These are fan blades to replace broken blades on the Noctua A-Series NF-A4x10 FLX 40mm. However, these should also work on most 40mm fans with minor changes to the inner diameter opening.

40mm fans tend to be preferred for a lot of 3D printer cooling fans. Due to their small size, and exposure to heat, the blades tend to become brittle and break off. To use these replacements, trim all remaining blades off the inner spindle. Make sure to use something like a utility knife to remove all remaining blade material. Once the spindle has been reduced to a cylinder, just press the printed blades over the spindle. They should be a tight fit. If needed, use a bit of super glue to affix the blades.

You may have to tinker with your printer settings to get this to come out right without support material. If you do have to use support material, make sure you sand the blades after removing the supports. Otherwise the leftover material will negatively impact the fan's performance.