Drape vacuum forming

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. 

The majority of the individual type of blisters are made by vacuum forming either into or over (drape type) moulds, which are frequently made from metals such as aluminium. The choice between female and male moulds depends on a number of factors, as follows. 

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. 

Compare and contrast drape forming and vacuum forming. 

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. 

Equipment differs with sheet thickness. There are generally three groups of machines—the general-purpose for drape and vacuum forming the sheet-fed machine which is usually a shuttle or rotary type, and the roll-fed machine for thin sheet (0.07 in.). It is doubtful if the roll-type machine would be used with reinforced thermoplastics. 

Ablasshahn, Ablaufhahn draining rack AbtropfgesteU drape and vacuum forming Vakunmstteckfomiveifahren drape assist (film/foil)... 

Drape forming type of vacuum forming (qv) process in which heated plastics sheet is pulled down over a male mould, or the mould is pushed into the sheet, and the vacuum applied. Drape forming is used mainly to produce a certain degree of performing. 

In vacuum forming, for example, the heat-plasticized sheet (either directly from an extruder or preheated in an oven) is drawn against the mold surface by the application of a vacuum from beneath the surface. Similarly, a positive pressure may be used to force the sheet against the mold surface, and where very deep draws are required, mechanical assists, plug forming, are used. In drape forming, the plasticized sheet is draped over a male mold, perhaps with a vacuum assist. 

FIGURE 17.9 Sheet-forming process (a) vacuum forming (b) drape forming. 

PURE VACUUM FORMING The term vacuum forming can be applied to drape forming as well, but in this case it is used to indicate the use of a female mold half. The clamped sheet is moved into place, secured, and then a vacuum is applied to the mold. The vacuum is particularly useful in this case, because the blank must be drawn into comers and deep pockets, and reproduce other features that are difficult or impossible to replicate with the force of gravity alone. 

Vacuum forming is a variation of a classic forming technique, where a sheet of material is heated slightly (not melted) to make it pliable, and then placed over a mold. In the classic technique, the material simply drapes over the mold, as a result of gravity and the mechanics of the material. (Think of a piece of leather, and a wooden shoe mold.) In vacuum... 

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