Injected TPU (thermoplastic polyurethane) is a molded plastic shell that makes up the outer structure of most modern snowboard boots. It's the most common shell construction you'll find across all riding styles, from freestyle to freeride.
The biggest advantage of TPU shells is flex consistency. Unlike softer materials that can stiffen up on freezing days or break down and get sloppy over time, TPU maintains a predictable feel across a wide range of temperatures and throughout the life of the boot. That means if you buy a medium-flex boot, it'll still feel medium-flex on a bitter January morning rather than turning into a stiff brick.
TPU shells are also highly durable and resist abrasion better than synthetic leather alternatives. For most riders, this is the ideal construction—you get reliability and consistent performance without having to think about it. The only downside is that TPU boots can feel slightly less broken-in right out of the box compared to softer-shell options, but they pack out naturally after a few days on snow.