Shear Form Process

In the sheet-forming process, stainless steel, bronze, nickel-base alloys, or titanium powders are mixed with a thermosetting plastic and presintered to polymerize the plastic. Sintering takes place in wide, shallow trays. The specified porosity is achieved by selecting the proper particle size of the powder. Sheet is available in a variety of thicknesses between 16 x 30 mm and as much as 60 x 150 cm. A sheet can be sheared, roUed, and welded into different configurations. 

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

Oxidative degradation of polymers is initiated by radicals (R ) generated in the polymer by heat or mechanical shear during processing or by exposure to UV light. These radicals, in turn, react with 02 to form peroxy and hydroperoxide radicals that promote radical reactions. 

The powder-forming processes are similar in many ways to those nsed for powder metallurgy described in the previons section. For example, pressing is a common method for processing ceramics however, ceramic powders can be pressed in either dry or wet form. In wet form, they can also be extended, just like metals, and cast in a variety of process variations. The nominal forming pressures and shear rates associated with some of these processing methods are snmmarized in Table 7.3. Yon may want to refer back to this table when each of the varions processing techniques is described in more detail. 

Table 7.3 Nominal Processing Pressures and Shear Rates in Some Ceramic Forming Processes...

Controlled crystalhsation (tempering) is a vital step in chocolate production. The aim of tempering is to generate sufficient seed crystals of form V to act as points on which the fats can crystallise. Tempering normally involves a comphcated temperature-shear-time process. Such a process involves ... 

In a study of initially incompatible blends of SAN and PC (30 70) extrusion at a high shear rate of up to 10 s produced a blend in the first extrusion run in which the minor constituent (SAN) was present as small spherical particles in PC (Takahashi et al, 1988). The apparent volume fraction of the spherical SAN decreased with the shear rate and repeated extrusion. The DMA measurements showed that the maximum of tan (5 for PC shifted to lower temperatures with repeated extrusion, which is consistent with some SAN being mixed with PC in a compatible form. This result suggests that compatibility is enhanced in extremely-high-shear-rate processing. Because of the rapid quench from an elevated temperature that occurs after processing, this new structure may be retained. 

The shearing forces deform, i.e. orient, the macromolecules. The degree of orientation reached in the molding depends on the magnitude of the shearing forces at work in the forming process, which are in turn influenced by the shear rate. 

Benefits for the forming process directly are, for instance, the possibilities of superposition of vibrations and counter shearing, movements with rest under load for processes which require time and acciuate lower return point. 

Incremental sheet forming (ISF) is a forming process similar to shear forming that allows a flexible, cost-efficient, and timely implementation of a CAD model to a real part. It was developed to meet the market demands of an efficient production of small batches, customized parts, or prototypes. 

Shear forming belongs to a group of processes used for production of hollow, rotationally symmetric parts. The term spinning is used to refer to the three... 

Components produced by shear forming are mainly parts for the automotive and aerospace industries, along with art objects, musical instm-ments, and kitchenware. Typical examples are jet engine and turbine components, dishes for radar antennas, and domestic utensils. The process is capable of forming components of diameters ranging from 3 mm to 10 m, and thicknesses of 0.4-25 mm (Brown 1998). Figure 1 shows the main features of the process and examples of obtainable geometries. 

Chapter 1, Mechanical Metallurgy of Flat-Rolled Metal Products (Sinha), describes what goes on inside and on the surface of sheet, plate, and coil stock in the shearing and forming process. This chapter will provide a technical base for understanding various types of equipment used by the metal-forming professional. 

In all cases, the product designer must keep in mind that the roll tooling must have access to pinch and drive the sheet through the entire forming process. Large embosses, louvers, shear forms, and so forth must leave areas for drive or roll grip. ... 

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