I have been trying out a soft filament from Rubber from Plastink and have managed some successful prints with it. The main issue I’m facing is that the filament tends to catch inside the extruder where it starts curling and gets stuck. There is lot of space around hobbed bit and idler bearing that can probably be closed of to provide support through the feeder. When extruding at even slow pace with no retraction the results can be very good. The bowden didn’t seem to be too much of a problem. At least not with the retraction off.
Before I start messing with this myself I thought Id ask if anyone here has done such a modification to the extruder enclosure or know of a suitable replacement I can print and try out?
I had been wondering why it was unexpectedly hard to insert the filament and the lack of alignment between the holes and the pulley explains that. I suspect that it is to increase the grip surface with the pulley. That is probably not a bad thing in general but seems offers more opportunity for a soft filament to catch at the moment.
I went ahead yesterday and set up the original extruder setup in OpenSCAD to better analyze the options available.
I’ve been doing lots of flexible filament prints with Filaflex and the results are good. Yes, occasionally it does catch before entering the bowden tube and initially you do have to keep your eye on it.
I tried a few solutions (most recent is some very fine stainless steel tubing to drop to just above the point where the filament leaves the hobbed wheel, but as the holes aren’t aligned this hasn’t worked so far and I’m still messing about with it).
So far none worked any better than just slowing the print speed down everywhere to 15mm/s. Very rarely get any issues at this speed and while the quality isn’t the absolute highest, it’s fine for me. Here’s my attempt at a soft plastic fishing lure successfully printed with 0.5mm walls (grey colour done by colouring the first metre of filament with a black marker pen):
@freyzor - do you know if Rubber more flexible than filaflex?
@biscuitlad I’m afraid I don’t have any other filament to compare to. The plastink rubber bends well but does not seem to compress much or stretch. I printed a couple of timing belts meant for my robot wheel drives and the were very promising. But I’m less optimistic for printed tires as it is still quite slippery and the not very springy (lack of compression). I have yet to experiment with tires though.
I too printed on 15mm/s and when I got past the initial feed problems it worked quite well.
Update: I have tried a couple of new extruders. And am currently using http://www.thingiverse.com/thing:321523 which has very solid filament support and uses an Mk7 drive gear. I added a quick release lever and so far I have pretty good results. No snagging with this one.
http://www.thingiverse.com/make:137932
I have noticed issues when loading that if there is left over plastic in the tube when loading the rubber filament, it can jam and never reach tip to make the filament melt. This is when the curling mostly happens for me. Loading and unloading some other (PLA) filament has resolved the issue and cleared the tube. After which the soft filament will load more easily.
Did you have to order the mk 7 drive wheel? It’s not marked which one comes with the Vertex.
I ordered the mk7 online some years back for a printer project that got nowhere 
The mk7 is smaller in diameter (13mm) than the vertex driver (16mm) and can drive more force through. I’d consider getting an mk8 (11mm) if I where ordering a new one as it should provide even greater driving force. The extruder can easily be modded to use a mk8 by adjusting the gear_dia variable in the scad script.
Remember to recalibrate your feed rate. https://www.youtube.com/watch?v=YUPfBJz3I6Y
You can set the firmware values directly from Repetier-Host (config > Firmeware EEPROM Configuration) and feed manually via command line by clicking the easy mode. You can also toggle the log to show the GCODE communications.
Note that the commands need spaces in Repetier-Host
resetting the extruder position to zero
G92 E0
moving extruder 100mm at feed rate of 60
G1 E100 F60
Also the hotend needs to be turned ON for the extruder to push due to safety as the log will tell you if you try.
Started 2 weeks ago with this litle Project based a litle on varius other feeders out there along with some minor ideas of my own. I have one problem i Think and that is i need to solv a 3rd bolt Point for it to be strong enouph to hold it together on the top side. Atm it is only holding itself together with the 2 bottom bolts. It might not need more then the 2 bottom bolts but i havent even tried Printing it yet so have no idea. Anyone have any ideas for improvements on this one are more then welcome to give a litle feedback Before i start taking upp this cad drawing again for Another revision.
As you can see it is like the feeder we have today a 2 part solution. The dif is that the feeder handle is switched around to be push down instead of push up. The filament is aligned strait. There is cone based entry Points for the filament and not much free air for the filament to travel thru.
Was considering building in a filament filter into it down the road. Could be useful maybe.
THIS IS A WORK IN PROGRESS!!! so have no idea what this thing can do or cant do. But i like learning things and trying things out.
You where correct slowfoot 
Managed to get it on the printer today. Looks solid enouph and is alot simpler to load the filament in.
The print quality on that part is awesome :o I’m not even close.
I have modified the firmware with a couple OF tweeks here and there and modified the printer itself with some things. Right OF the box i had alot OF issues with My prints also.
wow alot … most of it is firmware related … Remade the load and unload functions. was just stupid having a function for every filament type and every extruder. so i smashed them together to 1 function. Have done that with varius other things. Adding custom settings for filament support that isnt normaly supported in the printers menues. Cutting code making the firmware smaller. Basecly removing useless crap. Have changed some calc functions on how they did stepping calculation. Speed tweeking. Did also some lcd Changes. Changed varius default values. Put in my own splash screen and so on and on.
Might release the firmware at some Point as a sidetrack to vellemans versions. But atm im searching for a better pully solution where the belts dont have the chance to eat up each other. If i cant find any rdy made ill end up designing and making them myself i gues 
The feeder you have noticed i gues. Atm making a new revision on it with minor tweeks. Will probably release it soon for anyone else to be able to play with it.
But have minor irritating issues with some other things that bugs me. That need to be solved. Starting to Think that i have to many things going on at the same time
Nice! Do you have a programming background or so? Look forward to your version then 
If you need help designing the pulleys or so, I can always help? Maybe there’s another way I can contact you?
Andreas : I do have a developer bakground both in programing and hardware. Now days i work more as a Solutions Architect but i do code minor things stil when ever i feel like it need to keep the knowledge alive in an ageing brain.
Doesnt look like there exist the type of pulley i would have liked for this. Have searched a couple of Days now. Smells like we have to draw this one our self.
I uploaded the stl files for the feeders on thingiverse if someone want to try them out
http://www.thingiverse.com/thing:967475
The mk7 is smaller in diameter (13mm) than the vertex driver (16mm) and can drive more force through. I’d consider getting an mk8 (11mm) if I where ordering a new one as it should provide even greater driving force. The extruder can easily be modded to use a mk8 by adjusting the gear_dia variable in the scad script.
Managed to get it on the printer today. Looks solid enouph and is alot simpler to load the filament in.
The feeder you have noticed i gues. Atm making a new revision on it with minor tweeks. Will probably release it soon for anyone else to be able to play with it.