I think the other issue is that unlike the hard board, the cardboard has fibers that stick out and end up inside the melted nylon and you actually have to break the fibers to get the nylon off. With the hardboard, it might stick, but it is just surface contact.I think that's a large part of my problem. The bottom edge of corrugated cardboard actually sticks out slightly, and the hot knife is welding the nylon to that edge.