Printed in Unicorn Elixir PLA. I had to use an X-acto knife to cut the seam of the top of the lid near the hinge. I felt like it was going to break if I tried to pry it open any harder (the plastic started to bend a little and looked like it was going to snap). Once I got that first intital open, it worked perfectly. I use it everyday at work. :) It pops on and off easily, yet is tough enough to not worry about coming off.