Forming the Sheet

The process basically forms the sheet after it has been heated to the point at which it is soft and flowable, and then applying differential pressure (atmospheric pressure, air pressure, vacuum, or their combinations) to make the sheet or film conform to the shape of the male or female mold producing many different products (Table 8-23). The more precise and controlled pressure applied, the more efficient in reproducing products at the lowest cost occurs (Fig. 8-50),... 

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. 

Cubex [Westinghouse], TM for an oriented silicon-iron alloy in rolled sheet form for use as cores for transformers and other inductive devices. The alloy sheet comprises cubic grains with faces parallel to the sheet surface. In one form, the sheet is doubly oriented with two directions of easy magnetization parallel to the surface to the sheet. One direction of easy magnetization is parallel to the... 

The synthetically prepared polypropylene mixture is composed of 50% parent polymer and 50% extract. The substrate shape that is tested to examine the feasibility of the concept is a thin sheet. To form the sheet, a few grams of the synthetic mixture is pressed between heated platens maintained at 170°C. The thickness of the pressed preform is 4 mil. Depending upon the amount of polymer used, the sheets obtained were 3-5 in in diameter. 

Vacuum Forming - Vacuum forming is a type of thermoforming process consisting of preheating the plastic sheet prior to forming. The sheet is formed into the female mold by application of vacuum through holes in the mold. 

Here compressed air is used to form the sheet. In one variation, a plastic sheet is heated and sealed across the female cavity (Figure 11.14). Air at controlled pressure is introduced into the mold cavity. This blows the sheet upward into an evenly stretched bubble. A plug which fits roughly into the mold cavity descends on the sheet. When the plug reaches its lowest possible position, a vaeuum or, in some cases, air under pressure is used to complete part formation. 

In larger-volume production, preimpregnated sheet compounds are used. The fabricator forms the sheet compound to shape and initiates the cure. These compounds are known as prepregs (preimpregnated materials), SMCs (sheet molding compounds), and BMC (bulk molding compounds). There are many variations within each group (Chapter 4). 

It is worthwhile to add the following. The / -sheet structure also has intramolecular hydrogen bonds. For a peptide, if the conformational energy of the / -sheet structure is significantly lower than that of the a-helix, alcohol may induce the peptide to form the / -sheet. In fact, it was experimentally shown for some peptides [46] that the / -hairpin structure linking adjacent strands in an anti-parallel / -sheet is considerably more stabilized by addition of TFE. 

The blank is typically formed in a single roller pass. Blank shape can be either a flat sheet or a preform produced by other means. Generally, a single roller is used to form the sheet however, in case of high-strength and thick-walled components, a multiple roller configuration may be used. 

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