Hi Jarcode, thanks for your message. Mr. E is correct. Our boots are Grilamid. Which shell size do you have? How wide are your feet? We suggest you stand in the shells without any liner. Are your feet too wide just standing in the shells? If not, then it would be better to work with a thinner liner before strething or milling the shells. If your feet are too wide, have you tried one shell size larger? Hope this helps.