The problem is the material that your trying to dye… not the new dye forumla Rit is selling.
Quoted from the Rit Dye website "It will also not dye certain plastics, such as polyethylene, which are found in golf discs and components in RC Cars, and polycarbonates, which are hard plastics such as those found in eye glass frames. "
The plastic brand “Celcon” is Acetal… more specifically Acetal Copolymer. There are two types Acetal; Homopolymer/POM-H and Acetal Copolymer/POM-C. POM-C is more resistant to water than POM-H. It’s more resistant because it is less porous than POM-H.
What does all that mean.
Assuming (because i didn’t look it up myself) the caps are made of Celcon which is POM-C. It’s not a very porous material. Therefore it is extremely difficult for pigmented water to penetrate and dye the material. I’m not suprised it didn’t take any at all…
I have dyed several different things that are Delrin/POM-H/Acetal Homopolymer. Heat, Time, and concentrated pigment is the key and higher quantities than you’d expect you would need of each. Even then the pigment doesn’t go very deep into POM-H.