doggability
And which is better for Nylon 12?
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Jacquardmod Jacquardmod
The answer is it depends. 

Idye Poly is the only dye that will work on polyester.  It also works on protein fibers like silk and nylon(although it is synthetic, its structure is very similar to a protein).  So this protein dyeing is why Poly and Acid dye both work on nylon.  Acid dyes do not dye polyester at all though.

Acid dyes tend to be brighter and more vibrant and require lower temperatures than the poly dyes do.  Both have fairly good lighfastness, but it can vary dye to dye.  

For 3d printed parts i have success with both.  People seem to like the black Idye poly over the acid dye black.  The Turquoise poly is more light fast than the Acid dye and so is the red.  

So basically, you will get brighter colors from the Acid dyes, but some of the poly colors are more lightfast.  The black poly seems to be the go to over the jet black acid dye which can turn brown on nylon.  

I told you this in the email, but for everyone.  You need to use a lot less dye on 3d printed parts than you would on fabric.  1/2% to 1% by weight whereas on fabric you would use 4-6%.  The fabric is absorbent, 3d printed objects are not.  
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doggability
Do you have a formula to determine how much dye to use based on the surface area of a Nylon 12 3d printed part?
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Jacquardmod Jacquardmod
Weight of the item * .005 = Weight of the dye.  Then if too light double amount of dye.  1 tablespoon approx equal to 14g or 1/2 an ounce.  1 tablespoon = 3 teaspoons

That should help
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doggability
I'm assuming then that you do not have a formula using surface area. It seems potentially error prone to calculate this for a 3d part using weight, though it does makes sense for fabric. i.e. there is a closer correlation between weight and surface area for fabric, whereas this could vary greatly with a 3d part.

That having been said, I think any formula is just a starting point. (Probably an obvious statement to anybody but a novice like me).
I just did the following for iDye Poly Violet:
    2 tsp dye, 1 tsp intensifier, boil 20 minutes
It turned out almost exactly like the color on the package.
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Jacquardmod Jacquardmod
Yeah for sure.  Figuring out the surface area for complex shapes is not something anyone is going to want to.  Better to do it by weight, which will get you in the ballpark and then you can adjust as needed.  
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doggability
Surface Area is easily available from most any 3d modeling application.
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Jacquardmod Jacquardmod
Ahh, so you are actually better equipped to determine this than I am.  
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detobias detobias
As you know I have had much problem with colorfastness with poly red. Is lightfastness fading?  
And I find poly black way too blue. Any tip on what to add?
thanks!
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Jacquardmod Jacquardmod
Detobias, you are having trouble with washfastness for the red, correct?  Light fastness should be pretty good. 

Less blue in the black is easy.  Add a pinch of Orange and then increase until you get it more neutral.  You will probably want a little blue because eliminating all the blue will make a more brownish black instead of bluish black.  
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