The largest construction or workplace utility knives typically feature retractable and replaceable blades, made of either die-cast metal or molded plastic. Some use standard razor blades, others specialized double-ended utility blades. The user can adjust how far the blade extends from the handle, so that, for example, the knife can be used to cut the tape sealing a package without damaging the contents of the package. When the blade becomes dull, it can be quickly reversed or switched for a new one. Spare or used blades are stored in the hollow handle of some models, and can be accessed by removing a screw and opening the handle. Other models feature a quick-change mechanism that allows replacing the blade without tools, as well as a flip-out blade storage tray. The blades for this type of utility knife come in both double- and single-ended versions, and are interchangeable with many, but not all, of the later copies. Specialized blades also exist for cutting string, linoleum, and other materials.