Feed Extrusion Parameters and Formula: What Sets Puffing Degree?

The formula sets the limit before the machine is involved - starch at 20% or more for floating feed against 10-15% for sinking feed, fat at or below 12% on a single screw and 17% on a twin screw, crude protein 25-60%, incoming moisture 15-27%.

Process windows then work inside that limit: barrel 120-180 deg C with 140-160 deg C best for cereals, conditioner steam 7-8% on soybean and about 10% on corn at 0.1-0.4 MPa, conditioning 65-90 deg C, and staged cooling.

> > - Starch is the expansion skeleton; high-amylopectin, low-gelatinization-temperature sources expand most.

> - Fat above 8% belongs in post-extrusion spraying, not in the premix.

> - Grinding fineness by species: 20 mesh full pass for freshwater fish, 40 mesh for crab and shrimp, 60 mesh for eel and turtle.

> - Every 1% of fat between 12% and 17% adds about 16 kg/m3 to bulk density.

> - Mixed float and sink after drying means uneven cooling, not a bad formula.

What Decides Puffing Degree: Formula, Process or Post-Processing?

Puffing degree is the visible result of three events happening together: starch gelatinization, pressure release at the die, and water flashing to vapor. The factors affecting feed extrusion group into three tiers - formula, process settings, and post-processing - and the formula tier sets the limit that no machine setting can lift.

For ingredient-level data on starch, protein and fat sources, the Feedipedia feed ingredients database is a free reference that formulation teams can cite directly.

The formula-set limit

The three events form one chain, not three separate knobs. Starch has to gelatinize before the melt can hold gas, the die has to release pressure fast enough for that gas to expand, and enough water has to flash to vapor to fix the bubble structure. The process tier works inside the formula limit; post-processing only locks what the extruder has already produced.

How to judge whether the fault is formula or machine

A formula problem is there from the first batch and does not answer to temperature or screw speed, because the limit itself is too low. A machine problem shows up as a change on a line that was already running well, and it usually traces back to wear, moisture or the die. In the mills we supply, a dense pellet is far more often a formula or conditioning problem than a screw problem.

Which Starch Level and Starch Source Set the Expansion Rate?

Starch is the expansion skeleton: its content and gelatinization degree set the expansion rate directly. It does two jobs - it expands, and it binds - and its level is the main factor deciding whether a feed floats or sinks. Sinking aquafeed carries 10-15% starch; floating aquafeed needs at least 20%. Floating fish feed extrusion parameters follow the same windows as sinking feed, except for the starch floor: below 20% the expansion cannot overcome the density of the protein and mineral fraction.

For a complete line built around this window, see the floating fish feed extruder line.

Starch windows by feed type

Within those windows, starch type moves bulk density. Prefer sources high in amylopectin and low in gelatinization temperature, because they give the highest expansion. Practical sources run 75-82% starch in wheat flour, 70-75% in corn flour and about 81% in rice flour; tuber crops such as potato and cassava carry more amylose and bind well. The more amylopectin and denatured starch, the greater the puffing degree and the lower the bulk density, while high amylose with denatured starch raises crush strength and hardness. Extrusion puffing degree is set at the mixer.

How to judge the starch source by composition

Denatured starch builds a flint-like surface structure that slows water uptake and improves water stability - the property that keeps a floating pellet intact for hours instead of minutes. Judge the source on two readings: a high-amylopectin, low-gelatinization-temperature flour gives the most expansion for a given conditioning window, while a high-amylose flour trades expansion for crush strength. A feed that must float through a full feeding cycle is a water-stability problem first and an expansion problem second.

Where Is the Fat Limit, and When Should Fat Be Post-Sprayed?

Fat is the most common reason a formula will not puff. The working limit is 12% for single-screw extrusion and 17% for twin-screw extrusion. At or below 12%, fat has almost no effect on pellet quality; above the limit, the pellet loses expansion long before it loses stability, and that is the failure mode that shows up as a sinking "floating" feed. When a formula has to carry more fat than the extruder can handle, move the surplus to post-extrusion spraying, so the oil does not interfere with gelatinization.

For a side-by-side view of screw types, see our guide on how to choose an aquaculture or pet feed extruder.

If your formula sits above the single-screw fat limit, tell us your target bulk density and we will size the extruder - we reply with an itemised quotation, EXW/FOB basis; CIF available on request.

The 12% and 17% fat limits

Between 12% and 17%, every 1% of extra fat adds about 16 kg/m3 to finished bulk density - a linear penalty you can use to predict float behaviour before running the batch. From 17% to 22% the pellet barely expands, but pellet quality stays stable; above 22%, stability collapses. Routine practice keeps total fat at or below 8% and sprays the surplus onto the finished pellet, because fat native to raw materials interferes less than added oil. Hardness climbs with fat from 6% to 14%, then falls quickly above 14%.

The decision rule for single-screw and twin-screw frames

Use the 16 kg/m3 rule as a formula check: if the target bulk density is fixed and the formula drifts from 12% to 17% fat, expansion is what you lose first, and no barrel temperature rise buys it back. Two ways out - post-spray the surplus fat, or move the formula to a twin-screw frame. The decision rule: native fat under 12% is fine on a single-screw frame. Added fat above 12%, or a viscous high-fat formula, calls for the twin-screw series. That series covers high-fat aquafeed with 11-13% fat addition, pet formulas with 20-30% fresh meat and 20% or more fat, and cooked compound feeds with starch gelatinization above 95%.

Frame selection, with rated outputs from our machine data:

Size the frame on sustained output after cooling, not on the nameplate peak: the largest single-screw frame is rated 1800-2000 kg/h on a 132 kW drive with a dia. 200 mm screw, but that rate only holds if the dryer and cooler behind it are matched. A line rated 40-800 kg/h covers most farm and small-mill needs. Above that, work back from cooler capacity, not from the extruder brochure. Diesel-driven versions of the smaller frames run the same outputs without grid power, which suits field sites and areas with unreliable electricity.

What Does the 25-60% Protein Band Do to Puffing and to Extruder Load?

Fat sets the limit above; protein sets the skeleton inside it. In the finished pellet, protein is the structural partner to starch - holding shape while starch supplies the expansion. Where the formula sits inside the protein band decides whether the extruder spends its energy on puffing or on load. Protein and fat both work against starch expansion, which is why high-fat, high-protein formulas are the hardest to puff.

For dog and cat formulas at the top of the band, see the pet feed extruder range.

The 25-60% protein band

Crude protein in extruded formulas runs 25-60%, where it acts as a structural skeleton in the finished pellet. Within that band, raising protein lowers the friction coefficient, cuts barrel wear, improves product organization and adds elasticity. The trade-off sits at the top: high protein means lower puffing degree, a more viscous melt and a higher energy draw.

How to judge plant protein against animal protein

Plant protein absorbs water and binds well, forms and puffs easily, and is cheap - but its digestibility trails animal protein and it takes up a lot of formula space. That is the judgement call: plant sources extrude more easily, while animal sources digest better and leave more room for starch and fat. When protein climbs, raise extrusion temperature and trim moisture to hold gelatinization. The energy penalty is measurable: a high-protein melt needs more shear for the same expansion.

How Do Fiber and Fibrous Protein Meals Limit Puffing Degree?

Fiber is the one raw material factor with no fixed numeric limit - its effect on puffing degree is measured by trial on your own meal, not read off a table. High straw-type fiber pierces bubble walls, suppresses expansion and leaves a firm, dense pellet. Treat it as a limiting condition rather than a number to tune, because how far the grinder can take the fibrous meals sets the target.

The fiber and fibrous meal limit

Fiber caps the limit from the other side, without a dial. Where straw-type fiber runs high, suppression of expansion is immediate, so the puffing target has to be set lower before the batch is mixed; the exact limit for your formula is to be confirmed against the factory technical confirmation sheet. Fibrous protein meals add a second penalty: cottonseed and rapeseed meals are hard to grind and plug dies, which raises breakdown rates and leaves torn, poor-looking pellets.

How to judge a die-pressure spike

If those meals dominate your protein supply, budget for finer grinding and plan for more die changes - and treat any die-pressure spike as a grinding problem before you touch the formula. A spike with a rising motor load and torn pellet surfaces says the same thing: the meal is too coarse for the die.

What Grinding Fineness Do Freshwater Fish, Shrimp and Eel Feeds Require?

Fiber determines the lower limit; grinding sets how much of it the cook can reach. Surface area, digestibility and how completely starch gelatinizes all trace back to grinding fineness - and gelatinization is what makes a pellet puff. The windows below are the reference used in Chinese aquafeed mills.

A line built to these fineness specs is shown in the small floating fish feed pellet line, 40-800 kg/h.

Mesh windows by species

Finer grinding raises surface area for digestion and starch gelatinization, which helps both puffing and pelleting. For crab, shrimp, eel and turtle feeds, the fineness spec is often the difference between a pellet that holds water stability and one that clouds the tank.

How to judge grinder screen wear

Fineness works because gelatinization is a heat-and-water penetration problem: smaller particles cook through faster and more completely at the same conditioning time. If pellets puff poorly and the barrel temperature is already in window, check grinder screen wear before changing the process - a worn screen quietly raises particle size and cuts cook degree. Hammer mill screen selection is the cheapest way to improve quality on the whole line.

Why Is Raw Material Moisture 15-27% Not the Same as Conditioning Moisture?

Two readings get confused more often than any other pair in extrusion: raw material moisture and conditioning moisture. The 15-27% window describes the first one - the moisture of the meal entering the process, not a value measured after conditioning. Keep them apart - some literature quotes 12-14% for material at the extruder inlet, and mixing the two is the most common reason a proven window fails on the floor. 

The 15-27% incoming window

Inside the window, more moisture usually means lower bulk density and a higher expansion rate, because more water flashes to steam at the die. At the high end, shrinkage and final deformation increase; at the low end, energy use, mechanical wear and the surface-to-core texture difference rise. The practical setting is a compromise - enough water for expansion, but not so much that the dryer becomes the bottleneck. If your dryer already runs at capacity, trim the top of the moisture window first.

How to judge moisture on the running line

Judge the setting by pellet shape after drying and by dryer loading, not by how the meal feels in the mixer. A hand squeeze reads surface moisture, not how much water will flash at the die; sample pellets after the dryer and compare bulk density against the target instead.

Which Process Parameter Affects Puffing Most: Moisture, Temperature or Speed?

That window only sets the starting point; the moisture the conditioner adds is the first parameter that actually changes the pellet. Once the formula is fixed, process parameters matter in a clear order: conditioning moisture first, die temperature second, screw speed third. Feed extrusion moisture and temperature are the two conditioning parameters, and moisture is the stronger one - it moves bulk density and expansion rate faster than any temperature or speed change. That is why the conditioner deserves more attention than the control panel. Die temperature here is the melt temperature at the die, which rides on the barrel window covered next.

The impact ranking and the parameter windows

When a pellet is too dense, most operators reach for screw speed first; the correct first move is conditioning moisture, because moisture penetration decides how much starch actually gelatinizes before the die. Only then tune die temperature, then speed. Working in that order usually fixes a dense pellet in one shift. The table below collects the working window and the judgement call for every parameter named here.

The decision rule for conditioning moisture

Too little conditioning moisture gives high puffing but poor toughness; too much lowers bulk density, makes the pellet spongy and, at the extreme, too heavy. Adding moisture generally lowers bulk density and raises expansion rate - but only if conditioning time and temperature let the water penetrate evenly. A conditioner that runs full and hot does more for puffing than any screw change.

Steam dose, pressure, temperature and time have to be matched so water penetrates every particle evenly. Mismatch them and the extruder inherits a meal that is wet outside and dry inside, which shows up later as uneven density across the batch. Too much steam softens the meal and makes it sticky, so scale the dose with the starch level. Pressure that is too low gives poor penetration; too high over-wets the meal. Longer conditioning gives better cook and binding, limited by how much moisture the dryer can pull out afterwards.

What Barrel and Die Temperature Windows Give the Best Puffing?

For twin-screw processing of cereal-based formulas, the extrusion chamber runs 120-180 deg C, and 140-160 deg C gives the best result. Temperature is what turns a cooked meal into an expanding melt: moderate heat gelatinizes starch fully and raises overall expansion.

For frames with the thermal mass to hold this window at commercial rates, see the large extruder for fish feed.

The barrel band and the die setting

Below the band, starch does not gelatinize fully, protein does not denature, and cook degree stays low - the pellet comes out dense and its water stability suffers. Inside the band, expansion rises steadily with temperature, and 140-160 deg C is the sweet spot for cereal formulas. The die setting follows the same logic: a hot die promotes starch melting and gives a porous, light pellet, while a cool die gives a denser, water-stable pellet. The exact die temperature window is formula-specific and is to be confirmed against the factory technical confirmation sheet.

How to judge spray-out at the die

Above the band, the energy input exceeds the cohesive forces between particles and the melt sprays out at the die; heat-sensitive nutrients are damaged and puffing actually falls. Practical rule: hold 140-160 deg C on cereals, step down for heat-sensitive formulas, and treat visible steam at the die plus pressure swings as the signal that temperature is too high.

How Do Screw Configuration, Screw Speed and Die Design Split Radial and Longitudinal Expansion?

Radial and longitudinal expansion are controlled by different things: radial expansion follows screw configuration and shear, while longitudinal expansion follows screw speed.

For machines built around adjustable shear and die options, see the aquafeed expander.

The radial and longitudinal window

Shear elements - kneading blocks, reverse threads - store elastic energy in the melt, and that energy releases preferentially as radial expansion. A moderate fiber level combined with strong shear locks the bubble structure so the pellet grows outward instead of along its axis. High speed with strong shear lowers melt viscosity, which helps expansion but raises energy use; changing the screw element mix - for example adding kneading blocks - is how you tune shear and refine the bubble structure without changing the formula.

How to judge radial against longitudinal expansion

Longitudinal expansion is tuned with speed: high speed shortens residence time and reduces pressure, so the pellet stretches lengthways, while low speed favors radial expansion. On the die side, a small open area means high pressure and promotes puffing, while pellet size sets durability, water uptake and water stability - larger pellets absorb water more slowly and hold shape longer in the pond. Die changes are routine on dry single-screw frames, and frames with a sliding rail let the knife group move back quickly, so keep one spare die per product on the shelf.

How Should Cooling and Drying Be Staged to Retain Water or Reduce Moisture?

Cooling and drying decide final density. If pellets cool unevenly, density varies across the batch and the classic symptom appears: some pellets float and some sink in the same load. Staging the cooler is how you control which way the batch goes - and it is the last place to look when float behaviour drifts.

See how the cooler and dryer sit in the feed production line machinery sequence.

The FAO manual chapter on feed processing and product quality covers the same principle from the storage and product-quality side.

If your cooler is the bottleneck, ask for a cooling and drying layout review - we reply with an itemised quotation, EXW/FOB basis; CIF available on request.

The quick-cool and slow-cool windows

Two modes cover most needs. Quick cooling locks water in: the pellet surface sets fast, moisture stays inside, and the product keeps its aroma and heat-sensitive components. Slow cooling removes moisture: longer residence pulls water down toward the safe storage limit, which protects shelf life. The finished target is moisture at the high end of the safe storage standard, a smooth surface, moderate hardness, low fines and retained flavor - the checklist summarised in the September 2026 article from 新饲料工艺杂谈.

How to judge even drying by the float-or-sink check

Check the discharge directly: scoop samples from several points across the cooler and float them. A batch that splits is telling you there is a flow or air-distribution problem in the dryer or cooler, not a formula problem - fix the airflow and bed depth first, because re-tuning the extruder to compensate for a bad cooler just moves the fault downstream.

FAQ: Feed Extrusion Parameters and Puffing Degree