I e-mailed reprap.me and asked them what they where using for mixing.
Here is the reply, very intersting.
"Repetier firmware supports color mixing, (if you will call what the hotend does mixing)
Here's the programmers explanation, I will comment more below:
A combination of exactly one nozzle with inputs 2 - 6 filaments driven by
their own filament driver build a mixing extruder. It is not required that this
combination in deed mixes the colors. It could also be used to put only one
of the colors through the nozzle.
This combination gives us one real extruder that can in theory print many
different color combinations. Through the M163 command, it is possible to
change the mixing ratio at any time. Since slicers do not support this,
the firmware also introduces the concept of virtual extruders. The mixing
extruders can simulate 16 extruders (reachable over T0 until T15) with a preset
mixing ratio. The current mixing ratio can be stored as a virtual extruder
with the M164 command.
Set NUM_EXTRUDER to the number of filament drives. In addition set
MIXING_EXTRUDER to 1 in your Configuration.h. Now the steps per mm and
heater control settings are taken from extruder 0. The corresponding
settings in higher extruder numbers are ignored.
All N extruders get a weight Wx and steps are executed to preserve the weights
in optimum number. Therefore each extruder has a error counter Ex which is
initalized with Wx and we also Compute the sum S of all weights.
Then for each extrusion step:
Make step with highest value and subtract S from that value, then add Wx to Ex
Extruders with weight 0 are not considered.
The diamond hot end heats really fast due to it's low thermal mass. I used one successfully for ages on my rostock after I designed this as a proper fan cooled holder for it to also cool the PLA [www.thingiverse.com]
I changed though because the ceramic glue on the assembly made changing the nozzle a very risky process.
I now use a pretty standard heaterblock as sold by dx.com or robotdigg or geetech etc. My last order was from robotdigg, they have some crazy low pricing.
1.75mm is a more popular standard than 3mm or 2.85mm. I initially started with the diamondage filament but eventually switched to 1.75 because it was available from more places in better variety and pricing. Other than Diamond age, Ninjaflex also charges more by weight for 1.75mm but the vast majority of my printing is in PLA. Most places sell it for the same price by weight as 3mm filament.
steppersonline are great for steppermotor variety and pricing, I saw robotdigg is good for stepper motors too, good for lots of parts actually.
I have a printed nylon effector with a nylon cooling cowl and my hot end heat break stainless steel threaded tube screws into a custom aluminium water cooled block I made. This lets me print with pretty much any plastic but the biggest problem I have is thermal contraction as I don't have an enclosed chamber. taulman 910 specs look really good but I still haven't managed to get any yet.
It leaks like a sieve from the threads, once that runs down the hotend it catches on the print destroying it
I need to pull it apart and put some ptfe tape on the threads or some other method to stop the leaks.. Was also thinking of small soft copper washers.. but looks like I will have to make them.. probably going to do both... so its sealed on the base and the threads..
In the mean time I have had to put the normal hotend and extruder back on, so I could print some stuff.
I am a big fan of soft copper washers for keeping PLA in.
I am tempted to make up my own diamond hotend but with only two extruders so I don't need to update my electronics too. My main machine has a separate second extruder and I only ever use it for printing either one filament or the other. Two colours is just too much hassle with mediocre results due to one hot end dribbling while the other one prints. Having two extruders connected to one nozzle would solve that. I don't know how this would work with say ABS and PLA hooked up at the same time. I suspect that extruding ABS would cause too much heat to go into the PLA and cause it to swell and block or melt and dribble thru the ABS. Probably best to have similar materials in each feed.
My Prusa Mendel i2 inspired Repstrap with welded steel frame: [youtu.be]
And my Smartrap derived Briefcase 3D printer: [youtu.be]