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vacuum forming[′vak·yəm ′fȯrm·iŋ]
a method of molding items from plastic sheets. During vacuum forming the sheet materials up to 2 mm thick, based on thermoplastic polymers, are secured to the mold, heated to the point of softening, and then shaped by the action of atmospheric pressure, and a vacuum on the order of 10 kilonewtons per sq m (100 mm Hg) is simultaneously produced in the cavity created by the sheet and the surface of the form.
Articles with a great depth of drawing are produced by the positive method of vacuum forming (the material is formed on the surface of a convex punch mold); articles with a lesser depth are produced by the negative method (the material is pulled into a depression in a matrix mold). Optically transparent parts (for example, airplane cabin windows) are obtained by so-called free vacuum forming, in which the hemisphere that forms as a result of a pressure drop does not touch the walls of the chamber (in this method a mold is not necessary); because of this, the finished article has a smooth surface.
The advantage of vacuum forming as opposed to other methods of forming items from a sheet is the low specific pressure required to produce molds from easily processed materials (gypsum, wood, aluminum, and so on). Various containers, hemispherical items, refrigerator parts, and such are produced by means of vacuum forming.
REFERENCESBratsykhin, E. A., S. S. Mindlin, and K. N. Strel’tsov. Pererabotka plasticiheskikh mass v izdeliia. Moscow-Leningrad, 1966.
Bernhardt, E. (compiler). Pererabotka termoplastichnykh materialov. Moscow, 1965. (Translated from English).
R. V. TORNER