Plastic may be shaped in various ways. It can be extruded (pushed through a nozzle when liquid) to form sheets, tubes and fibers. Molten plastic can be poured into moulds. Vacuum forming is a way of making complicated plastic shapes. A sheet of warm plastic is placed over a mould, and then the air is sucked from under it so that the sheet is pulled firmly against the sides of the mould. When the plastic is cooled, it retains the mould’s shape. Disposable cups are often made in this way.
Metalworking using machines and machine tools includes cutting using a lathe, plastic forming, and welding. When grouped with other such metalworking techniques, plastic forming is also called stamping and makes the designed shapes by pressing the material into a die. This processing method utilizes the plasticity—the characteristic that a material remains in the shape it is changed to by the application of a certain force—of metals and other solids. Plastic forming is primarily used in the metalworking of steel materials such as those for automobile parts. Unlike cutting with a lathe, this method does not produce chips and also allows mass production of the same parts through mold pressing.
There are two types of plastic forming: Cold-plastic forming, which is performed at ambient temperatures, and hot-plastic forming, which uses heat. When heated, metal undergoes thermal expansion and changes shape. As such, cold-plastic forming is used whenever possible and hot-plastic forming is used only when the material of the target being produced is hard.
Some examples of other types of plastic forming include forging for manufacturing nuts and bolts; extrusion, wire drawing, and pultrusion for forming wire materials and pipes; deep drawing for creating spherical surfaces in metal sheets; bending for producing leaf springs; riveting for securing assemblies in place; and shearing for cutting metal sheets.
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