Bowden materials/multiple print heads

From what I hear, the PTFE tubing in a bowden extruder is bit springy and it creates more of an ooze control problem. I was wondering if anyone has tried one with a braided steel cover like these:
[www.summitracing.com]

These cables are designed for NOS plumbing, but they are Teflon lined. If the lining is very thin, it will probably rub through relatively quick. I don't see a spec for the Teflon thickness nor do I really know off the top of my head what thickness people use in bowden extruders. They include the fittings though, which seems more handy/stronger than trying to thread a nut on the PTFE tube. I'm thinking the steel mesh would help confine the PTFE to a specific length. I know the mesh will have a tendency to stretch a little as well, but does the combination stretch less than just a plain PTFE tube?

There has also been a lot of talk about using bowden extruders in a multiple print head setup. I'm wondering if anyone has an actual working prototype yet. I have seen that bits from bytes has a three head system, but it looks like they just use three complete print heads on a single carriage. That could be quite a lot of weight to sling around for little machines like these.

I'm trying to design a two head system for starters with the long-term goal of having six. I'm expecting problems with filament cables/heater wires and ooze primarily. I have yet to build my reprap, so I don't really know how much space I've got to work with either. Six heads might occupy too much space. I'm also thinking I need a way to adjust the print head alignment. The opening on the two nozzles I got are slightly off center, so if I don't mechanically adjust for it, then I have to somehow setup tool profiles to adjust the Gcode. Otherwise, the end of one thread will never line up with the start of the next after a tool change. I'm hoping someone has already solved at least some of these problems.

Someone mentioned using a configuration like the lenses on a microscope. Out of all the ideas I have seen, this seems like the best setup for a 4+ head system, so that is what my mental sketch-ups have centered around. The idea of rotating a barrel full of extruders sounds less complicated and less prone to failure than ejecting one tool and picking up another. Especially when you think about the wiring attached to the heater barrel. I already have plenty of ideas about hacking the firmware/hardware/software to run six heads, but that will all come after getting a two head system operational first. One step at a time.

I thought the ooze problem came from the force applied to the filament itself, and not the bowden cable? As in the principle of pushing a cable instead of pulling it. I may, of course, be very wrong.

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-Nudel
Blog with RepRap Comic

I hadn't considered the effect of compressing the filament itself. I'll have to take a look at the compressive properties of abs/pla and see how that compares to the tensile properties of PTFE. Depending on which deforms more, especially if there is a significant difference, changing the cable could be pointless. I was thinking the problem was more to do with the filament trying to push the two ends of the cable in opposite directions and stretching it out. I was thinking if the cable couldn't stretch as much, you wouldn't have to pull back on the filament as far to take the pressure off.

I don't want to complicate the design any more than it already will be, but in the end it looks like an active cut-off valve is the only sure way to control ooze. Maybe if I try to construct the hot end so that it comes up to temperature as quick as physically possible I could completely shut off the inactive print heads. If I re-heat them a layer or two in advance of needing them, maybe they could heat up in time to be usable? That would at least stop the oozing on inactive print heads.

I don't see what the test conditions were and my strength of materials knowledge is quite rusty, but it appears that PTFE deforms more than ABS. I have been comparing the tables at [www.matbase.com] and [www.matbase.com].

ABS reportedly has a compressive strength of 60 - 86Mpa but PTFE has a tensile strength of just 25 - 36Mpa. Likewise they show that PTFE has a drastically higher percentage of elongation. They show ABS at 5 - 25% and PTFE at 350 - 550% Judging by the tensile strength of the two materials, I don't think the difference in elongation could be accounted for by ABS simply failing before the PTFE sample either.

So, even though you can't compensate for the ABS deforming under compression, it might make a world of a difference to stop the PTFE from deforming. I'm not sure what the difference in results might be with PLA. The data at [www.matbase.com] covers a pretty wide range. It looks like some samples that were tested more closely matched PTFE while other samples more closely matched ABS.