Wet bag isostatic pressing

Further patents were taken out on the isostatic pressing of refractory metal powders by Coolidge in 1917 (for tubes of tungsten and molybdenum) and by Pfanstiehp82 in 1919, while Fehse described the wet bag isostatic pressing of tungsten tubes in 1928. However, very little further development appears to have followed. 

Shape is often also important. In many cases, spherical products of size enlargement are desired. The approximate shape can be obtained with all tumble/growth agglomeration methods. On the other hand, unless extremely accurate feed control is established in some punch-and-die machines or by using wet bag isostatic pressing, spherical products cannot be produced with pressure agglomeration. The nearest approximation would be pillow-, lens-, or almond-shaped compacts. 

A small wet-bag isostatic press, used to produce laboratory samples and low-volume production parts, might have an internal diameter of 150 mm and a depth of 460 mm. Large wet-bag presses may have cavity diameters >1.8 m and lengths up to 3.7 m. The wet-bag CIP process can be automated. 

Isostatic pressing is a compaction technique through the application of a uniform hydrostatic pressure to the powder confined in a flexible rubber container. There are two modes of isostatic pressing (i) wet-bag pressing and (ii) dry-bag pressing. In wet-bag isostatic pressing, the powder is filled in a flexible rubber mold, which is then submerged into a pressure vessel filled with oil as fluid, as shown... 

Fig. 4.16 Schematic of a wet-bag isostatic pressing system. Reproduced with permission from [115]. Copyright 2007, Springer...

Fig. 8.152 Photograph of an automated wet bag cold isostatic press (pressure chamber dimension 430 mm dia. 1,000 mm high, 200 MPa max. pressure) with mold washing and handling system (courtesy EPSI, Haverhill, MA, USA).

There are many variations on using the cold isostatic press (CIP) here we just emphasize some basic themes. Figure 23.3 illustrates the so-called wet-bag CIP process. Powder is weighed into a rubber bag and a metal mandrel is inserted that makes a seal with the mouth of the rubber bag. The sealed bag is placed inside a high-pressure chamber that is filled with a fluid (normally a soluble oil/ water mixture) and is hydrostatically pressed. The pressures used can vary from about 20 MPa up to IGPa depending upon the press and the application. For production units the pressure is usually <400 MPa. Once pressing is complete, the pressure is released slowly, the mold is removed from the pressure chamber, and the pressed component is removed from the mold. 

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