Try using CMYK/8.

[mac_heibu]

What is your document‘s color profile?

Works fine and as expected, if you use, for example the PDF/X-4 output set and use the correct(!) simulation profile in Acrobat’s print preview.

[thomaso]

22 hours ago, Rabari said:

a document Colour format which corresponds with the output I desire (Grey/8)

Which "corresponding" profile did you choose?

By default the Grayscale document color format applies the "Grayscale D50" profile, which is not CMYK but RGB > therefore it is not able to export K only values. Instead you must alter this profile to one which suits CMYK.

But actually there is no need for a grayscale document setup:

To me with Afpublisher version 1.7.2 the issue with K only + the default export preset "for print" (PDF v.1.7) seems to be fixed. That means there is no need any more to export as PDF/X to get true K only results in the PDF.  Also, with K only elements in your document, you can achieve a K only result even with a CMYK document color space:

The attached file is a PDF of preset "for print", notice the role of the "K only" button for image resources:   v172 k-only _v1.7.pdf

[Rabari]

Thank you for the illuminating responses!
Using the CMYK/8 Document Colour Format setup fixed the problem. Why didn't I try that in the first place?
(A bit strange that the Grey/8 setup produces four colour plates… I used the Generic grey profile. Seems like there is a great deal of RGB going on in the Affinity apps behind the scenes)

[thomaso]

I' guess you haven't tried CMYK as document color space because you don't need CMY at all in your PDF and therefore wanted to limit the color space for the complete document from the beginning? 

Note that you can export from a CMYK document with CMYK objects as K only by using on export a true CMYK grayscale profile only, as f.i. in my screenshot above below "D50", or (haven't tried), the "Black & White" profile on top of the pulldown list.