Screw Oil Press (Expeller)

How the screw press works

Inside the barrel, a hardened worm (screw) shaft turns within a cage of steel bars set with fine gaps. As seed advances, the shrinking free volume raises pressure; oil is forced out through the cage-bar gaps and drains away, while the compacted solids leave as cake through a choke whose gap the operator adjusts. The friction of compression generates heat, which lowers oil viscosity and helps extraction — the basis of hot pressing. Throughput, cake thickness and residual oil are all set by worm design, choke setting and how well the seed was prepared.

What sets yield

Yield on a screw press is governed, in order, by seed preparation (moisture in the 7–9% window, roasting for hot pressing), pressing-element condition, choke setting (a tighter choke thins the cake and lowers residual, up to the machine's limit), and only then machine size. A single pass typically leaves 10–18% oil in cake; multi-stage or double pressing reaches 5–8%. Cold-press operation deliberately runs slower and cooler to keep oil temperature low (commonly cited below ~60°C), trading yield for native flavor.

Wear parts and maintenance

The worm and cage bars are consumables: they abrade with throughput and seed hardness, and as clearances open the press can no longer hold pressure, so residual oil creeps up. This is the usual cause of a slow, unexplained yield decline. Treat replacement as a scheduled operating cost, keep a spare worm-and-bar set, and remember that good cleaning — removing abrasive grit and stones — directly extends element life.

Choke, cake and the yield you can actually reach

The most accessible control on a screw press is the choke — the adjustable restriction at the cake outlet. Tighten it and pressure rises, the cake thins, and more oil is driven out, lowering residual up to a machine-specific limit beyond which the press overloads, overheats or jams; loosen it and throughput and temperature ease at the cost of leaving more oil in the cake. Operators tune the choke by cake feel and appearance: a well-pressed cake exits as firm, relatively dry flakes or a hard collar, while an under-pressed cake is soft and visibly oily. Because the optimum shifts with seed, moisture and worm wear, choke setting is a running adjustment rather than a one-time value — a quick daily cake check belongs in every mill's routine, and it is the single habit that keeps a screw press near its best achievable yield.

Heat, single vs double pressing, and sizing

A screw press turns much of its drive power into friction heat, so the chamber runs well above ambient even without external heating — useful for yield but, in excess, a cause of darker oil, so temperature is a variable to steer via feed rate, choke and worm speed. On recovery, a single pass leaves roughly 10–18% oil in the cake; a second pass or a purpose-built multi-stage bar-cage press reaches about 5–8%. Whether the extra pass pays depends on the oil still in the cake, its price and the cake's value — a calculation the ROI calculator handles with your numbers. Size the press against your daily target with the capacity calculator, remembering that preparation and element condition move yield more than nameplate size does.

Buying and running a screw press well

When choosing a screw press, the specification that matters most is rarely headline capacity — it is suitability for your seed and the availability of wear parts. A press matched to your oilseed, with readily sourced replacement worms and cage bars and a controllable choke and drive, will out-earn a nominally larger machine that is hard to maintain. In operation, the discipline is simple and repeatable: prepare seed to the right moisture, keep it clean to protect the elements, tune the choke to the cake, watch outlet temperature, and replace wear parts on schedule. Do these and a screw press delivers steady yield for years; neglect them and yield drifts down invisibly. It is the most forgiving and versatile oil-extraction machine available, which is exactly why it anchors the majority of the world's small and mid-size oil mills.

Indicative specifications

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