Most cases are fabricated from panels joined by metal or plastic extrusions, molded plastic, or metal. Fabricated cases are typically made from panels constructed of two layers. An outer layer of ABS or fiberglass laminate is adhered to a middle layer of lightweight cabinet-grade plywood such as birch, poplar, or maple. These two layers are known as laminate panel, and high-grade cases are made from composite material. Inside the case, an internal shock-absorbing filler such as polyurethane or polyethylene foam has cavities that correspond to the shape of the components it is preserving. Alternate shock protection methods include mounting the case contents to racks or panels attached to the case walls with shock isolators. The corners of fabricated cases are commonly reinforced with stamped sheet steel case corners commonly finished in zinc, nickel, chrome.