I use Jacquard Acid Dye
by Dharma Trading. Note to soak the pieces in clean water (I use tap water) for a while before dying, otherwise the dye won't hold. I have tried soapy water, but that does not make any difference, other than the effort to get all the soap out of the part again, so I do not recommend it. I have tried adding salt to the dye, again with not difference other than the need for extra washing to get the salt out of the model (which crystallizes at the surface of the pieces and looks bad).
See the picture below for color reference.
-Red: 609 Bright Scarlet
-Yellow: 602 Bright Yellow
-Green: 629 Emerald
-Dark blue: 621 Sky Blue (Primary)
-Light blue: 624 Turquoise (Primary)
-Pink: 608 Pink (Primary)
-Orange: 605 Pumpkin Orange
Do not forget to add white vinegar, following the instructions. Despite the name "acid dye", you have to add the dye yourself.
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I have noticed no strong dependencies of the dye-to-water or dye-to-white-vinegar ratio. Only if there is too much dye that doesn't fully dissolve, then the pieces get dye stains (which you can rinse off). Even with little dye, you get a full solid color, until you have used up most dye from the bath.
Each of the colors has its own idiosyncrasies.
Bright Yellow, Emerald and Turquoise "bleed". The come nicely-looking out of the dye bath, but then start to discolor. This is caused by the dye being made of multiple components, and because of the trapped powder (and water) inside hollowed pieces. The different components travel at a different rate through the drying nylon in a process called chromatography
. I solve that by putting the pieces in the oven at 150 degrees Celcius to boil the water out. This process is not without risk. If you made your walls too thin and have trapped powder and water behind the wall, then the stream pressure could deform the wall. After that, rinse and possibly repeat.
Bright Scarlett has the property that it boils like milk, i.e. it immediately spills over the edge of your pan. I have had quite a few messy Bright Scarlett accidents. I tried 618 Fire Red, but 609 Bright Scarlet is better and brighter.
Bright Yellow spatters a lot. You need to keep a lid on that color, or otherwise your furnace becomes covered with yellow spatter. I have also tried 601 Sun Yellow (Primary), which does not bleed, but the resulting yellow is just a bit too pale to my taste.
I tried many greens, but only Emerald gives a good solid green result.
Sky Blue is my favorite color from a dyeing-process point of view. It always results in the same consistent color, also between dye batches.
Pumpkin Orange gets way to saturated if you dye for the recommended 20 minutes. I use 20 seconds (!) instead. Make sure that the dye is boiling when you dip, and get the pieces out in one go.
I have done some "undercooking" and "overcooking" experiments with Bright Yellow and Turquoise. 15 minutes is insufficient, you need the required 20 minutes. One the other hand, except it does not matter for any of the dyes if you use 20 minutes, 30 or more.
Do NOT use 639 Jet Black. It is no good, as it becomes a purplish gray. I use Dylon 8 Black
instead. The good thing about this black is that you can boil as long as you want, and the black (nearly) always comes out the same. An occasional correction with a sharpie marker is needed, which may have to do with the 3D-printing process.
I recycle all dye bath (except black), as they can be reused many times without any effect to the color quality. The reason that I do not recycle black, is that Dylon 8 is more sensitive to dye depletion. Also, I have a cheap local source.