When I save an illustrator file to pdf (press) that includes a mask and view the PDF preview or print the file, the black areas of the mask are shown in low opacity.

What's causing this?

enter image description here

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When using masks, especially opacity masks, in Illustrator it is important to use the proper black as opposed to just "black". I assume you're using an opacity mask and not a clipping mask, since this issue wouldn't show itself in a clipping mask. Although, based on your sample, a clipping mask would certainly work and be a bit easier.

For Opacity Masks, if you use just "black" (the black swatch) - that is a rich RGB black (generally R35G35B35 or close to that) or 100%k, you aren't actually masking everything. You are only masking those colors.

This is one instance where using the "Registration" swatch actually helps. The Registration swatch is a swatch which contains the total limit for the color mode. So in RGB, the Registration swatch is R0G0B0. In CMYK the Registration swatch is 100C100M100Y100K. You want the "black" of the mask to be 100% of all colors. You want to use the Registration swatch rather than the Black swatch.

For example... a CMYK image with an opacity mask... enter image description here

Note how only the actual black is hidden in the top image. That's because the mask only has 100%K in it. Once you add the other colors, the mask hides those colors as well.

I do realize that in print design you are taught to NEVER use 100% of all colors and in fact be mindful of ink limits above 300. However, realize this is a mask and not printed ink. You aren't applying a color to anything to be printed, you are hiding the colors in the documents. It's different.

I would speculate that your particular opacity mask is using a CMYk Rich black and that is why it's only partially hiding the colors. Bumping the "black" of your mask object to 100% for all colors should correct the issue.

If you are working on an RGB image, your mask "black" should be R0G0B0 not any other value.

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