COLD ISOSTATIC PRESSING

    Cold Isostatic Pressing (Cold Isostatic Pressing) refers to a molding method in which the powder sealed in a plastic mold is uniformly pressed in all directions at the same time at room temperature. There are two methods of cold isostatic pressing: dry bag method and wet bag method. The wet bag method is a molding method in which powder is sealed in a molding rubber mold, placed in a liquid in a high-pressure container, and isotropic pressure is applied. The dry bag method uses liquid as the pressure transmission medium, but the pressure is only applied to the radial outer wall of the mold, and the axial direction of the mold is basically unaffected.

    In the process of cold isostatic pressing, the surface of the powder particles in contact with the plastic rubber sleeve has no relative displacement during the molding period, and there is no friction effect of the mold wall on the powder in the normal scale pressing. If the plastic rubber sleeve contains rigid components, even if the relative displacement occurs between the powder particles and the rigid components, the magnitude is far lower than the displacement in the normal pressure, and the effect of the mold wall friction is obviously lower than that of the normal pressure. It can be considered that in the process of cold isostatic pressing, the molding pressure acts on the powder in all directions through the rubber-coated mold under the condition of no or little friction of the mold wall. Therefore, the cold isostatic pressing forming technology is more reasonable, and the obtained green compact is prepared by conventional scale pressing.

    The compact density is high and uniform. Under the same forming pressure, the density of cold isostatic pressing is 20-30% higher than that of conventional pressing. If two molding methods are used to make the green body reach the same density value, the molding pressure is higher than the cold isostatic pressing pressure. In addition, under the action of elastic after-effect, the dimensional expansion of the cold isostatic compact is uniform in all directions, while the expansion value of the molded compact in the pressing direction is greater than that in the vertical pressing direction.

As a molding process, cold isostatic pressing technology has the following characteristics compared with conventional molding technology:

(1) The density of products formed by cold isostatic pressing is high. Generally 5 to 15% higher than that of unidirectional and bidirectional compression molding.

(2) The density of the compact is uniform. In compression molding, whether it is unidirectional or bidirectional pressing, there will be uneven density distribution of green compacts. This density change can often reach more than 10% when pressing complex-shaped products. This is due to the frictional resistance between the powder and the steel mold. An isostatic fluid medium transmits pressure, equal in all directions. The rubber sleeve and the powder are generally compressed in the same way, the powder and the rubber sleeve have no relative movement, the frictional resistance between them is very small, and the pressure only decreases slightly. The bulk density is uniform.

(3) The length-diameter ratio of the product is less restricted, which is conducive to the production of rod-shaped and tubular products with a relatively large length-diameter ratio.

(4) The green body formed by cold isostatic pressing has high density, small shrinkage during sintering, and high sintered body density.

(5) The green body formed by cold isostatic pressing has high strength and can be directly transported and machined.

(6) The internal stress of the green body formed by cold isostatic pressing is small, which reduces the defects such as cracking and delamination of the green body.

(7) There is no need to add lubricant to the powder, which reduces the pollution to the product and simplifies the manufacturing process.

(8) It can realize the forming of the green body near and the net size, and reduce the processing cost and raw material waste.

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