Hello,  I'm sewing a small batch of face masks for a business in Michigan. They have asked if I'm able to dye the white polyester/nylon elastic ear-loops dark brown or black. I've never dyed anything before and found your products in my internet search.
3 Questions:
1)   Would the iDye be recommended for these ear-loops? Is it possible the dye may bleed onto their skin, behind their ears, especially while they are perspiring due to the nature of construction work?
2) Would silver plated metal attachments on the ear-loop elastic also be dyed or effected in some way?
3) When washed, can hot water temp be used? and would it bleed into the face-mask fabric?
Your advice is much appreciated, Thank you
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Jacquardmod Jacquardmod

It actually matters whether they are Nylon or Poly. 

Poly can only be dyed with Idye poly at boiling temp(which can be hard on elastic).  It will also dye Nylon though, so this is your "catch all" dye in this scenario.  It's possible it is a composite material and this is the only thing that will work on everything.  It really does have to be boiled, but this material should absorb fast, so I think if you boiled for no more than 20 min,you would be ok, and maintain stretch.  I have done it with Nylon successfully before.  Longer baths over 30 min seems to degrade stretch substantially.  

Nylon can be dyed with acid dyes at a lower temp between 160-180.  So this is better for maintaining the stretch.  You will get an overall darker color than you would on poly as well.  

#2 The silver plated attachments will be unaffected and undyed.

#3 This is a great question.  The poly dye is more likely to reactivate and stain something at high temp, but it won't on cotton or any natural fabric, just synthetics.  

The other thing you have to avoid with the poly dye is crocking.  It is possible to over dye polyester and nylon with poly dye.  If you use too much, there is nowhere for the dye to go and it piles up on the surface and then "crocks" off onto other stuff including skin.  You can completely avoid this by using less dye, so you need to do some tests before you just start cranking them out.  Rule of thumb is your want 4% dye by weight for most colors, and no more than 6% dye by weight for black.  

I think I would start personally by trying the acid dye and see if that is good enough.  It uses less energy, you will get better color, and there is less risk of crocking.  
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Thank you for such detailed and helpful information. Just ordered the acid dye to try first, as you've 
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