Thursday, April 19, 2012


Colour mixing

Here is Myles's latest mixer extruder.  On top is a 12V geared-down DC motor turning a vertical PEEK drive shaft.  The brass hot end is shrouded in insulating glassfibre.  Inside it is a cylindrical cavity with a length of brass hex rod that is turned by the motor.  This is the stirrer.  It is sealed by a silicone O ring.  Bowden feeds are routed in at the sides near the top of the hex rod.

The hex rod is about 0.2mm less across the corners than the diameter of the cylinder in which it turns.

And here is a much clearer picture of the results with the input colours either side:

This is with no stirrer control - it is simply on all the time the end is hot.  We think we will get  less dribble if we turn it off when no extrusion is required.

The obvious way to use the device will be to have a purge bucket (maybe with a nozzle wiper) and to set a new colour then run the device over that till the colour runs true.  Slightly wasteful, but the purge volume shouldn't be more than the volume of the void in the device, which is not great.

Rather than purging into a bucket, which would tend to need a nozzle wipe afterwards, perhaps it is better to build a dummy object during the purge. E.g. a cube where one layer is the correct volume for the most purges on a layer.
using the control software to designated an object on the plate for color purging would be a good feature
What about writing the software to do the part infill using the purge plastic? That way on bigish parts there would be no waste. Assuming the infill volume was greater than the purge volume.
What if you could also specify an actual second part to print, that's just "ugly" and made up of purges plus whatever other plastic was needed to make the part.

Imagine someone buying pretty RepRap parts from a seller (with multicolor logos etc), and during that same print the seller also made an 'ugly' set of equally functional parts, but they sell for a bargain discount price.

(If you don't specify an ugly part to print, it could always default back to a cube, as nophead suggests)

Maybe what I just said is exactly what jaf meant. :) If so, sorry.

Patrick, that idea is great. What you'd probably also want is some idea of whether certain plastics or mixes of plastic are transparent or opaque, so you could 'hide' the purge inside a section whose shell is opaque.
Yes - I like the idea of hiding the purge in the infill too. That's very neat.
Though thinking about it, one of the places where you are most likely to want multiple colours is on the top of a flat object. There you would have no infill to purge into, so an alternative arrangement would be needed for that case come what may.
As most objects are built with 25% or less infill so you may still have room to fit (squeeze) in some extra purged plastic in the object even if you are about to do a solid infill layer.
I bet with 3 solid layers the colour change is clean by the third on many/most objects.
Could you use a IR LED & sensor to auto detect the color is true & automate the process?
If you were to purge into a bucket of cold water, couldn't you just make a filament of the new color? That could then me fed back into the head at a later time for recycling. I'm thinking to pre-fill the mixer cavity for the next time you use that color.
a)vary the input transition speed, i.e. as you start to switch to 50/50 the ratio goes from 100/0 to 0/100 briefly then 25/75 then stabilizes at 50/50 to drive the ratio in the mixing chamber more rapidly, and maintain it.

b) as mentioned mixing areas for infill. where you need to bleed extra until you get a clean color there could be a setting for increase infill density to max.

vary where/how infill is applied and use it to eat up the difference. if you finish layer 3 perimeter in color 1, infill layer 3 starting the mix, infill layer 4 to stabilize the mix, and then perimeter layer 4 with the next color you've gained a LOT of plastic to blend.

a color specification parameter that will allow a % precision for transition. e.g. i want layers 1-4 to be 100% blue, and layer 5+ to be 50% blue and 50% white, but i'm ok with layer 4 shifting between colors so utilize that layer for the transition.

c)print small bits in a easy to break apart form that can be remelted for non color specific prints/infill feed filament
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