Screw (Expeller) Pressing

What screw pressing is

Screw pressing (the machine is universally called an expeller, after the original Anderson trade name) is the mechanical method behind most of the world’s pressed oil. Prepared seed — cracked, flaked and usually conditioned — is fed continuously into one end of a horizontal barrel or cage, and a rotating worm shaft conveys it toward a restricted discharge. Because the worm’s root diameter increases and its pitch tightens along the shaft, the space available to the material shrinks as it travels, so the seed is progressively compressed. The pressure squeezes liquid oil out through fine slots in the cage wall, while the compacted solids are extruded at the far end as a shaped cake. Unlike a batch hydraulic press, this happens continuously, which is why the expeller became the standard industrial press.

How the screw builds pressure

The heart of the machine is the way the worm and cage generate pressure without a separate pump. As the material is dragged forward by the flights of the screw into an ever-smaller volume, and is held back by the resistance of the choke at the discharge, it is squeezed hard against the cage bars. The cage is not solid: it is built from hardened bars or a slotted barrel with narrow gaps sized to let oil drain out but hold the solids in. The pressure is set mainly by the choke or cone at the outlet — tightening it restricts the cake, raises internal pressure and drives more oil out (but reduces throughput and raises temperature); opening it does the reverse. So an operator tunes yield against throughput and heat largely through this one adjustment, together with worm configuration and feed rate. It is a genuine operating skill, not a set-and-forget machine.

Heat, friction and quality

Screw pressing generates its own heat. The intense friction and shear of compressing seed against the cage raises the temperature of the oil and cake, often substantially, even without external cooking — which is why a machine run hard is effectively hot pressing. This heat helps yield (it lowers oil viscosity and ruptures more cells) but works against quality and flavour, and can degrade heat-sensitive oils. To run a genuine cold press, operators slow the machine, open the choke and often cool the barrel so the oil stays below a target temperature — accepting lower yield as the price of a cooler, better-tasting oil. Some presses circulate water through the shaft or cage to control temperature. The hot-versus-cold decision, in other words, is largely made at the screw press through how aggressively it is run and how much the temperature is allowed to climb.

Full-press versus pre-press

How far a screw press takes the oil out defines two modes. In full pressing, the expeller is run hard to recover as much oil as mechanically practical in one pass (sometimes two), leaving a cake with a residual oil of roughly 6–12% depending on seed and machine — this is the mode of standalone mills that sell both oil and cake and do not use solvent. In pre-pressing, the press is run more gently to remove only part of the oil, leaving a higher-oil cake (often around 15–20%) that then goes to solvent extraction to recover the rest. High-oil seeds such as sunflower, rapeseed and groundnut are usually pre-pressed then extracted, because trying to full-press them wastes oil and strains the machine. This split is important enough that pre-pressing has its own page.

Single and double pressing

Yield can also be raised by pressing more than once. In single pressing, the seed passes through the expeller a single time; in double pressing, the cake from the first pass is re-fed (often after re-conditioning) through the same or a second press to squeeze out more oil. Double pressing lifts recovery and lowers cake residual, at the cost of extra energy, throughput and wear — a trade-off small mills weigh with tools like a double-press calculator. Cold-press operations sometimes double-press precisely because each pass is run gently to keep the oil cool, so two mild passes recover what one hard pass would. Whether to single- or double-press is therefore another lever, alongside the choke setting and pressing temperature, for balancing yield, quality and cost.

The cage, bars and wear parts

A screw press is a wearing machine, and that shapes how it is owned and run. The two hardest-working parts are the worm (screw) flights and the cage bars (or slotted barrel), both of which abrade continuously as gritty, sometimes hull-bearing material is forced past them under pressure. As flights and bars wear, the internal clearances open, pressure falls and cake residual oil creeps up — so a press that once left 8% in the cake may quietly drift to 11% as parts age, silently costing yield. Good operations therefore treat wear parts as consumables, monitor cake oil as an early warning, and rebuild worms and re-bar cages on a schedule. Thorough cleaning upstream — de-stoning and magnetic separation — directly extends this life by keeping stones and metal out of the press. Wear is thus not a fault but a running cost to be managed, and one reason cake-oil monitoring is a standard mill discipline.

Throughput, energy and what limits yield

Several dials set how a given expeller performs, and they interact. Feed rate and choke setting trade throughput against yield and temperature; preparation quality (flake thickness and conditioning) sets the ceiling the press can reach; and seed moisture has a sweet spot — too wet and the press slips and the cake is soft, too dry and it presses poorly. Screw pressing is also energy-intensive, since driving the worm against high internal pressure draws real motor power, which is why energy per tonne is a live cost that operators track with tools like an electricity calculator. The practical limit on mechanical yield is not the press alone but the whole chain — seed, preparation, moisture, machine condition and how hard it is run — which is why a disappointing yield is diagnosed across all of these rather than blamed on the press by itself.

Where screw pressing fits

Screw pressing sits at the centre of the oil-extraction map. For small and medium mills, a screw press (or a bank of them) is the whole extraction step: seed in, oil and cake out, no solvent, no explosion-rated building. For large plants, the screw press is usually the pre-press ahead of solvent extraction, or is skipped entirely for low-oil soybean, which many big plants flake and send straight to solvent. Compared with hydraulic pressing, the screw press wins on throughput and continuity; compared with solvent extraction, it recovers less of the oil but needs no hexane, no explosion-proofing and far less scale to be viable — which is exactly why it dominates the small- and mid-scale world. To choose and size an actual machine rather than understand the process, see screw press machines.

Key figures (indicative)

FAQ