I was the first to come out with knife sheaths for kitchen knives back in 1982 as a plastic fabricator. The product’s major sales obstacle was its cleanability. I was told that professional cooks would put their dirty knives in them. So, the last thing you would want to do, was to line it with felt! My foodservice sales rep told me to have them tested by the NSF (National Sanitation Foundation). He wondered if they could be put through a commercial dishwasher. I didn’t say anything, but the answer is no. The sheaths, at that time, were made from a plastic sheet that was heated in a straight line and then folded. This creates stress in the bend. When the sheath is subject to a high temperature, the tension in the bend will cause it to open up. This is called the plastic’s memory. Luckily this didn’t end up being a lasting concern, but I now know, from personal experience, that some cooks do put dirty knives in their sheaths. The product survived all that and became so popular that the US Patent and Trademark Office considers knife sheaths a separate product category.