Billow forming and plug assist forming are occasionally combined with drape forming and vacuum forming to obtain unique wall thickness distributions. One
example is reverse draw forming with plug assist. illustrates the multiple step process, Where a bubble is blown first. The plug is then plunged into
the bubble, everting it under control. Once the plug has stretched the bubble nearly to the femle mold bottom, vacuum or pressure is applied to
force the sheet against the mold surface. This technied requires much patience since a stable bubble without excessive stretching is key to uniform and consistent
part wall thickness distribution. The male mold variation of this, with the mold initially acting as the plug. This technique is sometimes called pressure bubble
immersion forming or just inmersion forming. Again, the bubble can be formed with vacuum, in a vacuum box that also serves as a pressure box for pressure forming.
Table 1.6 [19] summarizes many aspects of the themoforming process.

Table 1.6 Characteristics of the Themoforming Process.  Thermoforming Machine

Adapted form [19]