Uniaxial pressing and isostatic pressing are both methods for compacting powder samples, but they have some important differences.
Uniaxial pressing involves applying a force along one axis, typically in an up/down direction. This method is used for pressing simple shapes that have two fixed dimensions, such as cylinders or squares/rectangles. It requires a mold and a hydraulic press and is a relatively inexpensive process. However, there are some limitations to uniaxial pressing. First, the aspect ratio of the samples should be relatively short, meaning they should not be too long. This is because the powder close to the moving pressing surfaces is compacted more than the powder further away from the pressing surface. Second, uniaxial pressing is only suitable for samples with simple shapes.
Isostatic pressing, on the other hand, applies pressure from all directions to the specimen, reducing the gradient effects between the powder particles and the die walls. In isostatic pressing, a uniform hydrostatic pressure is applied to the powder enclosed in a flexible rubber or plastic mold. There are two commonly used types of isostatic pressing: wet-bag and dry-bag. In wet-bag isostatic pressing, the powder is filled in a shaped and flexible mold, sealed, and immersed in liquid in a high-pressure vessel. The liquid is pressurized, and the pressure is transmitted through the flexible wall of the mold to the powder, resulting in compaction. Wet-bag isostatic pressing provides higher packing uniformity compared to uniaxial pressing. Pressures of up to 1000 GPa can be used, although the most common production units operate up to 200-300 MPa. Wet-bag isostatic pressing coupled with 3D green machining is used to fabricate complex ceramic parts at a very high quality. Dry-bag isostatic pressing is easier to automate than the wet-bag process. It has a rubber mold tightly connected to the pressure vessel, but the pressurized liquid does not act from all directions. The mold must be carefully designed to ensure homogeneous particle packing in the powder compact.
One of the main advantages of isostatic pressing is that it overcomes some limitations of uniaxial pressing. Isostatic pressing applies pressure from all directions, resulting in more uniform particle packing. However, because of the flexible mold used in isostatic pressing, it is not able to provide green bodies with the same accurate dimensions as uniaxial pressing. Isostatic pressing is particularly useful for shaping complex parts or producing high-density and isotropic green bodies.
In summary, uniaxial pressing involves applying force along one axis and is suitable for simple shapes, while isostatic pressing applies pressure from all directions and is useful for complex shapes and achieving high-density green bodies.
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