How to Work With Transparency in Illustrator for Printing

Transparency is not only the opacity slider. In an Illustrator file it covers every object below 100 percent opacity, every blending mode other than Normal, every opacity mask, and every live effect built on those: Drop Shadow, Inner Glow, Outer Glow, Feather and Gaussian Blur. A logo with a soft shadow is a transparent file even if nothing on the artboard looks see-through.

PostScript, the language most imaging devices still speak, has no way to describe a partly transparent object. It knows shapes, flat colour and pixels. So somewhere between your artboard and the plate, every soft edge has to be rewritten as opaque artwork. That rewrite is flattening, and it happens whether you run it or not. The only real question is which machine does it and at what resolution.

Open Window > Flattener Preview and set Highlight to Transparent Objects, then press Refresh. Whatever lights up is what the flattener will work on. It is worth doing on any file you inherited, because designers regularly hand over artwork with a 98 percent opacity rectangle nobody remembers making. For the wider file handover, our guide to creating commercial print-ready files covers the rest of the checklist.

Go to Edit > Transparency Flattener Presets. Illustrator ships three: High Resolution, Medium Resolution and Low Resolution. For anything that goes on paper, use High Resolution and leave the others for screen proofs.

High Resolution sets the raster and vector balance to 100, line art and text resolution to 1200 ppi, and gradient and mesh resolution to 300 ppi. Medium drops those to 300 and 150, which is a desktop proof setting, and Low sits at 288 and 144 for the web. The two resolution fields only apply to the parts the flattener has to rasterize, which is why line art can be set so much higher than gradients without exploding the file size.

The balance slider is the setting people misread. At 100 the flattener keeps as much artwork in vector as it possibly can and rasterizes only the regions where it has no choice. Lower it and more of the page is converted to pixels, which produces a simpler file that some old RIPs digest more happily, at the cost of softer type. Keep it at 100. The one exception is a genuinely complex page, a poster with dozens of blended objects stacked on a gradient mesh, where 100 can generate hundreds of atomic regions and a PDF that stalls the RIP. That is the moment to try 75, and only then.

Now the setting that causes more reprints than the flattener itself. Effect > Document Raster Effects Settings controls the resolution of every drop shadow, glow, feather and blur in the document. It is not part of the flattener and the flattener cannot rescue it. Many documents start life at 72 ppi. A shadow rendered at 72 ppi prints as visible squares no matter which preset you flatten with. Set it to 300 ppi at the start of the job, because raising it later re-renders existing effects and can change how heavy a glow looks. If you are already thinking about output resolution, our note on the best resolution for printing covers what each element on a page actually needs.

Four things account for nearly every transparency problem that reaches our preflight desk, and all four are visible before you send the file.

White set to Multiply disappears. Multiply keeps the darker value and white has none to give, so the object vanishes against whatever sits behind it. Black set to Screen does the same in reverse. It is not a flattener fault, but flattening makes it permanent, so check the Transparency panel on any object that looks wrong.

Drop shadows over gradients build a visible box. When a shadow with a soft edge is rasterized against a smooth background, the rasterized rectangle can show as a faint square, especially in light tints. Widening the shadow blur or moving the object onto a flat background fixes it faster than any preset change.

Blend modes over spot colors split the separation. A Multiply object crossing a Pantone forces that overlap area into process ink, so part of your spot separation quietly becomes CMYK and the colour shifts at the boundary. If the job runs a spot ink, keep transparency off it, or accept the conversion deliberately. The Pantone to CMYK conversion guide explains what the numbers do when you make that call.

Hairlines between regions are usually a screen artifact. Thin white lines across a flattened area in Acrobat are almost always anti-aliasing in the viewer. Turn off Smooth Line Art in Acrobat preferences and they normally vanish. If they persist in Illustrator's own Flattener Preview at high zoom, the region boundary is real and the artwork needs simplifying. Getting your colour build right in the first place helps here too, which is what the colour profile guide and the CMYK and RGB conversion guide are for.

Seven failures repeat, and six of them cost nothing to avoid.

• Flattening the master file. The live version is gone the moment you save and close. Always flatten a duplicate.
• Leaving raster effects at 72 ppi. The shadow prints blocky and the flattener preset makes no difference at all.
• Blending in RGB, converting later. Every overlap resolves to different CMYK values than you approved on screen.
• White objects left on Multiply. They look present on the artboard and print as nothing.
• Transparency crossing a spot ink. The overlap converts to process and the spot separation splits in two.
• A drop shadow that stops at the trim line. Extend it into the bleed, or the cut exposes a hard edge where the shadow should fade off the sheet.
• Sending a flattened and an unflattened file together with no note about which is current. The plant will pick one, and it may not be yours.

Before you upload anything, run a last look with fresh eyes. Preflight catches most of this automatically, and choosing the right file type prevents the rest by never putting the artwork in a format that has to be flattened in the first place. If a card job is what you are prepping, standard business cards start at $17.57 and print from the same PDF you just built.