One extruder. Two completely different products.

Most feed mills start with floating feed — it's more forgiving to produce, and easier to monitor fish feeding behavior. But many species, including shrimp, bottom-feeding fish, and crab, require sinking pellets. If you're running an extruder and want to produce both, the machine itself doesn't change. What changes is how you configure and operate it.

Here's what actually matters — based on production experience, not catalog specs.

The first thing that determines whether a pellet floats or sinks is the formula, before the extruder does anything.

Starch content drives expansion. For floating feed, around 20% starch is the target. For sinking feed, keep starch between 10–15% — around 15% works well in practice. Too little starch and water stability suffers; the pellet breaks apart and you get more fines.

Fat content works in the opposite direction. Higher fat content helps the pellet sink. For sinking feed, target fat at 5–8%, slightly higher than typical floating feed formulas. For floating feed, lower fat and higher starch give the expansion needed for buoyancy.

One more practical note: plant-based raw materials gelatinize more readily than animal-based materials. If your sinking feed formula is heavy on fish meal, expect the process to be less forgiving.

The die plate for sinking feed is meaningfully different from a floating feed die.

The open area ratio on a sinking feed die should be at least 40% higher than a floating feed die. The reason: sinking feed production requires a smaller pressure differential between the inside of the extruder barrel and the atmosphere outside. Less pressure differential means less expansion — which is exactly what you want. More die openings reduce that differential.

The length-to-diameter ratio of the die holes also matters. For sinking feed, a ratio of approximately 1:1.8 works well. Longer die holes allow more time for the material to set before exiting, which helps keep density up.

This is the most technically demanding adjustment when switching between floating and sinking production.

For sinking feed, material residence time inside the barrel needs to be shorter than for floating feed. On the same machine:

• Floating feed: approximately 30–35 seconds
• Sinking feed: approximately 20–30 seconds

Shear force also needs to be lower. When producing sinking feed, do not use slotted screws at the discharge end — slotted screws increase shear and mechanical energy input, which drives expansion. Instead, use steep-faced screws oriented toward the discharge end, so material exits quickly rather than circulating in the barrel.

Keep the pressure relief vent on the barrel open during sinking feed production. This reduces internal pressure and limits expansion at the die exit.

Moisture at the conditioner stage is higher for sinking feed than for floating feed:

• Floating feed leaving the conditioner: approximately 25% moisture
• Sinking feed leaving the conditioner: approximately 28% moisture

The higher moisture content helps suppress expansion. You can also add 2–3% water directly into the extruder barrel during sinking feed production — this further reduces the pressure build-up that causes expansion.

Both floating and sinking feed benefit from conditioning above 90°C — this is the threshold for adequate starch gelatinization.

Target gelatinization levels:

• Sinking feed: 70–85%
• Floating feed: 80–95%

When starch is cooked at sufficient temperature and moisture, the granules swell, burst, and form a gel that binds the feed ingredients together. For floating feed, this gel is inflated by steam pressure at the die exit, creating the porous structure that keeps the pellet buoyant. For sinking feed, you want gelatinization high enough for water stability, but not so high that the pellet becomes a foam structure.

Some sinking feed formulas — typically those with low starch or high fiber — cannot reach 90°C conditioning without causing processing problems. In those cases, reduce the water addition at the conditioner and increase water injection directly into the barrel. This often allows the conditioning temperature to reach its target.

Screw speed for sinking feed on an adjustable machine (twin screw): 250–400 rpm. Single screw extruders typically run at a fixed speed around 350 rpm. Higher speed increases shear force and mechanical energy absorption, which promotes expansion — so for sinking feed, stay toward the lower end of the range if your machine allows it.

Dryer temperature is where many operators make a costly mistake.

During extrusion, feed material is subjected to high temperature and pressure inside the barrel. When it exits the die, the sudden pressure drop causes any remaining trapped moisture to vaporize — and if enough starch has been gelatinized, the pellet expands. The degree of expansion is set at the die exit.

If your pellets exit the extruder at 80°C and enter a dryer set at 120°C, the heat differential will trigger secondary gelatinization of any remaining uncooked starch — and pellets that were sinking at the die exit will become floating pellets after drying.

For sinking feed, the dryer control principle is: low temperature, slow belt.

• Dryer temperature: around 100°C, ideally below 90°C
• Belt speed: slow enough for a drying time of 40–60 minutes

Rushing the drying stage with high heat is one of the most common reasons sinking feed turns into floating feed by the time it reaches the bag.

The most direct measurement of whether your process is achieving the right result:

If you're planning to produce both floating and sinking feed on the same line, the extruder itself is not the limiting factor — the configuration is. You'll need:

• A die plate designed for sinking feed (higher open area, appropriate L/D ratio)
• A screw set that can be reconfigured for lower shear
• A dryer with reliable low-temperature control and adjustable belt speed
• A conditioner with accurate moisture and temperature control

The extruder's ability to switch between product types — by adjusting temperature, moisture, screw profile, and speed — is one of the main reasons it has replaced the pellet mill in many aquatic feed operations. Floating feed, sinking feed, and slow-sinking feed are all achievable on the same machine with the right setup.

If you're evaluating extruder options for aquatic feed — whether for shrimp, fish, or specialty species — and want to understand how a specific machine configuration maps to your production requirements, contact us. We've configured lines for both floating and sinking aquatic feed across multiple markets.

You can also explore our equipment range and project solutions for reference configurations.