Powder Manufacturing Processes Machinery's Handbook, 31st Edition
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Screw
Blade
Blade
(a)
(b)
(c)
(d)
Fig. 12. Blending and Mixing Devices: a) Rotating Drum; b) Rotating Double Cone; c) Screw Mixer; d) Blade Mixer
In the drum and double cone methods, the container has bafflers inside to prevent the mixture from the free falling, an action that separates the particles by mass. This is the point at which other materials can be added to the mixture. The other materials consist of lubricants (used to lower the friction between the powder and the die), binders (used to hold some metals together between pressing and sintering), and deflocculants (used to allow for better powder flow in the die). Lubricants typically are stearic acid or zinc stearate in a proportion of 0.25 to 5 percent by weight. Because of their high surface area- to-volume ratio, metal powders are explosive; this is particularly true of aluminum (Al), magnesium (Mg), titanium (Ti), zirconium (Zr), and thorium (Th). For that reason, great care must be exercised to protect both workers’ health and the general environment dur- ing blending and handling. Compaction of Metallic Powder.— After the metallic powders have been blended, com pacting is the next step. The purposes of the compacting operation are to obtain the re- quired shape, ensure particle-to-particle interlocking, and make the part sufficiently strong for further processing. In compaction, the powder is fed into a die cavity, either automatically or by hand by a feed shoe. At the start the powder is at about 2.5 times the final part volume. The process generally is carried out at room temperature (cold compacting), although it can be done at elevated temperatures (hot compacting). After pressing, the part, which is called a green compact , is ejected from the die. Green parts are very frail and can crumble or be damaged very easily. To obtain higher green strengths, the powder must be fed prop erly into the die cavity, and proper pressure must be developed throughout the part. Fig. 13 shows various stage metallic powder compacting process.
v
v, F
v, F
Upper Punch
Compacted Shape (green)
Powder Feed Shoe
Die
v, F
Lower Punches
(a) (d) Fig. 13. Compacting Metallic Powder: a) Filled Die Cavity with Powder; b) Initial Position of Upper and Lower Punches; c) Final Position of Upper and Lower Punches; d) Ejection of Part (Green) (b) (c)
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