Plasticity forming methods

Vitreous silica is high purity Si02 glass that can withstand service temperatures above 1,000°C. As a metastable phase of silica, vitreous silica can be readily obtained in nature and synthetically. Silica glass can be produced in a pure and stable form, displaying useful properties, but is rigid and difficult to shape even at 2,000°C. Hence, it is not accessible to mass production plastic-forming methods. However, techniques have been developed to produce vitreous silica in various shapes and sizes [36-39],... 

Extrusion is a suitable plastic forming method for fabricating a variety of shapes with constant cross section, not only for flat shapes, but also for structural shapes, such as I-sections, channels, pipes, and tubes. 

Plastic forming methods in which a mixture of the ceramic powder and additives is deformed plastically through a nozzle or in a die provide a convenient route for the mass production of CCTamic green bodies. Extrusion is used extensively in the traditional ceramics industry and to a lesser extent in the advanced ceramics sector. Injection molding has been the subject of intense investigation in recent years, but it has not yet made any significant inroads in the forming of ceramics for industrial applications. 

Injection-molded articles can be decorated by in-mold labeling or by post-mold decoration. In the former method, printed film is inserted into the mold cavity before injection. The plastic forms an intimate contact with the graphic material. Post-mold decoration includes hot stamping, dry offset printing, and decal printing. 

Common consolidation methods include variations on pressing powders, plastic forming, slip casting and tape casting. The choice of which method to use for a given application is very dependent upon the shape, size and required dimensional tolerances of the final product, available capital outlay and desired productivity. Each of these methods is examined in turn below, followed by sintering, which turns the consolidated powders into a monolithic body with mechanical integrity. 

Unlike clay-based systems, modem ceramics require additives, termed binders, to provide the plasticity required for ductile-forming methods to be used. These organic additives serve to modify the rheological behavior of the ceramic suspensions and impart handling strength to the green, as-formed ceramic bodies. Their... 

Vacuum forming Method of sheet forming in which the plastic sheet is clamped in a stationary frame, heated, and drawn down by a vacuum into a mould. In a loose sense, it is sometimes used to refer to all sheet forming techniques, including Drape Forming involving the use of vacuum and stationary moulds. 

Sintered silicon carbides are formed by all the traditional methods as well as standard plastic forming techniques such as injection molding. These compacts are sintered with small amounts of additives at very high temperatures in inert atmospheres to form essentially a single phase silicon carbide structure. 

Of the forming methods mentioned, extrusion of bodies plasticized with methyl-cellulose, polyvinyl alcohol, etc., is the commonest. More recently, quite new processes have been developed on the basis of principles employed in other fields of industry, primarily injection moulding, known from the field of organic polymers. 

A number of new ceramics do not necessarily use clay raw materials as plastic components. This function is met either by suitable organic substances (the so-called plasticizers) or use is made of other forming methods. With these materials, which include in particular the newer types of technical ceramics, the traditional classification of raw materials has lost its significance. 

It should be noted that the various forming methods require somewhat different rheological parameters, so that the way of determining the optimum workability described above is just a rough approximation. In addition, one should also consider plastic Viscosity, which characterizes the resistance to the acting stress offered by the body being formed. 

The forming methods can be divided according to the nature of the material ben formed (casting slip, plastic mass, powdered mixture) or according to the nature of t external forming forces. The classification suggested by Spicak (1969) of the fonnii processes is suitable, for example. 

Two basic requirements must be satisfied for plastic forming to be successful. First, the mixture must flow plastically (above a certain yield stress) for formation of the desired shape. Second, the shaped article must be strong enough to resist deformation under the force of gravity or under stresses associated with handling. These two requirements are particularly important in extrusion, where the powder mixture contains a limited amount of binder and the shaped article must be dried after forming. As in casting methods, the... 

The vibration pressing method with wet powders was used for the forming of specimens in this investigation. This is a new plastic pressing method with vibration,and it could reduce to applied pressures for compacting in contrast to a conventional die pressing method with dry powders,because the binder content was sufficient to allow the freedom of the flow of powders. 

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