This tutorial explains how to set up your print dialogue window to print greyscale (Multi-channel separations) from Photoshop directly to your printer. Each spot color is essentially a greyscale separation but in order to print to the RIP, your files MODE must be a Milti-Channel file. It cannot be a greyscale mode (and contain additional spot colors (and print all colors). It will not recognize the other spot colors if the file is only a greyscale file. The mode must also not be RGB or CMYK. As a Multi-Channel mode, the channels then must be all spot colors. It cannot have any other channel in the file at the time that is not a spot color.
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This is different than some separation software where you may NEEd the RGB channels at the top and the file might need to be an RGB file. Other separation methods and software, require you to have the separations in the RGB “layers” That’s craziness! but hey, I guess you gotta do what you gotta do. For me working with hundreds of different print shops, and providing color separations for screen printing, I get requested to provide films in many strange ways that don’t make sense to me, but I’m not in their shop and I don’t have their setup. Printing films can be done in many ways so do what is best for you, but this method seems most easiest to me, when your art comes from Photoshop and your seps come from Photoshop. Why go anywhere else to print your films is my thought. It seems to be more efficient.
Down at the very bottom of the Print dialgue window, you can also choose to use Photoshops default registration marks, and add spot color name sot each separations et. I make my own...but my files are also at 600ppi resolution, so these registration marks and color info come out nice and crisp.
https://youtu.be/WJF_4YQIuH4