Dislocation of Invariant Molecular Structure or How a hot knife screws up atomic structure and causes your fiber to turn brown!Most fiber optic companies (many of them now out of business) advocate using a hot knife to cut acrylic fiber. Acrylic fiber is manufactured with long polymer chains in a wonderfully precise crystalline structure that is exceptionally transparent to visible light. Heating that fiber, after its manufacture (with a heat gun to relieve bends, with a hot knife to cut fiber ends or with an inefficiently IR filtered illuminator), invariably scrambles that crystalline structure. That scrambled structure traps photons producing heat. The fiber yellows slightly, trapping more photons, becoming more yellow and more opaque until it turns brown and melts.
Manufacturer instructions than say, "use a hot knife to cut fiber ends evenly" almost always include something like "leave a five-foot service loop at the projector so that you can cut and re-polish fiber as it caramelizes."
NoUVIR fiber, on the other hand, is cut and polished cold. We say, "NoUVIR polymethyl methacrylate "acrylic" fibers are warranted against YELLOWING OR LOSS OF TRANSMISSION for TEN YEARS when used with a NoUVIR Cold-Nose™ projector."