Dry Pressing

Dry pressing refers to forming methods that require up to 15 wt% water in which plastic deformation of the clay-water mixture is minimal. At the lower end of the water contents, water is present as a partially adsorbed layer. At the higher end, the particle surfaces will be completely covered by the adsorbed layer and some water will condense in fine pores. The amount of water needed for a pressing operation varies depending on the pressing characteristics desired, the state of hydration of the clay, how the clay interacts with water, and the particle size of the clay [1,22]. In dry pressing, water acts mainly as a binder that promotes green strength in a compacted body.

Dry pressing is defined as the simultaneous shaping and compaction of a powder in either a rigid die or a flexible container [28]. Common variations on the technique include uniaxial pressing and isostatic pressing [29]. The water content must be sufficient to pro­mote binding of the clay particles without forming a continuous water film that would allow for excessive plastic deformation under an applied load. Dry pressing is the most common forming technique used in the ceramics industry and it is used to form a variety of clay-based ceramics including floor and wall tile, bricks, and electrical insulators [29]. Shapes with a low aspect ratio (height to diameter) are commonly formed by pressing operations [29]. A schematic representation of a die used for uniaxial dry pressing, along with the resulting forces on the powder compact, is shown in Fig. 8. Compaction pres­sures range from 20 to 400MPa (3-60 ksi) with an upper pressure limit of around 100 MPa for uniaxial pressing. Fabrication of parts with high aspect ratios or the use of pressing pressures above 100 MPa can lead to the development of pressure gradients (Fig. 9) and other defects that affect the quality of parts after pressing and after firing [29]. As a side note, most nonclay ceramics require the addition of binders and plasticizers as

Dry Pressing

Fig. 8 Schematic representation of a common die geometry for dry pressing [1]

Dry Pressing

Fig. 9 Schematic representation of pressure gradients that are present in uniaxially pressed clay-based ceramics after pressing at (a) low and (b) high pressure (Reproduced by permission of John Wiley from J. S. Reed, Principles of Ceramic Processing, 2nd Edition, John Wiley, New York, 1995) [1]

forming aids [1]. Organic additives are commonly used as binders and plasticizers, but clays such as bentonites are also used as binders/plasticizers in many applications [14].