Over the months I have been searching for a rubber filament, without much luck. I had tried a couple but they proved very difficult to extrude due to being too soft.
I recently got some soft PLA to try, and to my delight it printed quite well on the UP and Deltasine R3dback printer. The trick I found on the UP was to bump up the temperature to about 250C. At normal PLA temps the filament would buckle and stop extruding within minutes.
I didn't hold much hope of extruding it on the bowden system of the delta either, but found with the same temps and extruding nice and slow ( ~15mm/s) I could get consistent extrusion. Although the retracts need to be dialled back too.
Once extruded the end result is quite flexible, but not overly stretchy like rubber would be, but more like really flexible plastic. Its quite like the material my iphone cover is made from. I will hopefully post some pictures of an iphone case printed in soft PLA soon.
So, without further ado here are a few pictures of the Pink panther women (http://www.thingiverse.com/thing:1216) , printed in black soft PLA on the UP at .2mm. Printed as shell to maintain flexibility.
Soft PLA is available for sale in black and white in both 3mm and 1.75mm at