Drape forming

There are many methods for molding the sheet of plastic, once it has been softened by heat. The simplest are drape forming, vacuum forming, and pressure forming. Other thermoforming techniques are modifications or combinations of these basic methods. 

In drape forming (Fig. 10.2), the main forming force is the influence of gravity on the hot plastic sheet. Typically, a positive, or male mold of the part, with a convex shape, is used. The forming is assisted by the application of vacuum to pull the material down around the shape. 

---

In drape forming, the thermoplastic sheet is clamped and heated and the assembly then sealed over a male mold. The mold may be forced into the sheet or the sheet may be pulled into the mold by introduction of a vacuum between the sealed sheet and mold. By draping the sheet over the mold, the part of the sheet touching the mold remains close to the original thickness. Foamed PS and polyolefins are generally used in this procedure. 

Drape forming process is the simplest technique for use with a male mold. The heated clamped sheet is lowered over the mold until it seals with the mold base. This action allows the heated sheet to conform by gravity or pressure. The mold form acts as a crude sheet pre-stretch plug. 

It combines drape forming with vacuum action. The sheet is clamped into a movable frame, heated, and draped over high points of a male mold. Vacuum is pulled to complete the forming operation. In this technique a male or female mold is closed into the hot sheet. 

It is a modified system of drape forming for producing more uniform wall thickness and minimizes the dangers of tearing over the corners of large moldings because of the protective cushion of compressed air... 

Vacuum (or pressure) is used both to billow pre-stretch and to form the sheet. The heated sheet is first clamped across a vacuum box or chamber, which is then partially evacuated, causing atmospheric pressure to billow and stretch the sheet. A male mold is then advanced into the billowed sheet, and forming is completed by drawing a vacuum (or pressure) on the mold while venting the vacuum chamber to the atmosphere. The process is a takeoff to drape forming but with the advantage of sheet pre-stretch, which produces a much more uniform distribution of wall thickness. 

Vacuum forming Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mould. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary moulds. 

Compare and contrast drape forming and vacuum forming. 

Drape forming (also called oven forming) is carried out commercially on small and large parts where the plastics material is pre-heated in an oven to the forming temperature, placed in a mould and held in place while it cools. Applications include windscreens, seat backs and bottoms, covers and doors. 

VACUUM FORMING—Heat-softened sheet is placed over a male or female mold air is evacuated form between sheet and mold, causing sheet to conform to contour of mold. Modifications include vacuum snapback forming, plus assist, drape forming, etc. 

---