Forming operations

Forging Rolling Extrusion Drawing Sand Die Investment Lost foam Continuous Powder Welding 

FigMi 11-8 Classification scheme of metal fabrication techniques discussed in this chapter. 

Rolling, the most widely used deformation process, consists of passing a piece of metal between two rolls a reduction in thickness results from compressive stresses exerted by the rolls. Cold rolling may be used in the production of sheet, strip, and foil with a high-quality surface finish. Circular shapes, as well as I-beams and railroad rails, are fabricated using grooved rolls. 

For extrusion, a bar of metal is forced through a die orifice by a compressive force that is applied to a ram the extruded piece that emerges has the desired shape and a reduced cross-sectional area. Extrusion products include rods and tubing that have rather complicated cross-sectional geometries seamless tubing may also be extruded. 

With sand casting, probably the most common method, ordinary sand is used as the mold material. A two-piece mold is formed by packing sand around a pattern that has the shape of the intended casting. A gating system is usually incorporated into the mold to expedite the flow of molten metal into the cavity and to minimize internal casting defects. Sand-cast parts include automotive cylinder blocks, fire hydrants, and large pipe fittings. 

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Industrial scale polymer forming operations are usually based on the combination of various types of individual processes. Therefore in the computer-aided design of these operations a section-by-section approach can be adopted, in which each section of a larger process is modelled separately. An important requirement in this approach is the imposition of realistic boundary conditions at the limits of the sub-sections of a complicated process. The division of a complex operation into simpler sections should therefore be based on a systematic procedure that can provide the necessary boundary conditions at the limits of its sub-processes. A rational method for the identification of the subprocesses of common types of polymer forming operations is described by Tadmor and Gogos (1979). 

Synthesis This is part of Kutney s quebrachamine synthesis (J. Amer. Chem. Soc., 1966, 3656). He foimd that if X = OEt, both C-N bond-forming operations could be... 

In Its overall design this procedure is similar to the Gabriel synthesis a nitrogen nude ophile IS used m a carbon-nitrogen bond forming operation and then converted to an ammo group m a subsequent transformation... 

Two-piece aluminum containers are also available. These consist of impact-extmded upper shells having a seamed-on aluminum bottom. The valve opening is machined from the soHd aluminum rather than roUed. AH coatings must be appHed to the can body after the forming operation. 

At the gob feeder, the forming operation begias. The gob feeder deUvers a glass gob shaped such that it enters the blank mold without excessive mold contact, distortion, or reshaping of the glass. Shears cut off a gob of glass. The gob is fed to either a blow and blow, press and blow, or press-only forming operation to produce a container. 

Fig. 13. Various forming operations (a) backward extmsion (b) deep drawing (c) spinning (d) bending (e) stretching (f) stamping (g) coining (h)...

In some metal-forming operations such as rod and wire drawing, various surface treatments are appHed to the workpiece. These include descaling, cleaning the apphcation of lubricant carriers, and the use of lubricants (see Lubrication and lubricants). Descaling can be mechanical or chemical (pickling). 

Specialty binders may be used in novel or unique forming operations. Cross-linking polymers are used to induce gelation in gelcasting. 

Liquids. Liquids (33) are common forming additives in plastic, paste, and slurry processing. In plastic forming operations, the Hquid aids forming and serves as the binder/plasticizer for the system. In pastes and slurries, other additives are also dissolved or dispersed in the Hquid/solvent. Water is a good, inexpensive solvent that can be recycled. Organic Hquids such as alcohols are used to process water-sensitive materials and to dissolve water-insoluble forming additives, however, at considerably more expense. 

Ceramic forming iavolves consoHdation and mol ding of ceramic powders to produce a cohesive body of the desired size and shape. Ceramic forming operations (38,40—66) are conducted with dry powders, plastic bodies, pastes, and slurries. 

Plastic Stra.ln Ra.tlo. The plastic strain ratio is the ratio of strains measured in the width over the thickness directions in tensile tests. This ratio characterizes the abhity of materials to resist thinning during forming operations (13). In particular, it is a measure of the abhity of a sheet material to resist the thinning and failure at the base of a deep drawn cup. The plastic strain ratio is measured at 0°, 45°, and 90° relative to the rolling direction. These three plastic strain ratios Rq, R, and R q, are combined to obtain the average strain ratio, cahed the R or the R value, and its variation in strain ratio, cahed... 

Porosity ranks next to thickness in importance, especially when the finishes must serve in polluted and/or humid environments which promote tarnish and corrosion. Pores, openings in the surface that extend to the underplate or substrate, can be intrinsic in the coating (14), or can be produced by mechanical wear or by forming operations involved in manufacturing. In some environments the substrate can tarnish or corrode at pore sites and can produce localized areas of insulating films which cause contact resistance to increase. Porosity is less important for connectors that operate indoors at moderate to low relative humidities and in the absence of corrosive pollutants (15). 

Cladding may be less expensive than selective electro deposition when coatings greater than 1 p.m of a noble metal are required, but may be more expensive than electro deposition for thinner coatings. Selective techniques are most easily used for sheet metal substrates that are to be machine stamped and formed into contacts. Clad noble metals are considerably more ductile (and less hard) than comparable electro deposits and, therefore, are better suited to forming operations. Contacts that are made into separate parts from rod by screw machining are usually coated on all exposed surfaces by barrel electroplating. 

Essentially large-scale, semicontin-uous tray drying. Suited to a wide variety of shapes and forms. Operation can be made continuous. Widely used Not apphcahle Suited for leather, wallboard, veneer. 

Steel Carbon steel is the most common, cheapest, and most versatile metal used in industry. It has excellent ductility, permitting many cold-forming operations. Steel is also very weldable. 

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