Using NinjaFlex, the Rubber-like-filament from FennerDrives (found at Creative Tools).

3D Printing
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Using NinjaFlex, the Rubber-like-filament from FennerDrives (found at Creative Tools). Prints excellent on any normal FFF/FDM printer.

Just imagine what rubberish thing you will be able to print. . .
https://www.youtube.com/watch?v=U_RZp7DXMT0