The reason why the powder flows, its essence is the imbalance of the force on the particles in the powder. The force of particles includes gravity, adhesion force between particles, friction force, electrostatic force, etc. The most important influence on powder flow is gravity and adhesion force between particles. By analyzing the influencing factors of powder fluidity, it is of great significance to use scientific methods to measure powder fluidity.

The factors that affect the fluidity of powder are as follows:

1. Granularity

The specific surface area of ??the powder is inversely proportional to the particle size. The smaller the particle size of the powder, the larger the specific surface area. As the particle size of the powder decreases, the molecular attraction and electrostatic attraction between the powders gradually increase, which reduces the fluidity of the powder particles; secondly, the smaller the particle size of the powder, the easier the particles are adsorbed, aggregated into agglomerates, and bonded together. The increase of the air permeability leads to the increase of the angle of repose and the deterioration of the fluidity. Thirdly, the particle size of the powder decreases, and it is easy to form a close packing between the particles, which reduces the air permeability, the compression rate and the fluidity of the powder.

2. Form

In addition to particle size, the effect of particle morphology on flowability is also very significant. Powders with the same particle size and different shapes have different fluidity. Obviously, spherical particles have the smallest contact area with each other and have the best fluidity. The surface of the needle-like particles has a large number of plane contact points and shear force between irregular particles, so the fluidity is poor.

3. Temperature

Heat treatment can increase the bulk density and tap density of the powder. Because the density of powder particles will increase after the temperature rises, but when the temperature rises to a certain level, the fluidity of the powder will decrease, because the adhesion of the powder increases significantly at high temperature, and the gap between the powder and the powder increases. Or adhesion occurs between the powder and the vessel wall, which reduces the fluidity of the powder. If the temperature exceeds the melting point of the powder, the powder will turn into a liquid, making the adhesion stronger.

4. Moisture content

When the powder is in a dry state, the fluidity is generally good. If it is too dry, the particles will attract each other due to electrostatic action, which will deteriorate the fluidity. When a small amount of water is contained, the water is adsorbed on the surface of the particles and exists in the form of surface adsorbed water, which has little effect on the fluidity of the powder. As the water continues to increase, a water film is formed around the particles adsorbed by the water, and the resistance to the relative movement between the particles increases, resulting in a decrease in the fluidity of the powder. When the water content exceeds the maximum molecular bound water, the more water content, the lower the fluidity index and the worse the powder fluidity.

5. Interaction between powder particles

Powders with different particle sizes and shapes have different influences on the powder fluidity by their cohesion and friction. When the powder particle size is larger, the powder fluidity mainly depends on the powder shape, which is Because the volume force is much greater than the cohesion between the powder particles, the powder particles with rough surface or the powder particles with uneven shape have poor fluidity. When the powder particles are small, the fluidity of the powder mainly depends on the cohesion force between the powder particles, because the volume force at this time is much smaller than the cohesion force between the particles.