Snap back forming

Snap-back forming See vacuum snap-back forming. 

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. 

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... 

In viw PAI and antithrombin are stabilized in their active forms by binding to vitronectin and heparin, respectively. These two serpins seem to have evolved what Max Perutz has called "a spring-loaded safety catch" mechanism that makes them revert to their latent, stable, inactive form unless the catch is kept in a loaded position by another molecule. Only when the safety catch is in the loaded position is the flexible loop of these serpins exposed and ready for action otherwise it snaps back and is buried inside the protein. This remarkable biological control mechanism is achieved by the flexibility that is inherent in protein structures. 

The rather low coordination in the (100) and (110) surfaces will clearly lead to some instability and it is perhaps not surprising that the ideal surface structures shown in Figure 1.2 are frequently found in a rather modified form in which the structure changes to increase the coordination number. Thus, the (100) surfaces of Ir, Pt and Au all show a topmost layer that is close-packed and buckled, as shown in Figure 1.3, and the (110) surfaces of these metals show a remarkable reconstruction in which one or more alternate rows in the <001 > direction are removed and the atoms used to build up small facets of the more stable (111) surface, as shown in Figure 1.4, These reconstructions have primarily been characterised on bare surfaces under high-vacuum conditions and it is of considerable interest and importance to note that chemisorption on such reconstructed surfaces can cause them to snap back to the unreconstructed form even at room temperature. Recently, it has also been shown that reconstructions at the liquid-solid interface also... 

Stretched with little tendency to snap back to original form... 

An elastomer with a low modulus will often have far better erosive wear than material with a higher modulus. An abrasive wear test (such as the DIN abrader) will show a poor result for a soft elastomer. This is also shown in field applications such as tire wear. When the application is changed from abrasive to erosive wear, the softer elastomer will wear very well. The reason for this is that the low modulus of the soft elastomer allows the stresses from each impact to be dissipated more readily than for hard polyurethanes. The soft material will stretch further and then snap back before damage is done. Any microcracks formed will have a slower growth rate and hence less erosion will occur. 

One of the prominent features of polymeric liquids is the property to recover partially the pre-deformation state. Such behaviour is analogous to a rubber band snapping back when released after stretching. This is a consequence of the relaxation of macromolecular coils in the system every deformed macro-molecular coil tends to recover its pre-deformed equilibrium form. In the considered theory, the form and dimensions of the deformed macromolecular coil are connected with the internal variables which have to be considered when the tensor of recoverable strain is to be calculated. Further on, we shall consider the simplest case, when the form and dimensions of macromolecular coils are determined by the only internal tensor. In this case, the behaviour of the polymer liquid is considered to describe by one of the constitutive equations (9.48)-(9.49) or (9.58). 

Like the snap back process, billow forming is a sheet pre-stretch process used with a male mold. The sheet is clamped across a pressure chamber and is billow pre-stretched by applying a low magnitude positive air pressure to the chamber. The male mold is then moved into the stretched sheet until the clamp seals on the mold periphery. Contact between the mold and controlling the rate at which the pressure chamber is vented to compensate for the advancing mold can regulate the sheet. 

Zero-time reassociation comes from palindromic sequences that can form "snap-back" or "hairpin" structures. Such molecules reassociate very rapidly, at a rate independent of their concentration, because they do not need to interact with another molecule to form a double-stranded structure. 

The Vc and Me values for crosslinked polymer networks can also be evaluated from stress-strain diagrams on the basis of theories for the rubber elasticity of polymeric networks. In the relaxed state the polymer chains of an elastomer form random coils. On extension, the chains are stretched out, and their conformational entropy is reduced. When the stress is released, this reduced entropy makes the long polymer chains snap back into their original positions entropy elasticity). Classical statistical models of entropy elasticity affine or phantom network model [39]) derive the following simple relation for the experimentally measured stress cr ... 

D. Vacuum forming with snap-back can reduce starting sheet size, aids material distribution, minimizes chill marks. 

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

G. Forming with billow snap-back is recommended for any parts requiring a uniform, controllable wall thickness. 

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. 

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