Dry-pressing

Dry-pressing is carried out in a die with movable top and bottom punches (Fig. 3.2). A cavity is formed with the bottom punch in a low position and this is filled with free-flowing granulated powder (Fig. 3.2(a)) which is then struck off level with the top of the die. The top punch then descends and compresses the powder to a predetermined volume (Fig. 3.2(b)) or, in more elaborate presses, to a set pressure (75-300 MPa or 750-300 atm). Both punches then move upwards until the bottom punch is level with the top of the die and the top punch is clear of the powder-feeding mechanism (Fig. 3.2(c)). The compact is then removed, the bottom punch is lowered and the cycle is repeated (Fig. 3.2(d)). 

The feedstock is usually contained in a hopper attached to the press, and if the granules have a range of sizes they may segregate as the die fills and cause density variations. Also, during pressing the granules must flow between the closing punches so that, finally, the space between them is uniformly filled. 

Alternatively, a stiff paste of ceramic and binder solution can be forced through a wire mesh and granules of the required size extracted from the dried product by sieving. The flow of the granules can be enhanced and die-wall friction reduced by tumbling with a small quantity of a powdered lubricant such as calcium stearate. 

Once the granules are properly distributed in the die they must crush readily into small fragments so that their structure is not apparent in the final pressing, since this would result in the formation of large pores. Compaction must not be 

Shapes with a uniform section in the pressing direction are the easiest to produce by dry pressing. Pieces that vary in section require very careful powder preparation and may need special press facilities such as floating dies, i.e. dies free to move relative to the punches, or dies that split open to allow easy extraction of the compact. The time taken by a pressing on an automatic machine varies from 0.2 s for pieces of diameter around 1mm to 5 s for large complex shapes. 

The pressure can be applied in single or two directions. The compaction in single direction is called single-ended compaction, and the second 

The size and shape of the part and the type of the press determine the production rate. For large parts, the rate goes down to a mere 15 parts per minute. Small parts may be produced at a rate of several thousands per minute. The presses can be mechanical or hydraulic. Mechanical presses operate at greater speeds than hydraulic ones. 

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Reed J S and Runk R B 1976 Dry pressing Ceramic Fabrication Processes, Treatise on Materials Science and Technology yo 9, ed F F Y Wang (New York Aoademio) pp 71-93... 

Thurnauer FI 1960 Controls required and problems enoountered in produotion dry pressing Ceramic Fabrication Processes ed W D Kingery (New York Wiley) pp 62-70... 

Bricks may be extmded or dry-pressed on mechanical or hydraulic presses. Formed shapes may be burned before use, or in the case of pitch or resin/chemically bonded bricks, may be cured (tempered) at a low temperature. 

The dry-press process is used to make good quaHty face brick, ie, brick used on exposed waHs. Very Httie water is added to the clay, less than 10% by weight. This is only enough water to make the clay damp. The damp clay is then pressed into molds under pressures of 3.4 to 10.3 MPa (500 to 1500 psi). The damp brick shapes ate removed from the molds when they have set up enough to handle and ate stacked for drying and burning. 

Binder selection depends on the ceramic powder, the size of the part, how it is formed, and the green density and strength requited. Binder concentration is deterrnined by these variables and the particle size, size distribution, and surface area of the ceramic powder. Three percent binder, based on dry weight, generally works for dry pressing and extmsion. 

As shown in Figures 1.36(a) and 1.36(b), the glycine nitrate process-derived powder is highly porous. The pore size ranges from tens of nanometers to several micrometers. The powder is thus named as foam powder, which shows extremely low fill densities of less than 1/100 of the theoretical density value. The low fill density makes it possible to prepare thin films of doped ceria by dry pressing, in which the films are processed by means of punches in a hardened metal die. Figures 1.36(c) and 1.36(d) show a cross-sectional view of an 8 /tm-thick GDC film fabricated by dry pressing. 

Smo.15Ce0 8502 5, 20 dry pressing Ni-Smo CeogjO and BaojSrojCoo.sFeo.2 3-5 electrode... 

Snio 2Ce0 gO 9, 30 Jim, dry pressing Ni-Sm CeogOj 9 and Sm05Sr05CoO3 electrode... 

Get a clean, dry press and two bolts. Screw one of the bolts into the press about halfway and call that the bottom of the press. 

G. Webb, 13, 2284 (1919). Incendiary composition, such as MgO 3.45p, Mg (wax- or resin-coated) 3.7, Ba(N03)2 7. This mixture was dry-pressed. The alkaline earth serves as a retarder of combustion and, by varying the quantity of this ingredient, the rapidity of combustion can be fixed as desired within wide limits... 

Powders. Powders have, in the past, been the principal form in which sulfur dyes were sold. In general, they are made from the dried press cake, finely ground, and standardized with common salt, sodium sulfate, or soda ash. They are prepared for dyeing by making a paste with water, which is dissolved by boiling with the necessary amount of reducing agent and further addition of water. 

The dried press cakes are broken up with a mallet into smaller lumps and thrown continuously into the corning mill (Figs. 97 and 98) consisting of several pairs of bronze rollers arranged one above the other. After being crushed between the up-... 

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