The “stripey toothpaste” effect of the Diamond hot end.

As readers of my bog will know, I have been using a Diamond hot end for quite some time. One of the issues with this hot end is that it doesn’t actually mix colours all that well. What happens is that the filament exits the nozzle somewhat like stripey toothpaste. The featured image above shows how this manifests itself. This was a simple hollow cylindrical container that I created by using a custom script to alter the mixing ratios in 1% increments at every layer change. So, the colours “fade” from 100% Red and 0% Blue to 0%Red and 100% Blue then from 100%Blue and 0% Yellow to 100% Yellow and 0%Blue and finally from 100%Yellow and 0%Red back to 100% Red and 0% Yellow.

The three pictures above are the same object viewed from different angles. The one on the left is roughly at the angle where the Blue filament enters the hot end, the one in the middle is roughly where the Yellow enters and the one on the right is roughly where the Red enters. You can see that the colour is biased towards the inlet position in the hot end for that particular filament.

The effect can be largely negated by using translucent filaments.

Or, it can be exploited. To explore this a bit further, I created a simple pyramid and orientated it on the build plate such that each face was in line with each of the filament inlet positions. I defined a single tool with mixing ratio of equal proportions for each of the 3 filaments. In this case, the filaments that happened to be loaded at the time were Red, Black and Gold. So the tool was using roughly 1/3rd of each colour. Then I printed the object just as one would using a “normal” hot end and took three pictures, each at 120 degrees around the object. Here they are:

blackredsmall

redgoldsmall

goldblacksmall

As you can see, each face is a different colour although they were all printed at the same time, using the same tool with no fancy tool changes or post processing. The top picture shows mostly Black on the left and mostly Red on the right. The next picture down with the object turned 120 degrees shows mostly Red now on the left and mostly Gold on the right. The last picture is with object turned another 120 degrees so the mostly Gold is now on the left and mostly Black is on the right. I say “mostly” because there is some mixing of the colours on each face but they are strongly biased towards the position where each filament enters the hot end.

I just wanted to show this as an illustration that sometimes what may be seen as defective or unwanted behaviour, can be exploited to ones advantage. (That is of course if one has a need for a 3 sided object with each face being a different colour).

Now on to the next picture and one that had me baffled for some time.

colourshiftsmall

I was printing something else when I came across this issue (always an issue – never a problem). So I made a simple rectangle with two holes in it, so that I could evaluate what was happening. As you can see, there is a distinct colour shift. Some sections are darker than others (or lighter depending on your point of view). It was printed mixing two filaments, Blue and White in equal proportions. It is consistent and reproducible. It will happen with any combination of extruders. i.e. extruders 0 and 1, extruders 1 and 2 or extrudes 0 and 2. There is no change in layer height or any sign of over or under extrusion. The extruder gears all appear to rotate at the same speed.

After a lot of head scratching and a lot of testing, I finally realised what was causing it. When it is printing, the infill is at 45 degrees and the head starts just below the left hand hole and moves diagonally in the direction of top left to bottom right whiles traversing toward the right hand side. This is the dark section. So it goes all around the bottom of the hole, then it reaches the right hand side of the hole after which it goes all the way to the top of the print. Then it reaches the right hand side of the right hand hole, and goes around it.

So for all of these moves, each new line of filament (apart from the first line) is supported by or squashed up against, the previous line on the left but has free space on the right.

Once the print head reaches the top right corner of the print, it then moves to the nearest unprinted section which is the top of the right hand hole, nearest to the right hand corner. To continue the print, the head still moves in the same direction diagonally but this time it is traversing from right to left instead of from left to right.

So now, for all these moves each new line of filament is supported by or squashed up against, the previous line on the right but has free space on the left.

Going back to what we now know about the filament coming out as stripey toothpaste, it is reasonable to deduce that the colour of the “mixed” filament changes depending on whether the filament is supported and/or squashed on the left with free space on the right compared to being supported on the right with free space on the left. To put it another way, it seems that the newly extruded filament rolls or twists either towards or away from the previous line of filament thus giving a predominance of one colour over another depending on the direction that the head is traversing.

At least that is my theory. It explains what I have observed happening but like a lot of theories that come up in the world of 3D printing, it isn’t actually proven. The only way to do that would be to put the prints under a microscope.

Looks like that might have to be something to add to my tool box…….watch this space……..

Ian

 

 

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2 thoughts on “The “stripey toothpaste” effect of the Diamond hot end.

  1. After reading your initial comments on this subject at the DuetWiFi forum, I gave some though to the design of a true mixing hot end. I created a cad drawing that uses a four filament design similar to the Diamond but employs a slightly larger central mixing chamber where a shaft driven mixing blade would blend the various colors together before extruding into the print. The shaft would be motor driven from above. Although I see this technically doable, I also see it to be problematic in practical terms considering the larger volume that would need to be passed from the hot end at every color change. Also I doubt we’d be able to see mixed color clarity on the order of what we see in CMYK color printing. Your thoughts Ian…

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    1. Hi Terry,
      Nice idea but you are not the first to think of this, see here http://forums.reprap.org/read.php?2,721308.
      Personally, I think it may be possible to achieve true mixing passively and thus save a lot of the complexity. I’m thinking along the lines of chemical anchors and such like that are used in the building industry. Basically it’s a sort of 2 part epoxy that is kept separately in a walled tube but has a nozzle that makes the two compounds spiral together as they travel down the nozzle. Of course, anything that increases the size of the mixing chamber will increase the purge requirement.

      As for true colour mixing my thoughts are that we need 6 inputs. CYMK is 4 but then we aren’t printing on white paper so we need white as well. That takes care of the hue (assuming we could get filaments that are true CYMK colours). My understanding of how inkjet printers works is that colour saturation is taken care of by altering the dot density. With 3d printing we can’t under extrude so the sum of all the filaments must always add up to 100%. Therefore to control colour saturation, we’d need to use transparent filament as well (to simulate dot density). Hence my reasoning that we’d need 6 inputs. I could be wrong on this and CYMK plus White might suffice.
      Ian

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