Is there a way to bond silicone to nylon?
I assume you use a molding silicone or a specialized prosthetic silicone for film and theater masks. The chemical compositions of those kinds of silicone are designed to not stick to anything.
Construction silicone, on the other hand, is designed to stick to as many materials as possible. You could try applying construction silicone to the mask, but I haven't tested if this actually works. There's also the risk that the construction silicone could &quot;melt&quot; through the prosthetic silicone. This melting process usually takes several days to months, so better try it on a waste piece.
A more promising approach is to embed an anchor in the mask where the muscle would be. The simplest anchor would be a key ring or a washer to which the fishing line is already tied. The mask must be thick enough to enclose the anchor on all sides or it might be pulled out. You can achieve a better bond and more natural movements by cuting a piece of mesh (any grid-like material made of metal, plastic or fabric) into the shape of the muscle, tying the fishing line to it and embedding the mesh where the natural muscle would be.