The Blueair Blue Pure 311 Auto air purifier requires a plastic cage to support the filter. Theoretically, if you accidentally throw away the plastic cage when you change the filter, you will be unable to use the air purifier. Theoretically, if you have a friend with a 3D printer, you could ask them to design and print a new cage for you. Theoretically, this actually happened, and I was the friend.

I designed the cage to closely mimic the original, although I used circular holes instead of hexagonal to eliminate the need for most support material.

I tried printing just a short end-cap for each end of the filter to save time and plastic, but it turned out that it really did need the full structure to stop it from collapsing once the purifier is running.

It prints in two parts, but they don’t need to be glued together; the friction against the filter is enough to hold them in place.

The OpenSCAD and STL files are available on GitHub.