Any suggestion on a workflow to export/render an After Effects frame to a printable file? I need to print a raster graphic realized whit Trapcode by Maxon and preserve the transparency. The AE composition is in 8K and the print output should be suitable for a large printing format.
AE is pixel based, as opposed to dots per inch, but as Mylenium says, once you get the frame into photoshop, it should all be fine. From your comp, select the frame you wish to use, then go to Composition > Save Frame As > Photoshop document or Save Frame As > File, which produces a single layer image. You could also try Video Copilot's FX Console, which captures the frame to a temporary PNG file. This works quite well, being based off the preview (I think).
Photoshop's upscaling is excellent these days, I recently saved a bunch of images out for an online merchandise store and on resizing in Photoshop they came out great. Depending on your print medium, you'll likely also need to alter the colour space from RGB to CYMK, which can change the colours somewhat.
It's a standard RGB to CMYK workflow (or whatever separations you need) like e.g. converting 3D rendered stuff. All that is required is to generate the pertinent metadata by opening it in Photoshop, changing the DPI to the required spec and adjusting the colors based on your color management. Outside that there is realyl nothing that would affect the AE side of things. If at all you may want to have an adjustment layer with e.g. a Levels or Curves effect somewhere to preview the gamut reduction, but otherwise there's really not much to it. A consistent CM workflow in PS is much more important than how you crank out your files in AE.