RepRap 3D Printer Color Mixing Nozzle by James Corbett

If you’ve done any desktop 3D printing and, lets be honest, who hasn’t, right, then you’re aware of the limitation imposed on your creativity by the color of the plastic you’re printing in. Makerbot has sort of addressed the issue with their dual extruder, but it’s really only a tease. Being able to switch from one color of plastic to another only highlights the fact that you can’t print in any of the colors in between.

At first people tried to solve the problem by coloring white ABS filament with markers just before it entered the extruder. This approach worked. For example, the Filament Colorizer by cyclone holds two sharpies and makes your prints new colors. Dry erase markers also work, as demonstrated by scocioba. Also RyGuy. As an alternative, James Corbett has developed an extruder that mechanically mixes different colors of plastic. The idea is that you could have a roll of plastic in basic colors (like CMYK and black) that you just push into the mixing chamber in different proportions to create the full color wheel.

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Testing the marker thing was pretty straight forward. People just rubbed a marker on the plastic filament and it did pretty much what you’d expect. Testing the mechanical mixer was an entirely different story. Corbett went through several passive and active designs before he found one that successfully mixed the plastic. Follow the jump to read the conclusion and way ahead from his paper on the subject.

8. Project Conclusions
Initial trials showed that the PLA plastic would mix to produce the expected colours (similar to paints) and the relative strengths of the pigments in each filament were approximately determined to be in the following order (strongest first); black, red, electric blue, yellow then white. These colours were selected as they are the closest representation to CMYK widely available.

This project showed that the use of a static mixing nozzle was Impractical for this home 3D printer application due to the very high pressures required to drive the filament which led to the drive mechanism stalling and slipping on the filament. Also the tests ran showed no promising mixing results that deemed this method of inducing mixing impracticable to pursue with development.

This led to the design of an active mixing system which consisted of a hexagonal bar rotating within a cylinder 0.3mm larger diameter than that across the points on the bar, this was driven by a 12V DC motor at a gear ratio of 200:1 and a turning speed of approximately 80rpm (max.). The results appeared to be homogeneously mixed with no visible streaks in any of the prints produced, the nozzle was tested briefly with various combinations of two filaments to produce a wider variety of colours.
Following these results a more practical configuration of the nozzle was fabricated to simplify the design and eliminate initial minimal leaking problems, the new nozzle performed faultlessly even over a longer print until the driven speed of the filaments were altered to give different feed ratios. The slowing of one filament led to a build up of heat and pressure in the end of the Bowden tubes which led to rupturing, this was temporarily solved using a PEEK sleeve but will require further development to resolve the issue.

While investigating the effect of varying the ratio of filament inputs it was made clear how much stronger the black filament than white, the ratio of white to black had to be 30/40% before any noticeable difference in the colour can be seen to the eye and to make a significant lightening to the black at least 80% white needs to be used. This would make calibration very difficult as the colour printed is very dependent on the black filament and
any varying in the drive could cause large fluctuations in the colour printed, therefore the black filament driver could be mounted closer (on the X-carriage) to increase the control of the filament, or a weaker black filament could be developed which would give more control of the colour as a greater change of ratio would be required to alter the colour. Unfortunately time did not allow for detailed testing of colour proportion mixing and
alternating filament feeds to determine the purge volume of the nozzle and printability which would lead into the investigation of the possibility of printing colour gradients. The next stages of the development are outlined in the following section.

9. Further Work
The design of the nozzle leaves much to be desired with further development, firstly the length of the mixer should be shortened in steps and subsequently tested at each length, this will help determine the minimum length of mixing insert that can be used to produce satisfactory mixing results. Following the optimisation of the length of the insert the original nozzle can be shortened to the equivalent length of the insert, with the internal design of
the nozzle modified any excess material of the nozzle can be milled off. This modification will reduce the thermal mass and the volume of plastic of the nozzle to a minimum which will increase the thermal control and printability of the nozzle.
As mentioned before the ballooning effect of the PTFE feed tubes needs to be addressed possibly by using a stainless steel barrel connected to a heat sink being actively cooled by a fan. This will require some careful design and manufacture to incorporate all the feed tubes into one heat sink to result in the need for just a single fan, also the flow of air around the nozzle will require special consideration to minimise the cooling of the nozzle which already has compromised heating performance due to the large thermal mass.

With the previous modification in place the printability of the nozzle can be tested, meaning the transition volume of plastic required to be extruded to alter between two mix ratios. This can be tested by printing the Colour Transition Ladder G-code and using the method found in Appendix J. The nozzle should be initially purged with black then the print should be started and only white filament being driven. The colour of the extruded filament
should then be analysed to test at what stage of the print the colour has reached a consistent composition. This will enable the determination of how much plastic needs to be purged during a print to create a sharp colour transition.
Firmware and electrical modifications require the most development to accommodate the new nozzle as they both need to incorporate the ability to independently control a minimum of 5 separate filament drives (Black, White, Cyan, Magenta and Yellow) this may be increased to also utilise colourless, water soluble or soft composition plastics. The electronics and software development were far beyond the scope of this project but once
these modifications are in place it will enable the more comprehensive testing and calibration of the nozzle drives. The control of the ratios of the filament drives would be controlled using a widespread map across the full available colour space that could be produced by the printer, this map would relate to CMYK percentage values as normal ink printing does.

Another feature that will be required of the firmware, will be a method to purge the nozzle with the correct printing colour before applying the colour to the print, there are a couple of theories to achieve this; the first is to print a block in tandem with the print which will be used to purge material on, this method will be somewhat wasteful with having to print this block for every print, also the size of this block will have to be increased for
multiple colour changes on the same layer. Another more economical solution would be to use the infill of a part to purge the plastic to the correct colour, and possible the inner boundary as the outer boundary will define the appearance of the object. This method will create much less waste although will not be as practical if parts of an object are desired to be clear as the infill will appear messy in terms of colour. Nor will this method be practical when determining the hardness of an object by using hard and soft plastics or by printing support material using water soluble material so a ‘purge print’ will have to be done alongside prints with these features.

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