A method of shaping a flat plastic sheet over a solid relief pattern (also called a male or positive mold). The plastic is heated until it is soft, and when a vacuum is created under the mold, the plastic is drawn down onto it, as if forming a new skin. The plastic sheet cools quickly, retaining the shape of the pattern. Vacuum forming is desirable when needing a "shell" type plastic form with a uniform thickness. A form can be made over molds made of wood, plaster, and many other materials. Also see cast and impression.