The file appeared fine on the screen but printed incorrectly.
Of all the ways a print job can go wrong, this feels the most unfair. The PDF opened correctly, with colours looking right and everything positioned as intended. But then the printed sheet came back with a pale box behind the logo, a headline that turned muddy, or a white line running across an image that wasn't there in the design.
Nearly all of these issues arise from three related aspects: transparency, overprint, and flattening. These elements are components of a print file that are invisible on screen but become apparent during printing. This guide elucidates what each of these elements is, how they contribute to the specific problems you encounter, and how to verify your file before it is processed by a printing machine.
Transparency refers to the effects that are not yet truly present.
Transparency in your artwork refers to any interaction between objects, rather than having them simply sit on top of each other. This includes drop shadows, glows, feathered edges, blend modes such as multiply or screen, and any object set below one hundred percent opacity.
The key point to understand is that these effects are live instructions, not finalized pixel images. Your design software calculates these effects on the fly each time it redraws the screen. A printing press cannot replicate this dynamic process. At some point before printing, all the dynamic elements must be converted into static, opaque objects that a press can reproduce. This conversion process is known as flattening.
The issue arises when flattening occurs in an unexpected location, such as a different application or with settings that were not anticipated.
The white line problem
The most recognizable symptom is a faint white or pale line cutting across an image or a colored panel, sometimes referred to as stitching. This occurs because the flattener divides the artwork into regions, some of which are rasterized and others that are kept as vectors. The joins between these regions do not always align perfectly.
Two factors can reduce this issue: exporting the file at a high enough flattener resolution, and using a PDF standard that maintains transparency rather than flattening it early. If you notice these lines on a proof, the file is indicating a problem, and increasing the print size will not eliminate them.
The colour shift problem
When a drop shadow or a transparent object is placed over a spot color or a rich black, flattening can alter the color underneath, making it slightly different. On screen, the transition appears seamless. However, in print, a visible rectangle can form where the shadow's bounding area was. This is why designers are advised to avoid placing transparent effects over spot colors, and this advice is well-founded.
Overprint: the setting that hides in plain sight.
Overprint instructs the press to print one ink on top of another, rather than knocking a hole in the layer below. Used deliberately, overprint is useful, most commonly for black text. This text is set to overprint so that any tiny misregistration on press does not show white edges around the letters.
Used accidentally, it causes two classic faults:
- White set to overprint disappears. There is no white ink in standard process printing. White is the absence of ink, so an object that is white and set to overprint simply vanishes on the printed sheet, while showing perfectly on your monitor.
- Coloured objects go muddy. A cyan panel set to overprint on top of a magenta background prints as both inks combined, giving a colour nobody chose.
Overprint settings often arrive in a file without anyone setting them, inherited from a template, an imported logo from a supplier, or a file that passed through several hands. This is one of the reasons a supplied logo is worth checking rather than trusting. The general checks are covered in the article on digital printing and in the wider preflight guidance under general printing guides.
How to check your own file
You do not need prepress software to catch most of these issues. Recent versions of common PDF readers and design applications include preview modes that simulate what will happen on press.
- Turn on overprint preview. This single setting reveals disappearing white objects and muddy overlaps immediately. Without it you are looking at a lie.
- Use separation preview. Switch individual inks off one at a time. Anything that should be on black but appears on cyan is a build problem.
- Look for unexpected plates. If your job is meant to be four-colour and a fifth spot colour appears in the list, something in the file is still referencing it.
- Check the flattener preview. Design applications can highlight which areas will be affected by transparency, which shows you where trouble is likely.
- Zoom to a high magnification on edges. Stitching lines and soft edges show up at four hundred percent that are invisible at fit-to-screen.
Do these changes on the exported PDF, not on your working file. The export is what the printer receives, and it may differ from your working file.
Export settings that avoid most of it
A few habits can prevent most of these faults:
- Export to a PDF standard that preserves transparency rather than flattening it during export. This allows the printer to flatten the transparency with their own settings.
- Keep the text as live text whenever possible to ensure it remains sharp and does not get rasterized.
- Avoid applying transparent effects over spot colors.
- Set only true black text to overprint, and never use white.
- For large black areas, use a rich black instead of single-ink black. Keep small text as single-ink.
- Place linked images at their final size rather than scaling them heavily in the layout.
On the last point, the resolution consequences are covered in the article on digital and colour printing, and the colour-space side sits in the guidance on getting colours right.
When the printer flattens for you
A reliable printer conducts an automated preflight check on every incoming file. This process reports issues related to transparency, overprint settings, resolution, color spaces, fonts, and bleed.
This argument supports the need to examine a proof thoroughly rather than just glancing at it. A digital proof generated after flattening displays the artwork exactly as it will be separated, which differs from what you see in your design file on your screen.
Send us the file, and we will check it first.
We conduct a comprehensive preflight check on every job before providing a start time. We inform you of what we find rather than quietly making fixes. This distinction is important because an automatic fix can be incorrect. For example, if a panel is intentionally set to overprint, silently switching it off alters your design.
When we identify an issue, we specify which object, which page, and what the printed result would be. This allows you to decide on the next steps. If you prefer that we correct the issue, we will do so and send a revised proof showing the change. Most of these issues can be resolved quickly at the file stage, but they become much more costly once the run is on the machine.
You can upload artwork through our ordering process and see the options under printing services, or ask us to look at a file before you commit to a run. If a job is time-critical, see express and fast printing, and get in touch through the contact page.
The short version
Transparency effects are live instructions that must be flattened before printing. Flattening in the wrong place can cause pale boxes and white stitching lines. Overprint prints one ink over another, which is suitable for black text but not for white objects, which can vanish entirely. To ensure proper printing, turn on overprint preview and separation preview on your exported PDF. Check the edges of the objects at high magnification.
If you do only one thing from this list, turn on overprint preview. It takes just a second and it shows you the file that the printer will see.
