Bleeds are the print that extends beyond the edge of the trim or fold lines of your packaging. Bleeds account for any slight movement in paper when it's being printed or cut, and therefore help minimize the likelihood of having any unprinted edges. It's important to have all designs at the edge of a cut or fold line 'bleed' over and into the next panel or empty space.
If the design of your packaging dieline includes full color anywhere there's a cut or fold line, please be sure to 'bleed' over into the empty space like in this example below.
Here's another example where bleeds have been taken into account. You'll see that in areas where there are colors or artwork at the edge of a cut line, the design extends beyond the dieline.
It's common for paper to shift slightly when it's printed or being cut - even 1 millimeter. Misalignment is much less obvious and practically not noticeable when there aren't any designs or artwork at the edge of a cut or fold line.
Even Apple has experienced the risk of having different colors at the edge of a fold line. In one of Apple's original rigid box packaging for their iPhones, the top lid of the box is black, and the sides of the box is white. Due to slight misalignment, you can see that this caused a shift in their artwork, which was right at the edge of the fold line. The result is an small yet obvious empty white line on the left side of the box.
Take a look at the dieline design below. Let's say the paper were to shift even 1mm downwards. Since all colors are right to the edge of the fold line, what will happen is: the yellow side of the box will 'bleed' into the light blue base of the box, and the light blue will 'bleed' into the pink side, the pink side will 'bleed' into the white portion of the lid, and the colorful patterns will 'bleed' into the front yellow portion. And if you have print on both sides of your box, it becomes even trickier to maintain this alignment.
For this reason, we usually recommend clients limit the number of sides of the box that have artwork right to the edge of a cut/fold line.
Ensure all artwork at the edge of a trim or fold line go beyond the line. Here are a few examples of folding carton boxes where bleeds have been accounted for. You can see that the artwork on the sides of the box bleed into the top or bottom flaps to ensure that the sides won't have any unprinted edges.
For more best practices on your packaging designs, check out our dieline design tips here.