Vacuum snap-back

Figure 8.17. Shaping of sheet, (a) Application of vacuum, (b) and (c) Air prressure. (d) Mechanical pressure, (e) Combination of methods (vacuum snap-back). (After Estevez and Powell )...

Figure 10.8 Vacuum snap-back thermoforming (reprinted with permission from [1])...

Snap-back forming See vacuum snap-back forming. 

E. Air-slip forming is similar to vacuum snap-back except that heated sheet is billowed up and mold rises to meet It. 

Vacuum snap-back forming (Fig. 10-9) is used for rubbery materials. The male mold can be moved into position as in part A of the figure or used with vacuum as in B. 

Refer to Fig. 4-28. After the plastic sheet is heated and sealed over the top of the female vacuum box (A), a vacuum applied at the bottom of the vacuum box pulls the plastic material into a concave shape. The latter can be controlled by turning the vacuum on and off to maintain a constant shape in the sheet. When the plastic has been prestretched, the male plug enters the sheet (B) and a vacuum is drawn through the male plug. Vacuum beneath the sheet is vented to the atmosphere or light air pressure is applied in place of the vacuum (C). External deep draws (D) can be obtained from the vacuum snap-back process for forming items like luggage, auto parts, etc. 

Snap-Back Forming See Vacuum Snap-Back Forming. 

Billow drape forming consists of a male mold pressed into a sheet prestretched by the billowing process (Fig. 1.74). A similar process is billow vacnnm forming, wherein a female mold is used (Fig. 1.75). In vacuum snap-back forming, vacnnm is nsed to prestretch the sheet, then a male mold is pressed into the sheet, and, finally, pressnre is nsed to force... 

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