- Hot plate
- Rit Dye (liquid)
- vinegar or other weak acid
- glass dish
- rubber gloves (not a necessity, but it can get messy and it does kinda dye your finger nails...)
- something to blot the model dry like a shop rag or paper towel
To add a little color, I used RIT liquid fabric dye. Start by pouring the dye into a glass dish. Then add your weak acid, (I used vinegar because that's what I had lying around). Make sure to stir the mixture thoroughly and continue to stir the dye periodically, even when the part is in the dye bath, as the dye can tend to settle. The main aspects that allow you to get good color from this method appear to be temperature and pH. If the dye isn't taking to the nylon, add a little vinegar and turn up the heat a little. You'll know when it starts to take because when you pull out the part you will see a sudden and drastic change in color as you hit that correct temperature/pH range. If its not within that range, it will look as though all the dye is just running off the part and it will appear mostly white when you pull it out.
The time it takes to dye the piece will depend on how dark you want the color. For reference, the green prosthetic hand took about 5-7 min submerged to achieve that color, but its worth noting that I waited until it was heated up to 65C before putting it in.
Another interesting feature of dyeing nylon this way is that the color appearance is a function of the surface area to volume ratio. That is, if there are holes through the model or multiple ridges, that portion will have a larger surface area exposed to the dye at one time and thus be darker relative to a portion of the model with a smooth even surface, for a given amount of dyeing time. This is something to keep in mind if you are dyeing something with a lot of irregular features to it. The other attributes that control the color outcome are concentration of dye to diluent (water + vinegar) and time spent in the dye bath.
A word on the actual color outcome vs what the packages claim, apparently dyeing 3D printed nylon doesn't translate exactly in the color world the way using the same dye on fabric would. They just don't match what they say they will be on the bottle. "Denim blue" was very black at the strong concentration, and gray a lower concentrations. "Cherry red" varied from pinkish to maroon depending on the time spent in the dye. In a way, using this dye is a little like mixing paint for color, its not always intuitive and often requires a few tests to get it right. That said, if you have printed a large complicated part that took a lot of effort to produce, I would suggest using a smaller print to do a test dye before dunking your hard work into the dye. You cannot undo the dye, so make sure you are satisfied with the color outcome before putting that 26-hour print in there. The nice thing about this dye is that, once you have found a concentration that gives you the color you like, you can store the dye at that concentration and reuse it. Granted I haven't reused a dye that was stored for more than a few weeks thus far, but it seems to keep at least for a little while.
Lastly, the print still needs to be dried after dyeing, otherwise you can have residual dye that can rub off onto other things. I have had success with my drybox over night, or if you are in a hurry, you could put it under a heat lamp or in an oven on low (~70-80C) . Although I'm not positive about the food safety of drying dyed nylon in an oven that you cook your food in, so I wouldn't suggest this if you don't have a non-food oven. If you used vinegar to lower the pH, as I did, you will notice a vinegar smell on the print when you first take it out and blot it dry, but don't worry, this will go away and after a day or so you won't be able to detect it at all.