How to Control Puffing Degree in Twin-Screw Fish Feed Extruders
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How to Control Puffing Degree in Twin-Screw Fish Feed Extruders
The puffing degree of a twin-screw fish feed extruder can not only be adjusted, but also precisely controlled - typically within an error of ±5%. This precision comes from three core design advantages.
1. Three Design Advantages of the Twin-Screw System
- Meshing twin-screw transmission: the two screws mesh and rotate in the same or opposite directions in the extrusion chamber, creating forced conveying and a strong shearing effect. Adjusting screw speed precisely controls the residence time and shear intensity of the raw material, thereby precisely regulating the puffing degree.
- Independent steam heating and temperature control: unlike single-screw extruders that rely solely on frictional heat, the twin-screw extruder injects a fixed amount of steam into the chamber through a steam-injection system. Combined with segmented heating, the chamber temperature is precisely controlled between 120–160 °C with a fluctuation of ≤ ±3 °C - a stable thermal basis for puffing control.
- Modular die-head design: dies with different apertures and structures can be replaced quickly; combined with an adjustable-speed cutting device, the range of puffing adjustment can be refined further.
2. The Working Principle of Puffing Control
Extrusion is the synergistic result of "extrusion shearing - high temperature and pressure - instantaneous pressure release". Adjusting the puffing degree means controlling how much the internal moisture of the raw material vaporizes and expands by changing key parameters: the greater the shearing intensity, the higher the temperature and pressure, and the longer the residence time, the more fully the starch gelatinizes and the more easily internal moisture vaporizes. When the material is instantaneously depressurized through the die, the expansion force generated by moisture vaporization is stronger - and the puffing degree is higher.
The twin-screw extruder controls this process through three parameters: screw speed, steam-injection rate and die orifice diameter. Increasing screw speed enhances shear strength but shortens residence time; combined with increased steam injection, a high puffing degree is achieved. Conversely, decreasing screw speed and steam injection reduces the puffing degree, producing sinking feed. Example: for sinking grass-carp feed, reducing screw speed and steam injection controls the puffing degree at 30–40%, giving compact pellets that sink rapidly with water stability exceeding 10 hours
3. Practical Verification
| Application example | Puffing degree | Result |
|---|---|---|
| Sea bass floating feed (large aquatic feed mill) | 85–90% | Floating rate over 98%, meeting sea bass farming needs |
| Juvenile carp feed (200 kg/h small extruder) | 50–55% | Fine, palatable pellets; 95% feed intake for juveniles |
| Adult carp sinking feed | 40–45% | Compact, water-resistant pellets; feed waste below 5% |
| Grass-carp sinking feed | 30–40% | Rapid sinking; water stability over 10 h |
Within 8 hours of continuous production, puffing fluctuation can be controlled within ±3% - far superior to the ±10% fluctuation typical of a single-screw extruder. This stability ensures consistent feed quality across batches, a core advantage for large-scale aquatic feed production.
4. Operating and Safety Notes
Match formula moisture to the target puffing range; starch content should normally be above 20% for reliable expansion.
Monitor steam pressure and temperature; the steam-injection system must include pressure gauges, safety valves and interlock protection.
Run trials with small batches when changing formulas or dies, recording parameters (screw speed, steam rate, temperature, die size) for the recipe library.
Values quoted are typical test results; confirm performance for your formula and machine model with the supplier.
