The Impact of Mold Design for Floating and Sinking Fish Feed in Twin-Screw Extruders

Date: 2026-01-09 Categories: Blog Views: 15397

Fish feed moulds

The core difference between molds for floating and sinking fish feed lies in the design of the die flow channel, compression ratio, hole formation/venting structure, and heating/cooling system. These molds must be highly compatible with the twin-screw extruder's screw configuration, temperature/pressure profiles, and formulations. Floating feed molds are primarily puffing heads (with venting/hole formation), while sinking feed molds use compact heads (high compression/low temperature). These molds are not interchangeable.

1. Core Logic of Twin-Screw Extruder-Specific Molds

The main advantage of the twin-screw extruder is its high conveying capacity, shear force, precise temperature/pressure control, and continuous production. The mold (die) is the terminal component that achieves "particle shaping and density control":

  • Floating Feed: The twin-screw extruder provides high temperature and pressure (120–140°C, 3–6 MPa), allowing the starch to fully gelatinize and produce gas. The core design of the die is venting, hole formation, and pressure expansion, forming enclosed air pockets with a density of <0.95 g/cm³.
  • Sinking Feed: The twin-screw extruder provides low temperature and high pressure (80–90°C, 6–10 MPa), avoiding excessive gelatinization. The die design focuses on high compression, compact flow channels, and no air pockets, achieving a particle density of >1.05 g/cm³.

Key Premise: The screw configuration (conveying, shearing, kneading sections), temperature zones, and feeding rate of the twin-screw must be matched with the die parameters. Otherwise, particle formation and density will be uncontrolled.

2. Core Parameter Comparison of Twin-Screw-Specific Sinking vs. Floating Feed Molds (Die + Die Plate)

ParameterTwin-Screw Sinking Fish Feed MoldTwin-Screw Floating Fish Feed Mold (Puffing Die)Core Impact (Twin-Screw Context)
Core FunctionDense forming, no air pockets, sinks immediately in waterPuffed forming, uniform air pockets, floats ≥30 minutes (National Standard)Directly determines feed density and buoyancy
Compression Ratio (Die Flow Channel / Die Holes)High (12:1–18:1, hard pellets); screw configuration: high compression section + compact dieLow (6:1–10:1, puffed pellets); screw configuration: shearing section + venting section + hole formation dieHigher compression results in denser pellets; low compression + venting aids in gas expansion
Die Flow Channel DesignStraight holes/micro-conical holes, no venting slots, smooth flow channels, no dead corners; die hole diameter: 0.8–4 mmStep holes/funnel holes/ring-shaped venting slots, flow channels with guiding cones, die holes with grooves/closed holes; hole diameter: 1.5–6 mmVenting slots discharge water vapor/gases, hole formation structures help expand the gelatinized feed, forming air pockets
Die Plate Material & Surface TreatmentTungsten carbide/alloy steel, HRC60+; mirror-polished inner wall, resistant to high-pressure wear, suitable for low-temperature high shearStainless steel/alloy steel + nitriding treatment, HRC55–60; inner wall resistant to temperature and fatigue, suitable for high-temperature puffing (120–140°C)Sinking die resists high-pressure wear; floating die resists thermal fatigue + caking
Heating/Cooling SystemLow-temperature control (80–90°C), die with cooling water channels, preventing gelatinization and gas productionHigh-temperature control (120–140°C), die with heating rings + insulation layer, some with zoned temperature control; puffing section requires overheated steam assistanceTemperature directly determines starch gelatinization degree, which in turn affects air pocket formation
Formula Adaptation (Twin-Screw-Specific)Low starch (15%–25%), high protein (30%–45%), high oil (8%–12%); moisture 12%–14%High starch (25%–35%), medium protein (25%–35%), low oil (<8%); can add leavening agents/binders; moisture 14%–16%Starch is the core ingredient for buoyancy; excessive oil will damage air pockets; high protein benefits density
Extrusion Parameters MatchScrew speed: 300–400 rpm, pressure: 6–10 MPa, low-temperature short extrusionScrew speed: 400–600 rpm, pressure: 3–6 MPa, high-temperature long extrusion (with venting section)Pressure/speed/temperature determine shear force and gelatinization degree

3. Key Types of Twin-Screw Molds and Their Suitable Applications

(a) Sinking Feed Molds (Twin-Screw Specific)

  • Standard Straight Hole Compact Die: Suitable for carp, crucian carp, and shrimp/crab opening feeds (0.8–1.2 mm), smooth flow channel, dense pellets, clog-resistant mold; ideal for medium to large aquatic feed plants with high batch production and uniform density.
  • Micro-Conical Hole Die: Suitable for cold-water fish (e.g., salmon juveniles), with larger entry and smaller exit holes for denser pellets, anti-wear, clog-resistant mold; ideal for premium aquatic feed.
  • Flat Dies (for Small Twin-Screw Extruders): Suitable for small-scale farms, smaller batches with controlled density, but lower capacity and uniformity compared to ring dies/molds.

(b) Floating Feed Molds (Twin-Screw Specific, Mainstream is Puffing Dies)

  • Step Hole/Funnel Hole Puffing Die: Mainstream die type, suitable for tilapia, grass carp, and other major freshwater fish; flow channels with guiding cones + venting slots, uniform air pockets, strong buoyancy; ideal for medium to large feed plants.
  • Ring-Shaped Venting Die (Twin-Screw Venting Type): Suitable for marine fish and ornamental fish; die with ring-shaped venting slots to discharge water vapor/gas, preventing air pocket rupture, buoyancy ≥60 minutes; ideal for premium aquatic feed.
  • Twin-Screw – Single-Screw Composite Die: Suitable for ultra-large capacities, where the twin-screw handles gelatinization and the single-screw handles shaping; die with hole formation structures for high capacity and stable buoyancy.

4. Mold Selection, Debugging, and Maintenance for Twin-Screw Extruders (Practical Guide)

(a) Selection Steps (Accurate Matching)

  1. Define the feed type (sinking/floating)
  2. Set the density target (sinking > 1.05, floating < 0.95)
  3. Confirm the formula (starch/protein/oil ratio)

Match Twin-Screw Parameters:

  • Screw configuration (conveying/shearing/kneading sections)
  • Temperature zones (sinking 80–90°C, floating 120–140°C)
  • Pressure range (sinking 6–10 MPa, floating 3–6 MPa)

Confirm Mold Parameters:

  • Compression ratio, die hole diameter, flow channel structure, material, heating/cooling system

Small Batch Trial Production:

  • Test particle density, floating time, and forming quality, adjust mold/screw/temperature parameters.

(b) Common Issues and Debugging Solutions

ProblemSinking FeedFloating Feed
Insufficient Density / Poor Buoyancy1. Increase compression ratio;
2. Lower granulation temperature;
3. Increase protein/oil ratio;
4. Adjust screw configuration to add shearing section		1. Decrease compression ratio;
2. Increase granulation temperature;
3. Add more starch/leavening agents;
4. Increase venting slot area;
5. Adjust screw configuration to add venting section
Clogging of Die Holes1. Control raw material moisture (12%–14%); 2. Reduce fiber content; 3. Polish die holes; 4. Use anti-clogging molds (e.g., micro-conical holes)1. Control raw material starch purity;
2. Lower temperature to avoid caking;
3. Increase venting;
4. Use die holes with grooves to prevent sticking
Uneven Particles1. Adjust screw speed for uniform feeding;
2. Polish die holes;
3. Use smooth flow channel molds		1. Adjust temperature zones for uniform gelatinization;
2. Add guiding cones for uniform flow;
3. Use step holes for even puffing

(c) Maintenance Key Points (Prolonging Mold Life)

  • Clean die holes after each shutdown to remove feed residue and avoid blockages; use dedicated cleaning tools to prevent die hole damage.
  • Regularly inspect die hole wear; sinking dies wear down, leading to reduced particle density; floating dies wear down, affecting buoyancy. Replace dies with wear > 0.1 mm.
  • Lubricate die heads regularly to prevent mold jamming; check heating/cooling systems periodically to prevent temperature control failure.
  • Die storage: After cleaning, apply anti-rust oil, store in sealed containers, and avoid humidity.

5. Summary and Selection Recommendations

Molds for twin-screw extruders in fish feed production are not "universal parts" but "customized components." For floating feed, prioritize low compression ratios, step holes, and ring-shaped venting slots for puffing dies, paired with high starch formulas and high-temperature venting processes. For sinking feed, prioritize high compression ratios, straight holes/micro-conical holes for compact dies, paired with low starch formulas and low-temperature high-pressure processes.