Update May 31 2019
The printed elastic hinge as function at the front plate, instead of a spring loaded filament pressure arm, works great but … after a few months the pressure becomes less and the knurled filament wheel starts to slip. I did consider another material, for example nylon, but still the risk of failing is to high.
A more robust solution of an elastic hinge is on his way. After testing I will publish :-)
For the rest I'm super happy with this dual extruder.

The development spec for this dual extruder was simple;

• must be under 250 grams
• universal applicable (Delta & the rest)
• minimum amount of parts (Latest revision = Housing front without lever and spring)
• no reaction forces to the outside

Extrusion force measurements by Mark Hywood did show that the feeding force for filament with the use of a 0.4 nozzle is anywhere between 12 and 25 N.
https://airtripper.com/1338/airtripper-extruder-filament-force-sensor-introduction/

A direct driven Nema 17 42mm stepper motor extruder can provides a brute force of 118 N (max) and enables a very simple extruder design. Designing a floating dual extruder with the same Nema 17 motors for a Delta printer, gives you a total floating mass of close to 900 grams, almost 1 Kg.
With extrusion wheel d=10 and 200 steps/rev, the mechanism extrudes 0.157 mm filament per step. (can be redused by 1/4, 1/16, 1/32 micro stepping)

If we want to reduce the weight of the floating mass, the use of a Nema 11 32 stepper motor with a gearbox 1:3 can be considered. A max filament feeding force of 50N can be achieved.
The gearbox housing can be 3d printed out of nGEN or ABS. (Spur gears material: Brass)
Conditions: Nema 11 32; extruder wheel contact d=7; i=2.9; ==> F2=49.7 N
Total estimated mass: 0.460 Kg (E3D dual nozzle, Cooler fan and plate not included)
Total realized mass: 0.480 Kg (E3D dual nozzle, Cooler fan and plate not included)
With extrusion wheel d=7 and 200 steps/rev, the mechanism extrudes 0.038 mm filament per step. (can be reduced by 1/4, 1/16, 1/32 micro stepping)
A higher stepping rate but a four times more presice extrusion will be a not unimportant side result.

I added HOUSING_FRONT_rev4 (LEFT and RIGHT). This part eliminates the lever and the spring because the pressure bearing holder acts as a elastic hinge. (Works great)
I now added "auto lift" hot ends to this design based on an idea launged 4 years ago in the RepRap comunity. I have no clue why this idea never took off.
If somebody has experiences with it, please share. (Se first 3 pictures)
Have fun.
Video: https://youtu.be/FTR852kITME

R9 home designed and build

No

No

0.2

30% to 45%

Material used nGEN from Colorfabb
Do not exceed recommended speed
The Housing Front (L+R) to be printed with 3x circumference shell, 0,2mm layer height and 45% infill.
Break loose the elastic hinge if stick's to the surface. Play with shell thickness and infill in order to get the right tension on your filament.

I'm using Alibre already for many years and it is one of my favorite CAD systems.
The 3D design tool as well as the 2D drawing tools are very down to earth.
Eport files to other CAD file formats as well as to .obj files or stl files is easy to do.