1. What Is Bulk Density? — A Fundamental Property of Powders
Bulk density is one of the most important physical properties when handling powders.
It represents the mass of a powder loosely filled into a container divided by its volume, including the air gaps between particles.
Bulk density varies greatly depending on particle size, moisture content, surface condition, and storage conditions.
Because these changes directly affect powder flow and equipment performance, bulk density is essential data for designing powder-handling systems.
2. How Bulk Density Affects Powder Conveying
Bulk density has a direct impact on feeding accuracy, flow stability, and screw feeder performance.
2-1. Directly affects feed rate calculation
The theoretical feed rate of a screw feeder is:
Screw rotation speed × Screw volume × Bulk density
Low bulk density → less mass per revolution
High bulk density → more mass per revolution
2-2. Influences feeding stability
Low bulk density powders tend to:
- trap air
- collapse inconsistently
- form bridges and ratholes
High bulk density powders:
- flow more evenly
- offer more stable feeding
- may cause adhesion or compaction depending on the material
2-3. Affects required torque and machine strength
Heavier powders increase mechanical load.
This may require stronger motors, optimized gear ratios, and reinforced construction.
3. How Powder Behavior Varies by Bulk Density
Understanding bulk density makes powder behavior more predictable.
Low bulk density powders
Examples: silica, light pigments, cellulose
Characteristics:
- low flowability
- easy air entrainment
- prone to bridging
- unstable feeding
High bulk density powders
Examples: metal powders, sulfates, calcium carbonate
Characteristics:
- more consistent flow
- stable feed rate
- risk of adhesion or compaction
- higher mechanical load on equipment
4. Why Bulk Density Matters in Screw Feeder Selection
At Seiwa Giken, bulk density is always reviewed when selecting or designing screw feeders.
4-1. Choosing screw diameter and pitch
Low bulk density powders often require:
- larger screw diameters
- wider pitch designs
This improves transport efficiency.
4-2. Optimizing hopper design
Low bulk density powders may need flow aids such as:
- agitators
- knockers
- air vibrators
These prevent bridging and maintain stable flow.
4-3. Selecting the correct motor
High bulk density powders increase torque demand.
A higher torque motor or optimized gear ratio may be necessary.
5. Bulk Density Changes Depending on Measurement Method
Bulk density is not a fixed number. Measurement conditions change the result.
- Loose bulk density: gently filled powder
- Tapped density: compacted by tapping or vibration
Both values are important when evaluating powder characteristics.
Seiwa Giken conducts real-condition feeding tests to determine practical feeding behavior.
6. Summary: Bulk Density Sets the Foundation for Powder Equipment Design
Bulk density determines:
- powder flowability
- feeding amount per revolution
- equipment load
- potential failure modes
Understanding bulk density is the starting point for selecting the right screw feeder and designing a stable powder conveying system.
Just like bulk density, another important piece of information is the angle of repose.You can find an article about it here ↓↓
