Machinery's Handbook, 31st Edition
1536 Powder Manufacturing Processes The amount of pressure necessary for a compaction operation is to a large degree based on the material. For example, compacting aluminum powder generally requires lower pressure, while compacting iron powder requires relatively higher pressure. The compac tion pressure also depends upon the powder characteristics, additives, and the desired density of the green compact. The density of the green compact depends on the applied pressure and the size distribution of the particles. If all particles are of the same size, there will always be some porosity when they are packed together. Friction force will oppose the movement of particles during processing; therefore, lubrication can reduce the required compaction pressure and also will cause a more uni- form distribution of particles during pressing. Lubricant should be applied in the correct quantities. As the compaction pressure increases, the density of the green compact increases, and the individual particles are pressed together to plastically deform them, increasing their contact and friction but decreasing the pore size. The higher density increases the strength and elastic modulus of a part. Fig. 14 shows density of the powders as a function of pressure.
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Fig. 14. Density of the Powders as a Function of Pressure: a) Filled Die Cavity with Powder; b) Initial Position of Upper and Lower Punches; c) Final Position of Upper and Lower Punches Typical pressed densities for soft iron components would be, e.g., about 85 percent of theoretical density. Compaction pressure rises significantly if higher pressed densities are required, and this practice becomes uneconomical due to higher costs for the larger presses and stronger tools to withstand the higher pressures. One disadvantage of this technique involves the differences in pressed density that can occur in different parts. Because of friction between metal particles in the powder and friction between the punches and die walls, the density within the part can vary consider- ably. Density variation can be minimized by proper punch and die design, by control of friction, and by keeping low the length-to-diameter ratio of the part.
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