It is known in the prior art that thermoplastic materials such as, for example, polystyrene foam sheet, may be thermoformed into various articles by preheating the sheet to an elevated forming temperature and thereafter deforming the preheated sheet to the desired configuration utilizing a pair of matched metal molds. Articles which have been formed utilizing such a technique include, for example, foam egg cartons, meat trays, produce containers, disposable table-wear items and the like. Such items are characterized by having structural features which include surrounding upstanding-wall member elements which are integral with the base of the item, and such items show little or no tendency to curl or warp following their removal from the thermoforming mold. However, it has been found that when relatively flat articles, which are devoid of surrounding upstanding wall elements, are thermoformed utilizing the above-described procedure there is a tendency for such articles to bend or curl upon removal from the thermoforming mold resulting in, not the desired flat structure when the longitudinal and transverse axis of the article lie substantially in the same plane, but rather an article which has a pronounced curve or bow. It has been found that regulation and control of the thermoforming process conditions, such as preheat and forming temperatures, forming pressures, and residence time in the forming mold do not remedy the undesirable bending or blowing effect assumed by the article when it is removed from the mold.
In accordance with the present invention, a method and apparatus are provided which allow for the formation of flat surfaces on products such as thermoplastic foam structures which have been thermoformed. The method includes controlling the cross section and surface area of the foam part being formed in such a manner as to produce parts or details of parts made from foam sheets which are flat.