Sheet forming fundamentals

The topologies of helices and pleated sheets involve fundamentally different intra- and interchain interactions. The a-helix is formed by a continuous segment... 

The final chapter (Chapter 10) focuses on web and sheet forming processes. It demonstrates how the statistical techniques can be applied to evaluate process and control performance for quality assurance and to acquire fundamental insight towards the operation of such processes. 

The operations sheet is fundamental to manufacturing estimating. It is also called a route sheet, traveler, or planner. There are many styles, and each plant has its own form. The purpose of the... 

Allan T, Tekkaya AE (2012) Sheet metal forming fundamentals. ASM International, Materials Park Schuler GmbH (1998) Metal forming handbook. Springer, Berlin/Heidelberg... 

Knowing the key objectives and interest in thermoplastic sheet forming, we next address the process and its fundamental considerations. 

In this section some fundamental issues concerning thermoplastic sheet forming appear. The content here builds on Chapter 1 but the focus is specific to sheet forming. 

Now that the fundamentals are covered, the next sections present typical sheet forming methods. The methods presented next are common however, new forming methods are still in development (Zampaloni et a/., 2004). 

Polypeptide chains are folded into one or several discrete units, domains, which are the fundamental functional and three-dimensional structural units. The cores of domains are built up from combinations of small motifs of secondary structure, such as a-loop-a, P-loop-p, or p-a-p motifs. Domains are classified into three main structural groups a structures, where the core is built up exclusively from a helices p structures, which comprise antiparallel p sheets and a/p structures, where combinations of p-a-P motifs form a predominantly parallel p sheet surrounded by a helices. 

Strand-turn-strand motifs in /1-solenoids differ fundamentally from those found in globular proteins. In globular structures, two adjacent strands with an intervening /l-turn form an antiparallel structure called a /1-hairpin (Fig. 10A). In /1-solenoids, the polypeptide chain also folds back on itself, but the flanking /1-strands make contact via their side chains rather than interacting via H-bonds of the backbone (Fig. 10A). As a result, consecutive strands find themselves in two different, parallel, /1-sheets. The latter strand-turn-strand structure is called a /1-arch, and its turn, a /1-arc (Hennetin et at., 2006 Yoder and Jurnak, 1995). In /1-solenoids, /1-arches stack in-register to form /1-arcades which have two parallel /1-sheets assembled from corresponding strands in successive layers. 

What is the nature of the insoluble forms of the prion protein They are hard to study because of the extreme insolubility, but the conversion of a helix to (3 sheet seems to be fundamental to the process and has been confirmed for the yeast prion by X-ray diffraction.11 It has been known since the 1950s that many soluble a-helix-rich proteins can be transformed easily into a fibrillar form in which the polypeptide chains are thought to form a P sheet. The chains are probably folded into hairpin loops that form an antiparallel P sheet (see Fig. 2-ll).ii-11 For example, by heating at pH 2 insulin can be converted to fibrils, whose polarized infrared spectrum (Fig. 23-3A) indicates a cross-P structure with strands lying perpendicular to the fibril axis >mm Many other proteins are also able to undergo similar transformation. Most biophysical evidence is consistent with the cross-P structure for the fibrils, which typically have diameters of 7-12 rnn."-11 These may be formed by association of thinner 2 to 5 nm fibrils.00 However, P-helical structures have been proposed for some amyloid fibrils 3 and polyproline II helices for others. 1 11... 

The use of templates that can nucleate secondary structures has also been studied [23], The fundamental idea is to attach one or more conformationally flexible peptides to a rigid template that is designed to initiate either a /f-sheet or an a-helix by forming the first crucial hydrogen bonds. These interactions compensate for the loss of entropy associated with the folding process and in particular in the initiation step. This strategy has been used to develop stable helices, sheets, and artificial proteins. 

Although it may seem reasonable that an increase in viscosity of the spray fluid should increase the drop size, there is little fundamental information on the relationship between the drop size of the sprays and the viscosity of the spray liquid. Moreover, the information that is available is conflicting. Besides the work of Yeo and Dorman, where a viscosity term was found unnecessary when working with liquids having relatively low viscosities, other workers have found that a function of viscosity was necessary to describe drop size (16,17, 20), but the value of this function has varied from v01 to v106 (where v is the kinematic viscosity of the liquid). More recently, Dombrowsld and Johns (9) have examined the breakup of sheets of viscous liquids formed from fan-jet nozzles and have derived a theoretical expression for the size of drops produced. The expression is very complex and includes viscosity terms, but it is difficult to use in a practical fashion to predict drop size and its variation with a particular physical parameter of the spray fluid. 

The third and fourth structures are sometimes called the zincblende and the wurtzite structures, on account of two forms of ZnS. The zinc-blende structure is also often called the diamond structure, since it is found in diamond and some other crystals. The fundamental features of both structures are similar each ion is tetrahedrally surrounded by four ions of the opposite sign, as in Fig. XXIII-3 (a). There arc a number of ways of joining such tetrahedra to form regular crystals, however. The diamond, or zincblende, lattice is the simplest of these. In the first place, tetrahedra like Fig. XXIII-3 (a), can be formed into sheets like... 

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