In flexographic printing, we love creative label designs that layer images and color, but things get tricky when transparent images are placed over backgrounds. Transparent images won’t trap to other elements in your design.
If you’re a designer, you don’t need to worry about trapping files yourself, but setting up your artwork correctly can make the process smoother and ensure your final label prints beautifully.
Here’s why transparent images can cause problems and how to fix them before your file reaches prepress.
Why Transparent Images Are a Problem
Trapping is the process of overlapping colors slightly so there are no white gaps if the stock shifts during printing.
When an image has a transparent background, our trapping software doesn’t have a defined color to work with. The program doesn’t know exactly where one color ends and the next begins.
So, even if your design looks perfect on screen, the software can’t calculate how to trap those edges. The result? Fine white lines, small gaps, or color mismatches around your image.
How to Fix It: Convert the Image for Trapping
To make a transparent image work in flexo printing, we must convert it to a solid image with a clipping mask and a filled background.
Here’s how that process works:
Step 1: Create a Clipping Mask in Photoshop
- Open your image in Photoshop.
- Create a new Alpha Channel and select the area you want to keep.
- Delete that area from your alpha channel.
- Invert the selection (Command/Ctrl + I) to define the background area.
- Copy that mask (Command/Ctrl + A, then Copy).
Step 2: Paste into Illustrator and Trace
- Paste the shape into Illustrator and size it to match your image.
- Use Image Trace (set to Silhouette) to create a clean outline.
- Expand the traced object, bring it to the front, and make it a Compound Path (Command/Ctrl + 8).
- Select the image and path, then make a Clipping Mask (Command/Ctrl + 7).
Now you have a clean shape, but it’s still transparent.
Step 3: Add a Solid Background for Trapping
- Go back to Photoshop and reselect the area around your image.
- Contract the selection slightly (about 2 pixels depending on the resolution of your image).
- Invert your selection to select the transparent area.
- Use Content-Aware Fill to fill in that space.
- This creates a subtle, color-matched fill behind your image.
- Save and relink the updated file in Illustrator.
You’ll now have a filled image that looks the same visually, but under the hood, it has a solid background that the trapping program can recognize.
Why It Works
Adding this subtle fill gives the prepress software the correct information to overlap colors during trapping. The result is:
- Clean edges
- No white gaps
- Smooth transitions between image and background color
Even though you can’t see the difference in your design file, this adjustment makes all the difference when your label runs on press.
The Takeaway
If you’re designing for flexographic printing, avoid sending files that rely on transparent backgrounds. Instead, use clipping masks and solid fills to ensure your artwork traps correctly.
These small steps help ensure your labels print beautifully without visible edges or color separation.
Watch the video below for a full demonstration of this process, including creating the clipping mask, filling the background, and setting up your file for trapping.
At Rose City Label, we take pride in helping designers and brand owners create stunning, press-ready artwork. If you’re unsure how to prepare a file, our prepress team is here to help.