**6.6 Thermoforming**

Thermoforming is a process which uses heat and pressure or vacuum to transform thermoplastic flat sheet into a desired three-dimensional parts. The sheet is drawn from large rolls or from an extruder and then transferred to an oven for heating to its softening temperature. The heated sheet is then transferred to a preheated, temperature-controlled mold. Vacuum is applied to remove the trapped air and deform the sheet into the mold cavity, where it is cooled to retain the formed shape. After that, a burst of reverse air pressure is applied to break the vacuum and assist the formed part out of the mold. The principal factors in this process include the forming force, mold type, sheet prestretching, the material input form and the process phase condition. These factors have a critical effect on the quality and properties of the final products.
