Powder density (specific gravity) plays a very important role in powder mixing. The smaller the difference in powder density, the easier it is to mix the powder. The larger the difference in powder density, the more difficult it is to mix the powder. Powder density also affects powder fluidity. Under the same conditions, the greater the powder density, the better the fluidity.
The concept of powder density (specific gravity)
The density of powder refers to the mass of powder per unit volume. In practical applications, the density of powder can be divided into bulk density and tap density.
1. Bulk density: refers to the ratio of the mass and volume of powder naturally accumulated in a small dose volume unit, and the powders are loosely combined.
2. Tap density: refers to the ratio of the mass and volume of powder after repeated vibration and compaction within a unit volume, and the powders are in a relatively tightly combined state. Usually the tap density is 1.2 to 1.3 times the bulk density.
3. The actual density of the material in the barrel: In practical applications, the comprehensive density of the powder in the mixing equipment barrel is between the bulk density and the tapped density. Therefore, the effects of both densities are taken into account when calculating the loading capacity of mixing equipment.
4. The relationship between powder specific gravity and powder density:
Density is the ratio of powder mass to volume (the measurement method of mass is very complicated); specific gravity is the ratio of powder weight to volume (the measurement method of weight is very simple). At low altitudes close to sea level, the specific gravity and density of powder are almost equal. Therefore, we usually use specific gravity to approximate density.
2. The influence of powder specific gravity on mixing
Powder specific gravity is an important factor affecting the mixing process, and its effects include the following aspects.
1. The impact of the difference in specific gravity between powders on powder mixing:
1). Under the same conditions, the smaller the difference in specific gravity between powders, the easier it is to achieve uniform mixing. This is because uneven phenomena such as weight segregation are less likely to occur when the powder is in motion;
2). Under the same conditions, the greater the difference in specific gravity between powders, the more difficult it is to achieve uniform mixing. This is because when the powder is in motion, the heavier powder particles tend to sink, while the lighter powder particles tend to float, making it difficult to mix uniformly.
2. The impact of powder specific gravity on fluidity can indirectly affect the ease of powder mixing:
1). The greater the specific gravity of the powder, the better the fluidity of the powder will be during mixing. This is because the weight increases the influence on the movement of the powder.
2) Generally speaking, the better the fluidity, the easier it is to homogenize and mix the powder.
Note: If the fluidity of the powder is too good, it will cause difficulties in uniform mixing. This problem will be discussed in the fluidity issue.
3. Typical mixing problem: "Light and heavy powders are not easy to mix evenly"
When the difference in specific gravity of powders is large, it will be difficult to mix light and heavy powders evenly.
Question asked:
The above example is that in the powder metallurgy industry, we often encounter the problem of mixing light and heavy powders. The classic representative is to add (alloying elements) non-metallic carbon powder to metal powder. The specific gravity of metal powder is usually between 1.0 and 3.0. Sometimes, some tungsten powder even reaches above 4.0, while the specific gravity of carbon powder is about 0.3. It is very difficult to evenly mix the lighter and less carbon powder into the heavier metal powder during mixing. This is a typical mixing problem of light and heavy powders. The powder is in the moving mixing container. Since the specific gravity of the light and fine carbon powder is much smaller than that of metal and other powders, some of the light powders are always suspended above the main powder. , will even float in the air. If there is no forced mixing means, even if the mixing time is extended, it will be difficult to mix into the theme powder.
