Artwork Bleed

This question about artwork bleed keeps coming up, so I thought I would address it.

When a printer says that they need artwork bleed, or just bleed, what does that mean?

Artwork bleed is required when a design has colour/ink that goes right to the very edge of the sheet. This ink could be a photo, or it could just be a colour panel, it doesn’t matter.

Printing is ultimately a mechanical process, and the machines need an area on the edge of the sheet that they can grip without damaging their output. The same as if you painted a sheet of wood on one side, and had to pick it up. You’d pick up the wood very carefully, touching only parts of it with no wet paint on it. Printing machines are the same.
The way that professional printers are able to supply you with an a4 sheet with ink right to the edges is very unremarkable. My aunt once asked me how it was done. She said “your print machines must be amazing, how do you get ink right to the edge? None of the desktop printers I’ve ever used have been able to do it.”
The secret is, we don’t print to the edge, we print onto bigger pieces of paper, and then cut it down. My aunt was so deflated.

The printing trade uses super sizes with codes like, SRA3, SRA4 etc, as opposed to A3 & A4.
SRA3: 320x450mm
A3: 297x420mm
I use SRA3 every day, and looking at those dimensions you can see how it is done. The super sizes are about 7% bigger than the standard sizes. This provides room for bleed and machine grip.

Artwork with bleed will look like this. Note the crop marks in the corners, this tells the guillotine operator where to cut. I’ve overlaid green lines on the right to show where the print will be trimmed.

These are photos of the artwork printed. You can see around the edge of the sheet there is a lot of white. This is the machines grip area, it’s about 10mm around the edge, then you see the artwork begin. The final trimmed sheet has ink right to the edges.

But why do I need it?
Artwork bleed gives printers a margin of error. The machines are not infallible and neither are the operators. There are small variations throughout the process that result in a small amount of movement of the artwork on the sheet. Paper stock can be slightly different sizes, the paper skews as the machine applies the ink, printed stacks are not jogged 100% correctly. Any or all of these combine to create a small amount of movement of the ink on the sheet. If you consider perfect placement to be position 0, then the ink will move between -1 and +1 throughout the run. This is an industry standard. Technology and operators strive to minimise it, and some printing techniques are more prone to it that others, but it happens is just a fact.

Artwork bleed allows for this reality and gives printers a margin of error.

Standard bleed is 3-5mm depending on what you are doing. How much bleed you are advised to give, is a good guide for how big your internal margins should be, though I recommend adding 1mm. If your printer asks you for 3mm bleed, give yourself a 4mm internal margin. This if for the exact same reason as bleed is required. Consider it internal bleed.