Page 47 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

INTRODUCTION TO POLYMERS AND PLASTICS 1.33

• Pressure forming
• Free blowing
• Matched die molding
Drape forming, as shown in Fig. 1.29, involves either lowering the heated sheet onto a
male mold or raising the mold into the sheet. Usually, either vacuum or pressure is used to
force the sheet against the mold. In vacuum forming (Fig. 1.30), the sheet is clamped to
the edges of a female mold, then vacuum is applied to force the sheet against the mold.
Pressure forming is similar to vacuum forming except that air pressure is used to form the
part (Fig. 1.31). In free blowing, the heated sheet is stretched by air pressure into shape,
and the height of the bubble is controlled using air pressure. As the sheet expands outward,
it cools into a free-form shape as shown in Fig. 1.32. This method was originally devel-
oped for aircraft gun enclosures. Matched die molding (Fig. 1.33) uses two mold halves to
form the heated sheet. This method is often used to form relatively stiff sheets.

FIGURE 1.29 Drape-forming process. 68 FIGURE 1.30 Vacuum-forming process. 66

FIGURE 1.31 Pressure forming. 69

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