I talked with some manufacturers for the 3D printing rubber shoe soles. They are saying the end material is still rubber-like as the base which normally are made up of A & B two parts ( one is the rubber, anther is the hardner). To implement 3D printing into it, companies mixed UV-sensitive resin with the AB material and use 3D printers to form up the shape of the sole.
However, the pour time for these AB rubber is like a few hours before it becomes hard. In order to finish a sole in such a short time, you need a high-speed 3D printer which later comes the Carbon 3D the high-speed DLP machine perfectly fits for this way.
Also, you need post-process to hard the sole. That’s why the newly printed sole has to be put into a heated oven to further cure the resin and also hard the material. This gives the sole as a final using product.
Apparently, Formlabs doesn’t have the speed. Based on my understanding, that’s why they didn’t do a full sole instead of only a small section. Also, they still haven’t released the material to the public. I guess it is a mixed material with other rubber/silicone which requires a lot of post finishing that normal person can’t achieve by just load and print.