1. Low contrast between colors
The number one cause. If the base and the relief have similar brightness, the phone can't separate them. Fix: reprint with a light and a dark color. If the part is already printed, coloring the top of the relief with a black permanent marker sometimes does the trick.
2. Modules too small
If the little squares are under 1 mm, the nozzle rounds them off and they merge. Fix: make the part bigger or shorten the link so the code has fewer modules. See minimum QR size for 3D printing.
3. Missing quiet zone
The code needs a plain border of about 4 modules. If the relief runs to the edge of the part or something sits right next to it, the reader can't find the code. Fix: leave more base around it.
4. Glare and shine
Silk or metallic filaments reflect light. Fix: scan without flash, tilting the phone a little, or reprint the relief in a matte color.
5. Squashed first layer (elephant's foot)
If the first layer is too squished, the base color bleeds at the edges and the modules deform. It matters less on a QR printed face up, but shows on small parts. Fix: recalibrate your first-layer height.
6. Tall relief with side light
A relief several millimeters tall casts shadows when light comes from the side, and the reader sees "ghost modules". Fix: keep the relief at 0.6–1 mm and use good front light.
Before printing, scan the generator preview straight off your computer screen. If it's already hard there, the printed part will be harder: increase size or contrast.
Final check
- Try two different phones (the stock camera app often reads better than some apps).
- Scan from different distances: very close sometimes won't focus.
- Try natural light and indoor light.
Frequently asked questions
Why won't my 3D-printed QR code scan?
Almost always because of low contrast between colors, modules that are too small, no quiet zone around the code, or glare from shiny filament.
Can I fix it without reprinting?
Sometimes: darkening the relief with a permanent marker, or scanning without flash and with front light, can be enough.