Old rubber typewriter feet turn rock hard, or become a sticky mess. They don't make your machine stay at the same place while typing. Keeping these feet will result in the machine sliding all over the place. So to make sure that the machine doesn't move, and/or scratch your typing surface, it is advisable to install new feet. These feet are all printed with Shore A70 TPU. This means that they have the same properties and flexibility as real rubber.

Shore A70 TPU typewriter feet with 100% infil density
Replacement typewriter feet flexibility

Here are a couple examples of the Shore A70 feet. As can be seen, they're completely solid, which costs more material and print time, but this means that they are the same as real rubber feet!

Some time ago, I found a great way to make the feet look even more like real rubber!

Shore A70 TPU typewriter feet with 100% infil density
Shore A70 TPU typewriter feet with 100% infil density
Shore A70 TPU typewriter feet with 100% infil density

It is possible to buff out the print lines with a large buffing wheel or small nylon brushes on a rotary tool. Note that you need to keep moving the printed part to not melt too much and damage the part