i think its similar to electroplating. somehow the take a plastic exhaust kit part, form it into some sort of cast core and electro plate it. whatever material the base is maust have to dissolve. reason i say this is i saw these before and specifically the 1/48 roland c.ii exhaust is identical to the kit part. so do they cast the part in plaster then do a galvanic deposition onto that then dissolve out the matrix. dont know. but it would be neat to have every exhust done this way.
